Compared to its value in the solar convective envelope, the helium abundance in the in-situ measurements of the fast and slow solar wind has long been known to be depleted relative to hydrogen, with occasional transient exceptions (Bochsler 1998, SSRv 85, 291). In the slow solar wind, the degree of depletion has more recently been shown to depend upon the wind speed and the level of solar activity (Aellig et al. 2001, GeoRL 28, 2767). Measurements of the helium abundance in the corona, associated to measurements of the coronal outflow velocity, will provide evidence for the degree of correlation between wind speed and helium abundance and allow identification of the source regions of the slow wind streams with different helium abundance. During quasi-corotation the intrinsic evolution of magnetic topology will be observed and thus its influence on the wind parameters (such wind outflow velocity and helium abundance) will be directly assessed. The abundance can be derived from simultaneous observations of the resonantly scattered component of singly ionized helium by EUI/FSI in its 30.4 nm channel and of that of neutral hydrogen by Metis in Lyα (121.6 nm). The analysis of the data will require also simultaneous Metis pB images and synoptic EUI/FSI 17,4.  The former can provide estimates of the electron density, while the latter can be used to asses the contribution of the Si XI 30.3 nm line in the EUI/FSI 30.4 bandpass.

While this is essentially an EUI/Metis sub-objective, in-situ instruments may be interested also. For instance, SWA/HIS will measure the α/p density ratio.

In addition, the upcoming ground-based Daniel K. Inouye Solar Telescope (DKIST) will in principle be capable of providing measurements of the emission from neutral helium in the corona below 1.5 R_Sun (coronagraphic observations in the He I 1083 nm line with Cryo-NIRSP). Although neutral helium is a very minor species in the corona and therefore does not add direct information on the total number of helium atoms and ions, measurements of that species will help constrain the ionization temperature of helium.  The preferred configuration will require Solar Orbiter in cojunction with Earth.  A Science Use Case (SUC 60) has been proposed for inclusion in the DKIST Critical Science Plan (link: https://nso-atst.atlassian.net/browse/UC-60.)

This SOOP is also designed to be run in support of the Herschel sounding rocket flights. The First instance of this SOOP was run during the second flight of the payload (LTP6), but the mission failed. A third launch is planned for March 29 2023.