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| Titel |
The effect of electron bite-outs on artificial electron heating and the PMSE overshoot |
| VerfasserIn |
M. Kassa, O. Havnes, E. Belova |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 23, no. 12 ; Nr. 23, no. 12 (2005-12-23), S.3633-3643 |
| Datensatznummer |
250015429
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| Publikation (Nr.) |
 copernicus.org/angeo-23-3633-2005.pdf |
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| Zusammenfassung |
| We have considered the effect that a local reduction in the
electron density (an electron bite-out), caused by electron absorption on to dust
particles, can have on the artificial electron heating in the height region between 80 to
90km, where noctilucent clouds (NLC) and the radar phenomenon PMSE (Polar
Mesospheric Summer Echoes) are observed. With an electron density profile
without bite-outs, the heated electron temperature Te,hot will generally
decrease smoothly with height in the PMSE region or there may be no significant heating
effect present. Within a bite-out Te,hot will decrease less rapidly
and can even increase slightly with height if the bite-out is strong. We have looked
at recent observations of PMSE which are affected by artificial electron heating,
with a heater cycling producing the new overshoot effect. According to the theory for
the PMSE overshoot the fractional increase in electron temperature Te,hot/Ti, where Ti is the unaffected ion temperature=neutral temperature,
can be found from the reduction in PMSE intensity as the heater is switched on.
We have looked at results from four days of observations with the EISCAT VHF radar
(224 MHz), together with the EISCAT heating facility. We find support for the
PMSE overshoot and heating model from a sequence of observations during one of
the days where the heater transmitter power is varied from cycle to cycle and
where the calculated Te,hot/Ti is found to vary in proportion to the
transmitter power. We also looked for signatures of electron bite-outs by
examining the variation of Te,hot/Ti with height for the three other days. We
find that the height variation of Te,hot/Ti is very different on the
three days. On one of the days we see typically that this ratio can increase with height,
showing the presence of a bite-out, while on the next day the heating factor mainly
decreases with height, indicating that the fractional amount of dust is low, so that the
electron density is hardly affected by it. On the third day there is little heating
effect on the PMSE layer. This is probably due to a sufficiently high electron density in the
atmosphere below the PMSE layer, so that the transmitted heater power is absorbed
in these lower layers. On this day the D-region, as given by the UHF (933MHz)
observations, extends deeper down in the atmosphere than on the other two
days, indicating that the degree of ionization in and below the PMSE layers is higher
as well. |
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