Network Loop Oscillations with EIS/Hinode

We analyze a time sequence of He II 256.32 angstrom images obtained with EIS/Hinode, sampling a small magnetic loop in magnetic network. Wavelet analysis indicates 11-min periodicity close to the loop apex. We interprete this oscillation as forcing through upward leakage by the fundamental acoustic eigenmode of the underlying field-free cavity. The observed loop length corresponds to the value predicted from this mechanism.Comment: 3 pages, 3 figures; To appear in "Magnetic Coupling between the Interior and the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200

Numerical Simulations of Magnetoacoustic-Gravity Waves in the Solar Atmosphere

We investigate the excitation of magnetoacoustic-gravity waves generated from localized pulses in the gas pressure as well as in vertical component of velocity. These pulses are initially launched at the top of the solar photosphere that is permeated by a weak magnetic field. We investigate three different configurations of the background magnetic field lines: horizontal, vertical and oblique to the gravitational force. We numerically model magnetoacoustic-gravity waves by implementing a realistic (VAL-C) model of solar temperature. We solve two-dimensional ideal magnetohydrodynamic equations numerically with the use of the FLASH code to simulate the dynamics of the lower solar atmosphere. The initial pulses result in shocks at higher altitudes. Our numerical simulations reveal that a small-amplitude initial pulse can produce magnetoacoustic-gravity waves, which are later reflected from the transition region due to the large temperature gradient. The atmospheric cavities in the lower solar atmosphere are found to be the ideal places that may act as a resonator for various oscillations, including their trapping and leakage into the higher atmosphere. Our numerical simulations successfully model the excitation of such wave modes, their reflection and trapping, as well as the associated plasma dynamics

Acoustic oscillations in the field-free, gravitationally stratified Acoustic oscillations in the field-free, gravitationally stratified cavities under solar bipolar magnetic canopies

The main goal is to study the dynamics of the gravitationally stratified, field-free cavities in the solar atmosphere, located under small-scale, cylindrical magnetic canopies, in response to explosive events in the lower-lying regions (due to granulation, small-scale magnetic reconnection, etc.). We derive the two-dimensional Klein-Gordon equation for isothermal density perturbations in cylindrical coordinates. The equation is first solved by a standard normal mode analysis in order to obtain the free oscillation spectrum of the cavity. Then, the equation is solved in the case of impulsive forcing associated to a pressure pulse specified in the lower-lying regions. The normal mode analysis shows that the entire cylindrical cavity of granular dimensions tends to oscillate with frequencies of 5-8 mHz and also with the atmospheric cut-off frequency. Furthermore, the passage of a pressure pulse, excited in the convection zone, sets up a wake in the cavity oscillating with the same cut-off frequency. The wake oscillations can resonate with the free oscillation modes, which leads to an enhanced observed oscillation power. The resonant oscillations of these cavities explain the observed power halos near magnetic network cores and active regions.Comment: 8 pages, 8 figures, accepted in Astronomy and Astrophysic

Magnetic field inference in active region coronal loops using coronal rain clumps

Aims. We aim to infer information about the magnetic field in the low solar corona from coronal rain clumps using high-resolution spectropolarimetric observations in the Ca II 8542 Å line obtained with the Swedish 1 m Solar Telescope. Methods. The weak-field approximation (WFA) provides a simple tool to obtain the line-of-sight component of the magnetic field from spectropolarimetric observations. We adapted a method developed in a previous paper in order to assess the different conditions that must be satisfied in order to properly use the WFA for the data at hand. We also made use of velocity measurements in order to estimate the plane-of-the-sky magnetic field component, so that the magnetic field vector could be inferred. Results. We have inferred the magnetic field vector from a data set totalling 100 spectral scans in the Ca II 8542 Å line, containing an off-limb view of the lower portion of catastrophically cooled coronal loops in an active region. Our results, albeit limited by the cadence and signal-to-noise ratio of the data, suggest that magnetic field strengths of hundreds of Gauss, even reaching up to 1000 G, are omnipresent at coronal heights below 9 Mm from the visible limb. Our results are also compatible with the presence of larger magnetic field values such as those reported by previous works. However, for large magnetic fields, the Doppler width from coronal rain is not that much larger than the Zeeman width, thwarting the application of the WFA. Furthermore, we have determined the temperature, T, and microturbulent velocity, ξ, of coronal rain clumps and off-limb spicules present in the same data set, and we have found that the former ones have narrower T and ξ distributions, their average temperature is similar, and coronal rain has microturbulent velocities smaller than those of spicules

