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Ann. Geophys., 20, 1225-1238, 2002
www.ann-geophys.net/20/1225/2002/
© European Geosciences Union 2002
On the nature of nighttime ionisation enhancements observed with the Athens Digisonde
I. Tsagouri1 and A. Belehaki1
1Institute for Space Applications and Remote Sensing, National Observatory of Athens, Metaxa and Vas. Pavlou, P. Penteli 15236, Greece
Correspondence to: I. Tsagouri (tsagouri@space.noa.gr)
Abstract. True height electron
density profiles observed with the Athens Digisonde were analyzed together with
geomagnetic data indices for six magnetic storms in September and October 2000
in an effort to define the ionospheric structure during events of nighttime
ionisation enhancements and to discuss the physical processes that may cause
them. An upwelling of the F2-layer, limited to the dark hemisphere, was evident
in all storm events as the ionospheric response to enhanced geomagnetic
activity, but nighttime positive effects and nighttime height enhancements are
attributed to two distinct mechanisms, according to our findings. Height
enhancements are wavelike disturbances and the time delay of their occurrence
at middle latitudes depends on the increase rate of the AE index, and
consequently, on the rate that the solar wind input energy dissipates in the
auroral ionosphere. Thus, they most probably originated in the auroral oval
region and propagated toward the equator-like TID disturbances, with a
periodicity that depends on the ionisation density. On the other hand, foF2
increases do not share the same wavy appearance, which could mean that they are
not connected to TIDs and are not of auroral oval origin. The increased
nighttime density can only be speculated to be due to increased downward fluxes
from the plasmasphere.
Key words. Ionosphere
(ionosphere-magnetosphere interactions; ionospheric disturbances; mid-latitude
ionosphere)
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