Ground disturbances of the ring, magnetopause, and tail currents on the day the solar wind almost disappeared

We study ring current dynamics on a day of exceptionally low solar wind den sity (of similar to0.1 cm(-3)). Inner magnetospheric measurements of geomag netic activity show geomagnetic quiet par excellence. On May 11, 1999, the values were \ Dst \ < 10 nT and Kp = 0(+). The field at geostationary orbit was very close to dipolar. We simulate the global evolution of the ring cu rrent ion population during this period and compare the effect of the magne topause, ring, magnetotail, and field-aligned currents on the Dst index. Me asured H+, He+, and 0+ energy and pitch angle distributions by the Magnetos pheric Ion Composition Spectrometer (MICS) and HYDRA instruments on Polar o n May 9 are used as initial conditions for our kinetic model. Comparing mod el results with Polar data on May 11, we find remarkable agreement, proving the applicability of our model for these magnetospheric conditions. We com pare modeled H+ and He+ ion distributions with quiet time ring current dist ributions inferred from statistical studies, and we find that as a general trend, the simulation results have lower values than the statistical patter ns. The ground magnetic field disturbances due to the ring and the magnetop ause currents decrease quasi-monotonically on May 11, reaching limiting mag nitudes of similar to5 and similar to3 nT, respectively. These values are s ubstantially smaller than the similar to 20 nT quiet time values obtained f rom statistical studies.