Showing papers on "Solar transition region published in 2009"

Active region transition region loop populations and their relationship to the corona

Abstract: The relationships among coronal loop structures at different temperatures are not settled. Previous studies have suggested that coronal loops in the core of an active region (AR) are not seen cooling through lower temperatures and therefore are steadily heated. If loops were cooling, the transition region would be an ideal temperature regime to look for a signature of their evolution. The Extreme-ultraviolet Imaging Spectrometer on Hinode provides monochromatic images of the solar transition region and corona at an unprecedented cadence and spatial resolution, making it an ideal instrument to shed light on this issue. Analysis of observations of AR 10978 taken in 2007 December 8-19 indicates that there are two dominant loop populations in the AR: (1) core multitemperature loops that undergo a continuous process of heating and cooling in the full observed temperature range 0.4-2.5 MK and even higher as shown by the X-Ray Telescope and (2) peripheral loops which evolve mostly in the temperature range 0.4-1.3 MK. Loops at transition region temperatures can reach heights of 150 Mm in the corona above the limb and develop downflows with velocities in the range of 39-105 km s–1.

Active region transition region loop populations and their relationship to the corona

Abstract: The relationships among coronal loop structures at different temperatures is not settled. Previous studies have suggested that coronal loops in the core of an active region are not seen cooling through lower temperatures and therefore are steadily heated. If loops were cooling, the transition region would be an ideal temperature regime to look for a signature of their evolution. The Extreme-ultraviolet Imaging Spectrometer (EIS) on Hinode provides monochromatic images of the solar transition region and corona at an unprecedented cadence and spatial resolution, making it an ideal instrument to shed light on this issue. Analysis of observations of active region 10978 taken in 2007 December 8 -- 19 indicates that there are two dominant loop populations in the active region: core multi-temperature loops that undergo a continuous process of heating and cooling in the full observed temperature range 0.4-2.5 MK and even higher as shown by the X-Ray Telescope (XRT); and peripheral loops which evolve mostly in the temperature range 0.4-1.3 MK. Loops at transition region temperatures can reach heights of 150 Mm in the corona above the limb and develop downflows with velocities in the range of 39-105 km/s.

Solar transition region above sunspots

Abstract: Aims. We study the transition region (TR) properties above sunspots and the surrounding plage regions, by analyzing several sunspot reference spectra obtained by the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument in March 1999 and November 2006. Methods. We compare the SUMER spectra observed in the umbra, penumbra, plage, and sunspot plume regions. The hydrogen Lyman line profiles averaged in each of the four regions are presented. For the sunspot observed in 2006, the electron densities, differential emission measure (DEM), and filling factors of the TR plasma in the four regions are also investigated. Results. The self-reversals of the hydrogen Lyman line profiles are almost absent in sunspots at different locations (at heliocentric angles of up to 49°) on the solar disk. In the sunspot plume, the Lyman lines are also not reversed, whilst the lower Lyman line profiles observed in the plage region are obviously reversed, a phenomenon found also in the normal quiet Sun. The TR densities of the umbra and plume are similar and one order of magnitude lower than those of the plage and penumbra. The DEM curve of the sunspot plume exhibits a peak centered at log( T / K) ~ 5.45, which exceeds the DEM of other regions by one to two orders of magnitude at these temperatures. We also find that more than 100 lines, which are very weak or not observed anywhere else on the Sun, are well observed by SUMER in the sunspot, especially in the sunspot plume. Conclusions. We suggest that the TR above sunspots is higher and probably more extended, and that the opacity of the hydrogen lines is much lower above sunspots, compared to the TR above plage regions. Our result indicates that the enhanced TR emission of the sunspot plume is probably caused by a large filling factor. The strongly enhanced emission at TR temperatures and the reduced continuum ensure that many normally weak TR lines are clearly distinctive in the spectra of sunspot plumes.

Benchmarking atomic data for astrophysics: Fe VII and other cool lines observed by Hinode EIS

Abstract: The EUV spectrum of Fevii is reviewed, using new rates for electron impact excitation, atomic structure calculations, and experimental data. In particular, solar observations of a Sunspot loop spectrum obtained from the Hinode EUV Imaging Spectrometer (EIS) are used. Previous line identifications, mostly based on lab oratory data, have been assessed. Large discrepancies between observed and predicted line intensities and wavelengths are found for the decays from the 3s 2 3p 5 3d 3 configuration, which are strong EUV lines. We ascribe these discrepancies to incorrect line ide ntifications. A number of new identifications are proposed. W ith these, very good agreement between theory and experimental data is found. A few transitions, in particular from the 3s 2 3p 6 3d 4s configuration, are observed for the first time, and are shown to provide a new important diagnostic for measuring the electron temperature in the solar transition region. The temperatures obtained at t he base of solar coronal loops are found to be close to the temperature of maximum abundance in ionization equilibrium (log T [K]= 5.4). The assessment of the Fevii lines was done in conjunction with an assessment of all the strongest cool lines observed with EIS. This spectrum is rich in transition region lines. Some new identifications are presented, in particular for Feix. Most of the strongest transitions are identified, however a large number of lines still awaits firm identification.

