Solar Eclipse and Radio: What Happens to HF Propagation

During a total solar eclipse the sky darkens, the temperature drops and the birds go quiet. Something similar happens high above, in the ionosphere, which depends on the radiation of the Sun for its very existence. For a short time the Moon’s shadow switches off part of the Sun’s ionising light over a limited region, and the layers that carry our HF signals react in a way that radio amateurs can hear and measure. This makes an eclipse one of the few natural experiments that hams can take part in. This guide explains what happens in each layer, which bands you can expect to change, how past eclipses have been used for amateur research, and how to prepare your own observations.

Why the Ionosphere Reacts to the Moon

The ionosphere is created by ultraviolet and X-ray radiation from the Sun, which strips electrons from atoms of the upper atmosphere. The result is a set of layers, named D, E and F, that bend or absorb radio waves according to their electron density. Our guide to the ionosphere layers D, E, F1 and F2 describes their roles. When the Moon passes in front of the Sun, the ionising radiation over the shadowed area falls off, sometimes almost to its night-time level, and the electrons begin to recombine with the ions. When the Sun returns, the process is reversed. The details are covered in the general article on a solar eclipse, and the principles of the effect on the ionosphere are the same as those of the daily sunset and sunrise, only faster and more local.

The speed of the reaction differs from layer to layer. The lower layers, where the air is denser, react quickly because the electrons recombine within minutes. The upper F layer reacts more slowly, because the air is thin and electrons survive longer. This difference is why the effect of an eclipse is not simply a night in the middle of the day, and also why the results are so interesting to researchers.

The D Layer: Less Absorption

In daytime the D layer absorbs signals on the lowest bands. Waves on 160, 80 and 40 metres lose most of their energy when they pass through it, which is why these bands are usually good for short distances only during the day. During an eclipse the D layer fades quickly, within a few minutes of the shadow arriving. The absorption falls, and low-band signals that would normally be lost may be heard over longer paths. Operators in the path of past eclipses have reported unusual daytime openings on 160 and 80 metres, with signals that normally need darkness.

The effect ends as soon as the Sun comes back. After the maximum phase, the D layer builds up again in a few minutes, and the low-band signals vanish once more. The change can be sharp, and you may hear a strong station disappear within seconds, so keep a recording of your receiver if you want to catch the moment.

The E and F Layers: Different Story for the Upper Bands

The E layer also weakens during an eclipse, so regular E-layer propagation, which at 100 kilometres of height helps with short hops on 40 to 20 metres, diminishes. The F layer, which carries most of the long-distance HF signals, loses electron density as well. The result is a lower maximum usable frequency, or MUF. A band that was open to a distant target before the eclipse, such as 15 or 10 metres, can close at the height of the event, because the layer is no longer dense enough to reflect that frequency. Lower bands such as 30 or 40 metres may stay open or improve, as they do at dusk.

In the F region there is more complexity. The F1 and F2 layers can approach each other in height, the electron profile changes, and there are reports of waves and travelling disturbances during and after the eclipse. These are the subject of scientific research, and amateur data have contributed. Because the effect on the F layer builds and fades more slowly, it can last longer than the eclipse and its influence extends beyond the zone of the shadow. The table gives a simplified overview of what to expect.

The Shadow Is Small, the Path Is Long

Keep in mind the geometry. The darkest part of the shadow, the umbra, is only a few hundred kilometres across at the most, and a partial eclipse covers a much larger area, but with a smaller effect. A radio path of 2000 kilometres between two stations may cross the shadow in the middle, at the end or not at all. The effect on the signal therefore depends on where the path lies relative to the shadow, which is a reason why the same eclipse looks different at different stations. Stations at the edge of the zone might notice only a slight shift in the signal, while a path that passes through the totality at its midpoint, where the reflection takes place, can show large changes.

Because the shadow moves at a speed of well over a thousand kilometres per hour, the effect travels along the surface. This is an advantage for researchers: the same path is influenced for only a short time, so a continuous record shows the change as a clear peak or dip. For this reason the station that sends or receives a steady signal during the whole period is more useful than a station that only makes a few contacts.

Amateur Experiments: What Has Been Done

The eclipses that crossed North America in 2017 and 2024 were used by the HamSCI community, a group of radio amateurs and scientists, for the Solar Eclipse QSO Party and for data collection by many receivers. Participants made contacts and sent reception reports, and the organisers analysed the results together with measurements of time signal stations and with data from ionosondes. Amateur data from WSPR and from reporting networks complement the work of professional instruments because they cover so many paths. You can find the background on the HamSCI website.

The idea is simple and fits our hobby: millions of reception reports are generated daily by digital modes. If you add the time of an eclipse to them, the change in the pattern shows how the layers reacted. Articles about WSPR and about PSK Reporter describe the two main sources of such data. A station that is active on these systems during an eclipse automatically contributes to the research, even if the operator does nothing special.

