On the origin of the dawn‐dusk asymmetry of toroidal Pc5 waves

Observations in space and on the ground have shown that azimuthally polarized (toroidal) magnetospheric Pc5 waves (frequency = 1.7–6.7 mHz) are more prominent on the dawnside than on the duskside, but the origin of this dawn‐dusk asymmetry is unknown. To determine the origin, we conducted a statistical analysis of toroidal Pc5 waves at the Time History of Events and Macroscale Interactions during Substorms (THEMIS) D spacecraft using ion bulk velocity measured during 2008–2014. We found that both the detection rate of narrowband Pc5 waves, which are attributed to fundamental toroidal‐mode standing Alfvén waves, and the amplitude of velocity oscillations in the fixed Pc5 band exhibit a dawn‐dusk asymmetry consistent with previous studies regardless of whether the interplanetary magnetic field is directed along the Parker spiral or orthogonal to it. These results imply that the asymmetry originates within the magnetosphere, not from a dawn‐dusk asymmetry of the Kelvin‐Helmholtz instability on the magnetopause. Based on theoretical studies published previously and a global magnetohydrodynamic simulation, we propose that the asymmetry originates from the local time dependence of the radial mass density variation and the associated gradient of the frequency of standing Alfvén waves. The THEMIS data indicate that in the L range 8–12 the average gradient is smaller in the dusk sector than in the dawn sector. This will result in weaker field line resonance (FLR), and thus weaker toroidal Pc5 waves, in the dusk sector. Density structures associated with drainage plumes may also suppress FLR in the dusk sector.