Oscillations In Solar Atmosphere Research Articles

Solar Atmospheric Oscillations as Measured by the GOES-R Series EXIS EUVS-C Instrument

This Letter presents first observations of distinct solar atmospheric oscillations from signals collected by the Extreme Ultraviolet and X-ray Irradiance Sensors EUVS-C instrument, which is part of each Geostationary Observational Environmental Satellite R-Series instrument payload. The EUVS-C instrument is a full-disk, normal-incidence spectrograph that covers a narrow band in the mid-ultraviolet between 276 and 284 nm, where it can measure the magnesium ii emission doublet at ∼280 nm and the photospheric continuum. The primary goal of EUVS-C data is to construct the well-known Mg ii index, which is often used as a proxy for chromospheric activity. Because of the high temporal and spectral resolution of EUVS-C measurements, the data provide a unique opportunity to observe discernible solar atmospheric waves that have definite signatures of 3 and 5 minute oscillation periods, where the frequency response of these signals is dependent on what part of the spectrum is analyzed (e.g., Mg emission lines). Furthermore, both photospheric and chromospheric waves can simultaneously be examined. With the recent increase in solar activity for Solar Cycle 25, these waves exhibit enhanced amplitudes and phase shifts during the impulsive shock of strong solar flares. This Letter will discuss the analysis for deriving these waves, and results from both a quiescent Sun and an X-class solar flare event will be presented.

Multi-wavelength observations of the 2014 June 11 M3.9 flare: temporal and spatial characteristics

We present multi-wavelength observations of an M-class flare (M3.9) that occurred on 2014 June 11. Our observations were conducted with the Dunn Solar Telescope (DST), employing adaptive optics, the multi-camera system Rapid Oscillations in Solar Atmosphere (ROSA), the new Hydrogen-Alpha Rapid Dynamics camera (HARDcam) in various wavelengths, such as Ca II K, Mg I b2 (at 5172.7 Å), and Hα narrow band and G-band continuum filters. Images were re-constructed using the Kiepenheuer-Institut Speckle Interferometry Package (KISIP) code, to improve our image resolution. We observed intensity increases of ≈120%–150% in the Mg, Ca K and Hα narrow band filters during the flare. Intensity increases for the flare observed in the SDO EUV channels were several times larger, and the X-rays, as recorded by GOES, increased over a factor of 30 for the harder band. Only a modest delay was found between the onset of flare ribbons of a nearby sympathetic flare and the main flare ribbons observed in these narrow band filters. The peak flare emission occurred within a few seconds for the Ca K, Mg and Hα bands. Time-distance techniques indicate propagation velocities of ≈60 km s−1 for the main flare ribbon and as high as 300 km s−1 for smaller regions, which we attribute to filament eruptions. This result and delays and velocities observed with SDO (≈100 km s−1) for different coronal heights agree well with the simple model of energy propagation versus height, although a more detailed model for the flaring solar atmosphere is needed. Finally, we detected marginal quasi-periodic pulsations (QPPs) in the 40–60 s range for the Ca K, Mg and Hα bands, and such measurements are important for disentangling the detailed flare-physics.

GLOBAL AND LOCAL CUTOFF FREQUENCIES FOR TRANSVERSE WAVES PROPAGATING ALONG SOLAR MAGNETIC FLUX TUBES

The propagation of linear transverse waves along a thin isothermal magnetic flux tube is affected by a global cutoff frequency that separates propagating and non-propagating waves. In this paper, wave propagation along a thin but non-isothermal flux tube is considered and a local cutoff frequency is derived. The effects of different temperature profiles on this local cutoff frequency are studied by considering different power-law temperature distributions as well as the semi-empirical VAL C model of the solar atmosphere. The results show that the conditions for wave propagation strongly depend on the temperature gradients. Moreover, the local cutoff frequency calculated for the VAL C model gives constraints on the range of wave frequencies that are propagating in different parts of the solar atmosphere. These theoretically predicted constraints are compared to observational data and are used to discuss the role played by transverse tube waves in the atmospheric heating and dynamics, and in the excitation of solar atmospheric oscillations.

