AMATEUR RADIO STATION
NEW ZEALAND
High-frequency (HF) radio communication (3–30 MHz) relies on the Earth’s ionosphere to refract signals over the horizon. The Sun is the engine that creates and energizes this ionospheric layer, enabling long-distance HF “skywave” propagation. Changes in solar activity can dramatically enhance or disrupt HF radio links. This report examines the ionosphere’s role and details how various facets of solar activity – from the 11-year sunspot cycle to sudden solar flares and geomagnetic storms – affect HF propagation. (In general, increased solar output raises ionospheric electron density and allows higher-frequency propagation, whereas intense solar disturbances can lead to signal absorption or blackouts.) The aim is to provide a comprehensive, data-backed overview for understanding and anticipating HF propagation conditions under different solar phenomena.
Ionospheric Formation: The ionosphere is a region of the upper atmosphere (~50–500 km altitude) that is ionized by solar ultraviolet (UV) and X-ray radiation. When solar photons collide with atmospheric atoms, they knock loose electrons, creating a plasma of free electrons and ions. These free electrons act to refract (bend) HF radio waves back toward Earth, enabling communications well beyond line-of-sight distances. The greater the electron density in a given layer, the higher the radio frequency that can be reflected by that layer. During daylight, intense solar radiation produces multiple ionospheric layers designated D (~50–90 km), E (~90–140 km), F1 (~140–210 km), and F2 (~210–400+ km). At night, with the Sun absent, the lower layers (D, E, F1) largely dissipate, leaving only a weakened F-region to support HF propagation. The F2 layer is the most important for long-range HF communication since it exists 24 hours a day (persisting at night via day-to-night winds) and has the highest altitude (yielding longest skip distances) and highest electron density (reflecting the highest usable frequencies).
Propagation Mechanism: HF signals transmitted upward are refracted by these ionospheric layers and can return to Earth beyond the horizon in one or multiple “hops,” a process known as skywave or skip propagation. Lower frequency HF bands (longer wavelength, e.g. 1.8–7 MHz) are bent more strongly and are easily returned to Earth under most conditions. Higher-frequency HF signals (shorter wavelength, e.g. 21–30 MHz) require a sufficiently dense ionosphere to refract them; otherwise, they pass through into space. The D layer, while too low to reflect HF, plays a crucial role by absorbing radio energy – especially at lower frequencies. In daylight, D-layer absorption can significantly weaken HF signals, particularly on the 3–10 MHz bands, whereas at night the disappearance of the D layer allows these low bands to propagate much farther. In summary, normal HF propagation is a product of steady solar radiation: day/night cycles and seasonal changes in solar illumination lead to daily and seasonal variations in ionospheric density, controlling which frequencies propagate best at a given time.
11-Year Solar Cycle: The Sun’s activity rises and falls in an approximately 11-year cycle, measured by observable sunspots on its surface. Sunspots are dark, magnetically active regions on the Sun; they themselves are cooler spots, but around them lie bright UV-emitting regions (plages) that pump out enhanced extreme ultraviolet (EUV) radiation. The number of sunspots (quantified by the sunspot number, SSN) varies from near zero at solar minimum to well over 100 at solar maximum in strong cycles. A related metric is the 10.7 cm solar flux index (SFI), a measure of solar radio noise that correlates with UV output; SFI values range from ~60 at sunspot minimum to 200–300+ at a solar peak. Higher SSN or SFI indicates a more intense solar output, meaning more ionizing radiation reaching Earth’s atmosphere. Consequently, during solar maxima the F2-layer ionospheric electron densities are much greater than during minima. As a result, the ionosphere can refract higher-frequency signals at solar max than it can at solar min. Observations confirm that F2 critical frequency (the highest frequency reflected by a vertical signal) tracks closely with the solar cycle – rising in step with sunspot numbers and falling when sunspots are few.
