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Relationship between electron field‐aligned anisotropy and dawn‐dusk magnetic field: Nine years of Cluster observations in the Earth magnetotail
Author(s) -
Yushkov E.,
Petrukovich A.,
Artemyev A.,
Nakamura R.
Publication year - 2017
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2016ja023739
Subject(s) - anisotropy , physics , population , electron , asymmetry , astrophysics , magnetic field , field (mathematics) , condensed matter physics , atomic physics , optics , quantum mechanics , sociology , demography , mathematics , pure mathematics
We investigate the distribution and possible origins of thermal anisotropic electrons in the Earth's magnetotail, using 9 years of Cluster observations. We mainly focus on relation between electron anisotropy and B z and B y magnetic field components (in GSM coordinates). The anisotropy of electron population is characterized by temperature ratio T ∥ / T ⊥ and by the maximum of phase space density ratio F ∥ / F ⊥ (∥ and ⊥ are relative to the background magnetic field). The population identified by large F ∥ / F ⊥ is organized as short‐time (dozens of seconds) bursts with enhanced F ∥ and can be observed even in the plasma sheet with small T ∥ / T ⊥ . The thermal anisotropy T ∥ / T ⊥ is larger for time intervals characterized by stronger B z and B y : the strong B y corresponds to the T ∥ / T ⊥ peak around the magnetotail neutral plane B x =0, whereas the strong B z corresponds to larger T ∥ / T ⊥ with a flat profile across the magnetotail. There is a dawn‐dusk asymmetry: large T ∥ / T ⊥ corresponds mostly to strong B z at the dusk flank and to strong B y at the dawn flank. Using these differences of the electron anisotropy dependence on B y and B z , we discuss two possible mechanisms responsible for the anisotropy formation.