On the Occurrence of Mirror Type Magnetic Depressions as Measured by ISEE 1/2

The occurrence of mirror type magnetic depressions was investigated based on the 10 year long magnetic field measurements aboard the ISEE 1/2 spacecraft. A numerical algorithm (including minimum variance calculation) was developed to identify the individual mirror type dips which were characterized by the magnetic flux missing in the depressions. It was found that mirror type intability most frequently occurs close to the magnetopause in the subsolar region of the magnetosheath. No significant dawn-dusk asymmetry was revealed. These results do not support the idea that the source of the instability is at the bow shock.