HF Band Openings: For radio operators, this means high solar activity greatly **extends the usable frequency range** for HF communications. At the peak of a cycle, frequencies on the order of 25–30 MHz (the 12 m and 10 m ham bands) can be bent back to Earth, enabling worldwide skip propagation on bands that would be “dead” at solar minimum. During solar minimum years, the F2 layer’s critical frequency at midday might only reach ~5 MHz, limiting the maximum usable frequency (MUF) for long-distance paths to perhaps 15 MHz (around the 20 m band) or lower. In contrast, at solar maximum the F2 critical frequency can exceed 10 MHz, pushing MUFs well above 30 MHz. For example, during the strong Cycle 22 peak (late 1980s), the smoothed sunspot number stayed above 100 for years and the 10 m band (28 MHz) opened almost all day, every day, to some part of the world. At such times, even relatively high-frequency HF and low-VHF signals can propagate via F2 ionosphere reflection. Conversely, when the Sun is quiet (sunspot counts near zero), upper HF bands like 15 m, 12 m, and 10 m may not open at all for long-distance work. Operators then must rely on lower frequencies (40 m, 80 m, etc.), especially at night, to reach distant stations. In summary, moderate increases in solar activity are beneficial for HF communications: more sunspots = more ionospheric ionization, higher MUFs, and longer potential skip distances. Indeed, an elevated solar flux index strongly correlates with improved HF propagation – as one source puts it, “a high SFI number means a high F2 critical frequency and better HF communications.”
Daytime vs. Nighttime Conditions: It’s worth noting that even during a solar maximum, HF propagation undergoes its normal diurnal rhythm due to Earth’s rotation. Daylight brings strong ionization of D, E, and F1 layers, which raises MUFs but also increases D-layer absorption on lower frequencies; nighttime sees those lower layers decay, reducing absorption but also lowering the overall electron density in the F region. At solar max, the night F2 layer remains enough ionized to support some high-frequency paths that would vanish at solar min. Seasonal effects also play a role (the ionosphere tends to be denser around the equinoxes, and some complex chemistry makes winter daytime F2 densities higher than summer in certain cases). But the solar cycle intensity is the dominant long-term factor: historically, HF enthusiasts eagerly await solar max years to enjoy reliable worldwide DX on the higher bands, whereas during solar minima those bands lie mostly dormant.
While a high average level of solar radiation improves HF propagation, **intense short-term outbursts from the Sun can have immediate negative effects**. The most rapid disturbances come from solar flares – explosive releases of energy often associated with sunspot regions. A solar flare unleashes a burst of electromagnetic radiation across the spectrum, including a surge of X-rays and extreme UV. These high-energy photons travel to Earth at light speed (reaching us in ~8 minutes) and abruptly increase ionization in the lower ionosphere | NOAA / NWS Space Weather Prediction Center](https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts). In particular, strong X-rays penetrate down to the D-region (~60–90 km altitude) and even the upper stratosphere, creating an over dense D-layer on the dayside of Earth | NOAA / NWS Space Weather Prediction Center](https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts). The result is a phenomenon called a Sudden Ionospheric Disturbance (SID), or more colloquially a short-wave fadeout: HF signals that normally would refract from higher layers instead get absorbed by the now-ionized dense lower atmosphere | NOAA / NWS Space Weather Prediction Center](https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts). During a major flare, HF radio communications on the sunlit side of Earth can experience severe degradation or complete blackout, typically affecting the entire 3–30 MHz range | NOAA / NWS Space Weather Prediction Center](https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts). Even VHF can be impacted in extreme cases, and operators might notice even the background shortwave noise floor drops to near zero — an eerie silence indicating that radio waves are being swallowed before they can reach a receiver.
– Daylight-Only Effect: HF fadeouts due to flares occur only on the sunlit half of the Earth. Any radio circuit that has its ionospheric reflection point in daylight will be affected, while night-side paths continue normally. (The Earth itself luckily blocks the flare’s X-rays from reaching the night-side ionosphere.)
– Rapid Onset and Short Duration: The increase in D-layer absorption begins essentially at the same time the flare is observed in X-rays, with no warning (aside from solar monitoring) because the radiation travels at light speed | NOAA / NWS Space Weather Prediction Center](https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts). Signal loss can occur within seconds to minutes of a strong flare’s peak. Most blackout events last from a few minutes to an hour or two, and then the ionosphere recovers. Fadeouts show a fast onset and a more gradual recovery as the ionization subsides. The duration of an SID correlates with the flare’s length and intensity – longer-duration flares produce longer disruptions.
– Frequency Dependence: Lower-frequency HF waves (longer wavelengths) are absorbed more strongly by the dense ionosphere. Thus, during a flare-induced SID, the lowest HF bands (MF and lower HF) are the first to fade out and the last to come back. For example, 5 MHz and 7 MHz signals might disappear early in a solar flare event. Higher HF frequencies (e.g. 18–28 MHz) are less susceptible to D-layer absorption and may only be moderately degraded or even remain usable during a smaller flare. In a massive flare (X-class), however, all frequencies up through 30 MHz can be completely blacked out for a period.
