Climatology of the cusp-related thermospheric mass density anomaly, as derived from CHAMP observations
We report on the thermospheric mass density anomaly in the vicinity of the ionospheric cusp. A systematic survey of the anomalies is presented, based on a statistical analysis of 4 years of data (2002–2005) obtained by the accelerometer onboard CHAMP. The anomalies are detected during all years a...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2008-09-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/26/2807/2008/angeo-26-2807-2008.pdf |
| Summary: | We report on the thermospheric mass density anomaly in the vicinity of the
ionospheric cusp. A systematic survey of the anomalies is presented, based on
a statistical analysis of 4 years of data (2002–2005) obtained by the
accelerometer onboard CHAMP. The anomalies are detected during all years and
seasons in both hemispheres but with stronger signatures in the Northern
Hemisphere. For the same geophysical conditions, solar flux and geomagnetic
activity the anomalies in the north are larger by a factor of about 1.35.
Over the course of the survey period the amplitude decreases by more than a
factor of 5 while the level of solar flux reduces by a factor of 2. The
anomaly strength also depends on the solar wind input. The merging electric
field, <I>E</I><sub>merg</sub>, is generally enhanced for about an hour before the anomaly
detection. There is a quadratic response of the anomaly amplitude to
<I>E</I><sub>merg</sub>. For geophysical conditions of P10.7<150 and <I>E</I><sub>merg</sub><1 mV/m
hardly any events are detected. Their amplitudes are found to be
controlled by an additive effect of P10.7 and <I>E</I><sub>merg</sub>, where the weight of
<I>E</I><sub>merg</sub>, in mV/m, is by about 50 times higher than that of the solar
flux level. The solar zenith angle and the influence of particle
precipitation are found to play a minor role as a controlling parameter of
seasonal variation. The well-known annual variation of the thermospheric
density with a minimum around June also influences the formation of the cusp
anomalies. This leads to a clear hemispheric asymmetry with very weak
anomalies in the south during June solstice, which is supposed to be a
combined effect of the minimum in annual variation and the seasonal decrease
of solar insolation in this region. |
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| ISSN: | 0992-7689 1432-0576 |