Abstract
AbstractObservations from the WHISPER (Waves of High frequency and Sounder for Probing of Electron density by Relaxation) instrument on board Cluster, for the interval spanning 2001–2012, are utilized to determine an empirical model describing the total electron density along closed geomagnetic field lines. The model, representing field lines in the region of 4.5≤L < 9.5, includes dependences on L and magnetic local time. Data verification tests ensured that the WHISPER data set provided unbiased measurements for low‐density regions, including comparisons with Plasma Electron and Current Experiment and Electric Field and Waves observations. The model was determined by modeling variations in the electron density along the field lines, which is observed to follow a power law distribution along the geomagnetic field at high latitudes, with power law index values ranging from approximately 0.0 to 1.2. However, a localized peak in electron density close to the magnetic equator is observed, which is described using a Gaussian peak function, with the electron density peak ranging as high as 10 cm−3 above the background power law dependence. The resulting model illustrates some key features of the electron density spatial distribution. The role of the number density distribution, represented by the empirical electron density model, in determining the total plasma mass density is also explored. By combining the empirical electron density model with an empirical average ion mass model, the total plasma mass density distribution is inferred, which includes contributions of both the number density and ion composition of the plasma in the region.
Highlights
Plasma mass density is a fundamental parameter of the magnetosphere, and variations in the plasma mass density provide information on magnetospheric morphology and dynamics
A previous study developed an empirical model for the average ion mass, for closed field lines in the range of 5.9 ≤ L< 9.5, over all magnetic local time (MLT), describing the ion composition contribution to the total plasma mass density
This study provides a useful description of the plasma number density, the key constraint of the empirical model is that MLT dependences are neglected
Summary
Plasma mass density is a fundamental parameter of the magnetosphere, and variations in the plasma mass density provide information on magnetospheric morphology and dynamics. A key motivation for developing magnetospheric mass density models is to estimate the frequencies of ultralow frequency waves, corresponding to the response time of the magnetosphere to perturbations. A previous study developed an empirical model for the average ion mass, for closed field lines in the range of 5.9 ≤ L< 9.5, over all magnetic local time (MLT), describing the ion composition contribution to the total plasma mass density. This study aims to determine a corresponding empirical model of the spatial distribution of total plasma electron density for the closed magnetosphere, in a region covering the outer plasmasphere, plasmatrough, and near-Earth plasma sheet. The resulting model will describe variations in the electron density in the equatorial plane and along magnetic field lines
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