First direct observation of a torsional Alfvén oscillation at coronal heights

Context. Torsional Alfvén waves are promising candidates for the transport of energy across different layers of the solar atmosphere. They have been predicted theoretically for decades. Previous detections of Alfvén waves so far have however mostly relied on indirect signatures. Aims. We present the first direct observational evidence of a fully resolved torsional Alfvén oscillation of a large-scale structure occurring at coronal heights. Methods. We analysed IRIS imaging and spectral observation of a surge resulting from magnetic reconnection between active region prominence threads and surrounding magnetic field lines. Results. The IRIS spectral data provide clear evidence of an oscillation in the line-of-sight velocity with a 180° phase difference between the oscillation signatures at opposite edges of the surge flux tube. This together with an alternating tilt in the Si IV and Mg II k spectra across the flux tube and the trajectories traced by the individual threads of the surge material provide clear evidence of torsional oscillation of the flux tube. Conclusions. Our observation shows that magnetic reconnection leads to the generation of large-scale torsional Alfvén waves

Excitation of vertical coronal loop oscillations by impulsively driven flows

Context Flows of plasma along a coronal loop caused by the pressure difference between loop footpoints are common in the solar corona. Aims We aim to investigate the possibility of excitation of loop oscillations by an impulsively driven flow triggered by an enhanced pressure in one of the loop footpoints. Methods We carry out 2.5D magnetohydrodynamic (MHD) simulations of a coronal loop with an impulsively driven flow and investigate the properties and evolution of the resulting oscillatory motion of the loop. Results The action of the centrifugal force associated with plasma moving at high speeds along the curved axis of the loop is found to excite the fundamental harmonic of a vertically polarised kink mode. We analyse the dependence of the resulting oscillations on the speed and kinetic energy of the flow. Conclusions We find that flows with realistic speeds of less than 100 km s−1 are sufficient to excite oscillations with observable amplitudes. We therefore propose plasma flows as a possible excitation mechanism for observed transverse loop oscillations

Excitation and evolution of vertically polarised transverse loop oscillations by coronal rain

Context. Coronal rain is composed of cool dense blobs that form in solar coronal loops and are a manifestation of catastrophic cooling linked to thermal instability. The nature and excitation of oscillations associated with coronal rain is not well understood. Aims. We consider observations of coronal rain in a bid to elucidate the excitation mechanism and evolution of wave characteristics. Methods. We analyse IRIS and Hinode/SOT observations of an oscillating coronal rain event on 17 Aug 2014 and determine the wave characteristics as a function of time using tried and tested time-space analysis techniques. Results. We exploit the seismological capability of the oscillation to deduce the relative rain mass from the oscillation amplitude. This is consistent with the evolution of the oscillation period showing the loop loosing a third of its mass due to falling coronal rain in a 10-15 minute time period. Conclusions. We present first evidence of the excitation of vertically polarised transverse loop oscillations triggered by a catastrophic cooling at the loop top and consistent with two thirds of the loop mass comprising of cool rain mass

Analysis of coronal rain observed by IRIS, HINODE/SOT and SDO/AIA : transverse oscillations, kinematics and thermal evolution

Coronal rain composed of cool plasma condensations falling from coronal heights along magnetic field lines is a phenomenon occurring mainly in active region coronal loops. Recent high resolution observations have shown that coronal rain is much more common than previously thought, suggesting its important role in the chromosphere-corona mass cycle. We present the analysis of MHD oscillations and kinematics of the coronal rain observed in chromospheric and transition region lines by IRIS, Hinode/SOT and SDO/AIA. Two different regimes of transverse oscillations traced by the rain are detected: small-scale persistent oscillations driven by a continuously operating process and localised large-scale oscillations excited by a transient mechanism. The plasma condensations are found to move with speeds ranging from few km s−1 up to 180 km s−1 and with accelerations largely below the free fall rate, with the likely reasons being pressure effects and the ponderomotive force resulting from the loop oscillations. The observed evolution of the emission in individual SDO/AIA bandpasses is found to exhibit clear signatures of a gradual cooling of the plasma at the loop top. We determine the temperature evolution of the coronal loop plasma using regularised inversion to recover the differential emission measure (DEM) and by forward modelling the emission intensities in the SDO/AIA bandpasses using a two-component synthetic DEM model. The inferred evolution of the temperature and density of the plasma near the apex is consistent with the limit cycle model and suggests the loop is going through a sequence of periodically repeating heating-condensation cycles

