Multiharmonic Toroidal Standing Alfven Waves in the Midnight Sector Excited by Solar Wind Source

Excitation of toroidal mode standing Alfven waves in the midnight sector in association with substorms is well documented, but studies are sparse on dayside sources for the waves. This paper reports observation of midnight toroidal waves by the Van Allen Probe B spacecraft during a geomagnetically quiet period on 12--13 May 2013. The spacecraft detected odd harmonics of the toroidal waves below 30 mHz as it moved within the plasmasphere from ~2100 magnetic local time (MLT) to ~0030 MLT through midnight in the L range 4.2--6.1. The frequencies and the relationship between the electric and magnetic field components of the waves are consistent with theoretical toroidal waves for a reflecting ionosphere. According to the solar wind OMNI data, the cone angle of the interplanetary magnetic field was small, a condition that favors excitation of upstream ultralow frequency waves. As expected, compressional waves as well as toroidal waves were observed by geostationary satellites located on the dayside. The nightside toroidal waves were most likely excited by the same mechanism as the dayside toroidal waves, that is, field line resonance driven by fast mode waves that resulted from transmission of the upstream waves into the magnetosphere.