Heliospheric Nose Research Articles

Whence the Interstellar Magnetic Field Shaping the Heliosphere?

Abstract Measurements of starlight polarized by aligned interstellar dust grains are used to probe the relation between the orientation of the ambient interstellar magnetic field (ISMF) and the ISMF traced by the ribbons of energetic neutral atoms discovered by the Interstellar Boundary Explorer spacecraft. We utilize polarization data, many acquired specifically for this study, to trace the configuration of the ISMF within 40 pc. A statistical analysis yields a best-fit ISMF orientation, B magpol, aligned with Galactic coordinates ℓ = 42°, b = 49°. Further analysis shows the ISMF is more orderly for “downfield” stars located over 90° from B magpol. The data subset of downfield stars yields an orientation for the nearby ISMF at ecliptic coordinates λ, β ≈ 219° ± 15°, 43° ± 9° (Galactic coordinates l, b ≈ 40°, 56°, ±17°). This best-fit ISMF orientation from polarization data is close to the field direction obtained from ribbon models. This agreement suggests that the ISMF shaping the heliosphere belongs to an extended ordered magnetic field. Extended filamentary structures are found throughout the sky. A previously discovered filament traversing the heliosphere nose region, “Filament A,” extends over 300° of the sky, and crosses the upwind direction of interstellar dust flowing into the heliosphere. Filament A overlaps the locations of the Voyager kilohertz emissions, three quasar intraday variables, cosmic microwave background (CMB) components, and the inflow direction of interstellar grains sampled by Ulysses and Galileo. These features are likely located in the upstream outer heliosheath where ISMF drapes over the heliosphere, suggesting Filament A coincides with a dusty magnetized plasma. A filament 55° long is aligned with a possible shock interface between local interstellar clouds. A dark spot in the CMB is seen within 5° of the filament and within 10° of the downfield ISMF direction. Two large magnetic arcs are centered on the directions of the heliotail. The overlap between CMB components and the aligned dust grains forming Filament A indicates the configuration of dust entrained in the ISMF interacting with the heliosphere provides a measurable foreground to the CMB.

Heliospheric effects caused by Sun-originating versus LISM-advected fluctuations

Context. We investigate the response of the heliosphere to fluctuations in the local interstellar medium (LISM) as compared to the influence of solar-cycle fluctuations. Aims. We discuss the differences between effects coming from the two types of drivers of time-dependent effects in the heliosphere in the context of the shape of the heliosphere, the thickness of the inner heliosheath, and the position of the ribbon of enhanced energetic neutral particle emission, as observed by the IBEX mission. Methods. Our study is based on a comparison of fully time-dependent simulations obtained with a three-dimensional (3D) model of the heliosphere. Results. We show that density fluctuations, taking the form of entropy waves and originating from the LISM, may reduce the thickness of the inner heliosheath to a similar extent as the solar-cycle effects. However, the relative motions of the termination shock and the heliopause in the two types of simulations are different. The amplitude of variation of the heliopause position is greater for the LISM fluctuation. The IBEX ribbon position is shown to be not significantly affected by the two types of drivers, although the effect of LISM fluctuation is stronger than that of the solar cycle. In this context, slight systematic changes of the position of the IBEX ribbon in its different sectors (i.e., changes in the heliospheric nose followed by variations in the heliospheric flanks) may serve as an indicator of the passage of a density fluctuation in the LISM, as suggested by our study. We also discuss the difficulties in fitting the LISM parameters in the presence of time-dependent effects.

Turbulent Acceleration of Interstellar Pickup Ions at the Heliospheric Termination Shock Forms the Global ENA Spectrum

