Education · Propagation
How the sun shapes the bands
The ionosphere is alive — rebuilt every day by the sun. Every number on the dashboard tells a piece of that story. This page explains all of them, so you can read conditions like an elmer.
On This Page
Concept 01
The Ionosphere — your invisible antenna
Every HF contact you make travels not through space but through a mirror — an ionized layer of atmosphere 60–1000 km above Earth. The sun builds that mirror fresh every day.
The sun constantly fires ultraviolet light and X-rays into Earth's upper atmosphere. When those photons slam into air molecules, they knock electrons loose, creating a plasma — charged particles that can bend and reflect radio waves the way glass bends light.
This ionized region is called the ionosphere, and it has distinct layers:
- D-layer (60–90 km) — exists only in daylight. It absorbs signals on 160 m and 80 m during the day, which is why those bands are noisy at noon and open at night. When the sun sets, the D-layer disappears and the absorber is gone.
- E-layer (90–150 km) — present by day, much weaker at night. Responsible for sporadic-E (Es) propagation — those sudden, brief openings on 6 m and 10 m that can make contacts across thousands of miles without warning.
- F-layer (150–1000 km) — the workhorse for DX. At night the F-layer merges into one layer; by day it splits into F1 and F2. The F2 layer, pushed higher by solar heating, provides the long-distance skip that makes 20 m and 15 m DX bands work.
The key insight: more sunlight → more ionization → higher bands open; less sunlight → D-layer drops → lower bands open at night. The greyline, the K-index, the SFI — all of them describe different aspects of what the sun is doing to this mirror right now.
Why HF and not VHF?
The ionosphere bends radio waves depending on frequency. Below roughly 30 MHz (HF), the plasma can refract signals back to Earth. Above 30 MHz, signals punch straight through into space — which is why VHF and UHF require line-of-sight or satellites.
License exam relevance
Ionospheric layers and HF propagation appear in Element 3, Group G3 of the General class question pool — skip distance, MUF, critical frequency, and greyline are all testable topics.
Concept 02
Live Data Tiles — four numbers to check first
The top of the dashboard shows four real-time measurements pulled from NOAA's Space Weather Prediction Center every 15 minutes. Here's what each one means and what you should do when you see it.
Solar Flux Index (SFI)
The SFI — also called the F10.7 index — is a daily measurement of the sun's radio emissions at a wavelength of 10.7 cm (2800 MHz), taken at the Dominion Radio Astrophysical Observatory in British Columbia. It has been measured continuously since 1947, making it the longest unbroken record of solar activity.
SFI tracks ionization of the F-layer almost perfectly without requiring direct UV/X-ray measurements in space. Higher SFI means the sun is pushing more energy into the ionosphere, lifting the maximum usable frequency (MUF) upward. When MUF rises above a band's frequencies, that band opens.
Operating tip: SFI above 150 is when 10 m and 12 m come alive and you can work stations with a wire antenna that would normally require a beam. Check the SFI before deciding which bands to call CQ on.
A-Index (Ap)
The A-index is a daily planetary index (Ap) of geomagnetic activity, derived by averaging eight 3-hour K-index measurements and converting them to a linear scale from 0 to 400. It's the "daily report card" for geomagnetic conditions — easier to compare day-to-day than the real-time K-index.
The dashboard shows the Ap value derived from the current real-time Kp reading, so it gives you an idea of how the day has been trending — not just the last 3 hours.
K-Index (Kp)
The K-index is a quasi-logarithmic scale from 0 to 9 that measures disturbance of Earth's magnetic field over a 3-hour window. It's collected by a worldwide network of magnetometers and averaged into the planetary Kp index. The dashboard shows the real-time 1-minute estimate from NOAA's GOES satellite network — it updates more frequently than the official 3-hour value.
Geomagnetic storms are caused by coronal mass ejections (CMEs) or high-speed solar wind streams hitting Earth's magnetosphere. As the disturbance compresses and distorts the magnetic field, induced currents in the ionosphere scatter and absorb HF signals — especially on paths that cross the polar regions.
Operating tip: A Kp of 3 or lower with a high SFI is the sweet spot for DX — good ionization with a calm magnetosphere. When Kp spikes to 5+, shift to local and regional contacts and work north-south paths carefully.
X-Ray Class
NOAA's GOES satellites continuously measure the sun's X-ray output in the 0.1–0.8 nm band. The X-ray class tells you the current solar flare intensity — or, when there are no active flares, the background radiation level. It's updated every minute from the primary GOES satellite.
The class system uses letters followed by a number. Each letter is 10× more intense than the previous:
Within each class, a decimal number multiplies the intensity (e.g., M5.0 is five times an M1.0). X-class flares go above X10 in the most extreme events (the 2003 "Halloween storms" produced an X28).
What to watch for: During an X or strong M flare, the sunlit hemisphere of Earth gets flooded with X-rays simultaneously — skip on east-west paths may vanish in minutes. The blackout usually recovers within an hour. Night-side paths are unaffected.
Concept 03
HF Band Conditions — the S-meter grid
Below the live tiles, the dashboard shows an S-meter–style grid for each major HF band. Here's how to read it and how the model works.
What the ratings mean
Each band tile shows a label — Excellent, Good, Fair, Poor, or Closed — along with filled bars like an S-meter. This is a modeled estimate of current conditions based on three inputs:
- SFI (Solar Flux Index) — determines whether the ionosphere has enough ionization to support the band at all. Each band has a minimum SFI threshold: 10 m needs SFI ≥ 90 to have a chance at all; 80 m works even when SFI is 65.
- Time of day (UTC) — the dashboard uses your browser's UTC clock. D-layer absorption is highest midday and absent at night, so 80 m and 40 m are poor by day and excellent at night. 10 m and 15 m need daytime F2 ionization, so they fade after sunset.
- Kp (geomagnetic index) — storm penalties are applied when Kp ≥ 4. Higher bands that depend on the F-layer take the biggest hit. Each step above Kp 4 drops conditions one level across all bands, with higher bands hit harder.
What it is — and isn't
This is a propagation model, not a measurement. Real HF conditions vary enormously with your location, the path you're working (short vs. long path, polar vs. equatorial), antenna, and local noise floor. The model gives you a starting expectation — think of it as the forecast, not the observation. Always trust a live cluster spot or a wsjt-x waterfall over a modeled estimate.
The model intentionally errs on the side of caution: a band rated "Fair" may actually be working beautifully if you're on the right path at the right time. "Closed" means conditions are against you, not that the band is silent — Es and transequatorial propagation (TEP) can open bands regardless of SFI.
Bands covered
- 10 m (28 MHz) — king of solar maximum
- 12 m (24 MHz) — often overlooked but rich at high SFI
- 15 m (21 MHz) — the daily workhorse near solar max
- 17 m (18 MHz) — reliable mid-cycle band
- 20 m (14 MHz) — the all-time DX standard
- 30 m (10 MHz) — CW/data only, usually open day and night
- 40 m (7 MHz) — night DX, day regional
- 80 m (3.5 MHz) — regional/continental; noisy by day
Maximum Usable Frequency (MUF)
The MUF is the highest frequency the ionosphere can refract back to Earth on a given path. When SFI rises, MUF rises. The band conditions model essentially asks: "Is this band's frequency below the current MUF for typical paths?" If yes, it can propagate; if no, the signal punches through and is lost in space.
Concept 04
Time Slider — plan ahead, look back
The slider below the band conditions grid lets you shift the simulated time ±12 hours from now, updating both the band conditions and the greyline map simultaneously.
Move the slider left and the model rewinds — showing you what conditions looked like (or how the model would have rated them) up to 12 hours ago. Move it right and the model fast-forwards — letting you see what to expect tonight, or plan tomorrow morning's contest operating.
