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Meeting ID: 954 6459 6287
Passcode: plasma

Electrons in Earth's magnetotail are significantly energized, both in the form of heating and in the form of acceleration to non-thermal energies. While magnetic reconnection is considered to play an important role, it still remains unclear how electrons are energized and partitioned between thermal and non-thermal components.  Here we show, based on in-situ observations by NASA's Magnetospheric Multiscale (MMS) mission combined with multi-component spectral fitting methods, that the average electron energy (or equivalently temperature) is substantially higher when the locally-averaged electric field magnitude |E| is also higher.  While the result is consistent with the  classification of 'plasma-sheet' and 'tail-lobe' reconnection during which reconnection is considered to occur on closed and open magnetic field lines, respectively, it further suggests that a stochastic Fermi acceleration in 3D, reconnection-driven turbulence is essential for the production and confinement of non-thermal electrons in the reconnection region. The classical picture of a coherent and uniform potential drop across the reconnection flow channel (typically a few Earth's radii) is unrealistic to explain the observed maximum energy of electrons (>430 keV). A puzzle is that the non-thermal power-law component can be quite small even when the electric field is large and the bulk population is significantly heated. The non-thermal fraction of electron energies varies from sample to sample between ~20% and ~60%, regardless of the electric field magnitude.  Interestingly, however, these values of non-thermal fractions are similar to those obtained for the above-the-looptop hard X-ray coronal sources for solar flares.