Eugene Parker, after whom the Parker Solar Probe was named, seems to have been the first to have accurately predicted the stream of particles emitted by the Sun that forms the ‘solar wind.’ Parker made the call in a 1958 paper, when solar sailing was just being noised about for the first time, so it wouldn’t have struck him that the term was a bit incautious. Today, when solar sailing is operational, people often assume the solar wind drives solar sails, when in fact the operating principle for solar sails is the momentum generated by photons, which are themselves massless. But streaming particles are indeed a kind of ‘wind,’ and there are magnetic sail concepts tailored for them too.

As always, we have to be careful about terminology, especially given the significance of the solar wind in defining our Solar System’s environment. Solar transients likewise have to be considered, because in addition to solar flares, we have to factor in coronal mass ejections (CMEs) and the particles accelerated by both of these. All of this as well as the interplanetary magnetic field shapes our star’s interactions with the local interstellar medium, creating the plasma envelope called the heliosphere. Only the Voyager spacecraft have returned data from the LISM, and they’re not very far into it.

We’ve looked at the Interstellar Probe concept in these pages before. The result of countless hours and numerous contributors at the Johns Hopkins Applied Physics Laboratory and elsewhere, the study was created for the Solar and Space Physics Decadal Survey and extensively analyzes everything from the launch vehicle to instrumentation for a mission that would exit the heliosphere and reach as far as hundreds of AUs within 50 years. Given that our knowledge of the realm beyond the heliosphere is almost entirely the result of remote sensing and indirect measurements, having an actual spacecraft on the scene would take us far beyond our modeling.

Image: The SWAP instrument aboard New Horizons has confirmed that the solar wind slows as it travels farther from the Sun. This schematic of the heliosphere shows that the solar wind begins slowing at approximately 4 AU radial distance from the Sun and continues to slow as it moves toward the outer solar system and picks up interstellar material. Current extrapolations reveal the termination shock may currently be closer than found by the Voyager spacecraft. However, increasing solar activity will soon expand the heliosphere and push the termination shock farther out, possibly to the 84-94 AU range encountered by the Voyager spacecraft. Credit: Southwest Research Institute; background artist rendering by NASA and Adler Planetarium.

Because I’m talking about the outer boundaries of the heliosphere today, a quick word about how remote sensing ‘sees’ them is appropriate. We have data from the IBEX satellite (Interstellar Boundary Explorer) covering an entire solar cycle from 2009 through 2019. Although it’s in Earth orbit, IBEX detects energetic neutral atoms (ENAs) from the outer regions of the heliosphere, the zone where solar wind particles begin to collide with those of the even less understood interstellar wind. We are in essence mapping a region by sending a signal – actually using the Sun’s ‘signal,’ the particles of the solar wind – deep into the edge of the system and trying to make sense out of the return echoes. The IMAP (Interstellar Mapping and Acceleration Probe), set for launch in 2025, will further examine this region from its own vantage at the L1 Lagrange point.

We can draw a lot of conclusions from the data from such missions, but not enough. Sarah Spitzer (University of Michigan), lead author of a paper on a just released study that analyzes how best to exit the heliosphere, notes the significance of the JHU/APL work: “Without such a mission, we are like goldfish trying to understand the fishbowl from the inside.”

Indeed. Considered as a kind of shield, the heliosphere acts as a brake on galactic radiation, but predicting its effects, and even more significantly its shape and size over time, is all but impossible. We tend to refer to the heliosphere as a ‘bubble,’ but it’s a poor term given that the shape changes with solar output and interactions with the LISM. A variety of potential shapes for the heliosphere remain in play in the literature. Have a look at the figure below, which represents just one of the numerous possibilities.

Image: One recent study of the heliosphere’s shape posited a croissant-shape or small spherical shape with tail lobes. Credit: Figure adapted from Merav Opher et al., 2020.

