PARAMETERIZED STUDY OF THE IONOSPHERIC MODIFICATION ASSOCIATED WITH SUN-ALIGNED POLAR-CAP ARCS

Sun-aligned (SA) arcs are a prevalent feature of the polar cap ionosphere during northward interplanetary magnetic field conditions. These arcs are, like the auroral arc, complex electrodynamic system coupling the ionosphere and magnetosphere. The electron precipitation and convection electric field associated with this system modify the polar cap ionospheric plasma distribution. In the past decade, a wealth of observational information has indicated the complexity of these ionospheric modifications, but only a limited number of model studies have been carried out to elucidate the changes associated with SA arc. In this investigation, an extensive parametric study has been conducted to identify and order these ionospheric modifications. Specifically, we are interested in modeling the local evolution and decay of ionospheric modification due to a source consisting of an idealized, though representative, SA arc structure. The Utah State University time-dependent ionospheric model (TDIM) has been used to define the ionospheric physics and its dependencies on both solar cycle and season. An SA arc model has been developed. This model enables one to systematically vary the key arc parameters; electric field strength, width, electron precipitation energy flux, and electron precipitation characteristic energy. Our findings indicate that the changes associated with the E and F regions are quite different. F region structures have relatively long lifetimes and are significantly affected by transport. This leads to a significant role for cross-arc convection. The E region is much less affected by transport due to short chemical recombination times and thus, E region modifications are confined to the arc structure itself where precipitation and enhanced electric fields are present. Furthermore, the response of the F region to changes in the arc parameters is not linear, and the largest ionospheric modifications occur for intermediate arc widths and electric field strengths. An important indicator of F region modification is the total electron content (TEC) for flux tubes convecting through SA arc structures. The magnitude and variation of the TEC enhancement with various arc parameters can be an important indicator of real processes which are occurring in the arc structure. In addition to TEC we examine the variation of T(e), T(i), N(e), SIGMAp, and SIGMA(H) for various arc parameters.