Empirical specification of field‐aligned plasma density profiles for plasmasphere refilling

Abstract

[1] Sounding measurements from the radio plasma imager (RPI) on the IMAGE satellite are used to derive electron density distributions along magnetic field lines in the nightside plasmasphere and plasmatrough for three midlatitude passes. These passes occurred during (1) a magnetic storm, (2) a prolonged quiet time, and (3) a sudden commencement of a storm, respectively. It is found that the density profiles of filled (in the inner plasmasphere) and depleted (in the plasmatrough or outer plasmasphere) flux tubes have different field line dependence. A multivariant least squares fit with a simple analytical function is used to model the density profiles. The fitting parameters in the function define the field line dependence of a density profile, i.e., the steepness of the density profile at high latitudes and the flatness at low latitudes. In each pass the density profiles along the filled and depleted flux tubes can be well modeled with the selected functional form, with two different sets of fitting parameter values for filled and depleted flux tubes. For the three passes examined, the fitting parameter values are not sensitive to the geomagnetic activity for the inner plasmasphere density profiles but vary slightly for the plasmatrough or outer plasmasphere density profiles from case to case. The equatorial densities extrapolated from the measured density profiles approximately have a power law relation with L values. The results suggest that the selected function has potential of being able to construct realistic global empirical plasmasphere/plasmatrough models. Furthermore, it is now feasible to empirically determine the density profiles along the depleted flux tubes for plasmasphere refilling studies.

Simulation of field-aligned H<sup>+</sup> and He<sup>+</sup> dynamics during late-stage plasmasphere refilling

“…6. Results for H + , including variations with solar conditions, are consistent with measured late-time refilling rates (Horwitz et al, 1990b;Rasmussen et al, 1993;Lawrence et al, 1999;Su et al, 2001;Tu et al, 2006). However, we find seasonal variations in H + refilling that differ from past results.…”

“…Observations (Carpenter, 1966;Horwitz et al, 1990a;Su et al, 2001) show that the differences between these values and more typical numbers -1000 cm −3 at L=3 and 500 cm −3 at L=4 (Corcuff et al, 1972;Horwitz et al, 1990b;Tu et al, 2006;Rasmussen and Schunk, 1990, and references therein) -lie in the dynamic nature of the plasmasphere. That is, these simulations correspond to a prolonged quiet period and, further, do not include a steady-state loss function such as the plasmaspheric wind of Lemaire and Schunk (1992).…”

Refilling process in the plasmasphere: a 3-D statistical characterization based on Cluster density observations

“…In that line of thought, several studies have been achieved to elaborate an empirical relationship between the PBL location and the geomagnetic indices (Carpenter and Anderson, 1992;Moldwin et al, 2002), and between the PBL location and the solar wind conditions (Larsen et al, 2007). However, observations of smooth plasmaspheric density transition extended to L = 7 and beyond have been reported after several days of quiet geomagnetic conditions (Chappell, 1972;Carpenter and Anderson, 1992;Tu et al, 2006). This implies either a PBL located beyond L = 7 or a smooth density transition from the inner plasmasphere up to the sub-auroral region without detectable signature of a distinct boundary.…”

“…Tu et al (2006) extended the technique used by Reinisch et al (2001Reinisch et al ( , 2004 and Huang et al (2004) to model the field-aligned electron density profile remotely from the Radio Plasma Imager (RPI) on board the IMAGE satellite. They concluded that the power law dependence quoted above is reliable enough to construct global empirical plasmasphere/trough models.…”

“…Little attention has been paid to model such a smooth density transition at the PBL. Actually, ignoring the density distribution along depleted flux tubes is a real difficulty when one wishes to undertake and validate simulations of plasmasphere filling/refilling (Tu et al, , 2006(Tu et al, , 2007.…”