Thermal Electron Behavior in Obliquely Propagating Whistler Waves: MMS Observations in the Solar Wind

Determining how electrons behave in whistler waves is fundamental for understanding whistler waves' role in space electron energization. Here, we report a whistler wave-electron interaction case observed by Magnetospheric Multiscale spacecraft in the solar wind. The observations suggest whistler waves can effectively trap/bunch thermal electrons. In the direction perpendicular to the background magnetic fields, thermal electrons are bunched by and rotate with wave perpendicular fields, manifesting as repeated inclined stripes in rhythm with the waves on electron gyrophase-time spectrograms. On the other hand, in the parallel direction, thermal electrons are trapped by wave parallel electric fields, resulting in trapping islands around wave parallel potential maxima. Here, the boundaries of these trapping islands are displayed as V-shaped structures of large phase space density on electron pitch angle-time spectrograms, since the waves propagate along electron density gradient. The observations presented here well demonstrate whistler waves' capability of modulating electron phase space trajectories.

Automated summary

Understanding electron behavior in whistler waves is crucial for comprehending their role in energizing electrons in space. Observations indicate that whistler waves can effectively trap thermal electrons, causing them to bunch and rotate in perpendicular and parallel directions, respectively.