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Multi-wavelength Study of Transition Region Penumbral Subarcsecond Bright Dots Using IRIS and NST  

Na Deng   Submitted: 2016-07-05 13:38

Using high-resolution transition region (TR) observations taken by the Interface Region Imaging Spectrograph (IRIS) mission, Tian et al. (2014b) revealed numerous short-lived subarcsecond bright dots (BDs) above sunspots (mostly located in the penumbrae), which indicate yet unexplained small-scale energy releases. Moreover, whether these subarcsecond TR brightenings have any signature in the lower atmosphere and how they are formed are still not fully resolved. This paper presents a multi-wavelength study of the TR penumbral BDs using a coordinated observation of a near disk-center sunspot with IRIS and the 1.6 m New Solar Telescope (NST) at the Big Bear Solar Observatory. NST provides high-resolution chromospheric and photospheric observations with narrow-band Hα imaging spectroscopy and broad-band TiO images, respectively, complementary to IRIS TR observations. A total of 2692 TR penumbral BDs are identified from a 37-minute time series of IRIS 1400 Å slitjaw images. Their locations tend to be associated more with downflowing and darker fibrils in the chromosphere, and weakly associated with bright penumbral features in the photosphere. However, temporal evolution analyses of the BDs show that there is no consistent and convincing brightening response in the chromosphere. These results are compatible with a formation mechanism of the TR penumbral BDs by falling plasma from coronal heights along more vertical and dense magnetic loops. The BDs may also be produced by small-scale impulsive magnetic reconnection taking place sufficiently high in the atmosphere that has no energy release in the chromosphere.

Authors: Na Deng, Vasyl Yurchyshyn, Hui Tian, Lucia Kleint, Chang Liu, Yan Xu, and Haimin Wang
Projects: BBSO/NST

Publication Status: accepted to ApJ
Last Modified: 2016-07-06 10:35
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Chromospheric Rapid Blueshifted Excursions Observed with IBIS and Their Association with Photospheric Magnetic Field Evolution  

Na Deng   Submitted: 2014-12-14 19:11

Chromospheric rapid blueshifted excursions (RBEs) are suggested to be the disk counterparts of type II spicules at the limb and believed to contribute to the coronal heating process. Previous identification of RBEs was mainly based on feature detection using Dopplergrams. In this paper, we study RBEs on 2011 October 21 in a very quiet region at the disk center, which were observed with the high-cadence imaging spectroscopy of the Ca II 8542 Å line from the Interferometric Bidimensional Spectrometer (IBIS). By using an automatic spectral analysis algorithm, a total of 98 RBEs are identified during a 11 minute period. Most of these RBEs have either a round or elongated shape, with an average area of 1.2 arcsec^2. The detailed temporal evolution of spectra from IBIS makes possible a quantitative determination of the velocity (~16 km s-1) and acceleration (~400 m s-2) of Ca II 8542 RBEs, and reveal an additional deceleration (~-160 m s-2) phase that usually follows the initial acceleration. In addition, we also investigate the association of RBEs with the concomitant photospheric magnetic field evolution, using coordinated high-resolution and high-sensitivity magnetograms made by Hinode. Clear examples are found where RBEs appear to be associated with the preceding magnetic flux emergence and/or the subsequent flux cancellation. However, a further analysis with the aid of the Southwest Automatic Magnetic Identification Suite does not yield a significant statistical association between these RBEs and magnetic field evolution. We discuss the implications of our results in the context of understanding the driving mechanism of RBEs.

Authors: Na Deng, Xin Chen, Chang Liu, Ju Jing, Alexandra Tritschler, Kevin P. Reardon, Derek A. Lamb, Craig E. Deforest, Carsten Denker, Shuo Wang, Rui Liu, Haimin Wang
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2014-12-15 13:50
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High-Cadence and High-Resolution Hα Imaging Spectroscopy of a Circular Flare's Remote Ribbon with IBIS  

