Geotail Measurements of Equatorial Pc3-4 Waves Near Midnight

A survey of more than 50 Geotail perigee passes through the inner magnetotail current sheet in 1997-1999 revealed two intervals of transverse waves near 9 Re with periods of one to two minutes. On both days the waves were confined to within 0.5 Re of the equatorial plane and they occurred during the recovery phase of magnetic storms. On August 21, 1999, 0100-0400 UT, the waves were measured near midnight and had two frequency bands harmonically related at 10 and 20 mHz. Magnetic and electric field fluctuations had peak to peak amplitudes up to 10 nT and 1 mV/m respectively. The lower frequency transverse waves appeared in both electric and magnetic field data but the higher frequency waves were present only in the electric field except for a small compressional magnetic field component. The magnetic field waves were largest in the radial field component and the electric and magnetic waves were 90 degrees out of phase, with the phase reversing as the equator was crossed. These characteristics are suggestive of standing waves along the dipolar-like field lines. Simultaneous data from GOES 8 at a location three hours earlier in local time, 11 degrees north of the geomagnetic equator and 2.5 Re earthward of Geotail revealed waves at a frequency twice that of the low frequency Geotail waves. The GOES 8 waves had smaller amplitudes with a maximum perturbation in the azimuthal direction. On August 7, 1998, 0100-0300, Geotail magnetic field waves were detected at 0300 LT with a frequency near 6 mHz, but without any clearly detectable electric field waves.