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Evidence for two-loop interaction from IRIS and SDO observations of penumbral brightenings  

Spiros Patsourakos   Submitted: 2017-04-26 04:56

We analyzed spectral and imaging data from the Interface Region Imaging Spectrograph (IRIS), images from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO), and magnetograms from the Helioseismic and Magnetic Imager (HMI) aboard SDO. We report observations of small flaring loops in the penumbra of a large sunspot on July 19, 2013. Our main event consisted of a loop spanning ~ 15 arcsec, from the umbral-penumbral boundary to an opposite polarity region outside the penumbra. It lasted approximately 10 minutes with a two minute impulsive peak and was observed in all AIA/SDO channels, while the IRIS slit was located near its penumbral footpoint. Mass motions with an apparent velocity of ~ 100 km s-1 were detected beyond the brightening, starting in the rise phase of the impulsive peak; these were apparently associated with a higher-lying loop. We interpret these motions in terms of two-loop interaction. IRIS spectra in both the C II and Si IV lines showed very extended wings, up to about 400 km s-1, first in the blue (upflows) and subsequently in the red wing. In addition to the strong lines, emission was detected in the weak lines of Cl I, O I and C I as well as in the Mg II triplet lines. Absorption features in the profiles of the C II doublet, the Si IV doublet and the Mg h and k lines indicate the existence of material with a lower source function between the brightening and the observer. We attribute this absorption to the higher loop and this adds further credibility to the two-loop interaction hypothesis. We conclude that the absorption features in the C II, Si IV and Mg II profiles originate in a higher-lying, descending loop; as this approached the already activated lower-lying loop, their interaction gave rise to the impulsive peak, the very broad line profiles and the mass motions.

Authors: C. E. Alissandrakis, A. Koukras, S. Patsourakos, A. Nindos
Projects: IRIS

Publication Status: 2017, Astronomy and Astrophysics, in press
Last Modified: 2017-04-26 12:02
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Microwave and EUV Observations of an Erupting Filament and Associated Flare and CME  

Spiros Patsourakos   Submitted: 2013-09-14 05:11

A filament eruption was observed with the Siberian Solar Radio Telescope (SSRT) on June 23 2012, starting around 06:40 UT, beyond the West limb. The filament could be followed in SSRT images to heights above 1 Rs, and coincided with the core of the CME, seen in LASCO C2 images. We discuss briefly the dynamics of the eruption: the top of the filament showed a smooth acceleration up to an apparent velocity of 1100 km s-1. Images behind the limb from STEREO-A show a two ribbon flare and the interaction of the main filament, located along the primary neutral line, with an arch-like structure, oriented in the perpendicular direction. The interaction was accompanied by strong emission and twisting motions. The microwave images show a low temperature component, a high temperature component associated with the interaction of the two filaments and another high temperature component apparently associated with the top of flare loops. We computed the differential emission measure from the high temperature AIA bands and from this the expected microwave brightness temperature; for the emission associated with the top of flare loops the computed brightness was 35% lower than the observed.

Authors: Alissandrakis, C. E.; Kochanov, A. A.; Patsourakos, S.; Altyntsev, A. T.; Lesovoi, S. V.; Lesovoya, N. N.
Projects: None

Publication Status: PASJ, Oct 2013, in press
Last Modified: 2013-09-14 17:27
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Hot coronal loops associated with umbral brightenings  

Spiros Patsourakos   Submitted: 2013-08-05 20:03

Aims: We aim to investigate the association of umbral brightenings with coronal structures. Methods: We analyzed AIA/SDO high-cadence images in all bands, HMI/SDO data, soft X-ray images from SXI/GOES-15, and Hα images from the GONG network. Results: We detected umbral brightenings that were visible in all AIA bands as well as in Hα. Moreover, we identified hot coronal loops that connected the brightenings with nearby regions of opposite magnetic polarity. These loops were initially visible in the 94 Å band, subsequently in the 335 Å band, and in one case in the 211 Å band. A differential emission measure analysis revealed plasma with an average temperature of about 6.5 ? 106 K. This behavior suggests cooling of impulsively heated loops.