Improving solar wind persistence forecasts: removing transient space weather events, and using observations away from the Sun-Earth line

This study demonstrates two significant ways of improving persistence forecasts of the solar wind, which exploit the relatively unchanging nature of the ambient solar wind to provide 27 day forecasts, when using data from the Lagrangian L1 point. Such forecasts are useful as a prediction tool for the ambient wind, and for benchmarking of solar wind models. We show that solar wind persistence forecasts can be improved by removing transient solar wind features such as coronal mass ejections (CMEs). Using CME indicators to automatically identify CME-contaminated periods in ACE data from 1998 to 2011, and replacing these with solar wind from a previous synodic rotation, persistence forecasts improve (relative to a baseline): skill scores for Bz, a crucial parameter for determining solar wind geoeffectiveness, improve by 7.7 percentage points when using a proton temperature-based indicator with good operational potential. We also show that persistence forecasts can be improved by using measurements away from L1, to reduce the requirement on coronal stability for an entire synodic period, at the cost of reduced lead time. Using STEREO-B data from 2007 to 2013 to create such a reduced lead time persistence forecast, we show that Bz skill scores improve by 17.1 percentage points relative to ACE. Finally, we report on implications for persistence forecasts from any future missions to the L5 Lagrangian point and on the successful operational implementation (in spring 2015) of the normal (ACE-based) and reduced lead time (STEREO-based) persistence forecasts in the Met Office's Space Weather Operations Centre, as well as plans for future improvements

Kinematics of coronal rain in a transversely oscillating loop : ponderomotive force and rain-excited oscillations

Context. Coronal rain are cool dense blobs that form in solar coronal loops and are a manifestation of catastrophic cooling linked to thermal instability. Once formed, rain falls towards the solar surface at sub-ballistic speeds, which is not well-understood. Pressure forces seem to be the prime candidate to explain this. In many observations rain is accompanied by transverse oscillations and the interaction between the two needs to be explored. Aims. Therefore, an alternative kinematic model for coronal rain kinematics in transversely oscillating loops is developed to understand the physical nature of the observed sub-ballistic falling motion of rain. It explicitly explores the role of the ponderomotive force arising from the transverse oscillation on the rain motion as well as the capacity of rain to excite wave motion. Methods. An analytical model is presented that describes a rain blob guided by the coronal magnetic field supporting a onedimensional shear Alfvén wave as a point mass on an oscillating string. The model includes gravity and the ponderomotive force from the oscillation acting on the mass, as well as the inertia of the mass acting on the oscillation. Results. The kinematics of rain in the limit of negligible rain mass are explored and falling and trapped regimes are found, depending on wave amplitude. In the trapped regime for the fundamental mode, the rain blob bounces back and forth around the loop top at a long period inversely proportional to the oscillation amplitude. The model is compared with several observational rain studies, including one in-depth comparison with an observation that shows rain with up-and down bobbing motion. The role of rain inertia in exciting transverse oscillations is explored in inclined loops. Conclusions. It is found that the model requires displacement amplitudes of the transverse oscillation that are typically an order of magnitude larger than observed to explain the measured sub-ballistic motion of the rain. Therefore, it is concluded that the ponderomotive force is not the primary reason for understanding sub-ballistic motion, but it plays a role in cases of large loop oscillations. The appearance of rain causes the excitation of small-amplitude transverse oscillations that may explain observed events and provide a seismological tool to measure rain mass

