Coronal loops are fundamental building blocks of the solar active regions and the corona. Therefore, a clear understanding of the physics of coronal loops will help us understand the physics of active region heating in particular and coronal heating in general. This requires a precise measurement of physical quantities such as electron densities and filling factors, temperatures, and flows in coronal loops. In this paper we have carried out an investigation of a spatially well resolved coronal loop using the EIS onboard Hinode to measure the above mentioned physical quantities. Based on this study we find that a nano-flare model could explain most of the observed characteristics of this loop.
Deep Dive into On active region loops: Hinode/EIS observations.
Coronal loops are fundamental building blocks of the solar active regions and the corona. Therefore, a clear understanding of the physics of coronal loops will help us understand the physics of active region heating in particular and coronal heating in general. This requires a precise measurement of physical quantities such as electron densities and filling factors, temperatures, and flows in coronal loops. In this paper we have carried out an investigation of a spatially well resolved coronal loop using the EIS onboard Hinode to measure the above mentioned physical quantities. Based on this study we find that a nano-flare model could explain most of the observed characteristics of this loop.
regions increased with temperature. For more detailed study we selected one particular well defined loop. Downward flows are detected along the coronal loop, being stronger in lower temperature lines (rising up to 60 km s -1 near the foot point). The downflow was localized towards the footpoint in transition region lines (Mg VII) and towards the loop top in high temperature line (Fe XV). By carefully accounting for the background emission we found that the loop structure was close to isothermal for each position along the loop, with the temperature rising from around 0.8 MK to 1.5 MK from the close to the base to higher up towards the apex (≈ 75Mm). We derived electron density using well established line ratio diagnostic techniques. Electron densities along the active region loop were found to vary from 10 10 cm -3 close to the footpoint to 10 8.5 cm -3 higher up. A lower electron density, varying from 10 9 cm -3 close to the footpoint to 10 8.5 cm -3 higher up, was found for the lower temperature density diagnostic. Using these densities we derived filling factors in along the coronal loop which can be as low as 0.02 near the base of the loop. The filling factor increased with projected height of the loop. These results provide important constraints on coronal loop modeling.
Subject headings: Sun: Corona -Sun: transition region -Sun: activity -Sun: coronal loops -Sun:active regions -Sun: fundamental parameters
Magnetically dominated solar plasma consists of a variety of structures, such as active regions, coronal loops, X-ray bright points, coronal holes etc. Anchored in the photosphere, coronal loops populate both active and quiet regions, and form the basic building blocks of the Sun’s atmosphere. The physics of all kinds of loops, therefore, holds the key to understanding coronal heating, solar wind acceleration and the flow of mass and energy in the region. Despite extensive work on the development of theoretical modelling (see e.g., Klimchuk 2006;Narain and Ulmschneider 1996, for a review) the energy source, structure maintenance and mass balance in coronal loops are not yet fully understood. The ultimate solution of this problem requires precise measurements of physical plasma parameters such as flows, electron temperature, density and filling factors in spatially resolved coronal structures. The measurement of above mentioned parameters have been performed using earlier spectrometers. However, the measurements were limited by spectral and spatial resolutions. Moreover the simultaneous temperature coverage has always been an issue.
The Extreme-ultraviolet Imaging Spectrometer (EIS; Culhane et al. 2007) on board Hinode spacecraft provides an excellent opportunity to measure these parameters in greater detail. In this paper we have studied overall intensity and flow structure in an active region. Moreover, we have studied flows, electron temperature, density and filling factor along a well resolved quiescent active region coronal loop. The rest of the paper is structured as follows.
In section 2, we provide observations and data reduction. We present out results in section 3 with a brief summary and conclusion in section 4.
The Extreme-ultraviolet Imaging Spectrometer (EIS; Culhane et al. 2007) on board Hinode is an off-axis paraboloid design with a focal length of 1.9 meter and mirror diameter of 15 cm. It consists of a multi-toroidal grating which disperses the spectrum on two different detectors covering 40 Å each. The first detector covers the wavelength range 170-210 Å and second covers 250-290 Å providing observations in a broad range of temperature (≈ 5.8-6.7 MK). The EIS has four slit/slot options available (1 ′′ , 2 ′′ , 40 ′′ and 266 ′′ ). The EIS provides monochromatic images of the transition region and corona at a high cadence using a slot (wide slit). High spectral resolution images can be obtained by rastering with a slit.
The EIS observed an active region on the Sun’s disk center on May 19, 2007 using the observation sequence AR velocity map. This sequence uses the 1 ′′ slit with an exposure time of 40 seconds with a delay of 12 seconds. The EIS raster used in this analysis started at 11:41:23 UT and finished at 16:35:01 UT. The left panel in Fig. 1 displays an image recorded by the Transition Region and Coronal Explorer (TRACE; Handy et al. 1999) The sequence used for this study comprises very many useful spectral lines. For this particular study, we have only selected a few windows with lines such as Si VII λ275
and Fe XV λ284 (log T[K] = 6.4). The selection of these lines provides diagnostics for the active region in a broad range of temperatures. The data were first processed using the standard processing routine eis prep.pro provided in Solar Software (SSW) tree. We have fitted all the lines at each pixel of the EIS raster using the routine eis auto fit.pro which is also provided in SSW. Note that the Mg VII λ278, and Fe XIV λ274 are blended with a Si VII line. This blendin
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