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Polar Telecon: February 2003

1. The combination of ISEE 1 and 2, CCE and Polar now has provided a complete survey of the magnetic field in the northern half of the magnetosphere; with sufficient redundancy that we can study it as a function of geomagnetic conditions. Figure 1 (PDF) shows at one local time where our coverage is concentrated and how much data is contributed by each of ISEE, CCE and Polar. The majority of the data were provided by Polar and we in the northern hemisphere. We have assumed that the magnetosphere is north-south symmetric in binning the data. Figure 2 (PDF)shows the coverage at midnight for 4 different ranges of Dst*, where the asterisk signifies that Dst has been corrected for the prevailing solar wind dynamic pressure.

2. We have binned all available ISEE, CCE and Polar data (10, 5 years respectively) and then found the curl of the statistical field in the azimuthal direction as a function of Dst*. At low Dst* we can see the eastward ring current at low L-values associated with the pressure gradient on the inner edge of the ring current and the main westward ring current at greater L values. The current is much weaker on the dayside, as shown in Figure 3 (PDF).

3. At high Dst* the currents are stronger (note the color scale has changed) and the day-night asymmetry persists, as shown in Figure 4 (PDF).

4. We can summarize these results by calculating the integrated current in the equatorial plane. These local time polar plots emphasize again the strong day-night asymmetry of the ring current. Note that each plot has its own color scale, as shown in Figure 5 (PDF).

5. Another way to visualize these results is a polar plot of integrated current intensity as a function of local time for each of our Dst* ranges. This plot shows quite dramatically the day-night asymmetry at each activity level, as shown in Figure 6 (PDF).

6. The azimuthal component of current is only part of the ring current story. If the ring current is mainly partial and not symmetric, then there must be a large current along the field direction and orthogonal to the field in the magnetic meridian. Figure 7 (PDF) shows this for -20>Dst*>-40nT; Figure 8 (PDF) for -40nT>Dst*>-60nT; Figure 9 (PDF) for -60>Dst*>-80nT; and Figure 10 (PDF) -80nT>Dst*>-100nT. There is clearly a strong current rising out of the ionosphere on the dawn meridian and a strong current flowing into the dusk meridian. Note that the current arrows on Figure 7 to 10 have a different calibration factor in each display.

7. Figure 11 (PDF) shows how the currents in the meridian planes at dawn and dusk evolve with the level of Dst*. The length of the arrow for fixed current density remains fixed in this display.

8. Conclusions

The strong day-night asymmetry in the equatorial ring current is very unexpected. The termination of the partial ring current occurs via meridional currents that have roughly the region 2 sense, up in the morning hours and down in the afternoon. We have checked that the currents are divergence free, and they are. Since the exosphere is nearly day-night symmetric with slightly greater density on the night side, charge exchange cannot explain this asymmetry. However, since the currents are strongest at lowest altitudes where the exosphere is densest they may play some role. More likely the ionospheric conductivity is playing a role, possibly resisting convection on the dayside. In fact the sense of the currents is like region 2 currents that help the ionospheric plasma circulate by overcoming atmospheric drag. Whatever the source of the equatorial asymmetry and the meridional currents, these currents all increase with increasing disturbance levels in the magnetosphere. There is not a level at which a new behavior of the ring current takes over.

Last updated: January 25, 2007