Global imaging of proton and electron aurorae in the far ultraviolet

S.B. Mende, H.U. Frey, T.J. Immel, J.-C. Gerard, B. Hubert, and S.A. Fuselier


The IMAGE spacecraft carries three FUV photon imagers, the Wideband Imaging
Camera (WIC) and two channels, SI-12 and SI-13, of the Spectrographic Imager.
These provide simultaneous global images, which can be interpreted in terms of
the precipitating particle types (protons and electrons) and their energies.
IMAGE FUV is the first space-borne global imager that can provide instantaneous
global images of the proton precipitation. At times a bright auroral spot, rich
in proton precipitation, is observed on the dayside, several degrees poleward of
the auroral zone. The spot was identified as the footprint of the merging region
of the cusp that is located on lobe field lines when IMF Bz was northward. This
identification was based on compelling statistical evidence showing that the
appearance and location of the spot is consistent with the IMF Bz and By
directions. The intensity of the spot is well correlated with the solar wind
dynamic pressure and it was found that the direct entry of solar wind particles
could account for the intensity of the observed spot without the need for any
additional acceleration. Another discovery was the observation of dayside
subauroral proton arcs. These arcs were observed in the midday to afternoon MLT
sector. Conjugate satellite observations showed that these arcs were generated
by pure proton precipitation. Nightside auroras and their relationship to
substorm phases were studied through single case studies and in a superimposed
epoch analysis. It was found that generally there is substantial proton
precipitation prior to substorms and the proton intensity only doubles at
substorm onset while the electron auroral brightness increases on average by a
factor of 5 and sometimes by as much as a factor of 10. Substorm onset occurs in
the central region of the pre-existing proton precipitation. Assuming that
nightside protons are precipitating from a quasi-stable ring current at its
outer regions where the field lines are distorted by neutral sheet currents we
can associate the onset location with this region of closed but distorted field
lines relatively close to the earth. Our results also show that protons are
present in the initial poleward substorm expansion however later they are over
taken by the electrons. We also find that the intensity of the substorms as
quantified by the intensity of the post onset electron precipitation is
correlated with the intensity of the proton precipitation prior to the
substorms, highlighting the role of the pre-existing near earth plasma in the
production of the next substorm. 

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Space Science Reviews, 109 (1-4), 211-254, 2003.