On the Formation of Extreme‐Ultraviolet Helium Lines in the Sun: Analysis ofSOHOData

Abstract

The resonance lines of helium in the Sun are several times stronger than expected, relative to lines of other ions. To explore the origins of this ''helium enhancement,'' we have studied data from the SUMER, CDS, MDI, and EIT instruments on the Solar and Heliospheric Observatory (SOHO). Time series data obtained in a quiet region and a coronal hole indicate that the spatio-temporal properties of the He i 584 8, and He ii 304 and 1084 8 lines are qualitatively unlike other chromospheric and transition region lines. Helium line intensities vary slowly compared to chromospheric oscillations and to transient transition region brightenings seen in other lines, yet they vary rapidly (both spatially and temporally) compared to the coronal radiation. This suggests that photoionization/recombination plays a minor excitation role in these lines. The Doppler shift of the 584 8 line shows a remarkably clear 4-8 mHz oscillation, with no accompanying line intensity oscillation. Transient brightenings are used to show that nonradial photon scattering might explain the previously found diffuse appearance of the network but cannot account for the reduced network/internetwork intensity contrast in the 584 8 line. We propose a new enhancement mechanism, based on arguments in a companion paper, in which predominantly neutral species such as helium diffuse across magnetic field lines into regions of hot coronal plasma, but charged ions do not. The enhanced internetwork helium emission may arise from the diffusion of helium atoms across the chromospheric canopy; the enhanced network emission may have contributions from diffusion from spicules. This mechanism may naturally explain why the 584 8 line shows Doppler shift, but not intensity, oscillations. Finally, if a mechanism such as ''velocity redistribution'' dominates helium emission, spectroheliograms of helium lines will be bright in regions of large temperature gradients parallel to the magnetic field. Differences between helium and other spectroheliograms may then reveal the relative contributions of ''classical'' and ''cool loop'' models to the observed emission.

Evidence of scattering effects influenced by plasma flows in C VI 1548 Å, 1550 Å spectral lines emitted from the Sun

“…There, the intensity ratio of lines doublets, such as the O vi 1032 Å and 1038 Å formed through radiative scattering, can be used to measure the solar wind velocity (Kohl & Withbroe 1982;Noci et al 1987). In active regions and also in impulsive events, such as flares, bright points, or jets, the resonance scattering of the radiation coming from these bright regions has a chance to compete with thermal emission (Judge & Pietarila 2004). Gontikakis et al (2013) showed that in active region NOAA 8541, the C iv 1548 Å and 1550 Å spectral lines' emission was caused by resonant scattering, in the vicinity of a microflare.…”

On the Formation of the Resonance Lines of Helium in the Sun

“…The companion paper (Judge & Pietarila 2004, hereafter Paper II) addresses some new solar observations from the Solar and Heliospheric Observatory (SOHO) spacecraft of relevance to the formation of the helium lines. As reviewed in Paper II, many earlier studies have addressed the problem of the solar helium lines.…”

On the nature of the unidentified solar emission near 117 nm

“…The generation processes of the helium lines He  at 30.38 nm and He  at 58.43 nm in the solar atmosphere are still a matter of debate (Jordan 1975;Andretta & Jones 1997;Macpherson & Jordan 1999;Andretta et al 2003;Ravindra & Venkatakrishnan 2003;Judge & Pietarila 2004). It is, however, undisputed that the upper chromosphere is optically dense for the He  58.43 nm line (Avrett 1999).…”