“Warwick Monash meeting”, 23rd – 25th February, 2014, University of Warwick, UK Abstracts list 1. A.N. Afanasyev: “Nonlinear Effects in the Shock-­‐Associated EUV Wave Propagation” Solar eruptions are sometimes accompanied by large-­‐scale wave-­‐like transients in the lower corona, the so-­‐called EUV (or EIT) waves. Several mechanisms are most likely responsible for such phenomena. We consider a class of EUV waves that are due to global coronal fast-­‐mode MHD shock waves and model their propagation with the method of nonlinear geometrical acoustics. This method is based on the ray-­‐tracing method (WKB approach) and allows one to take into account nonlinear properties of shock waves. Results of our modelling show the initial deceleration of an EUV wave as well as its damping and lengthening. These effects are due to nonlinearity of the coronal shock wave and agree with observations. We also consider the initial stage of propagation of the global coronal shock wave in the magnetosphere of a solar active region and analyse the evolution of the wave magnitude. Special attention is paid to the problem of the interaction of waves with coronal magnetic null points. The wave is captured by the null point and the amplitude of the wave increases considerably due to the Alfven speed decrease as well as the wave front convergence in the neighbourhood of the null point. Thus, nonlinear effects come to play and we model them, using the nonlinear geometrical acoustics method. 2. Tony Arber: “Title: MHD turbulent heating of the chromosphere” Recent models using either reduced MHD models or incompressible 1D models have suggested that the driven MHD wave flux into the magnetised chromosphere may be responsible for maintaining chromospheric temperatures. This talks summarises the 1D results highlighting the problems with 1D models and describes the minimum physics required for credible simulations. Intimately related to this is a description of the possible turbulent models so the talk will cover a brief introduction to the differences between Kolmogorov, Iroshnikov-­‐
Kraichnan and Goldereich-­‐Sridhar models and why these distinctions matter for chromospheric theory. Finally, and most importantly, can any of this be observed? What are the observational constraints and how might analysis of observations and turbulence simulations be compared. 3. Chris Brady: “Modelling chromospheric heating” The solar chromosphere is a notoriously difficult part of the solar atmosphere to work in, and yet the heating of this region is one of the great unanswered questions in solar physics. The mixture of partial ionisation, intermediate optical path lengths and changes of the plasma from low to high beta make attempts at modelling challenging. In this talk, work done at Warwick using a mixture of analytical models, simulations and observation is presented that attempts to explain the roles played by different mechanisms in the heating and dynamics of the chromosphere. 4. Anne-­‐Marie Broomhall: “Helioseismic data as a global parameter of the Sun’s magnetic activity cycle” It is well known that the properties of the Sun’s acoustic oscillations (p modes), such as their frequencies, vary with the Sun’s magnetic activity cycle. Here we consider the helioseismic properties of those oscillations observed in Sun-­‐as-­‐a-­‐star data, which are only sensitive to those oscillations with the largest horizontal scales, the truly global oscillations of the Sun. We compare helioseismic data with a number of well-­‐known global activity proxies, ranging from photospheric measures of the Sun’s magnetic flux (sunspot number and area), through the chromosphere and corona (10.7cm radio flux and green coronal index), and out into interplanetary regions (interplanetary magnetic flux and galactic cosmic ray intensity). The comparisons are made over cycles 22, 23, and the rising phase of cycle 24. We consider synergies and discrepancies observed over different timescales: both the 11yr solar cycle and mid-­‐
term (~2yr) variations. We find that the relationship between the different activity proxies and the helioseismic data changes from one cycle to the next. Since we consider unresolved solar data this work has natural implications for asteroseismic studies of activity cycles on other stars. 5. Philippa Browning: “Plasma heating and particle acceleration in twisted coronal magnetic fields” Twisted magnetic fields are likely to be common in the solar corona, and provide a reservoir of free magnetic energy. First, energy storage and release in single twisted magnetic flux ropes will be considered, with applications to solar coronal heating as well to small flares. I will show how reconnection can be triggered in the nonlinear phase of the ideal kink instability in twisted coronal loops, leading both to heating and acceleration of non-­‐thermal particles. Recently, we have developed models for more realistic configurations, representing confined solar flares, and some observational signatures of the reconnection are predicted, using a combination of 3D magnetohydrodynamic and test particle models. We then consider interacting magnetic flux ropes, representing multi-­‐