What changes with the slider:
- Band condition ratings — day/night D-layer absorption shifts based on the simulated UTC time. A band that's "Poor" now might be "Excellent" in 6 hours once darkness spreads across your target continent.
- Greyline map — the terminator line and day/night shading update to match the simulated time, so you can see where the greyline will be during your planned operating window.
The SFI and Kp used in the model are still today's live values — the slider only changes the time-of-day component. Real geomagnetic conditions 12 hours from now may differ. Use this as a planning tool, not a precise forecast.
To return to real time, click the Reset button that appears whenever you move the slider away from zero.
When to use the slider
- Planning a contest operating schedule
- Figuring out when Europe comes alive on 20 m from the US East Coast
- Checking when your target continent enters daylight (for high bands)
- Seeing when 80 m opens up for a midnight DX session
- Understanding why conditions were worse this morning than you expected
Concept 05
Greyline Map — the DXer's secret weapon
The greyline (also called the terminator or grey line) is the moving boundary between day and night. It's one of the most valuable tools in amateur radio propagation — and one of the most misunderstood.
Why the greyline matters
At the greyline, two things happen simultaneously:
- The D-layer is collapsing (on the dusk side) or hasn't reformed yet (on the dawn side). Without D-layer absorption, lower-frequency HF signals travel much farther.
- The F-layer remains strongly ionized — it doesn't disappear instantly at sunset. This creates a brief window where the ionosphere is both maximally reflective (strong F-layer) and minimally absorptive (no D-layer).
The result: stations near the greyline can work the entire world on 40 m, 30 m, and 80 m for 15–30 minutes each morning and evening. The closer your path runs along the terminator, the longer that window lasts.
Reading the map
The map on the dashboard renders in real-time (or simulated time if you've moved the slider) using pixel-by-pixel math. The bright area is dayside — F-layer ionized, D-layer absorbing low bands. The dark area is nightside — D-layer gone, F-layer present but cooling. The gradient band between them is the greyline itself.
The subsolar point is the dot where the sun is directly overhead right now — the point of maximum solar intensity. In summer it's in the Northern Hemisphere; in winter, Southern. The sun's position drives everything else on the page.
Using it for DX
- Find the greyline passing over or near your target country — that's your window.
- Both stations benefit most when the greyline is at or near both ends of the path simultaneously (long path).
- On 40 m, greyline contacts of 10,000+ miles are routine during the 15–30 minute window.
- Use the time slider to find exactly when the greyline reaches your target region and plan your operating time around it.
Long path vs. short path
Every path between two points on Earth has two routes: the short path (direct) and the long path (going the other way around, always 180° from short path). The long path often runs through different propagation conditions and can be surprisingly productive, especially around the greyline.
Greyline and the exam
The General class exam asks about the greyline in Group G3C. Key facts: the greyline moves at roughly 1,000 mph (the speed of Earth's rotation); DX is enhanced at the greyline because D-layer absorption is minimized; the effect is most pronounced on 40 m and 80 m.
The subsolar point
The subsolar point is where the sun is directly overhead. Its latitude is the sun's declination — it ranges from 23.5°N (summer solstice) to 23.5°S (winter solstice) and crosses the equator at the equinoxes. Its longitude moves westward at 15° per hour as Earth rotates.
Concept 06
Solar Imagery — seeing the sun live
Three live images from NASA and ESA spacecraft let you see what the sun is actually doing right now. Each image shows a different layer or region of the sun, revealing different types of solar activity.
Hot Coronal Plasma
This image comes from NASA's Solar Dynamics Observatory (SDO), using the Atmospheric Imaging Assembly (AIA) camera tuned to 193 Ångströms — extreme ultraviolet light. At this wavelength you're seeing plasma at approximately 1.5 million °C (the corona), plus hotter flare plasma around 20 million °C.
What to look for:
- Active regions — bright, complex areas, often in pairs. These are where sunspots are below and where flares and CMEs originate.
- Coronal holes — dark patches with relatively little plasma. They're regions of open magnetic field lines where solar wind streams outward at high speed. When a coronal hole faces Earth, geomagnetic activity often rises 1–3 days later.
- Loops and arcades — bright arches connecting opposite-polarity regions. When these erupt, they produce CMEs.
The image updates every 15 minutes (matching NASA's public release cadence) with a cache-bust parameter to ensure you see the freshest frame.
White-Light Sunspot Map
The HMI (Helioseismic and Magnetic Imager) on SDO captures the photosphere — the visible surface of the sun — in white light. This is the closest thing to what the sun would look like through a properly filtered telescope.
What to look for:
- Sunspots — the dark regions. Their dark appearance is because they're cooler (~3,500°C vs. ~5,500°C for surrounding photosphere), caused by intense magnetic fields suppressing convection. More and larger sunspot groups correlate with higher SFI.
- Sunspot groups (active regions) — catalogued as NOAA AR numbers. Complex groups with multiple polarities are the flare producers.
- Limb darkening — the edge of the visible disk appears darker. This is normal, not activity.
Counting sunspot groups is something amateur astronomers contribute to — the international sunspot number (SSN) tracks solar cycle progress and correlates directly with SFI.
Coronagraph — CME Detection
LASCO (Large Angle and Spectrometric Coronagraph) on ESA/NASA's SOHO spacecraft uses a disk (occulter) to block the blinding photosphere, revealing the faint corona and anything flying through it. The C3 field of view covers from about 3.7 to 32 solar radii — wide enough to see CMEs departing the sun.
What to look for:
- CMEs — expanding cloud-like bright structures blooming outward from behind the occulter disk. A halo CME (expanding in all directions) means it's heading toward Earth. Arrival typically takes 1–3 days depending on speed.
- Solar energetic particle (SEP) storms — during major events, high-energy protons bombard the LASCO CCD, creating a "snow storm" of bright speckles across the image. This happens fast (minutes after a large flare) and is a warning sign.
- Background streamers — the stable, radially oriented bright lines visible in quiet periods. These are helmet streamers — the ambient coronal structure that follows magnetic field lines out from the sun.
If you see a halo CME in the LASCO image, expect elevated Kp in 1–3 days and prepare for possible HF degradation.
Concept 07
History & Charts — reading the trend
The dashboard includes sparklines on each live tile and a full 24/72-hour trend chart below. Context matters as much as the current reading.
Sparklines on the tiles
Each live data tile shows a small line chart underneath the current value. This is a 24-hour sparkline drawn from hourly data stored in the database. It shows you at a glance whether conditions are rising, falling, or stable — without needing to read a number.
- An upward SFI trend means the sun is becoming more active — high bands may be opening.
- A rising K-index that just spiked is more concerning than a K of 4 that has been steady for 12 hours — the storm may still be building.
- A falling A-index after a storm peak is a good sign; conditions often recover quickly once the disturbance passes.
The trend chart
The full chart below the sparklines lets you select a 24-hour or 72-hour window and plots SFI, Kp, and X-ray flux over time. Use it to see:
- Whether today's SFI represents a change from yesterday, or if it has been stable
- When a geomagnetic storm started and how quickly Kp is recovering
- Whether there have been any significant flares in the last 72 hours
- Overall trends entering a contest weekend
All historical data is recorded from the live cron feed — one measurement roughly every 15 minutes, stored in a Cloudflare D1 (SQLite) database and served from the API endpoint.
Sudden Ionospheric Disturbances
A SID (Sudden Ionospheric Disturbance) appears in the X-ray chart as a sharp spike. It coincides exactly with a flare — X-rays travel at the speed of light, so the D-layer on the sunlit side is flooded instantaneously. This is different from a CME, which takes days to arrive.
