A global view of velocity fluctuations in the corona below 1.3 R_odot with CoMP |
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Richard Morton Submitted: 2016-09-06 03:48
The Coronal Multi-channel Polarimeter (CoMP) has previously demonstrated the presence of Doppler velocity fluctuations in the solar corona. The observed fluctuations are thought to be transverse waves, i.e. highly incompressible motions whose restoring force is dominated by the magnetic tension, some of which demonstrate clear periodicity. We aim to exploit CoMP's ability to provide high cadence observations of the off-limb corona to investigate the properties of velocity fluctuations in a range of coronal features, providing insight into how(if) the properties of the waves are influenced by the varying magnetic topology in active regions, quiet Sun and open fields regions. An analysis of Doppler velocity time-series of the solar corona from the 10,747 Å Iron XIII line is performed, determining the velocity power spectra and using it as a tool to probe wave behaviour. Further, the average phase speed and density for each region are estimated and used to compute the spectra for energy density and energy flux. In addition, we assess the noise levels associated with the CoMP data, deriving analytic formulae for the uncertainty on Doppler velocity measurements and providing a comparison by estimating the noise from the data. It is found that the entire corona is replete with transverse wave behaviour. The corresponding power spectra indicates that the observed velocity fluctuations are predominately generated by stochastic processes, with the spectral slope of the power varying between the different magnetic regions. Most strikingly, all power spectra reveal the presence of enhanced power occurring at ~3~mHz, potentially implying that the excitation of coronal transverse waves by p-modes is a global phenomenon.
Authors: Morton, R. J.; Tomczyk, S.; Pinto, R. F.
Projects: None
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Publication Status: In Press
Last Modified: 2016-09-07 12:12
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The generation and damping of propagating MHD kink waves in the solar atmosphere |
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Richard Morton Submitted: 2014-01-29 03:04
The source of the non-thermal energy required for the heating of the upper solar atmosphere to temperatures in excess of a million degrees and the acceleration of the solar wind to hundreds of kilometres per second is still unclear. One such mechanism for providing the required energy flux is incompressible torsional Alfvén and kink magnetohydrodynamic (MHD) waves, which are magnetically dominated waves supported by the Sun's pervasive and complex magnetic field. In particular, propagating MHD kink waves have recently been observed to be ubiquitous throughout the solar atmosphere, but, until now, critical details of the transport of the kink wave energy throughout the Sun's atmosphere were unclear. Here, the ubiquity of the waves is exploited for statistical studies in the highly dynamic solar chromosphere. This large-scale investigation allows for the determination of the chromospheric kink wave velocity power spectra, a missing link necessary for determining the energy transport between the photosphere and corona. Crucially, the power spectra contains evidence for horizontal photospheric motions being the main mechanism for kink wave generation in the quiescent Sun. In addition, a comparison to measured coronal power spectra is provided, revealing frequency-dependent transmission profiles suggesting there is enhanced damping of kink waves in the lower corona.
Authors: R. J. Morton, G. Verth, A. Hillier, R. Erdélyi
Projects: None
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Publication Status: In press - March 2014
Last Modified: 2014-01-29 12:20
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The dynamical behaviour of a jet in an on-disk coronal hole observed with AIA/SDO |
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Richard Morton Submitted: 2013-10-30 03:15
{EUV jets situated in coronal holes are thought to play an important role in supplying heated material to the corona and solar wind. The multi-wavelength capabilities and high signal-to-noise of detectors on-board SDO allows for detailed study of these jet's evolution. We aim to exploit SDO's capabilities to reveal information on the jet dynamics and obtain estimates for plasma properties associated with the jet.}
{We study the dynamics an EUV jet with AIA/SDO at a coronal hole boundary. The details of the jet evolution are discussed and measurements of the jet's parameters, e.g. length, width, life time, outward speed, are obtained. Further, automated emission measure analysis is exploited to determine estimates for the temperature and density of the jet. A propagating transverse wave supported by the jet spire is also observed. Measurement of the wave
properties are exploited for magneto-seismology and are used in conjunction with the emission measure results to estimate the magnetic field strength of the jet.}
{We present a detailed description of the jet's evolution, with new evidence for plasma flows, prior to the jet's initiation, along the loops at the base of the jet and also find further evidence that flows along the jet spire consist of multiple, quasi-periodic small-scale plasma ejection events. In addition, DEM analysis suggests that the jet has temperatures of log{5.89pm0.08} K and electron densities of log{8.75pm0.05}~cm-3. Measured properties of the transverse wave suggest the wave is heavily damped as it propagates along the jet spire with speeds of sim110 km s-1. The magneto-seismological inversion of the wave parameters provides values of B=1.21pm0.2~G along the jet spire, which is in line with previous estimates for open fields in coronal holes. }
Authors: K. Chandrashekhar, R. J. Morton, D. Banerjee and G. R. Gupta
Projects: SDO-AIA
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Publication Status: Accepted A&A
Last Modified: 2013-10-30 17:01
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Observations of ubiquitous compressive waves in the Sun's chromosphere |
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Richard Morton Submitted: 2013-06-19 06:29
The details of the mechanism(s) responsible for the observed heating and dynamics of the solar atmosphere still remain a mystery. Magnetohydrodynamic (MHD) waves are thought to play a vital role in this process. Although it has been shown that incompressible waves are ubiquitous in off-limb solar atmospheric observations their energy cannot be readily dissipated. We provide here, for the first time, on-disk observation and identification of concurrent MHD wave modes, both compressible and incompressible, in the solar chromosphere. The observed ubiquity and estimated energy flux associated with the detected MHD waves suggest the chromosphere is a vast reservoir of wave energy with the potential to meet chromospheric and coronal heating requirements. We are also able to propose an upper bound on the flux of the observed wave energy that is able to reach the corona based on observational constraints, which has important implications for the suggested mechanism(s) for quiescent coronal heating.
