A Turbulent Heliosheath Driven by the Rayleigh–Taylor Instability

Abstract

The heliosphere is the bubble formed by the solar wind as it interacts with the interstellar medium (ISM). The collimation of the heliosheath (HS) flows by the solar magnetic field in the heliotail into distinct north and south columns (jets) is seen in recent global simulations of the heliosphere. However, there is disagreement between the models about how far downtail the two-lobe feature persists and whether the ambient ISM penetrates into the region between the two lobes. Magnetohydrodynamic simulations show that these heliospheric jets become unstable as they move down the heliotail and drive large-scale turbulence. However, the mechanism that produces this turbulence had not been identified. Here we show that the driver of the turbulence is the Rayleigh–Taylor (RT) instability produced by the interaction of neutral H atoms streaming from the ISM with the ionized matter in the HS. The drag between the neutral and ionized matter acts as an effective gravity, which causes an RT instability to develop along the axis of the HS magnetic field. A density gradient exists perpendicular to this axis due to the confinement of the solar wind by the solar magnetic field. The characteristic timescale of the instability depends on the neutral H density in the ISM and for typical values the growth rate is ∼3 years. The instability destroys the coherence of the heliospheric jets and magnetic reconnection ensues, allowing ISM material to penetrate the heliospheric tail. Signatures of this instability should be observable in Energetic Neutral Atom maps from future missions such as the Interstellar Mapping and Acceleration Probe (IMAP). The turbulence driven by the instability is macroscopic and potentially has important implications for particle acceleration.

On the Energization of Pickup Ions Downstream of the Heliospheric Termination Shock by Comparing 0.52–55 keV Observed Energetic Neutral Atom Spectra to Ones Inferred from Proton Hybrid Simulations

“…Zank et al (2021) studied the interaction and transmission of quasi-2D turbulence through a collisionless perpendicular shock wave in the large beta regime and found that the downstream spectral amplitude is increased significantly. Simulations also demonstrate that a Rayleigh-Taylor-like instability (Opher et al 2021), which mixes the HS and LISM plasmas, can form turbulent heliospheric jets with scales of order 100 au and a turnover timescale of years (Opher et al 2015). How far downstream the turbulence ensues, whether the above processes could further accelerate the PUIs in sufficient numbers, depending on the source of energy, and how these accelerated PUIs get transported further into the HS should be investigated in future studies.…”

The Structure of the Large-Scale Heliosphere as Seen by Current Models

“…Another important conclusion recently arrived at (Opher et al. 2021 ) is that the kink instability is not the driving mechanism to the instability of the heliospheric jets (as argued in Sect. 8.2 and speculated in Pogorelov et al.…”

“…Opher et al. ( 2021 ) showed agreement of the split-tail model with both V1 and V2 data across the HP and argue that this has to do with reconnection occurring in the flanks, and as such a comet-tail model is not required for observational agreement. Lastly, one cannot say that the split-tail model disagrees with TeV observations given that the split-tail model has not yet been compared with TeV observations.…”

“…This instability destroys the coherence of the heliospheric jets and generates turbulence which leads to magnetic reconnection, allowing LISM material to enter the heliotail (Opher et al. 2021 ). There is still the issue to understand the exact path from the instability becoming highly non-linear to allowing reconnection to proceed and the ISM to penetrate between the two lobes.…”

“…8.3 are interesting, Opher et al. ( 2021 ) indicate that the newly found Rayleigh-Taylor instability along the axis of the solar magnetic jets is responsible for the ISM flowing between the jets – so effort should be done to understand why models such as Moscow do not present such instabilities, as well as understanding the evolution and impact of such instability on the global structure of the heliosphere and on acceleration of particles.…”