Abstract The heliospheric energetic neutral atom spectrum observed by the Interstellar Boundary Explorer (IBEX) reveals that the heliosheath proton distribution is consistent with a power law. The origin of the spectrum is likely from interstellar pickup ions (PUIs) accelerated at the heliospheric termination shock (HTS). We present an explanation of the proton spectrum origin using a test particle simulation of PUIs accelerated at the HTS. PUIs experience preferential heating by the motional electric field in the shock foot, but do not develop a power-law tail without the presence of turbulence at wavenumbers (k) close to the PUI gyroradius scale (R g). Voyager 2 observations of the magnetic field downstream of the HTS indicate a moderate amount of turbulence at kR g ≅ 1, δ B / B 0 2 ≅ 0.01 , which we find to be sufficient for producing a downstream suprathermal PUI tail but not at intensities observed by IBEX. Within the shock ramp, however, Voyager observed the turbulence power at much smaller scales to be nearly 100 times stronger, suggesting the possibility of strong turbulence at the PUI gyroradius scale. We show that a proton distribution can develop a power law downstream of the HTS consistent with IBEX observations if δ B / B 0 2 ≳ 0.1 at kR g ≅ 1 in the shock foot. Shock drift acceleration of PUIs by the motional electric field is aided by interactions with turbulence upstream of the shock overshoot. Steepening of the IBEX proton spectrum in directions farther from the heliospheric nose suggests the HTS compression ratio and/or turbulence power weakens near the heliotail.

Observations of a Radial Density Gradient in the Very Local Interstellar Medium by Voyager 2

Abstract Beginning on 2020 June 19, at a heliocentric distance of 124.2 au, the Voyager 2 Plasma Wave Science instrument began to observe radio emissions followed by electron plasma oscillations in its 3.11 kHz spectrum analyzer channel. Plasma oscillations at this frequency imply an electron density in the range of 0.12 cm−3 ± 15%, although some response in the 1.78 kHz channel near the peak of the plasma oscillations suggest a density of 0.087 cm−3 ± 8%. Shortly after Voyager 2 crossed the heliopause, in late 2019 January, the Voyager 2 Plasma Wave Science instrument detected plasma oscillations in its 1.78 kHz channel giving an electron density of 0.039 cm−3 ± 15%. While the Voyager spectrum analyzer affords relatively poor spectral resolution, the recent observation of plasma oscillations in the 3.11 kHz channel provides definitive evidence of a radial density gradient in the very local interstellar medium (VLISM), just beyond the heliopause with a magnitude similar to that observed by Voyager 1 obtained with higher spectral resolution measurements. Plasma oscillations observed by Voyager 1 range from frequencies as low as 2.1 kHz increasing to about 3.2 kHz, giving an electron density profile that increases from about 0.055 to about 0.13 cm−3 over a distance spanning some 20 au. Given the 67° difference in heliographic latitude and 43° difference in longitude between the two Voyagers, the new Voyager 2 observations imply that the density gradient is a large-scale feature of the VLISM in the general direction of the heliospheric nose.

SYMMETRY OF THEIBEXRIBBON OF ENHANCED ENERGETIC NEUTRAL ATOM (ENA) FLUX

The circular ribbon of enhanced energetic neutral atom (ENA) emission observed by the Interstellar Boundary Explorer (IBEX) mission remains a critical signature for understanding the interaction between the heliosphere and the interstellar medium. We study the symmetry of the ribbon flux and find strong, spectrally dependent reflection symmetry throughout the energy range 0.7-4.3?keV. The distribution of ENA flux around the ribbon is predominantly unimodal at 0.7 and 1.1?keV, distinctly bimodal at 2.7 and 4.3?keV, and a mixture of both at 1.7?keV. The bimodal flux distribution consists of partially opposing bilateral flux lobes, located at highest and lowest heliographic latitude extents of the ribbon. The vector between the ribbon center and heliospheric nose (which defines the so-called BV plane) appears to play an organizing role in the spectral dependence of the symmetry axis locations as well as asymmetric contributions to the ribbon flux. The symmetry planes at 2.7 and 4.3?keV, derived by projecting the symmetry axes to a great circle in the sky, are equivalent to tilting the heliographic equatorial plane to the ribbon center, suggesting a global heliospheric ordering. The presence and energy dependence of symmetric unilateral and bilateral flux distributions suggest strong spectral filtration from processes encountered by an ion along its journey from the source plasma to its eventual detection at IBEX.

THE INTERSTELLAR MAGNETIC FIELD CLOSE TO THE SUN. II.

