Magnetic holes in the vicinity of dipolarization fronts: Mirror or tearing structures?

Magnetic holes filled with isotropic energetic electrons (up to a few 10(5) eV) have been observed by THEMIS in the vicinity of dipolarization fronts. These structures can partially contribute to the initial seed population of energetic electrons within the magnetosphere; therefore finding their nature is important for understanding of the population of high energy electrons within the magnetosphere. Previously, these structures have been interpreted as the result of the mirror instability due to the similarity in their appearance with mirror dips observed in the terrestrial magnetosheath and solar wind. The THEMIS data shown here prove that the measured properties of these structures contradict to the interpretation as mirror waves. In the present study it is shown that these waves do not exhibit the effects on the ion population that are expected due to mirror wave structures. However, they do have a pronounced effect on the high energy electron population. The evolution of the high energy electron population within these structures is investigated. It is then argued that the tearing instability can be responsible for their generation.