#The midnight wave driver
Toroidal waves have been observed in the midnight sector, which poses an interesting question of whether the driver waves originate from sources in the solar wind or in the magnetotail. Possible dayside sources capable of generating the driver waves include variation of the solar wind dynamic pressure (Southwood & Kivelson, 1990), upstream ultralow frequency (ULF) waves (Yumoto & Saito, 1983), transient foreshock structures (Zhao et al., 2017), and the magnetopause Kelvin-Helmholtz instability (Chen & Hasegawa, 1974 Southwood, 1974). Statistical studies using satellite data indicate that toroidal waves are observed mainly on the dayside (Anderson et al., 1990 Junginger et al., 1984), suggesting that the driver waves are generated on the dayside. Toroidal waves are, in general, considered to be excited by coupling to driver fast mode waves through the field line resonance (FLR) mechanism (Hasegawa et al., 1983 Tamao, 1965). In this study, we investigate the source mechanism of toroidal waves observed in the midnight sector of the inner magnetosphere, which we define to be the region extending to the distances covered by geostationary satellites.
We are interested in finding out whether spacecraft detect nightside toroidal waves when ground magnetometers do not. On the ground, the rate of detection of toroidal waves with magnetometers is very low in the midnight sector (Del Corpo et al., 2019 Wharton et al., 2019), making magnetoseismology least effective in that region. Finally, the waves are used in magnetoseismic evaluation of the mass density and ion composition in the magnetosphere (Menk & Waters, 2013). Toroidal waves also play an important role in magnetosphere-ionosphere coupling (Greenwald & Walker, 1980 Samson et al., 1992). Although the basic properties of observed toroidal waves are theoretically well understood (Cummings et al., 1969 Radoski & Carovillano, 1966), the waves remain an important research topic because of their relevance to particle acceleration and energy and momentum transport in the magnetosphere (Elkington et al., 1999 Ukhorskiy et al., 2005 Zong et al., 2017). These waves are routinely observed in the magnetosphere (Engebretson et al., 1986 Lin et al., 1986 Takahashi et al., 2015) and are the source of ground magnetic pulsations detected primarily in the north-south component (Hughes, 1974 Inoue, 1973).
(the magnetic field shell parameter) and magnetic local time (MLT) and by magnetic field perturbations in the azimuthal direction. Toroidal mode standing Alfvén waves, hereinafter referred to as toroidal waves, are characterized by discrete frequencies that change with Ground magnetometers located near the footprint of the spacecraft did not detect toroidal waves. The nightside toroidal waves were likely driven by fast mode waves that resulted from transmission of upstream ultralow frequency waves into the magnetosphere. 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 frequencies and the relationship between the electric and magnetic field components of the waves are consistent with theoretical toroidal waves for a reflecting ionosphere. The spacecraft detected toroidal waves excited at odd harmonics below 30 mHz as it moved within the plasmasphere fromĠ030 magnetic local time through midnight in the dipole This paper reports observation of midnight toroidal waves by the Van Allen Probe B spacecraft during a geomagnetically quiet period on 12–.
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.
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