A key indicator of the galactic environment of the Sun is provided by the magnetic field in the interstellar medium (ISM), which influences the shape of the heliosphere. The direction of the nearby interstellar magnetic field (ISMF) is determined from starlight polarized in the ISM. The local ISMF direction is found from the ISMF direction that provides the best fit to the polarization position angles of nearby stars, using weighted fits to the data. New polarization observations are included in the analysis. The best-fitting ISMF is close to the magnetic field direction traced by the center of the Ribbon of energetic neutral atoms, discovered by the Interstellar Boundary Explorer spacecraft. Both the magnetic field and kinematics of the local ISM are consistent with a scenario where the local ISM is a fragment of the Loop I superbubble. An ordered component of the local ISMF is found in a region where PlanetPol data show that polarization increases with distance. It extends to within 8 parsecs of the Sun and implies a weak curvature in the nearby ISMF. Variations from the ordered component indicate turbulence of +/-23 deg. The local ISMF is generally uniform in direction over spatial scales of 8-200 parsecs so that it appears similar to interarm magnetic fields. The best-fitting ISMF direction also agrees with the position of tail-in spatial asymmetries in GeV-TeV galactic cosmic rays. The peculiar geometrical relation between the CMB dipole moment, the heliosphere nose, and local ISMF is supported by these new results. Radiative torques are not likely to play a role in grain alignment for these polarizations.

COMPARISONS OF THE INTERSTELLAR MAGNETIC FIELD DIRECTIONS OBTAINED FROM THEIBEXRIBBON AND INTERSTELLAR POLARIZATIONS

Variations in the spatial configuration of the interstellar magnetic field (ISMF) near the Sun can be constrained by comparing the ISMF direction at the heliosphere found from the Interstellar Boundary Explorer spacecraft (IBEX) observations of a 'Ribbon' of energetic neutral atoms (ENAs), with the ISMF direction derived from optical polarization data for stars within ~40 pc. Using interstellar polarization observations towards ~30 nearby stars within 90 deg of the heliosphere nose, we find that the best fits to the polarization position angles are obtained for a magnetic pole directed towards ecliptic coordinates of lambda, beta 263 deg, 37 deg (or galactic coordinates of L,B 38 deg, 23deg), with uncertainties of +/- 35 deg, based on the broad minimum of the best fits and the range of data quality. This magnetic pole is 33 deg from the magnetic pole that is defined by the center of the arc of the ENA Ribbon. The IBEX ENA ribbon is seen in sightlines that are perpendicular to the ISMF as it drapes over the heliosphere. The similarity of the polarization and Ribbon directions for the local ISMF suggest that the local field is coherent over scale sizes of tens of parsecs. The ISMF vector direction is nearly perpendicular to the flow of local ISM through the local standard of rest, supporting a possible local ISM origin related to an evolved expanding magnetized shell. The local ISMF direction is found to have a curious geometry with respect to the cosmic microwave background dipole moment.

Plasma flows in the heliosheath

Voyager 2 is making the first plasma measurements in the heliosheath. The radial flow speeds in the heliosheath vary between 80 and 200 km/s with an average speed of 138 km/s. The flow in the T (azimuthal) direction is fairly constant and averages about 48 km/s; the flow direction is consistent with flow away from the heliospheric nose. Flow in the N (meridional) direction is also away from the nose and averages −14 km/s. These flows suggest that the shock is blunter in the T than in the N direction, so that the heliosphere is wider than it is high. The flow in the RN plane has quasi‐periodic oscillation with a period of 110 days and an amplitude of 21 km/s. The oscillation in flow angle is about 6° in the RN plane and 17° in the TN plane and may result from periodic variations of the termination shock normal direction.

TIME-DEPENDENT ACCELERATION OF INTERSTELLAR PICKUP IONS AT THE HELIOSPHERIC TERMINATION SHOCK USING A FOCUSED TRANSPORT APPROACH

A time-dependent focused transport approach to modeling diffusive shock acceleration of interstellar pickup ions at the termination shock is discussed. By taking into account time variations in the magnetic field angle, and thus by implication in the shock obliquity and injection speed at the termination shock using Voyager 1 observations as guide, we show that unaccelerated core interstellar pickup protons can be accelerated by the nearly perpendicular termination shock at the heliospheric nose. Many features of anomalous cosmic rays, observed by the Voyager spacecraft at energies below the big spectral dip starting at ~3 MeV, can be successfully reproduced with this approach. This includes multiple power-law spectral slopes with stable breaking points, power-law spectral slopes that are harder than predicted by standard diffusive shock acceleration, large pitch-angle anisotropies with strong time variations upstream that converge to steady-state isotropy in the heliosheath, upstream pitch-angle anisotropies that peak at ~1 MeV, episodic strongly anisotropic intensity spikes at the termination shock, and strong spectral volatility upstream that is reduced downstream and almost completely disappears farther downstream.

