Whistler mode waves near the magnetic field intensity minimum of mirror structures in magnetosheath

Wave-particle interactions are thought to play a crucial role in energy transfer in collisionless space plasmas in which the motion of charged particles is controlled by electromagnetic fields. In the terrestrial magnetosheath, intense whistler mode waves, called 'Lion roars', are often detected by spacecraft around minima of semi-periodic fluctuations of magnetic field intensity due to motion of mirror mode structures. It is expected that whistler mode waves are efficiently generated near a local minimum of magnetic field intensity due to the smallest cyclotron resonance velocity. We show the detailed characteristics of such whistler mode waves using the data obtained by the four MMS (Magnetospheric Multiscale) spacecraft. Reversals of field-aligned component of Poynting flux of whistler mode waves in mirror mode structures corresponded to reversals of gradient of magnetic field intensity along magnetic field lines. It is consistent with the idea that the whistler mode waves are efficiently generated near local minima of magnetic field intensity and propagate toward regions of larger magnetic field intensity along the magnetic field lines on both sides. Because reversals of the field-aligned component of Poynting flux are measurable even by a single spacecraft, they are useful to find good candidates of effective wave generation regions along field lines. Because the anisotropy of electrons depends on energy in such Poynting flux reversal regions, which are near the centers of mirror mode structures, simple approximation of bi-Maxwellian distributions is not usable to estimate linear growth rates at such locations. Although parallel electron temperatures from moment calculations are usually higher than perpendicular temperatures in such regions, pancake or an outer edge of butterfly type anisotropy of electron distributions above ~100 eV can contribute to effective wave generation at the local minima of magnetic field intensity along field lines.