Solar abundances have been historically assumed to be representative of cosmic abundances. However, our knowledge of the solar abundance of helium, the second most abundant element, relies mainly on models and indirect measurements through helioseismic observations, because actual measurements of helium in the solar atmosphere are very scarce.
In 2009, NASA launched a sounding rocket investigation to measure helium in the extended solar atmosphere.
For the first time, scientists have gathered a full global map of the helium and hydrogen emission in the solar corona.
To measure the amount of atmospheric helium and hydrogen, NASA‘s Helium Resonance Scattering in the Corona and Heliosphere, or HERSCHEL, the sounding rocket took images of the solar corona.
HERSCHEL’s observations demonstrated that helium wasn’t evenly dispersed around the corona. The equatorial region had no helium, while the areas at mid-latitudes had the most. Comparing with images from ESA/NASA’s Solar and Heliospheric Observatory (SOHO), the scientists were able to show the abundance at the mid-latitudes covers with where Sun’s magnetic field lines open out into the solar system.
This shows that the ratio of helium to hydrogen is strongly connected with the magnetic field and the speed of the solar wind in the corona. The equatorial regions with low helium abundance measurements matched measurements from the solar wind near Earth. This points to the solar atmosphere being more dynamic than scientists thought.
HERSCHEL remotely investigates 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.
In the future, scientists are planning to take more observations to explain the difference in abundances. Two new instruments—Metis and EUI onboard ESA/NASA’s Solar Orbiter—can make similar global abundance measurements and will help provide further information about the helium ratio in the corona.
- Moses, J.D., Antonucci, E., Newmark, J. et al. Global helium abundance measurements in the solar corona. Nat Astron (2020). DOI: 10.1038/s41550-020-1156-6