Three-dimensional Features of the Outer Heliosphere Due to Coupling between the Interstellar and Heliospheric Magnetic Field. V. The Bow Wave, Heliospheric Boundary Layer, Instabilities, and Magnetic Reconnection

Abstract: The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW-LISM interaction. By performing three-dimensional, MHD plasma / kinetic neutral atom simulations, we determine the width of the outer heliosheath - the LISM plasma region affected by the presence of the heliosphere - and analyze quantitatively the distributions in front of the heliopause. It is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. We also demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by IBEX.

Three-dimensional Features of the Outer Heliosphere Due to Coupling between the Interstellar and Heliospheric Magnetic Field

The paper by Pogorelov et al. investigates the complex interaction between the solar wind (SW) and the local interstellar medium (LISM), focusing on the dynamic features of the outer heliosphere influenced by the interstellar and heliospheric magnetic field coupling. By employing three-dimensional magnetohydrodynamic (MHD) plasma simulations coupled with kinetic neutral atom models, the authors explore a range of phenomena including the heliospheric boundary layer, bow wave formations, instabilities, and magnetic reconnection.

Key Findings

The study reveals that charge exchange processes between ions and neutral atoms significantly alter the plasma environment preceding the heliopause—a critical boundary distinguishing the heliosphere from interstellar space. Notably, the charge exchange reduces the contribution of potential shock transitions in modifying overall plasma parameters even when the unperturbed LISM velocity surpasses the fast magnetosonic speed.

The simulations incorporate adaptive mesh refinement techniques allowing precise localization of the heliospheric boundary layer and identification of areas of reduced plasma density and enhanced magnetic fields. Such layers, according to the study, are consistent with plasma oscillations of increasing frequency as detected by Voyager 1's plasma wave instrument, evidencing the complex nature of interstellar plasma interactions with heliospheric boundaries.

Implications

The findings offer important insights into the behavior of the heliospheric boundary layer and its response to varying environmental conditions provided by LISM properties. The presence and variability of the bow wave region—affected by the SW–LISM interaction—underlines its significance in shaping solar system boundary dynamics and emphasizes the erosion effect on potential bow shocks due to charge exchange.

The simulations also explore instability phenomena and potential magnetic reconnection events near the heliopause. These processes potentially facilitate substantial transformations in heliospheric structures and invoke considerations regarding how observations made by spacecraft like Voyager relate to the broader spatial phenomena occurring across these interfaces.

Numerical Results and Hypotheses

Quantitative assessments in the paper highlight the variation in plasma density and the fast magnetosonic Mach number along crucial axes and planes relative to the heliosphere and LISM. The study proposes that increased ISMF strengths systematically dilute the contribution of shock-induced variations in the plasma, offering alternative interpretations on boundary layer dynamics due to kinetic treatments of neutral atoms.

Speculation on Future Developments

Future investigations, as suggested by the paper, should incorporate more detailed, data-driven models to enhance correlating observational spacecraft data with simulation outputs. Continuous advancement in adaptive simulation techniques will likely provide further granularity in predicting instabilities and magnetic reconnections around the heliopause.

The implications for cosmic ray modulation and solar boundary dynamics are significant as they unfold under the complex SW–LISM interactions. Evaluating more complete models that incorporate kinetic ion behaviors alongside MHD frameworks could enrich our understanding of astrological phenomena and inform ongoing exploration missions within and beyond the heliosphere.