Post-storm recovery
After a geomagnetic storm, conditions don't always snap back immediately. The ionosphere can remain disturbed for 1–2 days. Watching the A-index trend in the 72-hour chart tells you whether you're still in the storm or entering recovery.
Concept 09
Grid Map — Maidenhead locators and propagation
The Grid page overlays HF propagation conditions on a world map divided into the Maidenhead locator grid system. It's the same grid used in contest exchanges and DX spotting.
The Maidenhead grid
The Maidenhead Locator System divides Earth into a hierarchy of grid squares, each identified by a short code. For amateur radio purposes, a 4-character locator (e.g., FN42) identifies a 1°×2° area, which is accurate enough for most purposes. A 6-character locator (e.g., FN42aa) pinpoints a specific 2.5'×5' square.
Grid squares are used for:
- Contest exchanges — VHF/UHF contests, POTA activations, and satellite contests all use grid squares as the scoring unit.
- Antenna pointing — once you know a station's grid, you can compute beam heading and distance exactly.
- Awards — many awards (VHF Century Club, etc.) require contacts with a certain number of unique grid squares.
Propagation overlay
The grid map colors each cell based on the same HF propagation model used by the band conditions widget, combined with the current SFI, Kp, and time of day. The coloring reflects expected conditions from your location to that grid square on the selected band — not a global average.
The locator lookup tool lets you enter a Maidenhead grid to center the map and compute the short-path bearing and distance from a reference location. This is useful for:
- Finding a DX station's heading before turning your beam
- Checking which part of the world is currently sunlit and likely to have good F2
- Planning a POTA activation to maximize contacts from a given grid
Finding your grid square
Your 4-character grid square starts with two letters (field) and two digits (square). Enter your grid in the locator box on the Grid page. If you don't know your grid, websites like qrz.com show it in your profile, or your GPS coordinates can be converted instantly online.
Short path vs. long path
The Grid page shows short-path bearings by default. Long path is always 180° from short path. Long-path contacts are most productive when the short path runs through an absorbing region (polar path during a storm, over the sunlit hemisphere at midday for low bands) and the long path avoids it.
Concept 10
The Solar Cycle — where are we now?
Everything on this dashboard waxes and wanes over an 11-year cycle. Understanding where we are in the cycle tells you what to expect for the next several years.
The cycle basics
The sun's magnetic field reverses polarity roughly every 11 years, driving a cycle of activity from solar minimum (few sunspots, low SFI, quiet magnetosphere) to solar maximum (many sunspots, high SFI, frequent flares and CMEs) and back.
We're currently in Solar Cycle 25, which began in December 2019. Cycle 25 has been significantly more active than predicted — the official forecast expected a weak cycle like Cycle 24, but sunspot numbers and SFI have repeatedly exceeded predictions. Solar maximum is estimated around 2025–2026.
What the cycle means for HF
- Near maximum (now): SFI regularly above 150, sometimes above 200. 10 m and 12 m wide open globally for months at a time. Frequent moderate flares. Some geomagnetic storms from CMEs and coronal holes.
- Declining phase (2026–2028): SFI drops toward 120–130. 10 m still opens but less reliably. 15 m and 17 m remain productive. Flare frequency decreases.
- Near minimum (2029–2031): SFI drops to 70–80. 10 m rarely opens. Shift energy to 40 m, 80 m, and 160 m for reliable DX. The ionosphere is still there — you just need lower frequencies.
The key numbers to track over time
The Smoothed International Sunspot Number (SSN) is the official metric for cycle progress, published monthly by the World Data Center for the Sunspot Index (SILSO). The SFI on this dashboard tracks it daily — an SFI above 200 means we're in truly exceptional solar activity; below 70 is deep minimum.
Amateur radio operators who've been licensed across a full solar cycle develop an intuition for this rhythm — knowing that the great 10 m opening you're enjoying now won't last forever is part of the art. Working new countries on 10 m during the peak is valuable; when the cycle descends, those propagation paths close for a decade.
Solar Cycle 25 highlights
- Start: December 2019
- Predicted max SSN: ~115 (weak cycle)
- Actual max SSN: 230+ (strong cycle)
- Notable events: X9.0 flare (2024), multiple G4 storms
- Expected minimum: ~2030–2031
Best bands by phase
- Solar max: 10 m, 12 m, 15 m
- Rising/falling: 15 m, 17 m, 20 m
- Solar min: 20 m, 30 m, 40 m, 80 m, 160 m
Quick Reference
At-a-glance tables
Print these or bookmark this page. These numbers appear on every solar dashboard and in the General class exam.
Solar Flux Index (SFI)
- < 80Low activity. 10/12/15 m mostly closed. Work 40/80 m.
- 80–110Moderate. 15/17 m open regionally. 10 m spotty.
- 110–150Good. 10/12/15 m open to DX. Look for openings.
- > 150Excellent. All bands active. 10 m wide open worldwide.
K-Index (Kp)
- 0–2Quiet. Excellent conditions, stable polar paths.
- 3–4Unsettled. Minor polar degradation, watch high-latitude paths.
- 5–6Storm (G1–G2). HF degradation, aurora at mid-latitudes.
- 7–9Severe storm (G3–G5). HF largely unreliable.
X-Ray Flare Classes
- A, BBackground / very minor. No HF impact.
- CMinor flare. Rare HF impact on very long sunlit paths.
- MModerate. Brief short-wave fadeout possible. Monitor.
- XMajor. HF blackout on sunlit side. May last 1+ hour.
Geomagnetic Storm Scale (G-scale)
- G0Kp < 5. Quiet to unsettled. No storm.
- G1–G2Kp 5–6. Minor to moderate. Watch polar paths.
- G3Kp 7. Strong. HF degraded broadly. Aurora to 50°N.
- G4–G5Kp 8–9. Severe to extreme. Near-total HF blackout.
Concept 11
NOAA Storm Scales — R, S, and G
NOAA SWPC publishes three numbered scales (0–5) that describe the severity of different types of space weather events. The dashboard displays the current values from the WWV/WWVH geophysical alert bulletin, updated hourly.
R — Radio Blackout
Caused by solar X-ray flares ionizing the D-layer. The D-layer absorbs HF signals — stronger ionization means stronger absorption. R1 produces minor degradation on low HF at high latitudes; R3–R4 causes wide-area blackouts on the sunlit side of Earth; R5 (X20+ flare) causes complete HF blackouts on the sunlit hemisphere for hours. Watch for elevated X-ray flux (C, M, X class) as the precursor.
S — Solar Radiation Storm
High-energy protons from large flares or CMEs can reach Earth in minutes to hours. S1–S2 are minor; S3 affects satellite operations and can cause elevated noise on polar-path HF links; S4–S5 events are rare and may disrupt transpolar airline communications. The S scale is most relevant to operators running polar or transpolar paths.
G — Geomagnetic Storm
Caused by solar wind pressure pulses and coronal mass ejections interacting with Earth's magnetosphere. This is the scale HF operators watch most closely. G1 (Kp 5) causes minor absorption at high latitudes; G2 (Kp 6) brings aurora to 55° latitude and noticeable HF degradation at high latitudes; G3 (Kp 7) degrades mid-latitude HF and generates very active aurora; G4–G5 events are rare and can shut down HF across entire hemispheres. The K-index tile on the dashboard is the real-time proxy for the G scale.
Scale to Kp mapping
- G0 — Kp 0–4 — no storm
- G1 — Kp 5 — minor storm
- G2 — Kp 6 — moderate storm
- G3 — Kp 7 — strong storm
- G4 — Kp 8 — severe storm
- G5 — Kp 9 — extreme storm
Reading the dashboard badges
When an R, S, or G badge on the dashboard turns amber or red, conditions are active. All three at zero (R0, S0, G0) means no space weather event is in progress — a good time for DX on all bands.
