On the Origin of Magnetic Holes in the Inner Heliosheath

Trains of magnetic holes (or successive depressions in the magnetic field intensity) have been observed in the inner heliosheath by both Voyager 1 and 2 [Burlaga et al., 2007; Burlaga et al., 2016]. The origin and nature of these structures are still a matter of debate. It was hypothesized that magnetic holes observed in the solar wind are current sheets (planar pressure balance structures) [Burlaga and Lemaire, 1978]. Tsurutani et al. [1992] suggested that magnetic holes observed in the solar wind are produced by the mirror instability when the temperature is anisotropic and the plasma is high. Baumgartel [1999] interpreted magnetic holes as solitons and Avinash and Zank [2007] used a soliton model with pickup ions to explain magnetic holes in the heliosheath. Here we use a 1-D three-fluid model with solar wind ions, pickup ions and electrons to simulate fast magnetosonic turbulence and ion-ion instabilities in the inner heliosheath self-consistently. A virtual satellite is flown in the simulation at the speed of the Voyager 2 spacecraft to sample the simulated plasma turbulence 2 AU downstream of the termination shock at the cadence of the available Voyager MAG data (48 s). This allows direct comparison with the Voyager observations in time domain. A train of magnetic holes developed in time in the three-fluid simulation self-consistently, which were very similar in width and shape to the magnetic holes observed by Voyager 1 in a unipolar magnetic region in the inner heliosheath in 2006. We identified the simulated magnetic holes as packets of high-frequency fast mode waves propagating upstream in the plasma frame at a very low group velocity (~16 km/s). We also calculated the kinetic plasma dispersion relation for perpendicular waves in the inner heliosheath, and demonstrated that the high-frequency fast mode of the multi-fluid description corresponds to the fundamental ion Bernstein mode of the kinetic description at large wavelength. Ion Bernstein waves are electrostatic ion cyclotron waves propagating quasi-perpendicular to the magnetic field and because of this they are not damped by wave particle interaction (Landau damping). Based on these results, we suggest a new generation mechanism of magnetic holes in the inner heliosheath. Magnetic holes are likely produced by ion Bernstein wave packets propagating upstream in the non-equilibrium solar wind plasma.