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| Titel |
Observations of the response time of high-latitude ionospheric convection to variations in the interplanetary magnetic field using EISCAT and IMP-8 data |
| VerfasserIn |
H. Khan, S. W. H. Cowley |
| 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 ; 17, no. 10 ; Nr. 17, no. 10, S.1306-1335 |
| Datensatznummer |
250013826
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| Publikation (Nr.) |
 copernicus.org/angeo-17-1306-1999.pdf |
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| Zusammenfassung |
We have combined ~300 h of tristatic
measurements of the field-perpendicular F region ionospheric flow measured
overhead at Tromsø by the EISCAT UHF radar, with simultaneous IMP-8
measurements of the solar wind and interplanetary magnetic field (IMF) upstream
of the Earth's magnetosphere, in order to examine the response time of the
ionospheric flow to changes in the north-south component of the IMF (Bz).
In calculating the flow response delay, the time taken by field changes observed
by the spacecraft to first effect the ionosphere has been carefully estimated
and subtracted from the response time. Two analysis methods have been employed.
In the first, the flow data were divided into 2 h-intervals of magnetic local
time (MLT) and cross-correlated with the "half-wave rectifier"
function V2Bs, where V is the solar
wind speed, and Bs is equal to IMF Bz if the
latter is negative, and is zero otherwise. Response delays, determined from the
time lag of the peak value of the cross-correlation coefficient, were computed
versus MLT for both the east-west and north-south components of flow. The
combined data set suggests minimum delays at ~1400 MLT, with increased
response times on the nightside. For the 12-h sector centred on 1400 MLT, the
weighted average response delay was found to be 1.3 ± 0.8 min, while for the
12-h sector centred on 0200 MLT the weighted average delay was found to increase
to 8.8 ± 1.7 min. In the second method we first inspected the IMF data for
sharp and enduring (at least ~5 min) changes in polarity of the north-south
component, and then examined concurrent EISCAT flow data to determine the onset
time of the corresponding enhancement or decay of the flow. For the case in
which the flow response was timed from whichever of the flow components
responded first, minimum response delays were again found at ~1400 MLT, with
average delays of 4.8 ± 0.5 min for the 12-h sector centred on 1400 MLT,
increasing to 9.2 ± 0.8 min on the nightside. The response delay is thus found
to be reasonably small at all local times, but typically ~6 min longer on the
nightside compared with the dayside. In order to make an estimate of the
ionospheric information propagation speed implied by these results, we have
fitted a simple theoretical curve to the delay data which assumes that
information concerning the excitation and decay of flow propagates with constant
speed away from some point on the equatorward edge of the dayside open-closed
field line boundary, taken to lie at 77° magnetic latitude. For the combined
cross-correlation results the best-fit epicentre of information propagation was
found to be at 1400 MLT, with an information propagation phase speed of 9.0 km s–1.
For the combined event analysis, the best-fit epicentre was also found to be
located at 1400 MLT, with a phase speed of 6.8 km s–1.
Key words. Interplanetary physics (interplanetary
magnetic fields) · Magnetospheric physics (Plasma convection; solar wind ·
magnetosphere interactions) |
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