Where is the “cosmic shield” of the solar system thin enough to allow the high-energy particles to escape? The heliosphere has commonly been called a bubble which has been blown up by the solar wind, although the more appropriate metaphor is that of a living boundary: as the magnetized plasma escapes, it encounters the local interstellar medium and remodels itself as the activity of the Sun increases and decreases. According to NASA, it is a massive bubble that is formed by the wind of the Sun and that surrounds the planets and regulates what enters the planets out of the galaxy.
Voyager 1 and Voyager 2 made that abstraction geometry. The edge of the heliosphere was mapped in two directions and at two different times by each spacecraft, giving us two scarcely-obtained, hard-earned “pins” on a map that would otherwise have to be deduced. Those crossings, of Voyager 1 in 2012 and Voyager 2 in 2018, are often used since they are the only direct in situ measurements of the location of the boundary and its behavior. The dates are not important, but what the measurements tell us: that the heliosphere is not homogeneous, and that the laws which control cosmic rays are not the same everywhere.
The charged particles are cosmic rays and the magnetic structure of the heliosphere defines the ease with which they are threading in. The traditional history of transport physics explains that the heliospheric magnetic field deflects and dissipates these particles and varies the strength of these particles with energy, location, and time through a mechanism referred to as modulation. Practically that implies that there are areas in which cosmic rays are more effectively suppressed and areas in which they more readily enter a discontinuous “hit map” instead of having just one number that represents the total shielding of the solar system.
Of particular consequence is the outer heliosphere. Modulation can be produced in high conditions between the termination shock at which the solar wind is decelerated, and the heliopause, at which the solar and interstellar material is segregated. A number of galactic cosmic rays reaching the inner solar system is affected by turbulence, varying magnetic geometry as well as large-scale flows. Cosmic rays alone can also be used as diagnostics even in the absence of a camera at the boundary: the intensity of such rays can be used to determine the physical structure of the heliosphere in a region where spacecraft can hardly pass.
It is the reason why the heliosphere mapping had shifted to indirect imaging using “atomic messengers.” IMAP NASA Interstellar Mapping and Acceleration Probe is extremely dependent on elements of the energetic neutral atoms (ENAs), which are formed when speedy ions exchange charge with leisurely neutrals. The lack of charge causes them to move in straight lines, and this means that sensors close to Earth can follow them to the source regions that are far away and form a series of boundaries that cannot be seen at all. NASA underlines the fact that the imaging provided by IMAP will be 30 times more resolved than the previous world maps, and that the Voyager point samples will be viewed in context and not as isolated wonders.
The vantage position of IMAP also brings the distant boundary nearer to practical implications, being at Lagrange Point 1, which combines boundary imaging with near-real-time measurements of the solar wind and energetic particles. The same physics which determines the outer edge of the heliosphere also determines the variation in the radiation conditions across the inner solar system, with implications on technology and human risk in spaceflight.
The “map” of Voyager can be thought of as a framework thus two boundary crossings, which proved the shielding to be directional, decades of particle measurements which proved modulation to be dynamic. IMAP is a further development of that structure to a complete picture of the sky over time a picture which can determine where the cosmic rays have struck most fiercely, and where the defensive mechanisms of the solar system are strongest.
