Abstract
Abstract. 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.
Highlights
The general tendency for magnetic disturbances to be more stormy at equinoxes than at solstices has been recognised for more than 150 years (Sabine, 1852) and the cause of the semi-annual variation has been studied by many researchers since them
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
Mid-latitude range indices, such as am, have been shown in many studies to be closely correlated with the product of Bs Vx2, where Bs is the southward component of the interplanetary magnetic field (IMF) in geocentric solar magnetospheric (GSM) coordinates, and Vx is the solar wind velocity www.ann-geophys.net/27/1909/2009/
Summary
The general tendency for magnetic disturbances to be more stormy at equinoxes than at solstices has been recognised for more than 150 years (Sabine, 1852) and the cause of the semi-annual variation has been studied by many researchers since them. The key parameter in the equinoctial hypothesis, on the other hand, is the geomagnetic colatitude (9) of the sub-solar point, that is, the angle between the solar wind flow and the dipole axis of the earth It has not been elucidated why and in what way the geomagnetic colatitude is involved in the semi-annual modulation of geomagnetic activity. The explanation by the RM hypothesis has been generally accepted for many years This may be primarily because it related the semi-annual variation of geomagnetic activity to the change in the southward component of the IMF which plays an essential role in the dayside magnetic reconnection between the interplanetary magnetic field (IMF) and the geomagnetic field.
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