Frequency Distribution of Acoustic Oscillation in the Solar Atmosphere During Flare Event
The Astrophysical Journal, Volume 886, Issue 1, article id. 32, 17 pp. (2019).
We present a study of multi-wavelength observations, of a C 2.3 -class solar flare in Active Region NOAA 12353, observed on 2015 May 23, which reveal new properties of acoustic waves in the flaring region. The space-, and ground-based data measured by the HELioseismological Large Regions Interferometric Device, operating at the Vacuum Tower Telescope, the Atmospheric Imaging Assembly (AIA), and Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory, were used in this paper. First, using power spectra of solar oscillations, we identified the dominant frequencies and their location at seven different atmospheric levels before and after the flare event. Second, based on AIA observations taken in six Extreme Ultraviolet filters, we derived Differential Emission Measure (DEM) profiles and DEM maps of the flare. Finally, we confirm the sigma shape of the magnetic field in the active area, directly related to the flare. Our results are as follows: the high-frequency waves (ν > 5 mHz) in the photosphere, in both cases, before and after the flare, are generated at the footpoints of the chromospheric loop, while in the chromosphere (Hα line), before the event the power enhancement exhibits the maximum of flare emission, and after the eruption the enhancement by all frequencies is observed only in the post-flare loop area. Moreover, the power of oscillation in the pores surrounding the area before the flare has a random character, while after the flare oscillation power is concentrated in the pore, and weakened outside of. We conclude that accurate detection of high-frequency acoustic waves in active regions can lead to faster and easier prediction of high-energy events.
Observational Evidence for Variations of the Acoustic Cutoff Frequency with Height in the Solar Atmosphere
The Astrophysical Journal Letters, Volume 819,
Issue 2, article id. L23, 8 pp. (2016).
Direct evidence for the existence of an acoustic cutoff frequency in the solar atmosphere is given by observations performed by using the HELioseismological Large Regions Interferometric DEvice operating on the Vacuum Tower Telescope located on Tenerife. The observational results demonstrate variations of the cutoff with atmospheric heights. The observed variations of the cutoff are compared to theoretical predictions made by using five acoustic cutoff frequencies that have been commonly used in helioseismology and asteroseismology. The comparison shows that none of the theoretical predictions is fully consistent with the observational data. The implication of this finding is far reaching as it urgently requires either major revisions of the existing methods of finding acoustic cutoff frequencies or developing new methods that would much better account for the physical picture underlying the concept of cutoff frequencies in inhomogeneous media.
Multi-height spectroscopy for probing the solar atmosphere
Central European Astrophysical Bulletin, Vol. 39, p. 101-107
We present preliminary results from multi-height observations, taken with the HELLRIDE (HELioseismic Large Region Interferometric DEvice) instrument at the VTT (Vacuum Tower Telescope) in Izaña, Tenerife. The goal of this work is to study solar oscillations at different atmospheric heights. The data was obtained in May 2014 for 10 different wavelengths with high spatial, spectral and temporal resolution. In this paper we discuss the results from quiet sun measurements. The region was selected in such a way to be near to the disk center. Using spectral and cross-spectral analysis methods we derive phase differences of waves propagating between the atmospheric layers. The formation heights of the photospheric spectral lines were calculated by τ5000 = 1 in agreement with an LTE approximation and chromospheric lines with an NLTE method, respectively. We find that the acoustic cut-off frequency is a function of height in the solar atmosphere.
Improved High-resolution Fast Imager
Journal of Astronomical Telescopes, Instruments, and Systems, Volume 9, id. 015001 (2023).
The improved High-resolution Fast Imager (HiFI+) is a multiwavelength imaging filtergraph, which was commissioned at the GREGOR solar telescope at Observatorio del Teide, Izaña, Tenerife, Spain, in March 2022 - followed by science verification in April 2022, after which it entered routine observations. Three camera control computers with two synchronized sCMOS and CMOS cameras each provide near diffraction-limited imaging at high cadence in six wavelength bands (Ca II H at 396.8 nm, G-band at 430.7 nm, blue continuum at 450.6 nm, narrow- and broad-band Hα at 656.3 nm, and TiO bandhead at 705.8 nm). This unique combination of photospheric and chromospheric images provides "tomographic" access to the dynamic Sun and complements spectropolarimetric observations at the GREGOR telescope. High image acquisition rates of 50 and 100 Hz facilitate image restoration, where time series of restored images have a typical cadence of 6 and 12 s, which is sufficient to resolve the dynamics of the solar photosphere and chromosphere. In principle, all imaging channels can be restored individually using the speckle masking technique or multiframe blind deconvolution (MFBD). However, images recorded strictly simultaneously in the narrow-/broad-band Hα and the G-band/blue continuum channels can be pairwise subjected to multiobject multiframe deconvolution (MOMFBD) expanding the science capabilities of HiFI+. For example, the narrow-band (FWHM = 60 nm) Halle Hα Lyot filter isolates the Hα line core, which facilitates matching chromospheric fibrils and filamentary structures to photospheric bright points. Likewise, dividing G-band by blue continuum images enhances small-scale brightenings, which are often related to small-scale magnetic fields so that their evolution can be tracked in time. A detailed description of the improved high-cadence, large-format imaging system is presented and its performance is assessed based on first-light observations.
The European Solar Telescope
Astronomy & Astrophysics, Volume 666, id.A21, 36 pp.
The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems.