Transient Proton radiation belts
On 24 March 1991, the CRRES spacecraft observed the creation of a new radiation belt in a time period of less than 90 seconds. This belt was created by an intense shock in the interplanetary medium, and consisted of electrons with energies above 15 MeV and protons of comparable energy. This completely unexpected observation aroused intense interest and study, both experimental and theoretical. Several other, much less dramatic events subsequently were found in the CRRES data.
Naturally we have been carefully scrutinizing the SAMPEX data for shock-injection events now that the Sun has become more active. Two clear proton injections, creating new radiation belts, were observed by SAMPEX on 4 May and 26 August 1998. Nature once more had a surprise for us; on 26 August not one but two proton radiation belts, quite separate in L value, were created; see Figure 1.
The situation prior to the injection is shown in Figure 2. These belts only lasted a week or two; the rapid loss probably was due to the fact that these protons were near the limit of adiabatic trapping, and that geomagnetic activity subsequent to the injection lead to a breakdown of adiabatic motion. It is not clear yet if the shock event also injected a significant number of energetic electrons. The difficulty with the electron detection is the fact that the outer zone contains intense fluxes of electrons with energies of several MeV. Thus, unless the interplanetary shock injects electrons to energies above 10 MeV, any shock-injected electrons do not stand out in the crowd. At present the SAMPEX data are being analyzed in concert with data from other ISTP spacecraft, especially POLAR and WIND, to obtain an understanding of the physics of shock-injected radiation belts, and simulations are beginning by ISTP theory researchers.
Contributed by Bern Blake, Aerospace Corp. «Return to the Results page