Whistler Wave Generation by Anisotropic Tail Electrons During Asymmetric Magnetic Reconnection in Space and Laboratory

• Published in: Geophysical research letters Vol. 45; no. 16; pp. 8054 - 8061
• Main Authors: Yoo, Jongsoo, Jara‐Almonte, J., Yerger, Evan, Wang, Shan, Qian, Tony, Le, Ari, Ji, Hantao, Yamada, Masaaki, Fox, William, Kim, Eun‐Hwa, Chen, Li‐Jen, Gershman, Daniel J.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.08.2018; American Geophysical Union
• Subjects: Anisotropy; ASTRONOMY AND ASTROPHYSICS; Correlation; Dispersion; Dispersion relation; Distribution functions; Drift; Electron distribution; Electrons; Energy resources; Energy sources; Free energy; Heliospheric and Magnetospheric Physics; Instability; Laboratories; Linear analysis; Lower-hybrid drift instability; Magnetic field; Magnetic fields; Magnetic reconnection; Magnetopause; Magnetosphere; Magnetospheres; Particle energy; Plasma waves; Remote sensing; Satellites; Spaceborne remote sensing; Stability; Temperature; Temperature anisotropy; Velocity; Wave generation; Whistler wave; Whistler waves
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL079278

Whistler wave generation near the magnetospheric separatrix during reconnection at the dayside magnetopause is studied with data from the Magnetospheric Multiscale mission. The dispersion relation of the whistler mode is measured for the first time near the reconnection region in space, which shows that whistler waves propagate nearly parallel to the magnetic field line. A linear analysis indicates that the whistler waves are generated by temperature anisotropy in the electron tail population. This is caused by loss of electrons with a high velocity parallel to the magnetic field to the exhaust region. There is a positive correlation between activities of whistler waves and the lower hybrid drift instability both in laboratory and space, indicating the enhanced transport by lower hybrid drift instability may be responsible for the loss of electrons with a high parallel velocity. Plain Language Summary Magnetic reconnection is a fundamental process in magnetized plasma, during which magnetic energy is converted to particle energy. Due to this nature of magnetic reconnection, there are many free energy sources that can excite plasma waves such as lower hybrid and whistler waves. Whistler waves near the boundary between the magnetosphere and the exhaust region of magnetic reconnection have been observed over many decades. However, the propagation characteristic and the exact excitation mechanism associated with magnetic reconnection have not been well understood. Here the dispersion relation of the whistler wave is clearly measured for the first time by using correlations between four satellites of the Magnetospheric Multiscale mission. The measured dispersion shows that the whistler wave propagates mostly parallel to the background magnetic field toward the central reconnection region, which agrees well with a linear theory. A linear calculation with the measured electron distribution function verifies that the whistler wave is excited by temperature anisotropy in energetic electrons whose energy is much larger than that of bulk electrons. Observations both in space and laboratory suggest that lower hybrid drift instabilities may cause the anisotropy in energetic electrons, which is an interesting wave‐wave‐particle phenomenon. Key Points The whistler wave dispersion relation is measured for the first time by using correlation between multiple spacecraft The whistler wave propagating toward the X line is generated by temperature anisotropy in the electron tail Positive correlations between LHDI and whistler activities suggest that the anisotropy is generated by transport due to LHDI