Taking well-known boundaries at Earth as examples, by construction the plasmapause and magnetopause are well defined. The plasmapause (e.g., Pierrard et al., 2021) is defined as the boundary where a strong gradient in plasma density occurs in the inner magnetosphere such that there are almost no more low energy particles outside of this boundary. It is thus identified based on density measurements. The Earth’s magnetopause (e.g., Kivelson and Russell, 1995), by contrast, is defined as a magnetic boundary and thus typically identified using magnetic field data. It is the current sheet that separates regions where either the solar or the terrestrial magnetic field dominates. Importantly, it must be noted that for the magnetopause, because of the occurrence of magnetic reconnection, vast amounts of solar plasma are penetrating through the boundary and populating large regions of the Earth’s magnetosphere at all times (boundary layers, cusp, plasma mantle, etc.). Although there are regions on its inside with plasma from the external medium, the magnetopause is not defined based on which particle populations are observed, but is defined as the outermost current sheet where the magnetic field rotates to become solar-dominated.

The term “Heliopause”, by contrast, leaves some ambiguity on its definition. It refers to the outer boundary of the heliosphere but does not say if identification should be based on thermal plasma, energetic particles or field observations, and on what signature thereof. Voyager 1 and 2 have clearly entered a region that has much higher plasma density (Gurnett et al., 2013; Gurnett and Kurth 2019), contains significant amounts of interstellar plasma populations (e.g., GCR; Webber and MacDonald, 2013), where at least some particle populations of heliospheric origin (e.g., ACR; Webber and MacDonald, 2013) seem to have disappeared and where plasma flows have changed significantly (Richardson et al., 2019; Dialynas et al., 2021; Richardson, 2021; Giacalone et al., 2022; Richardson et al., 2022). The vast majority of the community holds these facts as overwhelming evidence that Voyager 1 and 2 have exited into the VLISM.

However, in addition to locations closer than initially predicted by models, the purported heliopause crossings at Voyager 1 and 2 show increases in magnetic field strength higher than expected (but cf. Fuselier and Cairns, 2013; Cairns and Fuselier, 2017), as well as magnetic shears across the heliopause that are much lower than remote observations and models would predict at both spacecraft (Burlaga et al., 2013; 2019). The magnetic field past the heliopause was found to be very stable with unexpectedly high magnetic field and solar-like orientation (Burlaga and Ness 2016), and this remains true to the current day for both Voyager 1 and 2 for many AUs beyond the heliopause. The chances for both spacecraft to have crossed the heliopause, if defined based on magnetic field measurements alone, at locations where the magnetic shear is so low appears extremely unlikely. In addition, recent analyses suggest there exists a radial plasma flow at the heliopause at both Voyager 1 and 2, outward from the inner heliosheath out into the VLISM. Such observations are at odds with theoretical expectations for the outer heliosphere and heliopause (Cummings et al., 2021; Dialynas et al., 2021; Fuselier et al., 2021; Richardson et al., 2022), although they are proposed to be consistent with the model of Fisk and Gloeckler (2022), as discussed below. Note, however, that there are instrumental uncertainties with the particle measurements near the heliopause at Voyager 2 (Richardson, 2021).

There is currently no consensus on the interpretation of these magnetic field and flow observations. It points to the lack of appropriate low energy plasma measurements on Voyager 1 and 2, which are critical to determine bulk plasma properties in the few eV to few tens-of-keV range (thermal particles in solar wind, heliosheath, and VLISM, as well as PUIs). We argue in this WP that magnetic reconnection plays a pivotal role in solar-interstellar interaction and in structuring the neighborhood of the heliopause, whatever its definition.

Many recent works have studied the occurrence and impact of magnetic reconnection at the heliopause (e.g., Swisdak et al., 2010; 2013; Fisk and Gloeckler, 2014; 2022; Strumik et al., 2014; Fuselier and Cairns, 2017; Opher et al., 2017; Fuselier et al., 2020). A prime characteristic of magnetic reconnection is that despite being triggered at small scales (electron and ion scales), it affects the system on a global scale (MHD and much beyond, e.g., the dynamics of whole magnetospheres, solar corona, etc.). In particular, magnetic reconnection allows plasma to protrude across discontinuities that otherwise would be impermeable. A second fundamental impact of magnetic reconnection is that it affects the system’s magnetic topology at very large scales (e.g., Hesse and Cassak, 2019).

