Twangs: Eddy-Currents in Accelerometer due to Charge Redistributions caused by induced Photo-Currents?

The Gravity Recovery and Climate Experiment (GRACE) has been launched more than 9 years ago to determine Earth' gravity field including its temporal variations. Since the beginning of the mission so called twangs can be found in 10 Hz accelerometer data of both GRACE satellites. A twang is the notation for a so far unsufficient explicable phenomenon in accelerometer data, usually formed by a sudden spike, consisting of two or more peaks with a relatively high amplitude, and an oscillating decay. In our studies we were able to distinguish between several types of twangs, for which we were able to build models with a high sampling. With these models the corresponding type of twang can be found within accelerometer data by cross correlation. Furthermore, using the built models twangs can be on the one hand better characterised as well as helpful for a successful reduction of twangs from accelerometer data. Also, we were able to determine a geographical as well as a temporal distribution of the twangs, and for certain types of twangs now a prediction of occurrence might be possible. We can relate most types of twangs to external forces, whereas we state the hypothesis that they are due to an electrical charge redistribution within the satellite body. In this hypothesis, satellite non-conductive surface planes are charged by the surrounding ionosphere/ thermosphere and this charge is being redistributed in the satellites conductive body as soon as the material gets photo-conductive, leading to a discharge current. This discharge current evokes a magnetic field, which may result in an eddy current within the accelerometer, according to an increasing or decreasing current. Moreover, these investigations could contribute to a better understanding of satellite sensorics as used for gravity field determination from space, especially with regard to upcoming missions. Also, the results may improve our knowledge about ionosphere/ thermosphere in GRACE altitudes.