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Line-of-sight geometrical and instrumental resolution effects on intensity perturbations by sausage modes  

Patrick Antolin   Submitted: 2013-03-26 02:44

Diagnostics of MHD waves in the solar atmosphere is a topic which often encounters problems of interpretation, due partly to the high complexity of the solar atmospheric medium. Forward modeling can significantly guide interpretation, bridging the gap between numerical simulations and observations, and increasing the reliability of mode identification for application of MHD seismology. In this work we aim at determining the characteristics of the fast MHD sausage mode in the corona on the modulation of observable quantities such as line intensity and spectral line broadening. Effects of line-of-sight angle, and spatial, temporal and spectral resolutions are considered. We take a cylindrical tube simulating a loop in a low-{eta} coronal environment with an optically thin background, and let it oscillate with the fast sausage mode. A parametric study is performed. Among other results, we show that regardless of the ionisation state of the plasma, the variation of spectral line broadening can be significant, even for low intensity modulation. The nature of this broadening is not thermal but is mostly turbulent. This places spectrometers in clear advantage over imaging instruments for the detection of the sausage mode. The modulation of all quantities is considerably affected by the line-of-sight angle, and especially by the spatial and temporal resolution when these are on the order of the mode's wavelength and period. This places high constraints on instrumentation.

Authors: P. Antolin, T. Van Doorsselaere
Projects: None

Publication Status: Accepted for publication in A&A
Last Modified: 2013-03-26 10:36
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Transverse oscillations of loops with coronal rain observed by Hinode/SOT  

Patrick Antolin   Submitted: 2011-05-12 02:26

The condensations composing coronal rain, falling down along loop-like structures observed in cool chromospheric lines such as Hα and ion{Ca}{2} H, have long been a spectacular phenomenon of the solar corona. However, considered a peculiar sporadic phenomenon, it has not received much attention. This picture is rapidly changing due to recent high resolution observations with instruments such as extit{Hinode}/SOT, CRISP of extit{SST} and extit{SDO}. Furthermore, numerical simulations have shown that coronal rain is a loss of thermal equilibrium of loops linked to footpoint heating. This result has highlighted the importance that coronal rain can play in the field of coronal heating. In this work, we further stress the importance of coronal rain by showing the role it can play in the understanding of the coronal magnetic field topology. We analyze extit{Hinode}/SOT observations in the ion{Ca}{2} H line of a loop in which coronal rain puts in evidence in-phase transverse oscillations of multiple strand-like structures. The periods, amplitudes, transverse velocities and phase velocities are calculated, allowing an estimation of the energy flux of the wave and the coronal magnetic field inside the loop through means of coronal seismology. We discuss the possible interpretations of the wave as either standing or propagating torsional Alfvén or fast kink waves. An estimate of the plasma beta parameter of the condensations indicates a condition that may allow the often observed separation and elongation processes of the condensations. We also show that the wave pressure from the transverse wave can be responsible for the observed low downward acceleration of coronal rain.

Authors: Patrick Antolin and Erwin Verwichte
Projects: Hinode/SOT

Publication Status: Accepted for publication in The Astrophysical Journal
Last Modified: 2011-05-12 15:06
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Coronal Rain as a Marker for Coronal Heating Mechanisms  

Patrick Antolin   Submitted: 2010-05-15 05:42

Reported observations in Hα, Ca II H, and K or other chromospheric lines of coronal rain trace back to the days of the Skylab mission. Corresponding to cool and dense plasma, coronal rain is often observed falling down along coronal loops in active regions. A physical explanation for this spectacular phenomenon has been put forward thanks to numerical simulations of loops with footpoint-concentrated heating, a heating scenario in which cool condensations naturally form in the corona. This effect has been termed ''catastrophic cooling'' and is the predominant explanation for coronal rain. In this work, we further investigate the link between this phenomenon and the heating mechanisms acting in the corona. We start by analyzing observations of coronal rain at the limb in the Ca II H line performed by the Hinode satellite, and derive interesting statistical properties concerning the dynamics. We then compare the observations with 1.5-dimensional MHD simulations of loops being heated by small-scale discrete events concentrated toward the footpoints (that could come, for instance, from magnetic reconnection events), and by Alfvén waves generated at the photospheric level. Both our observation and simulation results suggest that coronal rain is a far more common phenomenon than previously thought. Also, we show that the structure and dynamics of condensations are far more sensitive to the internal pressure changes in loops than to gravity. Furthermore, it is found that if a loop is predominantly heated from Alfvén waves, coronal rain is inhibited due to the characteristic uniform heating they produce. Hence, coronal rain may not only point to the spatial distribution of the heating in coronal loops but also to the agent of the heating itself. We thus propose coronal rain as a marker for coronal heating mechanisms.