CHIANTI—AN ATOMIC DATABASE FOR EMISSION LINES. XI. EXTREME-ULTRAVIOLET EMISSION LINES OF Fe VII, Fe VIII, AND Fe IX OBSERVED BY HINODE/EIS

Abstract: A detailed study of emission lines from Fe VII, Fe VIII, and Fe IX observed by the EUV Imaging Spectrometer on board the Hinode satellite is presented. Spectra in the ranges 170-212 A and 246-292 A show strongly enhanced lines from the upper solar transition region (temperatures 5.4 <= log T <= 5.9) allowing a number of new line identifications to be made. Comparisons of Fe VII lines with predictions from a new atomic model reveal new plasma diagnostics, however there are a number of disagreements between theory and observation for emission line ratios insensitive to density and temperature, suggesting improved atomic data are required. Line ratios for Fe VIII also show discrepancies with theory, with the strong lambda185.21 and lambda186.60 lines underestimated by 60%-80% compared to lines between 192 and 198 A. A newly identified multiplet between 253.9 and 255.8 A offers excellent temperature diagnostic opportunities relative to the lines between 185 and 198 A, however the atomic model underestimates the strength of these lines by factors of 3-6. Two new line identifications are made for Fe IX at wavelengths 176.959 A and 177.594 A, while seven other lines between 186 and 200 A are suggested to be duemore » to Fe IX but for which transition identifications cannot be made. The new atomic data for Fe VII and Fe IX are demonstrated to significantly modify models for the response function of the Transition Region And Coronal Explorer 195 A imaging channel, affecting temperature determinations from this channel. The data will also affect the response functions for other solar EUV imaging instruments such as SOHO/EIT, STEREO/EUVI, and the upcoming AIA instrument on the Solar Dynamics Observatory.« less

The nascent fast solar wind observed by EIS onboard Hinode

Abstract: The origin of the solar wind is one of the most important unresolved prob- lems in space and solar physics. We report here the first spectroscopic signatures of the nascent fast solar wind on the basis of observations made by the EUV Imaging Spectrometer (EIS) on Hinode in a polar coronal hole, in which patches of blueshift are clearly present on dopplergrams of coronal emission lines with a formation temperature of lg(T/K)=6.0. The corresponding upflow is associated with open field lines in the coronal hole, and seems to start in the solar transition region and becomes more prominent with increasing temperature. This temperature-dependent plasma outflow is interpreted as evidence of the nascent fast solar wind in the polar coronal hole. The patches with significant upflows are still isolated in the upper transition region but merge in the corona, in agreement with the scenario of solar wind outflow being guided by expanding magnetic funnels.

Solar transition region above sunspots

Abstract: We study the TR properties above sunspots and the surrounding plage regions, by analyzing several sunspot spectra obtained by SUMER in March 1999 and November 2006. We compare the SUMER spectra observed in the umbra, penumbra, plage, and sunspot plume regions. The Lyman line profiles averaged in each region are presented. For the sunspot observed in 2006, the electron densities, DEM, and filling factors of the TR plasma in the four regions are also investigated. The self-reversals of the Lyman line profiles are almost absent in umbral regions at different locations (heliocentric angle up to $49^\circ$) on the solar disk. In the sunspot plume, the Lyman lines are also not reversed, whilst the lower Lyman line profiles observed in the plage region are obviously reversed. The TR densities of the umbra and plume are similar and one order of magnitude lower than those of the plage and penumbra. The DEM curve of the sunspot plume exhibits a peak centered around $\log(T/\rm{K})\sim5.45$, which exceeds the DEM of other regions by one to two orders of magnitude at these temperatures. We also find that more than 100 lines, which are very weak or not observed anywhere else on the Sun, are well observed by SUMER in the sunspot, especially in the sunspot plume. We suggest that the TR above sunspots is higher and probably more extended, and that the opacity of the hydrogen lines is much smaller above sunspots, as compared to the TR above plage regions. Our result indicates that the enhanced TR emission of the sunspot plume is very likely to be the result of a large filling factor. The strongly enhanced emission at TR temperatures and the reduced continuum make many normally weak TR lines stick out clearly in the spectra of sunspot plumes.

Solar transition region in the quiet Sun and active regions

Abstract: The solar transition region (TR), in which above the photosphere the tempera- ture increases rapidly and the density drops dramatically, is believed to play an important role in coronal heating and solar wind acceleration. Long-lasting up-flows are present in the upper TR and interpreted as signatures of mass supply to large coronal loops in the quiet Sun. Coronal bright points (BPs) are local heating phenomena and we found a different Doppler-shift pattern at TR and coronal temperatures in one BP, which might be related to the twisted loop system. The dominant energy loss in the lower TR is the Ly-alpha emission. It has been found that most Ly-alpha radiance profiles are stronger in the blue peak, an asymmetry opposite to higher order Lyman lines. This asymmetry is stronger when the downflow in the middle TR is stronger, indicating that the TR flows play an important role in the line formation process. The peak separation of Ly-alpha is found to be larger in coronal holes than in the quiet Sun, reflecting the different magnetic structures and radiation fields between the two regions. The Lyman line profiles are found to be not reversed in sunspot plume and umbra regions, while they are obviously reversed in the surrounding plage region. At TR temperatures, the densities of the sunspot plume and umbra are a factor of 10 lower than of the plage, indicating that the sunspot plasma emitting at TR temperatures is higher and possibly more extended above sunspots than above the plage region.