Concept 13
X-Ray Flux Chart — tracking solar flares in real time
The X-ray flux chart on the dashboard shows 6 hours of data from NOAA's GOES satellite. It's the most sensitive indicator of impending radio blackouts — a flare shows up in the X-ray data minutes before any propagation effect is felt on the bands.
Reading a logarithmic scale
X-ray flux spans several orders of magnitude, so the chart uses a logarithmic Y-axis. The labels A, B, C, M, and X correspond to the standard solar flare classification system:
- A (≈ 10⁻⁸ W/m²) — background quiet sun; no propagation effect.
- B (10⁻⁷) — slightly elevated; occasional minor HF enhancement at the subionospheric level.
- C (10⁻⁶) — weak flares; possible brief HF degradation at very high latitudes (R0–R1).
- M (10⁻⁵) — moderate flares; temporary HF blackouts on the sunlit side (R1–R2). May affect low HF bands for 10–30 minutes.
- X (10⁻⁴ and above) — major flares; wide-area HF blackouts (R3–R5). X1 and above can shut down 10 m through 15 m on the dayside for hours.
What to watch for
A sharp spike on the chart — rising steeply within 5–15 minutes — is a flare in progress. The R-scale badge on the dashboard will update as NOAA publishes the event. If you lose a path suddenly during the day and the X-ray chart shows a recent spike, the flare is the likely cause. Most flares are short-lived (minutes to an hour); conditions recover quickly once the flux drops back to background levels.
Flare class and duration
- C flares — 5–15 minutes, minor effect
- M flares — 15–60 minutes, moderate
- X1–X5 — 1–3 hours, significant blackout
- X5+ — hours, severe blackout on dayside
Only on the sunlit side
X-ray flares only affect the D-layer on the hemisphere facing the sun. If it's nighttime at your location, an ongoing flare may not affect your local bands — but paths to the sunlit hemisphere will be degraded.
Concept 14
Great-Circle Path Tool — planning a specific contact
The Grid page includes a path calculator that draws the great-circle arc between two Maidenhead grid squares and shows band conditions for that specific path.
Why great-circle matters for antennas
Radio waves travel the shortest path between two points on a sphere — the great-circle route — not straight lines on a flat map. For DX contacts, pointing your beam antenna along the great-circle bearing (not the direction that looks straight on a Mercator map) is essential. A station in Japan from the US East Coast is to the northwest on a Mercator map, but the great-circle bearing is actually closer to due north.
How to use the tool
Enter a TX (transmit) and RX (receive) Maidenhead grid square — 4 or 6 characters each. Click Draw and the tool:
- Plots the great-circle arc on the map, colour-coded yellow.
- Marks TX (cyan) and RX (pink) endpoints on the map.
- Calculates the great-circle distance in km and miles.
- Opens the band conditions panel for the midpoint of the path, showing which bands are most likely open on that particular route.
Short path vs. long path
The tool plots the short path (the shorter of the two great-circle routes). Long path is always the complementary arc — 360° minus the short-path distance. Long-path contacts are productive when the short path traverses a polar region during a storm, or when the long path is entirely on the night side for a low-band contact.
Finding grid squares
Your own grid square is shown on the Grid page after clicking "Locate me". For a DX station, their grid square appears in contest exchanges, DX cluster spots, and QRZ.com profiles. A 4-character locator (e.g. FN31) is accurate to ±1° longitude and ±0.5° latitude.
Path midpoint conditions
The band panel that opens on "Draw" shows conditions at the midpoint of the path — roughly where F2 reflection is most important for that contact. If the midpoint is in polar or high-latitude regions, expect increased storm sensitivity on polar paths.
Concept 15
Solar Wind Monitor — live DSCOVR data
The solar wind row on the dashboard shows real-time data from NOAA's DSCOVR satellite, positioned at the Sun–Earth L1 Lagrange point about 1.5 million km from Earth — roughly 30–60 minutes upstream of any solar wind that will hit our magnetosphere.
Three numbers to watch
- Bz (nT) — The north–south component of the interplanetary magnetic field (IMF). This is the single most important value for predicting geomagnetic storms. When Bz is strongly negative (southward), it reconnects with Earth's magnetosphere and allows solar wind energy to pour in. Sustained Bz below −10 nT typically leads to a geomagnetic storm. Positive Bz (northward) means Earth's field is mostly shielded. The mini-chart shows the 1-hour Bz trend — a sudden southward turn warrants attention.
- Speed (km/s) — Solar wind plasma speed. Normal is 300–500 km/s. After a coronal mass ejection, the shock front can arrive at 600–800 km/s or faster. High speed alone won't cause a storm — the Bz direction matters more — but a high-speed stream combined with southward Bz is a recipe for a severe G3+ storm.
- Density (p/cm³) — Proton density. Normal is 3–10 p/cm³. A density spike, especially in front of a CME, can compress the magnetosphere and briefly enhance storm effects.
Why DSCOVR matters
Before DSCOVR and its predecessor ACE, storms arrived with little warning. Now, the ~30–60 minute lead time from L1 means operators can see a negative Bz arriving before it affects the ionosphere — enough time to make a note that conditions may deteriorate, switch to a backup path, or wait out the storm.
Bz colour coding
On the dashboard, a strongly negative Bz turns the badge red — southward, storm-driving. Strongly positive Bz turns it green — northward, shielded. Near zero stays neutral grey. The mini-chart's zero-line makes trend direction easy to read at a glance.
Exam relevance
The IMF, solar wind speed, and CME lead time appear in General class Element 3, Group G3 as propagation-influencing factors. Understanding Bz direction is key to understanding why some CMEs cause storms and others pass harmlessly.
Concept 16
Radio Blackout Indicator — the R-scale decoded
The radio blackout indicator on the dashboard derives the NOAA R-scale in real-time from the live X-ray flux reading, and tells you which HF frequencies are currently affected on the sunlit side of Earth.
From X-ray flux to blackout scale
Solar flares flood the dayside ionosphere with intense X-ray radiation, dramatically increasing ionisation in the D-layer. The D-layer normally absorbs HF signals during the day, but during a flare the absorption can become so intense that signals are completely blocked — a short-wave fadeout (SWF). The NOAA R-scale maps flare class to impact:
Key operating insight
Blackouts only affect the sunlit hemisphere — if it's nighttime at your location, an X-class flare may not affect you directly, but paths to the dayside will be disrupted. After a flare, conditions usually recover within minutes to a couple of hours as X-ray flux drops back to background. Watch the X-ray chart for the characteristic rise-and-fall spike shape.
Low vs high bands
Higher HF frequencies are affected first and most severely. A moderate R2 event may only degrade 15–20 m, leaving 40 m and 80 m usable. An R4+ event can wipe out the entire 3–30 MHz range on the dayside.
Recovery time
- R1 — recovers in 5–15 minutes
- R2 — 15–60 minutes
- R3 — 1–2 hours
- R4/R5 — several hours
Concept 17
Solar Cycle 25 Chart — where are we in the cycle?
The Solar Cycle 25 chart in the Trends section shows the observed smoothed sunspot number (SSN) alongside NOAA's predicted curve and uncertainty band — placing today's conditions in the context of the entire 11-year cycle.
What the chart shows
- Solid green line — Observed monthly smoothed SSN, from the start of Solar Cycle 25 in December 2019 to the present. The smoothing averages 13 months to remove noise.
- Dashed line — NOAA's predicted SSN for the remainder of the cycle, updated monthly.
