Electron Thermalization and Electrostatic Turbulence Caused by Flow Reversal in Dipolarizing Flux Tubes

During magnetic reconnection in Earth's magnetotail, a dipolarizing flux tube (DFT) is formed and carries large amounts of energy toward the Earth to produce the aurora. Electrons inside the entire DFT are generally hot and tenuous, because they originate from the low-density lobe region and subsequently are heated by reconnection. Here, we report a special DFT hosting both hot-tenuous and cold-dense electrons, and we observe unique electron thermalization and associated electrostatic turbulence inside such a DFT. Analyses of the ion dynamics indicate that formation of the special phenomenon might be due to the flow reversal on the DFT flank, which is found to be an isobaric process in the direction perpendicular to the magnetic field. Correlation analysis shows that electrostatic waves at frequencies of 2-70 Hz are well correlated with the temperature anisotropy of electrons in the range of 300-27,000 eV, and waves at a frequency above one electron gyrofrequency (f ce) have a strong negative correlation with the electron temperature anisotropy as well. This study can improve our understanding of electron dynamics in the magnetotail.