The instruments of the voyager discovered an area at the system edge that had an appraised temperature of particles of about 30,00050,000 in kelvin. The term “wall” might be confused with the word masonry, yet the engineering fact is in a less obvious form: a demarcation where particles are rarefied having tremendous energy and also transfer very less heat to a passing spacecraft.
The outer edge of the heliosphere, the bubble of the Sun, which encloses the planets and flattens the onslaught of high-energy particles in the interstellar space, is that boundary. The heliopause is created when the repulsive force of solar wind is balanced with the force of the interstellar medium. Practically, it acts not like a line on a map, but like an active interface that is moulded by the solar activity, magnetic fields and the galactic environment around it.
The Voyager crossings transformed an imaginary frontier into a measured passage. The data of the spacecraft highlighted the reason behind the label the “wall of fire”: particle populations in these are sometimes very energetic, resulting in spectacular temperature readings in a region that is so sparse that a probe does not heat up as in the case of dense gas. That distinction of temperature as particle energy and temperature as experienced heat is an aspect of the conceptual shock that Voyager presented to the public, and an aspect of the technical difficulty of engineers who develop sensors to read plasma in regimes much more extreme than that experienced on earth.
Another finding that was facilitated by Voyager was the unexpected magnetic continuity at the boundary. It was found that the magnetic field immediately outside the heliopause is parallel to the field within the heliosphere, a finding which made it challenging to draw simple images of a sudden magnetic “flip” at the edge. The designers of spacecrafts would find these results important since particle transport and the noise environment of antennas as well as the behavior of plasma waves requires a strong dependence on magnetic geometry.
The second jump was made not by a distant probe but by a satellite that was close to the earth through an ingenious type of remote sensing. IBEX allowed researchers to construct a 3D distance map of the locations of such interactions by observing energetic neutrons of neutral atoms formed as charged solar-wind particles exchange the electrons with interstellar atoms. The method consisted of the time-of-return signaling: the patterns in the solar wind conditions are replicated in the number of the neutral atoms arriving at the device in several years. The finding limited the heliopause to a more or less 110-120 AU on the direction of the “nose” of the heliosphere and the downwind direction stretching at least 350 AU within the reach of the method. And that is the headline implication of the new map that is asymmetry.
Instead of taking the shape of a pretty oval, the heliosphere seems to be deformed, elongated, stretched, and sensitive to the winds and calm periods of the solar wind. Recent overviews have characterized the net shape as a “deflated crescent,” underlining that even the Sun conditions as well as the local interstellar medium have moulded the shield of the Sun. This view also associates heliosphere with astrospheres of other stars. X-ray observations of a young Sun-like star reveal an encircling bubble that is pushed by an outflow of gas that is more rapid and more concentrated than the present day solar wind that provides a relative perspective how stellar winds and the gas surrounding the system may reshape its protective bubble as time progresses.
The overlaying of Voyager crossing into the heliopause and IBEX mapping the entire world leads to the redefinition of the heliopause as a far-off milestone to a man-made domain: a quantifiable, changing interface with its structure capable of mapmaking. That chart is not an outline any longer it is a working geometry that helps to instruct the next generation of voyages that will explore interstellar space using more purposely oriented and focused instrumentation and a clearer goal than Voyager ever had.