This latter fact is illustrated in Figure 1A for the Earth’s magnetosphere. The context displayed is that of an interplanetary magnetic field (IMF) that has an orientation similar (nearly parallel) to that at the nose of the Earth’s magnetosphere. In such a case, the magnetic shear across the magnetopause is low. Magnetic reconnection does not occur near the nose (as it does for southward IMF), but instead occurs at high latitudes above both polar cusps (as marked with blue arrows) and may lead the same IMF field line to reconnect twice with the Earth’s magnetic field (green field line labelled 3). Although reconnection occurs over extended X-lines (out of the plane of the figure), the reconnection region is very small in comparison to the large-scale topological changes that affect the whole dayside magnetopause surface (here the magnetopause is defined as the current sheet separating the solar and Earth’s magnetic fields, just inside of field line 3). This large-scale change in topology leads to significant changes in the plasma properties in the dayside regions. Particles flow across the open magnetopause and form various types of boundary layers on both sides of the current sheet (inside and outside), with signatures of the penetration, disappearance and mixing of populations of various types and energies as a function of time, location, species, etc. These effects are very well documented at Earth (e.g., Gosling et al., 1990).

The outer heliospheric system is obviously different, but similarities also exist. This is illustrated in Figure 1B that displays the complex, wound structure of the magnetic field in the outer heliosphere (adapted from Opher et al., 2017). Although Voyager 1 and 2 observations show that the magnetic shear across the heliopause is lower than expected, the magnetic field orientation in the VLISM is expected to have a dominant orientation mostly parallel to that in the inner heliosheath at the nose. For that reason, recent modeling found that reconnection is not expected to occur along the nose (e.g., Swisdak et al., 2010; Fuselier and Cairns, 2017). It is instead expected on the flanks of the heliopause, as marked with blue arrows in Figure 1B. As shown by Opher et al. (2017), in such a configuration the same wound (Parker spiral) heliospheric field line may reconnect twice with the VLISM at widely separated locations (blue reconnection regions marked with arrows). Because it allows the same field lines to reconnect twice, this process bears an interesting resemblance with the Earth’s magnetosphere under northward IMF, as depicted in Figure 1A. As explained next, such topologies would mean that there exists complex, intricate signatures in particle and field measurements in the vicinity of the heliopause (cf; Fuselier and Cairns (2017) for instance).

Figure 1A illustrates the complex signatures expected in suprathermal electron flow (i.e., pitch angle) anisotropies at Earth, as an example. Toward the top of Figure 1A, next to field lines numbered (1 - black), (2 - blue) and (3 - green) the small blue and red vectors highlight pristine (i.e., rather cold) and heated, respectively, suprathermal electron flows. Heated electrons are the signature of the spacecraft being located on a reconnected field line, and the direction of the suprathermal electron flow signals the direction of the reconnection region. The small red arrows pointing northward next to field lines 2) and 3) at the top of Figure 1A signal the fact that reconnection has occurred southward along those field lines. However, the reconnection region may very well be at a very distant location (given the very high speed of suprathermal electrons) as is the case here with the reconnection region being located all the way above the cusp in the opposite, southern hemisphere. Field line 3) shows heated electrons also coming from the north (southward red arrow) because it is reconnected there also (field line 3) is doubly reconnected), but field line 2) shows pristine (cold, blue arrow) electrons coming from the north because this field line has not reconnected (yet) at this end. Depending on the timing of magnetic field line reconnection at both locations, as the spacecraft traverses the magnetopause it typically observes heated solar wind electrons streaming in both parallel and anti-parallel directions outside the magnetopause on doubly reconnected field lines (field line 3)) as well as unidirectional heated electrons which signal singly reconnected field lines (field line 2). The latter unidirectional heated electrons are observed on the outermost, sunward side of this boundary layer. Although not detailed here, the newly closed field lines eventually sink into the magnetosphere and form broad boundary layers inside the magnetopause (e.g., Lavraud et al., 2006; Lavraud et al., 2018; Øieroset et al., 2008). In other words, crossing the magnetopause from any given side one expects to observe a very complex, layered structure with variable particle properties corresponding to the mixing of particles from both sides (such complex signatures are observed at Earth also at other energies and for other species).

