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Search Results: 1 - 10 of 208530 matches for " L. Teriaca "
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SUMER observations of the inverse Evershed effect in the transition region above a sunspot
L. Teriaca,W. Curdt,S. K. Solanki
Physics , 2009, DOI: 10.1051/0004-6361:200810209
Abstract: Aims. We analyse SUMER spectral scans of a large sunspot within active region NOAA 10923, obtained on 14-15 November 2006, to determine the morphology and dynamics of the sunspot atmosphere at different heights/temperatures. Methods: The data analysed here consist of spectroheliograms in the continuum around 142.0 nm and in the Si iv 140.2 nm, O iii 70.3 nm, N iv 76.5 nm, and O iv 79.0 nm spectral lines. Gaussian-fitting of the observed profiles provides line-of-sight velocity and Doppler-width maps. Results: The data show an asymmetric downflow pattern compatible with the presence of the inverse Evershed flow in a region within roughly twice the penumbral radius at transition-region temperatures up to 0.18 MK. The motions, highly inhomogeneous on small scales, seem to occur in a collar of radially directed filamentary structures, with an average width less than the 1 Mm spatial resolution of SUMER and characterised by different plasma speeds. Assuming that the flows are directed along the field lines, we deduce that such field lines are inclined by 10 deg to 25 deg with respect to the solar surface.
Propagating MHD waves in coronal holes
D. Banerjee,G. R. Gupta,L. Teriaca
Physics , 2010, DOI: 10.1007/s11214-010-9698-z
Abstract: Coronal holes are the coolest and darkest regions of the solar atmosphere, as observed both on the solar disk and above the solar limb. Coronal holes are associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind. During the years of the solar minima, coronal holes are generally confined to the Sun's polar regions, while at solar maxima they can also be found at lower latitudes. Waves, observed via remote sensing and detected in-situ in the wind streams, are most likely responsible for the wind and several theoretical models describe the role of MHD waves in the acceleration of the fast solar wind. This paper reviews the observational evidences of detection of propa- gating waves in these regions. The characteristics of the waves, like periodicities, amplitude, speed provide input parameters and also act as constraints on theoretical models of coronal heating and solar wind acceleration.
Solar extreme ultraviolet variability of the quiet Sun
F. Shakeri,L. Teriaca,S. K. Solanki
Physics , 2015, DOI: 10.1051/0004-6361/201424491
Abstract: The last solar minimum has been unusually quiet compared to the previous minima (since space-based radiometric measurements are available). The Sun's magnetic flux was substantially lower during this minimum. Some studies also show that the total solar irradiance during the minimum after cycle 23 may have dropped below the values known from the two minima prior to that. For chromospheric and coronal radiation, the situation is less clear-cut. The Sun's 10.7\,cm flux shows a decrease of $\sim4\%$ during the solar minimum in 2008 compared to the previous minimum, but \ion{Ca}{II} K does not. Here we consider additional wavelengths in the extreme ultraviolet (EUV), specifically transitions of \ion{He}{I} at 584.3\,\AA\ and \ion{O}{V} at 629.7\,\AA , of which the CDS spectrometer aboard SOHO has been taking regular scans along the solar central meridian since 1996. We analysed this unique dataset to verify if and how the radiance distribution undergoes measurable variations between cycle minima. To achieve this aim we determined the radiance distribution of quiet areas around the Sun centre. Concentrating on the last two solar minima, we found out that there is very little variation in the radiance distribution of the chromospheric spectral line \ion{He}{I} between these minima. The same analysis shows a modest, although significant, 4\% variation in the radiance distribution of the transition region spectral line \ion{O}{V}. These results are comparable to those obtained by earlier studies employing other spectral features, and they confirm that chromospheric indices display a small variation, whereas in the TR a more significant reduction of the brighter features is visible.