thread structure within loops as well as reconnections between different loops. Resistive and Hall MHD simulations of merging flux ropes will be described, as well as a relaxation model of energy release in interacting flux ropes. I will also show that instability in one flux tube can trigger energy release in a stable neighbour, leading potentially to an avalanche of heating events. 6. Paul Cally: “Aspects of Mode Conversion in MHD Wave Theory” The solar magnetic environment is very inhomogeneous, meaning that the usual fast/slow/Alfvén categorisation of MHD waves is not sufficient. Mode conversion and resonant absorption (they are actually the same thing in different guises) operate widely throughout the solar atmosphere, with implications for observational interpretation, helioseismology, and atmospheric heating. I will report on several aspects of this process in gravitationally stratified atmospheres, including insights to be gained from exact solutions, implications for multiple scattering in large ensembles of thin flux tubes, and prospects for a directional Time-­‐
Distance helioseismology in active regions. The work discussed is work joint with variously Shelley Hansen, Chris Hanson, and Hamed Moradi. 7. Giulio Del Zanna: “Heating in active regions” 8. Chris Goddard: “Detection and analysis of fast wave trains in the solar corona” This recently started project aims to use SDO/AIA data for the detection and analysis of fast wave trains in the solar corona. This will involve identifying events which produce observable examples of this phenomena. Image enhancement is carried out with smoothing, contrast adjustment and fast fourier transform filtering to highlight the wave train. Further analysis including time distance maps and wavelet analysis can then be used to determine the characteristics of the wave train. 9. Viktor Fedun: “Model of multi magnetic flux tube configurations in the Lower Solar Atmosphere” In this talk we will show the possibility of analytical construction of realistic magnetic field configurations, typical of the lower solar atmosphere. The magneto-­‐hydrostatic equilibrium is obtained by taking into account the presence of external forces. Systems incorporating open single and multiple flux tubes and closed magnetic loops can be combined to form magnetic structures that could even represent complex solar active regions. The developed model successfully spans the Interface Region of the solar atmosphere, from the photosphere up to the solar corona across the challenging transition region, while retaining physically valid plasma pressure, density and magnetic flux. Modelling magnetic structures can depict the main characteristics of solar intergranular lanes or active regions. HMI data can be used, as an initial magnetic field distribution, to construct a realistic magnetic field distribution. The model includes a number of free parameters, which makes the solution applicable to a variety of other physical problems, and it may therefore be of more general interest. 10. Bogdan Hnat “Temperature anisotropy in the presence of ultra low frequence waves in the terrestrial foreshock” We present the first study of the correlation between elevated solar wind core plasma temperatures and temperature anisotropy in the terrestrial foreshock. Plasma temperature is enhanced near the fire hose marginal stability threshold in the presence of ultra low frequency (ULF) large amplitude magnetic perturbations, which are intrinsically right-­‐hand circularly polarized. Direct comparison of contemporaneous anisotropic temperatures in the upstream solar wind and the foreshock suggests that the net heating of plasma is mediated via increase of the parallel temperature in the foreshock region where the ULF waves are present. We consider the possibility that a mechanism based on Landau damping, where solar wind plasma temperature parallel to the background magnetic field is increased by interaction with oblique compressible fast magneto-­‐acoustic ULF waves, influences temperature anisotropy. 