The double reconnection scenario of heliospheric and VLISM field lines unveiled in the simulation of Figure 1B should similarly lead to specific particle signatures in the broad vicinity of the heliopause. Such topological considerations have been put forth to explain the fact that the drop in ACR fluxes is not collocated (it occurs further outside) with the main magnetic field increase at the Voyager 1 and 2 heliopause crossings (e.g., Krimigis et al., 2013; Fuselier et al., 2020). Because ACRs are produced within the Heliosphere, their drop outside the reported heliopause crossings (as well as the timings and anisotropies of various species at different energies) is taken as evidence of particle leakage due to reconnected field lines in the vicinity of the heliopause, but remote from Voyager 1 and 2 (e.g., Fuselier et al., 2020), or other instabilities (cf. introduction). But the complete disappearance (in all directions) of ACRs has also been interpreted as an escape (in both parallel and anti-parallel directions) along doubly reconnected field lines as shown in Figure 1B (since the ACRs originate inside the Heliosphere). In this scenario, even past the drop in ACRs, the Voyager 1 and 2 spacecraft would remain magnetically connected to an open heliopause and thus remain inside the main current sheet that would signal the effective transition into the pristine VLISM (e.g., Fisk and Gloeckler, 2022). Particle measurements with complete directionality information are critical to fully investigate the large-scale heliopause magnetic topology.

To first order, the analogy between Figures 1A and 1B stops at the fact that the same field lines may reconnect twice at widely separated distances (when conditions at the nose are not favorable for reconnection to occur there). This is because the signatures in terms of magnetic field (pile-up, orientation) and particles species, energies, and anisotropies are widely different in both contexts (different particle sources, coexistence of multiple populations (e.g., PUIs), magnetic geometries, system size, presence/absence of shocks, instabilities at play, etc.). As further discussed in the next section, the width of these possible heliopause boundary layers, although not known, could be substantial. Furthermore, the non-vanishing outward radial flow observed at both Voyager 1 and 2 near the heliopause is not consistent with most models, and comparison to the Earth’s magnetosphere does not provide insight in that respect. Determining where reconnection occurs, how plasma flows near the heliopause, and the ensuing large-scale magnetic topology requires more complete observations, in particular in the few eV to few tens-of-keV range (thermal and PUI). These are critical to better define the heliopause, its location and its shape.

Finally, it should be noted that magnetic reconnection along the heliopause has been proposed as one of the possible mechanisms to explain the ribbon structure of energetic neutral atom fluxes in global observations from the Interstellar Boundary Explorer (IBEX). However, given the alternating polarity structure of the heliospheric magnetic field along significant portions of the heliopause it is unclear how this process could explain the configuration and steadiness of the ribbon (McComas et al., 2009; McComas et al., 2011). The latter point has another important implication, and that is that the locations of magnetic reconnection on the heliopause surface, and therefore the large-scale magnetic topology of the heliopause, should change over the course of the solar cycle given the alternating magnetic field polarity.

The plasma properties of the outer heliosphere differ greatly from those of the inner heliosphere where magnetic reconnection has been widely studied. In particular, the pressure in the accumulated PUIs becomes larger than the solar wind thermal pressure already upstream of the termination shock, and at the termination shock the pickup ions are heated more than the thermal solar wind (Richardson et al., 2008; Mostafavi et al., 2017; Mostafavi et al., 2018). The plasma Beta conditions also largely change in the outer Heliosphere, past the termination shock, at and beyond the heliopause in the VLISM. Also, while asymmetric reconnection should be expected at the heliopause, symmetric reconnection may be occurring at current sheets in the inner (Drake et al., 2017) and outer heliosheath, as well as in the VLISM (although surprisingly no evidence has been reported for reconnection in the inner heliosheath so far, and in the VLISM current sheets are expected to be much less frequent). Studying magnetic reconnection in all these regions would thus provide significant new insights into our understanding of magnetic reconnection in new plasma regimes.