Dynamics and evolution of an eruptive flare
L. Teriaca,A. Falchi,R. Falciani,G. Cauzzi,L. Maltagliati
Physics , 2009, DOI: 10.1051/0004-6361:20065065
Abstract: We study the dynamics and evolution of a C2.3 two-ribbon flare, developed on 2002 August 11, during the impulsive and the long gradual phase. To this end we obtained multiwavelength observations using the CDS spectrometer aboard SOHO, facilities at the NSO/Sacramento Peak, and the TRACE and RHESSI spacecrafts. CDS spectroheliograms in the Fe XIX, Fe XVI, O V and He I lines allows us to determine the velocity field at different heights/temperatures during the flare and to compare them with the chromospheric velocity fields deduced from H alpha image differences. TRACE images in the 17.1 nm band greatly help in determining the morphology and the evolution of the flaring structures. During the impulsive phase a strong blue-shifted Fe XIX component (-200 km/s) is observed at the footpoints of the flaring loop system, together with a red-shifted emission of O V and He I lines (20 km/s). In one footpoint simultaneous H alpha data are also available and we find, at the same time and location, downflows with an inferred velocity between 4 and 10 km/s. We also verify that the "instantaneous" momenta of the oppositely directed flows detected in Fe XIX and H alpha are equal within one order of magnitude. These signatures are in general agreement with the scenario of explosive chromospheric evaporation. Combining RHESSI and CDS data after the coronal upflows have ceased, we prove that, independently from the filling factor, an essential contribution to the density of the post-flare loop system is supplied from evaporated chromospheric material. Finally, we consider the cooling of this loop system, that becomes successively visible in progressively colder signatures during the gradual phase. We show that the observed cooling behaviour can be obtained assuming a coronal filling factor between 0.2 and 0.5.
The SUMER Lyman-alpha line profile in quiescent prominences
W. Curdt,H. Tian,L. Teriaca,U. Schühle
Physics , 2010, DOI: 10.1051/0004-6361/200913875
Abstract: Aims: Out of a novel observing technique, we publish for the first time, SoHO-SUMER observations of the true spectral line profile of hydrogen Lyman-alpha in quiescent prominences. With SoHO not being in Earth orbit, our high-quality data set is free from geocoronal absorption. We study the line profile and compare it with earlier observations of the higher Lyman lines and recent model predictions. Methods: We applied the reduced-aperture observing mode to two prominence targets and started a statistical analysis of the line profiles in both data sets. In particular, we investigated the shape of the profile, the radiance distribution and the line shape-to-radiance interrelation. We also compare Ly-a data to co-temporal 1206 Si III data. Results: We find that the average profile of Ly-a has a blue-peak dominance and is more reversed, if the line-of-sight is perpendicular to the field lines. The contrast of Ly-a prominence emission rasters is very low and the radiance distribution differs from the log-normal distribution of the disk. Features seen in the Si III line are not always co-spatial with Ly-a emission. Conclusions: Our empirical results support recent multi-thread models, which predict that asymmetries and depths of the self-reversal depend on the orientation of the prominence axis relative to the line-of-sight.
Solar transition region above sunspots
H. Tian,W. Curdt,L. Teriaca,E. Landi,E. Marsch
Physics , 2009, DOI: 10.1051/0004-6361/200912114
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.
Evolution of microflares associated with bright points in coronal holes and in quiet regions
S. Kamio,W. Curdt,L. Teriaca,D. E. Innes
Physics , 2010, DOI: 10.1051/0004-6361/201015715
Abstract: We aim to find similarities and differences between microflares at coronal bright points found in quiet regions and coronal holes, and to study their relationship with large scale flares. Coronal bright points in quiet regions and in coronal holes were observed with Hinode/EIS using the same sequence. Microflares associated with bright points are identified from the X-ray lightcurve. The temporal variation of physical properties was traced in the course of microflares. The lightcurves of microflares indicated an impulsive peak at hot emission followed by an enhancement at cool emission, which is compatible with the cooling model of flare loops. The density was found to increase at the rise of the impulsive peak, supporting chromospheric evaporation models. A notable difference is found in the surroundings of microflares; diffuse coronal jets are produced above microflares in coronal holes while coronal dimmings are formed in quiet regions. The microflares associated with bright points share common characteristics to active region flares. The difference in the surroundings of microflares are caused by open and closed configurations of the pre-existing magnetic field.
