Substorm and Storm Time Ion Injections: Similarities, Differences, and 
Implications for the Storm/Substorm Relationship 

G. D. Reeves and M. G. Henderson 

We have analyzed isolated and storm-time ion injections using in situ
geosynchronous particle data, remotely-sensed Energetic Neutral Atom
data, and the Dst index. We find that there are both surprising
similarities between the two classes of events as well as important
differences that bear directly on the relationship between storms and
substorms. Our analysis is based on a superposed epoch study of seven
isolated ion injection events and seven storm-time ion injection
events. We find that the average geosynchronous ion responses during
the growth phase, at onset, and in the approximately 15 min following
onset are nearly identical in intensity, spectral hardness, and
temporal profile for the two classes of events. ENA observations
confirm that similarity and additionally show that the two classes of
injections span nearly the same extent in local time. The two classes
of injection events differ primarily in the subsequent behavior of
the ion fluxes. For the isolated injections the geosynchronous fluxes
return to pre-event levels within about an hour and the ENA
observations show the expected westward drift and dispersion of the
injected ions. For the storm-time injections the geosynchronous
fluxes remain at elevated levels for at least several hours following
the initial injection - longer for lower energies (e.g., 50 keV) than
for higher energies (e.g., 250 keV). The ENA observations in the 1-3
hours following the onset also show clear differences for the two
classes of events. For the storm-time events the ENA fluxes remain
elevated and even intensify for at least 3 hours. Additionally the
ENA observations show new evidence that the region of new particle
injections expands eastward (opposite to the ion drift direction) to
encompass most of the night side. Within three hours ENA emissions
are observed coming from most of the inner magnetosphere but have
still not formed a symmetric, trapped distribution. Within that same
three hours the Dst index decreased an average of 40 nT with the
initial decrease observed in the same hour as the initial injection.
The isolated injections did not produce a measurable Dst signature.
These results show that, despite many remarkable similarities,
storm-time ion injection events are different than isolated injection
events.

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Presented at the Chapman Conference on the Storm-Substorm Relationship, 
Lonavala/Mumbai, India, March 12-16, 2001.