Universal Time Seasonal Variations of Auroral Zone Magnetic Activity and VHF Scintillations.

摘要

The AL index, which is the negative component of the auroral electrojet index, AE, is utilized to study the universal time seasonal variations of northern hemisphere auroral zone magnetic activity in the midnight sector on quiet days. The solstitial AL patterns are found to be consistent with the predictions of the Kelvin-Helmholtz instability theory for geomagnetic activity which is based on the variation of dipole tilt angle psi with respect to the solar wind. Further, it is found that the best ordering of the seasonal variation is obtained by dividing the year into two broad seasons, one 6-month period centered around the June solstice, when the plane of symmetry of the plasma sheet in the magnetotail is above the solar magnetospheric equatorial plane, and the other 6-month period centered around the December solstice, when the plane of symmetry is below the solar magnetospheric equatorial plane. The observed AL patterns are thus interpreted as a seasonal modulation of low-energy particle precipitation caused by the asymmetric topology of the plasma sheet. The nature of this seasonal pattern is found to vary in different sectors of the auroral zone. Since auroral scintillations are associated with this particle precipitation, a comparative study between scintillations and AL index was undertaken. It is found that the long-term seasonal behavior of 137-MHz scintillation data from ATS 3 at an auroral station in the North Atlantic sector is consistent with the seasonal behavior of AL data in this sector. (Author)