Storm Associated Pc4 Waves at Synchronous Orbit in May 2017

Abstract:

A magnetic storm with a sudden commencement and prolonged initial phase began at 15:42 UT on May 27, 2017. Its main phase began at 21:59 UT and ended at 08:00 UT on May 28 with a minimum Dst of -130 nT. Asymmetry of the ring current vanished by 13:00 UT, and the ring current had recovered to quiet levels by 09:30 UT on May 29. This storm was driven by a flux rope in a very low speed solar wind. Solar wind coupling strength vanished at 18:00 UT on May 28. At this time the GOES 13 (east) spacecraft was 1 hour past local noon. Large amplitude Pc 4 waves of 110 sec period appeared and persisted for 12 hours ending at 06:00 on May 29 as GOES 13 passed midnight. The GOES 15 (west) spacecraft encountered these waves at 20:00 UT when it was one hour before noon. The wave frequency was higher with a period of 70 sec. The waves ended at 10 UT as GOES west passed midnight. The waves at both spacecraft were linearly polarized with a principal axis along the V (radial) direction. At GOES 13 the main magnetic field has a significant earthward tilt implying a large angle between the principal axis and the main field. Similar behavior was seen at GOES 15. Spectral analysis demonstrates the waves at both spacecraft had a significant second harmonic. Coherency analysis indicates slight variations in the polarization properties with frequency. Dynamic analysis indicates a systematic shift in frequencies and polarization properties with time. We have examined the flux of 40 keV electrons at GOES13 and find one brief sequence of eight cycles of 100 sec fluctuation at 00:20 on May 29. We will obtain additional magnetic field data from two other GOES spacecraft, and additional electron and proton data from all spacecraft. We will use the proton data to examine the hypothesis that these waves are generated by a bounce-drift resonance of waves with drifting ring current protons.