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In the ionospheric E-region, the electrons are magnetized because their frequency of rotation around the magnetic field is much greater than their frequency of collisions with the neutrals. Thus, the electrons drift mostly perpendicular to the electric fields. In contrast, the ions are unmagnetized because their frequency of rotation around the magnetic field is smaller than the frequency of collisions between the ions and neutrals, and thus neutral winds can drag the ions to move in the direction of winds. This differential motion between the electrons and the ions becomes much larger during geomagnetic storms and can excite the Farley-Buneman (two-stream) instability. This instability leads to turbulent electric fields and plasma density perturbations. The interaction of the electrons with the turbulent electric fields caused by the Farley-Buneman (F-B) instability produces anomalous electron heating (AEH) in the auroral region. Incoherent Scatter Radar observations have shown dramatic enhancements of the electron temperature in the auroral electroject region as a result of AEH during strong geomagnetic storms. This electron temperature enhancement typically takes place at around 105-125 km in height and raises the electron temperature by up to a factor of 8, from 300-500 K to ~4000 K.
To fully assess the impact of AEH on the E-region ionosphere and on the coupling between the magnetosphere and ionosphere, HAO postdoctoral scientist Jing Liu, in collaboration with the scientists from HAO, Boston University, Dartmouth College, and the Applied Physics Laboratory in the Johns Hopkins University, has implemented the AEH rate in the Thermosphere- Ionosphere- Electrodynamics General Circulation Model (TIEGCM). The AEH rate formula is obtained from a kinetic model of the electron energization within the high-latitude electrojet due to the Farley-Buneman instability that is developed by the scientists at the Boston University. Two TIEGCM simulations were carried out for an idealized strong geomagnetic storm event: the first one was the default TIEGCM run without AEH and the second one with AEH caused by the F-B instability (Figure 1). The TIEGCM simulation with AEH reveals large electron temperature enhancement effects (red lines) at high latitudes. This enhancement occurs at locations where large high-latitude convection electric fields are present. The magnitude of the enhancement can be greater than 2000 K, which is consistent with radar observations.
The TIEGCM also shows that there are noticeable increases in E-region electron densities and conductivity when AEH is included in the model simulation. This change of ionospheric density and conductivity, in turn, affects the coupling between the magnetosphere and ionosphere by modifying the field-aligned currents and high-latitude convection pattern. This can further alter Joule heating and ion drag, and the global thermospheric and ionospheric response to major geomagnetic storms.
This work is supported by the following grants: NASA grants NNX14AI13G and NNX14AE06G.