- Shaded band — The high/low uncertainty envelope around the prediction. Solar cycle prediction is still imprecise — the actual peak may arrive earlier, later, higher, or lower than the central forecast.
Why this matters for propagation
The SSN directly drives the F2-layer's critical frequency. Near solar maximum — when SSN is highest — the MUF is elevated for most of the day, opening 10 m, 12 m, and 15 m to worldwide DX. Near solar minimum, only 20 m and below remain reliably open for long-distance contacts. Cycle 25 is tracking significantly above the initial low predictions — the current peak suggests conditions similar to moderate Cycle 24 years, with excellent 10 m activity.
The current SSN label in the chart header shows the most recent observed monthly value. Compare it against the chart to see how close we are to the predicted maximum — and whether the trend is still rising or beginning to decline.
Solar Cycle 25 facts
- Cycle start: December 2019 (solar minimum)
- Predicted peak: 2025–2026
- Cycle 25 is running above initial predictions
- Expected end: ~2030–2031
SSN vs SFI
The Sunspot Number (SSN) and Solar Flux Index (SFI) are closely correlated but not identical. SSN counts visible sunspots; SFI measures radio emissions at 10.7 cm. The daily SFI you see on the dashboard is the operational value — the SSN is the historical benchmark used for cycle tracking.
Concept 18
MUF Estimator — finding the highest usable frequency
The Grid page great-circle tool now calculates the Maximum Usable Frequency (MUF) and Frequency of Optimum Traffic (FOT) for the specific path you draw, using the current Solar Flux Index.
What MUF means
The Maximum Usable Frequency (MUF) is the highest frequency that the ionosphere can refract back to Earth for a given path length and solar conditions. Above the MUF, signals pass through the ionosphere into space instead of bouncing back — the path is lost. Operating close to the MUF gives the strongest signals and longest skip distances, but also the most variability.
The Frequency of Optimum Traffic (FOT) is typically 85% of the MUF — the frequency that provides a reliable daily-median circuit while still being close enough to the MUF for good signal strength. Operators planning schedules should use the FOT as their target frequency; use the MUF as an upper bound to explore.
How the estimate is calculated
The estimator uses a simplified relationship between SFI and the F2-layer critical frequency (foF2):
foF2 ≈ 4.35 + 0.0141 × SFIMHz (vertical-incidence critical frequency)- For an oblique path of distance d km, the MUF is elevated by a geometry factor:
MUF = foF2 × √(1 + (d / 600)²) - The 600 km denominator corresponds to a nominal F2-layer height of 300 km and a single-hop reflection.
This is a rough estimate — actual MUF depends on geographic latitude, time of day, season, and ionospheric irregularities. For precise planning, tools like VOACAP or PropLab Pro use full ionospheric models. The dashboard estimate is useful for a quick sanity check on which bands are worth trying for a given path.
Using the result
After drawing a path on the Grid page, the info bar shows: distance · MUF ≈ X MHz · FOT ≈ Y MHz · Try [band]. Start on the recommended band, then work up toward the MUF for stronger signals — but expect more flutter near the top.
Exam relevance
MUF, FOT, and critical frequency appear prominently in General class Group G3. Understanding that MUF increases with SFI and path length — and that operating above MUF loses the path — is tested directly.
Concept 19
Push Notifications — space weather alerts on your phone
The Alerts button on the dashboard lets you subscribe to browser push notifications for space weather events — geomagnetic storms and major solar flares — so you never miss an opening or a blackout while away from the screen.
How it works
- Service worker — Your browser registers a background script (service worker) that can receive push messages even when the tab is closed, as long as the browser is running.
- VAPID keys — The server uses Web Push Protocol (RFC 8030) with VAPID authentication (RFC 8292). Your subscription is stored securely; no email or account is required.
- Encryption — Push payloads are end-to-end encrypted using RFC 8291 (aes128gcm content encoding). Only your browser can decrypt the notification — the push relay (Google, Mozilla, Apple) sees only ciphertext.
- Cron trigger — Every 15 minutes, the NOAA data cron checks Kp against your configured threshold, and X-ray flux for X-class events. If conditions cross your threshold, a push is sent.
Setting your threshold
The Kp threshold (default 5) determines when you're notified. Kp 5 is a G1 minor storm — already disrupting polar paths. If you only care about severe events, raise it to 6 or 7. If you want early warning for any disturbance affecting high-latitude paths, set it to 4. X-class flare alerts fire regardless of the Kp threshold.
To subscribe: click the Alerts button on the dashboard, choose your Kp threshold, and click Enable alerts. Your browser will ask for notification permission. To unsubscribe, click the same button and select Disable alerts — the subscription is removed from the server immediately.
Browser support
- Chrome / Edge / Firefox — full support
- Safari on macOS 13+ / iOS 16.4+ — supported
- Older browsers — button is hidden gracefully
Privacy
Your push endpoint, encryption key, and Kp threshold are stored in a Cloudflare D1 database. No personal information (email, location, call sign) is collected. Subscriptions can be deleted at any time via the Disable alerts button.
Concept 20
Aurora Visibility Forecast — where the lights might show
The Aurora row on the dashboard calculates the equatorward boundary of the auroral oval in real time from the live Kp index. When Kp rises, the aurora expands toward lower latitudes and more people can see it.
How the boundary is calculated
The auroral oval — the ring around each magnetic pole where charged particles from the solar wind collide with the atmosphere — shifts equatorward during geomagnetic storms. The dashboard uses the simplified empirical formula:
Equatorward boundary ≈ 67° − (Kp × 2°) latitude
This is a rough estimate based on statistical observations of the oval during different Kp levels. Real aurora visibility also depends on sky darkness, local magnetic declination, cloud cover, and how close you are to the boundary — being within a few degrees of the boundary gives the best chance of a display on the horizon.
What aurora means for radio
Aurora is simultaneously beautiful and disruptive for HF. When the oval expands, polar paths — Europe to North America via the Arctic, or any trans-polar route — can suffer severe absorption and scatter. VHF operators running EME or meteor scatter may notice aurora scatter on 2 m and 6 m, which produces a distinctive rasping sound on CW signals (aurora scatter is not audiophile-quality SSB territory).
Operating tip: When Kp climbs to 5 or above, abandon trans-polar DX attempts and work east-west paths or equatorial directions instead. On 6 m, deliberately try aurora scatter — tune to the edges of the aurora oval and listen for the characteristic watery echo.
The auroral oval
The oval is not centered on the geographic poles but on the magnetic poles, which are offset by several degrees. This is why aurora is more common in northern Scandinavia and northern Canada than at equivalent latitudes elsewhere — those regions sit closer to the magnetic pole.
Ham radio connection
Kp ≥5 is the standard threshold for push notifications on this dashboard. Setting your alert threshold to Kp 5 means you get a heads-up when aurora visibility is plausible for much of the northern hemisphere — useful both for astrophotography and for checking polar path openings.
Concept 21
Space Weather Outlook — 3-day probabilities from NOAA
NOAA's Space Weather Prediction Center issues probability forecasts for solar flare activity (R-scale) and geomagnetic storms (G-scale) for the current and next two days. The 3-Day Outlook row translates those forecasts into radio-operator terms.
What the probabilities mean
NOAA issues two probabilities for each day:
- C-flare / R1+ probability — the chance that any solar flare will reach R1 (minor radio blackout) or higher. Even a 40% probability doesn't mean a blackout is certain — it means conditions are elevated and the chance of at least a minor R1 event is about 40%. On high-SFI days with active regions, this number can stay above 30% for days at a time.
- M-flare / R3+ probability — the chance of a moderate-to-major radio blackout (R3 or worse). This matters more operationally: R3 means 10–30 MHz is blacked out across most of the sunlit hemisphere for 10–60 minutes. Watch for this rising during major active region transits.
