Coronal loop oscillations and diagnostics with Hinode/EIS

Context.Standing slow (acoustic) waves commonly observed in hot coronal loops offer a unique opportunity to understand the properties of the coronal plasma. The lack of evidence for similar oscillations in cooler loops is still a puzzle. Aims.The high cadence EIS instrument on board recently launched Hinode has the capability to detect wave motion in EUV lines both in the imaging and spectroscopy modes. The paper aims to establish the distinct characteristics of standing and propagating acoustic waves and to predict their footprints in EIS data. Methods.A 1D hydrodynamic loop model is used and the consequences of various types of heating pulses are examined. In each case, the resulting hydrodynamic evolution of the loop is converted into observables using a selection of available EIS spectral lines and windows. Results.Propagating/standing acoustic waves are a natural response of the loop plasma to impulsive heating. Synthetic EIS observations of such waves are presented both in the imaging and spectroscopy modes. The waves are best seen and identified in spectroscopy mode observations. It is shown that the intensity oscillations, unlike the Doppler shift oscillations, continuously suffer phase shifts due to heating and cooling of the plasma. It is therefore important to beware of this effect when interpreting the nature of the observed waves

Alfvén instability of steady state flux tubes. II. Upflows in stratified atmospheres

Context. MHD instabilities play an important role in the dynamics and energetics of the solar atmosphere. Aims. An open vertical magnetic flux tube is permeated by an upflow in a stratified atmosphere with variable temperature. The stability of the tube is investigated with respect to small-amplitude torsional perturbations generated at the footpoint by random convective motions. Methods. A steady state equilibrium incorporating the effects of a vertical body force, heating, and losses is derived analytically. The governing equations for torsional motions are integrated with a fourth-order Runge-Kutta method and matched with the analytical solutions in the upper regions to obtain a numerical dispersion relation. The dependence of the eigenmode frequencies on different parameters is analysed. Unstable modes are found for a range of Alfvén and flow speeds in the photosphere, as well as expansion factors of the flux tubes. Both supersonic and subsonic flows are considered. Results. It is shown that torsional perturbations are exponentially amplified in time if a section of the tube exists where the upflowing plasma decelerates and the tube expands. The flow speeds required for the instability are sub-Alfvénic. Conclusions. The instability may be important for understanding the abundance of Alfvén waves seen in recent observations and the associated heating in magnetic regions of the solar atmosphere

Alfvén instability of steady state flux tubes. Isothermal flow

Context. MHD instabilities are expected to play an important role in the dynamics and energetics of the solar atmosphere. Aims. One application of a recently discovered MHD instability to an open magnetic flux tube is investigated. The flux tube is gravitationally stratified and permeated by a smooth isothermal flow. Methods. The equilibrium structure is derived, and the stability of the tube with respect to small amplitude torsional perturbations generated at the footpoint by random convective motions analysed. Results. It is shown that torsional perturbations are exponentially amplified in time if a narrow region exists where the flux tube rapidly expands while the plasma is flowing away from the footpoint. No high flow speeds are required for the instability to set in. Conclusions. These obtained results may account for the nonthermal broadenings associated with upflows in magnetic regions of the lower solar atmosphere. However, additional studies that incorporate temperature variations are needed for more robust conclusions

Alfven Instability in coronal loops with siphon flows

A new magnetohydrodynamic (MHD) instability in coronal loops is presented. It is demonstrated that small-amplitude torsional Alfvénic disturbances generated by random photospheric motions are inevitably amplified in asymmetrically stratified loops with siphon flows. The loop asymmetry and the flow speeds can be arbitrarily small. The growth rates linked with the instability increase with increasing flow speeds. The instability is caused by over-reflection of Alfvén waves at the transition region where steep variations in density exist

Formation of a Dense Flux Rope by a Siphon Flow

The interaction of siphon ow with an initially linear Alfven wave within an isolated chromospheric loop is investigated. The loop is modelled using 1.5D magnetohydrodynamics (MHD). The siphon flow undergoes a hydrodynamic (HD) shock, which allows the Alfven instability to amplify the propagating waves as they interact with the shock and loop footpoints. The amplification leads to non-linear processes strongly altering the loop equilibrium. Azimuthal twists of 50 km/s are generated and the loop becomes globally twisted with an azimuthal magnetic field 5 times stronger than the longitudinal field. The flow is accelerated to 70 km/s due to the propagating shock waves that form. Near the end of the simulation, where the non-linear processes are strongest, flow reversal is seen within the descending leg of the loop, generating up flows up to 28 km/s. This flow reversal leads to photospheric material being `pulled' into the loop and spreading along its entirety. Within about 2.5 h the density increases by a factor of about 30 its original value.publishersversionPeer reviewe

Alfvén Instability in a Compressible Flow

A new ideal magnetohydrodynamic instability is presented. It is shown that linear incompressible Alfvénic disturbances can get over-reflected and exponentially amplified in compressible plasma flows. A simple and transparent stability criterion for a two-layer model is derived. The instability does not require a shear in the flow and may arise for rather moderate sub-Alfvénic flow speeds. It is therefore important to be aware of the Alfvén instability when dealing with laboratory or astrophysical plasma flows

Individual Differences in Performance Speed Are Associated With a Positivity/Negativity Bias. An ERP and Behavioral Study

There is a current dispute over the origins, incidence, and development of Positivity Bias, i.e., preferential processing of positive relative to negative information. We addressed this question using a multi-method technique of behavioral, psychometric and event-related potential (ERP) measures in a lexical decision task (LDT). Twenty-four university students (11 female) participated (age range 18–26), but four were omitted owing to data issues. Participants were classified as Positivity Biased (PB) if their LDT responses to positive words were faster than negative words, and vice versa for those classified as Negativity Biased (NB), leading to a group of 11 PB participants and a group of 9 NB participants. Interestingly, the PB group was significantly faster overall than the NB group and had significantly shorter P2 component ERP latencies in the left occipital region. Furthermore, the PB group had significantly higher scores for expressive suppression (ES), together with higher scores for Crystallized Knowledge and for cognitive reappraisal (CR). These results suggest that around 55% of the students had Positivity Bias, and these were more efficient in processing information and had better emotion regulation abilities than those with a Negativity Bias

Hinode EUV spectroscopic observations of coronal oscillations

Context. Waves offer a unique opportunity to understand and diagnose the properties of the solar coronal plasma. Aims. Hinode/EIS observations are analysed to detect wave and oscillatory motions in the solar corona. Methods. The EIS observations were carried out using a selection of EUV lines. Hinode/XRT images are taken concurrently. Results. Two examples of oscillations in active regions are presented. We analysed the evolution of the intensities and Doppler shifts. The Hinode/XRT images suggest that both events occurred along loop-like structures. The first event is interpreted as a slow sausage (acoustic) type wave with a period of 1.2 mHz. The second example is associated with a transverse, most likely kink type, wave with a period of 3 mHz. The EUV line ratios were then used to determine the value of the coronal magnetic field