Nose Of Heliopause Research Articles

Influence of the interstellar magnetic field and 11-year cycle of solar activity on the heliopause nose location

Context. The heliosphere is formed by the interaction between the solar wind (SW) plasma emanating from the Sun and a magnetised component of local interstellar medium (LISM) inflowing on the Sun. A separation surface called the heliopause (HP) forms between the SW and the LISM. Aims. In this article, we define the nose of the HP and investigate the variations in its location. These result from a dependence on the intensity and direction of the interstellar magnetic field (ISMF), which is still not well known but has a significant impact on the movement of the HP nose, as we try to demonstrate in this paper. Methods. We used a parametric study method based on numerical simulations of various forms of the heliosphere using a time-dependent three-dimensional magnetohydrodynamic (3D MHD) model of the heliosphere. Results. The results confirm that the nose of the HP is always in a direction that is perpendicular to the maximum ISMF intensity directly behind the HP. The displacement of the HP nose depends on the direction and intensity of the ISMF, with the structure of the heliosphere and the shape of the HP depending on the 11-year cycle of solar activity. Conclusions. In the context of the planned space mission to send the Interstellar Probe (IP) to a distance of 1000 AU from the Sun, our study may shed light on the question as to which direction the IP should be sent. Further research is needed that introduces elements such as current sheet, reconnection, cosmic rays, instability, or turbulence into the models.

The Twist of the Draped Interstellar Magnetic Field Ahead of the Heliopause: A Magnetic Reconnection Driven Rotational Discontinuity

Abstract Based on the difference between the orientation of the interstellar B ISM and the solar magnetic fields, there was an expectation that the magnetic field direction would rotate dramatically across the heliopause (HP). However, the Voyager 1 spacecraft measured very little rotation across the HP. Previously, we showed that the B ISM twists as it approaches the HP and acquires a strong T component (east–west). Here, we establish that reconnection in the eastern flank of the heliosphere is responsible for the twist. On the eastern flank the solar magnetic field has twisted into the positive N direction and reconnects with the southward pointing component of the B ISM. Reconnection drives a rotational discontinuity (RD) that twists the B ISM into the −T direction and propagates upstream in the interstellar medium toward the nose. The consequence is that the N component of B ISM is reduced in a finite width band upstream of the HP. Voyager 1 currently measures angles ( ) close to solar values. We present MHD simulations to support this scenario, suppressing reconnection in the nose region while allowing it in the flanks, consistent with recent ideas about reconnection suppression from diamagnetic drifts. The jump in plasma β (the plasma to magnetic pressure) across the nose of HP is much greater than in the flanks because the heliosheath β is greater there than in the flanks. Large-scale reconnection is therefore suppressed in the nose but not at the flanks. Simulation data suggest that B ISM will return to its pristine value 10–15 au past the HP.

VOYAGER 1 NEAR THE HELIOPAUSE

Recent observations from the Voyager 1 spacecraft show that it is sampling the local interstellar medium (LISM). This is quite surprising because no realistic, steady-state model of the solar wind (SW) interaction with the LISM gives an inner heliosheath width as narrow as ~30 AU. This includes models that assume a strong redistribution of the ion energy to the tails in the pickup ion distribution function. We show that the heliopause (HP), which separates the SW from the LISM, is not a smooth tangential discontinuity, but rather a surface subject to Rayleigh-Taylor-type instabilities which can result in LISM material penetration deep inside the SW. We also show that the HP flanks are always subject to a Kelvin-Helmholtz instability. The instabilities are considerably suppressed near the HP nose by the heliospheric magnetic field in steady-state models, but reveal themselves in the presence of solar cycle effects. We argue that Voyager 1 may be in one such instability region and is therefore observing plasma densities much higher than those in the pristine SW. These results may explain the early penetration of Voyager 1 into the LISM. They also show that there is a possibility that the spacecraft may start sampling the SW again before it finally leaves the heliosphere.

Electric Sail Mission Analysis for Outer Solar System Exploration

Missions towards the boundaries of the Solar System require long transfer times and advanced propulsion systems. An interesting option is offered by electric sails, a new propulsion concept that uses the solar wind dynamic pressure for generating a continuous thrust without the need for reaction mass. The aim of this paper is to investigate the performance of such a propulsion system for obtaining escape conditions from the Solar System and planning a mission to reach the heliosphere boundaries. The problem is studied in an optimal framework, by minimizing the time to reach a given solar distance or a given hyperbolic excess speed. Depending on the value of the sail characteristic acceleration, it is possible that, in an initial mission phase, the sailcraft may approach the Sun to exploit the increased available thrust due to the growing solar wind electron density. The corresponding optimal trajectory is constrained to not pass inside a heliocentric sphere whose admissible radius is established by thermal constraints. Once the escape condition is met, the sail is jettisoned and the payload alone continues its journey without any propulsion system. A medium performance electric sail is shown to have the potentialities to reach the heliosheath, at a distance of 100 AU, in about fifteen years. Finally, the Interstellar Heliopause Probe mission is used as a reference mission to further quantify the electric sail capabilities for an optimal transfer towards the heliopause nose (200 AU).

