Low‐altitude observations of the evolution of substorm injection boundaries

Abstract

We have analyzed the properties of the evolution of the ion and electron precipitation and the magnetic field perturbations in the auroral oval during substorms using the low‐altitude polar‐orbiting satellites DMSP F6 and F7. We show two examples where the magnetosphere‐ionosphere coupling, based on the character of the particle precipitation and field‐aligned current regions, exhibits different substorm responses. The main difference in the coupling response is reflected in the intensification of the ion keV plasma sheet precipitation flux. In one example (November 29, 1984) the intensification is confined to a narrow latitudinal region at the poleward edge of the region 2 current system. In the other example (November 1, 1984) the intensification encompasses a much wider latitudinal span over which typical estimated ion thermal energies are 15 keV and where there is a proliferation of discrete electron precipitation with peak energies of 500 eV. During the recovery phase, that region develops a very uniform diffuse electron precipitation that can last for long periods (>1 hour) subjected to electrostatic potential drops of ≈450 eV. There are, however, characteristics common to both examples. For instance, the energy distribution in the equatorward edge of the midnight auroral oval ion precipitation is transformed a few minutes into the expansion phase from that expected qualitatively from a steady state earthward convection into an energy distribution where the flux of keV ions is dramatically reduced and only ions and electrons with energies below 1 keV are detected. This precipitation region is colocated with a system of region 2 polarity field‐aligned currents and has a well‐defined poleward boundary that defines the location where the average energy of the ion and electron precipitation starts to decrease monotonically with decreasing latitude. Also, during the expansion phase the intense diffuse electron precipitation that occurs poleward of the region 2 system coincides with strong region 1 field‐aligned currents. Inside this region, ion and electron flux depletion (≈2° wide) develops around the magnetic latitude where the enhancement of the westward electrojet is first detected. During expansion and recovery additional ion precipitation appears in the midnight and dusk passes equatorward of the electron equatorward precipitation boundary in the two cases presented here. The similarity between the ion spectra of this ion precipitation and the spectra of the plasma sheet suggests that portions of plasma sheet plasma become detached from the bulk of the plasma sheet population. The equatorward detachments are separated from the rest of the plasma sheet by a dispersionless boundary. The ion energy cutoff of the dusk detachments exhibits the dispersion in latitude characteristic of steady state earthward transport produced by an enhanced convection electric field. This suggests that the particle transport in the magnetosphere is disrupted at onset in the inner magnetosphere where region 2 currents are generated but continues unimpeded in the earthward region.