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CoMP observations of Coronal Mass Ejections  

Hui Tian   Submitted: 2013-03-21 08:39

CoMP measures not only the polarization of coronal emission, but also the full radiance profiles of coronal emission lines. For the first time, CoMP observations provide high-cadence image sequences of the coronal line intensity, Doppler shift and line width simultaneously in a large field of view. By studying the Doppler shift and line width we may explore more of the physical processes of CME initiation and propagation. Here we identify a list of CMEs observed by CoMP and present the first results of these observations. Our preliminary analysis shows that CMEs are usually associated with greatly increased Doppler shift and enhanced line width. These new observations provide not only valuable information to constrain CME models and probe various processes during the initial propagation of CMEs in the low corona, but also offer a possible cheap and low-risk means of space weather monitoring.

Authors: Tian, H.; Tomczyk, S.; McIntosh, S. W.; Bethge, C.; de Toma, G.; Gibson, S.
Projects: None

Publication Status: Will appear in the special issue of Coronal Magnetism, Sol. Phys
Last Modified: 2013-03-21 10:27
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THE NASCENT FAST SOLAR WIND OBSERVED BY THE EUV IMAGING SPECTROMETER ON BOARD HINODE  

Hui Tian   Submitted: 2012-12-05 08:26

The origin of the solar wind is one of the most important unresolved problems 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 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) > 5.8. 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.

Authors: Hui Tian, Chuanyi Tu, Eckart Marsch, Jiansen He, and Suguru Kamio
Projects: None

Publication Status: The Astrophysical Journal Letters, 709:L88?L93, 2010 January 20
Last Modified: 2012-12-05 14:05
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HYDROGEN Lyα AND Lyβ RADIANCES AND PROFILES IN POLAR CORONAL HOLES  

Hui Tian   Submitted: 2012-12-05 08:24

The hydrogen Lyα plays a dominant role in the radiative energy transport in the lower transition region, and is important for the studies of transition-region structure as well as solar wind origin. We investigate the Lyα profiles obtained by the Solar Ultraviolet Measurement of Emitted Radiation spectrograph on the Solar and Heliospheric Observatory spacecraft in coronal holes and the quiet Sun. In a subset of these observations, the Hi Lyβ, Si iii, and Ovi lines were also (quasi-)simultaneously recorded. We find that the distances between the two peaks of Lyα profiles are larger in coronal holes than in the quiet Sun, indicating a larger opacity in coronal holes. This difference might result from the different magnetic structures or the different radiation fields in the two regions. Most of the Lyβ profiles in the coronal hole have a stronger blue peak, in contrast to those in quiet-Sun regions while in both regions the Lyα profiles are stronger in the blue peak. Although the asymmetries are likely to be produced by differential flows in the solar atmosphere, their detailed formation processes are still unclear. The radiance ratio between Lyα and Lyβ decreases toward the limb in the coronal hole, which might be due to the different opacity of the two lines. We also find that the radiance distributions of the four lines are set by a combined effect of limb brightening and the different emission level between coronal holes and the quiet Sun.

Authors: Hui Tian, Luca Teriaca, Werner Curdt, and Jean-Claude Vial
Projects: None

Publication Status: The Astrophysical Journal, 703:L152?L156, 2009 October 1
Last Modified: 2012-12-05 14:05
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Sizes of transition-region structures in coronal holes and in the quiet Sun  

Hui Tian   Submitted: 2012-12-05 08:22

Aims. We study the height variations of the sizes of chromospheric and transition-region features in a small coronal hole and the adjacent quiet Sun, considering images of the intensity, Doppler shift, and non-thermal motion of ultraviolet emission lines as measured by SUMER (Solar Ultraviolet Measurements by Emitted Radiation), together with the magnetic field as obtained by extrapolation from photospheric magnetograms. Methods. In order to estimate the characteristic sizes of the different features present in the chromosphere and transition region, we have calculated the autocorrelation function for the images as well as the corresponding extrapolated magnetic field at different heights. The Half Width at Half Maximum (HWHM) of the autocorrelation function is considered to be the characteristic size of the feature shown in the corresponding image. Results. Our results indicate that, in both the coronal hole and quiet Sun, the HWHM of the intensity image is larger than that of the images of Doppler-shift and non-thermal width at any given altitude. The HWHM of the intensity image is smaller in the chromosphere than in the transition region, where the sizes of intensity features of lines at different temperatures are almost the same. But in the upper part of the transition region, the intensity size increases more strongly with temperature in the coronal hole than in the quiet Sun. We also studied the height variations of the HWHM of the magnetic field magnitude B and its component |Bz|, and found they are equal to each other at a certain height below 40 Mm in the coronal hole. The height variations of the HWHM of |Bz/B| seem to be consistent with the temperature variations of the intensity size. Conclusions. Our results suggest that coronal loops are much lower, and magnetic structures expand through the upper transition region and lower corona much more strongly with height in the coronal hole than in the quiet Sun.

