Local Interstellar Medium Velocity Research Articles

Three-dimensional Features of the Outer Heliosphere Due to Coupling between the Interstellar and Heliospheric Magnetic Field. V. The Bow Wave, Heliospheric Boundary Layer, Instabilities, and Magnetic Reconnection

Abstract The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW–LISM interaction. By performing three-dimensional, MHD plasma/kinetic neutral atom simulations, we determine the width of the outer heliosheath—the LISM plasma region affected by the presence of the heliosphere—and analyze quantitatively the distributions in front of the heliopause. It is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. We also demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by Interstellar Boundary Explorer.

OBSERVATIONS OF THE INTERPLANETARY HYDROGEN DURING SOLAR CYCLES 23 AND 24. WHAT CAN WE DEDUCE ABOUT THE LOCAL INTERSTELLAR MEDIUM?

Observations of interstellar helium atoms by the Interstellar Boundary Explorer (IBEX) spacecraft in 2009 reported a local interstellar medium (LISM) velocity vector different from the results of the Ulysses spacecraft between 1991 and 2002. The interplanetary hydrogen (IPH), a population of neutrals that fills the space between planets inside the heliosphere, carries the signatures of the LISM and its interaction with the solar wind. More than 40 yr of space-based studies of the backscattered solar Lyα emission from the IPH provided limited access to the velocity distribution, with the first temporal evolution map of the IPH line-shift during solar cycle 23. This work presents the results of the latest IPH observations made by the Hubble Space Telescope's Space Telescope Imaging Spectrograph during solar cycle 24. These results have been compiled with previous measurements, including data from the Solar Wind Anisotropies instrument on the Solar and Heliospheric Observatory. The whole set has been compared to physically realistic models to test both sets of LISM physical parameters as measured by Ulysses and IBEX, respectively. This comparison shows that the LISM velocity vector has not changed significantly since Ulysses measurements.

THE STRUCTURE OF THE LOCAL INTERSTELLAR MEDIUM. VI. NEW Mg II, Fe II, AND Mn II OBSERVATIONS TOWARD STARS WITHIN 100 pc

We analyze high-resolution spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope toward 34 nearby stars (<100 pc) to record MgII, FeII and MnII absorption due to the local interstellar medium (LISM). Observations span the entire sky, probing previously unobserved regions of the LISM. We detected one to six individual absorption components along any given sight line, and the number of absorbers roughly correlates with the pathlength. This high-resolution near-ultraviolet (NUV) spectroscopic survey was specifically designed for sight lines with existing far-UV (FUV) observations. The FUV spectra include many intrinsically broad absorption lines (i.e., of low atomic mass ions) and often observed at medium resolution. The LISM NUV narrow-line absorption component structure presented here can be used to more accurately interpret the archival FUV observations. As an example of this synergy, we present a new analysis of the temperature and turbulence along the line of sight toward epsilon Ind. The new observations of LISM velocity structure are also critical in the interpretation of astrospheric absorption derived from fitting the saturated HI Lyman-alpha profile. As an example, we reanalyze the spectrum of lambda And and find that this star likely does have an astrosphere. Two stars in the sample that have circumstellar disks (49 Cet and HD141569) show evidence for absorption due to disk gas. Finally, the substantially increased number of sight lines is used to test and refine the three-dimensional kinematic model of the LISM, and search for previously unidentified clouds within the Local Bubble. We find that every prediction made by the Redfield & Linsky (2008) kinematic model of the LISM is confirmed by an observed component in the new lines of sight.

Heliospheric asymmetries due to the action of the interstellar magnetic field

We discuss the asymmetry of the heliospheric discontinuities obtained from the analysis of 3D modeling of the solar wind (SW) interaction with local interstellar medium (LISM). The flow of charged particles is governed by the ideal MHD equations and the flow of neutral particles is described by the Boltzmann equation. The emphasis is made on the asymmetries of the termination shock (TS) and the heliopause under the combined action of the interstellar and interplanetary magnetic fields (ISMF and IMF) in the presence of neutral hydrogen atoms whose transport through the heliosphere is modeled kinetically, using a Monte Carlo approach. We show that the deflection of neutral hydrogen flow from its original direction in the unperturbed LISM is highly anisotropic and evaluate a possible angle between the hydrogen deflection plane measured in the SOHO SWAN experiment and the plane containing the ISMF and LISM velocity vectors for different ISMF strengths. It is shown that the ISMF of a strength greater than 4 μG can account for the 10 AU difference in the TS heliocentric difference observed during its crossing by the Voyager 1 and Voyager 2 spacecraft, which however results in a larger discrepancy between the calculated and observed velocity distributions. The effect of a strong ISMF on the distribution of plasma quantities in the inner heliosheath and on 2–3 kHz radio emission is discussed.

