The Near Disappearance of the Solar Wind at Earth: May 10-12, 1999

Modeling Ring Current Dynamics

We studied ring current dynamics during the May 10-12, 1999, ISTP interval of exceptionally low solar wind density, when inner magnetospheric measurements of geomagnetic activity show geomagnetic quiet par excellence (|Dst|<10 nT and Kp=0+). The interplanetary medium was monitored by the MFI and SWE instruments on the GGS spacecraft WIND. We simulated the global evolution of the ring current ion population during this period using measured H+, He+, and O+ energy and pitch angle distributions by the MICS and HYDRA instruments on Polar on May 9 as initial conditions for our kinetic model. Comparing model results with Polar data on May 11, we found remarkable agreement, proving the applicability of our model for these magnetospheric conditions. The modeled ring current energy density indicated initially moderately asymmetric conditions for O+ with the nightside ring current energy density being more intense. This asymmetry was weaker for the other two ions. These asymmetries all but have disappeared by the end of May 11, when the ring current has become symmetric. These results contrast sharply with the calculations during the main phase of the storm on May 4, 1998, one of the most disturbed days on the rising phase of this solar cycle, where pronounced asymmetry is visible for all 3 species and the energy density maximizes in the dusk-premidnight sector (note the difference in the color bar).

We calculated the magnetopause currents effects on ground magnetometers following Burton et al. [1975] and using dynamic pressure values derived from WIND/SWE data on May 10-11, 1999. We used the Dessler-Parker-Sckopke relation and the modeled total energy of the ring current to obtain the associated magnetic field perturbation at the surface of the Earth. We found that the ground magnetic field disturbances due to the ring and the magnetopause currents decreased quasi-monotonically on May 11, reaching limiting magnitudes of about 5 and 3 nT, respectively. These values are substantially smaller than the 20 nT quiet time values obtained from statistical studies.

References

Jordanova, V. K., C. J. Farrugia, J. F. Fennell, and J. D. Scudder, Ground disturbances of the ring, magnetopause, and tail currents on the day the solar wind almost disappeared, J. Geophys. Res., in press, 2000.

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