After the termination shock (TS) crossing, the Voyager 2 spacecraft has been observing strong variations of the magnetic field and solar wind parameters in the heliosheath. Anomalous cosmic rays, electrons, and galactic cosmic rays present strong int
ensity fluctuations. Several works suggested that the fluctuations might be attributed to spatial variations within the heliosheath. Additionally, the variability of the solar wind in this region is caused by different temporal events that occur near the Sun and propagate to the outer heliosphere. To understand the spatial and temporal effects in the heliosheath, it is important to study these effects separately. In this work we explore the role of shocks as one type of temporal effects in the dynamics of the heliosheath. Although currently plasma in the heliosheath is dominated by solar minima conditions, with increasing solar cycle shocks associated with transients will play an important role. We used a 3D MHD multi-fluid model of the interaction between the solar wind and the local interstellar medium to study the propagation of a pair of forward-reverse shocks in the supersonic solar wind, interaction with the TS, and propagation to the heliosheath. We found that in the supersonic solar wind the interaction region between the shocks expands, the shocks weaken and decelerate. The fluctuation amplitudes of the plasma parameters vary with heliocentric distance. The interaction of the pair of shocks with the TS creates a variety of new waves and discontinuities in the heliosheath, which produce a highly variable solar wind flow. The collision of the forward shock with the heliopause causes a reflection of fast magnetosonic waves inside the heliosheath.
All the current global models of the heliosphere are based on the assumption that the magnetic field in the heliosheath, in the region close to the heliopause is laminar. We argue that in that region the heliospheric magnetic field is not laminar but
instead consists of magnetic bubbles. Recently, we proposed that the annihilation of the sectored magnetic field within the heliosheath as it is compressed on its approach to the heliopause produces the anomalous cosmic rays and also energetic electrons. As a product of the annihilation of the sectored magnetic field, densely-packed magnetic islands/bubbles are produced. These magnetic islands/bubbles will be convected with the ambient flows as the sector region is carried to higher latitudes filling the heliosheath. We further argue that the magnetic islands/bubbles will develop upstream within the heliosheath. As a result, the magnetic field in the heliosheath sector region will be disordered well upstream of the heliopause. We present a 3D MHD simulation with very high numerical resolution that captures the north-south boundaries of the sector region. We show that due to the high pressure of the interstellar magnetic field a north-south asymmetry develops such that the disordered sectored region fills a large portion of the northern part of the heliosphere with a smaller extension in the southern hemisphere. We suggest that this scenario is supported by the following changes that occur around 2008 and from 2009.16 onward: a) the sudden decrease in the intensity of low energy electrons detected by Voyager 2; b) a sharp reduction in the intensity of fluctuations of the radial flow; and c) the dramatic differences in intensity trends between GCRs at V1 and 2. We argue that these observations are a consequence of V2 leaving the sector region of disordered field during these periods and crossing into a region of unipolar laminar field.
Recently, surface magnetic field maps had been acquired for a small sample of active M dwarfs, showing that fully convective stars (spectral types ~M4 and later) host intense (~kG), mainly axi-symmetrical poloidal fields. In particular, the rapidly r
otating M dwarf V374Peg (M4), believed to lie near the theoretical full convection threshold, presents a stable magnetic topology on a time-scale of 1 yr. The rapid rotation of V374Peg (P=0.44 days) along with its intense magnetic field point toward a magneto-centrifugally acceleration of a coronal wind. In this work, we aim at investigating the structure of the coronal magnetic field in the M dwarf V374Peg by means of three-dimensional magnetohydrodynamical (MHD) numerical simulations of the coronal wind. For the first time, an observationally derived surface magnetic field map is implemented in MHD models of stellar winds for a low-mass star. We self-consistently take into consideration the interaction of the outflowing wind with the magnetic field and vice versa. Hence, from the interplay between magnetic forces and wind forces, we are able to determine the configuration of the magnetic field and the structure of the coronal winds. Our results enable us to evaluate the angular momentum loss of the rapidly rotating M dwarf V374Peg.
The rapid rotation (P=0.44 d) of the M dwarf V374Peg (M4) along with its intense magnetic field point toward magneto-centrifugal acceleration of a coronal wind. In this work, we investigate the structure of the wind of V374Peg by means of 3D magnetoh
ydrodynamical (MHD) numerical simulations. For the first time, an observationally derived surface magnetic field map is implemented in MHD models of stellar winds for a low mass star. We show that the wind of V374Peg deviates greatly from a low-velocity, low-mass-loss rate solar-type wind. We find general scaling relations for the terminal velocities, mass-loss rates, and spin-down times of highly magnetized M dwarfs. In particular, for V374Peg, our models show that terminal velocities across a range of stellar latitudes reach ~(1500-2300) n_{12}^{-1/2} km/s, where n_{12} is the coronal wind base density in units of 10^{12} cm^{-3}, while the mass-loss rates are about 4 x 10^{-10} n_{12}^{1/2} Msun/yr. We also evaluate the angular-momentum loss of V374Peg, which presents a rotational braking timescale ~28 n_{12}^{-1/2} Myr. Compared to observationally derived values from period distributions of stars in open clusters, this suggests that V374Peg may have low coronal base densities (< 10^{11} cm^{-3}). We show that the wind ram pressure of V374Peg is about 5 orders of magnitude larger than for the solar wind. Nevertheless, a small planetary magnetic field intensity (~ 0.1G) is able to shield a planet orbiting at 1 AU against the erosive effects of the stellar wind. However, planets orbiting inside the habitable zone of V374Peg, where the wind ram pressure is higher, might be facing a more significant atmospheric erosion. In that case, higher planetary magnetic fields of, at least, about half the magnetic field intensity of Jupiter, are required to protect the planets atmosphere.
By means of numerical simulations, we investigate magnetized stellar winds of pre-main-sequence stars. In particular we analyze under which circumstances these stars will present elongated magnetic features (e.g., helmet streamers, slingshot prominen
ces, etc). We focus on weak-lined T Tauri stars, as the presence of the tenuous accretion disk is not expected to have strong influence on the structure of the stellar wind. We show that the plasma-beta parameter (the ratio of thermal to magnetic energy densities) is a decisive factor in defining the magnetic configuration of the stellar wind. Using initial parameters within the observed range for these stars, we show that the coronal magnetic field configuration can vary between a dipole-like configuration and a configuration with strong collimated polar lines and closed streamers at the equator (multi-component configuration for the magnetic field). We show that elongated magnetic features will only be present if the plasma-beta parameter at the coronal base is beta<<1. Using our self-consistent 3D MHD model, we estimate for these stellar winds the time-scale of planet migration due to drag forces exerted by the stellar wind on a hot-Jupiter. In contrast to the findings of Lovelace et al. (2008), who estimated such time-scales using the Weber & Davis model, our model suggests that the stellar wind of these multi-component coronae are not expected to have significant influence on hot-Jupiters migration. Further simulations are necessary to investigate this result under more intense surface magnetic field strengths (~2-3 kG) and higher coronal base densities, as well as in a tilted stellar magnetosphere.