When the Next Ones Come

Total eclipses are rare for any single place, but there are usually one or two on Earth every year, and a path often crosses a region with many amateurs. The total eclipse of August 2026 crossed Greenland, Iceland and Spain, and the one on 2 August 2027 is expected to cross southern Spain and North Africa, as the astronomical tables show. Partial eclipses are much more common, and they also give measurable effects, though smaller. Consult an eclipse calendar a few months before the date, check the times of the phases for your location and plan your station activity around them.

How to Prepare Your Own Observation

You do not need special equipment to take part. The important thing is a plan and a good record. Use the following steps.

• Choose the bands. Pick one low band, such as 80 or 40 metres, and one high band, such as 20 or 15 metres, so that you can see the opposite behaviours.
• Use a reference. Listen to a steady signal on a known path: a beacon from the NCDXF beacon network, a time signal or a WSPR transmitter. Note the signal strength at fixed intervals before, during and after the eclipse.
• Keep exact time. Set the clock of your computer to UTC and note the times of the first contact, the maximum and the last contact of the eclipse for your location.
• Record the audio. A recording of the receiver lets you compare the signals later and catch changes that you miss in the moment.
• Do the same on the day before and the day after, at the same hours, to know what a normal day looks like.

Then compare the logs. A drop of the MUF should appear as a fall in the high-band signal near the maximum, and a quick recovery afterwards. For weak signals, a digital mode will give you a clear number in the report, while for stronger signals the S-meter is sufficient. If you do not operate yourself, you can still follow the reports of other stations in the days after the event.

Reading Your Results

When the eclipse is over, plot the signal strength against UTC for each band and mark the first contact, the maximum and the last contact on the same graph. A few patterns are typical. On a low band, a daytime path that is normally weak may rise by several S units as the maximum approaches and fall back afterwards. On a high band, a path that is strong before the event may drop towards the maximum and recover some time after the last contact. The delay between the maximum of the eclipse and the maximum of the radio effect tells you how slowly the layer responds.

Do not expect every path to show a clean change. Fading, changes in the noise level and the operation of other stations all hide the effect, and an unusual result in one record may be a coincidence. The strength of an amateur dataset is in numbers: many stations with simple observations give a pattern that no single record could. Write your notes clearly, including the antenna, the power and the mode, so that your data can be compared with those of other stations.

Other Effects You Might Notice

The medium-wave broadcast band shows the changes more easily than the amateur bands, because the stations are fixed and the signals are strong. Distant stations on the AM band, which normally appear only at night, may come in during a total eclipse in the daytime, which is a nice test for a simple receiver. On the VHF bands there is generally no direct effect, but some observers have reported changes in sporadic E and in tropospheric conditions due to the cooling of the air near the ground. A local drop in temperature and a change in the wind can slightly alter the conditions for 144 MHz and higher paths, though the data are limited.

Time signal stations are a good reference, because they transmit steady carriers on known frequencies. A receiver that records the Doppler shift of such a carrier can show the movement of the reflecting layer during the eclipse, as the layer moves up and down. This is a more advanced observation, but simple software and a stable receiver are all that is required. The accuracy of your receiver clock and frequency matters, as discussed in our guide to calibrating a transceiver with WWV.

Safety and Common Sense

Never look at the Sun without proper eclipse glasses or filters, and do not point a camera or binoculars at it without a certified filter. The partial phases are as dangerous for the eyes as a normal day. It is also useful to remember that the temperature falls during the totality, so people who operate outdoors, as on a portable site, should take a jacket. Equipment near its thermal limits, such as a hot power amplifier, will notice the sudden change too, though only slightly.

The Bottom Line

A solar eclipse briefly removes the ionising light of the Sun from a part of the ionosphere. The D layer fades within minutes, so low-band absorption falls and daytime openings on 160, 80 and 40 metres may appear. The E and F layers lose density, so the MUF drops and the high bands can fade or close. The effects depend on the position of the path relative to the shadow. You can contribute by logging a steady reference signal on a low and a high band, with exact UTC times and a record on the days before and after, and by using the digital reporting systems that scientists use.

Frequently Asked Questions

Does a solar eclipse improve HF propagation?

It improves some bands and spoils others. The low bands can improve because the D layer absorbs less, while the high bands can fade as the F layer loses density and the MUF falls.

Which layer reacts fastest to an eclipse?

The D layer, which loses its electrons within minutes of the shadow arriving and rebuilds quickly afterwards.

Is an eclipse like night for the ionosphere?

Partly. The D layer behaves like night, but the F layer reacts more slowly and does not reach the same state in the short time of an eclipse.

Do I need to be in the path of totality to see an effect?

No. A radio path that crosses the shadow, or a partial eclipse, can also show changes, although they are smaller than in the totality.

How can I contribute to eclipse research as a ham?

Operate or listen on a steady signal, log exact UTC times, compare with the days before and after, and use digital reporting systems such as WSPR and PSK Reporter.

Is it safe to operate outdoors during an eclipse?

Yes, provided you use certified eclipse glasses for any observation of the Sun and dress for the drop in temperature. The radio work itself is no more dangerous than usual.