- G-storm probability — the chance of at least a G1 geomagnetic storm. As with flares, even a 30% G-storm probability should put you on alert — a G1 at the wrong time on a polar path can wipe out a DX contact you've been trying for days.
Kp max per day
Each day card shows "Kp≤N" or "Kp≥N" — the forecasted maximum Kp derived from the predicted G scale. G0 = Kp below 5 (quiet), G1 = Kp 5, G2 = Kp 6, G3 = Kp 7, G4 = Kp 8, G5 = Kp 9. Use this to plan multi-day DX trips or Field Day preparation: if Kp≥6 is forecast for Saturday, polar and high-latitude paths may be compromised.
Operating tip: Check the 3-Day Outlook before scheduling a DX pile-up session or an international contest. G1+ probability above 25% is your cue to have a backup plan — either shorter paths, lower bands, or later in the day when storm effects sometimes ease.
Source: NOAA SWPC
The dashboard fetches noaa-scales.json from services.swpc.noaa.gov every hour. The file contains entries for yesterday (-1), today (0), tomorrow (1), and the day after (2). The dashboard displays periods 0, 1, and 2 to give a 3-day window.
Forecast accuracy
24-hour NOAA flare forecasts reach roughly 75–85% accuracy for the correct class. Storm forecasting is harder — CMEs can arrive early, late, or with different orientations than predicted. The Bz component (shown in the Solar Wind row) is only measurable ~30–60 minutes before impact at the ACE/DSCOVR L1 point.
Concept 22
Active Solar Regions — the sources of flares and CMEs
NOAA assigns sequential numbers to clusters of sunspots on the solar disk that show potential for flares. The Active Regions table on the dashboard lists the current numbered regions, their magnetic classification, and flare probability for the next 24 hours.
Mount Wilson magnetic classifications
The magnetic structure of a sunspot group determines how much energy it can store and release. NOAA uses the Mount Wilson classification system:
- Alpha — a single polarity sunspot group. Low flare potential. Rarely produces significant events.
- Beta — a bipolar group with positive and negative polarities separated by a clear dividing line. Moderate potential. The most common active region class.
- Gamma — complex polarity distribution that doesn't fit into a simple bipolar pattern. Higher potential than Beta.
- Beta-Gamma — a bipolar group complex enough to show gamma characteristics. Can produce M-class flares.
- Beta-Delta — a bipolar group with opposite-polarity umbrae within the same penumbra — very close together. High M-class and X-class potential.
- Beta-Gamma-Delta — the most complex and dangerous classification. Major X-class flares are most likely from this type. Extremely rare outside solar maximum.
Flare probability percentages
The C%, M%, and X% columns give the 24-hour probability that the listed region will produce a flare of at least that class. These are per-region probabilities, not overall solar probabilities. A region with M 40% and X 10% is the dominant event driver for the day — watch for its transit across the solar disk.
The dashboard sorts regions by an internal flare potential score (M probability × 1 + X probability × 10) so the highest-risk region always appears at the top.
Operating tip: When a Beta-Gamma-Delta region is on the disk and X probability is above 10%, X-ray flux peaks become more likely — watch for the Radio Blackout indicator to light up. If you're in a pile-up or a contest, keep an eye on the X-ray class tile and be ready for sudden short-wave fade.
Carrington numbers
NOAA Active Region numbers are sequential — the first AR of each solar cycle restarts from ~1 (or continues from the previous cycle at NOAA's discretion). The naming traces back to Richard Carrington, who drew the first systematic sunspot maps in the 1850s. The 1859 Carrington Event — which disrupted telegraphs worldwide — remains the most powerful geomagnetic storm in recorded history.
Source and update rate
The dashboard fetches solar_regions.json from NOAA SWPC every 30 minutes. The source is updated by NOAA's Space Weather Forecasters as they analyze the latest SDO/HMI sunspot maps throughout the day. A blank table means the disk is currently spotless — not rare during cycle minimum.
Concept 23
Ham Contest Calendar — operating events and propagation windows
The contest calendar on the dashboard shows the next several major HF operating events. Contests aren't just competitive — they reliably populate the bands with stations worldwide, making them excellent propagation checkpoints regardless of whether you're contesting.
Why contests matter for propagation
During a major contest like CQ WW or ARRL DX, 10,000–40,000 stations may be active simultaneously on HF bands. This density of signals makes it easy to hear faint-path openings that would otherwise go unnoticed — if a weak DX signal is detectable at all, someone during a contest will find it. Monitoring contest weekends is the fastest way to map current band conditions worldwide.
Major contests on the calendar
- CQ WW DX (SSB + CW) — the world's largest amateur radio contest. Last full weekend of October (SSB) and November (CW). 48 hours, all HF bands, work anyone, exchange: signal report + CQ zone. Draws stations from every DXCC entity. If 10 m or 15 m are open anywhere on Earth, you'll hear it here.
- ARRL DX (CW + SSB) — third weekend of February (CW) and first weekend of March (SSB). W/VE stations work DX; DX stations work W/VE. Classic benchmark for North American HF propagation at the start of spring.
- ARRL Sweepstakes (CW + Phone) — first full weekend of November (CW) and third full weekend (Phone). North-American-only, 160–10 m. Requires working all 83 ARRL/RAC sections — a propagation workout that exercises every band from 160 m to 10 m.
- CQ WPX (SSB + CW) — last full weekend of March (SSB) and May (CW). Exchange includes a serial number and your prefix. Draws an enormous variety of rare prefixes onto the air — useful for checking low-band DX openings.
- ARRL Field Day — last full weekend of June. Emergency-preparedness exercise and social event. Stations operate from temporary/portable setups. Not a DX contest but a strong propagation indicator — if you can hear stations across the US on 15 m or 10 m on Field Day, those bands are open.
- IARU HF World Championship — second full weekend of July. IARU member-society HQ stations are multipliers. International participation. Good benchmark for summer HF propagation and ionospheric conditions post-solstice.
Operating tip: Open the dashboard before a contest weekend and check SFI, Kp, and the 3-day outlook together. SFI above 120 + Kp below 3 = prime conditions. If the forecast shows G1+ activity for contest weekend, decide early whether to focus on lower bands (40 m, 80 m) that are more storm-resistant than 10 m and 15 m.
Solar cycle and contests
At solar maximum (SFI routinely above 150), the highest HF bands dominate contest scores — 10 m contacts across thousands of miles with a wire antenna. At solar minimum, successful contesters shift down to 20 m and 40 m and rely on their antennas and operating skill more than ionospheric luck.
Finding more contests
The calendar shows major global events. For regional contests, club competitions, and specialty events (digital modes, VHF, SOTA activations), see the WA7BNM Contest Calendar at contestcalendar.com — a comprehensive, community-maintained database updated weekly.
Concept 24
Proton Flux & Solar Radiation Storms (S-Scale)
When the sun erupts, it can accelerate protons to a significant fraction of the speed of light. The GOES satellite's proton flux tile tracks these particles before they arrive — giving a warning window of minutes to hours.
What GOES is measuring
The Proton Flux tile shows the integral proton flux in the >10 MeV energy channel, measured in particle flux units (PFU = protons/cm²/s/sr). NOAA's GOES satellites orbit at 35,786 km and carry solid-state particle detectors pointed away from Earth. They sample the solar wind directly, detecting energetic proton streams that accompany major solar flares and fast CMEs.
The 10 MeV channel is the threshold for the NOAA S-scale (Solar Radiation Storm scale). Protons above 10 MeV are energetic enough to penetrate satellite electronics and ionize the polar ionosphere at low altitudes — affecting HF polar paths.
