Distribution Of Field-aligned Currents Research Articles

Equivalent ionospheric current systems representing IMF sector effects on the polar geomagnetic field

Equivalent ionospheric current systems representing IMF sector effects on the geomagnetic field in high latitudes are examined for each of the twelve calendar months by spherical harmonic analyses of geomagnetic hourly data at 13 northern polar stations for seven years. The main feature of obtained equivalent current systems includes circular currents at about 80° invariant latitude mostly in the daytime in summer and reversed circular currents at about 70° invariant latitude mainly at night in winter. Field-aligned current distributions responsible for equivalent currents, as well as vector distributions of electric fields and ionospheric currents, are approximated numerically from current functions of equivalent current systems by taking assumed distributions of the ionospheric conductivity. Two sets of upward and downward field-aligned current pairs in the auroral region, and also a field-aligned current region near the pole show seasonal variations. Also, ionospheric electric-field propagation along geomagnetic field lines from the summer hemisphere to the winter hemisphere with auroral Hall-conductivity effects may provide an explanation for the winter reversal of sector effects.

Geomagnetic variations and field-aligned currents at Northern high-latitudes, and their relations to the solar wind parameters

By using statistical analysis to study the relation between the interplanetary and geomagnetic fields, geomagnetic variations have been detected that are controlled by different parameters of the solar wind. The equivalent current systems for these geomagnetic variations have been determined for the 1968 summer and winter seasons from data from 14 high latitude stations. It has been shown that the geomagnetic field at high latitudes (0> 60°) can be represented by a sum of fields controlled by components of the Interplanetary Magnetic Field (I.M .F.) and by the velocity and density of the solar wind plasma. A correlation model is proposed by means of which hourly-mean values of the three components of the geomagnetic field vector at high latitudes may be determined from hourly-mean values of the components B x ,B y and B z of the I.M .F. The space—time distribution of field-aligned currents has been reconstructed from the horizontal com ponent of the ground level geomagnetic field variation. The correlation model of field-aligned currents has been obtained for high latitudes (0 ^ 60°). According to the model the value and direction of the field-aligned current j , are determined by I.M .F . components and spatial coordinates. The proposed model of field-aligned currents enables the nature of the actual magnetic disturbance registered during a given pass of the TRIAD and ISIS-2 satellites through the high latitude region to be interpreted. Moreover it enables some discrepancies between observations and the models of field-aligned currents proposed in the literature to be understood.

Electric fields in the ionosphere produced by polar field-aligned currents

Two regions of field-aligned currents of magnetospheric origin have recently been found1–3 along the auroral oval and region 1 currents flow at a slightly higher latitude than those in region 2. Figure 1 shows the distribution and flow directions of such field-aligned currents determined from Triad data4. We discuss here the effects that these currents seem to have on the distribution of electric fields in the ionosphere, in addition to the Sq electric field of ionospheric origin.

Field-aligned currents and ionospheric electric fields

It is shown that the observed distribution of the ionospheric electric field can be deduced from an equation combining Ohm's law with the current continuity equation by using the 'observed' distribution of field-aligned currents as the boundary condition for two models of the ionosphere. The first model has one conductive annular ring representing the quiet-time auroral precipitation belt; the second has two conductive annular rings that simulate the discrete and diffuse auroral regions. An analysis is performed to determine how well the electric-field distribution can be reproduced. The results indicate that the first model reproduces the Sq(p)-type distribution, the second model reproduces reasonably well a substorm-type potential and ionospheric current patterns together with the Harang discontinuity, and that the distribution of field-aligned currents is the same for both models.

The amplitude distribution of field-aligned currents at northern high latitudes observed by Triad

The spatial distribution and magnitudes of field-aligned currents at 800-km altitude over northern high latitudes were determined from Triad magnetometer data recorded at College, Alaska, during the period from July 1973 to October 1974. The characteristics that were determined include the following: (1) Large-scale field-aligned currents are concentrated in two principal areas encircling the geomagnetic pole: region 1, located near the poleward part of the field-aligned current region; and region 2, located near the equatorward part. (2) In region 1 during moderately disturbed conditions (2− ≤ Kp ≤ 4+) the largest current densities occur on the forenoon sector, with currents flowing into the ionosphere (with a peak value of ∼2 µA/m² between 0700 and 0800 MLT), and in the afternoon sector, with currents flowing away from the ionosphere (with a peak value of ∼1.8 µA/m² between 1500 and 1600 MLT). These areas of maximum current density in region 1 are approximately coincident with the location of the foci of the Sqp current system. (3) In region 2 for 2− ≤ Kp ≤ 4+ the largest current densities occur on the night side where auroral electrojets are usually most active, namely, in the evening to premidnight sector, with currents flowing into the ionosphere (with a peak value of ∼ 1 µA/m² between 2100 and 2300 MLT), and in the midnight to morning sector, with currents flowing away from the ionosphere (with a peak value of ∼ 1.3 µA/m² between 0200 and 0300 MLT). (4) The currents in region 1 are statistically larger than the currents in region 2 at all local times except for the sector near midnight (∼2100–0300 MLT), where the region 2 currents are comparable to or slightly larger than the region 1 currents. (5) The region 2 currents in the midnight to morning sector are correlated with the intensity of the westward electrojet, and the region 2 currents in the evening to premidnight sector are correlated with the intensity of the eastward electrojet. (6) The region 1 currents appear to persist, especially on the day side, even during very low geomagnetic activity with a value of current density ≳0.6 µA/m² for Kp = 0. We suggest that the magnetosphere-ionosphere current system which contains field-aligned currents consists of two distinct parts: a permanent part with field-aligned currents in region 1 and the other part having field-aligned currents in region 2, which is an important element of the auroral electrojets.