Major scientific challenges and opportunities in understanding magnetic reconnection and related explosive phenomena throughout the universe

This is a group white paper of 100 authors (each with explicit permission via email) from 51 institutions on the topic of magnetic reconnection which is relevant to 6 thematic areas. Grand challenges and research opportunities are described in observations, numerical modeling and laboratory experiments in the upcoming decade.https://ui.adsabs.harvard.edu/abs/2019BAAS...51c...5J/abstractAccepted manuscrip

The formation of magnetic depletions and flux annihilation due to reconnection in the heliosheath

The misalignment of the solar rotation axis and the magnetic axis of the Sun produces a periodic reversal of the Parker spiral magnetic field and the sectored solar wind. The compression of the sectors is expected to lead to reconnection in the heliosheath (HS). We present particle-in-cell simulations of the sectored HS that reflect the plasma environment along the Voyager 1 and 2 trajectories, specifically including unequal positive and negative azimuthal magnetic flux as seen in the Voyager data. Reconnection proceeds on individual current sheets until islands on adjacent current layers merge. At late time, bands of the dominant flux survive, separated by bands of deep magnetic field depletion. The ambient plasma pressure supports the strong magnetic pressure variation so that pressure is anticorrelated with magnetic field strength. There is little variation in the magnetic field direction across the boundaries of the magnetic depressions. At irregular intervals within the magnetic depressions are long-lived pairs of magnetic islands where the magnetic field direction reverses so that spacecraft data would reveal sharp magnetic field depressions with only occasional crossings with jumps in magnetic field direction. This is typical of the magnetic field data from the Voyager spacecraft. Voyager 2 data reveal that fluctuations in the density and magnetic field strength are anticorrelated in the sector zone, as expected from reconnection, but not in unipolar regions. The consequence of the annihilation of subdominant flux is a sharp reduction in the number of sectors and a loss in magnetic flux, as documented from the Voyager 1 magnetic field and flow data.This work has been supported by NASA Grand Challenge NNX14AIB0G, NASA awards NNX14AF42G, NNX13AE04G, and NNX13AE04G, and NASA contract 959203 from JPL to MIT. The simulations were performed at the National Energy Research Scientific Computing Center. We acknowledge fruitful discussions with Dr. Len Burlaga on the Voyager observations and with Dr. Obioma Ohia on outer heliosphere reconnection. This research benefited greatly from discussions held at the meetings of the Heliopause International Team Facing the Most Pressing Challenges to Our Understanding of the Heliosheath and its Outer Boundaries at the International Space Science Institute in Bern, Switzerland. (NNX14AIB0G - NASA; NNX14AF42G - NASA; NNX13AE04G - NASA; 959203 - NASA)Accepted manuscrip

A predicted small and round heliosphere

The shape of the solar wind bubble within the interstellar medium, the so-called heliosphere, has been explored over six decades(1-7). As the Sun moves through the surrounding partially-ionized medium, neutral hydrogen atoms penetrate the heliosphere, and through charge-exchange with the supersonic solar wind, create a population of hot pick-up ions (PUIs). The Voyager 2 (V2) data demonstrated that the heliosheath pressure is dominated by PUIs. Here we use a novel magnetohydrodynamic model that treats the PUIs as a separate fluid from the thermal component of the solar wind. Unlike previous models (8-10), the new model reproduces the properties of the PUIs and solar wind ions based on the New Horizon(11) and V2(12) spacecraft observations. The model significantly changes the energy flow in the outer heliosphere, leading to a smaller and rounder shape than previously predicted, in agreement with energetic neutral atom observations by the Cassini spacecraft.Accepted manuscrip

A Predicted Small and Round Heliosphere

The shape of the solar wind bubble within the interstellar medium, the so-called heliosphere, has been explored over six decades. As the Sun moves through the surrounding partially-ionized medium, neutral hydrogen atoms penetrate the heliosphere, and through charge-exchange with the supersonic solar wind, create a population of hot pick-up ions (PUIs). The Termination Shock (TS) crossing by Voyager 2 (V2) data demonstrated that the heliosheath (HS) (the region of shocked solar wind) pressure is dominated by suprathermal particles. Here we use a novel magnetohydrodynamic model that treats the freshly ionized PUIs as a separate fluid from the thermal component of the solar wind. Unlike previous models, the new model reproduces the properties of the PUIs and solar wind ions based on the New Horizon and V2 spacecraft observations. The PUIs charge exchange with the cold neutral H atoms of the ISM in the HS and are quickly depleted. The depletion of PUIs cools the heliosphere downstream of the TS, "deflating" it and leading to a narrower HS and a smaller and rounder shape, in agreement with energetic neutral atom observations by the Cassini spacecraft. The new model, with interstellar magnetic field orientation constrained by the IBEX ribbon, reproduces the magnetic field data outside the HP at Voyager 1(V1). We present the predictions for the magnetic field outside the HP at V2.Comment: submitted to publication - new version after revie

Dynamic Screening in Thermonuclear Reactions

It has recently been argued that there are no dynamic screening corrections to Salpeter's enhancement factor in thermonuclear reactions, in the weak-screening limit. The arguments used were: 1) The Gibbs probability distribution is factorable into two parts, one of which, $exp(-\beta \sum e_{i}e_{j}/r_{ij})$ ($\beta=1/k_{B}T$), is independent of velocity space; and 2) The enhancement factor is $w=1+\beta^{2}e^{2}Z_{1}Z_{2}$ with ${}_{k}={}_{k}/k^{2}$ and ${< E^{2} >}_{k}/(8\pi)=(T/2)[1-\epsilon^{-1} (0,k)]$. We show that both of these arguments are incorrect.Comment: Accepted for publication in The Astrophysical Journa

Global Asymmetry of the Heliosphere

Opher et al. 2006 showed that an interstellar magnetic field parallel to the plane defined by the deflection of interstellar hydrogen atoms can produce a north/south asymmetry in the distortion of the solar wind termination shock. This distortion is consistent with Voyager 1 and Voyager 2 observations of the direction of field-aligned streaming of the termination shock particles upstream the shock. The model also indicates that such a distortion will result in a significant north/south asymmetry in the distance to the shock and the thickness of heliosheath. The two Voyager spacecraft should reveal the nature and degree of the asymmetry in the termination shock and heliosheath.Comment: 6 pages, 5 figures, AIP Proceedings of the 5th IGPP "The Physics of the Inner Heliosheath: Voyager Observations, Theory and Future Prospects