Abstract
AbstractUnderstanding changes in the magnetospheric mass density during disturbed geomagnetic conditions provides valuable insight into the dynamics and structure of the environment. The mass density plays a significant role in a variety of magnetospheric processes, such as wave propagation, magnetic reconnection rates, and radiation belt dynamics. In this study, the spatial variations of total plasma mass density are explored through the analysis of Cluster observations. Data from the WHISPER (Waves of High frequency and Sounder for Probing of Electron density by Relaxation) and CODIF (ion Composition and Distribution Function analyzer) instruments, on board the four Cluster spacecraft for a time interval spanning 2001–2012, are used to determine empirical models describing the distribution of the total plasma mass density along closed geomagnetic field lines. The region considered covers field lines within 5.9≤L < 9.5, corresponding to the outer plasmasphere, plasmatrough, and near‐Earth plasma sheet. This study extends previous work to examine and quantify spatial variations in the electron density, average ion mass, and total plasma mass density with Dst index. The results indicate that during periods of enhanced ring current strength, electron density is observed to decrease and average ion mass is observed to increase, compared with quiet geomagnetic conditions. The combination of these variations shows that although heavy ion concentration is enhanced, the decrease in plasma number density results in a general decrease in total plasma mass density during disturbed geomagnetic conditions. The observed decrease in mass density is in contrast to prevailing understanding and, due to the dependence of the Alfvén speed on mass density, has important implications for a range of plasma processes during storm time conditions (e.g., propagation of wave modes).
Highlights
The magnetospheric mass density, and its variations under different conditions, is a fundamental parameter of the magnetosphere
Data from the WHISPER (Waves of High frequency and Sounder for Probing of Electron density by Relaxation) and CODIF instruments, on board the four Cluster spacecraft for a time interval spanning 2001–2012, are used to determine empirical models describing the distribution of the total plasma mass density along closed geomagnetic field lines
As the previous study examined the dependences of the electron density, average ion mass, and total mass density on L value and magnetic local time (MLT) in detail [Sandhu et al, 2016b, 2016a], this discussion will predominantly focus on the dependences of the field line distributions on Dst index
Summary
The magnetospheric mass density, and its variations under different conditions, is a fundamental parameter of the magnetosphere. The mass density is a significant factor for the propagation of wave modes concerned with radiation belt energization and decay [Li et al, 1993; Elkington et al, 1999; Meredith et al, 2003; O’Brien et al, 2003]. An understanding of how the mass density varies with geomagnetic activity is of importance and will provide useful information on the dynamical processes and morphology of the magnetosphere during disturbed conditions. A key motivation of this study is the application of a mass density model to understand the response time of the magnetosphere to both internal and external perturbations, through variations in the frequencies of magnetospheric ultralow-frequency waves
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