IMPROVED BOUNDARY NORMAL CALCULATIONS USING MINIMUM VARIANCE ANALYSIS AND WAVELET DE-NOISING TECHNIQUES

摘要

We have tested a method to improve the determination of boundary normals of shocks and discontinuities in space plasma. Usually, the presence of wave activity and oscillations in boundary regions prevents an exact determination of a normal. The method described here is based on wavelet de-noising of the time series measurements in order to reduce such effects. The boundary normal is thereafter determined from a minimum variance analysis (MVA) of the de-noised time series. A frequently used quality measurement of this procedure is the eigenvalue ratio of the variance matrix. We have applied the method to a number of observations of magnetopause crossings made by the two AMPTE satellites, and the result shows that an improvement of the eigenvalue ratio, and hence a better defined boundary normal can be achieved for many of the cases. The method also gives a better agreement between the boundary normal directions determined at the two satellites for about half of the cases.