Magnitude and direction of the local interstellar magnetic field inferred from Voyager 1 and 2 interstellar data and global heliospheric model

In this Letter, we provide constraints on the direction and magnitude of the pristine (i.e., unperturbed by the interaction with the Sun) local interstellar magnetic field. The constraints are based on analysis of the interstellar magnetic field components at the heliopause measured by magnetometer instruments on board Voyager 1 and 2 spacecraft. The analysis was performed with the help of our kinetic-magnetohydrodynamical (MHD) model of the global heliosphere. The model shows that the solar-induced disturbances of the interstellar magnetic field are extended relatively far from the Sun up to 400-500 AU. The field is draped around the heliopause and compressed. By comparison of the model results with Voyager data we found that the model provides results comparable with the data for the interstellar magnetic field of $B_{LISM}$ = 3.7-3.8 $\mu$G in magnitude and directed towards $\approx$125$^\circ$ in longitude, and $\approx$37$^\circ$ in latitude in the heliographic inertial (HGI) coordinate system.Comment: 5 pages + 2 appendixe

Lyman-alpha Absorption from Heliosheath Neutrals

We assess what information HST observations of stellar Ly-alpha lines can provide on the heliosheath, the region of the heliosphere between the termination shock and heliopause. To search for evidence of heliosheath absorption, we conduct a systematic inspection of stellar Ly-alpha lines reconstructed after correcting for ISM absorption (and heliospheric/astrospheric absorption, if present). Most of the stellar lines are well centered on the stellar radial velocity, as expected, but the three lines of sight with the most downwind orientations relative to the ISM flow (Chi1 Ori, HD 28205, and HD 28568) have significantly blueshifted Ly-alpha lines. Since it is in downwind directions where heliosheath absorption should be strongest, the blueshifts are almost certainly caused by previously undetected heliosheath absorption. We make an initial comparison between the heliosheath absorption and the predictions of a pair of heliospheric models. A model with a complex multi-component treatment of plasma within the heliosphere predicts less absorption than a model with a simple single-fluid treatment, which leads to better agreement with the data. Finally, we find that nonplanetary energetic neutral atom (ENA) fluxes measured by the ASPERA-3 instrument on board Mars Express, which have been interpreted as being from the heliosheath, are probably too high to be consistent with the relative lack of heliosheath absorption seen by HST. This would argue for a local interplanetary source for these ENAs instead of a heliosheath source.Comment: 27 pages, 7 figures, AASTEX v5.0, accepted by Ap

The Heliosphere and Local Interstellar Medium from Neutral Atom Observations at Energies Below 10 keV.

As the heliosphere moves through the surrounding interstellar medium, a fraction of the interstellar neutral helium, hydrogen, and heavier species crossing the heliopause make it to the inner heliosphere as neutral atoms with energies ranging from few eV to several hundred eV. In addition, energetic neutral hydrogen atoms originating from solar wind protons and from pick-up ions are created through charge-exchange with interstellar atoms. This review summarizes all observations of heliospheric energetic neutral atoms and interstellar neutrals at energies below 10 keV. Most of these data were acquired with the Interstellar Boundary Explorer launched in 2008. Among many other IBEX breakthroughs, it provided the first ever all-sky maps of energetic neutral atoms from the heliosphere and enabled the science community to measure in-situ interstellar neutral hydrogen, oxygen, and neon for the first time. These observations have revolutionized and keep challenging our understanding of the heliosphere shaped by the combined forces of the local interstellar flow, the local interstellar magnetic field, and the time-dependent solar wind

Heliospheric boundary in the backscattered solar Lyman-alpha radiation: analysis of Voyager-1/UVS data