Draping of the local interstellar magnetic field over the heliopause

A recent theory for radio events at 2–3 kHz observed by the Voyager spacecraft suggests that the emission is generated when shocks associated with global merged interaction regions (GMIRs) enter a region just beyond the heliopause nose that is primed with an enhanced level of superthermal electrons. In this GMIR/priming theory the superthermal electrons are accelerated by lower hybrid waves generated by pick‐up ions. For this acceleration to be efficient the pick‐up ion ring speed vr and the Alfvên speed vA must satisfy the inequality vr/vA ≲ 5, implying that the local magnetic field B must be sufficiently large. Here this constraint is used to predict which regions generate radio emission by calculating the draping of the interstellar magnetic field B∞ over the heliopause using the convected field equations and a gas‐dynamic simulation of the solar wind–VLISM interaction. The size and shape of the regions with large ∣B∣ are predicted to depend on the orientation of B∞ relative to the interstellar flow velocity. For sufficiently perpendicular orientations the high ∣B∣ region is a linear band parallel to B∞ in the plane of the sky, centered near where the surface is closely parallel to B∞, but the band shape is only a ∼10% effect compared with a circular surrounding region. The magnetic amplification factor increases with decreasing distance to the heliopause nose and increasingly perpendicular orientation of B∞, with factors ≳5 typical within axial and transverse distances to the nose of 5 and 35 AU, respectively. Combining the magnetic amplification with plausible neutral and plasma parameters, the constraint vr/vA ≲ 5 requires B∞ ≳ 0.06 nT for the GMIR/priming theory to operate within the draping region. A recently proposed constraint, that B be nearly perpendicular to the normal vector to the GMIR surface for effective electron acceleration by the GMIR shock, is also considered. A supporting argument is provided for the previous claim that this constraint predicts strong emission in a band perpendicular to B∞: Calculations show that the shock‐accelerated electrons produce significant emission only for distances parallel to B that are small (≈1 AU) compared with the macroscopic regions on the shock where B · ≈ 0. This predicted source orientation agrees well with observations of the source and an independent estimate of the direction of B∞ based on Lyman‐α observations. It is argued that the B · ≈ 0 constraint is a natural component of the GMIR/priming theory. The large, relatively circular nature of the draping region where vr/vA ≲ 5 will plausibly lead to the constraint B · ≈ 0 determining the intrinsic source shape in the plane of the sky.

Reconnection at the heliopause

In this MHD-model of the heliosphere, we assume a Parker-type flow, and a Parker-type spiral magnetic field, which is extrapolated further downstream from the termination shock to the heliopause. We raise the question whether the heliopause nose region may be leaky with respect to fields and plasmas due to non-ideal plasma dynamics, implying a breakdown of the magnetic barrier. We analyse some simple scenarios to find reconnection rates and circumstances, under which the heliosphere can be an “open” or a “closed” magnetosphere. We do not pretend to offer a complete solution for the heliosphere, on the basis of non-ideal MHD theory, but present a prescription to find such a solution on the basis of potential fields including the knowledge of neutral points. As an example, we imitate the Parker-spiral as a monopole with a superposition of homogeneous asymptotical boundary conditions. We use this toy model for x < − R , where R = 100 AU is the distance of the termination shock to describe the situation in the nose region of the heliopause.

3D high-speed escape heliocentric trajectories by all-metallic-sail low-mass sailcraft

Considering a photon solar-sail spacecraft with a lightness number in the range [12–1) (all-metallic sail), this paper analyses a class of heliocentric trajectories, characterized by orbital angular-momentum reversal, delivering a considerable cruise speed (10–20 AU/yr) and allowing a sailcraft to access any celestial latitude [−π2, +π2], depending primarily on the perihelion distance. It is found out that, if the lightness vector is kept constant in the heliocentric orbital frame, the sailcraft in the solar gravitational field is able to decrease, to zero and reverse its orbital angular momentum, then to accelerate while swinging-by the Sun safely and, finally, to get a cruise speed considerably higher than its heliocentric initial one. No planetary launch window is involved. The present 3D theory and its numerical check by a full sailcraft dynamics computer code represent the extension of the two-dimensional theory developed in the last two years. Some examples of deep-space missions are presented. Current and near-term technological aspects are indicated. A demonstration mission to 50 AU and to ecliptic latitude of 20 ° could be accomplished in 5.6 years (perihelion at 0.25 AU). A flight to 200 AU to the heliopause nose direction can last 14.3 years (perihelion at 0.20 AU), exhibiting a launch window of 15 days every year and with a max flight time penalty less than 1 yr.