Helium is the second most abundant element in the universe after hydrogen. But scientists are not sure how much there is actually in the Sun’s atmosphere, where it is difficult to measure. Knowing the amount of helium in the solar atmosphere is important to understanding the origin and acceleration of the solar wind – the constant stream of particles charged by the Sun.
In 2009, NASA launched a rocket sound probe to measure helium in the expanded solar atmosphere – the first time we have compiled a complete global map. Results, recently published in Natural Astronomy, are helping us better understand our spatial environment.
Previously, when measuring the ratios of helium to hydrogen in the solar wind as it reached the Earth, observations found the ratios much lower than expected. Scientists suspected that the missing helium may have been left behind in the Sun’s largest atmospheric layer – the corona – or perhaps in a deeper layer. Discovering how this happens is essential to understanding how the sun’s wind is accelerating.
To measure the amount of helium and atmospheric hydrogen, NASA’s Corona and Heliosphere Helium Resonance Discharge, or HERSCHEL, rocket sound took pictures of the solar corona. Led by the Marine Research Laboratory in Washington, DC, HERSCHEL was an international collaboration with Astrophysical Observatory of Turin in Italy and Institute of Spatial Astrophysics in France.
HERSCHEL observations showed that helium was not evenly distributed around the corona. The equatorial region had almost no helium while the mid-latitude areas had the most. Comparison with images from ESA / NASA Solar and Heliospheric Observatory (SOHO), scientists were able to show the abundance in the middle latitude of overlaps with places where the lines of the Sun’s magnetic field open in the solar system.
This indicates that the ratio of helium to hydrogen is strongly related to the magnetic field and the speed of the solar wind in the corona. The equatorial regions, which had low measurements of helium abundance, matched the measurements from the solar wind near Earth. This shows that the solar atmosphere is more dynamic than scientists thought.
The HERSCHEL sound rocket probe adds a working organ that seeks to understand the origin of the slow component of the solar wind. HERSCHEL remotely investigates the elementary composition of the region where the solar wind has accelerated, which can be analyzed at the same time as in situ measurements of the internal solar system, such as those of the Parker Solar Test. While the heat of the Sun is enough to power the lightest element – ionized hydrogen protons – to escape the Sun as a supersonic wind, other physics should help power the acceleration of heavier elements like helium. Thus, understanding the elemental abundance in the Sun’s atmosphere provides additional information as we try to learn the full story of how the sun’s wind is accelerating.
In the future, scientists plan to take more observations to explain the change in abundance. Two new instruments – Metis and EUI aboard ESA / NASA’s Solar Orbiter – are able to make similar global measurements with abundance and willingness to help provide new information about the helium-to-corona ratio.
Reference: “Measurements of helium abundance in the solar corona” by John D. Moisiu, Ester Antonucci, Jeffrey Newmark, Frédéric Auchère, Silvano Fineschi, Marco Romoli, Daniele Telloni, Giuseppe Massone, Luca Zangrilli, Mauro Focardio Land, , Guglielmo Rossi, Andrea M. Malvezzi, Dennis Wang, Jean-Christophe Leclec’h, Jean-Pierre Moalic, Frédéric Rouesnel, Lucia Abbo, Aurélien Canou, Nicolas Barbey, Chloé Guennou, John M. Laming, James Lemen, Jean-Pierre Wuelser, John L. Kohl and Lawrence D. Gardner, July 27, 2020, Natural Astronomy.
DOI: 10.1038 / s41550-020-1156-6