Author: Nathalia Alzate and Huw Morgan at Aberystwyth University, Shadia R. Habbal at the Institute for Astronomy (U of Hawai’i), Miloslav Druckmüller at Brno University of Technology and Constantinos Emmanouilidis at K@stro Observatory.
Ground based white light images taken during total solar eclipses have many unique advantages, particularly their very high dynamic range spanning tens of solar radii starting from the solar surface. When processed, these high resolution images reveal the finest details of coronal structures down to the spatial resolution of the instrument, which is currently 1-2 arcsec. In this nugget we present an analysis of eclipse data for 2012 and 2013 in the context of co-temporal and space-based observations in order to identify the dynamic events leading up to the appearance of “atypical” large scale structures in the two eclipse images. We find that these structures were formed as a consequence of the passage of CMEs several hours before totality, which were not associated with a filament eruption and lacked a core. The CMEs were clearly distinct from the ones visibly captured during totality in both years.
Shown in Figure 1, are the images acquired by C. Emmanouilidis and processed by M. Druckmüller. Totality started at 20:37 UT on 13 November 2012 in Australia, and at 13:52 UT on 3 November 2013 in Gabon, where they were acquired respectively. The insets in both examples are SDO/AIA 17.1 nm images taken at the same time as the corresponding eclipses. There were several striking features in both eclipse images, in particular, a tethered prominence eruption east of North in 2012, and two ejected plasmoids, almost diametrically opposite, forming the cores of two CMEs in 2013. Since capturing a CME in the corona during a total solar eclipse is rather rare, the cases of 2012 and 2013 are quite fortuitous. However, when not captured directly,  have shown that the passage of CMEs leaves a well-defined trail in the corona, which can be traced back to its origin with cotemporaneous observations from LASCO C2. In our study, we focused on the “atypical” regions seen in the south east quadrant of both images/dates, and encircled by a white ellipse in Figure 1.
To place the atypical regions within the context of dynamic events, SDO/AIA data were processed using the Multiscale Gaussian Normalization technique (MGN; ) and coronagraph data from SOHO/LASCO were processed with the Dynamic Separation Technique (DST; ; ). For the application of these image processing techniques, see  and . With the time series afforded by the space-based data, the evolution of the large scale coronal structures following flaring events in the low corona, could be explored.
In 2012, several disturbances were captured within the FoV of the LASCO C2 coronagraph starting at 10:48 UT on 12 November, one day before totality. The region from which these CMEs appear to originate (indicated by the white ellipse in Figure 1, top) contains an active region which produced several CMEs and flares, as well as jets that might be responsible for the bright ray seen in the eclipse image. The work by , currently under preparation, provides full analysis details of the observations made during both eclipses.
Figure 2 is a composite for 2013. In this case, a flaring activity led to a huge, yet faint, CME that left its mark in the coronal structures, as seen by the curvature of the lines indicated by the arrows (this CME is distinctly different from the very bright one captured in the south during the eclipse). The series of flaring events that arose from the active region close to the east limb, did not seem to cause any significant changes in the loop structures observed in 17.1 and 19.3 nm. However, some changes appeared in 17.1 nm away from these loops (Fig 2, panel g), indicated by the arrow, which ended up forming the southern edge of the CME in the low corona. There was no activity observed in 19.3 nm (Fig 2, panel h), and an eruption along an arched-like feature appeared in 30.4 nm (Fig 2, panel i).
Discussion and Conclusions
We find that the “shape” of these atypical regions is not a coronal hole. Instead, it is the direct consequence of a series of flaring activities at the east limb in both eclipses followed by jets and the subsequent appearance of faint CMEs in the LASCO C2 FoV. While CMEs were abundant at that time, the largest ones did not originate in this region, but rather innocuous ones that would not have been necessarily reported by LASCO C2. In both 2012 and 2013, these CMEs left enough of an imprint to change the large scale structure of the corona. However, they lacked a bright core, probably because they were associated with flaring activity and not a filament eruption, which typically forms the core of a CME. These observations suggest that CMEs associated with flares originate very close to the solar surface and can often be missed. As such they differ from CMEs triggered by prominence eruptions which seem to form higher in the corona.
CME activity began approximately 34 hours before totality in 2012. Similarly, in 2013 CME activity began approximately 30 hours prior to totality. This activity, with its long lasting effects, created the atypical structures seen in the eclipse images. These observations, in addition to the 2010 example , show that imprints of the disturbed large scale structures last a minimum of 12 to 48 hours. Given the frequency of CMEs during solar maximum (approximately 3 events per day), atypical structures such as the ones described here will unavoidably be captured during total solar eclipses or during observations spanning the same FoV.
White light eclipse images underscore the importance of acquiring coronal data over a large FoV to capture and follow the dynamic evolution and expansion of the coronal magnetic fields and plasmas, and to reliably establish their origin as they present themselves in these images. Although limited by the very short observing time, complementing them with space-based observations yields a unique tool, at present, for exploring the dynamics of the corona, in particular the long-lasting imprint of the passage of CMEs on coronal structures, starting from the solar surface.
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