Automated Detection of Coronal Loops using a Wavelet Transform Modulus Maxima Method

We propose and test a wavelet transform modulus maxima method for the au- tomated detection and extraction of coronal loops in extreme ultraviolet images of the solar corona. This method decomposes an image into a number of size scales and tracks enhanced power along each ridge corresponding to a coronal loop at each scale. We compare the results across scales and suggest the optimum set of parameters to maximise completeness while minimising detection of noise. For a test coronal image, we compare the global statistics (e.g., number of loops at each length) to previous automated coronal-loop detection algorithms

Coronal loop detection from solar images and extraction of salient contour groups from cluttered images.

This dissertation addresses two different problems: 1) coronal loop detection from solar images: and 2) salient contour group extraction from cluttered images. In the first part, we propose two different solutions to the coronal loop detection problem. The first solution is a block-based coronal loop mining method that detects coronal loops from solar images by dividing the solar image into fixed sized blocks, labeling the blocks as Loop or Non-Loop , extracting features from the labeled blocks, and finally training classifiers to generate learning models that can classify new image blocks. The block-based approach achieves 64% accuracy in IO-fold cross validation experiments. To improve the accuracy and scalability, we propose a contour-based coronal loop detection method that extracts contours from cluttered regions, then labels the contours as Loop and Non-Loop , and extracts geometric features from the labeled contours. The contour-based approach achieves 85% accuracy in IO-fold cross validation experiments, which is a 20% increase compared to the block-based approach. In the second part, we propose a method to extract semi-elliptical open curves from cluttered regions. Our method consists of the following steps: obtaining individual smooth contours along with their saliency measures; then starting from the most salient contour, searching for possible grouping options for each contour; and continuing the grouping until an optimum solution is reached. Our work involved the design and development of a complete system for coronal loop mining in solar images, which required the formulation of new Gestalt perceptual rules and a systematic methodology to select and combine them in a fully automated judicious manner using machine learning techniques that eliminate the need to manually set various weight and threshold values to define an effective cost function. After finding salient contour groups, we close the gaps within the contours in each group and perform B-spline fitting to obtain smooth curves. Our methods were successfully applied on cluttered solar images from TRACE and STEREO/SECCHI to discern coronal loops. Aerial road images were also used to demonstrate the applicability of our grouping techniques to other contour-types in other real applications

Motion magnification in coronal seismology

We introduce a new method for the investigation of low-amplitude transverse oscillations of solar plasma non-uniformities, such as coronal loops, individual strands in coronal arcades, jets, prominence fibrils, polar plumes, and other contrast features, observed with imaging instruments. The method is based on the two-dimensional dual tree complex wavelet transform (DT$\mathbb{C}$WT). It allows us to magnify transverse, in the plane-of-the-sky, quasi-periodic motions of contrast features in image sequences. The tests performed on the artificial data cubes imitating exponentially decaying, multi-periodic and frequency-modulated kink oscillations of coronal loops showed the effectiveness, reliability and robustness of this technique. The algorithm was found to give linear scaling of the magnified amplitudes with the original amplitudes provided they are sufficiently small. Also, the magnification is independent of the oscillation period in a broad range of the periods. The application of this technique to SDO/AIA EUV data cubes of a non-flaring active region allowed for the improved detection of low-amplitude decay-less oscillations in the majority of loops.Comment: Accepted for publication in Solar Physic

Segmentation of Loops from Coronal EUV Images

We present a procedure which extracts bright loop features from solar EUV images. In terms of image intensities, these features are elongated ridge-like intensity maxima. To discriminate the maxima, we need information about the spatial derivatives of the image intensity. Commonly, the derivative estimates are strongly affected by image noise. We therefore use a regularized estimation of the derivative which is then used to interpolate a discrete vector field of ridge points ``ridgels'' which are positioned on the ridge center and have the intrinsic orientation of the local ridge direction. A scheme is proposed to connect ridgels to smooth, spline-represented curves which fit the observed loops. Finally, a half-automated user interface allows one to merge or split, eliminate or select loop fits obtained form the above procedure. In this paper we apply our tool to one of the first EUV images observed by the SECCHI instrument onboard the recently launched STEREO spacecraft. We compare the extracted loops with projected field lines computed from almost-simultaneously-taken magnetograms measured by the SOHO/MDI Doppler imager. The field lines were calculated using a linear force-free field model. This comparison allows one to verify faint and spurious loop connections produced by our segmentation tool and it also helps to prove the quality of the magnetic-field model where well-identified loop structures comply with field-line projections. We also discuss further potential applications of our tool such as loop oscillations and stereoscopy.Comment: 13 pages, 9 figures, Solar Physics, online firs

Solar feature tracking in both spatial and temporal domains

A new method for automated coronal loop tracking, in both spatial and temporal domains, is presented. The reliability of this technique was tested with TRACE 171A observations. The application of this technique to a flare-induced kink-mode oscillation, revealed a 3500 km spatial periodicity which occur along the loop edge. We establish a reduction in oscillatory power, for these spatial periodicities, of 45% over a 322 s interval. We relate the reduction in oscillatory power to the physical damping of these loop-top oscillations

