 |
| Titel |
Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity |
| VerfasserIn |
A. Yoshida |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
0992-7689
|
| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 27, no. 5 ; Nr. 27, no. 5 (2009-05-04), S.1909-1914 |
| Datensatznummer |
250016509
|
| Publikation (Nr.) |
 copernicus.org/angeo-27-1909-2009.pdf |
|
|
|
|
|
| Zusammenfassung |
| Physical meaning of the equinoctial effect for semi-annual variation in
geomagnetic activity is investigated based on the three-hourly am index and
solar wind parameters. When the z component of the interplanetary magnetic
field (IMF) in geocentric solar magnetospheric (GSM) coordinates is
southward, am indices are well correlated with BsVx2, where
Bs is the southward component of the IMF and Vx is the solar wind
velocity in the sun-earth direction. The am-BsVx2 relationship,
however, depends on the range of Vx2: the am in higher ranges of
Vx2 tends to be larger than am in lower ranges of Vx2 for
the same value of BsVx2 for both equinoctial and solstitial
epochs. Using the data sets of the same Vx2 range, it is shown that
distribution of points in the am-BsVx2 diagram at the solstitial
epochs overlaps with that at the equinoctial epochs and the average am values
in each BsVx2 bin in solstitial epochs are closely consistent
with those in equinoctial epochs, if Vx2 for each point at
solstices are reduced to Vx2sin2 (Ψ) where Ψ is
the geomagnetic colatitude of the sub-solar point. Further, it is shown that
monthly averages of the am index in the long period is well correlated with
the values of sin2(ψ) for the middle day of each month. These
findings indicate that the factor that contributes to the generation of
geomagnetic disturbance is not the velocity of the solar wind, but the
component of the solar wind velocity perpendicular to the dipole axis of the
geomagnetic field. The magnitude of the perpendicular velocity component
varies semi-annually even if the solar wind velocity remains constant, which
is considered to be the long-missed key factor causing the equinoctial
effect. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|