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Identification of multiple hard X-ray sources in solar flares: A Bayesian analysis of the February 20 2002 event  

Michele Piana   Submitted: 2018-02-08 02:48

Hard X-ray emission in solar flares is typically characterized by a number of discrete sources, each with its own spectral, temporal, and spatial variability. Establishing the relationship amongst these sources is critical to determine the role of each in the energy release and transport processes that occur within the flare. In this paper we present a novel method to identify and characterize each source of hard X-ray emission. In particular, the method permits a quantitative determination of the most likely number of subsources present, and of the relative probabilities that the hard X-ray emission in a given subregion of the flare is represented by a complicated multiple source structure or by a simpler single source. We apply the method to a well-studied flare on 2002 February 20 in order to assess competing claims as to the number of chromospheric footpoint sources present, and hence to the complexity of the underlying magnetic geometry/toplogy. Contrary to previous claims of the need for multiple sources to account for the chromospheric hard X-ray emission at different locations and times, we find that a simple two-footpoint-plus-coronal-source model is the most probable explanation for the data. We also find that one of the footpoint sources moves quite rapidly throughout the event, a factor that presumably complicated previous analyses. The inferred velocity of the footpoint corresponds to a very high induced electric field, compatible with those in thin reconnecting current sheets.

Authors: Federica Sciacchitano, Alberto Sorrentino, A Gordon Emslie, Anna Maria Massone, Michele Piana
Projects: RHESSI

Publication Status: submitted to ApJ
Last Modified: 2018-02-08 16:22
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Solar hard X-ray imaging by means of Compressed Sensing and Finite Isotropic Wavelet Transform  

Michele Piana   Submitted: 2017-08-16 02:14

This paper shows that compressed sensing realized by means of regularized deconvolution and the Finite Isotropic Wavelet Transform is effective and reliable in hard X-ray solar imaging. The method utilizes the Finite Isotropic Wavelet Transform with Meyer function as the mother wavelet. Further, compressed sensing is realized by optimizing a sparsity-promoting regularized objective function by means of the Fast Iterative Shrinkage-Thresholding Algorithm. Eventually, the regularization parameter is selected by means of the Miller criterion. The method is applied against both synthetic data mimicking the Spectrometer/Telescope Imaging X-rays (STIX) measurements and experimental observations provided by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The performances of the method are compared with the results provided by standard visibility-based reconstruction methods. The results show that the application of the sparsity constraint and the use of a continuous, isotropic framework for the wavelet transform provide a notable spatial accuracy and significantly reduce the ringing effects due to the instrument point spread functions.

Authors: M. A. Duval-Poo, M. Piana, A. M. Massone
Projects: RHESSI

Publication Status: A&A (submitted)
Last Modified: 2017-08-16 09:23
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A hybrid supervised/unsupervised machine learning approach to solar flare prediction  

Michele Piana   Submitted: 2017-06-23 16:12

We introduce a hybrid approach to solar flare prediction, whereby a supervised regularization method is used to realize feature importance and an unsupervised clustering method is used to realize the binary flare/no-flare decision. The approach is validated against NOAA SWPC data.

Authors: Federico Benvenuto, Michele Piana, Cristina Campi, Anna Maria Massone
Projects: None

Publication Status: Submitted to ApJ
Last Modified: 2017-06-28 08:41
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DESAT: an SSW tool for SDO/AIA image de-saturation  

Michele Piana   Submitted: 2015-03-10 03:55

Saturation affects a significant rate of images recorded by the Atmospheric Imaging Assembly on the Solar Dynamics Observatory. This paper describes a computational method and a technological pipeline for the de-saturation of such images, based on several mathematical ingredients like Expectation Maximization, image correlation and interpolation. An analysis of the computational properties and demands of the pipeline, together with an assessment of its reliability are performed against a set of data recorded from the Feburary 25 2014 flaring event.

Authors: Richard A Schwartz, Gabriele Torre, Anna Maria Massone, Michele Piana
Projects: SDO-AIA

Publication Status: submitted to Inverse Problems
Last Modified: 2015-03-11 14:48
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Inverse diffraction for the Atmospheric Imaging Assembly in the Solar Dynamics Observatory  

Michele Piana   Submitted: 2015-02-03 01:14

The Atmospheric Imaging Assembly in the Solar Dynamics Observatory provides full Sun images every 1 seconds in each of 7 Extreme Ultraviolet passbands. However, for a significant amount of these images, saturation affects their most intense core, preventing scientists from a full exploitation of their physical meaning. In this paper we describe a mathematical and automatic procedure for the recovery of information in the primary saturation region based on a correlation/inversion analysis of the diffraction pattern associated to the telescope observations. Further, we suggest an interpolation-based method for determining the image background that allows the recovery of information also in the region of secondary saturation (blooming).