– Occurrence and Severity: Solar flares are more frequent around the peak of the 11-year cycle (when sunspots are abundant) and rare during solar minimum. Large flares capable of causing global HF blackouts (NOAA R3–R5 events) typically occur dozens to a few hundred times per cycle. In fact, one analysis noted that flares big enough to trigger total short-wave fadeouts happen on roughly 300 days in an average 11-year cycle, mostly clustered near solar maximum. An extreme X-class flare (such as X20+, an R5 “extreme” radio blackout) can knock out HF communications on the entire dayside of Earth, potentially for several hours. Fortunately, such extreme flares are relatively infrequent. More common M-class flares (R1–R2 minor/moderate blackouts) might cause localized or band-specific degradations (for instance, a brief 7 MHz outage but 21 MHz still functioning). In any case, when a strong flare hits, there is little one can do except wait it out – as ham operators say, “our HF bands go “poof” until the ionosphere returns to normal.
(Note: Space weather agencies categorize these radio blackout events on a 5-level scale. For example, an R3 “Strong” blackout corresponds to an X1-class flare and entails a wide-area HF communications blackout for about an hour on the sunlit side. An R5 “Extreme” event, from an enormous flare (X20 or higher), means complete HF signal loss across the sunlit hemisphere for several hours.)
Major solar eruptions can also hurl high-energy charged particles (protons and electrons) into space. When a fast coronal mass ejection or a significant solar flare accelerates solar energetic particles (SEPs), some of those charged particles spiral along the Sun’s magnetic field and reach Earth within tens of minutes to a few hours of the event. Unlike electromagnetic radiation, these particles are deflected by Earth’s magnetic field and primarily funnel into the polar regions near the magnetic poles. The result is a Solar Radiation Storm, which for HF radio is manifest as a Polar Cap Absorption (PCA) event. Essentially, a swarm of solar protons ionizes the upper atmosphere in the high-latitude regions, producing intense D-region absorption like a prolonged, regional SID – but concentrated over the poles.
Important features of proton-induced HF blackouts include:
– Delayed Onset and Long Duration: Unlike X-ray flares which cause immediate effects, SEP events typically begin some minutes to hours after the solar eruption. A blast of energetic protons can start increasing polar ionization as soon as ~10–20 minutes after a major flare in extreme cases, or more commonly a few hours later. Once begun, polar cap absorption can persist for days as protons continue to flood in. Large radiation storms (classified as S4 or S5 on NOAA’s scale) have caused HF disruptions lasting up to a week or more in polar regions. For example, a severe S4 radiation storm can lead to complete HF blackout for high-altitude flights in polar routes for several days.
– Geographic Extent – Polar Regions: PCA effects are essentially confined to high latitudes (auroral and polar zones). The proton flux guided by Earth’s dipolar field rains down into the atmosphere within about 60–90° of the poles. HF radio circuits that traverse polar areas – such as trans-polar airline flights, Arctic research stations, or intercontinental paths that go near the poles – “experience deep signal absorption” or total loss of communications during a PCA. It’s notable that unlike flare SIDs, PCAs can affect polar regions even if they are in darkness. A polar night sky offers no protection, because the ionization is driven by energetic particles (protons) rather than solar UV light. Thus, an ongoing proton storm will shut down HF links through the polar cap both day and night.
– HF Frequency Impact: Similar to flare-induced absorption, the lower HF frequencies (longer paths) suffer the most. During a strong PCA, practically all HF bands may become unusable on polar routes. For instance, communication with an aircraft near 80°N or a scientific base in Antarctica might be impossible except via satellite, as even 20 MHz signals get absorbed. Lower frequencies (say 5 MHz) might be absorbed for days on end. Minor radiation storms (S1) have only a small effect (perhaps a slight degradation on polar paths), but at S3 (strong) and above, polar HF blackout is a serious concern. This is why aviation authorities carefully monitor solar radiation storms – during an S4/S5 event, many commercial airlines will avoid polar routes because HF radio (used as a backup comm link) can be completely unreliable.
– Frequency of Occurrence: The worst radiation storms tend to accompany the largest solar flares or fast CMEs, which are more common near solar maximum. However, major PCA events are relatively infrequent compared to day-to-day flares. We might see only a handful of significant (S3+) proton events in a given solar cycle. They are, nonetheless, very disruptive when they occur. For HF operators, the only “workaround” during a polar cap absorption is to reroute communications away from the polar region. For instance, signals can sometimes be relayed through lower latitudes (skirting the affected zone). But for paths that must go over the pole, one must simply wait until the radiation storm subsides. Space weather services issue alerts (NOAA S-scale) for these events; an S4 (severe) warning means essentially no HF comms through the polar caps for some time.