11. Alan Hood “” 12. Chris Hornsey “Diagnostics of coronal heating in solar active regions” The coronal heating problem has been a central problem in solar physics since the 1940s. The mechanism by which energy is transferred to and deposited in the corona has been speculated on for a long time, and many mechanisms have been proposed. We aim to reconstruct coronal loops using data from SDO/HMI, and model the EUV emission of these loops to be able to find a diagnostic for the heating rate and its relationship with the magnetic field strength. We used a non-­‐linear force free magnetic field model to model the coronal magnetic field. Then considered loops that matched well with observations, these were modelled using a 1D hydrostatic model, with a heating rate dependant on magnetic field strength along the loop. The emission from these loops was then modelled in various EUV wavelengths, and its behaviour under different heating rates was observed. We were able to successfully model the EUV emission along these loops, and found that varying the heating parameters caused a predictable change in the modelled emission. We then performed tests to find a diagnostic for the dependence of the heating rate on the magnetic field strength. 13. Dimitri Kolotkov: “Hilbert-­‐Huang analysis of periodicities in the last two solar activity cycles” The periodicities of the last two solar activity cycles were investigated with the novel Hilbert-­‐Huang Transform (HHT) method. Raw data signals of five different observational proxies: the 10.7 cm radio flux intensity, the helioseismic frequency shift, and the sunspot area signals, recorded from the whole Sun disk, from the Northern solar hemisphere, and from the Southern hemisphere, were expanded into a set of intrinsic modes with Ensemble Empirical Mode Decomposition (EEMD) technique. The mean period of each empirical mode was estimated from the Hilbert spectrum, constructed for each separate mode. The periodicities were found to be allocated in three distinct groups: short-­‐term variations (with periods less than 0.5 year), quasi-­‐biennial oscillations (with typical periods from 0.5 year up to 3.9 years), and longer periodicities, e.g. such as 11-­‐year cycle. 14. Mihalis Mathioudakis: “The structure and evolution of solar photospheric magnetic bright points" 15. Valery Nakariakov: “Oscillations of CMEs” 16. Giuseppe Nisticò: “N-­‐S asymmetry of the solar magnetic field from coronal hole jets” The solar corona is an environment where the magnetic field controls the dynamics of plasma. Coronal jets, which are narrow ejections of plasma, are assumed to follow the magnetic field lines during their propagation. Starting from a catalogue of polar coronal jets observed with STEREO, we studied the average jet deflection both in the plane perpendicular to the line of sight, and, for a reduced number of jets, in the three dimensional space. 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 of the order of 25-­‐40%. The found asymmetry is attributed to the presence in the solar magnetic field of a non-­‐negligible quadrupole moment, which is responsible of the southward deflection of the heliospheric current sheet of the order of 10 deg, consistent with that inferred from other, independent, datasets and instruments. 17. David Pascoe: “Excitation and damping of kink waves in the solar corona” Observations have revealed that standing and propagating broadband kink oscillations are ubiquitous in the solar corona, providing the opportunity to use these oscillations as a seismological tool. We perform numerical simulations of footpoint-­‐driven transverse waves propagating in a low beta plasma. When density structuring is present, mode coupling in inhomogeneous regions leads to the coupling of kink and Alfvén modes. The frequency-­‐dependent decay of the propagating kink wave is observed as energy is transferred to the local Alfvén mode. 18. Chloe Pugh: “Quasi-­‐periodic pulsations in stellar flares” Quasi-­‐periodic pulsations (QPPs) in solar flares have been widely observed, and can be used as a coronal plasma diagnostics tool. More recently, QPPs have been observed in flares of other stars, many of which are millions of times more powerful than anything observed on the Sun. These 'superflares' have been identified in the short-­‐cadence light curves of over 200 Kepler stars, and have been examined for evidence of QPPs. Those showing evidence of oscillations have been analysed using the autocorrelation and wavelet techniques. Clear damped oscillations have been found in 14 flares; the periods of which range from 12 to 92 minutes. With a large sample of QPPs in stellar flares, it might be possible to infer whether the underlying physics of stellar superflares is similar to that of solar flares, and thus indicate whether a superflare could occur on the Sun. 19. Sergiy Shelyag: “The Sun off the disk centre” Solar observations off-­‐disk centre are difficult due to lower light levels and spatial resolution, compared to