Proton flares and the topology of the magnetic field in the...

1. Introduction

[2] Since charged particles most easily move along the field lines of the interplanetary magnetic field, the characteristics of a proton event depend strongly on the difference in heliographic longitudes of the flare itself and the base of the field line of the interplanetary magnetic field connecting the space station to the solar surface [Shea and Smart, 1990].

[3] However, it is widely known that solar proton events well correlate to strong bright H_a flares. Comparing the proton events registered from 1970 to 1980 (total 323 events) to H_a flares, Su et al. [2001] showed that not only strong H_a flares but also subflares SB and SN might be followed by proton events. Even H_a flares of the SF class may initiate small proton events, though the probability of their occurrence is less than 0.5%. The H_a flares of the 2B and 3B classes are the most productive for occurrence of proton events, in the case of two ribbon flares the occurrence of energetic particle (protons, in particular) being the most probable [Jain, 1986]. The probability of proton acceleration during the latter flares W_p is of the order of unity. The 2N and 3N flares are slightly less productive ( W_p sim 10% ). For the H_a flares of the importance 1N W_p < 2%, whereas for the H_a flares of the 1B class W_p sim 33%. This means that both characteristics of H_a flares (area and brightness) play an important role.

[4] The brighter a H_a flare and the larger its area, the more probable is an appearance of accelerated protons. For example, the probability of appearance of a proton event in a flare of the SB or 1N class is nearly the same. It is almost by a factor of 50 less than the probability of appearance of a proton event during a flare of the 3B class and by about a factor of 5 less than W_p for a H_a flare of the 3N class. Since the number of weak H_a flares is much higher than the number of strong flares and the probability of a proton event generation is directly proportional to the brightness and area of H_a flares, the observed maximum number of proton events corresponds to some intermediate class of H_a flares. The statistical analysis of the data for the 1970-1980 period showed [Su et al., 2001] that the maximum number of proton events was related to H_a flares of the 1B class (~26% of all proton flares).

[5] Having considered the processes of particle acceleration in the current sheet in flares at the given parameters (magnetic field strength, temperature, velocity of plasma flowing into the current sheet), Su et al. [2001] proposed a possible explanation of the observed correlation of the number of proton events and the class of H_a flares.

[6] Shea and Smart [1990] analyzed the data on proton events (more than 200) registered during the 19th, 20th, and 21st solar activity cycles. There is no unambiguous relation between the number of proton events and sunspot number; that is, the increase of proton events does not correspond to the increase of sunspots, though one can state that less proton flares are observed in the solar activity minimum and more in the solar activity maximum. If one compares the values averaged over three cycles, one can see that the maximum of the proton events distribution is observed a few years later than the maximum of sunspots. The cause of this phenomenon is not yet clear.