Simulating White-Light Images of Coronal Structures for Parker Solar Probe/WISPR: Study of the Total Brightness Profiles |
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Giuseppe Nistico Submitted: 2020-04-14 09:57
The Wide-field Imager for Parker Solar Probe (WISPR) captures unprecedented white-light images of the solar corona and inner heliosphere. Thanks to the uniqueness of Parker Solar Probe's (PSP) orbit, WISPR is able to image ``locally'' coronal structures at high spatial and time resolutions. The observed plane of sky, however, rapidly changes because of the PSP's high orbital speed. Therefore, the interpretation of the dynamics of the coronal structures recorded by WISPR is not straightforward. A first study, undertaken by citet{Liewer2019}, shows how different coronal features (e.g., streamers, flux ropes) appear in the field of view of WISPR by means of raytracing simulations. In particular, they analyze the effects of the spatial resolution changes on both the images and the associated height-time maps, and introduce the fundamentals for geometric triangulation. In this follow-up paper, we focus on the study of the total brightness of a simple, spherical, plasma density structure, to understand how the analysis of Thomson-scattered emission by the electrons in a coronal feature can shed light into the determination of its kinematic properties. We investigate two cases: {it (a)} a density sphere at a constant distance from the Sun for different heliographic longitudes; {it (b)} a density sphere moving outwardly with constant speed. The study allows us to characterize the effects of the varying heliocentric distance of the observer and scattering angle on the total brightness observed, which we exploit to contribute to a better determination of the position and speed of the coronal features observed by WISPR.
Authors: Giuseppe Nistic̣, Volker Bothmer, Angelos Vourlidas, Paulett Liewer, Arnaud Thernisien, Guillermo Stenborg, Russell Howard
Projects: PSP-WISPR
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Publication Status: Accepted for publication in Solar Physics
Last Modified: 2020-04-14 11:18
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Multi-instrument observations of a failed flare eruption associated with MHD waves in a loop bundle |
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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
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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 |
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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
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Publication Status: accepted for publication in A&A
Last Modified: 2015-07-23 10:11
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Observations of a high-quality quasi-periodic rapidly-propagating wave train using SDO/AIA |
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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
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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 |
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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
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Publication Status: Submitted
Last Modified: 2013-10-11 12:31
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