2005 9 16.
Solar cosmic rays during the extremely high ground level (GLE) enhancement on 23 February 1956.
It was a long-lasting event is the most famous among the proton events observed since 1942. The year 1956 belongs to the ascending phase of solar cycle 19, after a deep minimum of solar activity which took place in 1954. Over the 15 months between October 1953 and December 1954, the monthly average sunspot number remained less than 10. New sunspots began to be generated faster towards the end of 1955. In January–February 1956 the sunspot number became about 120, which is a typical value for periods of high solar activity. The GLE on 23 February 1956 (GLE 05) was the first ground level proton enhancement in the cycle. The event on February 1956 occurred in sunspot group 17 351 (Greenwich catalogue), which was located in the northern portion of the solar disk (N22) and on 16 February 1956 had a size that exceeded 1700 millionths of the solar surface. From 12 to 26 February another sunspot group of a smaller size, 17 353, crossed the solar disk. It appeared almost at the same heliolongitude area as the 17 351 sunspot group, but in the Southern Hemisphere. It is noteworthy that the maximum of flare activity for the sunspot group 17 351 manifested during the period when both AR 17 351 and AR 17 353 were very large and were close to their peak sizes. It is interesting that on 21 February another sunspot group, 17 349, located very close to AR 17 351 was also activated, producing a 3-B flare at N40 latitude near the limb. The most important flare occurred when group 17 351 was passing over the limb. This 3-B importance flare started on 23 February at 03:34 UT at coordinates N25W85 and produced the GLE 05. The most outstanding feature of this proton enhancement was a narrow and extremely intensive beam of ultra-relativistic particles arriving at Earth during the first minutes of the event. This unique beam was observed during the short time and its width did not exceed 30–40°. Thus, its contribution to the solar particle density, as well as to their fluency, was not very significant. In the present work all available data from neutron monitors operating in 1956 were analyzed, in order to develop a model of the solar cosmic ray behavior during the event. Cosmic ray data was obtained exclusively by ground level detectors of small size and in some cases of a non-standard design. The narrow and extremely intense beam of ultra-relativistic particles was arriving at Earth just after the onset and the unusually high maximum solar particle energy. The contribution of this beam to the overall solar particle density and fluency was not significant because of its very short duration and small width. The number of accelerated low energy particles was closer to a record value, but these particles passed mainly to the west of Earth. Since it was observed so close to the limb, we cannot exclude that it was partly behind the limb and could have started somewhat earlier. In conjunction with minimum solar activity, the intensity of galactic cosmic rays peaked in 1954, and almost the same high level remained during 1955. A decrease in cosmic ray flux started in February 1956. Just before GLE 05 the level of the galactic cosmic ray intensity was 8–9% lower than that in solar activity minimum. Geomagnetic activity was relatively low in the time period 1953–1955. Eight magnetic storms took place in January 1956, but they all were minor or moderate. In February only one magnetic storm (on 11–12 February) was recorded before 25 February. Sudden storm commencements (SSC) occurred on 11, 19, 21 and 22 February, evidence of strong interplanetary disturbances. However, in the last three cases there were no magnetic storms; the negative Bz component of the IMF was small during these times. After the flare on 23 February (near the limb) a magnetic storm did actually start. The shock arrival was registered on 25 February at 03:06 UT and it was a relatively fast interplanetary disturbance considering the remote western place of the source. This magnetic storm was the biggest one since April 1952, reaching the level of Kp=8+. The enhancement on 23 February 1956 revealed itself to be extremely anisotropic. During the first 20 min of the enhancements the anisotropy reached almost 100% and exceeded the value of 30% even at 04:30 UT. A fast arrival of the first particles is evident just after the onset. The maximum flux was reached relatively quickly: in 10–15 min at favorably located stations (Leeds, Stokholm) whose asymptotic cones of acceptance viewed in the direction of the maximum of the anisotropy flux. At the other stations the intensity peaked approximately in one hour. The energy range of the accelerated particles extended far above 15 GeV. The maximum flux of the particles arriving in a very narrow beam during the first 15–20 min exceeds the maximum diffusion flux by a factor greater than 20. The mean flux of solar cosmic rays, which may be considered as their density, increases quickly at the beginning of the event and reaches maxium in about 55 min. In the beginning of the event the spectrum of solar cosmic rays is hard: from 03:50 to 03:55 UT the spectral index γ reached the value of –3.1±0.5. Throughout the next 45 min the absolute γ value gradually increased and during the rest of the time exceeded 5. Such a behavior of the spectral index is typical for a GLE. The effective energy seems to be high only at the beginning of the event. In less than one hour it decreased to 4–5 GeV. The peak spectrum within the energy range from 500 MeV to 25 GV (Fig.12) – the energy dependence of the maximum fluxes of solar protons. The anisotropy dominates during the first 30 min of the effect and then decreases quickly with time. The narrowest beam is observed at the beginning of the effect. Only 15 min after the onset the spectrum started to soften gradually. The source of anisotropy was placed between 154◦ and 164◦ longitudes, which corresponds to the direction along the classic Archimedean spiral path for a high solar wind velocity. (Source: AnnalesGeophysicae): https://tinyurl.com/yav6pd8z
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