Statistical investigation of mirror type magnetic field depressions observed by ISEE-1

The occurrence of mirror type magnetic field depressions was investigated in the 10-year long data set measured by the magnetometer aboard the ISEE-1 satellite. The individual mirror type magnetic depressions were located by a self-developed numerical algorithm including minimum variance calculation. The spatial distribution of more than 33 thousand selected magnetic field depressions was statistically examined in a special parabolic coordinate system. The results of the analysis showed that mirror type depressions most frequently occurred in the magnetosheath around the average location of the magnetopause while the frequency of the events did not grow around the bow shock. Depressions observed at the nose of the magnetopause were larger on average than those observed towards the flanks. No significant dawn–dusk asymmetry was revealed in the number of the observed magnetic field depressions. According to these results pressure anisotropy raised by quasi-perpendicular bow shock compression provides a less likely source for the mirror mode instability compared to pressure anisotropy caused by magnetic field draping at the magnetopause.