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| Titel |
Ionospheric and geomagnetic responses to changes in IMF BZ: a superposed epoch study |
| VerfasserIn |
C. J. Davis, M. N. Wild, M. Lockwood, Y. K. Tulunay |
| 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 ; 15, no. 2 ; Nr. 15, no. 2, S.217-230 |
| Datensatznummer |
250012629
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| Publikation (Nr.) |
 copernicus.org/angeo-15-217-1997.pdf |
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| Zusammenfassung |
| Superposed epoch studies have been carried
out in order to determine the ionospheric response at mid-latitudes to southward
turnings of the interplanetary magnetic field (IMF). This is compared with the
geomagnetic response, as seen in the indices Kp, AE and
Dst. The solar wind, IMF and geomagnetic data used were hourly averages
from the years 1967–1989 and thus cover a full 22-year cycle in the solar
magnetic field. These data were divided into subsets, determined by the
magnitudes of the southward turnings and the concomitant increase in solar wind
pressure. The superposed epoch studies were carried out using the time of the
southward turning as time zero. The response of the mid-latitude ionosphere is
studied by looking at the F-layer critical frequencies, foF2,
from hourly soundings by the Slough ionosonde and their deviation from the
monthly median values, δfoF2. For the
southward turnings with a change in Bz of δBz > 11.5 nT accompanied by a solar wind dynamic pressure P
exceeding 5 nPa, the F region critical frequency, foF2,
shows a marked decrease, reaching a minimum value about 20 h after the southward
turning. This recovers to pre-event values over the subsequent 24 h, on average.
The Dst index shows the classic storm-time decrease to about –60 nT. Four
days later, the index has still to fully recover and is at about –25 nT. Both
the Kp and AE indices show rises before the southward
turnings, when the IMF is strongly northward but the solar wind dynamic pressure
is enhanced. The average AE index does register a clear isolated pulse
(averaging 650 nT for 2 h, compared with a background peak level of near 450 nT
at these times) showing enhanced energy deposition at high latitudes in
substorms but, like Kp, remains somewhat enhanced for several
days, even after the average IMF has returned to zero after 1 day. This AE
background decays away over several days as the Dst index recovers,
indicating that there is some contamination of the currents observed at the AE
stations by the continuing enhanced equatorial ring current. For data averaged
over all seasons, the critical frequencies are depressed at Slough by 1.3 MHz,
which is close to the lower decile of the overall distribution of δfoF2
values. Taking 30-day periods around summer and winter solstice, the largest
depression is 1.6 and 1.2 MHz, respectively. This seasonal
dependence is confirmed by a similar study for a Southern Hemisphere station,
Argentine Island, giving peak depressions of 1.8 MHz and 0.5 MHz for summer and
winter. For the subset of turnings where δBz >
11.5 nT and P ≤ 5 nPa, the response of the
geomagnetic indices is similar but smaller, while the change in δfoF2
has all but disappeared. This confirms that the energy deposited at high
latitudes, which leads to the geomagnetic and ionospheric disturbances following
a southward turning of the IMF, increases with the energy density (dynamic
pressure) of the solar wind flow. The magnitude of all responses are shown to
depend on δBz. At Slough, the peak depression
always occurs when Slough rotates into the noon sector. The largest ionospheric
response is for southward turnings seen between 15–21 UT. |
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