Spectropolarimetric Inversions of the Ca ii 8542 Å Line in an M-class Solar Flare

We study the M1.9 class solar flare SOL2015-09-27T10:40 UT using high-resolution full-Stokes imaging spectropolarimetry of the Ca ii 8542 {\AA} line obtained with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the non-LTE code NICOLE are used to construct semi-empirical models of the flaring atmosphere to investigate the structure and evolution of the flare temperature and magnetic field. A comparison of the temperature stratification in flaring and non-flaring areas reveals strong heating of the flare ribbon during the flare peak. The polarization signals of the ribbon in the chromosphere during the flare maximum become stronger when compared to its surroundings and to pre- and post- flare profiles. Furthermore, a comparison of the response functions to perturbations in the line-of-sight magnetic field and temperature in flaring and non-flaring atmospheres shows that during the flare the Ca ii 8542 {\AA} line is more sensitive to the lower atmosphere where the magnetic field is expected to be stronger. The chromospheric magnetic field was also determined with the weak-field approximation which led to results similar to those obtained with the NICOLE inversions.Comment: 11 pages, 9 figures, accepted in Ap

Erratum: Temporal evolution of small-scale internetwork magnetic fields in the solar photosphere

While the longitudinal field that dominates photospheric network regions has been studied extensively, small scale transverse fields have recently been found to be ubiquitous in the quiet internetwork photosphere. Few observations have captured how this field evolves. We aim to statistically characterise the magnetic properties and observe the temporal evolution of small scale magnetic features. We present two high spatial/temporal resolution observations that reveal the dynamics of two disk centre internetwork regions taken by the new GRIS/IFU (GREGOR Infrared Spectrograph Integral Field Unit) with the highly magnetically sensitive Fe I line pair at 15648.52 {\AA} and 15652.87 {\AA}. With the SIR code, we consider two inversion schemes: scheme 1 (S1), where a magnetic atmosphere is embedded in a field free medium, and scheme 2 (S2), with two magnetic models and a fixed stray light component. S1 inversions returned a median magnetic field strength of 200 and 240 G for the two datasets, respectively. We consider the median transverse (horizontal) component, among pixels with Stokes Q or U, and the median unsigned longitudinal (vertical) component, among pixels with Stokes V, above a noise threshold. We determined the former to be 263 G and 267 G, and the latter to be 131 G and 145 G, for the two datasets, respectively. We present three regions of interest (ROIs), tracking the dynamics of small scale magnetic features. We apply S1 and S2 inversions to specific profiles, and find S2 produces better approximations when there is evidence of mixed polarities. We find patches of linear polarization with magnetic flux density between 130 and 150 G, appearing preferentially at granule/intergranular lane (IGL) boundaries. The weak hG magnetic field appears to be organised in terms of complex loop structures, with transverse fields often flanked by opposite polarity longitudinal fields.Comment: Accepted for publication in A&A, 22 pages, 17 figures. Abstract abridged for mailing list, full abstract included in PD

The excitation of 5-min oscillations in the solar corona

We aim to study excitation of the observed 5-min oscillations in the solar corona by localized pulses that are launched in the photosphere. We solve the full set of nonlinear one-dimensional Euler equations numerically for the velocity pulse propagating in the solar atmosphere that is determined by the realistic temperature profile. Numerical simulations show that an initial velocity pulse quickly steepens into a leading shock, while the nonlinear wake in the chromosphere leads to the formation of consecutive pulses. The time interval between arrivals of two neighboring pulses to a detection point in the corona is approximately 5 min. Therefore, the consecutive pulses may result in the 5-min oscillations that are observed in the solar corona. The 5-min oscillations observed in the solar corona can be explained in terms of consecutive shocks that result from impulsive triggers launched within the solar photosphere by granulation and/or reconnection.Comment: 5 pages, 2 figures, accepted in A&

Newly identified properties of surface acoustic power

The cause of enhanced acoustic power surrounding active regions, the acoustic halo, is not as yet understood. We explore the properties of the enhanced acoustic power observed near disk center from 21 to 27 January 2002, including AR 9787. We find that (i) there exists a strong correlation of the enhanced high frequency power with magnetic-field inclination, with greater power in more horizontal fields, (ii) the frequency of the maximum enhancement increases along with magnetic field strength, and (iii) the oscillations contributing to the halos show modal ridges which are shifted to higher wavenumber at constant frequency in comparison to the ridges of modes in the quiet-Sun.Comment: 16 pages, 10 figures, submitted to solar physic