Probing Heliospheric Asymmetries with an MHD-Kinetic model

New solar wind data from the Voyager 1 and Voyager 2 spacecraft, together with the SOHO SWAN measurements of the direction that neutral hydrogen enters into the inner heliosheath and neutral helium measurements provided by multiple observations are expected to provide more reliable constraints on the ionization ratio of the local interstellar medium (LISM) and the direction and magnitude of the interstellar magnetic field (ISMF). In this paper we use currently the most sophisticated numerical model of the heliospheric interface, which is based on an MHD treatment of the ion flow and kinetic modeling of neutral particles, to analyze an ISMF-induced asymmetry of the heliosphere in the presence of the interplanetary magnetic field and neutral particles. It is shown that secondary hydrogen atoms modify the LISM properties leading to its shock-free deceleration at the heliopause. We determine the deflection of hydrogen atoms from their original trajectory in the unperturbed LISM and show that it occurs not only in the plane defined by the ISMF and LISM velocity vectors, but also, to a lesser extent, perpendicular to this plane. We also consider the possibility of using 2-3 kHz radio emission data to further constrain the ISMF direction.

Termination Shock Asymmetries as Seen by theVoyagerSpacecraft: The Role of the Interstellar Magnetic Field and Neutral Hydrogen

We show that asymmetries of the termination shock due to the influence of the interstellar magnetic field (ISMF) are considerably smaller in the presence of neutral hydrogen atoms, which tend to symmetrize the heliopause, the termination shock, and the bow shock due to charge exchange with charged particles. This leads to a much stronger restriction on the ISMF direction and its strength. We demonstrate that in the presence of the interplanetary magnetic field the plane defined by the local interstellar medium (LISM) velocity and magnetic field vectors does not exactly coincide with the plane defined by the interstellar neutral helium and hydrogen velocity vectors in the supersonic solar wind region, which limits the accuracy of the inferred direction of the ISMF. We take into account the tilt of the LISM velocity vector with respect to the ecliptic plane and show that magnetic fields as strong as 3 μG or greater may be necessary to account for the observed asymmetry. Estimates are made of the longitudinal streaming anisotropy of energetic charged particles at the termination shock caused by the nonalignment of the interplanetary magnetic field with its surface. By investigating the behavior of interplanetary magnetic field lines that cross the Voyager 1 trajectory in the inner heliosheath, we estimate the length of the trajectory segment that is directly connected by these lines to the termination shock. A possible effect of the ISMF draping over the heliopause is discussed in connection with radio emission generated in the outer heliosheath.

Three‐dimensional Features of the Outer Heliosphere due to Coupling between the Interstellar and Interplanetary Magnetic Fields. II. The Presence of Neutral Hydrogen Atoms

We investigate the effects of coupling the interstellar and interplanetary magnetic fields (ISMF and IMF) at the heliospheric interface while taking into account the momentum and energy transfer between the plasma and neutral components of the interacting solar wind (SW) and local interstellar medium (LISM). Investigation is performed on the basis of the multifluid model that treats different populations of neutral particles as fluids governed by separate sets of the Euler equations. As the properties of the local interstellar cloud (LIC) are rather poorly examined, with the exception of the interstellar plasma velocity and temperature, we assume several possible orientations of the ISMF. In particular, the case is considered where an ISMF vector is perpendicular to the LISM velocity vector and inclined at an angle of 60° toward the ecliptic plane of the Sun, as suggested in recent publications relating LIC properties to the radio emission observed by Voyager 1. Special attention is paid to the distribution of magnetic fields that can affect the cosmic-ray modulation throughout the heliosphere and the possibility of the heliospheric current sheet bending into one of the hemispheres. Differences are discussed between ideal MHD and multifluid calculations. Possibilities are analyzed for the IMF lines to cross the termination shock multiple times. Parameters are determined that affect the divergence between the neutral hydrogen and helium streamlines in the inner heliosphere.

Coupling of the interstellar and interplanetary magnetic fields at the heliospheric interface: The effect of neutral hydrogen atoms

Abstract We present numerical results showing the effect of neutral hydrogen atoms on the solar wind (SW) interaction with the local interstellar medium (LISM), where the interstellar magnetic field (ISMF) is coupled to the interplanetary magnetic field (IMF) at the surface of the heliopause. The IMF on the inner boundary surrounding the Sun is specified in the form of a Parker spiral and self-consistently develops in accordance with the SW motion inside the heliopause. The model of the SW–LISM interaction involves both plasma and neutral components which are treated as fluids. The configuration is chosen where the ISMF is orthogonal to the LISM velocity and tilted 60° to the ecliptic plane. This orientation of the magnetic field is a possible explanation of the 2–3 kHz emission data which is believed to originate ahead of the heliopause. It is shown that the topology of the heliospheric current sheet is substantially affected by the ISMF. The interaction pattern dependence on the neutral hydrogen density is analyzed.