Authors: Gabriele Torre, Richard A Schwartz, Federico Benvenuto, Anna Maria Massone, Michele Piana
Projects: SDO-AIA

Publication Status: submitted to Inverse Problems
Last Modified: 2015-02-03 10:58
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The process of data formation for the Spectrometer/Telescope for Imaging X-rays (STIX) in Solar Orbiter  

Michele Piana   Submitted: 2014-12-25 12:05

The Spectrometer/Telescope for Imaging X-rays (STIX) is a hard X-ray imaging spectroscopy device to be mounted in the Solar Orbiter cluster with the aim of providing images and spectra of solar flaring regions at different photon energies in the range from a few keV to around 150 keV. The imaging modality of this telescope is based on the Moire pattern concept and utilizes 30 sub-collimators, each one containing a pair of co-axial grids. This paper applies Fourier analysis to provide the first rigorous description of the data formation process in STIX. Specifically, we show that, under first harmonic approximation, the integrated counts measured by STIX sub-collimators can be interpreted as specific spatial Fourier components of the incoming photon flux, named visibilities. Fourier analysis also allows the quantitative assessment of the reliability of such interpretation. The description of STIX data in terms of visibilities has a notable impact on the image reconstruction process, since it fosters the application of Fourier-based imaging algorithms.

Authors: Sara Giordano, Nicola Pinamonti, Michele Piana, Anna Maria Massone
Projects: None

Publication Status: submitted to SIAM Journal on Imaging Sciences
Last Modified: 2014-12-29 11:15
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A Systematic Approach to the Reconstruction of Saturated SDO/AIA Images  

Michele Piana   Submitted: 2014-07-29 01:14

EUV images of solar flares provided by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory (SDO/AIA) are often affected by saturation effects in their core, physically most interesting region. We introduce an image reconstruction procedure that allows recovering information in the primary saturation domain using the secondary images produced by the diffraction fringes as input data. Such a procedure is based on standard image-processing tools like correlation, convolution, and back-projection. Its effectiveness is tested in the case of SDO/AIA observations of the July 8 2013 flaring event.

Authors: Richard A Schwartz, Gabriele Torre, Michele Piana
Projects: None

Publication Status: submitted to ApJ Letters
Last Modified: 2014-07-29 08:56
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Return currents and energy transport in the solar flaring atmosphere  

Michele Piana   Submitted: 2013-04-26 07:58

According to a standard ohmic perspective, the injection of accelerated electrons into the flaring region violates local charge equilibrium and therefore, in response, return currents are driven by an electric field to equilibrate such charge violation. In this framework, the energy loss rate associated to these local currents has an ohmic nature and significantly shortens the acceleration electron path. In the present paper we adopt a different viewpoint and, specifically, we study the impact of the background drift velocity on the energy loss rate of accelerated electrons in solar flares. We first utilize the Rutherford cross-section to derive the formula of the energy loss rate when the collisional target has a finite temperature and the background instantaneously and coherently moves up to equilibrate the electron injection. We then use the continuity equation for electrons and imaging spectroscopy data provided by RHESSI to validate this model. Specifically, we show that this new formula for the energy loss rate provides a better fit of the experimental data with respect to the model based on the effects of standard ohmic return currents.

Authors: Anna Codispoti, Gabriele Torre, Michele Piana, Nicola Pinamonti
Projects: RHESSI

Publication Status: Submitted to ApJ
Last Modified: 2013-04-26 10:15
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Subject will be restored when possible  

Michele Piana   Submitted: 2008-02-14 02:46

We address an important inverse problem in solar plasma physics by means of an "ad hoc" implementation of the Tikhonov regularization method. Our approach is validated in the case of different modifications of the mathematical model of the problem based on physical motivations, and for data sets obtained with synthetic experiments. Finally, an application to a real observation recorded by the NASA mission ?Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)? is considered.

Authors: A. M. Massone, M. Piana, M. Prato
Projects: RHESSI

Publication Status: Inverse Problems in Science and Engineering, in press
Last Modified: 2008-02-14 09:53
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Abstracts by Author
Identification of multiple hard X-ray sources in solar flares: A Bayesian analysis of the February 20 2002 event
Solar hard X-ray imaging by means of Compressed Sensing and Finite Isotropic Wavelet Transform
A hybrid supervised/unsupervised machine learning approach to solar flare prediction
DESAT: an SSW tool for SDO/AIA image de-saturation
Inverse diffraction for the Atmospheric Imaging Assembly in the Solar Dynamics Observatory
The process of data formation for the Spectrometer/Telescope for Imaging X-rays (STIX) in Solar Orbiter
A Systematic Approach to the Reconstruction of Saturated SDO/AIA Images
Return currents and energy transport in the solar flaring atmosphere
Subject will be restored when possible

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