Na Deng   Submitted: 2013-04-16 07:46

We present an unprecedented high-resolution ha imaging spectroscopic observation of a C4.1 flare taken with the Interferometric Bidimensional Spectrometer (IBIS) in conjunction with the adaptive optics system at the 76~cm Dunn Solar Telescope on 2011 October 22 in active region NOAA 11324. Such a two-dimensional spectroscopic observation covering the entire evolution of a flare ribbon with high spatial (0.1arcpix image scale), cadence (4.8 s) and spectral (0.1~AA stepsize) resolution is rarely reported. The flare consists of a main circular ribbon that occurred in a parasitic magnetic configuration and a remote ribbon that was observed by the IBIS. Such a circular-ribbon flare with a remote brightening is predicted in 3D fan-spine reconnection but so far has been rarely observed. During the flare impulsive phase, we define ``core'' and ``halo'' structures in the observed ribbon based on IBIS narrowband images in the ha line wing and line center. Examining the ha emission spectra averaged in the flare core and halo areas, we find that only those from the flare cores show typical nonthermal electron beam heating characteristics that have been revealed by previous theoretical simulations and observations of flaring ha line profiles. These characteristics include: broad and centrally reversed emission spectra, excess emission in the red wing with regard to the blue wing (i.e., red asymmetry), and redshifted bisectors of the emission spectra. We also observe rather quick timescales for the heating (sim30 s) and cooling (sim14-33 s) in the flare core locations. Therefore, we suggest that the flare cores revealed by IBIS track the sites of electron beam precipitation with exceptional spatial and temporal resolution. The flare cores show two-stage motion (a parallel motion along the ribbon followed by an expansion motion perpendicular to the ribbon) during the two impulsive phases of the flare. Some cores jump quickly (30 kms) between discrete magnetic elements implying reconnection involving different flux tubes. We observe a very high temporal correlation (gtrsim0.9) between the integrated ha and HXR emission during the flare impulsive phase. A short time delay (4.6 s) is also found in the ha emission spikes relative to HXR bursts. The ionization timescale of the cool chromosphere and the extra time taken for the electrons to travel to the remote ribbon site may contribute to this delay.

Authors: Na Deng, Alexandra Tritschler, Ju Jing, Xin Chen, Chang Liu, Kevin Reardon, Carsten Denker, Yan Xu, and Haimin Wang
Projects: Hinode/SOT,National Solar Observatory (Sac Peak),RHESSI,SDO-AIA,SDO-HMI

Publication Status: The Astrophysical Journal, accepted
Last Modified: 2013-04-17 12:22
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RAPID ENHANCEMENT OF SHEARED EVERSHED FLOW ALONG THE NEUTRAL LINE ASSOCIATED WITH AN X6.5 FLARE OBSERVED BY HINODE  

Na Deng   Submitted: 2011-05-10 12:41

We present G-band and Ca II H observations of NOAA AR 10930 obtained by Hinode/SOT on 2006 December 6 covering an X6.5 flare. The Local Correlation Tracking technique was applied to the foreshortening-corrected G-band image series to acquire horizontal proper motions in this complex ??? active region. With the continuous high-quality, spatial and temporal resolution G-band data, we not only confirm the rapid decay of outer penumbrae and darkening of the central structure near the flaring neutral line, but also unambiguously detect for the first time the enhancement of the sheared Evershed flow (average horizontal flow speed increased from 330 ± 3.1 to 403 ± 4.6 m s–1) along the neutral line right after the eruptive white-light flare. Post-flare Ca II H images indicate that the originally fanning out field lines at the two sides of the neutral line get connected. Since penumbral structure and Evershed flow are closely related to photospheric magnetic inclination or horizontal field strength, we interpret the rapid changes of sunspot structure and surface flow as the result of flare-induced magnetic restructuring down to the photosphere. The magnetic fields turn from fanning out to inward connection causing outer penumbrae decay, meanwhile those near the flaring neutral line become more horizontal leading to stronger Evershed flow there. The inferred enhancement of horizontal magnetic field near the neutral line is consistent with recent magnetic observations and theoretical predictions of flare-invoked photospheric magnetic field change.