The S-Scale
Ham radio impact
The primary HF effect of an S1+ event is a polar cap absorption (PCA) event. The energetic protons ionize the D-layer at polar latitudes — adding absorption across all frequencies on transpolar paths. During a major S3+ event, signals on 14–28 MHz over the poles can drop 20–40 dB. Low-latitude and mid-latitude paths are largely unaffected.
Operator tip: If you're working polar paths (Europe-to-west-coast USA via the pole, transpolar DXpeditions), watch for S1+ and have a plan to switch to southern-hemisphere paths or lower frequencies if the polar cap absorbs your signal.
Lead time
Once a large flare fires, light takes 8 minutes to reach Earth. Energetic protons travel much slower — they typically arrive 15 minutes to several hours after the associated X-ray burst. GOES gives you that window to react before polar path absorption peaks.
Data source
The dashboard fetches the NOAA GOES primary integral proton flux JSON, filtered to the >10 MeV channel. The S-scale is computed in real-time and updates every 5 minutes.
Concept 25
IMF Bt — Total Interplanetary Magnetic Field
The solar wind carries the sun's magnetic field all the way to Earth. Bt is the total strength of that field. When it's high and pointed south (negative Bz), geomagnetic storms develop fast.
Bz vs. Bt — two sides of the same field
The dashboard's Solar Wind row already shows Bz — the north-south component of the interplanetary magnetic field (IMF). A negative Bz means the field points southward, which is the key driver of geomagnetic activity: it allows magnetic reconnection with Earth's magnetosphere and pumps energy into the ring current, raising the Kp index.
Bt (total field) is the magnitude of the complete IMF vector — the combination of all three components (Bx, By, Bz). It tells you how strong the field is overall:
- Bt < 5 nT — background solar wind; typical quiet conditions
- Bt 5–15 nT — moderate field; storm possible if Bz turns south
- Bt > 15 nT — strong field; often associated with CME arrival; elevated storm potential regardless of Bz sign
- Bt > 25 nT — intense; G2+ storms likely if Bz stays negative
Why Bt matters for storm prediction
A high Bt event that arrives with northward Bz is harmless — but it tells you the field could rotate south at any moment. CME magnetic fields are complex and can rotate over hours. When you see a high Bt reading, the next Bz observation is critical. Operators monitoring for aurora or HF disruption should watch Bt and Bz together in real-time.
Operator tip: High Bt + Bz near zero = storm is a coin flip. High Bt + Bz turning negative = storm is developing fast. Plan your operating around the next 1–2 hour window.
Units
Both Bz and Bt are measured in nanoteslas (nT). Earth's surface magnetic field is about 25,000–65,000 nT; the interplanetary field is far weaker, but its direction matters more than its magnitude at that distance.
Data source
Bt is derived from the DSCOVR magnetometer (mag-7-day JSON) — the same instrument that provides real-time Bz. The dashboard's /api/solarwind endpoint computes Bt from the Bx, By, Bz field components.
Concept 26
Flare Event Log — reading the X-ray record
The flare log shows every solar flare detected by NOAA's GOES satellite in the last 7 days. Each row tells you when the X-ray burst started, when it peaked, when it ended, and how strong it was — information you need to connect a dead band to its cause.
The flare classification system
Flares are classified by peak X-ray flux in the 1–8 Å band. The letters follow a logarithmic scale with the same structure as the Richter scale — each class is 10× more powerful than the previous:
Each class also has a decimal suffix — X2.5 is 2.5× more powerful than X1.0. The largest flares on record were around X45 (2003 Halloween storms).
Begin, peak, end times
A flare event has three phases recorded in the log:
- Begin — X-ray flux starts rising above the pre-event background; this is when HF absorption on the sunlit hemisphere begins.
- Peak (max) — highest flux; worst blackout conditions. For large flares this can last 10–30 minutes.
- End — flux returns to the pre-event level. HF conditions recover, though secondary effects (elevated Kp from CMEs) can persist for days.
The "Region" column links the flare to a NOAA-numbered Active Region — the same numbered sunspot groups shown in the Active Regions table. If a region produces multiple flares, it is elevated to watch it for subsequent larger events.
Operator tip: If you suddenly notice poor HF conditions on the sunlit side of Earth, scroll the flare log first. An M or X flare from the past hour is the most common cause. Conditions typically recover within 30–60 minutes after the end time.
Night side is unaffected
Flare-induced radio blackouts only affect the hemisphere facing the sun at the time of the flare. If the flare occurs when your path is over the dark side of Earth, you won't notice anything — the X-rays can't penetrate the planet. Check your greyline map alongside the flare log to determine if you were in the sunlit zone.
CME lag
A major flare often launches a CME, but the CME impact arrives 1–3 days later — long after the immediate X-ray blackout has cleared. The geomagnetic storm from the CME is a separate event and affects the night side too. Watch the Bz/Bt row for the CME arrival signature.
Concept 27
3-Day Kp Bar Chart — storm timeline at a glance
The 3-day Kp chart shows 72 hours of geomagnetic activity as individual 3-hour bars — the native resolution of the planetary K-index. It answers a question the 7-day trend chart can't: exactly when did the storm peak, and is it over?
How the K-index is measured
The K-index is derived from 13 ground-based magnetometers distributed around the world (the Kp, or planetary K, is the average). Each station measures the maximum deviation in Earth's horizontal magnetic field over each 3-hour window, then converts it to a quasi-logarithmic 0–9 scale. The 3-hour bucket is not adjustable — it is a fundamental property of how the index is computed.
This means each bar on the chart represents exactly three hours of geomagnetic conditions. A storm that peaks between 06:00–09:00 UTC will show up as one red bar; the neighboring bars may be amber or green. That time precision matters for planning:
- Green (Kp ≤ 2) — quiet; stable propagation, polar paths open
- Amber (Kp 3–4) — unsettled; HF mostly fine; high-latitude paths slightly degraded
- Red (Kp ≥ 5) — storm (G1+); polar paths disrupted; aurora possible
Recovery vs. onset
A geomagnetic storm has three phases: sudden commencement (CME arrives and Kp spikes), main phase (Kp peaks, ring current builds), and recovery (Kp declines over 12–48 hours). The 3-day chart shows all three phases in one view. Looking for a sequence of red bars followed by declining amber bars means you're watching a storm in recovery — conditions are improving, not worsening.
The 7-day trend chart shows a longer history; the 24-hour chart shows the most recent 8 buckets with time labels. The 3-day chart hits the middle ground: enough history to see storm context while keeping the 3-hour resolution that tells you what hour conditions shifted.
Operator tip: After a geomagnetic storm, watch the 3-day chart for two consecutive green bars (6 quiet hours) before committing to a long-path DX session. Post-storm recovery is real but not instant — one quiet bucket can be followed by another disturbed period.
Bar label format
Each bar is labeled with its UTC start hour (e.g. "12Z" for the 12:00–15:00 UTC window). Bars at 00:00 UTC show the date instead of the hour to mark the day boundary — making it easy to find where one day ends and the next begins.
Data source
The chart fetches the NOAA planetary K-index JSON in real-time. The K-index is published by NOAA's Space Weather Prediction Center within minutes of each 3-hour window closing.
Concept 28
STEREO-A EUVI 195 Å — the sun from a different angle
NASA's STEREO-A spacecraft orbits the sun ahead of Earth, providing a view of solar regions that SDO cannot yet see. Active regions rotating toward Earth show up on STEREO-A days before they reach the Earth-facing disk.
What STEREO is and why it exists
The Solar TErrestrial RElations Observatory (STEREO) mission launched two identical spacecraft in 2006. STEREO-A (Ahead) orbits slightly inside Earth's orbit, lapping Earth over a ~22-month cycle and drifting ahead in heliolongitude. STEREO-B (Behind) was lost to contact in 2014.