Excitation of vertical coronal loop oscillations by plasma condensations

Context. Coronal rain composed of downfalling cool plasma condensations occurs in thermally unstable loops as a consequence of catastrophic cooling. Such loops contain significant quantities of dense plasma out of hydrostatic equilibrium. Transverse oscillations traced by coronal rain blobs are often observed in rainy loops. Aims. We aim to investigate the possibility of excitation of loop oscillations by the presence of condensation plasma. Methods. We carried out 2.5D magnetohydrodynamic simulations of a coronal loop containing a cool and a dense condensation region near the loop apex and investigated the properties and evolution of the resulting oscillatory motion of the loop. Results. The presence of dense condensation region at the apex of the coronal loop is found to excite fundamental harmonic of a vertically polarised kink mode. As the condensations fall towards the loop footpoints under the influence of gravity, the fundamental mode period decreases as a result of the change in distribution of mass along the loop. Conclusions. We propose coronal rain as a possible excitation mechanism for transverse loop oscillations

Magnetohydrodynamic oscillations in solar coronal rain

Coronal rain composed of cool plasma condensations falling from coronal heights along magnetic field lines is a phenomenon occurring in active region coronal loops. This work combines high-resolution observations and numerical simulations to understand the interplay between coronal rain and MHD oscillations. We analyse oscillations and kinematics of the coronal rain using high resolution observations. Two different regimes of transverse oscillations traced by the rain are detected: smallscale persistent oscillations driven by a continuously operating process and localized large-scale oscillations excited by a transient mechanism. The plasma condensations are found to move with accelerations largely below the free-fall rate. The observed evolution of the emission of the plasma at the loop top is found to exhibit clear signatures of a gradual cooling consistent with the limit cycle model and suggests the loop is going through a sequence of periodically repeating heating-condensation cycles. We further investigate the evolution and dynamics of coronal rain using 2.5D MHD simulations. We model the evolution of a cool plasma condensation in a gravitationally stratified coronal loop. The motion of plasma condensations is found to be strongly affected by the pressure of the coronal loop plasma. High coronal magnetic field or low condensation mass are found to lead to damped oscillatory motion of the condensations. The combined effect of plasma pressure gradients and magnetic tension force can therefore explain observed sub-ballistic motion and longitudinal oscillations of coronal rain. We finally address the possibility of excitation of loop oscillations by coronal rain. We carry out MHD simulations of a coronal loop containing a cool and dense condensation region near the loop apex. This is found to excite fundamental harmonic of a vertically polarised kink mode. As the condensations fall towards the loop footpoints, the fundamental mode period is found to decrease as a result of the change in distribution of mass along the loop. We also carry out simulations of a coronal loop with a siphon ow between the footpoints which is likely to arise in asymmetrically heated loops. The action of the centrifugal force associated with plasma moving along the curved axis of the loop is found to excite vertically polarised loop oscillations. We find that flows with realistic speeds are sufficient to excite oscillations with observable amplitudes. We therefore propose coronal rain as a possible excitation mechanism for transverse loop oscillations

Formation of coronal rain triggered by impulsive heating associated with magnetic reconnection

Context. Coronal rain consists of cool plasma condensations formed in coronal loops as a result of thermal instability. The standard models of coronal rain formation assume that the heating is quasi-steady and localised at the coronal loop footpoints. Aims. We present an observation of magnetic reconnection in the corona and the associated impulsive heating triggering formation of coronal rain condensations. Methods. We analyse combined SDO/AIA and IRIS observations of a coronal rain event following a reconnection between threads of a low-lying prominence flux rope and surrounding coronal field lines. Results. The reconnection of the twisted flux rope and open field lines leads to a release of magnetic twist. Evolution of the emission of one of the coronal loops involved in the reconnection process in different AIA bandpasses suggests that the loop becomes thermally unstable and is subject to the formation of coronal rain condensations following the reconnection and that the associated heating is localised in the upper part of the loop leg. Conclusions. In addition to the standard models of thermally unstable coronal loops with heating localised exclusively in the footpoints, thermal instability and subsequent formation of condensations can be triggered by the impulsive heating associated with magnetic reconnection occurring anywhere along a magnetic field line