Solar radio emission as a criterion for solar proton event warning

Abstract

The use of solar radio emission as an indication of impending proton arrival from a solar flare is discussed. Specific characteristics of solar radio emission on fixed frequencies have been found to be associated with solar flares that are proton emitters. These signal characteristics are distinguishable from nonproton emitting flares. Fixed frequencies between 1000 and 3750 Me tend to show recognizable signal characteristics that will give a warning time from a minimum of 30 min to an average of 2 hr before the arrival of protons near the earth. The correlation between solar radio emission and solar flares and between solar flares and solar cosmic ray or proton events has been known for quite some time and has been well established. If the knowledge were available that a particular solar flare was capable of producing protons, and that, within a relatively short period of time, the protons would be observed in the vicinity of the earth, the warning time gained would be used effectively in many ways. B OISCHOT1 described a particular type of radio emission whose characteristics showed a burst that lasted for tens of minutes and whose source did not remain fixed in the solar atmosphere. The source appeared to move outward with speeds up to 1000 km/sec. It followed that any continuum radiation (i.e., radiation showing smooth, even characteristics over a broad band of frequencies) would be called type IV, which orginally pertained to a specific characteristic in the meter wavelengths. The definition was later extended to include any long-period continuum emission in any part of the radio spectrum that follows a flare (Boischot and Pick7). These definitions of type IV, however precise, were not adequate for solar proton event warning. Many solar investigators—Boischot,1 Thompson and Maxwell,2 Pick-Gutmann,3 Kundu,4 Wild,5 and Bell6—have found that a significant relation exists between the occurrence of a spectral type IV solar radio outburst in association with a flare and an impending bombardment of protons in the vicinity of the earth. The very high correlation between solar proton events and type IV radio emission appeared to be very encouraging as a means of developing a solar proton event warning system. It appears from the preceding definitions that a type IV event can be recognized by the use of spectral solar radio receiving equipment. The use of solar radio spectrum analyzers, however, would entail the use of equipment whose complexity would not allow simple analytical procedures. It was necessary to investigate the development of a simplified technique of reception and analysis of the solar radio emission. It was also realized that, if the radio signal associated with a proton emitting flare could be recognized at a single frequency, a system might be developed which would be compact enough for use onboard spacecraft or for widespread terrestrial observations. The literature was nearly void of cases where investigators had sought for the relationship between the characteristics associated with a fixed frequency solar radio signal and the existence of associated protons. One exception was