Anchor Point of Electron Acceleration around Dipolarization Fronts in Space Plasmas

An anchor point, which is an energy threshold above which electrons are accelerated and below which electrons are decelerated, has recently been reported within the dipolarizing flux bundles behind dipolarization fronts (DFs) both in observations and simulations. However, what determines this point and how it is formed remain unclear. In this study, we investigate for the first time the formation of this point and the relation between this point and the plasma properties by considering a large amount of DF events measured by Cluster. We find a good correlation between this anchor point and the plasma-sheet density and temperature. We notice that such a point appears primarily in the DF events associated with strong whistlers, suggesting that it is formed due to wave-particle interactions near DFs. Quantitatively, we establish a model for the anchor point, ${E}_{\mathrm{AP}}={10}^{2.2\pm 0.3}\times {(N/T)}^{-0.6\pm 0.1}$ eV, where N and T are the normalized plasma-sheet density and temperature, respectively. With this model, we can predict the electron acceleration features behind DFs, by monitoring plasma properties in the plasma sheet. Such a model can be crucial for understanding electron acceleration regions elsewhere in space, such as reconnection diffusion region and collisionless shocks.