The shape of magnetic hole in affecting electron distribution function and wave properties

Abstract

Magnetic holes (MHs) have been widely observed in astrophysical and space plasmas. However, due to the lack of reconstruction method, the effects of MH shape on electron distribution function and wave properties are still unclear. In this study, we report a series of MHs in Earth’s magnetotail. We particularly focus on two of them with the clearest data features, reconstruct their topologies using the Second-Order Taylor Expansion (SOTE) method, and find their shapes to be bulging and deflated. Comparatively, the bulging MH exhibits a donut electron distribution, which may be attributed to the combined effects of internal expansion-induced betatron cooling and boundary contraction-induced betatron acceleration, while the deflated MH presents a pancake electron distribution. The beam instability and temperature anisotropy inside the bulging MH also excite electron cyclotron waves and whistler waves, respectively, while the deflated MH does not exhibit these types of waves. All these findings help us understand the geometric properties and evolutions of MHs.