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Multi-instrument observations of a failed flare eruption associated with MHD waves in a loop bundle  

Giuseppe Nistico   Submitted: 2016-12-06 17:44

We present observations of a B7.9-class flare that occurred on the 24th January, 2015, using the Atmopsheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO), the EUV Imaging Spectrometer (EIS) and the X-Ray Telescope of Hinode. The flare triggers the eruption of a dense cool plasma blob as seen in AIA 171Å,  which is unable to completely break out and remains confined within a local bundle of active region loops. During this process, transverse oscillations of the threads are observed. The cool plasma is then observed to descend back to the chromosphere along each loop strand. At the same time, a larger diffuse co-spatial loop observed in the hot wavebands of SDO/AIA and Hinode/XRT is formed, exhibiting periodic intensity variations along its length. The formation and evolution of magnetohydrodynamic (MHD) waves depend upon the values of the local plasma parameters (e.g. density, temperature and magnetic field), which can hence be inferred by coronal seismology. In this study we aim to assess how the observed MHD modes are affected by the variation of density and temperature. We combined analysis of EUV/X-ray imaging and spectroscopy using SDO/AIA, Hinode/EIS and XRT. The transverse oscillations of the cool loop threads are interpreted in terms of vertically polarised kink oscillations. The fitting procedure provides estimates for a period of ~ 3.5 to 4 min, and an amplitude of ~ 5 Mm. The oscillations are strongly damped showing very low quality factor (1.5-2), which is defined as the ratio of the damping time and the oscillation period. The weak variation of the period of the kink wave, which is estimated from the fitting analysis, is in agreement with the density variations due to the presence of the plasma blob inferred from the intensity light curve at 171Å. The coexisting intensity oscillations along the hot loop are interpreted as a slow MHD wave with a period of 10 min and phase speed of approximately 436 km s-1. Comparison between the fast and slow modes allows for the determination of the Alfvén speed, and consequently magnetic field values. The plasma-β inferred from the analysis is estimated to be approximately 0.1-0.3. We show that the evolution of the detected waves is determined by the temporal variations of the local plasma parameters, caused by the flare heating and the consequent cooling. We apply coronal seismology to both waves obtaining estimations of the background plasma parameters.

Authors: G. Nistico, V. Polito, V. M. Nakariakov, G. del Zanna
Projects: None,Hinode/EIS,Hinode/XRT,SDO-AIA

Publication Status: accepted for publication in A&A
Last Modified: 2016-12-07 12:17
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North-South asymmetry in the magnetic deflection of polar coronal hole jets  

Giuseppe Nistico   Submitted: 2015-07-22 18:07

Context. Measurements of the magnetic field in the interplanetary medium, of the sunspots area, and of the heliospheric current sheet position, reveal a possible North-South asymmetry in the magnetic field of the Sun. This asymmetry could cause the bending of the heliospheric current sheet of the order of 5-10 deg in the southward direction, and it appears to be a recurrent characteristic of the Sun during the minima of solar activity.
Aims. We study the North-South asymmetry as inferred from measurements of the deflection of polar coronal hole jets when they propagate throughout the corona.
Methods. Since the corona is an environment where the magnetic pressure is greater than the kinetic pressure (β ≪ 1), we can assume that magnetic field controls the dynamics of plasma. On average, jets during their propagation follow the magnetic field lines, highlighting its local direction. The average jet deflection is studied both in the plane perpendicular to the line of sight, and, for a reduced number of jets, in three dimensional space. The observed jet deflection is studied in terms of an axisymmetric magnetic field model comprising dipole (g1), quadrupole (g2), and esapole (g3) moments.
Results. We measured the position angles at 1 R and at 2 R of the 79 jets from the catalogue of Nistico09, based on the STEREO ultraviolet and white-light coronagraph observations during the solar minimum period March 2007-April 2008. We found that the propagation is not radial, in agreement with the deflection due to magnetic field lines. Moreover, the amount of the deflection is different between jets over the north and those from the south pole. Comparison of jet deflections and field line tracing shows that a ratio g2/g1 ≃ -0.5 for the quadrupole and a ratio g3/g1 ~eq 1.6-2.0 for the esapole can describe the field. The presence of a non-negligible quadrupole moment confirms the North-South asymmetry of the solar magnetic field for the considered period.
Conclusions. We find that the magnetic deflection of jets is larger in the North than in the South of the order of 25-40%, with an asymmetry which is consistent with a southward deflection of the heliospheric current sheet of the order of 10 deg, consistent with that inferred from other, independent, datasets and instruments.

