The Solar Helium Abundance in the Outer Corona Determined from Observations with SUMER/SOHO

Abstract

At altitudes of about 1.05 solar radii or more, the corona above quiet solar regions becomes essentially isothermal. This obviates many of the difficulties associated with the inverse problem of determining emission measure distributions, and allows for fairly straightforward relative element abundance measurements. We present new values for the He abundance. The first is based on a reanalysis of the He/O ratio studied previously using data acquired by SUMER. A more thorough evaluation of the atomic physics for He II, including a detailed treatment of radiative recombination, increases the predicted emission in the He II Balmer series compared to earlier analyses. We use a recently revised value of the O abundance to derive an He/H abundance ratio of 0.038, (mass fraction, Y = 0.13), with an error of ∼ 17% coming mainly from the O abundance uncertainty. We demonstrate that this result may be affected by gravitational settling of O relative to He. We also derive an abundance for He by direct comparison to emission lines of the H I Lyman series, with the result He/H = 0.052 ± 0.005 (Y = 0.17), a value similar to He abundances determined in the slow speed solar wind.

Observations of the 2019 April 4 Solar Energetic Particle Event at the Parker Solar Probe

“…However, He/H in SEPs can vary greatly from event to event and even in SIRs throughout the solar cycle (Lario et al 2003). The He/H ratio in this event does match that of 0.052±0.005 measured spectroscopically in the corona (Laming & Feldman 2001). Although it also agrees well with the ∼0.05 measured in high-speed solar wind or at solar maximum at all speeds, it is large compared to the ∼0.02 found in slow solar wind at solar minimum (Aellig et al 2001), conditions that describe the actual observing environment.…”

Neon and Oxygen Absolute Abundances in the Solar Corona

“…The emissivities of the H i lines have been calculated using the method and atomic data described in Feldman et al (2005) and Laming & Feldman (2001), where the reader is referred for further details; calculations included full treatment of electron impact ionization, proton and electron collision excitation rates.…”

Morphology, dynamics and plasma parameters of plumes and inter-plume regions in solar coronal holes

“…The composition of the solar photosphere can, with the exception of the noble gases, be directly determined from spectroscopic observations (cf., Grevesse and Sauval 1998). In the corona, attempts to measure the helium abundance have been made by Gabriel et al (1986) and Laming and Feldman (2001); for argon and neon abundances see Fig. 25(a).…”