Analysis of the Fe X and Fe XIV line width in the solar corona using LASCO-C1 spectral data
M. Mierla,R. Schwenn,L. Teriaca,G. Stenborg,B. Podlipnik
Physics , 2009, DOI: 10.1051/0004-6361:20078329
Abstract: The purpose of this paper is to analyze the variation in the line width with height in the inner corona (region above 1.1 Rsun), by using the spectral data from LASCO-C1 aboard SOHO. We used data acquired at activity minimum (August - October 1996) and during the ascending phase of the solar cycle (March 1998). Series of images acquired at different wavelengths across the Fe X 637.6 nm (red) and Fe XIV 530.3 nm (green) coronal lines by LASCO-C1 allowed us to build radiance and width maps of the off-limb solar corona. In 1996, the line width of Fe XIV was roughly constant or increased with height up to around 1.3 Rsun and then it decreased. The Fe X line width increased with height up to the point where the spectra were too noisy to allow line width measurements (around 1.3 Rsun). Fe X showed higher effective temperatures as compared with Fe XIV. In 1998 the line width of Fe XIV was roughly constant with height above the limb (no Fe X data available).
Propagating waves in polar coronal holes as seen by SUMER and EIS
D. Banerjee,L. Teriaca,G. R. Gupta,S. Imada,G. Stenborg,S. K. Solanki
Physics , 2009, DOI: 10.1051/0004-6361/200912059
Abstract: To study the dynamics of coronal holes and the role of waves in the acceleration of the solar wind, spectral observations were performed over polar coronal hole regions with the SUMER spectrometer on SoHO and the EIS spectrometer on Hinode. Using these observations, we aim to detect the presence of propagating waves in the corona and to study their properties. The observations analysed here consist of SUMER spectra of the Ne VIII 770 A line (T = 0.6 MK) and EIS slot images in the Fe XII 195 A line (T = 1.3 MK). Using the wavelet technique, we study line radiance oscillations at different heights from the limb in the polar coronal hole regions. We detect the presence of long period oscillations with periods of 10 to 30 min in polar coronal holes. The oscillations have an amplitude of a few percent in radiance and are not detectable in line-of-sight velocity. From the time distance maps we find evidence for propagating velocities from 75 km/s (Ne VIII) to 125 km/s (Fe XII). These velocities are subsonic and roughly in the same ratio as the respective sound speeds. We interpret the observed propagating oscillations in terms of slow magneto-acoustic waves. These waves can be important for the acceleration of the fast solar wind.
Accelerating waves in polar coronal holes as seen by EIS and SUMER
G. R. Gupta,D. Banerjee,L. Teriaca,S. Imada,S. Solanki
Physics , 2010, DOI: 10.1088/0004-637X/718/1/11
Abstract: We present EIS/Hinode & SUMER/SoHO joint observations allowing the first spectroscopic detection of accelerating disturbances as recorded with coronal lines in inter-plume and plume regions of a polar coronal hole. From time-distance radiance maps, we detect the presence of propagating disturbances in a polar inter-plume region with a period of 15 to 20 min and a propagation speed increasing from 130+/-14 km/s just above the limb, to 330+/-140 km/s around 160" above the limb. These disturbances can also be traced to originate from a bright region of the on-disk part of the coronal hole where the propagation speed was found to be in the range of 25+/-1.3 to 38+/-4.5 km/s, with the same periodicity. These on-disk bright regions can be visualized as the base of the coronal funnels. The adjacent plume region also shows the presence of propagating disturbance with the same range of period but with propagation speeds in the range of 135+/-18 to 165+/-43 km/s only. To our knowledge, this result provides first spectroscopic evidence of acceleration of propagating disturbances in the polar region close to the Sun (within 1.2 solar radii). We suggest that the waves are likely either Alfv'enic or fast magnetoacoustic in the inter-plume and slow magnetoacoustic in plume regions. This may lead to the conclusion that inter-plumes are preferred channel for the acceleration of the fast solar wind.
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