The source regions and a mechanism obtained by REIMEI observations
Takanori Nishiyama
Planetary Plasma and Atmospheric Research Center
Graduate School of Science
Tohoku University
Japan
Abstract
Pulsating aurora is a phenomenon which shows periodic emission variation in diffuse aurora. The emission is characterized by
not a sinusoidal change but a pulsation, and its typical period ranges from a few seconds to a few tens of seconds. Energy range
of precipitating electrons which generate pulsating aurora was estimated from a rocket observation by Sandahl et al., [1980].
Because pulsating aurora appears in diffuse aurora, electrons are thought to undergo cyclotron resonance with whistler mode
waves in the equatorial region of the magnetosphere and to precipitate into Earth?s upper atmosphere by pitch angle scattering.
This concept is widely accepted, but there is a few observations conflicting this idea. Sato et al., [2004] recently suggested that
the source region of pulsating aurora is located earthward, far from the equatorial plane, raising a question about the source
region of pulsating aurora.
The purpose of this study is to search for the source regions of pulsating aurora using simultaneous image and particle observation
data from REIMEI satellite in statistical basis. A great advantage of REIMEI is that it can make simultaneous observation
of aurora image and particle flux by an attitude control that makes it possible to point the field of view of Multi-spectral Aurora
Camera (MAC) to a footprint of magnetic field line threading the satellite. We used mainly MAC and Electron/Ion energy Spectrum
Analyzer (E/ISA) in this study. MAC takes a picture with three wavelengths; 427.8 (N2+ 1st Negative Band), 557.7 (OGreen line)
and 670.0 (N2 1st Positive Band) nm. The field of view is 7.6 degrees and the time and spatial resolutions are 120 ms
and 1 km, respectively. E/ISA is tophat type electrostatic analyzer with energy range from 10 eV to 12 keV and time resolution
of 40 ms. In observation of pulsating aurora, energy dispersion of electron flux associated with pulsating aurora is seen. From
the difference of energy and time, we carried out Time of Flight analysis and calculated a distance of the source region from
REIMEI. The distance was traced along a magnetic field line using Tsyganenko-89 model and the source regions were identified.
We analyzed 15 paths from November 2005 to November 2007 and 38 source regions were identified. The results revealed
that the source regions are not necessarily located close to the equatorial plane but distribute continuously in an extent of 30
degrees from the equatorial plane. In the outer zone of radiation belt, it is reported that whistler mode wave frequently appears in
an extent of 20 degrees from the equatorial plane at recovery phase (Meredith et al., [2001]) and it is consistent with our results.
So, we carried out numerical calculation for cyclotron resonance between whistler mode wave (ELF-VLF) and electrons in order
to examine the results. We used the code which takes account of high order resonance by Albert,[1999] and calculated pitch
angle scattering rate Daa as changed the location where the resonance takes place from equator to 30 degrees. As a result, the
peak values of Daa for equatorial pitch angles are about 10?2[1/s] and show no change in the values to the location, but as latitude
of resonance region is higher, Daa is larger at lower pitch angle near the loss cone. This suggests the resonance occurs
in the region from a equator to latitude of 30 degrees and supports the model that pulsating aurora is produced
by interactions between whistler mode waves and electrons.
Created 2008-12-15 13:18:52 by Mats Holmström Last changed 2008-12-15 13:18:52 by Mats Holmström
