Solar Dynamics Observatory discovers thin high temperature strands in coronal active regions

One scenario proposed to explain the million degrees solar corona is a finely-stranded corona where each strand is heated by a rapid pulse. However, such fine structure has neither been resolved through direct imaging observations nor conclusively shown through indirect observations of extended superhot plasma. Recently it has been shown that the observed difference in appearance of cool and warm coronal loops (~1 MK, ~2-3 MK, respectively) -- warm loops appearing "fuzzier" than cool loops -- can be explained by models of loops composed of subarcsecond strands, which are impulsively heated up to ~10 MK. That work predicts that images of hot coronal loops (>~6 MK) should again show fine structure. Here we show that the predicted effect is indeed widely observed in an active region with the Solar Dynamics Observatory, thus supporting a scenario where impulsive heating of fine loop strands plays an important role in powering the active corona.Comment: 11 pages, 4 figures, accepted for publicatio

Hinode/EIS spectroscopic validation of very hot plasma imaged with Solar Dynamics Observatory in non-flaring active region cores

We use coronal imaging observations with SDO/AIA, and Hinode/EIS spectral data, to explore the potential of narrow band EUV imaging data for diagnosing the presence of hot (T >~5MK) coronal plasma in active regions. We analyze observations of two active regions (AR 11281, AR 11289) with simultaneous AIA imaging, and EIS spectral data, including the CaXVII line (at 192.8A) which is one of the few lines in the EIS spectral bands sensitive to hot coronal plasma even outside flares. After careful coalignment of the imaging and spectral data, we compare the morphology in a 3 color image combining the 171, 335, and 94A AIA spectral bands, with the image obtained for CaXVII emission from the analysis of EIS spectra. We find that in the selected active regions the CaXVII emission is strong only in very limited areas, showing striking similarities with the features bright in the 94A (and 335A) AIA channels and weak in the 171A band. We conclude that AIA imaging observations of the solar corona can be used to track hot plasma (6-8MK), and so to study its spatial variability and temporal evolution at high spatial and temporal resolution.Comment: 10 pages, 2 figures, accepted for publication on ApJ Letter

More on the determination of the coronal heating function from Yohkoh data

Two recent works have analyzed a solar large and steady coronal loop observed with Yohkoh/SXT in two filter passbands to infer the distribution of the heating along it. Priest et al. (2000) modelled the distribution of the temperature obtained from filter ratio method with an analytical approach, and concluded that the heating was uniform along the loop. Aschwanden (2001) found that a uniform heating led to an unreasonably large plasma column depth along the line of sight, and, using a two component loop model, that a footpoint-heated model loop (with a minor cool component) yields more acceptable physical solutions. We revisit the analysis of the same loop system, considering conventional hydrostatic single loop models with uniformly distributed heating, and with heating localized at the footpoints and at the apex, and an unstructured background contribution extrapolated from the region below the analyzed loop. The flux profiles synthesized from the loop models have been compared in detail with those observed in both filter passbands with and without background subtraction; we find that background-subtracted data are fitted with acceptable statistical significance by a model of relatively hot loop (~3.7 MK) heated at the apex, with a column depth ~1/10 of the loop length. In discussing our results, we put warnings on the importance of aspects of data analysis and modeling, such as considering diffuse background emission in complex loop regions.Comment: 17 pages, 4 figures, refereed pape

Magic and loss. Notes on an international design workshop dedicated to the waterfront of Kolkata

Designing water spaces and their margins is a very critical operation, especially in a megalopolis like Kolkata, one of the ten most populous cities in the world, with great previsions of demographic growth in the coming years (15.5 million in 2010, 18.5 million in 2020). In Europe, in recent years water has been attracting growing consideration from the planning standpoint. For several decades waterfronts were abandoned, used to locate industrial facilities, sometimes blocked by highways, closed off by walls, used for storage places, and warehouses. Post-industrial cities are evidently turning the face towards their rivers by creating new waterfronts with pleasant environments for leisure and work, places for bars and restaurants, urban beaches and sports clubs, promenades along the river, new landscapes, including the restoration of riparian vegetation. All activities are part of a developing process and aimed at improving the overall quality of urban life. Is it conceivable to renew the relationship between the city of Kolkata and the Hooghly River, which will necessarily be enhanced in the coming years, without destroying its special feeling of place