Are heliospheric flows magnetic line- or flux-conserving?

This article discusses and tests the validity of the frozen in magnetic field paradigm (or 'ideal magnetohydro- dynamics (MHD) constraint') which is usually adopted by many authors dealing with heliospheric physics. To show the problem of using ideal MHD in such a coun- terflow configuration like the heliosphere, we first recapit- ulate the basic concepts of freezing-in of magnetic fields, respectively magnetic topology conservation and its viola- tion (= magnetic reconnection) in 3-D, already done by other authors with different methods with respect to derivations and interpretations. Then we analyse different heliospheric plasma environments. As a model of the stagnation re- gion/stagnation point in front of the heliospheric nose, we present and discuss the general solution of the ideal MHD Ohm's law in the vicinity of a 2-D stagnation point, which was found by us. We show that ideal MHD either leads necessarily to a di- verging magnetic field strength in the vicinity of such a stag- nation point, or to a vanishing mass density on the heliopause boundaries. In the case that components of the electric field parallel to the magnetic field do not exist due to the cho- sen form of the non-ideal Ohm's law, it is always possible to formulate the transport equation of the magnetic field as a modified ideal Ohm's law. We find that the form of the Ohm's law which is often used in heliospheric physics (see e.g. Baranov and Fahr, 2003), is not able to change magnetic topology and thus cannot lead to magnetic reconnection, which necessarily has to occur at the stagnation point. The diverging magnetic field, for in- stance, implies the breakdown of the flux freezing paradigm for the heliosphere. Its application, especially at the helio- spheric nose, is therefore rather doubtful. We conclude that it is necessary to search for an Ohm's law which is able to violate magnetic topology conservation.

Effects of Different Solar Wind Speed Profiles in the Heliosheath on the Modulation of Cosmic‐Ray Protons

An asymmetric solar wind termination shock (TS) model is used to study the effects on the modulation of cosmic-ray protons for different scenarios of the solar wind speed (V) in the heliosheath. This two-dimensional model is applied using predictions for V in the heliosheath that were calculated with a time-dependent three-dimensional hydrodynamic model. Decreases stronger than the generally assumed V ∝ 1/r2 in the heliosheath are studied, as well as an extreme case, V ∝ r2, where r is the radial distance.The effect of the TS is enhanced under certain circumstances, and barrier-type modulation in the heliosheath also depends on the chosen V-profiles. Significant changes occur mostly for the A < 0 solar magnetic polarity cycle, at all distances in the equatorial plane, when the V is changed from an incompressible fluid (V ∝ 1/r2, ∇ V = 0) in the heliosheath, to V ∝ 1/r8, in a symmetrical model. For the asymmetrical case the TS is predicted to be more effective in the heliospheric nose than in the tail, especially for the A < 0 cycle during solar minimum conditions. The different profiles for V do not have a significant effect on the intensities inside the TS, but in the heliosheath the difference can be quite significant. It is found that V ∝ 1/r2 in the heliosheath is an oversimplification. The consequent effects of having ∇ V ≠ 0 in the heliosheath prove to be relevant for cosmic-ray modulation and acceleration, especially now that the Voyager 1 spacecraft encountered the TS and entered the heliosheath.

Modulation of Galactic Protons in an Asymmetrical Heliosphere

A previously used two-dimensional model of the heliospheric modulation of cosmic rays including a solar wind termination shock is extended to include an outer modulation boundary that is asymmetrically shaped with respect to the Sun. The modulation process is described kinetically using the Parker transport equation. The model includes drifts, adiabatic energy changes, diffusion, convection, a solar wind termination shock, and a heliosheath and is used here to compute modulation differences between a symmetrical and an asymmetrical modeled heliosphere, as well as differences between an asymmetrical heliosphere including a termination shock and one without such a shock for Galactic cosmic-ray protons. The solutions are for solar minimum and moderate maximum conditions for both heliospheric magnetic field polarity cycles. It is found that the modulation produced for cosmic-ray protons with an asymmetrical heliospheric model differs from that produced with a symmetrical model, but significantly mostly for the A < 0 polarity cycle, especially in the tail region of the heliosphere. The results of the symmetrical model are, however, surprisingly reasonable for studies of the heliospheric nose region, measured against the asymmetrical model.

