GEOPHYSICAL RESEARCH LETTERS, VOL. 22, NO. 3, PAGES 291-294, FEBRUARY 1, 1995 Simulation of proton radiation belt formation during the March 24, 1991 SSC M. K. Hudson •, A.D. Kotelnikov •, X. Li •, I. Roth 2 J Wygant3 j B Blake4 M S GussenhovenS , M. Temerin 2 Abstract. The rapid formation of a new proton radia- tion belt at L •_ 2.5 following the March 24, 1991 Storm Sudden Commencement (SSC) observed at the CRRES satellite is modelled using a relativistic guiding center test particle code. The SSC is modelled by a bipolar electric field and associated compression and relaxation in the magnetic field, superimposed on a dipole mag- netic field. The source population consists of both solar and trapped inner zone protons. The simulations show that while both populations contribute to drift echoes in the 20-80 MeV range, primary contribution is from the solar protons. Proton acceleration by the SSC differs from relativistic electron acceleration in that different source populations contribute and nonrelativistic con- servation of the first adiabatic invariant leadsto greater energizationof protons for a given decrease in L. Model drift echoes and flux distribution in L at the time of injection compare well with CRRES observations. Introduction Protons were accelerated simultaneously with elec- trons to form new radiation belts following the Storm Sudden Commencement (SSC)of March24, 1991[see Mullen e! al., 1991, Blake e! al., 1992].Previously [œi e! al., 1993], weshowed that a simplified model of theSSC compression of the magnetosphere accelerates trapped outer zone electrons on a time scale short compared to their drift period. The model reproduced the drift echoes reported by Blake e! al. [1992] from measure- ments made on the CRRES satellite, which fortuitously passed through the inneredge of the newradiation belt as it formed. In this Letter we showthat the same pulse reproduces the proton drift echoes, including the dou- ble peaks reported by Blakee! a/.[1992], and the broad spectral peak observed above 20 MeV by the Protel in- strument on CRRES [Gusscnhoven ½!al., 1993]. Two distinct ion source populations, solar protons and the inner zoneprotons, contribute to the new protonbelt. The CRRES satellite was located at a radial distance of 2.55RE geocentric nearthe equatorial plane and0300 1Dartmouth College, Hanover, NewHampshire 2University of California, Berkeley, California 3 School of Physics andAstronomy, University of Minnesota 4The Aerospace Corporation, Los Angeles, California s Phillips Laboratory, Hanscorn AFB, Massachusetts Copyright 1995by the American Geophysical Union. Paper number 95GL00009 0094-8534/95/95GL-00009503.00 MLT at the time of the SSC [Mullen et al., 1991, Vam- pola and Korth, 1992, Blakeet al., 1992], and observed the new radiation belt form in less than 150 seconds, along with the electric and magnetic field signatures of the SSC [Wygant et al., 1994]. Modelling the SSC with a bipolar propagating electric field and associated compression and relaxation in the magnetic field, su- perimposedon a backgrounddipole magnetic field, Li et a/.[1993] followed the trajectories of 336,720 equato- rially mirroring electron guiding centers using a rela- tivistic test particle codewhich determines the guiding center velocity. We have speededup the original code (25x) by vectorization and replacing the energy conser- vation equation((2) in œi et a/.[1993])with the adia- batic change in energy along the ion trajectory. The magnetosphericcompression is modelled by an induc- tion electric field exactlyasin œiet a/.[1993]. Faraday's law determinesthe magnetic field, which is purely com- pressional in the model, but has an azimuthal as well as radial gradient, while two exponentials describein- bound and outbound pulses which approximate com- pressionand relaxation of the magnetosphere. Figure i showsthe maximum azimuthal electric field wavefront (a)inbound and (b) outbound at successive times. At t=0 the pulse maximum is at about 25 RE at •50, and it reaches10, 1.05 and 10 RE again at 48, 76 and 105 seconds, respectively. The pulse is assumed to strike the magnetopause at 1500 MLT, motivated by conclusions drawn from the arrival times of the first newly accelerated electrons and protons observed at CRRES [Blake et al., 1992],and the electron simula- tionsof Liet a/.[1993]. The initial dipolaffield remains partially compressed after the pulse leavesthe system. In the model, all reflection occursat the ionosphere, al- though partial reflectionwill occurfrom various gradi- ents within the •nagnetosphere. Also, the pulsevelocity is constant in the model, and taken to be a characteris- / sec 50 sec 60 70 1500MLT L----6. 1500 MLT Figure 1. Maximum azimuthal electricfield wavefront at successive times. a) Shows inbound and b) shows outbound pulse inside L=12 with times labeled from beginning of simulation (t=0) whenpulse is at about 25 RE. Dotted L=2.5 and L=6.5 contours are labeled. 291