Spectroscopic Observations of Fe XVIII in Solar Active Regions

The large uncertainties associated with measuring the amount of high temperature emission in solar active regions represents a significant impediment to making progress on the coronal heating problem. Most current observations at temperatures of 3 MK and above are taken with broad band soft X-ray instruments. Such measurements have proven difficult to interpret unambiguously. Here we present the first spectroscopic observations of the Fe XVIII 974.86 AA emission line in an on-disk active region taken with then SUMER instrument on SOHO. Fe XVIII has a peak formation temperature of 7.1 MK and provides important constraints on the amount of impulsive heating in the corona. Detailed evaluation of the spectra and comparison of the SUMER data with soft X-ray images from the XRT on Hinode confirm that this line is unblended. We also compare the spectroscopic data with observations from the AIA 94 AA channel on SDO. The AIA 94 AA channel also contains Fe XVIII, but is blended with emission formed at lower temperatures. We find that is possible to remove the contaminating blends and form relatively pure Fe XVIII images that are consistent with the spectroscopic observations from SUMER. The observed spectra also contain the Ca XIV 943.63 AA line that, although a factor 2 to 6 weaker than the Fe XVIII 974.86 AA line, allows us to probe the plasma around 3.5 MK. The observed ratio between the two lines indicates (isothermal approximation) that most of the plasma in the brighter Fe XVIII active region loops is at temperatures between 3.5 and 4 MK.Comment: 12 pages, 5 figures. Submitted as letter to Ap

Coronal fuzziness modelled with pulse-heated multistranded loop systems

Coronal active regions are observed to get fuzzier and fuzzier (i.e. more and more confused and uniform) in harder and harder energy bands or lines. We explain this evidence as due to the fine multi-temperature structure of coronal loops. To this end, we model bundles of loops made of thin strands, each heated by short and intense heat pulses. For simplicity, we assume that the heat pulses are all equal and triggered only once in each strand at a random time. The pulse intensity and cadence are selected so as to have steady active region loops ($\sim 3$ MK), on the average. We compute the evolution of the confined heated plasma with a hydrodynamic loop model. We then compute the emission along each strand in several spectral lines, from cool ($\leq 1$ MK), to warm ($2-3$ MK) lines, detectable with Hinode/EIS, to hot X-ray lines. The strands are then put side-by-side to construct an active region loop bundle. We find that in the warm lines ($2-3$ MK) the loop emission fills all the available image surface. Therefore the emission appears quite uniform and it is difficult to resolve the single loops, while in the cool lines the loops are considerably more contrasted and the region is less fuzzy. The main reasons for this effect are that, during their evolution, i.e. pulse heating and slow cooling, each strand spends a relatively long time at temperatures around $2-3$ MK, and that it has a high emission measure during that phase, so the whole region appears more uniform or smudged. We make the prediction that the fuzziness should be reduced in the hot UV and X-ray lines.Comment: 27 pages, 14 figure

Prominence plasma diagnostics through EUV absorption

In this paper we introduce a new diagnostic technique that uses prominence EUV and UV absorption to determine the prominence plasma electron temperature and column emission measure, as well as He/H relative abundance; if a realistic assumption on the geometry of the absorbing plasma can be made, this technique can also yield the absorbing plasma electron density. This technique capitalizes on the absorption properties of Hydrogen and Helium at different wavelength ranges and temperature regimes. Several cases where this technique can be successfully applied are described. This technique works best when prominence plasmas are hotter than 15,000 K and thus it is ideally suited for rapidly heating erupting prominences observed during the initial phases of coronal mass ejections. An example is made using simulated intensities of 4 channels of the SDO/AIA instrument. This technique can be easily applied to existing observations from almost all space missions devoted to the study of the solar atmosphere, which we list.Comment: 17 pages, 4 figures, submitted to Ap

Analysis of a multi-wavelength time-resolved observation of a coronal loop

Several items on the diagnostics and interpretation of coronal loop observations are under debate. In this work, we analyze a well-defined loop system detected in a time-resolved observation in several spectral bands. The dataset includes simultaneous images in the TRACE 171 A, 195 A and 284 A bands, and Yohkoh/SXT, and two rasters taken with SoHO/CDS in twelve relevant lines. The loop is initially best visible in the TRACE 195 A filter band, and later in the 171 A filter band, with correspondence with the CDS raster images at log T \~ 6.0-6.1. We have taken as pixel-by-pixel background the latest TRACE, Yohkoh and CDS images where the loop has faded out. We examine the loop morphology evolution, the light curves, the TRACE filter ratio distribution and evolution, the images and emission measure from the CDS spectral lines. Our analysis detects that, after background subtraction, the emission along the loop and its evolution are non-uniform, especially in the 171 A filter band, and that the TRACE 195/171 filter ratio has a moderately non-uniform distribution along the loop and evolves in time. Both the light curves and the filter ratio evolution indicate a globally cooling loop. Relatively hot plasma may be present at the beginning while, during the first CDS raster, the data indicate a rather moderate thermal structuring of the loop. Our data analysis supports a coherent scenario across the different bands and instruments, points out difficulties in diagnostic methods and puts quantitative basis for detailed forward modeling.Comment: 20 pages, 13 figs, refereed, in pres