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.