A multiline spectrometer for the analysis of solar atmospheric oscillations and flows at the VTT, Tenerife

Context. Despite longstanding observational efforts, the origins of the chromospheric temperature rise and the coronal heating are still not well understood. There is reason to believe that the limitations of existing observational devices might be contributing to this lack of experimental evidence. Aims. We intended to develop a multiline spectrometer capable of observing velocity fields simultaneously at more height levels of the solar atmosphere than previously possible. System design and handling would be optimized for the 3D-analysis of atmospheric waves and flows. Methods. The number of optical components was kept to a minimum in order to achieve high optical throughput and short scanning times. A new type of bandpass preselection unit was developed. We successfully tested this Fabry-Perot based multiline device at the Vacuum Tower Telescope (VTT). Results. During a proof-of-concept run we were able to observe 16 spectral lines at a cadence of 60 s sustained over several hours. The field of view was 100-by-100 arcsecs. Multiple diagnostic diagrams from closely spaced height levels were derived. Conclusions. A new instrument of this type will be installed permantently at the VTT. We expect to be able to collect new 3Dinformation about atmospheric waves and flows.

Solar atmospheric oscillations and the chromospheric magnetic topology

We investigate the oscillatory properties of the quiet solar chromosphere in relation to the underlying photosphere, with particular regard to the effects of the magnetic topology. We perform a Fourier analysis on a sequence of line-of-sight velocities measured simultaneously in a photospheric (Fe I 709.0 nm) and a chromospheric line (Ca II 854.2 nm). The velocities were obtained from full spectroscopic data acquired at high spatial resolution with the Interferometric BIdimensional Spectrometer (IBIS). The field of view encompasses a full supergranular cell, allowing us to discriminate between areas with different magnetic characteristics. We show that waves with frequencies above the acoustic cut-off propagate from the photosphere to upper layers only in restricted areas of the quiet Sun. A large fraction of the quiet chromosphere is in fact occupied by ``magnetic shadows'', surrounding network regions, that we identify as originating from fibril-like structures observed in the core intensity of the Ca II line. We show that a large fraction of the chromospheric acoustic power at frequencies below the acoustic cut-off, residing in the proximity of the magnetic network elements, directly propagates from the underlying photosphere. This supports recent results arguing that network magnetic elements can channel low-frequency photospheric oscillations into the chromosphere, thus providing a way to input mechanical energy in the upper layers.

Magnetic coupling of waves and oscillations in the lower solar atmosphere: can the tail wag the dog?

Can the ubiquitously magnetic solar atmosphere have any effect on solar global oscillations? Traditionally, solar atmospheric magnetic fields are considered to be somewhat less important for the existence and characteristic features of solar global oscillations (p,f and the not-yet-observed g-modes). In this paper, I demonstrate the importance of the presence of magnetism and plasma dynamics for global resonant oscillations in the solar atmosphere. In particular, in the lower part of the solar atmosphere there are both coherent and random components of magnetic fields and velocity fields, each of which contribute on its own to the line widths and frequency variations of solar global acoustic waves. Changes in the coherent large-scale atmospheric magnetic fields cause frequency shifts of global oscillations over a solar cycle. The random character of the continuously emerging, more localized, magnetic carpet (i.e. small-scale, possibly even sub-resolution, loops) gives rise to additional frequency shifts. On the other hand, random and organized surface and sub-surface flows, like surface granulation, meridional flows or differential rotation, also affect the coupling mechanism of global oscillations to the lower magnetic atmosphere. The competition between magnetic fields and flows is inevitable. Finally, I shall discuss how solar global oscillations can resonantly interact with the overlaying inhomogeneous lower solar atmosphere embedded in a magnetic carpet. Line width broadening and distorsion of global acoustic modes will be discussed. The latter is suggested to be tested and measured by using ring-analysis techniques.

Tunnel-effect and propagation of 5-min oscillations in the solar atmosphere

Tunneling of surface waves (which are also called ‘non-propagating’ or ‘evanescent mode’) in isothermal atmosphere is considered. Tunneling of 5-min oscillations in solar atmosphere is discussed. Phase lead of chromospheric oscillations with respect to photospheric oscillations (Tanenbaum et al., 1971) can be explained by tunneling only.