Non-gravitational accelerations and magnetic disturbances: What can be observed from the residual series of accelerometers on GRACE C?

The purpose of this study is to diagnose how the magnetic disturbances and the flow of the vertical currents can be identified from the non-gravitational acceleration measurements of GRACE C. Whereas the non-gravitational measurements are routinely used for the gravity field modelling, they can be used for the study of the upper atmosphere. Thus, separating the dominant forces of Solar Radiation Pressure (SRP) and the drag acting on the satellite is crucial in the calibration of the accelerometers and in understanding how the non-gravitational forces affect the satellite. In our analysis, we use an alternative weighted 1B dataset (ACW1B) of non-gravitational accelerations, which comprises the standard deviations of each measurement derived from the 1A data (10 Hz) using a weighted Gaussian filter with a cut-off frequency of 35mHz. Subsequently, we extract the atmospheric drag and the SRP, acting on the GRACE C satellite, directly from the accelerometer measurements. The weighted residual series along the three axes of the Science Reference Frame are analyzed based on their latitudinal, longitudinal, and local time variations and are compared with the field-aligned currents (FAC) dataset derived from the magnetic observations of GRACE C, provided by GFZ. We analyze the residual series in Magnetic Local Time (MLT) during the periods of combined Solstices, Equinoxes and on monthly basis in different frequency bands. In the higher frequency bands (f > 0.02 Hz) high correlation between the accelerometer residual series and the FACs is revealed, especially in the radial direction. The measurements in the along-track direction are the most disturbed during the geomagnetic storms, while in the radial direction we can distinctly identify the disturbances caused by the Earth Radiation pressure in the lower frequency bands. In the cross-track direction, the residual series reveal a strong signal in the equatorial region due to thruster activations. High dependency on Magnetic Local Time along the three axes of the accelerometer and consistent monthly differences between the ascending and descending tracks are investigated and presented.