Authors: Na Deng, Chang Liu, Debi Prasad Choudhary and Haimin Wang
Projects: Hinode/SOT

Publication Status: Published in ApJL
Last Modified: 2011-05-11 09:12
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On the Doppler Shift and Asymmetry of Stokes Profiles of Photospheric FeI and Chromospheric MgI Lines  

Na Deng   Submitted: 2010-06-20 19:22

We analyzed the full Stokes spectra using simultaneous measurements of the photospheric (FeI 630.15 and 630.25 nm) and chromospheric (MgI b2 517.27 nm) lines. The data were obtained with the HAO/NSO Advanced Stokes Polarimeter, about a near disc center sunspot region, NOAA AR 9661. We compare the characteristics of Stokes profiles in terms of Doppler shifts and asymmetries among the three spectral lines, which helps us to better understand the chromospheric lines and the magnetic and flow fields in different magnetic regions. The main results are: (1) For penumbral area observed by the photospheric FeI lines, Doppler velocities derived from Stokes I (Vi) are very close to those derived from linear polarization profiles (Vlp) but significantly different from those derived from Stokes V profiles (Vzc), which provides direct and strong evidence that the penumbral Evershed flows are magnetized and mainly carried by the horizontal magnetic component. (2) The rudimentary inverse Evershed effect observed by the MgI b2 line provides a qualitative evidence on its formation height that is around or just above the temperature minimum region. (3) Vzc and Vlp in penumbrae and Vzc in pores generally approach their Vi observed by the chromospheric MgI line, which is not the case for the photospheric FeI lines. (4) Outer penumbrae and pores show similar behavior of the Stokes V asymmetries that tend to change from positive values in the photosphere (FeI lines) to negative values in the low chromosphere (MgI line). (5) The Stokes V profiles in plage regions are highly asymmetric in the photosphere and more symmetric in the low chromosphere. (6) Strong red shifts and large asymmetries are found around the magnetic polarity inversion line within the common penumbra of the Delta spot. We offer explanations or speculations to the observed discrepancies between the photospheric and chromospheric lines in terms of the three-dimensional structure of the magnetic and velocity fields. This study thus emphasizes the importance of spectro-polarimetry using chromospheric lines.

Authors: Na Deng, Debi Prasad Choudhary, K. S. Balasubramaniam
Projects: National Solar Observatory (Sac Peak)

Publication Status: accepted to ApJ
Last Modified: 2010-06-21 09:29
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Subject will be restored when possible  

Na Deng   Submitted: 2007-09-23 21:17

We study the evolution of the flows and horizontal proper motions in and around a decaying follower sunspot based on time sequences of two-dimensional spectroscopic observations in the visible and white light imaging data obtained over six days from June~7 to~12, 2005. During this time period the sunspot decayed gradually to a pore. The spectroscopic observations were obtained with the Fabry-P'{e}rot based Visible-Light Imaging Magnetograph (VIM) in conjunction with the high-order adaptive optics (AO) system operated at the 65 cm vacuum reflector of the Big Bear Solar Observatory (BBSO). We apply local correlation tracking (LCT) to the speckle reconstructed time sequences of white-light images around 600 nm to infer horizontal proper motions while the Doppler shifts of the scanned FeI line at 630.15 nm are used to calculate line-of-sight (LOS) velocities with sub-arcsecond resolution. We find that the dividing line between radial inward and outward proper motions in the inner and outer penumbra, respectively, survives the decay phase. In particular the moat flow is still detectable after the penumbra disappeared. Based on our observations three major processes removed flux from the sunspot: (a) fragmentation of the umbra, (b) flux cancelation of moving magnetic features (MMFs; of the same polarity as the sunspot) that encounter the leading opposite polarity network and plages areas, and (c) flux transport by MMFs (of the same polarity as the sunspot) to the surrounding network and plage regions that have the same polarity as the sunspot.

Authors: Na Deng, Debi Prasad Choudhary, Alexandra Tritschler, Carsten Denker, Chang Liu, Haimin Wang
Projects: None

Publication Status: The Astrophysical Journal, accepted September, 2007
Last Modified: 2007-09-24 04:01
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Abstracts by Author
Multi-wavelength Study of Transition Region Penumbral Subarcsecond Bright Dots Using IRIS and NST
Chromospheric Rapid Blueshifted Excursions Observed with IBIS and Their Association with Photospheric Magnetic Field Evolution
High-Cadence and High-Resolution Halpha Imaging Spectroscopy of a Circular Flare's Remote Ribbon with IBIS
RAPID ENHANCEMENT OF SHEARED EVERSHED FLOW ALONG THE NEUTRAL LINE ASSOCIATED WITH AN X6.5 FLARE OBSERVED BY HINODE
On the Doppler Shift and Asymmetry of Stokes Profiles of Photospheric FeI and Chromospheric MgI Lines
Subject will be restored when possible

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