61 research outputs found
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
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, (), is independent of velocity space; and
2) The enhancement factor is with
and . We show that both of these
arguments are incorrect.Comment: Accepted for publication in The Astrophysical Journa
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
Globally Distributed Energetic Neutral Atom Maps for the "Croissant" Heliosphere
A recent study by Opher et al. (2015) suggested the heliosphere has a
"croissant" shape, where the heliosheath plasma is confined by the toroidal
solar magnetic field. The "croissant" heliosphere is in contrast to the
classically accepted view of a comet-like tail. We investigate the effect of
the "croissant" heliosphere model on energetic neutral atom (ENA) maps.
Regardless of the existence of a split tail, the confinement of the heliosheath
plasma should appear in ENA maps. ENA maps from the Interstellar Boundary
Explorer (IBEX) have shown two high latitude lobes with excess ENA flux at
higher energies in the tail of the heliosphere. These lobes could be a
signature of the confinement of the heliosheath plasma, while some have argued
they are caused by the fast/slow solar wind profile. Here we present ENA maps
of the "croissant" heliosphere, focusing on understanding the effect of the
heliosheath plasma collimation by the solar magnetic field while using a
uniform solar wind. We incorporate pick-up ions (PUIs) into our model based on
Malama et al. (2006) and Zank et al. (2010). We use the neutral solution from
our MHD model to determine the angular variation of the PUIs, and include the
extinction of PUIs in the heliosheath. In the presence of a uniform solar wind,
we find that the collimation in the "croissant" heliosphere does manifest
itself into two high latitude lobes of increased ENA flux in the downwind
direction.Comment: 14 pages, 1 table, 7 figures, Accepted for publication in Ap
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