Seasonal dependence of large‐scale Birkeland currents

Abstract

The seasonal dependence of large‐scale Birkeland currents has been determined from the analysis of vector magnetic field data acquired by the TRIAD satellite in the northern hemisphere. Statistical characteristics of single sheet (i.e., net currents) and double sheet Birkeland currents were determined from 555 TRIAD passes during the summer, and 408 passes during the winter (more complicated multiple‐sheet current systems were not included in this study). The average Kp value for the summer events is 1.9 and for the winter events is 2.0. The principal results include the following: (1) The single sheet Birkeland currents are statistically observed more often than the double sheet currents in the dayside of the auroral zone during any season. The single sheet currents are also observed more often in the summer than in the winter (as much as 2 to 3 times as often depending upon the MLT sector). (2) The intensities of the single and double sheet Birkeland currents on the dayside, from approximately 1000 MLT to 1800 MLT, are larger during the summer (in comparison to winter) by a factor of about 2. (3) The intensities of the double sheet Birkeland currents in the nightside (the dominant system in this local time) do not show a significant difference from summer to winter. (4) The single and double sheet currents in the dayside (between 0600 and 1800 MLT) appear at higher latitudes (by about 1° to 3°) during the summer in comparison to the winter. These characteristics suggest that the Birkeland current intensities are controlled by the ionospheric conductivity in the polar region. The greater occurrence of single sheet Birkeland currents during the summertime supports the suggestion that these currents close via the polar cap when the conductivity there is sufficiently high to permit it. Since the intensities of Birkeland currents are larger during periods of greater ionospheric conductivity, an important source (but perhaps not the only source) of these currents must be a voltage generator in the magnetosphere, possibly related to the convective electric field.

Fast Auroral Snapshot observations of the dependence of dayside auroral field‐aligned currents on solar wind parameters and solar illumination

“…For the events between 8 -11 MLT, the mean current for sunlit (dark) events was 8.6 (6.0); for 13-16 MLT sector, the mean was 7.7 (5). This dependence has been used by previous studies, including Fujii et al [1981], as evidence that the currents are due to a voltage source. Other studies have also addressed the question of whether the magnetosphere acts as a constant current source or a voltage source.…”

“…They found good correlation with IMF B z for B z < 0 and with similar parameters, consistent with reconnection as the current source. Their data set was separated by the sign of IMF B y because previous studies showed changes in the region of maximum current with this parameter [Potemra et al, 1979] and restricted to summer because Fujii et al [1981] showed changes in the size of current with season.…”

Numerical analysis of global ionospheric current system including the effect of equatorial enhancement

“…6 background ionospheric conductivity due to solar radiation is higher in the former. However, the dependence on the ionospheric conductivity may be di erent between the voltage generator and the current one, as discussed for the quiet time geomagnetic variations by Fujii et al (1981) and Fujii and Iijima (1987). The northsouth asymmetry is not basically expected for the current generator, such as discussed by Vickrey et al (1986) for Birkeland currents of intermediate scale size.…”

Observations of long‐term field‐aligned flow of O+ ions near the equator during summer and winter

“…During the summer and winter seasons, or more specifically, when the magnitude of the dipole tilt angle is appreciable, Birkeland currents become asymmetric about the low-latitude boundary layer. In the summer hemisphere this current is stronger mainly because the conductivity of the polar ionosphere is higher [Fujii et al, 1981]. Because of this Birkeland current asymmetry, a net magnetic flux, westward on the dawn side and eastward on the dusk side of the inner magnetosphere, is enclosed by geomagnetic field lines.…”