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New Evidence that Magnetoconvection Drives Solar-Stellar Coronal Heating  

Sanjiv Tiwari   Submitted: 2017-06-28 12:52

How magnetic energy is injected and released in the solar corona, keeping it heated to several million degrees, remains elusive. Coronal heating generally increases with increasing magnetic field strength. From comparison of a non-linear force-free model of the three-dimensional active-region coronal field to observed extreme-ultraviolet loops, we find that (1) umbra-to-umbra coronal loops, despite being rooted in the strongest magnetic flux, are invisible, and (2) the brightest loops have one foot in an umbra or penumbra and the other foot in another sunspot?s penumbra or in unipolar or mixed-polarity plage. The invisibility of umbra-to-umbra loops is new evidence that magnetoconvection drives solar-stellar coronal heating: evidently the strong umbral field at both ends quenches the magnetoconvection and hence the heating. Broadly, our results indicate that, depending on the field strength in both feet, the photospheric feet of a coronal loop on any convective star can either engender or quench coronal heating in the loop?s body.

Authors: Sanjiv K. Tiwari, Julia K. Thalmann, Navdeep K. Panesar, Ronald L. Moore, Amy R. Winebarger
Projects: SDO-AIA,SDO-HMI

Publication Status: ApJ Letters, in press
Last Modified: 2017-06-29 12:34
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Hi-C Observations of Sunspot Penumbral Bright Dots  

Sanjiv Tiwari   Submitted: 2016-03-23 19:54

We report observations of bright dots (BDs) in a sunspot penumbra using High Resolution Coronal Imager (Hi-C) data in 193 Å and examine their sizes, lifetimes, speeds, and intensities. The sizes of the BDs are on the order of 1arcsec and are therefore hard to identify in the Atmospheric Imaging Assembly (AIA) 193 Å images, which have 1.2arcsec spatial resolution, but become readily apparent with Hi-C's five times better spatial resolution. We supplement Hi-C data with data from AIA's 193 Å passband to see the complete lifetime of the BDs that appeared before and/or lasted longer than Hi-C's 3-minute observation period. Most Hi-C BDs show clear lateral movement along penumbral striations, toward or away from the sunspot umbra. Single BDs often interact with other BDs, combining to fade away or brighten. The BDs that do not interact with other BDs tend to have smaller displacements. These BDs are about as numerous but move slower on average than Interface Region Imaging Spectrograph (IRIS) BDs, recently reported by cite{tian14}, and the sizes and lifetimes are on the higher end of the distribution of IRIS BDs. Using additional AIA passbands, we compare the lightcurves of the BDs to test whether the Hi-C BDs have transition region (TR) temperature like that of the IRIS BDs. The lightcurves of most Hi-C BDs peak together in different AIA channels indicating that their temperature is likely in the range of the cooler TR (1-4x105 K).

Authors: Shane E. Alpert, Sanjiv K. Tiwari, Ronald L. Moore, Amy R. Winebarger, Sabrina L. Savage
Projects: None

Publication Status: in press, ApJ
Last Modified: 2016-03-30 20:56
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Transition-region/Coronal Signatures and Magnetic Setting of Sunspot Penumbral Jets: Hinode (SOT/FG), Hi-C, and SDO/AIA Observations  

Sanjiv Tiwari   Submitted: 2016-01-28 09:02

Penumbral microjets (PJs) are transient narrow bright features in the chromosphere of sunspot penumbrae, first characterized by Katsukawa et al. using the Ca ii H-line filter on Hinode's Solar Optical Telescope (SOT). It was proposed that the PJs form as a result of reconnection between two magnetic components of penumbrae (spines and interspines), and that they could contribute to the transition region (TR) and coronal heating above sunspot penumbrae. We propose a modified picture of formation of PJs based on recent results on the internal structure of sunspot penumbral filaments. Using data of a sunspot from Hinode/SOT, High Resolution Coronal Imager, and different passbands of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, we examine whether PJs have signatures in the TR and corona. We find hardly any discernible signature of normal PJs in any AIA passbands, except for a few of them showing up in the 1600 Å images. However, we discovered exceptionally stronger jets with similar lifetimes but bigger sizes (up to 600 km wide) occurring repeatedly in a few locations in the penumbra, where evidence of patches of opposite-polarity fields in the tails of some penumbral filaments is seen in Stokes-V images. These tail PJs do display signatures in the TR. Whether they have any coronal-temperature plasma is unclear. We infer that none of the PJs, including the tail PJs, directly heat the corona in active regions significantly, but any penumbral jet might drive some coronal heating indirectly via the generation of Alfvén waves and/or braiding of the coronal field.