In summary, solar radiation storms triggered by intense solar activity cause **weeks-long HF silence in polar areas** due to excess D-layer absorption. These events underscore the need for multiple communication paths – for example, satellites or rerouted HF – when operating in high latitudes during active solar periods.
Beyond the immediate radiation impacts of flares and proton events, the Sun can also induce geomagnetic storms that disturb the Earth’s magnetic field and ionosphere on a global scale. These are typically tied to coronal mass ejections (CMEs) or coronal hole high-speed streams:
– A CME is a colossal cloud of magnetized plasma ejected from the Sun, often associated with a strong flare or erupting filament. When a CME is directed toward Earth, it usually arrives 1–3 days after the solar eruption, impacting Earth’s magnetosphere with a shock wave and a flood of charged particles.
– A coronal hole is a region on the Sun where the magnetic field opens into space, allowing a continuous stream of solar wind to escape at high speed. Recurrent coronal hole streams can strike Earth and disturb the geomagnetic field, especially during the declining phase of the solar cycle.
When these solar eruptions reach Earth, they can rattle the Earth’s magnetosphere and induce a geomagnetic storm – marked by large fluctuations in the planetary magnetic field. Geomagnetic storms, in turn, have complex effects on the ionosphere (often called ionospheric storms). The heightened geomagnetic activity energizes currents and particle precipitation in the polar and auroral zones, alters global wind patterns in the thermosphere, and can either increase or decrease ionospheric electron densities in various regions and altitudes.
For HF propagation, a major geomagnetic storm is generally bad news – especially once the storm’s main phase sets in. The typical scenario during a strong geomagnetic storm (associated with a CME impact) is as follows:
– Initially, there might be a short-lived ionospheric enhancement (a positive phase) as the sudden compression of Earth’s magnetic field can temporarily increase electron density in some regions. In some cases, for a few hours, unexpectedly high frequencies might propagate (“MUFs spike” briefly). However, this is usually followed by…
– A prolonged ionospheric depression (negative phase). The F2-layer electron density drops well below normal, owing to atmospheric heating, turbulent mixing of molecular gases (which increases recombination), and magnetospheric electric fields disrupting the usual solar-produced ionization. The F2 critical frequencies plummet, meaning the maximum usable frequencies for HF communication are greatly reduced. Frequencies that one would typically use successfully (say 14–21 MHz for a certain path) might now go straight through the depleted ionosphere instead of reflecting, forcing operators to migrate to much lower bands (e.g. 5–7 MHz) to find reliable reflections.
When a geomagnetic storm is underway, HF propagation conditions can deteriorate in multiple ways:
– Reduced MUF and High-Frequency Loss: A significant ionospheric storm can cause the *MUF to drop by several MHz*. Stations find that higher HF bands (20 m, 17 m, 15 m, 10 m) no longer support long-distance communication that they did prior to the storm. Often, only the lower bands (40 m or even 80 m) still function for long haul, and even those paths may be shorter than usual. This reduction in usable frequencies can last 24–48 hours or more, until the ionosphere recovers. Major geomagnetic storms (Kp 8–9, NOAA G4–G5) are particularly debilitating — at G5 (extreme) level, HF radio propagation “may be impossible in many areas for one to two days” during the storm’s peak.
– Signal Fading and Phase Distortion: Geomagnetic storms invariably introduce irregularities and turbulence in ionospheric layers. Instead of nice smooth layers, the ionosphere breaks up into blobs and gradients. This causes incoming skywave signals to arrive over multiple paths and at varying times (multipath propagation). The result is deep fading, where signals fluctuate in strength, and often rapid phase or polarization shifts. Listeners might hear a Morse code tone warble or a single-sideband voice sound distorted. Fading can be especially pronounced on near-vertical incidence skywave paths or along paths through the auroral zone. Overall signal-to-noise ratio worsens, and weaker signals get buried in the din.
– Geographic Dependence – Aurora and High Latitudes: The brunt of geomagnetic storm effects is felt at higher latitudes. The auroral zones (roughly 60–70° geomagnetic latitude) and anywhere poleward bear the most severe ionospheric disruptions. HF circuits that traverse polar or auroral regions may experience complete blackout (this is sometimes called a polar blackout, distinct from PCA – here caused by auroral electrojet particle precipitation filling the D-region with electrons). Even mid-latitude paths can suffer, but typically to a lesser degree. Near the equator, the ionosphere is somewhat less disturbed during geomagnetic storms, so low-latitude paths might still be partly operational. In practical terms, a station in Scandinavia or Canada will see its HF connectivity degraded far more during a geomagnetic storm than a station in Indonesia or Brazil. Another dramatic effect at high latitudes is auroral propagation: HF and VHF signals can sometimes scatter off the aurora itself, but such modes are very lossy and typically manifest as distorted, fluttery signals when the K-index is high.
