G. Le and C. T. Russell
Institute of Geophysics and Planetary Physics
University of California, Los Angeles
J. G. Luhmann and Frances Fenrich
Space Science Laboratory
University of California, Berkeley
During the January 1997 magnetic cloud events, the POLAR spacecraft experienced three successive perigee passes with dramatically different interplanetary magnetic field (IMF) and solar wind conditions:
The POLAR spacecraft is in a highly inclined, elliptical orbit around the Earth. Its apogee is about 9 RE in the northern high latitude magnetosphere. Its perigee is about 1.8 RE in the southern high latitude magnetosphere. With time the plane of the orbit "precesses" with respect to the Earth-sun line due to the motion of the Earth around the sun with approximately a one-year period. During the January 10-11, 1997 magnetic cloud event, the POLAR orbit is nearly on the dawn-dusk meridian. We plot here the POLAR orbit in solar magnetic coordinates in which the Z-axis is along the magnetic dipole direction and the X-Z plane contains the solar direction. Note the POLAR spacecraft is in the southern hemisphere. During these perigee passes, the POLAR spacecraft moves from dawn to dusk across the polar cap.
|Figure 1a||Figure 1b||Figure 1c|
Figure 1(a), Figure 1(b), and Figure 1(c) are the orbit projections on the XY plane (view from the magnetic north) for the three intervals of interest on January 9, 10 and 11, respectively.
|Figure 2a||Figure 2b||Figure 2c|
Figure 2(a), Figure 2(b), and Figure 2(c) are the orbit projections on the YZ plane (view from the Sun) for the same intervals on January 9, 10 and 11, respectively.
Tsygangenko 1996 Model
In this study we use the magnetic field data for the three perigee passes to examine how the low altitude, high latitude magnetosphere responses to the different solar wind and IMF conditions. We are interested in deviations of the magnetic field from the average field configuration. Herein, we use Tsyganenko 1996 empirical magnetosphere model [Tsygangenko, 1996] with Dst=0, IMF By=0, IMF Bz=0, and solar wind dynamic pressure=2 nPa as the average magnetosphere configuration. In the model, the 1995 International Geomagnetic Reference Field at epoch of date (IGRF 95 model) is used for the internal field. We use the model field as the baseline for the data and display the residual field after subtracting the model field from the data.
Magnetic Field Inclination and Declination Angles - Comparison with T96 Model
First we compare the observed magnetic field inclination and declination angles with those predicted by the model. The inclination is defined as the angle between the observed magnetic field and the local horizontal plane consistent with its definition in ground-based studies. The declination is the angle of the field projection in the local horizontal plane as measured from the local dipole magnetic meridian.
Figure 3 shows the magnetic field inclination and declination angles during the three perigee passes (Red: model; Blue: data). The observed inclination angle agrees with the model prediction very well throughout all three perigee passes. The observed and predicted declination angle agrees best throughout the perigee pass under normal IMF and solar wind conditions (January 9); and they agree fairly well for strongly northward IMF and high dynamic pressure (January 11). The largest deflection of declination angle occurs for strongly southward IMF (January 10). Since the magnetic field is largely perpendicular to the local horizontal plane near the perigee, the deflection in declination angle indicates that the deflection in the magnetic field is mainly in the local horizontal plane, or in the direction transverse to the local magnetic field. Thus the deflection is caused by the field-aligned current, and the IMF Bz plays the most important role in determining the strength of the field aligned current.
Residuals of the Magnetic Field
Field-Aligned Currents at Low Altitudes
|Figure 4||Figure 5|
Figure 4 shows the magnetic field residuals in field aligned coordinates (FAC), where Z is along the local model magnetic field, Y is perpendicular to the model field and eastward, and X is perpendicular to the model field and completes the right-handed system. Figure 5 shows the field residuals in SM coordinates. Residuals in FAC X and FAC Y components are much stronger under strongly southward IMF conditions (January 10).
|Figure 6a||Figure 6b||Figure 6c|
Figure 6a, Figure 6b, and Figure 6c show the magnetic field vector residuals along the POLAR orbit track projected on SM XY plane for January 9, 10 and 11 perigee passes, respectively. On January 10 (Figure 6b), the sunward residual vectors in the dawn and dusk sectors are separated by the antisunward vectors in the polar cap. This is the signature one would observe when passing through unbalanced Region 1/Region 2 field aligned current sheets in the dawn and dusk sector. The Region 1 current in the higher latitudes is stronger than the Region 2 current. The Region 1 current flows into the ionosphere in the dawn sector and out of the ionosphere in the dusk sector. In the southern hemisphere, the unbalanced Region 1/Region 2 current sheets will produce the magnetic signature in Figure 6b.
Effects of the Ring Current and the Magnetopause Current on Polar Magnetosphere
The bottom panels of Figure 4 and Figure 5 also show the residual of the magnetic field strength near the perigee. On January 9 and 10, the observed magnetic field strength is larger than the model field strength, whereas on January 11, the observed field strength is smaller than the model field strength. This appears to be the the effect of Dst, or combined effects of the ring current and the magnetopause current. The magnetic field perturbation due to the ring current is nearly parallel to the Earth's field near the polar cap, i.e., it adds to the Earth's internal field and increases the magnetic field strength. The perturbation field due to the magnetopause current near the polar cap is nearly anti-parallel to the Earth's field, i.e., it decreases the magnetic field strength. The contribution to Dst due to the magnetopause currents is assumed proportional to the square root of the solar wind dynamic pressure. Figure 7 shows the WIND observations of IMF Bz and the solar wind dynamic pressure and the hourly Dst index as well as hourly Dst index corrected by the solar wind dynamic pressure (Dst*). The corrected Dst index (Dst*) is the one with the magnetopause current effect removed, and thus contains mainly the ring current contribution.
Following features are shown in the residuals of the magnetic field strength: