Simulations Of High-latitude Ionosphere-magnetosphere region plasma density structures and the Alfvén waves effects
Jaafari, Fajer Bitar
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O+ density structures in the polar cap ionosphere-magnetosphere region near 6000 km altitude have been observed with numerous spacecraft, with O+ densities ranging from above 10 cm-3 to lower than 0.01 cm-3. Regions with densities in the low range have been referred to as O+ density troughs [Zeng et al., 2004], or ion depletion zones [Horita et al., 1993]. Regions with high O+ densities are likely caused by processes such as soft auroral electron precipitation and transverse wave heating. In this simulation work, we use the UT Arlington Dynamic Fluid-Kinetic (DyFK) ionospheric plasma transport model to model O+ density profiles and other aspects of the ionosphere-magnetosphere plasma distribution under various conditions for two different studies. The first type of situation concerns the O+ troughs in the polar cap, which were observed by the Thermal Ion Dynamics Experiment (TIDE) on the Polar spacecraft. We use solar wind parameters and IMF conditions as inputs to drive a time-varying high-latitude electric potential model [Weimer, 2001]. We also incorporate auroral processes involving the effects of soft electron precipitation and wave-driven transverse ion heating, we simulate the ionosphere-magnetosphere plasma transport and the associated O+ bulk parameter profiles of different flux tubes convecting in the high-latitude region. The simulation results for realistic parameters and specific geophysical conditions agree reasonably well with the O+ density variations observed. The second study involves examination of the effects of inertial Alfvén waves propagating along auroral field lines producing parallel electric fields which accelerate auroral precipitating electrons. We examine the propagation of Alfvén waves within O+ auroral ionosphere-magnetosphere density profiles. For the Alfvén wave description, a linear one-dimensional Gyro-Fluid code [Jones et al., 2003] is used, in which electron inertia, electron pressure gradient and finite ion gyroradius effects are incorporated. This Alfvén wave propagation code determines the characteristics of propagating Alfvén waves which generate the inertial parallel electric field responsible for energizing electrons. A test particle model developed by Su et al  is then used to simulate the response of a distribution of electrons to these Alfvén wave electric fields. The integrated energy flux of the resulting precipitating electrons will be obtained to link back with DyFK model, which shows the effect of such Alfvénic electrons on the plasma evolution in the DyFK code and on Alfvénic phase speed profile.