Thermal Electron Behavior in Obliquely Propagating Whistler Waves: MMS Observations in the Solar Wind. Issue 14 (23rd July 2021)

• Record Type: Journal Article
• Authors: Liu, Z.‐Y.; Wang, B.; Zong, Q.‐G.; Yao, S. T.; Pollock, C. J.; Le, G.
• Is Part Of: Geophysical research letters. Volume 48:Issue 14(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-07-23
• Subjects: electron acceleration -- wave‐particle interaction -- whistler waves -- Landau trapping -- phase bunching; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL094099
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: 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. Plain Language Summary: Whistler waves, a branch of R mode plasma waves occurring ubiquitously in space, have been suggested to play an important role in space electron energization and associated space phenomena. To correctly evaluate whistler waves' role, determining how electrons behave in whistler waves is required. To this end, here we report a whistler wave‐electron interaction case observed by NASA's Magnetospheric Multiscale spacecraft in the solar wind. The observations demonstrate that whistler waves can significantly modulate thermal electron phase space density (PSD) distributions via phase bunching/trapping. In the direction perpendicular to the background magnetic fields, thermal electrons are bunched by and rotate with wave perpendicular fields, manifesting as a series of inclined stripes in rhythm with the waves on electron PSD 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. In this event, the boundaries of trapping islands are displayed as "V"‐shaped structures of large PSDs on electron PSD pitch angle‐time spectrograms, as the waves propagate along electron density gradient. These observations provide direct evidence for the energy and momentum exchange between whistler waves and electrons. Key Points: The instantaneous response of solar wind thermal electrons to whistler waves is observed The observations demonstrate whistler waves' capability of trapping/bunching thermal electrons The boundaries of trapping islands are identified in electron pitch angle‐time spectrograms