The solar proton event of April 16, 1970, 3. Evolution of pitch angle distribution as ≲1‐MeV protons propagate into the high‐latitude magnetotail

Abstract

The solar proton event of April 16, 1970 was monitored by Vela satellites, of orbit r=18 RE, in the solar wind and high‐latitude magnetotail (lobe). Intensity structure at ≲1 MeV indicates a delay of 85–102 min in access of protons to near the center of the north lobe, corresponding to entry points at 340–370 RE from the earth. In three sequential periods, of 16, 181, and 124 min duration, the average intensity in the north lobe was lower, higher, and lower, respectively, than that in interplanetary space by factors which varied from 2 to 5. These reversals were a consequence of reversals in field‐aligned anisotropy in interplanetary space, the interplanetary magnetic field remaining southward. Pitch angle distributions were measured in three dimensions in interplanetary space and in the north lobe. In the lobe the distributions were essentially isotropic at r=18 RE. Comparison is made with theoretical propagation of solar particles along field lines in an open tail model, under the following conditions along a trajectory: (1) adiabatic motion all the way (Liouville theorem)—the ‘adiabatic access model’ or (2) isotropization at the magnetopause followed by adiabatic motion—the ‘nonadiabatic access model.’ Neither mode of access explains the observations adequately. A hybrid mode is proposed, in which a minimal amount of scattering occurs as particles enter the tail, followed by amplification (attenuation) of intensity as the pitch distribution is transformed to near 18 RE in the favored (unfavored) lobe. In this mode a large part of the isotropization at Vela orbit is accomplished by the Liouville transformation, since particles entering the tail beyond ≈100 RE will see an increase in magnetic field by a factor of 3 as they propagate along the tail. The amount of scatter at the magnetopause is estimated to be Δµ(rms)=0.3, where µ is cosine of pitch angle. Intensity discrepancies later in the event are interpreted as (1) a brief adjustment in the topology of the magnetotail in which the field line through Vela 6A changed its exit point to ≈530 RE, compared to ≈350 RE before and ≈420 RE after and (2) interconnection between interplanetary and magnetotail fields determined by the radial component of the interplanetary field, as distinct from the southward component during the main portion of the event (the particle access window, however, remained at the same distance from the earth).