Excitation of toroidal mode standing Alfvén waves in the midnight sector of the inner magnetosphere 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 toroidal waves excited at odd harmonics below 30 mHz as it moved within the plasmasphere from (Formula presented.) 2100 magnetic local time to (Formula presented.) 0030 magnetic local time through midnight in the dipole (Formula presented.) 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. At the time of the nightside toroidal waves, compressional waves were observed by geostationary satellites located on the dayside, and the amplitudes of both types of waves varied with the cone angle of the interplanetary magnetic field. The nightside toroidal waves were likely driven by fast mode waves that resulted from transmission of upstream ultralow frequency waves into the magnetosphere. Ground magnetometers located near the footprint of the spacecraft did not detect toroidal waves.
Bibliographical noteFunding Information:
Work at JHU/APL was supported by NASA Grant NNX17AD34G and NSF Grant 1840970. Work at The Aerospace Corporation was supported by RBSP-ECT funding provided by JHU/APL Contract 967399 under NASA's Prime Contract NAS501072. International Space Science Institute, Bern, facilitated collaboration of K.?T., M.?V., and A.?D.?C. by hosting Magnetoseismology Team meetings (Peter Chi, lead). Data used in this study are available from the following sources: NASA/GSFC Space Physics Data Facility Coordinated Data Analysis Web (https://cdaweb.gsfc.nasa.gov) for Van Allen Probes; Zenodo (https://doi.org/10.5281/zenodo.3376790) for EMMA; NOAA National Geophysical Data Center (http://satdat.ngdc.noaa.gov) for GOES; Zenodo (https://doi.org/10.5281/zenodo.3385024) for ETS-VIII; Space Sciences Laboratory, University of California, Berkeley (http://themis.ssl.berkeley.edu/index.shtml) for THEMIS; and World Data Center for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp) for geomagnetic indices.
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- Van Allen Probe
- ion foreshock
- nightside magnetosphere
- toroidal Alfvén waves