Solar eclipse and propagation: More questions than answers for now

May 2nd, 2018, Published in Articles: EngineerIT

In August 2017 more than 600 000 radio amateurs, mainly in the USA, took part in a Solar Eclipse QSO Party (SEQP) in a coordinated initiative to research the possible influences of an eclipse of the sun on radio propagation. The solar eclipse caused a shadow of the moon traversing the USA from Oregon to South Carolina in just over 90 minutes. It was the most significant solar eclipse experienced over the USA in 100 years.

The objective of the SEQP was to flood the airwaves with radio contacts, all measured by automated receivers of the reverse beacon network (RBN), PSK reporter, and WSPR net.

The RBN is a revolutionary new idea where, instead of beacons actively transmitting signals, a network of stations listen to the bands and report what stations they hear, when and how well. These receiving stations are automated. The PSK reporter network started out as a project to automatically gather reception records of  phase-shift keying (PSK) activity and then make those records available in near real-time to interested parties, typically amateurs who initiated the communication so they could check propagation conditions to a certain area. The weak signal propagation reporter network (WSPR net) is a group of radio amateurs using K1JT’s MEPT-JT digital mode to probe radio frequency propagation conditions using very low power transmissions.


When those observations were combined with the logs from individual stations, the result constituted one of the largest ionospheric experiments ever performed. The SEQP started before the eclipse began on the western coast of Oregon. Is was important to start the operation well before the eclipse to establish what the bands were like before the shadow started digging a trench through the ionisation in the atmosphere. As the penumbra started cutting into the solar disc, it was recommended that radio amateurs make as many calls as possible so that the automatic networks could record the activity from as many individual stations as possible.

Although the ionospheric effects of solar eclipses have been studied for over 50 years, many unanswered questions remain: How much of the ionosphere is affected by a solar eclipse? How long is it effected for? Why?

In some ways, the shadow of a solar eclipse is similar to the darkness of night. However, there are significant differences between solar eclipses and typical day-night variations. For example, an eclipse shadow moves faster and in the opposite direction of the dusk or dawn terminators. Additionally, an eclipse shadow is relatively localised compared to night. Because the ionosphere does not respond instantaneously to changes in solar inputs, several processes in addition to simple ion production and recombination are at play. It is not possible to assume that the ionosphere will respond to an eclipse in the same manner as dusk or dawn.

Previous solar eclipse studies have found that ionospheric density at lower altitudes (D and E regions 60 to 150 km altitude) deplete rather quickly. Conflicting results have been reported for the F region (150 to 600 km altitude) which is affected more by plasma transport processes than photo-ionisation. It has been reported that typically these conditions allow for better radio wave propagation of frequencies below 10 MHz during the eclipse, as D and E regions absorption disappears during this time. 

The 2017 Solar Eclipse QSO Party (SEQP) made history and, although the final numbers are not yet in, preliminary reports show that over 670 000 spots were detected by the RBN and over 542 000 spots were reported to PSK reporter.

Nathaniel Frissell, an associate research professor at the New Jersey Institute of Technology, and others are investigating whether the sudden absence of sunlight during the eclipse – and especially solar ultra-violet and x-rays – would briefly change the properties of the upper atmosphere..

An initial analysis of the solar eclipse RF seismograph (a real-time HF propagation monitoring tool) measurements by Alex Schwarz, VE7DXW, and his modulation-demodulation software radio (MDSR) group suggested that the effect of the brief interruption in solar radiation within an approximately 115 km wide strip had minimal overall effect on radio propagation. The solar eclipse RF seismograph exclusively showed that propagation changed, but not to the extent that was expected. Schwarz and the MDSR team said that during the eclipse they measured in three locations, and two did not show any changes in the way propagation behaved. On the third station, at an elevation of 900 metres, the 40 m band came up, but that is not any different from regular 40 m behaviour.

The team believes that increased absorption on the low bands from high solar activity may have been a factor in the measurement’s not yielding expected results. The small band of darkness could not compensate for the thicker D Layer.

Frissell said that he would be hesitant to make these conclusions based on observations from a single point of reference. Observations made during the 1999 eclipse in the UK at the Rutherford Appleton Laboratory has shown that there are significant ionospheric changes resulting from an eclipse.

All the input is still being evaluated by the Ham Radio Science Citizen Investigation (HamSCI) group. HamSCI is a platform for the publicity and promotion of projects that are consistent with advance scientific research and understanding through amateur radio activities.

Results will be the published in a research paper to be published later this year. Early results have resulted in many more questions than answers.  

Q codes for communication

QSO in one of the Q codes, a standardised collection of three-letter codes all of which start with the letter “Q”. It was initially developed for commercial radiotelegraph communication and later adopted by other radio services, especially amateur radio. Although Q codes were created when radio amateurs used Morse code exclusively, they continued to be employed after the introduction of voice transmissions. To avoid confusion, call signs are restricted; no country has ever issued an ITU prefix starting with “Q”. QSO means “Can you communicate with…” or, as a statement, “I can communicate with…” Today it means having a contact (discussion) with another radio amateur.