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Ann. Geophys., 20, 1181-1191, 2002
www.ann-geophys.net/20/1181/2002/
© European Geosciences Union 2002
Instantaneous ionospheric global conductance maps during an isolated substorm
A. Aksnes1, J. Stadsnes1, J. Bjordal1,*, N. Østgaard2,3, R. R. Vondrak2, D. L. Detrick4, T. J. Rosenberg4, G. A. Germany5, and D. Chenette6
1Department of Physics, University of Bergen, Bergen, Norway
2NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
3University of California, Berkeley, CA 94720-7450, USA
4University of Maryland, College Park, USA
5University of Alabama in Huntsville, USA
6Lockheed Martin Advanced Technology Center, Palo Alto, USA
*deceased 17 July 2001
Correspondence to: A. Aksnes (arve.aksnes@fi.uib.no)
Abstract. Data from the Polar
Ionospheric X-ray Imager (PIXIE) and the Ultraviolet Imager (UVI) on board the
Polar satellite have been used to provide instantaneous global conductance
maps. In this study, we focus on an isolated substorm event occurring on 31
July 1997. From the PIXIE and the UVI measurements, the energy spectrum of the
precipitating electrons can be derived. By using a model of the upper
atmosphere, the resulting conductivity values are generated. We present global
maps of how the 5 min time-averaged height-integrated Hall and Pedersen
conductivities vary every 15 min during this isolated substorm. The method
presented here enables us to study the time development of the conductivities,
with a spatial resolution of ~ 700 km. During the substorm, a single region of
enhanced Hall conductance is observed. The Hall conductance maximum remains
situated between latitudes 64 and 70 corrected geomagnetic (CGM) degrees and
moves eastward. The strongest conductances are observed in the pre-midnight
sector at the start of the substorm expansion. Toward the end of the substorm
expansion and into the recovery phase, we find the Hall conductance maximum in
the dawn region. We also observe that the Hall to Pedersen conductance ratio
for the regions of maximum Hall conductance is increasing throughout the event,
indicating a hardening of the electron spectrum. By combining PIXIE and UVI
measurements with an assumed energy distribution, we can cover the whole
electron energy range responsible for the conductances. Electrons with energies
contributing most to the Pedersen conductance are well covered by UVI while
PIXIE captures the high energetic component of the precipitating electrons
affecting the Hall conductance. Most statistical conductance models have
derived conductivities from electron precipitation data below approximately 30
keV. Since the intensity of the shortest UVI-wavelengths (LBHS) decreases
significantly at higher electron energies, the UVI electron energy range is
more or less comparable with the energy ranges of the statistical models. By
calculating the conductivities from combined PIXIE and UVI measurements to
compare with the conductivities from using UVI data only, we observe
significant differences in the Hall conductance. The greatest differences are
observed in the early evening and the late morning sector. We therefore suggest
that the existing statistical models underestimate the Hall conductance.
Key words. Ionosphere (auroral
ionosphere, particle precipitation) – Magnetospheric physics (storms and
substorms)
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