June 10, 2020
The Interstellar Medium is sparse.
As written in previous Pictures of the Day, particles in space are far more diffuse than any vacuum on Earth. The best vacuums created on Earth reach a 0.1 millimeter distance between individual atoms. Between stars, however, there is only one atom per cubic centimeter, while in the intergalactic voids it is thought that there is only one atom for every ten cubic meters.
The Interstellar Medium (ISM), the region between stars, is composed of gas and dust grains that are less than one-tenth of a micron in size. One micron is equal to one-millionth of a meter, so they are four times smaller than the frequency of blue light (0.450 microns). Since the dust particles are that small, blue light is scattered by the ISM, so more long wavelengths reach Earth than without the dust. Called “interstellar reddening”, it is the same phenomenon that makes red sunrises and sunsets.
An important point is that the ISM contains ionized particles. Since the ISM is not electrically neutral, it should be thought of as a plasma. It is those electrons and positive ions that are critical to understanding the behavior of the ISM and how the Solar System interacts with it. If electric charge separation takes place in different regions of the ISM, weak electric fields develop. Electric fields, no matter how weak, initiate electric charge flow, or electric currents.
As plasma pioneer, Irving Langmuir determined, plasma in a laboratory isolates itself with thin walls of oppositely charged double layers, so it is probable that the same thing is happening around the Sun in the ISM.
Since the Sun is positively charged with respect to the ISM, what is known as a “Langmuir sheath” forms around it. In the common interpretation of stars, the sheath is thought to be a “shock front”, because the heliosphere is “plowing through” the ISM. So, as conventionally assumed, it must be shaped like a teardrop, since the “pressure” of the ISM ought to compress it on the leading side and then stream away from the following side. Artistic renderings see it as comet-like.
However, IBEX satellite observations did not conform to the assumptions. IBEX scanned the sky, looking for the quantity and magnitude of energetic neutral atoms (ENA). ENAs form when protons combine with electrons and become electrically neutral. As ionized particles, they spiral in the Sun’s electromagnetic field, but once they combine, they fly away in a straight line.
In the consensus view, solar wind protons acquire electrons in collisions with hydrogen atoms already located in the heliosheath. If it is teardrop shaped, ENAs would be distributed around the sky in a more-or-less uniform pattern. IBEX observations revealed a “ribbon” of increased ENAs perpendicular to the galactic magnetic field. The teardrop is actually a sphere.
As mentioned, Langmuir sheaths, in which opposite charges build up near each other, create electric fields between them, so those double layers accelerate ions. Where the solar magnetosphere, or heliosphere, meets the dissimilar charge of the ISM, the two regions form a Langmuir sheath, which leads to a spherical formation.
Stephen Smith
The Thunderbolts Picture of the Day is generously supported by the Mainwaring Archive Foundation.
