Evolution of Field‐Aligned Electron and Ion Densities From Whistler Mode Radio Soundings During Quiet to Moderately Active Period and Comparisons With SAMI2 Simulations

Abstract

AbstractKnowledge of field‐aligned electron and ion distributions is necessary for understanding the physical processes causing variations in field‐aligned electron and ion densities. Using whistler mode sounding by Radio Plasma Imager/Imager for Magnetopause‐to‐Aurora Global Exploration (RPI/IMAGE), we determined the evolution of dayside electron and ion densities along L ∼ 2 and L ∼ 3 (90–4,000 km) during a 7 day (21–27 November 2005) geomagnetically quiet to moderately active period. Over this period the O+/H+ transition height was ∼880 ± 60 km and ∼1000 ± 100 km, respectively, at L ∼ 2 and L ∼ 3. The electron density varied in a complex manner; it was different at L ∼ 2 and L ∼ 3 and below and above the O+/H+ transition height. The measured electron and ion densities are consistent with those from Challenging Minisatellite Payload (CHAMP) and Defense Meteorological Satellite Program (DMSP) and other past measurements, but they deviated from bottomside sounding and International Reference Ionosphere (IRI) 2012 empirical model results. Using SAMI2 (Naval Research Laboratory (NRL) ionosphere model) with reasonably adjusted values of inputs (neutral densities, winds, electric fields, and photoelectron heating), we simulated the evolution of O+/H+ transition height and field‐aligned electron and ion densities so that a fair agreement was obtained between the simulation results and observations. Simulation studies indicated that reduced neutral densities (H and/or O) with time limited O+‐H charge exchange process. This reduction in neutral densities combined with changes in neutral winds and plasma temperature led to the observed variations in the electron and ion densities. The observation/simulation method presented here can be extended to investigate the role of neutral densities and composition, disturbed winds, and prompt penetration electric fields in the storm time ionosphere/plasmasphere dynamics.