Because the sun rotates (one full rotation takes ~27 days as seen from Earth), regions on the solar limb — just rotating into view — are often the most flare-active. A region that fired off M and X flares while on the far side will be electrically charged and ready to erupt by the time it rotates into the Earth-facing disk. STEREO-A sees those regions before SDO and Earth-based observatories do.
The 195 Å (EUVI) channel
The Extreme Ultraviolet Imager (EUVI) on STEREO captures the sun at 195 Ångströms — sensitive to Fe XII emission at about 1.5 million Kelvin. This is the same temperature range as SDO's AIA 193 Å channel, so the two images show comparable features: coronal loops, active regions, and coronal holes appear in similar contrast. The key difference is the viewing angle, not the physics.
- Bright compact regions — magnetically active areas; flare-prone. If bright on STEREO-A but not yet visible on SDO, it will rotate into view within days.
- Dark areas (coronal holes) — open magnetic field lines; source of fast solar wind streams. A large coronal hole rotating toward the Earth-facing disk predicts a high-speed stream arrival in 3–5 days.
- Faint diffuse arcs or halos — coronal mass ejections lifting off the limb.
Reading STEREO-A alongside SDO
Compare the STEREO-A image to the SDO AIA 193 image side by side. Features visible only on STEREO-A are on the far-side or limb — they'll rotate into Earth view soon. Features visible on SDO's limb that are also bright on STEREO-A are confirmed active — they're being seen from two angles simultaneously, confirming the region is large and bright enough to matter.
Operator tip: If you see a large bright active region on STEREO-A that isn't prominent on SDO yet, check back in 3–7 days. That region may be rotating toward Earth — keep an eye on the flare log for increasing M and X flare activity as it approaches central meridian.
Beacon data
The STEREO-A image on the dashboard comes from the spacecraft's beacon mode — a low-resolution real-time downlink used when the main antenna is not pointed at Earth. Beacon images update every few hours and have slightly lower quality than archived full-res data, but they're available with minimal delay.
Spacecraft status
STEREO-A was launched in 2006 and is still operational as of 2026, far past its 2-year design life. Its orbit means it periodically passes behind the sun from Earth's perspective — during those windows (solar conjunctions), imagery may be temporarily unavailable.
Concept 29
Observed band activity — live WSJT-X decodes
While predicted propagation tells you what should work, observed band activity tells you what is actually happening right now. This panel captures real FT8 and FT4 decodes from a receive station and presents them as a band-by-band activity snapshot updated every 15 minutes.
FT8 and FT4 — weak-signal digital modes
FT8 and FT4 are weak-signal digital modes developed by Joe Taylor (K1JT) and Steve Franke (K9AN) and distributed as part of the WSJT-X software suite. They are designed to complete two-way contacts under conditions where SSB voice and even CW would fail.
- FT8 (14.074 MHz and other band segments) — 15-second transmit/receive cycles; works down to −24 dB SNR relative to a 2.5 kHz noise floor. One complete QSO takes about 90 seconds.
- FT4 — a faster variant with 7.5-second cycles, used primarily in contests. Slightly less sensitive than FT8 but moves more traffic per hour.
Because these modes are so sensitive, they decode signals that are completely inaudible to the human ear. A station that cannot be heard at all on 40m SSB will often still produce a clean FT8 decode — making the decode count a far more honest measure of actual propagation than "can you hear anything on the band?"
What WSJT-X logs
Every time WSJT-X decodes a transmission, it logs: the UTC timestamp, the signal-to-noise ratio in dB, the sending callsign, and the sending station's 4-character Maidenhead grid locator. The dashboard station (W4GGJ) runs WSJT-X receive-only and relays those decode records in real time.
The four tile metrics
Each band tile on the dashboard shows a 15-minute rolling window of decode data aggregated by band:
- Decodes — total number of individual transmissions decoded in the window. High decode counts indicate an open band with many stations. A band with zero decodes is either closed or quiet.
- Unique calls — distinct callsigns heard. This filters out repeated transmissions from the same station, giving a cleaner count of how many stations are actually reachable.
- Best SNR — the strongest signal received in the window, in dB relative to the 2.5 kHz noise floor. Values above 0 dB are strong; −10 dB is moderate; −20 dB and below is weak but still decodable. A best SNR near −20 dB with many decodes means the band is marginal but open.
- Grids — unique Maidenhead grid squares heard. Grids heard from multiple directions (e.g., EN, FM, FN in the Northeast US; EM in the Midwest; DM in the Southwest; EL in the Gulf Coast; FK in the Caribbean) confirm the band is open in multiple paths, not just one.
The 15-minute window
Data is bucketed into discrete 15-minute intervals aligned to the UTC clock (00:00–00:15, 00:15–00:30, etc.). The current window closes at :00, :15, :30, and :45 each hour. Because FT8 operates in 15-second cycles, up to 60 decoding attempts occur per transmitter per 15 minutes — the window is wide enough to catch intermittent propagation that might otherwise miss a shorter snapshot, yet short enough to track conditions as they evolve during a grey-line pass or afternoon opening.
Observed vs predicted — reading both together
The band activity panel is most useful when read alongside the predicted HF conditions panel above it. Predicted conditions are model outputs — they show what ionospheric science says should be possible based on solar flux, K-index, and time of day. Observed decodes show what is actually happening at this station right now.
Common patterns:
- Predicted "Good" + many decodes — model and reality agree. Go operate.
- Predicted "Good" + few or no decodes — conditions may be degrading ahead of a geomagnetic event, or the band has faded locally. Watch K-index for a rising trend.
- Predicted "Poor" + many decodes — the model is conservative or a sporadic-E opening is in progress. Observed beats predicted; get on the air.
- Predicted "Poor" + no decodes — band is genuinely closed. Try a lower band or wait for greyline.
Operator tip: Watch the Grids metric during the first 30 minutes after sunrise or before sunset. A sudden jump from a handful of grids to 40+ grids across multiple directions is a greyline opening — often the best propagation of the day on 40m and 80m.
Data source
All decode data comes from a single receive station (W4GGJ) located in Florida. W4GGJ runs WSJT-X full-time and relays decoded records via Mission Control, a local relay agent, to the dashboard API. The data reflects one station's receive horizon — not a global aggregate.
Geographic bias
W4GGJ's Florida location means 40m decodes skew toward stations in the eastern United States, the Caribbean, and South America on ground-wave or short-hop F2. Trans-Atlantic paths open regularly on 40m at night and near greyline. West Coast stations and Pacific paths are less frequently heard due to skip distance — if you're operating from the western US, treat 40m decode counts as a lower bound on actual band activity.
SNR reference level
The SNR values shown are relative to a 2.5 kHz noise bandwidth as measured at the W4GGJ receiver. Antenna gain, local noise floor, and receiver noise figure all affect the absolute numbers. The relative trend — improving vs degrading SNR across consecutive 15-minute windows — is more useful than the absolute value.
Update cadence
Decode records are ingested and bucketed every 15 minutes by the backend cron. The dashboard tile refreshes every 10 minutes client-side. There may be up to a ~25-minute lag between a live decode at the station and its appearance in the current window — but the previous window's data is always complete and final.
TavaOne Education
Studying for your license?
These propagation concepts — SFI, K/A index, MUF, greyline, and ionospheric layers — appear on the General class exam (Element 3, Group G3). TavaOne Education's free online course covers the full question pool, with this tool as a live reference so you can see the numbers in action while you study.
Check today's numbers
Theory is useful. Current data is better. Head back to the dashboard to see what the sun is doing right now — and use the time slider to plan your next operating session.