the disk centre. Nevertheless, such observations are of a great interest, since the radial structure of the solar atmosphere becomes more apparent, and horizontal motions and magnetic fields in the solar plasma turn into the line-­‐of-­‐sight motions and magnetic fields. Therefore, off-­‐disk observations together with the disk centre data would allow constructing a more detailed, three-­‐dimensional picture of the solar atmosphere. While large-­‐scale observational facilities allowing high-­‐resolution and high-­‐cadence solar limb spectropolarimetry, such as DKIST, are being constructed, it is compelling to use numerical modelling to predict the spectropolarimetric properties of radiation for various features in the solar atmosphere at different positions at the solar disk, including the limb. In my talk I will discuss our recent simulations and LTE radiative diagnostics of active solar granulation and acoustic properties of sunspots, which include center-­‐to-­‐limb variation. I will show general properties of the radiation across the solar limb, such as the continuum and line limb darkening, as well as the granulation contrast. I will demonstrate the presence of profiles with both emission and absorption features at the simulated solar limb, and pure emission profiles above the limb, and discuss physical origins of the emission wings in the Stokes profiles at the limb in LTE approximation. I will also demonstrate effects of non-­‐locality of radiative transport in the solar atmosphere and centre-­‐to-­‐limb variation on helioseismic measurements. 20. Erwin Verwichte : “MHD oscillations in coronal rain” Coronal rain are cool dense blobs that form in warm active region loops and fall along the guiding magnetic field to the solar surface. Antolin & Verwichte (2011) presented a first observational study with Hinode/SOT in In Ca II H in which multiple coronal rain blobs are seen to oscillate transversely in phase. I will discuss observations and theory of magnetohydrodynamic oscillations in coronal rain and what they can reveal about the dynamics of coronal rain and the structure of the magnetic loop supporting it. 21. Sergei Zharkov: “Flare seismology: what do sunquakes tell us about energy transport in flares?” Solar flares and coronal mass ejections (CMEs) are believed to be manifestations of a sudden and rapid release of accumulated magnetic energy in the corona. Only recently, the photospheric changes due to the reconnection and coronal magnetic field reconfiguration have been seriously considered from the theoretical point of view. Analysis of seismic emission (sun-­‐quakes) observed in the solar interior in the vicinity of flares offers us an opportunity to explore some of the most severe constraints on our understanding of how energy and momentum are transferred from the solar atmosphere to the interior. Only a limited number of M and X-­‐class flares have been reported to show seismic signatures in the form or ripples or egression sources, revealing that some of the most powerful flares often do not produce any seismic signatures. In addition, the most powerful signatures were recorded from an M-­‐class flare. This raises important questions about how the flare energy and momentum are transported to the solar surface and interior in order to produce sun-­‐quakes. Using recent observations by Hinode, RHESSI and SDO we review, analyse and test the new theories behind seismic emission in solar flares. 22. Ding Yuan “Predicting the observational features of standing slow modes of a flaring coronal loop” Standing slow mode waves in hot flaring loops are exclusively observed by spectrographs and are used to diagnose magnetic field strength and temperature of the loop structure. Due to the lack of spatial information, longitudinal mode identification is limited, and so is MHD seismology. In this study, we simulate a standing slow mode wave in a flaring loop and synthesize the spectrograph SUMER and imager SDO/AIA observations. We found that the emission intensity and line width oscillations are a quarter period out of phase with the Doppler shift velocity, which could be used to identify a standing slow mode wave. However, the longitudinal overtunes could be only measured with the assistance of an imager. If a flaring loop is heated to a temperature such that positive temperature modulation results in excess emission enhancement than the same amount negative temperature variation would reduce it, one may observe a double periodicity close to the loop apex. The MHD seismology is reliable very reliable in coronal plasma, provided the loop density and temperatures are effectively measured. 23. Sergiy Shelyag (on behalf of Alina Donnea): “From large scale to small scale jets in the Sun”