A heliospheric hybrid model: hydrodynamic plasma flow and kinetic cosmic ray transport

In this paper we present a new five particle species hybrid model for calculating cosmic ray particle transport and acceleration in a dynamic heliospheric environment. In particular the effects of solar cycle related changes in the solar wind speed on the heliospheric geometry, solar wind flow and cosmic ray distribution are discussed, when a polar- ecliptic asymmetry at the inner boundary is modeled. It is shown that the disappearance of the fast solar wind over the solar poles toward solar maximum influences the geometry of the termination shock which is an important structure for cosmic ray acceleration. For solar maximum conditions, the shock radius is smaller in the polar regions and in the he- liospheric tail compared to solar minimum. These changes influence cosmic ray transport and acceleration in these re- gions, especially for the polarity cycle where positive parti- cles drift in along the heliospheric current sheet. For this po- larity cycle, and for both the anomalous and galactic cosmic ray protons, an increase in particle intensities at the shock in the heliospheric tail is computed as the shock moves in- ward toward the Sun. For the heliospheric nose, it is also shown that both the plasma speed and cosmic ray intensities are relative insensitive to changes in the latitudinal profile of the solar wind speed. Therefore toward solar maximum conditions there is a decrease in the nose-tail asymmetry of the computed cosmic ray distribution compared to solar min- imum conditions. shock fronts, the inner, the termination shock of the solar wind, and the outer, the bowshock of the interstellar ionized matter. The interstellar wind is a consequence of the rela- tive motion between the solar system and the local interstel- lar medium, while the inner solar wind results from the solar coronal outflow with asymptotically supersonic velocities. In the interface region between the two shocks a contact discon- tinuity forms, the heliopause, which separates both media by a surface which cannot penetrated by the plasmas.

Effects of the position of the solar wind termination shock and the heliopause on the heliospheric modulation of cosmic rays

Abstract The effects of changing the position of the solar wind termination shock and the position of the heliopause, and therefore the extent of the heliosheath, on the modulation of cosmic ray protons are illustrated. An improved numerical model with diffusive termination shock acceleration, a heliosheath and drifts is used. The modulation is computed in the equatorial plane and at 35 heliolatitude using recently derived diffusion coefficients applicable to a number of cosmic ray species during both magnetic polarity cycles of the Sun. It was found that qualitatively the modulation results for the different heliopause positions are similar although they differ quantitatively, e.g., clearly different radial gradients are predicted for the regions beyond the termination shock compared to inside the shock. The difference between the modulation for the two solar polarity cycles are less significant at a heliolatitude of 35° than in the equatorial plane. We found that moving the termination shock from 90 to 100 AU, with the heliopause fixed at 120 AU, caused only quantitative differences so that the exact position of the TS in the outer heliosphere seems not crucially important to global modulation. Moving the heliopause outwards, to represent the modulation in the tail region of the heliosphere, causes overall decreases in the cosmic ray intensities but not linearly as a function of energy, e.g., at 1 GeV the effect is insignificant. We conclude from this modelling that the modulation of protons in the heliospheric nose and tail regions are qualitatively similar although, clear quantitative and interesting differences occur.

The interstellar heliopause probe technology reference study

Abstract The interstellar heliopause probe (IHP) is one of ESA’s technology reference studies (TRS). The TRS aim to focus the development of strategically important technologies of relevance to future science missions by studying technologically demanding and scientifically interesting missions that are currently not part of the science mission programme. Equipped with a highly integrated payload suite (HIPS), the IHP will perform in situ exploration of the heliopause and the heliospheric interface. The HIPS, which is a standard element in all TRSs, miniaturize payloads through resource reduction by using miniaturized components and sensors, and by sharing common structures and payload functionality. To achieve the scientific requirements of the mission, the spacecraft is to leave the heliosphere as close to the heliosphere nose as possible and reach a distance of 200 AU from the Sun within 25 years. This is possible by using a trajectory with two solar flybys and a solar sail with characteristic acceleration of 1.1 mm/s2, which corresponds to a 245 × 245 m2 solar sail and a sail thickness of 1–2 μm. The trajectory facilitates a modest sail design that could potentially be developed in a reasonable timeframe. In this paper, an update to the results of studies being performed on this mission will be given and the current mission baseline and spacecraft design will be described. Furthermore, alternative solar sail systems and enabling technologies will be discussed.