Authors: Alissandrakis, C. E.; Patsourakos, S.
Projects: SDO-AIA

Publication Status: Astronomy & Astrophysics, 2013, Volume 556, id.A79, 5 pp
Last Modified: 2013-08-06 14:53
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Direct Evidence for a Fast CME Driven by the Prior Formation and Subsequent Destabilization of a Magnetic Flux Rope  

Spiros Patsourakos   Submitted: 2012-12-14 03:59

Magnetic flux ropes play a central role in the physics of Coronal Mass Ejections (CMEs). Although a flux rope topology is inferred for the majority of coronagraphic observations of CMEs, a heated debate rages on whether the flux ropes pre-exist or whether they are formed on-the-fly during the eruption. Here, we present a detailed analysis of Extreme Ultraviolet observations of the formation of a flux rope during a confined flare followed about seven hours later by the ejection of the flux rope and an eruptive flare. The two flares occurred during 18 and 19 July 2012. The second event unleashed a fast (> 1000 km s-1) CME. We present the first direct evidence of a fast CME driven by the prior formation and destabilization of a coronal magnetic flux rope formed during the confined flare on 18 July.

Authors: S. Patsourakos, A. Vourlidas, G. Stenborg
Projects: SDO-AIA

Publication Status: ApJ, 2013 in press
Last Modified: 2012-12-14 16:10
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On the Nature and Genesis of EUV Waves: A Synthesis of Observations from SOHO, STEREO, SDO, and Hinode (Invited Review)  

Spiros Patsourakos   Submitted: 2012-05-19 01:29

A major, albeit serendipitous, discovery of the SOlar and Heliospheric Observatory mission was the observation by the Extreme Ultraviolet Telescope (EIT) of large-scale extreme ultraviolet (EUV) intensity fronts propagating over a significant fraction of the Sun's surface. These so-called EIT or EUV waves are associated with eruptive phenomena and have been studied intensely. However, their wave nature has been challenged by non-wave (or pseudo-wave) interpretations and the subject remains under debate. A string of recent solar missions has provided a wealth of detailed EUV observations of these waves bringing us closer to resolving the question of their nature. With this review, we gather the current state-of-the-art knowledge in the field and synthesize it into a picture of an EUV wave driven by the lateral expansion of the CME. This picture can account for both wave and pseudo-wave interpretations of the observations, thus resolving the controversy over the nature of EUV waves to a large degree but not completely. We close with a discussion on several remaining open questions in the field of EUV waves research.

Authors: S. Patsourakos, A. Vourlidas
Projects: None

Publication Status: Solar Physics, 2012, On-line first
Last Modified: 2012-05-20 08:22
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The Genesis of an Impulsive Coronal Mass Ejection observed at Ultra-High Cadence by AIA on SDO  

Spiros Patsourakos   Submitted: 2010-10-28 11:23

The study of fast, eruptive events in the low solar corona is one of the science objectives of the Atmospheric Imaging Assembly (AIA) imagers on the recently launched Solar Dynamics Observatory (SDO), which take full disk images in ten wavelengths with arcsecond resolution and 12 sec cadence. We study with AIA the formation of an impulsive coronal mass ejection (CME) which occurred on June 13, 2010 and was associated with an M1.0 class flare. Specifically, we analyze the formation of the CME EUV bubble and its initial dynamics and thermal evolution in the low corona using AIA images in three wavelengths (171, 193 and 211 A). We derive the first ultra-high cadence measurements of the temporal evolution of the CME bubble aspect ratio (=bubble-height/bubble-radius). Our main result is that the CME formation undergoes three phases: it starts with a slow self-similar expansion followed by a fast but short-lived (~ 70 sec) period of strong lateral over-expansion which essentially creates the CME. Then the CME undergoes another phase of self-similar expansion until it exits the AIA field of view. During the studied interval, the CME height-time profile shows a strong, short-lived, acceleration followed by deceleration. The lateral overexpansion phase coincides with the deceleration phase. The impulsive flare heating and CME acceleration are closely coupled. However, the lateral overexpansion of the CME occurs during the declining phase and is therefore not linked to flare reconnection. In addition, the multi-thermal analysis of the bubble does not show significant evidence of temperature change.