– Duration and Recovery: Geomagnetic/ionospheric storms last longer than flare events – usually on the order of **one to three days**, but sometimes even longer for slow-recovering disturbances. Conditions gradually improve as geomagnetic activity subsides (characterized by the K-index dropping back to low values). There are often day-to-day aftereffects; for instance, the ionosphere might remain in a depressed state for a couple of days even after the geomagnetic indices return to quiet. Radio operators must adapt to these changing conditions in real-time, often resorting to lower frequencies at night and possibly having no good propagation on certain paths until the ionosphere restores.
Because geomagnetic storms are so impactful, radio users closely monitor the K-index, which is a 3-hourly index of geomagnetic activity (0 = quiet, 9 = extreme storm). Generally, Kp 0–3 indicates calm conditions with minimal geomagnetic influence on HF – this is when one can expect excellent propagation on higher bands and stable signals. Kp of 4–5 (unsettled to minor storm) means the ionosphere is getting disturbed: one might see increased absorption on the lower bands (due to mildly enhanced D-layer absorption from auroral particle rain) and some fading on trans-polar paths. By the time Kp reaches 6, 7, or more, a full geomagnetic storm is in progress and HF propagation, especially on higher frequencies and high-latitude routes, will significantly degrade. At Kp 7–8 (G3–G4 storm), for example, HF signals may become intermittent and unreliable even at mid-latitudes. And as noted, at the extreme Kp 9 (G5) level, HF radio can effectively be knocked out over large areas.
In such conditions, the recommended strategy is to shift to lower frequency bands which can still be reflected by the weakened ionosphere. Communication that normally might use 14 MHz might need to move down to 5 MHz, for instance. Additionally, avoiding paths that go near the poles (where possible) can improve reliability during geomagnetic storms. Anecdotally, ham operators know a sudden jump in the K-index can “turn a thriving DX band into a graveyard” – signals that were booming in can fade away completely as the storm hits. On the other hand, once the geomagnetic unrest calms (K-index falls back to 1 or 2), HF conditions usually rebound, with MUFs climbing back to normal over a day or two.
(Monitoring: Space weather services provide a NOAA G-scale for geomagnetic storms, from G1 (minor, Kp=5) to G5 (extreme, Kp=9). These come with HF impact descriptions: e.g., G2 storms can cause high-latitude HF fadeout, G3 storms cause intermittent HF problems, and G5 storms result in widespread HF outages for up to 48 hours. By keeping an eye on real-time K-index updates or alerts, radio communicators can anticipate degradation. Many operators will pre-emptively adjust frequencies or schedules if they see a storm warning, much like taking an umbrella when a weather forecast predicts a downpour.)
Solar activity is a double-edged sword for HF radio propagation. On one side, a more active Sun (with abundant sunspots) greatly improves the baseline HF propagation conditions – boosting ionospheric densities and allowing higher-frequency, longer-distance communication. On the other side, the same active Sun is prone to flare-ups and magnetic storms that can severely disrupt HF signals, from short-lived blackout events on the dayside to multi-day depression of worldwide communications. Understanding these phenomena is crucial for anyone relying on HF radio. By studying solar indexes and space weather reports, operators can optimize their chances: for example, taking advantage of high solar flux periods for DX on 10 m, but having contingency plans (like lower-frequency channels or non-polar routes) during geomagnetic storm forecasts. Modern technology and forecasting have made it possible to know when the Sun is likely to “hamper or aid” HF propagation. In practice, amateur and professional communicators treat space weather much like terrestrial weather – something to be respected and planned for. As one ARRL expert put it, the Sun can do “unexpected and dramatic things” to our radio environment, and there’s not much we can do to change it. Instead, we prepare by keeping informed: monitoring real-time solar data (sunspot numbers, solar flux, X-ray flux) and geomagnetic indices (Kp, A-index) to adjust our operations accordingly. With comprehensive knowledge of how solar activity affects HF propagation, operators can maximize the “good days” of worldwide communication and endure the stormy days when our ionospheric link to the world is temporarily severed.