6:45 AM | *The greatest geomagnetic storm of the 20th Century…May 13-15, 1921…New York City/New York State were especially hard hit* — Arcfield Weather

Interesting impact on radio propagation

One of the interesting features of this solar storm according to a recent publication (Hapgood, 2019) was its variable impact on the propagation of long distance radio signals. There were reports showing both disruption and enhancement of radio propagation, with reports of enhancement gaining much attention because they were in stark contrast to the disruption of other telecommunications systems. One example of enhancement was a report in the New York Times (NYT, 1921a) that radio signals reaching New York from Berlin (some 6,400 km distant) and Bordeaux (5,800 km) were much stronger than usual between 02:30 and 04:00 GMT on 15 May. This report has particular credibility because the New York Times was one of a number of U.S. newspapers that then operated their own radio stations to receive news from Europe (Hudson et al., 2000). The good performance of radio links in the United States and at Bordeaux was also confirmed in statements by the Radio Corporation of America (Telegraph and Telephone Age, 1921b). Another example of enhancement came from the Pacific region, where Angenheister and Westland (1921a) and Gibbs (1921) reported unusually good conditions around 06:15 on radio links between radio stations at Apia in Samoa and Awanui in the north of New Zealand (a distance of 2,700 km).

To understand the enhancement of radio signals during the storm, it is essential to appreciate that the radio systems in use in 1921 operated in low‐frequency radio bands below 300 kHz. For example, the radio link between New Zealand and Samoa operated at 150 kHz (Gibbs, 1921). At this frequency, radio signals couple to the conductive surface of the Earth, both land and sea, and propagate along that surface, following the curvature of the Earth in a so‐called “ground wave.” The signals are gradually attenuated by the finite conductivity of the surface with less attenuation where conductivity is higher, mostly obviously over the salt water that forms the oceans (International Telecommunications Union, 2007). However, the signals can also propagate into the upper atmosphere and be reflected from the ionosphere giving a “sky wave” that can interfere with the ground wave signal, causing problems with signal reception. Sky wave interference can also arise from distant sources of natural radio signals such as lightning and other electrical activity in the atmosphere. Thus, good conditions for signal propagation at 150 kHz will arise when sky waves are heavily attenuated by absorption due to significant plasma density in the lower ionosphere below 90 km (credit Hapgood, 2019).

Potential impact on today’s world

Some recent research (e.g., Love, et al, 2019) now suggests that this great solar storm of May 1921 was about as equally intense as the “Carrington Event of 1859” which has been dubbed the strongest solar storm in recorded history. The super solar storm of 1859 took place during solar cycle #10 and was named for the British astronomer, Richard Carrington, as he observed from his own private observatory the solar flare which caused a major coronal mass ejection (CME) to travel directly toward Earth. Perhaps the most intense storm since the May 1921 super storm was the magnetic storm of March 1989 which caused an electricity blackout in Quebec, Canada.  

In today’s world, electronic technologies have become embedded into everyday life and are, of course, quite vulnerable to solar activity. Power lines, long-distance telephone cables, radar, cell phones, GPS, and satellites – all of which could be significantly affected by an event like the one of 1859 or the storm of 1921.  In other words, the world’s high-tech infrastructure could grind to a halt disrupting daily activities from purchasing a gallon gas to using the Internet.

Of particular concern is the fear about what this kind of solar storm could do to the electrical grid since power surges caused by solar particles can blow out giant transformers.  If numerous transformers happened to be destroyed at once, it would likely take a painfully long time to replace them.  The eastern US is especially vulnerable since the power infrastructure is highly interconnected so that failures in one location could cause failures in other regions. One long-term solution to this vulnerability would be to rebuild the aging power grid to be less susceptible to solar disruptions.

On the positive side, there is comfort in the fact that observations of the sun in today’s world are a constant with a fleet of spacecraft in position to monitor the sun and gather data on solar flares. Also, there is better forecasting today and solar scientists could give some sort of warning as to when solar flares might appear and whether a given storm is pointed at Earth.  Improved forecasting can allow for mitigating actions to be taken since the most damaging emissions travel slowly enough to be detected by satellites well before the particles strike the Earth.  For example, power companies could protect valuable transformers by taking them offline before a solar storm strikes.

One thing is certain, we should be prepared for another massive solar storm of the magnitude of the “Carrington Event of 1859” or the great geomagnetic storm of May 1921 – the most powerful solar storm of the 20th century.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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