Authors: H. Tian, E. Marsch, C.-Y. Tu, L.-D. Xia, and J.-S. He
Projects: None

Publication Status: A&A 482, 267?272 (2008)
Last Modified: 2012-12-05 14:05
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Solar transition region above sunspots  

Hui Tian   Submitted: 2012-12-05 08:19

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.

Authors: H. Tian, W. Curdt, L. Teriaca, E. Landi, and E. Marsch
Projects: None

Publication Status: A&A 505, 307?318 (2009)
Last Modified: 2012-12-05 14:04
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Horizontal supergranule-scale motions inferred from TRACE ultraviolet observations of the chromosphere  

Hui Tian   Submitted: 2012-12-05 08:17

Aims. We study horizontal supergranule-scale motions revealed by TRACE observation of the chromospheric emission, and investigate the coupling between the chromosphere and the underlying photosphere. Methods. A highly efficient feature-tracking technique called balltracking has been applied for the first time to the image sequences obtained by TRACE (transition region and coronal explorer) in the passband of white light and the three ultraviolet passbands centered at 1700 Å, 1600 Å, and 1550 ?. The resulting velocity fields have been spatially smoothed and temporally averaged in order to reveal horizontal supergranule-scale motions that may exist at the emission heights of these passbands. Results. We find indeed a high correlation between the horizontal velocities derived in the white-light and ultraviolet passbands. The horizontal velocities derived from the chromospheric and photospheric emission are comparable in magnitude. Conclusions. The horizontal motions derived in the UV passbands might indicate the existence of a supergranule-scale magnetoconvection in the chromosphere, which may shed new light on the study of mass and energy supply to the corona and solar wind at the height of the chromosphere. However, it is also possible that the apparent motions reflect the chromospheric brightness evolution as produced by acoustic shocks which might be modulated by the photospheric granular motions in their excitation process, or advected partly by the supergranule-scale flow towards the network while propagating upward from the photosphere. To reach a firm conclusion, it is necessary to investigate the role of granular motions in the excitation of shocks through numerical modeling, and future high-cadence chromospheric magnetograms must be scrutinized.

Authors: H. Tian, H. E. Potts, E. Marsch, R. Attie, J.-S. He
Projects: None

Publication Status: A&A 519, A58 (2010)
Last Modified: 2012-12-05 14:04
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PERSISTENT DOPPLER SHIFT OSCILLATIONS OBSERVED WITH HINODE/EIS IN THE SOLAR CORONA: SPECTROSCOPIC SIGNATURES OF ALFVENIC WAVES AND RECURRING UPFLOWS  

Hui Tian   Submitted: 2012-12-05 08:12

Using data obtained by the EUV Imaging Spectrometer on board Hinode, we have performed a survey of obvious and persistent (without significant damping) Doppler shift oscillations in the corona. We have found mainly two types of oscillations from February to April in 2007. One type is found at loop footpoint regions, with a dominant period around 10 minutes. They are characterized by coherent behavior of all line parameters (line intensity, Doppler shift, line width, and profile asymmetry), and apparent blueshift and blueward asymmetry throughout almost the entire duration. Such oscillations are likely to be signatures of quasi-periodic upflows (small-scale jets, or coronal counterpart of type-II spicules), which may play an important role in the supply of mass and energy to the hot corona. The other type of oscillation is usually associated with the upper part of loops. They are most clearly seen in the Doppler shift of coronal lines with formation temperatures between one and two million degrees. The global wavelets of these oscillations usually peak sharply around a period in the range of three to six minutes. No obvious profile asymmetry is found and the variation of the line width is typically very small. The intensity variation is often less than 2%. These oscillations are more likely to be signatures of kink/Alfvén waves rather than flows. In a few cases, there seems to be a π/2 phase shift between the intensity and Doppler shift oscillations, which may suggest the presence of slow-mode standing waves according to wave theories. However, we demonstrate that such a phase shift could also be produced by loops moving into and out of a spatial pixel as a result of Alfvénic oscillations. In this scenario, the intensity oscillations associated with Alfvénic waves are caused by loop displacement rather than density change. These coronal waves may be used to investigate properties of the coronal plasma and magnetic field.