Authors: S. Patsourakos, A. Vourlidas, G. Stenborg
Projects: None

Publication Status: ApJL 2010, in press. Movies available at http://dl.dropbox.com/u/3971111/movies_aia_cme.tar.gz.
Last Modified: 2010-10-30 01:51
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Evidence for a current sheet forming in the wake of a Coronal Mass Ejection from multi-viewpoint coronagraph observations  

Spiros Patsourakos   Submitted: 2010-10-05 03:20

Ray-like features observed by coronagraphs in the wake of Coronal Mass Ejections (CMEs) are sometimes interpreted as the white light counterparts of current sheets (CSs) produced by the eruption. The 3D geometry of these ray-like features is largely unknown and its knowledge should clarify their association to the CS and place constraints on CME physics and coronal conditions. With this study we test these important implications for the first time. An example of such a post-CME ray was observed by various coronagraphs, including these of the SECCHI instrument suite of the STEREO twin spacecraft and the Large Angle Spectrometric Coronagraph LASCO onboard the Solar and Heliospheric Observatory (SOHO). The ray was observed in the aftermath of a CME which occurred on 9 April 2008. The twin STEREO spacecraft were separated by about degrees on that day. This significant separation combined with a third ''eye'' view supplied by LASCO allow for a truly multi-viewpoint observation of the ray and of the CME. We applied 3D forward geometrical modeling to the CME and to the ray as simultaneously viewed by SECCHI-A and B and by SECCHI-A and LASCO, respectively. We found that the ray can be approximated by a rectangular slab, nearly aligned with the CME axis, and much smaller than the CME in both terms of thickness and depth (~ 0.05 and 0.15 Rsun respectively). We found that the ray and CME are significantly displaced from the associated post-CME flaring loops. The properties and location of the ray are fully consistent with the expectations of the standard CME theories for post-CME current sheets. Therefore, our multi-viewpoint observations supply strong evidence that the observed post-CME ray is indeed related to a post-CME current sheet.

Authors: S. Patsourakos, A. Vourlidas
Projects: STEREO

Publication Status: A&A, 2010, in press
Last Modified: 2010-10-05 08:00
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Toward understanding the early stages of an impulsively accelerated coronal mass ejection  

Spiros Patsourakos   Submitted: 2010-08-09 08:03

The expanding magnetic flux in coronal mass ejections (CMEs) often forms a cavity. A spherical model is simultaneously fit to STEREO EUVI and COR1 data of an impulsively accelerated CME on 25 March 2008, which displays a well-defined extreme ultraviolet (EUV) and white-light cavity of nearly circular shape already at low heights ~ 0.2 Rs. The center height h(t) and radial expansion r(t) of the cavity are obtained in the whole height range of the main acceleration. We interpret them as the axis height and as a quantity proportional to the minor radius of a flux rope, respectively. The three-dimensional expansion of the CME exhibits two phases in the course of its main upward acceleration. From the first h and r data points, taken shortly after the onset of the main acceleration, the erupting flux shows an overexpansion compared to its rise, as expressed by the decrease of the aspect ratio from k=h/r ~ 3 to k ~ (1.5-2.0). This phase is approximately coincident with the impulsive rise of the acceleration and is followed by a phase of very gradual change of the aspect ratio (a nearly self-similar expansion) toward k ~ 1.5 at h ~ 10 Rs. The initial overexpansion of the CME cavity can be caused by flux conservation around a rising flux rope of decreasing axial current and by the addition of flux to a growing, or even newly forming,flux rope by magnetic reconnection. Further analysis will be required to decide which of these contributions is dominant. The data also suggest that the horizontal component of the impulsive cavity expansion (parallel to the solar surface) triggers the associated EUV wave, which subsequently detaches from the CME volume.

