THE IMPACT OF DOUBLE LAYERS ON ELECTRON TRANSPORT IN THE SOLAR CORONA
Li, Tak Chu
Drake, James F
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The problem of flare-heated electron transport from the corona to the chromosphere is crucial to understanding energy release in solar flares. Observations of coronal X-ray emissions suggested two opposite scenarios: free propagation and confinement of energetic electrons. Confinement is implied in the unexplained prolonged lifetime of the electrons compared to their transit time across the source region. Theoretical modeling of electron transport in solar flares has invoked anomalous resistivity resulting in anomalous conduction. However, there has been no clear evidence for anomalous conduction in numerical simulations. We explore the mechanisms of energetic electron transport in the solar corona by particle-in-cell (PIC) simulations. We demonstrate that hot electron transport is significantly inhibited by the formation of nonlinear, highly localized electrostatic potential drops, in the form of double layers (DLs). The electrons are either reflected by the potential and therefore confined in the source or decelerated and can therefore escape. An increasing number of DLs are generated in larger-scale simulations, pointing to a more likely picture for future space missions probing the corona. Hot electrons stream along magnetic fields through regions of cold plasma. A stream of cold return current electrons develops to maintain a system with zero net current. The DLs are generated through an ion-electron streaming instability due to the drift of the return current electrons interacting with the ions. The effectiveness of transport suppression by a DL is linked to the strength of the DL as defined by its potential drop. We demonstrate that the strength is limited by the formation of parallel shocks. Using PIC simulations and analytic modeling, we show that the maximum strength scales linearly with the hot electron temperature. At the maximum strength, a DL is capable of confining a substantial fraction of electrons in the source. This study has important implications for electron transport during solar flares. It shows transport suppression begins when the energetic electrons start to propagate away from a coronal acceleration site. It also reveals confinement of electrons with kinetic energies less than the total potential of the DLs for the DLs' lifetime, which is much longer than the electron transit time through the source region. Our results are consistent with observations of both free propagation and confinement of X-ray producing electrons in the corona, corresponding to the escaping and reflected electrons, respectively.