On the Characteristics and Source Regions of Dayside Proton Precipitation

D.H. Chua, K.F. Dymond, S.A. Budzien, J.C. Gerard, V. Coumans, and H.U. Frey


The source regions of precipitating protons on the dayside and their dependence
on solar wind conditions are studied using far-ultraviolet (FUV) spectral
observations and imaging. The High-resolution Ionospheric and Thermospheric
Spectrograph (HITS) aboard the Advanced Research and Global Observation
Satellite (ARGOS) observes Doppler-shifted H Lyman-alpha emissions from
precipitating protons with a spectral resolution of 1.5 Angstroms. The shapes of
these Doppler spectra are indicative of the energy and pitch angle distributions
of the proton precipitation. Global images of H Lyman-alpha emissions
obtained by the SI-12 instrument on the IMAGE spacecraft are examined to relate
the spectral observations to the dayside morphology of the proton aurora. During
periods of sustained southward interplanetary magnetic field (IMF), the dayside
proton aurora spectra exhibit broad Doppler shifts and are similar to those
observed on the nightside with inferred mean energies typical of plasma sheet
protons of magnetospheric origin. Global images of proton aurorae under these
conditions show continuous regions of H Lyman-alpha emissions across the
dayside extending from the nightside. In contrast, during periods of northward
or variable IMF, proton aurora emissions on the dayside often appear in an
isolated spot in the noon to late afternoon MLT sector. The Doppler-spectra of
the proton emissions in these regions are narrow, indicating precipitation with
low mean energies and from a different origin than that observed in the
southward IMF cases. These spectra may be indicative of magnetosheath protons
that have direct access to the ionosphere through high-latitude dayside
reconnection. This study further quantifies the characteristics of dayside
proton precipitation under various states  of the magnetosphere and highlights
the importance of IMF orientation on the coupling between the high-latitude,
dayside ionosphere and its plasma sources at higher altitudes.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004