International audienceUVS instrument onboard Voyager-1 has been performing measurements of the backscattered solar Lyman-alpha emissions for more than 35 years. Since 2003 when the scan platform movements were stopped, it measures Lyman-alpha radiation in one line of sight close to the upwind direction. The most interesting feature of these data is radial dependence of measured intensity. Namely, Voyager-1 data shows almost constant intensity (after correction for the solar flux variations) between 90 and 115 AU, while the numerical models of global heliosphere and radiative transfer predict decreasing intensity. Between 115 AU (beginning of 2011) and about 124 AU (beginning of 2013), the data show a decrease of the intensity again. This last period corresponds to motion of Voyager-1 in the transition region (Krimigis et al., 2011) before crossing the heliopause at 122 AU in August, 2012 (Stone et al., 2013). This behavior of the Lyman-alpha intensity observed by Voyager can be a tracer of complicated structure of the heliospheric boundary. Plasma instruments onboard Voyagers provide information about charged component of the solar and interstellar winds and energetic particles, while only the UVS data for the Lyman-alpha intensity reflect the distribution of the neutral hydrogen component. Therefore, interpretation of Voyager-1/UVS measurements is critical to understand the complete picture of the interaction between the solar wind and surrounding interstellar matter. In this work we will present preliminary results of analysis of Voyager-1/UVS data in the frame of the kinetic-MHD heliospheric model and radiative transfer code. Possible scenarios for hydrogen distribution consistent with Voyager-1 data will be proposed

Voyager 1/UVS Lyman α measurements at the distant heliosphere (90-130 AU): unknown source of additional emission

International audienceIn this work, we present for the first time the Lyman α intensities measured by Voyager 1/UVS in 2003-2014 (at 90-130 AU from the Sun). During this period Voyager 1 measured the Lyman α emission in the outer heliosphere at an almost fixed direction close to the upwind (that is towards the interstellar flow). The data show an unexpected behavior in 2003-2009: the ratio of observed intensity to the solar Lyman α flux is almost constant. Numerical modelling of these data is performed in the frame of a state-of-art self-consistent kinetic-MHD model of the heliospheric interface (Izmodenov & Alexashov, 2015). The model results, for various interstellar parameters, predict a monotonic decrease of intensity not seen in the data. We propose two possible scenarios that explain the data qualitatively. The first is the formation of a dense layer of hydrogen atoms near the heliopause. Such a layer would provide an additional backscattered Doppler shifted Lyman α emission, which is not absorbed inside the heliosphere and may be observed by Voyager. About 35 R of intensity from the layer is needed. The second scenario is an external non-heliospheric Lyman α component, which could be galactic or extragalactic. Our parametric study shows that ∼25 R of additional emission leads to a good qualitative agreement between the Voyager 1 data and the model results

Interstellar hydrogen fluxes measured by IBEX-Lo in 2009 : Numerical modeling and comparison with the data

International audienceIn this paper, we perform numerical modeling of the interstellar hydrogen fluxes measured by IBEX-Lo during orbit 23 (spring 2009) using a state-of-the-art kinetic model of the interstellar neutral hydrogen distribution in the heliosphere. This model takes into account the temporal and heliolatitudinal variations of the solar parameters as well as the non-Maxwellian kinetic properties of the hydrogen distribution due to charge exchange in the heliospheric interface. We found that there is a qualitative difference between the IBEX-Lo data and the modeling results obtained with the three-dimensional, time-dependent model. Namely, the model predicts a larger count rate in energy bin 2 (20–41 eV) than in energy bin 1 (11–21 eV), while the data shows the opposite case. We perform study of the model parameter effects on the IBEX-Lo fluxes and the ratio of fluxes in two energy channels. We show that the most important parameter, which has a major influence on the ratio of the fluxes in the two energy bins, is the solar radiation pressure

A NEW DETECTION OF LYα ABSORPTION FROM THE HELIOTAIL *

ABSTRACT We present new Hubble Space Telescope observations of H i Lyα absorption toward the F8 V star HD 35296. This line of sight is only a few degrees from the downwind direction of the local interstellar medium flow vector. As a consequence, Lyα absorption from the heliotail is detected in the spectrum, consistent with three previous downwind detections of heliotail absorption. The clustering of the heliotail absorption detections around the downwind direction demonstrates that the heliotail is pointed close to that direction, limiting the extent to which the interstellar magnetic field might be distorting and deflecting the heliotail. We explore this issue further using three-dimensional MHD models of the global heliosphere. The three computed models represent the first three-dimensional MHD models with both a kinetic treatment of neutrals and an extended grid in the tail direction, both of which are necessary to model Lyα absorption downwind. The models indicate only modest heliotail asymmetries and deflections, which are not large enough to be inconsistent with the clustering of heliotail absorption detections around the downwind direction. The models are reasonably successful at reproducing the observed absorption, but they do overpredict the Lyα opacity by a factor of 2-3. We discuss implications of these results in light of observations of the heliotail region from the Interstellar Boundary Explorer mission