Characteristic plasma properties during dispersionless substorm injections at geosynchronous orbit

The substorm-associated behavior of the thermal plasma (30eV < E < 40keV) in the plasma sheet is examined by means of a superposed epoch analysis, using a full year of data from a spacecraft in geosynchronous orbit. The zero epoch time is taken to be substorm onset as indicated by a dispersionless energetic particle injection observed on the same spacecraft. Five classes of injection events are found to be well ordered by their average local times. These range from pure ion injections ∼3 hours prior to local midnight, to ion injections followed a few minutes later by an electron injection ∼2 hours before midnight, to simultaneous ion and electron injections close to midnight, to electron injections that are followed by an ion injection ∼1 hour postmidnight, and finally to pure electron injections ∼2 hours postmidnight. The thermal electrons show a significant increase in temperature (from ∼1 to ∼2keV) and pressure at substorm onset, while the density and the thermal ion signatures (below ∼30keV) are typically weak and may even vary with local time. However, energetic ions (above ∼30keV), which contribute significantly to the total ion pressure, show clear flux enhancements, leading to a rise in the total temperature from ∼10 to ∼16keV. Preexisting perpendicular anisotropies in the thermal electrons are reduced during the substorm growth phase but become enhanced again after onset, sometimes after a brief period of parallel anisotropy. Similar anisotropy signatures are found for the thermal ions, although somewhat less pronounced.

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