Local time propagation of electron and proton auroras in substorms

S.B. Mende, B.J. Morsony, H.U. Frey


Latitudinal expansion of substorm auroras is explained by the poleward motion of
the dipolarization region during the expansive phase. However the cause of the
expansion of the aurora in local time is not fully understood and we used IMAGE
FUV observations of 154 substorms to investigate it. Images of the Wideband
Imaging Camera and the SI-12 proton auroral imager were re-projected to a
rectangular magnetic latitude local time coordinate grid during 154 substorms. A
representative function of the intensity distribution as a function of magnetic
latitude, composed of two Gaussian peaks, was fitted to the projected images and
the fitting parameters were obtained for all magnetic local times (MLT-s). This
fitting permitted the effective removal of dayglow and other non auroral
artifacts and yielded simple parameters to examine the properties of the aurora.
The integrated intensity of the fitted double Gaussian functions were used to
represent the intensity of the aurora at a particular MLT. Using this, intensity
MLT-keograms were produced to examine the MLT development of the proton and
electron aurora. These keograms show that the nightside aurora expands after
substorm onset, but in addition to the expansion, the luminosity exhibits a bulk
motion in the (MLT) direction towards dawn or dusk. No regular trends were
derived in the direction or speed of the luminosity of the aurora in either the
proton or the electron case. The MLT motion of the post substorm aurora
therefore does not represent the drift motion of the injected particles in the
magnetosphere but rather it represents a gradual drift of the particle injection
source region. In previous case studies, comparison of energetic neutral atom
(ENA) data with FUV showed that there is good agreement between lower energy
trapped and precipitating fluxes. From our study therefore we propose that the
substorm expansion in local time is a motion of the source of the particle
injection during the process of the substorm.

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Presentation, Fall Meeting, American Geophysical Union, San Francisco, USA, 
8-12 December 2003