As Spitzer and colleagues note:

Small variations in model parameters and properties measured in the nose of the heliosphere, the leading edge in the direction of the Sun’s motion through the LISM, lead to significant differences in the projected shape. The global response of the heliosphere is additionally expected to fluctuate with solar activity and therefore solar cycle…, which is yet another element in the interplay of heliosphere–interstellar interactions and another factor in our uncertainty of the overall shape. The main features of the shape of the heliosphere include the nose; the tail, which can be defined relative to the Sun as the region of the heliosphere found in the direction opposite the Sun’s motion through the LISM; the size of the heliosphere; and the behavior of the magnetic field lines at the heliospheric boundaries. All of these properties, including the size of the heliosphere, vary vastly in different proposed shapes and models of the heliosphere, which range in size anywhere from hundreds to thousands of au and in shape from comet- or magnetosphere-like or otherwise with extended tails… to spherical to croissant-shaped with multiple tail lobes…

Bear in mind that the heliosphere is a moving target. As recently as 60,000 years ago, the Sun entered what is called the Local Interstellar Cloud, and is now considered to be at that cloud’s edge or perhaps beyond it and in contact with surrounding clouds. The JHU/APL Interstellar Probe study notes that within 2000 years, our system will likely be within a completely different interstellar environment. The heliosphere will adjust. Let me quote the Interstellar Probe report on this, because the effects are startling when we consider the Sun’s 20 revolutions around the galactic core since its formation:

The orders-of-magnitude differences in interstellar properties have had dramatic consequences for the penetration of interstellar gas, dust, and galactic cosmic rays (GCRs) that have affected elemental and isotopic abundances, chemical atmospheric evolution, and perhaps even biological evolution. Along the evolutionary path, high interstellar cloud densities and ionization fractions have likely compressed the heliosphere down to below 25 au… Evidence is emerging for supernovae explosions as recent as 3 million years ago at only 20–50 pc from the Sun that probably compressed the heliosphere even below the orbit of Saturn and perhaps more, exposing the terrestrial planets to almost the full force of interstellar material and GCRs…

The GCR’s referenced above are galactic cosmic rays, high energy particles and heavy nuclei that can prove lethal to biology. The shielding effects of the heliosphere are all too easy to take for granted until we consider its malleable nature, so the more we learn about what affects it the better. The JHU/APL work highlights a probe trajectory that will not only sample the local interstellar medium but give us the ‘look back’ capability to see it as a whole, with the probe exiting the heliosphere at approximately 45 degrees off the heliopause nose direction.

This trajectory is attractive because it allows the heliopause and local interstellar medium to be reached within a reasonable timeframe, which in this context means something less than 50 years with current propulsion technologies. It also offers what the authors call “somewhat of a side view of the heliopause,” though one in the direction of the nose, and it allows the IBEX ribbon, a still puzzling region of enhanced Energetic Neutral Atom (ENA) emission in the outer heliosphere, to be probed.

But a trajectory through the IBEX ribbon center is only one of those that Spitzer and team analyze, ranging from the heliosphere nose to various angles out of the heliotail, the trailing part of the heliosphere in relation to the Sun’s motion through the medium. Here again I’m reminded of how little we know of the heliosphere’s shape, for some estimates of the heliotail have it extending more than 5000 AU downwind of the Sun. Hence the value of this paper, which assembles what our indirect observing methods have so far produced by way of data on the various defined parts of the heliosphere.

To facilitate mission planning, the paper proposes continued indirect measurements of ENA and the pickup ions (PUI) that facilitate a stronger ENA flux, emphasizing the heliotail, and measurements of interstellar ions that penetrate the heliosphere, including cosmic rays in the heliotail region. For the in situ measurements, the authors point out that the largest differences between the suggested shapes of the heliosphere would appear in the tail region. Our probe would thus do best to exit through the side of the heliosphere’s tail. Better still, the authors say, would be a two-spacecraft Interstellar Probe mission option reminiscent of the twin Voyager missions, one moving toward the nose, the other in the direction of the heliotail.

A great idea, but try to get that through the various funding entities… Even so:

Only a study of the tailward region of the heliosphere will give definitive evidence for the complete shape, which impacts how the heliosphere interacts with the LISM and therefore how the LISM impacts the composition of the heliosphere. Therefore, complementary in situ and indirect interstellar measurements must be made tailward within the heliosphere. These measurements can be made through the use of intentional instrumentation requirements for outer heliosphere missions. Additionally, it would be beneficial for the Interstellar Probe mission to either consider a trajectory through the heliotail, via the flank, or to comprise a unified mission of two spacecraft, in which a second Interstellar Probe would be launched with a tailward trajectory, perhaps intersecting one of the proposed…tail lobes.

The paper is Spitzer et al, “Complementary interstellar detections from the heliotail,” Frontiers in Astronomy and Space Sciences (08 February 2024). Full text.