[IGPP Everyone] TODAY: Space Physics Seminar - Friday January 15th - 3:30pm - Beam-driven electron cyclotron harmonic waves in Earth’s magnetotail (Xu Zhang, UCLA EPSS)

[Emmanuel V. Masongsong]

SPACE PHYSICS SEMINAR 




DEPARTMENT OF EARTH, PLANETARY, AND SPACE SCIENCES 

DEPARTMENT OF ATMOSPHERIC AND OCEANIC SCIENCES 

UNIVERSITY OF CALIFORNIA, LOS ANGELES 




ZOOM Link: https://ucla.zoom.us/j/94454128469?pwd=Qm9OSmZZekN5S0RoVTFITEs0N296QT09 

Meeting ID: 944 5412 8469 Passcode: 263337 



Beam-driven electron cyclotron harmonic waves in Earth’s magnetotail 

Xu Zhang, UCLA EPSS 

Although electron cyclotron harmonic (ECH) waves are the primary contributor to plasma sheet electron scattering loss, experimental verification of their most widely accepted excitation mechanism, loss-cone instability, has been lacking for decades. Using ten years of THEMIS satellite observations, we investigate ECH wave properties near dipolarization fronts, the predominant source of such waves. To our surprise we find that more than 30% of observed ECH waves have moderately oblique (~70⁰) wave normal angles (WNA), much less than the ~85º expected from classical loss-cone instability. These moderately oblique WNA ECH waves carry a strong field-aligned electric field that is used to identify them. They are often observed with cold, dense electrons that exhibit enhanced parallel flux at a few hundred eV energy, which suggests that low-energy counterstreaming beams (likely of ionospheric origin) might be their free energy source. By solving the linear dispersion relation for parameters representative of such plasma sheet electron distributions, we confirm that ECH waves at WNA ~ 70º can indeed be driven unstable by such beams. The positive growth rate of beam-driven ECH waves is mainly contributed from cyclotron resonance with electron beams at first-order and at higher-orders. Beam-driven ECH waves are unstable under various plasma conditions. The growth rate of beam-driven ECH wave increases with beam density, beam velocity, thermal electron temperature and decreases with beam temperature, beam temperature anisotropy, thermal electron density, cold electron density, cold electron temperature. Our work reveals a previously unknown excitation mechanism for ECH waves and significantly improve our understanding how ECH waves are generated and how they affect plasma sheet electron dynamics. 



Friday, January 15, 2021 

3:30 - 5:00 PM 




In-Charge: Vassilis Angelopoulos 

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