Magnetic Observations of the Low Altitude, High Latitude Magnetosphere During the January 1997 Magnetic Cloud Event

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

  1. Introduction
  2. Orbit Plots
  3. POLAR Magnetic Field Observations
  4. Summary

1. Introduction

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:

  1. January 9 (Perigee at 1415 UT): During the perigee pass on Jan 9, before the magnetic cloud arrived the Earth, the solar wind has a nominal dynamic pressure of ~ 1.6 nPa, and a small magnetic field of ~ 3nT with an average Bz of ~ 0 nT.

  2. January 10 (Perigee at 0751 UT): The next perigee pass was on Jan 10, shortly after the magnetic cloud hits the Earth and the IMF turns strongly southward ( Bz ~ -15 nT). During this period, the solar wind dynamic pressure is ~ 3 nPa.

  3. January 11 (Perigee at 0128 UT): The third perigee pass was on Jan 11 during the period of an extremely high density solar wind filament observed by WIND. The IMF is very large with a strongly northward component ( Bz ~ +15 nT) and a large negative By component ( By ~ -10 nT). The solar wind dynamic pressure is extremely large (~ 40 nPa).

2. Orbit Plots

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.

3. POLAR Magnetic Field Observations

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

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.

Figure 7

Following features are shown in the residuals of the magnetic field strength:

  1. Under quiet time (January 9), the observed magnetic field strength is greater than the model field strength due to the quiet time ring current (Dst ~ -10 nT, Dst* ~ 30 nT).

  2. On January 10, the perigee pass occurs during the period of strongly southward IMF Bz and the strong ring current buildup in the magnetic storm (Dst ~ -50 nT, Dst* ~ -90 nT). The field strength is greater than the model field strength and the residual is about twice of that in January 9.

  3. On January 11, we see a sudden decrease in the residual of the field strength at ~ 0115 UT. This time corresponds the leading edge of the high density solar wind filament. The compression of the magnetosphere by the large density solar wind filament produces a strong magnetopause current almost instantaneouly, which overcomes the ring current effect and causes a positive Dst index (Dst ~ +40 nT). Its effect near the polar cap is to decrease the field strength, as evidenced by the negative residual in field strength after ~ 0115 UT. The negative residual lasts about 20 mimutes, similar to the duration of the high density solar wind filament. During this period there is still a ring current (Dst* ~ -50 nT). The residual of the magnetic field strength gradually goes back to positive at the end of the high density solar wind filament.

4. Summary

The magnetic field observations during three successive perigee passes have been used to examine the response of the high latitude, low altitude magnetosphere to different IMF and solar wind conditions. We find that field-aligned currents are greatly enhanced during strongly southward IMF conditions. But the solar wind dynamic pressure has little effect on their strength. Effects of the ring current and magnetopause current can be identified in the magnetic field data near the polar cap. The ring current increases the field strength, whereas the magnetopause current decreases the field strength near the polar cap.
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