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

For the first time, we present and discuss EUV radiances of the solar transition region (TR) and corona obtained during a solar cycle. The measurements were obtained with the SOHO/coronal diagnostic spectrometer (CDS) during the period from 1996 to 2010. We find that limb-brightening significantly affects any characterisation of the solar radiances. We present the limb-brightening function for the main lines and find that it does not change measurably during the cycle. We confirm earlier findings that the radiance histogram of the cooler lines have a well defined, log-normal quiet-Sun component, although our results differ from previous ones. The width of the lowest-radiance log-normal distribution is constant along the cycle. Both the analysis of the centre-to-limb variation and of the radiance statistical distribution point to a constant quiet Sun emission along solar cycle 23. Lines formed above 1 MK are dramatically affected by the presence of active regions, and indeed, no "quiet Sun" region can be defined during periods of maximum activity. Much of the irradiance variability in lines formed below 1.5 MK is due to a change in the emitting area. For hotter lines, the emitting area saturates to almost 100% of full solar disk at the maximum of activity, while simultaneously the emission due to active regions increases by more than an order of magnitude. We show that structures around active regions, sometimes referred to as dark halos or dark canopies, are common and discuss their similarities and differences with coronal holes. In particular, we show how they are well visible in TR lines, contrary to coronal holes.

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“…To facilitate comparisons with the results of Pietarila & Judge (2004), we also computed power-law fits:…”

Section: Centre-to-limb Variation In the Quiet Sun Radiancesmentioning

confidence: 99%

The EUV spectrum of the Sun: SOHO CDS NIS radiances during solar cycle 23

Andretta

Zanna

2014

A&A

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“…In this, the second of two papers, we study some time series data, some spectroheliograms, and a magnetogram from various instruments on the Solar and Heliospheric Observatory (SOHO) spacecraft. In the first paper (Pietarila & Judge 2004, henceforth ''Paper I''), our emphasis was on reviewing the different models that have been proposed to explain why the helium lines are anomalously bright. The first quantitative demonstration that something was seriously amiss came from Jordan (1975), who built on the atomic excitation calculations by Hearn (1969aHearn ( , 1969b.…”

Section: Introductionmentioning

confidence: 99%

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

Judge

Pietarila

2004

ApJ

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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.

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“…The STEREO mission observes He II 30.4 nm emission. The problem is that helium lines are atypical (see, e.g Pietarila & Judge 2004)…”

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The Magnetic Future of the Sun

Judge

Egeland

Metcalfe

et al. 2017

ApJ

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We analyze space-and ground-based data for the old (7.0 ± 0.3 Gyr) solar analogs 16 Cyg A and B. The stars were observed with the Cosmic Origins UV Spectrographs on the Hubble Space Telescope (HST) on 23 October 2015 and 3 February 2016 respectively, and with the Chandra X-ray Observatory on 7 February 2016. Time-series data in Ca II data are used to place the UV data in context. The UV spectra of 18 Sco (3.7±0.5 Gyr), the Sun (4.6±0.04 Gyr) and α Cen A (5.4 +1.2 −0.2 Gyr), appear remarkably similar, pointing to a convergence of magnetic heating rates for G2 main-sequence stars older than ≈ 2 − 4 Gyr. But the B component's X-ray (0.3-2.5 keV) flux lies 20× below a well-known minimum level reported by Schmitt. As reported for α Cen A, the coronal temperature probably lies below that detectable in soft X-rays. No solar UV flux spectra of comparable resolution to stellar data exist, but they are badly needed for comparison with stellar data. Center-to-limb (C-L) variations are re-evaluated for lines such as Ca II through to X-rays, with important consequences for observing activity cycles in such features. We also call into question work that has mixed solar intensity-intensity statistics with flux-flux relations of stars.

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

Cited by 50 publications

References 58 publications

The EUV spectrum of the Sun: SOHO CDS NIS radiances during solar cycle 23

The EUV spectrum of the Sun: SOHO CDS NIS radiances during solar cycle 23

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

The Magnetic Future of the Sun

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