 |
| Titel |
Effect of the IMF By component on the ionospheric flow overhead at EISCAT: observations and theory |
| VerfasserIn |
H. Khan, S. W. H. Cowley |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
0992-7689
|
| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 18, no. 12 ; Nr. 18, no. 12, S.1503-1522 |
| Datensatznummer |
250014097
|
| Publikation (Nr.) |
 copernicus.org/angeo-18-1503-2000.pdf |
|
|
|
|
|
| Zusammenfassung |
We have analysed a database of ∼300 h of
tristatic ionospheric velocity measurements obtained overhead at Tromsø (66.3°
magnetic latitude) by the EISCAT UHF radar system, for the presence of flow
effects associated with the y-component of the IMF. Since it is already
known that the flow depends upon IMF Bz, a least-squares
multivariate analysis has been used to determine the flow dependence on both IMF
By and Bz simultaneously. It is found that
significant flow variations with IMF By occur, predominantly
in the midnight sector (∼2100–0300 MLT), but also pre-dusk (∼1600–1700 MLT),
which are directed eastward for IMF By positive and westward
for IMF By negative. The flows are of magnitude 20–30 m s–1
nT–1 in the midnight sector, and smaller, 10–20 m s–1 nT–1,
pre-dusk, and are thus associated with significant changes of flow of order a
few hundred m s–1 over the usual range of IMF By of
about ±5 nT. At other local times the IMF By-related
perturbation flows are much smaller, less than ∼5 m s–1 nT–1,
and consistent with zero within the uncertainty estimates. We have investigated
whether these IMF By-dependent flows can be accounted for
quantitatively by a theoretical model in which the equatorial flow in the inner
magnetosphere is independent of IMF By, but where distortions
of the magnetospheric magnetic field associated with a "penetrating"
component of the IMF By field changes the mapping of the field
to the ionosphere, and hence the ionospheric flow. We find that the principal
flow perturbation produced by this effect is an east-west flow whose sense is
determined by the north-south component of the unperturbed flow. Perturbations
in the north-south flow are typically smaller by more than an order of
magnitude, and generally negligible in terms of observations. Using equatorial
flows which are determined from EISCAT data for zero IMF By,
to which the corotation flow has been added, the theory predicts the presence of
zonal perturbation flows which are generally directed eastward in the Northern
Hemisphere for IMF By positive and westward for IMF By
negative at all local times. However, although the day and night effects are
therefore similar in principle, the model perturbation flows are much larger on
the nightside than on the dayside, as observed, due to the day-night asymmetry
in the unperturbed magnetospheric magnetic field. Overall, the model results are
found to account well for the observed IMF By-related flow
perturbations in the midnight sector, in terms of the sense and direction of the
flow, the local time of their occurrence, as well as the magnitude of the flows
(provided the magnetic model employed is not too distorted from dipolar form).
At other local times the model predicts much smaller IMF By-related
flow perturbations, and thus does not account for the effects observed in the
pre-dusk sector.
Key words: Magnetospheric physics (magnetosphere ·
ionosphere interactions) – Ionosphere (plasma convection; auroral ionosphere) |
| |
|
| Teil von |
|
|
|
|
|
|
|
|