The outer edge of the solar system has never been an easy thing to name that hard to measure. This boundary became a physical location with instruments present at Voyager, as it crossed into interstellar space, and even more recent sky maps have begun to refine the boundary. The short-term reward is not a more beautiful picture of the heliosphere, but better knowledge of how the cosmic rays of the galaxy creep through the protective envelope of the Sun and when that protection becomes weaker or stronger.
The point is that the heliosphere is not a solid wall. It is a contact point at which the magnetized plasma of the solar wind pushes into a partially ionized local interstellar medium creating a sequence of structures that culminates in the termination shock, the inner heliosheath and the heliopause. Voyager 2 also gave one of the closest boundary probes in place when it produced a abrupt transition of particle-population: a decrease in low-energy ions were accompanied by an increase in higher-energy cosmic rays, in line with crossed the heliopause at 119 au. The same crossing was also not like that of Voyager 1, which highlights the fact that the boundary is not uniform, but is structured and variable. Beyond the heliopause, Voyager 2 detected a transition region as the outbound, low-energy particles continued to flow and the intensity of the cosmic rays decreased as compared to their levels even farther away, and this is a reminder that even the zone around the heliopause is “interstellar space.”
Remote sensing has now started to complete the geometry of the interface of the modulation by cosmic rays on a global scale: what two point measurements cannot provide. The most significant approach is the use of energetic neutral atoms (ENAs) in which fast ions trade charge with neutral atoms and then proceed in straight line directed directional information. IBEX was the first to take this step and unveiled the “ribbon” a thin belt of amplified ENA emission whose geometry follows the interstellar magnetic field. An in-depth study of ribbon variations in 20092019 revealed that the brightness of the ribbon is strongly associated with the previous conditions in the solar wind, with a maximum correlation coefficient of a pixel of about 0.9 and an average lag of approximately 4 years in at least one ecliptic direction of the ribbon-a signature that is observable and is expected of the secondary-ENA mechanism, where the solar-wind-borne neutrals travel far beyond the heliopause and subsequently return as ENAs subsequently as a result of re-ionization and re
Such time-delay method transcends correlation. By disaggregating the lag into outward travel, reprocessing time beyond the heliopause and inbound travel, investigators are able to determine the distance of the effective region of the ENA sources relative to the surface of Earth and draw a distance scale of the heliopause in the various directions. One of them obtained a heliopause-distance map, which drastically changes with latitude, with big implied distances on the north polar region and significantly shorter ones on areas of the upwind hemisphere. The same article stated that these patterns do not resemble a plain, symmetrical single-tail image, but instead resemble more the shape of a “croissant” with two tail lobes, instead of one. These shape constraints are significant since cosmic-ray access also requires magnetic topology and minimum thickness of the boundary as much as it requires distance.
The sharper “edge map” makes the meaning of cosmic-ray measurements different. The in situ detectors on Voyager reveal the changing intensities and anisotropies in fine layers; ENA maps can tell how such layers and magnetic geometries are probably repeated throughout the sky. Practically, the combination can mitigate ambiguity: a minimum in the strength of the cosmic-ray density at a boundary can be considered within a global context of the most favourable regions of magnetic field orientation and charge-exchange source in general to transport, instead of being a purely local anomaly.
The following advance in the clarity is the NASA IMAP mission which aims to expand the ENA imaging to a much broader range of energies and with higher resolution than IBEX. IMAP is constructed at a fixed point close to the SunEarth L1 point, to convert the weak signals of the particles into global structure, with the same ENA “straight line messenger” concept that has been used to make the ribbon visible. IMAP will bridge the gap between solar-cycle variations nearby the Earth with local measurements of particles by imaging the boundary, the combination of which will link the delayed effects at the outer boundary of the heliosphere, precisely the type of coupling suggested by the multi-year lag of the ribbon.
The constant hazard of deep-space travel includes cosmic rays which are the tracer of interaction between the heliosphere and the galaxy. Since the measurement of the boundaries of Voyager in terms of ENA derived geometry is getting more and more detailed, the shielding of the heliosphere is turning into a quantifiable system with specific vectors of pathways, delays and boundary layers not merely a far boundary line on a graph.
