Relationship between electron field‐aligned anisotropy and dawn‐dusk magnetic field: Nine years of Cluster observations in the Earth magnetotail

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.