Authors: R. J. Morton, G. Verth, D. B. Jess, D. Kuridze, M. S. Ruderman, M. Mathioudakis, R. Erdelyi
Projects: None
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Publication Status: Published Nature Communications, 3, 1315 (2012)
Last Modified: 2013-06-19 11:44
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Observations of quasi-periodic phenomena associated with a large blowout solar jet |
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Richard Morton Submitted: 2012-04-24 02:13
A variety of periodic phenomena have been observed in conjunction with large solar jets. We aim to find further evidence for {(quasi-)}periodic behaviour in solar jets and determine what the periodic behaviour can tell us about the excitation mechanism and formation process of the large solar jet. Using the 304 {AA} (He-II), 171 {AA} (Fe IX), 193 {AA} (Fe XII/XXIV) and 131 {AA} (Fe VIII/XXI) filters on-board the Solar Dynamic Observatory (SDO) Atmospheric Imaging Assembly (AIA), we investigate the intensity oscillations associated with a solar jet. Evidence is provided for multiple magnetic reconnection events occurring between a pre-twisted, closed field and open field lines. Components of the jet are seen in multiple SDO/AIA filters covering a wide range of temperatures, suggesting the jet can be classified as a blowout jet. Two bright, elongated features are observed to be co-spatial with the large jet, appearing at the jet's footpoints. Investigation of these features reveal they are defined by multiple plasma ejections. The ejecta display (quasi-)periodic behaviour on timescales of 50 s and have rise velocities of 40-150 km,s-1 along the open field lines. Due to the suggestion that the large jet is reconnection-driven and the observed properties of the ejecta, we further propose that these ejecta events are similar to type-II spicules. The bright features also display (quasi)-periodic intensity perturbations on the timescale of 300 s. Possible explanations for the existence of the (quasi-)periodic perturbations in terms of jet dynamics and the response of the transition region are discussed.
Authors: R. J. Morton, A. K. Srivastava, R. Erd'elyi
Projects: SDO-AIA
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Publication Status: In print
Last Modified: 2012-04-25 12:19
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MHD waves in a compressible magnetic flux tube with elliptical cross-section |
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Richard Morton Submitted: 2008-10-30 07:21
The propagation of magnetohydrodynamic (MHD) waves in a
finite, compressible magnetic flux tube with an elliptical
cross-section embedded in a magnetic environment is
investigated.
Presented is the derivation of the general dispersion
relation of linear magneto-acoustic wave propagation for a
compressible magnetic flux tube with elliptical cross-section in a plasma with finite beta. The wave modes of propagation for the n=0 (symmetric) sausage and n=1 (anti-symmetric) kink oscillations are then examined in the limit of the thin flux tube approximation.
It is shown that a compressible magnetic tube with elliptical cross-section supports slow and fast magneto-acoustic waves. In the thin tube approximation the slow sausage mode and the slow and fast kink modes are found in analogue to a circular cross-section. However, the kink modes propagate with different phase speeds depending on whether the axial displacement takes place along the
major or minor axis of the ellipse. This feature is present in both the slow and the fast bands, providing two infinite sets of slow kink modes and two infinite sets of fast kink modes, i.e. each corresponding cylindrical mode splits into two sets of modes due to the ellipticity. The difference between the phase speeds along the different axis is dependent on the ratio of the lengths of the two axis. Analytical expressions for the phase speeds are found. It is
shown that the sausage modes do not split due to the introduced ellipticity, only the phase speed is modified when compared to the appropriate cylindrical counterpart. The percentage difference between the periods of the circular and elliptical cross-sections is also calculated, which reaches up to 21% for oscillations along the major axis approximately twice the length of the minor axis. The
level of difference in period could be very important in
magneto-seismological applications, when observed periods are inverted into diagnostic properties (e.g. magnetic field strength, gravitational scale height, tube expansion parameter, ect.). Also shown is the perturbation of focal points of the elliptical cross-section for different modes. It is found that the focal points are unperturbed for the sausage mode, but are perturbed for all higher modes.
Authors: R. Erdelyi, R.J. Morton
Projects:
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Publication Status: A&A, Accepted
Last Modified: 2008-11-05 06:41
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