Authors: Tiwari, Sanjiv K.; Moore, Ronald L.; Winebarger, Amy R.; Alpert, Shane E.
Projects: Hi-C

Publication Status: published in ApJ, Jan 14, 2016
Last Modified: 2016-02-03 09:19
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Depth-dependent global properties of a sunspot observed by Hinode (SOT/SP)  

Sanjiv Tiwari   Submitted: 2015-08-26 13:32

The three dimensional structure of sunspots has been extensively studied for the last two decades. A recent advancement of the Stokes inversion technique prompts us to revisit the problem. In the present work, we aim to investigate the global depth-dependent thermal, velocity and magnetic properties of a sunspot, as well as the interconnection between various local properties. High quality Stokes profiles of the disk centered, regular, leading sunspot of NOAA AR 10933 acquired by the Solar Optical Telescope/Spectropolarimeter onboard the Hinode spacecraft are analyzed. To obtain the depth-dependent stratification of the physical parameters, we have utilized the recently developed spatially coupled version of the SPINOR inversion code. We first study the azimuthally averaged physical parameters of the sunspot. The vertical temperature gradient in the lower to mid-photosphere is smallest in the umbra, it is considerably larger in the penumbra and still somewhat larger in the spot's surroundings. The azimuthally averaged field becomes more horizontal with radial distance from the center of the spot, but more vertical with height. At continuum optical depth unity, the line-of-sight velocity shows an average upflow of ~ 300 ms-1 in the inner penumbra and an average downflow of ~ 1300 ms-1 in the outer penumbra. The downflow continues outside the visible penumbral boundary. The sunspot shows at most a moderate negative twist of < 5^\circ at log(τ) = 0, which increases with height. The sunspot umbra and the spines of the penumbra show considerable similarity in their physical properties albeit with some quantitative differences (weaker, somewhat more horizontal fields in spines, commensurate with their location further away from the sunspot's core). The temperature shows a general anticorrelation with the field strength, with the exception of the heads of penumbral filaments, where a weak positive correlation is found. The dependence of the physical parameters on each other over the full sunspot shows a qualitative similarity to that of a standard penumbral filament and its surrounding spines. The large-scale variation of the physical parameters of a sunspot at various optical depths is presented. Our results suggest that the spines in the penumbra are basically the outward extension of the umbra. The spines and the penumbral filaments are together the basic elements forming a sunspot penumbra.

Authors: Tiwari, Sanjiv K.; van Noort, Michiel; Solanki, Sami K.; Lagg, Andreas
Projects: Hinode/SOT

Publication Status: A&A, accepted
Last Modified: 2015-08-27 10:27
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Near-Sun Speed of CMEs and the Magnetic Non-potentiality of their Source Active Regions  

Sanjiv Tiwari   Submitted: 2015-08-05 13:14

We show that the speed of the fastest coronal mass ejections (CMEs) that an active region (AR) can produce can be predicted from a vector magnetogram of the AR. This is shown by logarithmic plots of CME speed (from the SOHO LASCO CME catalog) versus each of ten AR-integrated magnetic parameters (AR magnetic flux, three different AR magnetic-twist parameters, and six AR free-magnetic-energy proxies) measured from the vertical and horizontal field components of vector magnetograms (from the Solar Dynamics Observatory's Helioseismic and Magnetic Imager) of the source ARs of 189 CMEs. These plots show: (1) the speed of the fastest CMEs that an AR can produce increases with each of these whole-AR magnetic parameters, and (2) that one of the AR magnetic-twist parameters and the corresponding free-magnetic-energy proxy each determine the CME-speed upper-limit line somewhat better than any of the other eight whole-AR magnetic parameters.