Three‐dimensional Features of the Outer Heliosphere Due to Coupling between the Interstellar and Interplanetary Magnetic Fields. I. Magnetohydrodynamic Model: Interstellar Perspective

The properties of the Local Interstellar Cloud (LIC) are rather poorly examined, with the exception of the interstellar plasma velocity and temperature. The poorest known quantities are the strength and direction of the interstellar magnetic field (ISMF). We review available observational data and perform three-dimensional, time-dependent parametric modeling of the solar wind (SW) interaction with the local interstellar medium (LISM) for a number of typical strengths and directions of the ISMF. The effect of coupling the ISMF and the interplanetary magnetic field on the SW-LISM interaction pattern is investigated. In addition, we consider the case of an ISMF vector perpendicular to the LISM velocity vector and inclined at an angle of 60° toward the ecliptic plane, as suggested in recent publications relating LIC properties to the radio emission observed by Voyager 1. Since the existence of nonevolutionary MHD shocks remains controversial, special attention is paid to the case where the ISMF strength ahead of the bow shock falls into the region of nonevolutionary parallel shocks. It is shown that a complicated, multishocked solution, which exists in this case in the axially symmetric statement, acquires a regular evolutionary structure in a genuinely three-dimensional formulation of the problem. Current views regarding the existence and character of nonevolutionary shocks in space are presented, and an attempt is made to distinguish important physical questions of the uniqueness of MHD shock-wave structures from numerical artifacts. This is the first in a series of papers investigating the three-dimensional structure of the outer heliosphere. Here we present the results of purely MHD calculations, adopting an interstellar medium perspective. Subsequent papers will address the combined effect of charge exchange between neutral and charged particles and magnetic fields.

Signatures of the Interplanetary Helium Cone Reflected by Pick-up Ions

As known for a long time, interstellar wind neutral helium atoms deeply penetrate into the inner heliosphere and, when passing through the solar gravity field, form a strongly pronounced helium density cone in the downwind direction. Helium atoms are photoionized and picked-up by the solar wind magnetic field, but as pick-up ions they are not simply convected outwards with the solar wind in radial directions as assumed in earlier publications. Rather they undergo a complicated diffusion-convection process described here by an appropriate kinetic transport equation taking into account adiabatic cooling and focusing, pitch angle scattering and energy diffusion. In this paper, we solve this equation for He+pick-up ions which are injected into the solar wind mainly in the region of the helium cone. We show the resulting He+pick-up ion density profile along the orbit of the Earth in many respects differs from the density profile of the neutral helium cone: depending on solar-wind-entrained Alfvenic turbulence levels, the density maximum when looking from the Earth to the Sun is shifted towards the right side of the cone, the ratio of peak-densities to wing-densities varies and a left-to-right asymmetry of the He+-density profile is pronounced. Derivation of interstellar helium parameters from these He+-structures, such as the local interstellar medium (LISM) wind direction, LISM velocity and LISM temperature, are very much impeded. In addition, the pitch-angle spectrum of He+pick-up ions systematically becomes more anisotropic when passing from the left to the right wing of the cone structure. All effects mentioned are more strongly pronounced in high velocity solar wind compared to the low velocity solar wind.

Influence of the interstellar magnetic field direction on the shape of the global heliopause

In this paper we study the influence of the angle between the local interstellar medium velocity and magnetic field vectors on the interaction of this medium with the solar wind. Both winds are supposed supersonic. Applicability of the two‐shock model of the interaction is discussed. Investigation is performed by numerical solution of the MHD equations for an ideal plasma in the one‐fluid approximation.

Characteristics of the helium component of the local interstellar medium

Extreme ultraviolet instruments on the Solrad 11 B satellite observed helium in the local interstellar medium (LISM) flowing through the solar system through resonantly scattered 584 A solar radiation. Since this satellite orbited the Earth at 130,000 km altitude and, thus, was far removed from the geocoronal airglow, reliable measurements over many directions at a given time were made. As a result these data, together with ''hot gas'' density-resonance scatter computations and our previous results, permit accurate determination of quite narrow ranges of LISM velocity, V = 22--28 km s/sup -1/, and temperature, T = 9,000--15,000 K. The derived LISM helium density of 1.1--2.7 x 10/sup -2/ cm/sup -3/, when compared with the derived density of the inflowing LISM hydrogen 4--6 x 10/sup -2/ cm/sup -3/, suggests that the local interstellar gas is substantially ionized.