Authors: S. Patsourakos, A. Vourlidas, B. Kliem
Projects: STEREO

Publication Status: A&A, 2010, in press
Last Modified: 2010-08-09 08:03
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What is the Nature of EUV Waves? First STEREO 3D Observations and Comparison with Theoretical Models  

Spiros Patsourakos   Submitted: 2009-05-18 08:18

One of the major discoveries of the Extreme ultraviolet Imaging Telescope (EIT) on SOHO were intensity enhancements propagating over a large fraction of the solar surface. The physical origin(s) of the so-called `EIT' waves is still strongly debated. They are considered to be either wave (primarily fast-mode MHD waves) or non-wave (pseudo-wave) interpretations. The difficulty in understanding the nature of EUV waves lies with the limitations of the EIT observations which have been used almost exclusively for their study. They suffer from low cadence, and single temperature and viewpoint coverage. These limitations are largely overcome by the SECCHI/EUVI observations on-board the STEREO mission. The EUVI telescopes provide high cadence, simultaneous multi-temperature coverage, and two well-separated viewpoints. We present here the first detailed analysis of an EUV wave observed by the EUVI disk imagers on December 07, 2007 when the STEREO spacecraft separation was approx 45^circ. Both a small flare and a CME were associated with the wave. We also offer the first comprehensive comparison of the various wave interpretations against the observations. Our major findings are: (1) high-cadence (2.5 min) 171AA, images showed a strong association between expanding loops and the wave onset and significant differences in the wave appearance between the two STEREO viewpoints during its early stages; these differences largely disappeared later, (2) the wave appears at the active region periphery when an abrupt disappearance of the expanding loops occurs within an interval of 2.5 minutes, (3) almost simultaneous images at different temperatures showed that the wave was most visible in the 1-2 MK range and almost invisible in chromospheric/transition region temperatures, (4) triangulations of the wave indicate it was rather low-lying (approx90 Mm above the surface), (5) forward-fitting of the corresponding CME as seen by the COR1 coronagraphs showed that the projection of the best-fit model on the solar surface was not consistent with the location and size of the co-temporal EUV wave and (6) simulations of a fast-mode wave were found in a good agreement with the overall shape and location of the observed wave. Our findings give significant support for a fast-mode interpretation of EUV waves and indicate that they are probably triggered by the rapid expansion of the loops associated with the CME.

Authors: Patsourakos, S.; Vourlidas, A.; Wang, Y. -M.; Stenborg, G.; Thernisien, A.
Projects: STEREO

Publication Status: Sol Phys, 2009, Special STEREO Issue, in press
Last Modified: 2009-05-18 08:23
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Spectroscopic Observations of Hot Lines Constraining Coronal Heating in Solar Active Regions  

Spiros Patsourakos   Submitted: 2009-03-24 09:03

EUV observations of warm coronal loops suggest that they are bundles of unresolved strands that are heated impulsively to high temperatures by nanoflares. The plasma would then have the observed properties (e.g., excess density compared to static equilibrium) when it cools into the 1-2 MK range. If this interpretation is correct, then very hot emission should be present outside of proper flares. It is predicted to be vey faint, however. A critical element for proving or refuting this hypothesis is the existence of hot, very faint plasmas which should be at amounts predicted by impulsive heating. We report on the first comprehensive spectroscopic study of hot plasmas in active regions. Data from the EIS spectrometer on Hinode were used to construct emission measure distributions in quiescent active regions in the 1-5 MK temperature range. The distributions are flat or slowly increasing up to approximately 3 MK and then fall off rapidly at higher temperatures. We show that active region models based on impulsive heating can reproduce the observed EM distributions relatively well. Our results provide strong new evidence that coronal heating is impulsive in nature.

Authors: S. Patsourakos, J. A. Klimchuk
Projects: Hinode/EIS

Publication Status: ApJ 2009, in press
Last Modified: 2009-03-24 09:31
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Static and Impulsive Models of Solar Active Regions  

Spiros Patsourakos   Submitted: 2008-08-21 13:24

The physical modeling of active regions (ARs) and of the global coronal is receiving increasing interest lately. Recent attempts to model ARs using static equilibrium models were quite successful in reproducing AR images of hot soft X-ray (SXR) loops. They however failed to predict the bright EUV warm loops permeating ARs: the synthetic images were dominated by intense footpoint emission. We demonstrate that this failure is due to the very weak dependence of loop temperature on loop length which cannot simultaneously account for both hot and warm loops in the same AR. We then consider time-dependent AR models based on nanoflare heating. We demonstrate that such models can simultaneously reproduce EUV and SXR loops in ARs. Moreover, they predict radial intensity variations consistent with the localized core and extended emissions in SXR and EUV AR observations respectively. We finally show how the AR morphology can be used as a gauge of the properties (duration, energy, spatial dependence, repetition time) of the impulsive heating.