Authors: Sanjiv K. Tiwari, David A. Falconer, Ronald L. Moore, P. Venkatakrishnan, Amy R. Winebarger, Igor G. Khazanov
Projects: SDO-HMI

Publication Status: Published in GRL
Last Modified: 2015-08-06 08:46
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Trigger Mechanism of Solar Subflares in a Braided Coronal Magnetic Structure  

Sanjiv Tiwari   Submitted: 2014-10-17 10:40

Fine-scale braiding of coronal magnetic loops by continuous footpoint motions may power coronal heating via nanoflares, which are spontaneous fine-scale bursts of internal reconnection. An initial nanoflare may trigger an avalanche of reconnection of the braids, making a microflare or larger subflare. In contrast to this internal triggering of subflares, we observe external triggering of subflares in a braided coronal magnetic field observed by the High-resolution Coronal Imager (Hi-C). We track the development of these subflares using 12 s cadence images acquired by SDO/AIA in 1600, 193, 94 Å, and registered magnetograms of SDO/HMI, over four hours centered on the Hi-C observing time. These data show numerous recurring small-scale brightenings in transition-region emission happening on polarity inversion lines where flux cancellation is occurring. We present in detail an example of an apparent burst of reconnection of two loops in the transition region under the braided coronal field, appropriate for releasing a short reconnected loop downward and a longer reconnected loop upward. The short loop presumably submerges into the photosphere, participating in observed flux cancellation. A subflare in the overlying braided magnetic field is apparently triggered by the disturbance of the braided field by the reconnection-released upward loop. At least 10 subflares observed in this braided structure appear to be triggered this way. How common this external trigger mechanism for coronal subflares is in other active regions, and how important it is for coronal heating in general, remain to be seen.

Authors: Sanjiv K. Tiwari, Caroline E. Alexander, Amy R. Winebarger, Ronald L. Moore
Projects: Hi-C

Publication Status: ApJ Letters, in press
Last Modified: 2014-10-22 12:37
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Structure of sunspot penumbral filaments: a remarkable uniformity of properties  

Sanjiv Tiwari   Submitted: 2013-07-25 07:24

The sunspot penumbra comprises numerous thin, radially elongated filaments that are central for heat transport within the penumbra, but whose structure is still not clear. We aim to investigate the fine-scale structure of these penumbral filaments. We perform a depth-dependent inversion of spectropolarimetric data of a sunspot very close to solar disk center obtained by Solar Optical Telescope/Spectropolarimeter on board the Hinode spacecraft. We have used a recently developed, spatially coupled 2D inversion scheme, which allows us to analyze the fine structure of individual penumbral filaments up to the diffraction limit of the telescope. Filaments of different sizes in all parts of the penumbra display very similar magnetic field strengths, inclinations, and velocity patterns. The temperature structure is also similar, although the filaments in the inner penumbra have cooler tails than those in the outer penumbra. The similarities allowed us to average all these filaments and to subsequently extract the physical properties common to all of them. This average filament shows upflows associated with an upward-pointing field at its inner, umbral end (head) and along its axis, as well as downflows along the lateral edge and strong downflows in the outer end (tail) associated with a nearly vertical, strong, and downward-pointing field. The upflowing plasma is significantly, i.e., up to 800,K, hotter than the downflowing plasma. The hot, tear-shaped head of the averaged filament can be associated with a penumbral grain. The central part of the filament shows nearly horizontal fields with strengths in the range of 1,kG. The field above the filament converges, whereas a diverging trend is seen in the deepest layers near the head of the filament. The fluctuations in the physical parameters along and across the filament increase rapidly with depth. We put forward a unified observational picture of a sunspot penumbral filament. It is consistent with such a filament being a magneto-convective cell, in line with recent magnetohydrodynamic simulations. The uniformity of its properties over the penumbra sets constraints on penumbral models and simulations. The complex and inhomogeneous structure of the filament provides a natural explanation for a number of long-running controversies in the literature.

Authors: S. K. Tiwari, M. van Noort, A. Lagg, S. K. Solanki
Projects: None

Publication Status: in press, A&A
Last Modified: 2013-07-25 09:55
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Abstracts by Author
New Evidence that Magnetoconvection Drives Solar-Stellar Coronal Heating
Hi-C Observations of Sunspot Penumbral Bright Dots
Transition-region/Coronal Signatures and Magnetic Setting of Sunspot Penumbral Jets: Hinode (SOT/FG), Hi-C, and SDO/AIA Observations
Depth-dependent global properties of a sunspot observed by Hinode (SOT/SP)
Near-Sun Speed of CMEs and the Magnetic Non-potentiality of their Source Active Regions
Trigger Mechanism of Solar Subflares in a Braided Coronal Magnetic Structure
Structure of sunspot penumbral filaments: a remarkable uniformity of properties

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