For example, concerning the inner heliosheath, Opher et al. (2011) proposed that the magnetic field and flows are not laminar, but instead consist of magnetic islands that develop upstream at the heliopause through reconnection (see also Schwadron and McComas (2013) regarding flux ropes at the heliopause). As discussed above the location of reconnection at the heliopause remains unclear, but it has been suggested that large pressure gradients across the heliopause due to PUIs produce a diamagnetic drift that may stabilize magnetic reconnection. This adds to the low magnetic shear observed at the Voyager 1 and 2 crossings, which likely precludes reconnection from occurring locally at the nose (Swisdak et al., 2010; 2013; Fuselier and Cairns, 2017; Opher et al., 2017; Fuselier et al., 2020). Magnetic reconnection may, however, develop at other locations on the flanks of the heliopause where conditions are satisfied thanks to higher magnetic shear (Opher et al., 2017).

Near the Earth’s magnetosphere and in the solar wind, where high accuracy thermal plasma measurements are available, detailed studies of the energy conversion and partitioning associated with reconnection can be made (e.g., Burch et al., 2016). Although there are obvious payload limitations compared to near-Earth missions, studying such questions in the vastly different outer heliosphere regimes would require dedicated low energy thermal plasma measurements, including PUIs, on a future Interstellar Probe. Studying the properties of reconnection in different regimes would also benefit from different types of wave measurements. This is true for example for the study of wave—particle interactions, but also because wave data can provide complementary measurements of plasma moments (such as density, as has been done on Voyager 1 and 2 in the absence of density measurement from thermal particle instruments (e.g., Gurnett and Kurth 2019; Kurth and Gurnett, 2020).

Using again an analogy with the Earth’s magnetopause: the timing, location, geometry and rate of magnetic reconnection should produce various layers of different widths as a function of species, energy and pitch angle. These boundary layers are not solely confined to one side (i.e., magnetosheath boundary layer marked on the outside of the magnetopause in Figure 1A), but rather are present on both sides of the current sheet (solar wind populates boundary layers on the inside of the magnetopause and the entire cusp region). If magnetic reconnection affects the topology of the heliopause on very large scales as illustrated in the simulation of Figure 1B, there is no doubt that significant effects in particle and field properties should be observed near the heliopause, but also possibly up to large distances inside and outside, although that remains unknown.

Indeed, the widths of the reconnection boundary layers on both sides of the current sheet depend on the reconnection rate (which depends on local plasma properties), on the distance from the reconnection region, as well as on the profile in Alfvén speed along the field lines as the reconnection kinks propagate on either side of the boundary. Again, the expected widths of the layers remain unclear for the heliopause. Some studies propose rather thin boundary layers, based on particle properties around the heliopause (Fuselier and Cairns, 2017; Fuselier et al., 2020). But other studies suggest the observed ACR leakage is actually occurring well inside the actual heliopause (which would not be crossed yet), in a much broader inner heliopause boundary layer that results from a reconnection process akin to that depicted in Figure 1B (e.g.; Fisk and Gloeckler, 2016; Fisk and Gloeckler, 2022). Again, in an analogy to the case of the Earth depicted in Figure 1A, it should be noted that the boundary layer that forms on the Earthward side (the high magnetic field side) of the magnetopause as a result of the process of double magnetic reconnection is typically of the order of one Earth radius, which is a significant size compared to the scale size of the dayside magnetosphere (∼10% of the ∼10 Earth radii magnetopause stand-off distance). If similar types of boundary layers were forming at the heliopause, they could be of very significant extents, possibly tens of AUs inside or outside the heliopause (depending on its definition). However, we lack the appropriate measurements on Voyager 1 and 2, particularly thermal plasma properties, to undoubtedly estimate the key parameters and decipher the complex structuring of all the expected types of boundary layers and their extents.

