FEATURE ARTICLE
The Galactic Environment of the Sun
The heliosphere appears to protect the inner solar system from the vagaries of the interstellar medium
Priscilla Frisch
Material Inside the Heliosphere
Although the ions in the interstellar medium are deflected around the heliosphere, a majority of the neutral interstellar atoms (mostly hydrogen and helium) flow cleanly into the solar system. Remarkably, about 98 percent of the diffuse gas within the heliosphere (excluding material associated with planetary bodies and comets) is interstellar material. In fact, the densities of the interstellar material and the solar wind are equal near the orbit of Jupiter. These surprising results can be understood if one appreciates that the solar wind must fill an increasingly large volume of space in the outer solar system, so that its density decreases with the inverse square of its distance from the sun. In contrast, the density of the neutral component of the interstellar wind changes very little as it flows through the heliosphere, until it is finally ionized.
The first discovery of interstellar matter within the solar system was made in the 1960s by a spacecraft observing the earth's geocorona, a layer of neutral hydrogen atoms that forms in the outermost part of the planet's atmosphere. The spacecraft detected a weak fluorescent glow of Lyman-alpha ultraviolet radiation—effectively a "marker" for neutral hydrogen—that had a different spatial distribution than the geocorona. A Lyman-alpha photon is emitted when an electron in a neutral hydrogen atom falls from the first excited energy level of the atom to the ground level. In interstellar space, the hydrogen is comparatively "cold" so that electrons are in the ground state. However, when neutral interstellar atoms flow into the solar system and approach the sun, the intense Lyman-alpha photon radiation from the solar atmosphere pumps the electron into the first excited state. The electron then naturally decays down to the lowest energy level again and emits a Lyman-alpha photon in the process, creating a weak interplanetary ultraviolet glow. (Recent results from the TRACE instrument aboard the SOHO satellite provide sensitive maps of the interplanetary Lyman-alpha glow, showing active regions of Lyman-alpha emissions on the backside of the sun.)
Since this discovery in the 1960s, many other manifestations of interstellar matter have been discovered within the solar system. Astronomers now know that most of the interstellar hydrogen atoms are ionized within several AU of the sun, partly by photo-ionization from solar radiation and partly by charge exchange with the solar wind. The helium atoms, on the other hand, penetrate to within a fraction of an AU of the sun before they are ionized by the solar photons. Some neutral helium atoms escape ionization, however, and are attracted by the sun's gravitation to form a focusing cone downwind of the sun. The earth passes through this focusing cone at the end of November every year.
As the interstellar atoms are ionized, they are "picked up" by the solar wind plasma and swept out to the heliosphere's termination shock. Since these pickup ions are products of the interaction between the solar wind and the neutral atoms of the interstellar medium, their measurement offers clues to the composition of the interstellar medium. Helium pickup ions were originally discovered near the earth by a team led by Eberhard Möbius, now at the University of New Hampshire, in the mid-1980s. More recently, as the Ulysses spacecraft left the inner solar system, the onboard SWICS instrument (of George Gloeckler at the University of Maryland and Johannes Geiss at the International Space Sciences Institute in Maryland) was able to detect and identify additional elements in the pickup-ion population, including nitrogen, neon and oxygen, as well as isotopes of helium and neon. Each of these elements is found partially in neutral form in interstellar gas, and the neutrals can enter the heliosphere without diversion by the Lorentz forces. Comparing the abundances of pickup ions with the abundances of ions in the nearby interstellar gas provides important clues about the original ionization level of the cloud feeding interstellar material into the solar system.
Once the pickup ions reach the termination shock they are accelerated up to cosmic-ray energies, forming a component known as the anomalous cosmic-ray population. This anomalous population is seen as a bump tacked onto the low-energy end of the galactic cosmic-ray spectrum. These particles are "anomalous" because their energies are too low for them to have entered the heliosphere from the outside, indicating that they must have formed within the solar system. As it happens, these anomalous cosmic rays return to the inner solar system where some are captured by the earth's magnetosphere. In other words, these particles zip back and forth through the heliosphere: They are blown into the solar system as interstellar neutral atoms, blown out to the termination shock as pickup ions and then returned to the inner solar system as anomalous cosmic rays!
Atomic particles are not the only visitors from outer space that find their way into the solar system. A team led by Eberhard Gruen of the Max-Planck Institute discovered "large" dust grains (between 0.2 and 6 micrometers in diameter) inside the heliosphere with dust detectors aboard the Ulysses and Galileo satellites. These dust grains were flowing with the same velocity and direction as the Local Interstellar Wind. (Smaller dust grains are charged and therefore excluded from the solar system by Lorentz forces just outside the heliopause.) The largest dust grains have trajectories that are relatively unaffected by the solar wind or solar-activity cycles. Much like the interstellar helium atoms, these dust particles are focused downwind of the sun, and the earth passes through this focusing cone at the end of November every year. Dust grains of intermediate sizes may be focused in the plane of the ecliptic or diverted from the plane, according to the polarity of the magnetic field embedded in the solar wind, which changes every 11 years with the phase of the solar cycle. (Once again the Lorentz force is important since it binds these charged interstellar grains to the solar wind.)
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