Authors: Hui Tian, Scott W. McIntosh, Tongjiang Wang, Leon Ofman, Bart De Pontieu, Davina E. Innes, and Hardi Peter
Projects: None

Publication Status: The Astrophysical Journal, 759:144 (17pp), 2012 November 10
Last Modified: 2012-12-05 14:04
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What can we learn about solar coronal mass ejections, coronal dimmings, and Extreme-Ultraviolet jets through spectroscopic observations?  

Hui Tian   Submitted: 2012-01-10 16:02

Solar eruptions, particularly coronal mass ejections (CMEs) andextreme-ultraviolet (EUV) jets, haverarely been investigated with spectroscopic observations. We analyzeseveral data sets obtained by theEUV Imaging Spectrometer onboard Hinode and find various types offlows during CMEs and jet eruptions.CME-induced dimming regions are found to be characterized bysignificant blueshift and enhancedline width by using a single Gaussian fit. While a red-blue (RB)asymmetry analysis and a RB-guideddouble Gaussian fit of the coronal line profiles indicate that theseare likely caused by the superposition ofa strong background emission component and a relatively weak (~10%)high-speed (~100 km s-1) upflowcomponent. This finding suggests that the outflow velocity in thedimming region is probably of theorder of 100 km s-1 not ~20 km s-1 as reported previously. Density andtemperature diagnostics of thedimming region suggest that dimming is primarily an effect of densitydecrease rather than temperaturechange. The mass losses in dimming regions as estimated from differentmethods are roughly consistentwith each other and they are 20%-60% of the masses of the associatedCMEs. With the guide of RBasymmetry analysis, we also find several temperature-dependentoutflows (speed increases with temperature)immediately outside the (deepest) dimming region. These outflows maybe evaporation flows whichare caused by the enhanced thermal conduction or nonthermal electronbeams along reconnecting fieldlines, or induced by the interaction between the opened field lines inthe dimming region and the closedloops in the surrounding plage region. In an erupted CME loop and anEUV jet, profiles of emissionlines formed at coronal and transition region temperatures are foundto exhibit two well-separated components,an almost stationary component accounting for the background emissionand a highly blueshifted(~200 km s-1 component representing emission from the erupting material.The two components caneasily be decomposed through a double Gaussian fit and we can diagnosethe electron density, temperatureand mass of the ejecta. Combining the speed of the blueshiftedcomponent and the projected speedof the erupting material derived from simultaneous imagingobservations, we can calculate the real speedof the ejecta.

Authors: Hui Tian, Scott W. McIntosh, Lidong Xia, Jiansen He, Xin Wang
Projects: None

Publication Status: ApJ, in press
Last Modified: 2012-01-11 09:11
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Two components of the coronal emission revealed by EUV spectroscopic observations  

Hui Tian   Submitted: 2011-06-06 12:10

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Hui Tian, Scott W. McIntosh, Bart De Pontieu, Juan Martínez-Sykora, Marybeth Sechler, Xin Wang
Projects: None

Publication Status: ApJ, in press
Last Modified: 2011-06-07 08:50
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Observation of High-speed Outflow on Plume-like Structures of the Quiet Sun and Coronal Holes with SDO/AIA  

Hui Tian   Submitted: 2011-05-17 14:55

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Hui Tian, Scott W. McIntosh, Shadia Rifal Habbal, Jiansen He
Projects: Hinode/EIS,SDO-AIA,STEREO

Publication Status: ApJ, in press
Last Modified: 2011-05-17 19:26
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Abstracts by Author
CoMP observations of Coronal Mass Ejections
THE NASCENT FAST SOLAR WIND OBSERVED BY THE EUV IMAGING SPECTROMETER ON BOARD HINODE
HYDROGEN Lyα AND Lyβ RADIANCES AND PROFILES IN POLAR CORONAL HOLES
Sizes of transition-region structures in coronal holes and in the quiet Sun
Solar transition region above sunspots
Horizontal supergranule-scale motions inferred from TRACE ultraviolet observations of the chromosphere
PERSISTENT DOPPLER SHIFT OSCILLATIONS OBSERVED WITH HINODE/EIS IN THE SOLAR CORONA: SPECTROSCOPIC SIGNATURES OF ALFVENIC WAVES AND RECURRING UPFLOWS
What can we learn about solar coronal mass ejections, coronal dimmings, and Extreme-Ultraviolet jets through spectroscopic observations?
Two components of the coronal emission revealed by EUV spectroscopic observations
Observation of High-speed Outflow on Plume-like Structures of the Quiet Sun and Coronal Holes with SDO/AIA

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