57 research outputs found
An automated classification approach to ranking photospheric proxies of magnetic energy build-up
We study the photospheric magnetic field of ~2000 active regions in solar
cycle 23 to search for parameters indicative of energy build-up and subsequent
release as a solar flare. We extract three sets of parameters: snapshots in
space and time- total flux, magnetic gradients, and neutral lines; evolution in
time- flux evolution; structures at multiple size scales- wavelet analysis.
This combines pattern recognition and classification techniques via a relevance
vector machine to determine whether a region will flare. We consider
classification performance using all 38 extracted features and several feature
subsets. Classification performance is quantified using both the true positive
rate and the true negative rate. Additionally, we compute the true skill score
which provides an equal weighting to true positive rate and true negative rate
and the Heidke skill score to allow comparison to other flare forecasting work.
We obtain a true skill score of ~0.5 for any predictive time window in the
range 2-24hr, with a TPR of ~0.8 and a TNR of ~0.7. These values do not appear
to depend on the time window, although the Heidke skill score (<0.5) does.
Features relating to snapshots of the distribution of magnetic gradients show
the best predictive ability over all predictive time windows. Other
gradient-related features and the instantaneous power at various wavelet scales
also feature in the top five ranked features in predictive power. While the
photospheric magnetic field governs the coronal non-potentiality (and
likelihood of flaring), photospheric magnetic field alone is not sufficient to
determine this uniquely. Furthermore we are only measuring proxies of the
magnetic energy build up. We still lack observational details on why energy is
released at any particular point in time. We may have discovered the natural
limit of the accuracy of flare predictions from these large scale studies
Evidence of a Plasmoid-Looptop Interaction and Magnetic Inflows During a Solar Flare/CME Eruptive Event
Observational evidence is presented for the merging of a downward-propagating
plasmoid with a looptop kernel during an occulted limb event on 2007 January
25. RHESSI lightcurves in the 9-18 keV energy range, as well as that of the 245
MHz channel of the Learmonth Solar Observatory, show enhanced nonthermal
emission in the corona at the time of the merging suggesting that additional
particle acceleration took place. This was attributed to a secondary episode of
reconnection in the current sheet that formed between the two merging sources.
RHESSI images were used to establish a mean downward velocity of the plasmoid
of 12 km/s. Complementary observations from the SECCHI suite of instruments
onboard STEREO-Behind showed that this process occurred during the acceleration
phase of the associated CME. From wavelet-enhanced EUVI, images evidence of
inflowing magnetic field lines prior to the CME eruption is also presented. The
derived inflow velocity was found to be 1.5 km/s. This combination of
observations supports a recent numerical simulation of plasmoid formation,
propagation and subsequent particle acceleration due to the tearing mode
instability during current sheet formation.Comment: 8 pages, 9 figures, ApJ (Accepted
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
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