Energetic ion mass composition as observed at near‐geosynchronous and low altitudes during the storm period of February 21 and 22, 1979
Authors: Strangeway R. J. , Johnson R. G. 
Published: February 2008

Mass composition data acquired during the storm period of February 21 and 22, 1979, are presented and analyzed. The data were obtained from the near‐geosynchronous SCATHA spacecraft and the polar‐orbiting S3‐3 spacecraft at altitudes below 8000 km. The data from both spacecraft show that significant amounts of ionospheric plasma were observed to be injected around the main phase of the two storms on February 21, 1979. At geosynchronous altitudes the increase in ionospheric plasma was found to be significant in both number density and energy density. Moreover, multiple dispersionlike signatures in the particle spectrograms were observed during the second storm, indicating that this plasma was recently injected into the magnetosphere. At lower altitudes the S3‐3 data also showed significant enhancements of ionospheric plasma, as determined from number density data. It was found that the density enhancement in the plasma population moved to progressively lower L shells during the recovery phase of the storms. As it is unlikely that the plasma is injected at the point of observation, at least during the recovery phase, we consider drift effects to be responsible for this signature. We hence summarize some of the simpler convection theory, specifically addressing the dependence of the boundaries between drift regimes as a function of L shell. To do this, a steady state convection model has been employed, but we assume that this “steady state” only applies during the recovery phase of the storm. Since we consider shielding to be important only later in the recovery phase, it is further assumed that the cross‐tail electric field is uniform. On comparing the data with the convection boundaries we find that we can usually choose a cross‐tail electric field strength which models the particle signatures quite closely. The major feature present in the particle spectra is an energy‐dependent minimum which, we presume, marks those ions that either have been lost or drift so slowly that they are not observed at the spacecraft. As a consequence of the comparison of the S3‐3 particle signatures with the predicted convection boundaries, we find that the apparent movement to lower L shells of the density enhancement during the recovery phase is due to time of flight effects on a low‐energy plasma population at these L shells (below L = 4). Time of flight also implies that these ions were in the morning local time sector at the time of the main phase of the storms. At the same time that these low‐energy ions drifting eastward are observed, large numbers of ions convecting westward are also seen. This plasma population contains a large amount of ionospheric plasma. Furthermore, the ionospheric plasma as labeled by singly charged oxygen appears to have been injected over quite a large range in local time during the first storm. However, the proton signatures imply that most of the protons observed at the higher L shells were confined to the nightside sector during the main phase of the storm. While there are many similar features associated with the second storm in the S3‐3 data, the oxygen ions display a signature consistent with injection only in the nightside magnetosphere. This is in agreement with the SCATHA observations where the particle spectra show multiple dispersion signatures.