Boundary layers produced by magnetic reconnection are often intricately linked to draping and pile-up processes. This is true at the Earth’s magnetopause and is likely also true at the heliopause. Draping has been proposed to occur in the outer heliosheath, outside the heliopause, with an associated magnetic pile-up and plasma depletion region (e.g., Schwadron et al., 2009; Burlaga et al., 2013; Fuselier et al., 2020; Kornbleuth et al., 2021; Mostafavi et al., 2022), the thickness of which has been estimated as ∼5 AU (Fuselier and Cairns, 2013). The location of the plasma depletion layer has been proposed to be displaced from the nose, unlike at Earth, based on calculations that account for the different partitioning between thermal, kinetic and magnetic pressures in a high Beta environment (Fuselier and Cairns, 2013). As is the case for the Earth’s magnetosphere, the width, extent and location of this layer may impact where reconnection is triggered, as well as the stability of the reconnection process (Fuselier et al., 2000; Lavraud et al., 2005). An open question regarding these layers, however, is that while Gurnett et al. (2013) first reported densities of ∼0.09–0.11 cm⁻³ past the heliopause, higher densities up to ∼0.14 cm⁻³ were actually measured at distances ∼20 AU beyond, as reported in Gurnett and Kurth (2019) and Kurth and Gurnett (2020) for both Voyager spacecraft.

A plasma depletion layer has also been studied (Izmodenov and Alexashov 2015) on the inside of the heliopause. As for the outer heliosheath pile-up region, the magnetic field pile-up forms gradients that decelerate and deflect plasma flows around and to the side of the obstacle formed by the heliopause. As a result, plasma density is depleted from the middle of the inner heliosheath to the heliopause. However, the effects of charge exchange between interstellar ions and neutrals should be significant and affect plasma flows and density depletion both inside and outside the heliopause. How these plasma depletion and magnetic pile-up layers develop and affect the interactions at the heliopause remain open questions. Proper low energy plasma measurements are required to address these questions.

Finally, using MHD simulations Opher and Drake (2013) suggested that the draping outside the heliopause is strongly affected by the solar magnetic field, although the underlying physical mechanism was not understood. They showed that as the VLISM magnetic field approaches the heliopause, it twists and acquires an east–west component, an effect that did not occur if the solar magnetic field was not present in the simulation. This draping could explain the twist of the magnetic field and the unexpectedly low magnetic shear at Voyager 1 and 2 (cf. also Grygorczuk et al., 2014; Isenberg et al., 2015; Opher et al., 2017). This possibility, however, remains to be confronted by observations and such observations, in particular in terms of low energy particles, are lacking.

To tackle science questions related to reconnection at the heliopause, a number of important and complementary measurements are needed on such a mission. These include magnetic field, plasma wave, suprathermal and energetic particles (up to ACR and GCR energies) instruments (since such are available on Voyager 1 and 2 they are not the focus of this WP), but also importantly low energy particle measurements that are not present onboard Voyager 1 and 2 (although operating the Faraday Cups do not have appropriate fields-of-view (FoV) on Voyager 2 at and beyond the heliopause). Low-energy particle measurements should cover the few-eV to few-tens-of-keV range, which include the thermal solar wind, heliosheath and VLISM populations, as well as PUIs. Other science questions and requirements related to PUIs are addressed in other WPs. We thus focus on measurement requirements for the lowest energy thermal populations here.

Bulk flow and thermal energies of low energy ions and electrons typically fall in the following ranges as a function of regions: solar wind from a few eV to 20 keV, inner heliosheath from eV to keV and outer heliosheath and VLISM are likely only in the few eV range, although this is not well constrained (e.g., Zank, 2015). Interstellar Probe shall fly further than 200 AU (possibly beyond 400 AU) into interstellar space. This leads to constraints in terms of requirements and design, the most critical being as follows.