Authors: Nistico G., Zimbardo G., Patsourakos S., Bothmer V., Nakariakov V. M.
Projects: STEREO

Publication Status: accepted for publication in A&A
Last Modified: 2015-07-23 10:11
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Dynamics of a Multi-Thermal Loop in the Solar Corona  

Giuseppe Nistico   Submitted: 2014-07-02 16:44

Context. We present an observation of a long-living multi-thermal coronal loop, visible in different EUV wavebands of SDO/AIA in a quiet-Sun region close to the Western solar limb. Aims. Analysis of persistent kink displacements of the loop seen in different bandpasses that correspond to different temperatures of the plasma allows to reveal fine, sub-resolution structuring of the loop. Methods. A vertically oriented slit is taken at the loop top and time-distance maps are made from it. Loop displacements in time-distance maps are automatically tracked with the Gaussian fitting technique and fitted with a ``guessed'' sinusoidal function. Wavelet transform are further used in order to quantify the periodicity variation in time of the kink oscillations. Results. The loop strands are found to oscillate with the periods ranging between 3-15 minutes. The oscillations are observed in intermittent regime with changes of the period and phase in time. The oscillations are different at three analysed wavelengths. Conclusions. This finding suggests that the loop-like threads seen at different wavelengths are not co-spatial, and hence that the loop consists of several multi-thermal strands. The detected irregularity of the oscillation can be associated with a stochastic driver acting at the footpoints of the loop.

Authors: G. Nistico, S. Anfinogentov, and V. M. Nakariakov
Projects: None

Publication Status: Accepted for publication in A&A
Last Modified: 2014-07-06 20:35
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Observations of a high-quality quasi-periodic rapidly-propagating wave train using SDO/AIA  

Giuseppe Nistico   Submitted: 2014-06-19 11:19

Context. We present a new event of quasi-periodic wave trains observed in EUV wavebands, rapidly-propagating away from an active region after a flare. Aims. We measure parameters of a wave train observed on 7 December 2013 after an M2.1 flare, i.e. phase speeds, periods and wavelengths, in relationship to the local coronal environment and the energy sources. Methods. We compare our observations with a numerical simulation of fast magnetoacoustic waves undergoing dispersive evolution and leakage in a coronal loop embedded in a potential magnetic field. Results. The wave train is observed to propagate as several arc-shaped intensity disturbances, for almost half an hour, with a speed greater than 1000 km s-1 and a period of about 1 min. The wave train followed two different patterns of propagation, in accordance with the magnetic structure of the active region. The oscillatory signal is found to be of high quality, i.e. there is a large number (10 or more) of subsequent wave fronts observed. The observations are found to be consistent with the numerical simulation of a fast wave train generated by a localised impulsive energy release. Conclusions. Transverse structuring in the corona can efficiently create and guide high quality quasi-periodic propagating fast wave trains.

Authors: G. Nistico, D. J. Pascoe, V. M. Nakariakov
Projects: None

Publication Status: Accepted for publication in A&A
Last Modified: 2014-06-20 06:31
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3D Reconstruction of Coronal Loops by the Principal Component Analysis  

Giuseppe Nistico   Submitted: 2013-10-11 12:31

Knowing the three dimensional structure of plasma filaments in the uppermost part of the solar atmosphere, known as coronal loops, and especially their length, is an important parameter in the wave-based diagnostics of this part of the Sun. The combination of observations of the Sun from different points of observations in space, thanks to the most recent missions including SDO and STEREO, allows us to infer information on the geometrical shape of coronal loops in the 3D space. Here, we propose a new method to reconstruct the loop shape starting from stereoscopically determined 3D points, which sample the loop length, by the Principal Component Analysis. This method is shown to retrieve in an easy way the main parameters that define loop, e.g. the minor and major axes, the loop plane, the azimuthal and inclination angles, for the special case of a coplanar loop.

Authors: Giuseppe Nistico, Erwin Verwichte, Valery M. Nakariakov
Projects: SDO-AIA,STEREO

Publication Status: Submitted
Last Modified: 2013-10-11 12:31
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Abstracts by Author
Multi-instrument observations of a failed flare eruption associated with MHD waves in a loop bundle
North-South asymmetry in the magnetic deflection of polar coronal hole jets
Dynamics of a Multi-Thermal Loop in the Solar Corona
Observations of a high-quality quasi-periodic rapidly-propagating wave train using SDO/AIA
3D Reconstruction of Coronal Loops by the Principal Component Analysis

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