Authors: S. Patsourakos, J. A. Klimchuk
Projects: None

Publication Status: ApJ, 2008, in press
Last Modified: 2008-08-22 13:37
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Subject will be restored when possible  

Spiros Patsourakos   Submitted: 2008-05-01 10:42

We report on the first stereoscopic observations of polar coronal jets made by the EUVI/SECCHI imagers on board the twin STEREO spacecraft. The significantly separated viewpoints (sim 11circ) allowed us to infer the 3D dynamics and morphology of a well-defined EUV coronal jet for the first time. Triangulations of the jet's location in simultaneous image pairs led to the true 3D position and thereby its kinematics. Initially the jet ascends slowly at approx10-20 mathrm{{km} {s}-1} and then, after an apparent 'jump' takes place, it accelerates impulsively to velocities exceeding 300 mathrm{{km} {s}-1} with accelerations exceeding the solar gravity. Helical structure is the most important geometrical feature of the jet which shows evidence of untwisting. The jet structure appears strikingly different from each of the two STEREO viewpoints: face-on in the one viewpoint and edge-on in the other. This provides conclusive evidence that the observed helical structure is real and is not resulting from possible projection effects of single viewpoint observations. The clear demonstration of twisted structure in polar jets compares favorably with synthetic images from a recent MHD simulation of jets invoking magnetic untwisting as their driving mechanism. Therefore, the latter can be considered as a viable mechanism for the initiation of polar jets.

Authors: Patsourakos, S., Pariat, E. Vourlidas, A., Antiochos, S. K., Wuelser, J. P.
Projects: STEREO

Publication Status: ApJL in press
Last Modified: 2008-09-23 21:02
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The Quiet Sun Network at Sub-Arcsecond Resolution: VAULT Observations and Radiative Transfer Modeling of Cool Loops  

Spiros Patsourakos   Submitted: 2007-05-02 08:58

One of the most enigmatic regions of the solar atmosphere is the transition region (TR), corresponding to plasmas with temperatures intermediate of the cool, few thousand K, chromosphere and the hot, few million K, corona. The traditional view is that the TR emission originates from a thin thermal interface in hot coronal structures, connecting their chromosphere with their corona. This paradigm fails badly for cool plasmas (approx T < {10}5 K) since it predicts emission orders of magnitude less than what it is observed. It was therefore proposed that the ''missing'' TR emission could originate from tiny, isolated from the hot corona, cool loops at TR temperatures. A major problem in investigating this proposal is the very small sizes of the hypothesized cool loops. Here, we report the first spatially resolved observations of sub-arcsec-scale loop-like structures seen in the Ly α line made by the Very High Angular Resolution Ultraviolet Telescope (VAULT). The sub-arcsec (approx 0.3 arcsec) resolution of VAULT allows us to directly view and resolve loop-like structures in the quiet Sun network. We compare the observed intensities of these structures with simplified radiative transfer models of cool loops. The reasonable agreement between the models and the observations indicates that an explanation of the observed fine structure in terms of cool loops is plausible.

Authors: S. Patsourakos, P. Gouttebroze, and A. Vourlidas
Projects: None

Publication Status: ApJ, 2007, in press, July 20, v664n 1 issue
Last Modified: 2007-05-02 11:04
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Coronal Loop Heating by Nanoflares: The Impact of the Field-aligned Distribution of the Heating on Loop Observations  

Spiros Patsourakos   Submitted: 2006-05-06 03:35

Nanoflares occurring at sub-resolution strands with repetition times longer than the coronal cooling time are a promising candidate for coronal loop heating. To investigate the impact of the spatial distribution of the nanoflare heating on loop observables, we compute hydrodynamic simulations with several different spatial distributions (uniform, loop top, randomly localized, and footpoint). The outputs of the simulations are then used to calculate density and temperature diagnostics from synthetic TRACE and SXT observations. We find that the diagnostics depend only weakly on the spatial distribution of the heating, and therefore are not especially useful for distinguishing among the different possibilities.

