A full description of the cosmic ray experiment on Explorer 12 is given and cosmic ray measurements made during the solar event of September 28, 1961, are reported and discussed. Galactic cosmic ray measurements are also reported. A few hours before the class 3 flare of September 28, two short counting rate increases were observed and are interpreted as electron bursts. The anisotropy of the medium-and low-energy solar protons early in the event and their intensity throughout the event are described. It is found that the history of the intensity of the solar protons is consistent, once isotropy is established, with their having diffused through interplanetary space with an effective mean free path of 0.04 AU. This result is discussed and is shown to be not obviously in disagreement with the generally accepted views regarding the configuration of the interplanetary magnetic field. An estimate of the distance from the sun at which diffusion becomes unimportant and particles escape gives 2 to 3 AU. It is pointed out that simple diffusion (where the particles are scattered from discrete scattering centers and the influence of a general magnetic field is negligible) does not account for the behavior of the anisotropy before isotropy is reached. Two days after the flare, and beginning just before the sudden commencement of a magnetic storm, there was a large increase in the intensity of protons between 2 and 15 Mev, the lower energy limit being determined by the sensitivity of the detectors. As most of these particles, which we have called 'energetic storm particles,' arrived after the sudden commencement occurred, we suggest that they were solar protons trapped within the plasma cloud which caused the magnetic storm. The outline of a possible trapping mechanism is given. Explorer 12 measurements of the Forbush decrease of September 30, 1961, are compared with neutron monitor measurements at I)eep River. The decrease is larger at Explorer 12 by a factor of 1.7 _ 0.3. This paper describes the detectors and discusses the measurements made during the solar cosmic ray event initiated by a class 3 flare on September 28, 1961. Fortunately, the satellite was at apogee both at the time of the arrival of the solar cosmic rays and when the magnetic storm began two days later. The rise and decay of the solar proton intensity were recorded as a function of energy from 2 to 600 Mev. A plasma cloud, apparently emitted at the time of the flare, produced a large magnetic storm and a moderate Forbush decrease about 46 hours later. At the time of the sudden commencement of the magnetic storm, an increase of intensity of low-energy (( 15 Mev) protons was observed. The other four solar cosmic ray 4983 4984 BRYANT, CLINE, DESAI, AND McDONALD

Entry of heated solar wind plasma into the magnetosphere is examined using electron distributions measured by AMPTE UKS and HEOS 2. In particular, the angular structure of the electron distributions is studied within the transition region separating the magnetosheath from the inner magnetosphere. The measurements suggest that electrons in the outer part of the transition region originate in the magnetosheath, whilst the population closer to the Earth consists of electrons from the magnetosphere combined with an energized magnetosheath component. This energized component contains “counterstreaming” electrons, which are confined to directions closely parallel and antiparallel to the magnetic field direction. The possibilities, that the energization of the counterstreaming electrons is cumulatively gained from either waves, electric fields perpendicular to the magnetic field, or quasi‐Fermi acceleration, are discussed. It is not possible to identify the topology of the magnetic fields of the outer part of the region, but there is strong evidence that the inner part is on closed magnetic field lines, which map to the day side auroral zone. The outer part of the transition region is a plasma depletion/magnetic field compression layer. The structure of the transition region is similar to that surrounding flux transfer events, which leads to the deduction that the plasma and field signatures of flux transfer events may be the result of displacement of the transition region earthward. Cases where the displacement is such that the field maximum of the depletion/compression region is encountered may well explain “crater” flux transfer event signatures.

The Debye length, the characteristic shielding distance in a plasma, is, when the electrons and ions have Maxwellian velocity distributions, determined by the ratio of the temperatures of these components, to the electron (or ion) number density. Plasmas encountered in space, however, commonly exhibit non-Maxwellian velocity distributions, where the evaluation of an appropriate ‘temperature’ from an observed velocity distribution is no longer a recognized procedure. This paper evaluates the shielding distance for a plasma having a modified power-law, or kappa, family of distribuitons characteristic of some space plasmas.

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We have made a complete survey of all electron data recorded during low‐latitude boundary layer crossings of the AMPTE‐UKS spacecraft. In 27 of the 31 cases there is a clear anticorrelation between the density and the mean energy of the electrons during the transition across the boundary layer. The simplest interpretation of this anticorrelation is that, in the majority of cases, there is a smooth change of plasma state between the bounding solar and terrestrial plasmas, and that the abrupt changes frequently seen in time series data arise from boundary layer motions. Our data indicate that there are always two distinct sections to the anticorrelation. We suggest that these represent two physically distinct regions within the boundary layer and indicate how electron density and energy might vary across them. We briefly discuss an alternative interpretation in which filaments of magnetosheath plasma penetrate the boundary layer. We show that our results provide a constraint on such a model.