The Solar Chromosphere/Corona Interface. I. Far‐Ultraviolet to Extreme‐Ultraviolet Observations and Modeling of Unresolved Coronal Funnels

Abstract

A coronal funnel model, developed by D. Rabin, was tested against a calibrated spectroheliogram recorded in the 170-175 Å bandpass. This image was recorded on board a sounding-rocket experiment flown on 1994 November 3, called the Multi-Spectral Solar Telescope Array II (MSSTA II). MSSTA, a joint project of Stanford University, the NASA Marshall Space Flight Center, and the Lawrence Livermore National Laboratory, is an observing platform composed of a set of normal-incidence, multilayer-coated optics designed to obtain narrow-bandpass, high-resolution images (1 00 -3 00 ) at selected far-ultraviolet (FUV), extreme-ultraviolet (EUV), and soft X-ray wavelengths (44-1550 Å ). Using full-disk images centered at 1550 Å (C iv) and 173 Å (Fe ix/x), the funnel model, which is based on coronal back-heating, was tested against the data incorporating observed constraints on global coverage and measured flux. Found was a class of funnel models that could account for the quiescent, globally diffuse and unresolved emission seen in the 171-175 Å bandpass, where the funnels are assumed to be rooted in the C iv supergranular network. These models, when incorporated with the CHIANTI spectral code, suggest that this emission is mostly of upper transition region origin and primarily composed of Fe ix plasma. The funnels are found to have constrictions, À $ 6 20, which is in good agreement with the observations. Further, the fitted models simultaneously satisfy global areal constraints seen in both images; namely, that a global network of funnels must cover $70%-95% of the total solar surface area seen in the 171-175 Å image, and j45% of the disk area seen in the 1550 Å bandpass. These findings support the configuration of the EUV magnetic network as suggested by Reeves et al. and put forth in more detail by Gabriel. Furthermore, the models are in good agreement with differential emission measure estimates made of the transition region by J. C. Raymond & J. G. Doyle for temperatures 250;000 K T 650;000 K, based on full-disk observations made on board Skylab.

On the Generation, Propagation, and Reflection of Alfven Waves from the Solar Photosphere to the Distant Heliosphere

“…The spatial scale of the canopy is set by the typical distance between network flux bundles (i.e., the size of supergranulation cells) in the chromosphere. Observational evidence for preferential wind acceleration in the rapidly expanding network "funnels" is growing (Rottman, Orrall, & Klimchuk 1982;Hassler et al 1999;Peter & Judge 1999;Aiouaz et al 2004; see also Martínez-Galarce et al 2003) but is still not definitive (e.g., Dupree, Penn, & Jones 1996).…”

Astrophysics in 2003

“…The era of hydrostatic atmospheric models for the chromosphere is gone. Spatial inhomogeneities (moss, spicules, Kline and G-band bright points, jets visible in EUV, Ha, C iv, Lya), MHD shocks (Ryutova & Tarbell 2003), obliquely-propagating shear Alfve ´n waves excited by newborn ions (Chen & Zhou 2003), spicular dynamics (James & Erde ´lyi 2002;James et al 2003;Whitelam et al 2002), temporal variations of up to 40% in the continuum radiance (Wilhelm & Kalkofen 2003), correlated variabilities in EUV intensity and Doppler shifts (Brkovic et al 2003), super-hydrostatic density scale heights that entail an extended chromosphere up to heights of ≈5000 km (probed now with RHESSI, Brown et al 2002a;Aschwanden et al 2002), intermittent heating from chromospheric reconnection processes (Chae et al 2003;Lee et al 2003c), the non-forcefreeness and funnel structure of the chromospheric magnetic field (Leka & Metcalf 2003;Martinez-Galarce et al 2003), the nonequilibrium CO chemistry (Asensio-Ramos et al 2003), non-Maxwellian electron distributions that cause helium enhancements ), and many more other "unorthodox" observations constantly cry out for revisions of chromospheric models. So we live in an era of permanent revamping of atmospheric models, which probably will last as long as new data pour in.…”

Implausibility of Hydrostatic Funnels Constituting the Sun’s Upper Transition Region

“…The Sun's upper TR cannot be dominated by both unresolved small loops (Oluseyi et al, 1999a(Oluseyi et al, , 1999bFeldman, Widing, and Warren, 1999;Warren and Winebarger, 2000;Xia, Marsch, and Wilhelm, 2004) and also unresolved funnels (Dowdy, Rabin, and Moore, 1986;Patsourakos et al, 1999;Martínez-Galarce et al, 2003). To address this apparent paradox, we construct hydrostatic funnel models and compare them to unresolved observations of the solar atmosphere.…”

“…So-called backheating models, in which the corona heats the transition region via thermal conduction, dominated early modeling efforts (Athay, 1966;Moore and Fung, 1972). Gabriel's (1976) hydrostatic funnel model, which included the effects of the persistent photospheric magnetic-field distribution being maintained in the downflow lanes of the supergranulation network, has been particularly influential among backheating models.…”