• Ion and electron populations range from the dense solar wind at one AU to the very tenuous ionized medium at 200 AU and beyond, with a factor 10^–6 lower fluxes/count rates expected. Measurements thus require low noise, high dynamic range detectors.

Two types of instruments can be foreseen to address these requirements, in a complementary fashion: Faraday cup (FC) and electrostatic analyzer (ESA). Having both would permit important redundancy of parts of the critical measurements (basic moments of the distribution function such as density, velocity and temperatures).

FCs are simpler in design and require fewer resources. FCs have proven to be a robust design for long term use on many missions, as for example on Voyager 2. An advantage of FCs is that it is straightforward to increase their geometry factor, as would be critical to detect low plasma densities. FCs are typically designed for measuring low energy ions, but given the low cadences required in the outer heliosphere the FCs for an interstellar mission should also include an electron mode. Because the spacecraft is expected to be spin-stabilized, three to four FC heads with appropriate FoV should be sufficient to make three-dimensional sampling and provide appropriate plasma moments. Each head may be sectorized to perform flow measurements. FCs have naturally a low response to noise from energetic particles, as these do not provide a significant current. But appropriate electronics should be used to suppress internal noise sources, and accurate high-voltage power supplies should be designed for the very low energies of the inner and outer heliosheath and VLISM.

An ESA instrument is fundamental as it provides information on the full particle distribution function (temperature anisotropies, pitch angles measurements, easier separation of species, etc.). It should have a 360° planar FoV (possibly 180°) and measure the full three-dimensional distribution function thanks to the spacecraft spin. A critical design aspect for that type of instrument concerns the high dynamic range of fluxes expected, which impacts the type of detector—Channel Electron Multipliers or Micro-Channel Plates—that should be used. While the former is typically more robust with time (lifetime) and has higher intrinsic dynamic range, the latter requires significantly lower resources (power). Another critical design aspect for such type of instrument is the signal-to-noise ratio, which owing to penetrating radiation and electronics clearly will require appropriate mitigation measures. An obvious concept to be used is a coincidence scheme, which should allow to reduce system background noise down to <10^–4 s⁻¹

As for the FCs, the ESA should be designed with low noise electronics and accurate high-voltage supply system down to low voltages (3 eV baseline, which is consistent with measurements to below the expected spacecraft potential of +5 V). To save significant resources, the instrument ideally should be able to measure both ions and electrons with the same detector head. State-of-the-art methods should be designed to perform such alternated measurements of ions and electrons to optimize front-end and high-voltage resources. Performing such alternated measurements of ions and electrons does not impact science requirements since measurement cadences do not need to be high in the outer heliosphere and VLISM.

The ESA instrument may be designed with a Time-of-Flight system to perform composition measurements down to low energies. In such a case, a clever design would be needed to perform both ions and electrons with the same head. Alternatively, half of the 360° FoV could be devoted to electrons and the other for ions. Or yet two separate heads/instruments could be used to ensure that ion composition is performed. It may be argued, however, that a low energy ESA instrument could be limited to measuring all ions without composition, but yet providing some composition information through energy separation (if the bulk flow is larger than the thermal speed), given that a dedicated PUI instrument with composition is required anyway and may go down to complementary low energies. While a PUI instrument could have limited angular resolution (e.g., 22.5°), an ESA instrument dedicated to the solar wind and VLISM should ideally have higher angular resolution (≤10°), at least in appropriate portions of the FoV. This aspect requires more detailed trade-off studies to find the appropriate design. Finally, we shall also note that the ESA instrument could be designed with a deflection system at the entrance to increase the FoV of the instrument when the spacecraft will be three-axis stabilized during planetary fly-bys, but this would be at the expense of reasonable, but non-negligible resources. We remind that more details on the measurement requirements (resolutions in energy, angle, time, mass etc.) are provided in the Mission Concept study Report (MCR) at https://interstellarprobe.jhuapl.edu/Interstellar-Probe-MCR.pdf).