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Ann. Geophys., 22, 1613-1622, 2004
www.ann-geophys.net/22/1613/2004/
© European Geosciences Union 2004
Energy distribution of precipitating electrons estimated from optical and cosmic noise absorption measurements
H. Mori1, M. Ishii1, Y. Murayama1, M. Kubota1, K. Sakanoi1, M.-Y. Yamamoto2, Y. Monzen3, D. Lummerzheim4, and B. J. Watkins4
1Communications Research Laboratory, 4-2-1, Nukui-Kitamachi, Koganei, Tokyo, Japan
2Kochi University of Technology, 185, Miyanokuchi, Tosayamada, Kochi, Japan
3University of Electro-Communications, 1-5-1, Chofugaoka, Chofu, Tokyo, Japan
4Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska 99775-7320, USA
Abstract. This study is a statistical analysis on energy distribution of precipitating
electrons, based on CNA (cosmic noise absorption) data obtained from the
256-element imaging riometer in Poker Flat, Alaska (65.11° N, 147.42° W),
and optical data measured with an MSP (Meridian Scanning
Photometer) over 79 days during the winter periods from 1996 to 1998. On the
assumption that energy distributions of precipitating electrons represent
Maxwellian distributions, CNA is estimated based on the observation data of
auroral 427.8-nm and 630.0-nm emissions, as well as the average atmospheric
model, and compared with the actual observation data. Although the
observation data have a broad distribution, they show systematically larger
CNA than the model estimate. CNA determination using kappa or double
Maxwellian distributions, instead of Maxwellian distributions, better
explains the distribution of observed CNA data. Kappa distributions
represent a typical energy distribution of electrons in the plasma sheet of
the magnetosphere, the source region of precipitating electrons. Pure kappas
are more likely during quiet times – and quiet times are more likely than
active times. This result suggests that the energy distribution of
precipitating electrons reflects the energy distribution of electrons in the
plasma sheet.
Key words. Ionosphere (auroral ionosphere; particle precipitation;
polar ionosphere)
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