Authors: P. Antolin, K. Shibata and G. Vissers
Projects: Hinode/SOT

Publication Status: Published in ApJ
Last Modified: 2010-05-17 09:16
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The Role Of Torsional Alfvén Waves in Coronal Heating  

Patrick Antolin   Submitted: 2010-03-18 04:55

In the context of coronal heating, among the zoo of magnetohydrodynamic (MHD) waves that exist in the solar atmosphere, Alfvén waves receive special attention. Indeed, these waves constitute an attractive heating agent due to their ability to carry over the many different layers of the solar atmosphere sufficient energy to heat and maintain a corona. However, due to their incompressible nature these waves need a mechanism such as mode conversion (leading to shock heating), phase mixing, resonant absorption, or turbulent cascade in order to heat the plasma. Furthermore, their incompressibility makes their detection in the solar atmosphere very difficult. New observations with polarimetric, spectroscopic, and imaging instruments such as those on board the Japanese satellite Hinode, or the Crisp spectropolarimeter of the Swedish Solar Telescope or the Coronal Multi-channel Polarimeter, are bringing strong evidence for the existence of energetic Alfvén waves in the solar corona. In order to assess the role of Alfvén waves in coronal heating, in this work we model a magnetic flux tube being subject to Alfvén wave heating through the mode conversion mechanism. Using a 1.5 dimensional MHD code, we carry out a parameter survey varying the magnetic flux tube geometry (length and expansion), the photospheric magnetic field, the photospheric velocity amplitudes, and the nature of the waves (monochromatic or white-noise spectrum). The regimes under which Alfvén wave heating produces hot and stable coronae are found to be rather narrow. Independently of the photospheric wave amplitude and magnetic field, a corona can be produced and maintained only for long (>80 Mm) and thick (area ratio between the photosphere and corona >500) loops. Above a critical value of the photospheric velocity amplitude (generally a few km s-1) the corona can no longer be maintained over extended periods of time and collapses due to the large momentum of the waves. These results establish several constraints on Alfvén wave heating as a coronal heating mechanism, especially for active region loops.

Authors: Antolin P., Shibata K.
Projects: None

Publication Status: Published - The Astrophysical Journal, Volume 712, Issue 1, pp. 494-510 (2010)
Last Modified: 2010-03-18 08:06
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Predicting observational signatures of coronal heating by Alfvén waves and nanoflares  

Patrick Antolin   Submitted: 2008-12-13 01:58

Alfvén waves can dissipate their energy by means of nonlinear mechanisms, and constitute good candidates to heat and maintain the solar corona to the observed few million degrees. Another appealing candidate is nanoflare reconnection heating, in which energy is released through many small magnetic reconnection events. Distinguishing the observational features of each mechanism is an extremely difficult task. On the other hand, observations have shown that energy release processes in the corona follow a power-law distribution in frequency whose index may tell us whether small heating events contribute substantially to the heating or not. In this work we show a link between the power-law index and the operating heating mechanism in a loop. We set up two coronal loop models: in the first model Alfvén waves created by footpoint shuffling nonlinearly convert to longitudinal modes which dissipate their energy through shocks; in the second model numerous heating events with nanoflare-like energies are input randomly along the loop, either distributed uniformly or concentrated at the footpoints. Both models are based on a 1.5-dimensional MHD code. The obtained coronae differ in many aspects; for instance, in the flow patterns along the loop and the simulated intensity profile that Hinode XRT would observe. The intensity histograms display power-law distributions whose indexes differ considerably. This number is found to be related to the distribution of the shocks along the loop. We thus test the observational signatures of the power-law index as a diagnostic tool for the above heating mechanisms and the influence of the location of nanoflares.

Authors: P. Antolin, K. Shibata, T. Kudoh, D. Shiota, D. Brooks
Projects: None

Publication Status: Published in ApJ 688, 669-682
Last Modified: 2008-12-14 17:43
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Abstracts by Author
Line-of-sight geometrical and instrumental resolution effects on intensity perturbations by sausage modes
Transverse oscillations of loops with coronal rain observed by Hinode/SOT
Coronal Rain as a Marker for Coronal Heating Mechanisms
The Role Of Torsional Alfv?n Waves in Coronal Heating
Predicting observational signatures of coronal heating by Alfv?n waves and nanoflares

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