A composite image of the sun showing the hydrogen (left) and helium (center and right) in the empty corona. The helium at depletion at the equatorial regions is evident. Credit: NASA
Helium is the second most abundant element in the universe after hydrogen. But scientists are not sure exactly how much there is in the atmosphere of the sun, where it is difficult to measure. Knowing the amount of helium in the solar atmosphere is important for understanding the origin and acceleration of the solar wind – the constant flow of charged particles from the sun.
In 2009 NASA launched a sound rocket study to measure helium in the extended solar atmosphere – the first time we have collected a complete global map. The results, recently published in Natural Astronomy, help us better understand our space environment.
HERSCHEL sounding rocket launches from the White Sands Missile Range, New Mexico. Credit: Missile Range for White Sands
Earlier, when measuring ratios of helium to hydrogen in the solar wind as it reached Earth, observations found much lower ratios than expected. Scientists suspected that the missing helium may have been left in the extreme atmospheric layer of the sun – the corona – or perhaps in a deeper layer. Discovering how this happens is key to understanding how the solar wind accelerates.
To measure the amount of atmospheric helium and hydrogen, Helium Resonance Scattering in the Corona and Heliosphere, as HERSCHEL, took sounding rocket images of the solar corona. Led by the Naval Research Lab in Washington, DC, HERSCHEL was an international collaborator with the Turin Astrophysical Observatory in Italy and the Institute of Spatial Astrophysics in France.
HERSCHEL’s observations showed that helium was not evenly distributed over the corona. The equatorial region had almost no helium, while the mid-latitude regions had the most. Comparison with images from ESA / NASA’s Solar and Heliospheric Observatory (SOHO) the scientists were able to show the abundance at the medium latitudes that overlap with where Sun’s magnetic field lines emerge in the solar system.
This shows 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, with a low measure of helium abundance, corresponded to measurements of the solar wind at Earth. This suggests that the solar atmosphere is more dynamic than scientists thought.
The HERSCHEL sound rocket study adds to a body of work that seeks to understand the origin of the slow component of the solar wind. HERSCHEL remotely examines the elemental composition of the region where the solar wind is accelerated, which can be analyzed in tandem with in situ measurements of the inner solar system, such as those of the Parker Solar Probe. While the heat of the sun is enough to power the lightest element – ionized hydrogen protons – to escape the sun like a supersonic wind, other physics should help to accelerate heavier elements such as helium. Thus, understanding elemental abundance in the atmosphere of the sun provides additional information as we attempt to learn the full story of how the sun accelerates wind.
A composite image shows the sun with open magnetic field lines (colored) that overlap with regions with enhanced helium abundance. Credit: NASA
In the future, scientists plan to take more observations to explain the difference in abundance. Two new instruments – Metis and EUI aboard ESA / NASA’s Solar Orbiter – are capable of making similar measurements for abundance worldwide and willing to provide new information on the helium ratio in the corona.
Reference: “Global helium measurements in the solar corona” by John D. Moses, Ester Antonucci, Jeffrey Newmark, Frédéric Auchère, Silvano Fineschi, Marco Romoli, Daniele Telloni, Giuseppe Massone, Luca Zangrilli, Mauro Focardi, Federico Landini, Maurizio Pancrazzi, 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
