On the Origin of the Basal Emission from Stellar Atmospheres: Analysis of Solar CiiLines

Abstract

Combining a variety of data with radiation hydrodynamic simulations, we examine the heating of the Sun's internetwork chromosphere and the hypothesis that the chromospheric '' basal '' emission arises because of acoustic wave dissipation. We focus on the 2s 2p 2 2 D 2s 2 2p 2 P o multiplet of C ii near 1335 Å , whose basal level of chromospheric emission has been reliably determined for stars and the Sun by Schrijver and colleagues. By accounting for center-to-limb variations and the different spectral bandpasses of the instruments used, we find that Schrijver's C ii solar basal intensity substantially exceeds stellar values, and that it can be identified with intensities seen in typical internetwork regions with the SUMER instrument on the SOHO spacecraft. Some time-series data sets of internetwork regions are then examined and compared with simulations made specifically for a typical observational data set, with vertical velocities at the lower boundaries fixed from observations with the MDI instrument on SOHO. The simulations can qualitatively account for the observed internetwork UV continuum fluctuations seen with SUMER, formed 0.6-0.85 Mm above the photosphere. However, they fail to capture almost any property of the observed internetwork C ii multiplet, which is formed substantially higher. The time-averaged simulations can account for between 1 7 and 1 4 of the C ii basal intensities; they predict oscillatory power between 5 and 10 mHz, whereas internetwork observations are dominated by low-frequency (<2 mHz) power of solar origin. The average simulated C ii intensities, which have a large contribution from the transition region heated by conduction down from a coronal upper boundary, fall short even of the smaller stellar basal intensities by a factor of !2. Together with known properties of weak, internetwork photospheric magnetic fields, we conclude that the internetwork upper chromosphere is probably dominated by magnetic heating. Thus, the solar basal (and internetwork) intensities of the C ii 1335 Å multiplet originate from magnetic, and not acoustic, mechanisms, in contradiction to the commonly accepted picture

Semiempirical Models of the Solar Atmosphere. II. The Quiet‐Sun Low Chromosphere at Moderate Resolution

“…It is possible that wave motions driven from the photosphere would increase in amplitude and develop into strong shocks as they propagate upward. The observations do not confirm the existence of shocks that can provide enough heating to explain radiative losses of the upper chromosphere (e.g., Judge et al 2003 andFossum &Carlson 2006), although some heating of the low chromosphere appears possible from the numerical simulations. However, motions in the low chromosphere with velocities compatible with the observed line broadening or shift (which is usually not more than $1Y2 km s À1 in lines formed near the temperature minimum) cannot completely explain the observed extension of the CO line emission above the solar limb for about 5 scale heights (Ayres & Rabin 1996).…”

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

“…The computed behavior is not in obvious disagreement with these observations. If anything, data exist to suggest that, at least for the C ii 1335 multiplet, such models as we have constructed underestimate the effects of photon scattering (Judge, Carlsson, & Stein 2003 Consider a plane-parallel slab of thickness Z that is ''effectively thin'' in all spectral lines, for which the incident radiation at frequencies in the lines can be neglected. This is a valid approximation for the formation of most ions in the solar transition region and corona, because the bulk of the solar radiative flux at visible and IR wavelengths streams through these plasmas, and the UV and EUV flux from the lower atmosphere is small compared with the line emission.…”

“…Assuming that this line is formed under the VR process, say, at T e % 3 ; 10 4 K, this would mean that the 584 8 line is blueshifted by between 0 and 5 km s À1 relative to the redshifted C ii 1037 8 lines (Fig. 6 of Peter & Judge 1999) but is not redshifted at all relative to the 1335 8 multiplet of C ii (Judge et al 2003). These He i 584 8 Doppler shifts are substantially less than the value of v NT % 10-14 km s À1 near 2 ; 10 4 K (Mariska et al 1978).…”

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

“…It seems likely that something is missing from the physics of the formation of upper chromospheric features that is not captured by the one-dimensional semiempirical models developed by Vernazza et al (1981). (Judge, Carlsson, & Stein 2003 have recently discussed this point. )…”