The plasmasphere is a critical region of the magnetosphere. It is important for the evolution of Earth's radiation belts. Waves in the plasmasphere interior (hiss) and vicinity (electromagnetic ion cyclotron, chorus) help control the acceleration and loss of radiation belt particles. Thus, understanding the extent, structure, content, and dynamics of the plasmasphere is crucial to understanding radiation belt losses. The Van Allen Probes mission uses two methods to determine the total plasma density. First, the upper hybrid resonance frequency can provide electron density; this determination is the most accurate and robust. However, it requires significant analysis and is challenging during geomagnetically active times: It becomes difficult to interpret the wave spectrum, and the amount of available data is severely limited. Second, the spacecraft potential is a proxy for the plasma density. These high-resolution measurements are available with high duty cycle. However, environmental effects can limit the accuracy of this method. The relation between spacecraft potential and density is empirical, requiring an independent density measurement and repeated checks. We perform a quantitative comparison of these two in situ techniques during the first 3.5 years of the Van Allen Probes mission. We show how to calibrate potential-based density measurements using only publicly available wave-derived densities to provide high-fidelity results even if upper hybrid measurements are sparse or unavailable. We quantify the level of uncertainty to expect from potential-derived density data. Our approach can be applied to any in situ spacecraft mission where reliable absolute density and spacecraft potential data are available.
Bibliographical noteFunding Information:
This work was supported at Southwest Research Institute by Van Allen Probes Energetic Particle, Composition, and Thermal Plasma Suite (ECT) funding, at the University of Iowa by Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) funding, and at the University of Minnesota by Electric Field and Waves (EFW) instrument funding, all under NASA's prime mission Contract NAS5-01072. All Van Allen Probes observations used in this study, along with display and analysis software, are publicly available at the websites for EFW (http://www.space.umn.edu/missions/rbspefw-home-university-of-minnesota/) and EMFISIS (http://emfisis.physics.uiowa.edu/). The results of the B-spline fit is provided in ASCII format as supporting information. Curve fitting was performed using the curve_fit routine of the SciPy standard Python library (https://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.curve_fit.html). The nonparametric kernel regression was performed using the KernelReg routine from the Python StatsModels library (http://www.statsmodels.org/devel/generated/statsmodels.nonparametric.kernel_regression.KernelReg.html). The B-spline fits were performed using the set of appropriate routines in the scikit-learn machine learning library for Python (https://scikit-learn.org/stable/index.html). The results of the B-spline fits are available as supporting information for this article and available for download from the Open Science Framework (https://osf.io) under the https://doi.org/10.17605/OSF.IO/UTPE7.
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- Van Allen Probes
- cold plasma density
- spacecraft charging
- wave resonances