On the Properties of Slow MHD Sausage Waves Within Small-Scale Photospheric Magnetic Structures

Abstract

The presence of magnetoacoustic waves in magnetic structures in the solar atmosphere is well-documented. Applying the technique of solar magneto-seismology (SMS) allows us to infer the background properties of these structures. Here, we aim to identify properties of the observed magnetoacoustic waves and study the background properties of magnetic structures within the lower solar atmosphere. Using the Dutch Open Telescope and Rapid Oscillations in the Solar Atmosphere instruments, we captured two series of high-resolution intensity images with short cadences of two isolated magnetic pores. Combining wavelet analysis and empirical mode decomposition (EMD), we determined characteristic periods within the cross-sectional (i.e., area) and intensity time series. Then, by applying the theory of linear magnetohydrodynamics (MHD), we identified the mode of these oscillations within the MHD framework. Several oscillations have been detected within these two magnetic pores. Their periods range from 3 to 20 minutes. Combining wavelet analysis and EMD enables us to confidently find the phase difference between the area and intensity oscillations. From these observed features, we concluded that the detected oscillations can be classified as slow sausage MHD waves. Furthermore, we determined several key properties of these oscillations such as the radial velocity perturbation, the magnetic field perturbation, and the vertical wavenumber using SMS. The estimated range of the related wavenumbers reveals that these oscillations are trapped within these magnetic structures. Our results suggest that the detected oscillations are standing harmonics, and this allows us to estimate the expansion factor of the waveguides by employing SMS. The calculated expansion factor ranges from 4 to 12.

Long-period quasi-periodic oscillations of a small-scale magnetic structure on the Sun

“…Similar periodicities and the corresponding dependence of the oscillation period upon the magnetic properties of a facular region were found in Kobanov et al (2015). Freij et al (2016) detected oscillations with periods from 3 to 20 min in the intensity of two isolated magnetic pores and interpreted them in terms of a slow magnetoacoustic sausage wave. Similarly, facular brightness was found to be dependent on the convective and wave motions in a facular region in Kostik & Khomenko (2016).…”

Photospheric Observations of Surface and Body Modes in Solar Magnetic Pores

“…They used the energy equations of Moreels et al (2015b) to determine that the energy carried by the modes decreases substantially with height and, thus, may release significant energy into the surrounding chromospheric plasma. A recent study of two pores (Freij et al 2016) using magneto-seismology techniques suggested that sausage modes in pores can be standing harmonics, with strong reflection at the transition region, indicating a chromospheric resonator.…”

“…To observe sausage modes in ground-based data, several studies (Dorotovič et al 2008;Moreels et al 2015a;Grant et al 2015;Freij et al 2016) searched for oscillatory signals in the cross-sectional area and intensity of pores. A key conclusion in each of these works was that the fractional variations in both area and intensity was so minor, that groundbased data was essential for studies of sausage modes in lower atmospheric regions.…”

“…Recent theoretical work (Yu et al 2017a,b) showed that resonant damping of slow surface sausage modes could be efficient under conditions usually observed in pores. Some previous studies of pore oscillations (Moreels et al 2015a;Grant et al 2015;Freij et al 2016) infer the surface/body characteristics of sausage modes using a combination of semi-empirical models, theory and the derived parameters of the modes from observations. Understanding the surface and body properties, however, is crucial in determining how energy is dissipated by modes in higher regions (Yu et al 2017b).…”

The Frequency-dependent Damping of Slow Magnetoacoustic Waves in a Sunspot Umbral Atmosphere

“…With the advent of highresolution observations from ground-and space-based instruments, both standing and propagating SMAWs have been discovered in a multitude of structures in the solar atmosphere including magnetic pores (Morton et al 2011;Dorotovič et al 2014;Moreels et al 2015;Freij et al 2016), chromospheric network (Vecchio et al 2007; Kontogiannis et al 2010Kontogiannis et al , 2014, coronal loops (Wang et al 2015;Jess et al 2016), and polar plumes (Krishna Prasad et al 2011;Su 2014). Recent multiwavelength observations of sunspots reveal that umbral flashes (Beckers & Tallant 1969;Rouppe van der Voort et al 2003) and running penumbral waves (Giovanelli 1972;Bloomfield et al 2007;Freij et al 2014) observed in the chromosphere, and quasi-periodic propagating disturbances observed in the coronal loops (Berghmans & Clette 1999;De Moortel et al 2000;De Moortel 2009), are just different manifestations of the SMAWs that propagate from the photosphere through to the corona (Jess et al 2012a;Krishna Prasad et al 2015;Zhao et al 2016).…”