J. K. Sandhu

The dependence of magnetospheric plasma mass loading on geomagnetic activity using Cluster

Understanding 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 analyser) 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 show 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 Alfven speed on mass density, has important implications for a range of plasma processes during storm time conditions (e.g. propagation of wave modes).

A statistical study of magnetospheric electron density using the Cluster spacecraft

Observations 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.

Variations of high-latitude geomagnetic pulsation frequencies: A comparison of time-of-flight estimates and IMAGE magnetometer observations: GEOMAGNETIC PULSATION FREQUENCIES

The fundamental eigenfrequencies of standing Alfven waves on closed geomagnetic field lines are estimated for the region spanning 5.9≤L < 9.5 over all MLT (Magnetic Local Time). The T96 magnetic field model and a realistic empirical plasma mass density model are employed using the time-of-flight approximation, refining previous calculations that assumed a relatively simplistic mass density model. An assessment of the implications of using different mass density models in the time-of-flight calculations is presented. The calculated frequencies exhibit dependences on field line footprint magnetic latitude and MLT, which are attributed to both magnetic field configuration and spatial variations in mass density. In order to assess the validity of the time-of-flight calculated frequencies, the estimates are compared to observations of FLR (Field Line Resonance) frequencies. Using IMAGE (International Monitor for Auroral Geomagnetic Effects) ground magnetometer observations obtained between 2001 and 2012, an automated FLR identification method is developed, based on the cross-phase technique. The average FLR frequency is determined, including variations with footprint latitude and MLT, and compared to the time-of-flight analysis. The results show agreement in the latitudinal and local time dependences. Furthermore, with the use of the realistic mass density model in the time-of-flight calculations, closer agreement with the observed FLR frequencies is obtained. The study is limited by the latitudinal coverage of the IMAGE magnetometer array, and future work will aim to extend the ground magnetometer data used to include additional magnetometer arrays.

Cross‐Phase Determination of Ultralow Frequency Wave Harmonic Frequencies and Their Associated Plasma Mass Density Distributions

Latitudinally-spaced ground-based magnetometers can be used to estimate the eigenfrequencies of magnetic field lines using the cross-phase technique. These eigenfrequencies can be used with a magnetic field model and an assumed plasma mass density distribution to determine the plasma mass density in the magnetosphere. Automating this process can be difficult and so far, it has not been possible to distinguish between the different harmonics. Misidentification of the harmonic mode will lead to incorrect estimations of the plasma mass density. We have developed an algorithm capable of identifying multiple harmonics in cross-phase spectrograms, using IMAGE magnetometers. Knowledge of multiple harmonics allows the distribution of plasma mass density to be estimated instead of assumed. A statistical study was performed that showed clear common bands of eigenfrequencies, interpreted as different harmonics. These eigenfrequencies were lowest in the early afternoon and at higher latitudes. There was also a greater occurrence of measurements in the dayside. We then modeled the plasma mass density distribution with a power law characterised by the exponent p, and compared the model eigenfrequencies to the data. This suggested that the even modes did not form during the interval of this study. Examination of the harmonic spacing and the high occurrence of the third harmonic supported this suggestion. We attribute the absence of the even modes to the driving mechanisms. Finally, we show that an equatorial bulge in plasma mass density was not present in our study. c ©2018 American Geophysical Union. All Rights Reserved. Keypoints: • Created an automated cross-phase search algorithm capable of detecting multiple harmonics simultaneously • Performed a statistical survey of harmonic modes and deduced only the odd modes were present • Modeled a bulge in equatorial plasma and showed that a power law was sufficient to describe the plasma mass density distribution c ©2018 American Geophysical Union. All Rights Reserved.