Seed Magnetic Fields Generated by Primordial Supernova Explosions

The origin of the magnetic field in galaxies is an open question in astrophysics. Several mechanisms have been proposed related, in general, with the generation of small seed fields amplified by a dynamo mechanism. In general, these mechanisms have difficulty in satisfying both the requirements of a sufficiently high strength for the magnetic field and the necessary large coherent scales. We show that the formation of dense and turbulent shells of matter, in the multiple explosion scenario of Miranda and Opher (1996, 1997) for the formation of the large-scale structures of the Universe, can naturally act as a seed for the generation of a magnetic field. During the collapse and explosion of Population III objects, a temperature gradient not parallel to a density gradient can naturally be established, producing a seed magnetic field through the Biermann battery mechanism. We show that seed magnetic fields $\sim 10^{-12}-10^{-14}G$ can be produced in this multiple explosion scenario on scales of the order of clusters of galaxies (with coherence length $L\sim 1.8Mpc$) and up to $\sim 4.5\times 10^{-10}G$ on scales of galaxies ($L\sim 100 kpc$).Comment: Accepted for publication in MNRAS, 5 pages (MN plain TeX macros v1.6 file). Also available at http://www.iagusp.usp.br/~oswaldo (click "OPTIONS" and then "ARTICLES"

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

The Vector Direction of the Interstellar Magnetic Field Outside the Heliosphere

We propose that magnetic reconnection at the heliopause only occurs where the interstellar magnetic field points nearly anti-parallel to the heliospheric field. By using large-scale magnetohydrodynamic (MHD) simulations of the heliosphere to provide the initial conditions for kinetic simulations of heliopause (HP) reconnection we show that the energetic pickup ions downstream from the solar wind termination shock induce large diamagnetic drifts in the reconnecting plasma and stabilize non-anti-parallel reconnection. With this constraint the MHD simulations can show where HP reconnection most likely occurs. We also suggest that reconnection triggers the 2-3 kHz radio bursts that emanate from near the HP. Requiring the burst locations to coincide with the loci of anti-parallel reconnection allows us to determine, for the first time, the vector direction of the local interstellar magnetic field. We find it to be oriented towards the southern solar magnetic pole.Comment: Submitted to ApJ; incorporates minor referee-suggested revision

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

The Orientation of the Local Interstellar Magnetic Field

The orientation of the local interstellar magnetic field introduces asymmetries in the heliosphere that affect the location of heliospheric radio emissions and the streaming direction of ions from the termination shock of the solar wind. We combine observations of radio emissions and energetic particle streaming with extensive 3D MHD computer simulations of magnetic field draping over the heliopause to show that the plane of the local interstellar field is ~ 60-90 degrees from the galactic plane. This suggests that the field orientation in the Local Interstellar Cloud differs from that of a larger scale interstellar magnetic field thought to parallel the galactic plane