Authors: S. Patsourakos, J. A. Klimchuk
Projects: None

Publication Status: ApJ, 2005, 628, 1023-1030
Last Modified: 2006-05-10 08:19
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A Model for Bright Knots in Solar Flare Loops  

Spiros Patsourakos   Submitted: 2004-06-28 07:32

EUV observations often indicate the presence of bright knots in flare loops. The temperature of the knot plasma is of order 1MK, and the knots themselves are usually localized somewhere near the loop tops. We propose a model in which the formation of EUV knots is due to the spatial structure of the non-flare active region heating. We present the results of a series of 1D hydrodynamic, flare-loop simulations, which include both an impulsive flare heating and a background, active region heating. The simulations demonstrate that the formation of the observed knots depends critically on the spatial distribution of the background heating during the decay phase. In particular, the heating must be localized far from the loop apex and have a magnitude comparable with the local radiative losses of the cooling loop. Our results, therefore, provide strong constraints on both coronal heating and post-flare conditions.

Authors: S. Patsourakos, S. K. Antiochos, and J. A. Klimchuk
Projects: TRACE

Publication Status: ApJ (in press)
Last Modified: 2004-06-28 07:32
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The Inability of Steady Flow Models to Explain the EUV Coronal Loops  

Spiros Patsourakos   Submitted: 2003-12-03 11:43

Recent observations from TRACE and EIT show that warm (1-1.5 MK) EUV coronal loops in active regions generally have enhanced densities, enhanced pressure scale heights, and flat filter ratio (temperature) profiles in comparison with the predictions of static equilibrium theory. It has been suggested that mass flows may explain these discrepancies. We investigate this conjecture using 1D hydrodynamic simulations of steady flows in coronal loops. The flows are driven by asymmetric heating that decreases exponentially along the loop from one footpoint to the other. We find that a sufficiently large heating asymmetry can produce density enhancements consistent with a sizable fraction of the observed loops, but that the pressure scale heights are smaller than the corresponding gravitational scale heights and that the filter ratio profiles are highly structured, in stark contrast with the observations. We conclude that most warm EUV loops cannot be explained by steady flows. It is thus likely that the heating in these loops is time dependent.

Authors: S. Patsourakos, J. A. Klimchuk & P. J. MacNeice
Projects: None

Publication Status: ApJ (in press)
Last Modified: 2003-12-03 11:43
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Abstracts by Author
Evidence for two-loop interaction from IRIS and SDO observations of penumbral brightenings
Microwave and EUV Observations of an Erupting Filament and Associated Flare and CME
Hot coronal loops associated with umbral brightenings
Direct Evidence for a Fast CME Driven by the Prior Formation and Subsequent Destabilization of a Magnetic Flux Rope
On the Nature and Genesis of EUV Waves: A Synthesis of Observations from SOHO, STEREO, SDO, and Hinode (Invited Review)
The Genesis of an Impulsive Coronal Mass Ejection observed at Ultra-High Cadence by AIA on SDO
Evidence for a current sheet forming in the wake of a Coronal Mass Ejection from multi-viewpoint coronagraph observations
Toward understanding the early stages of an impulsively accelerated coronal mass ejection
What is the Nature of EUV Waves? First STEREO 3D Observations and Comparison with Theoretical Models
Spectroscopic Observations of Hot Lines Constraining Coronal Heating in Solar Active Regions
Static and Impulsive Models of Solar Active Regions
Subject will be restored when possible
The Quiet Sun Network at Sub-Arcsecond Resolution: VAULT Observations and Radiative Transfer Modeling of Cool Loops
Coronal Loop Heating by Nanoflares: The Impact of the Field-aligned Distribution of the Heating on Loop Observations
A Model for Bright Knots in Solar Flare Loops
The Inability of Steady Flow Models to Explain the EUV Coronal Loops

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