Modulation of Cosmic‐Ray Electrons in the Outer Heliosphere

A new cosmic-ray transport model, which investigates the modulation of cosmic-ray electrons in the outer heliospheric regions, including all major modulation mechanisms and the asymmetry of the heliospheric shock structures, is developed. The transport of Galactic cosmic rays into the heliosphere is directly calculated from the local interstellar medium, while for the mutual interaction between the solar wind and the interstellar medium, the three-fluid two-dimensional hydrodynamic Bonn model is used to calculate the geometry of the heliosphere and solar wind flow profiles as input to the transport model. There are considerable differences in the modulation that cosmic rays experience between the heliospheric nose and tail regions, but because of large transport parameters, no modulation of Galactic cosmic rays occurs beyond the heliopause. It is shown that for energetic particles in the range of 1 GeV the termination shock has a significant influence on the modeled Galactic cosmic-ray flux intensities in the equatorial regions in direction to the heliospheric nose, while it is decreasing toward the poles and in the tail. Depending on energy, most of the modulation that the particles experience occurs at radial distances less than the shock radius. It is also shown that the cosmic-ray distribution in the inner heliosphere is not sensitive to the nose-tail asymmetry of the termination shock and heliopause. Finally, the effect of perpendicular transport parameter in the polar region is calculated. It is shown that depending on energy, this parameter influence computed latitudinal gradients, especially at, and to a lesser extend beyond, the termination shock, and in the heliospheric tail.

On the source location of low‐frequency heliospheric radio emissions

We used a combination of three different types of source localization to determine the two‐dimensional source positions for low‐frequency heliospheric radio emissions in ecliptic coordinates. These techniques include time of flight, rotating dipole direction finding, and a comparison of received power from two widely spaced receivers. Most of the source locations are within a few tens of degrees of the heliospheric nose, although some sources are located as far as 90° from the nose. The distribution is roughly linear and aligned generally parallel to the galactic plane. We conclude that the magnetic field in the local interstellar medium is the only obvious means by which to impose an asymmetry to the set of source locations about the nose; it is likely these measurements confirm that the previously proposed model for the galactic magnetic field with a primarily longitudinal orientation parallel to the galactic plane is present just upstream of the heliosphere. The apparent sizes of the radio sources range from very small (a few degrees at the most) to a few tens of degrees. In a couple of cases the source half‐angle could approach 80 degrees. We suggest that the true source size is quite small and that the larger sizes found here are likely due to scattering in the intervening medium. The fact that the apparent source size varies considerably implies that scattering in the outer heliosphere is a function of time, position, or both.

Far‐Ultraviolet Echoes from the Frontier between the Solar Wind and the Local Interstellar Cloud

We propose that first-order Fermi acceleration of Lyα photons should occur at the interface region between the solar and interstellar winds. In this process, photons are Doppler-shifted toward shorter wavelengths each time they are scattered by hydrogen neutrals that have been decelerated by charge exchange with protons in the transition region wrapping the heliopause. An additional component should therefore appear on the blue side of the line profile of the Lyα sky background emission. Its detection would not only reveal the existence of the heliopause interface but also trace its spatial distribution. We report here Hubble Space Telescope (HST) high-resolution Lyα line profiles of the sky background showing that the inner heliospheric hydrogen has a mean velocity of 23.2 ± 0.5 km s-1 in the upwind direction. This velocity corresponds to an averaged deceleration along the line of sight of the interstellar hydrogen of 2.8 ± 0.5 km s-1 after crossing the transition region. We show that an extra component, which we interpret to be the spectral signature of the Fermi process in the upwind direction, is present in the blue wing of the Lyα sky line profile. In addition, we interpret here the enigmatic excess in the Lyα sky brightness that was recently detected by Voyager deep in the inner heliosphere as being due to the additional Fermi emission. According to this interpretation, the distribution of this excess emission should also represent the spatial variation of the neutral velocity drop-off across the interface region. In this scope, we show that the region of the heliosphere's nose is ~12° away from the upwind axis—a good indication that the interstellar magnetic field deviates ~40° from the interstellar flow direction.