The multi-thermal and multi-stranded nature of coronal rain

In this work, we analyse coordinated observations spanning chromospheric, TR and coronal temperatures at very high resolution which reveal essential characteristics of thermally unstable plasmas. Coronal rain is found to be a highly multi-thermal phenomenon with a high degree of co-spatiality in the multi-wavelength emission. EUV darkening and quasi-periodic intensity variations are found to be strongly correlated to coronal rain showers. Progressive cooling of coronal rain is observed, leading to a height dependence of the emission. A fast-slow two-step catastrophic cooling progression is found, which may reflect the transition to optically thick plasma states. The intermittent and clumpy appearance of coronal rain at coronal heights becomes more continuous and persistent at chromospheric heights just before impact, mainly due to a funnel effect from the observed expansion of the magnetic field. Strong density inhomogeneities on spatial scales of 0.2"-0.5" are found, in which TR to chromospheric temperature transition occurs at the lowest detectable scales. The shape of the distribution of coronal rain widths is found to be independent of temperature with peaks close to the resolution limit of each telescope, ranging from 0.2" to 0.8". However we find a sharp increase of clump numbers at the coolest wavelengths and especially at higher resolution, suggesting that the bulk of the rain distribution remains undetected. Rain clumps appear organised in strands in both chromospheric and TR temperatures, suggesting an important role of thermal instability in the shaping of fundamental loop substructure. We further find structure reminiscent of the MHD thermal mode. Rain core densities are estimated to vary between 2x10^{10} cm^{-3} and 2.5x10^{11} cm^{-3} leading to significant downward mass fluxes per loop of 1-5x10^{9} g s^{-1}, suggesting a major role in the chromosphere-corona mass cycle.Comment: Abstract is only short version. See paper for full. Countless pages, figures (and movies, but not included here). Accepted for publication in the Astrophysical Journa

How Many CMEs Have Flux Ropes? Deciphering the Signatures of Shocks, Flux Ropes, and Prominences in Coronagraph Observations of CMEs

We intend to provide a comprehensive answer to the question on whether all Coronal Mass Ejections (CMEs) have flux rope structure. To achieve this, we present a synthesis of the LASCO CME observations over the last sixteen years, assisted by 3D MHD simulations of the breakout model, EUV and coronagraphic observations from STEREO and SDO, and statistics from a revised LASCO CME database. We argue that the bright loop often seen as the CME leading edge is the result of pileup at the boundary of the erupting flux rope irrespective of whether a cavity or, more generally, a 3-part CME can be identified. Based on our previous work on white light shock detection and supported by the MHD simulations, we identify a new type of morphology, the `two-front' morphology. It consists of a faint front followed by diffuse emission and the bright loop-like CME leading edge. We show that the faint front is caused by density compression at a wave (or possibly shock) front driven by the CME. We also present high-detailed multi-wavelength EUV observations that clarify the relative positioning of the prominence at the bottom of a coronal cavity with clear flux rope structure. Finally, we visually check the full LASCO CME database for flux rope structures. In the process, we classify the events into two clear flux rope classes (`3-part', `Loop'), jets and outflows (no clear structure). We find that at least 40% of the observed CMEs have clear flux rope structures. We propose a new definition for flux rope CMEs (FR-CMEs) as a coherent magnetic, twist-carrying coronal structure with angular width of at least 40 deg and able to reach beyond 10 Rsun which erupts on a time scale of a few minutes to several hours. We conclude that flux ropes are a common occurrence in CMEs and pose a challenge for future studies to identify CMEs that are clearly not FR-CMEs.Comment: 26 pages, 9 figs, to be published in Solar Physics Topical Issue "Flux Rope Structure of CMEs

Evidence for collapsing fields in corona and photosphere during the 15 February 2011 X2.2 flare: SDO AIA and HMI Observations

We use high-resolution images of the sun obtained by the SDO/AIA instrument to study the evolution of the coronal loops in a flaring solar active region. During 15 February 2011 a X-2.2 class flare occurred in NOAA 11158, a $\beta\gamma\delta$ sunspot complex. We identify three distinct phases of the coronal loop dynamics during this event: (i) {\it Slow rise phase}: slow rising motion of the loop-tops prior to the flare in response to slow rise of the underlying flux rope, (ii) {\it Collapse phase}: sudden contraction of the loop-tops with lower loops collapsing earlier than the higher loops, and (iii) {\it Oscillation phase}: the loops exhibit global kink oscillations after the collapse phase at different periods, with period decreasing with decreasing height of the loops. The period of these loop oscillations is used to estimate the field strength in the coronal loops of different loop lengths in this active region. Further, we also use SDO/HMI observations to study the photospheric changes close to the polarity inversion line (PIL). The longitudinal magnetograms show step-wise permanent decrease in the magnetic flux after the flare over a coherent patch along the PIL. Further, we examine the HMI Stokes I,Q,U,V profiles over this patch and find that the Stokes-V signal systematically decreases while the Stokes-Q and U signal increases after the flare. These observations suggest that close to the PIL the field configuration became more horizontal after the flare. We also use HMI vector magnetic field observations to quantify the changes in the field inclination angle and found an inward collapse of the field lines towards the polarity inversion line (PIL) by $\sim$ 10$^\circ$. These observations are consistent with the "coronal implosion" scenario and its predictions about flare related photospheric field changes.Comment: 27 pages, 7 figures, in press (Astrophysical Journal

Discovery of spatial periodicities in a coronal loop using automated edge-tracking algorithms

A new method for automated coronal loop tracking, in both spatial and temporal domains, is presented. Applying this technique to TRACE data, obtained using the 171 Å filter on 1998 July 14, we detect a coronal loop undergoing a 270 s kink-mode oscillation, as previously found by Aschwanden et al. However, we also detect flare-induced, and previously unnoticed, spatial periodicities on a scale of 3500 km, which occur along the coronal loop edge. Furthermore, we establish a reduction in oscillatory power for these spatial periodicities of 45% over a 222 s interval. We relate the reduction in detected oscillatory power to the physical damping of these loop-top oscillations