First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP: The Anatomy of an Umbral Flash

Abstract

The Visible Spectro-Polarimeter of the NSF Daniel K. Inouye Solar Telescope collected its Science Verification data on 2021 May 7–8. The instrument observed multiple layers of a sunspot atmosphere simultaneously, in passbands of Ca ii 397 nm (H line), Fe i 630 nm, and Ca ii 854 nm, scanning the region with a spatial sampling of 0.″041 and an average temporal cadence of 7.76 s, for a duration of 38.8 minutes. The slit moved southward across the plane of sky at 3.83 km s−1. The spectropolarimetric scans exhibit prominent oscillatory “ridge” structures that lie nearly perpendicular to the direction of slit motion (north to south). These ridges are visible in the maps of line intensity, central wavelength, line width, and both linear and circular polarization. Contemporaneous Atmospheric Imaging Assembly observations indicate that these ridges are purely temporal in character and are likely attributed to the familiar chromospheric 3 minute umbral oscillations. We observe in detail a steady umbral flash near the center of the sunspot umbra. Although bad seeing limited the spatial resolution, the unique high signal-to-noise ratio data enable us to estimate the shock Mach numbers (≈2), propagation speeds (≈9 km s−1), and their impacts on the longitudinal magnetic field (ΔB ≈ 50 G), gas pressure, and temperature (ΔT/T ≈ 0.1) of subshocks over 30 s. We also find evidence for rarefaction waves situated between neighboring wave train shocks. The Ca ii 854 nm line width is fairly steady throughout the umbral flash, except for a sharp 1.5 km s−1 dip immediately before, and a comparable spike immediately after, the passage of the shock front. This zigzag in line width is centered on the subshock and extends over 0.″4.

Observational characteristics of oscillations and waves in and around sunspots. Difficulties in observing and interpreting

“…Many physical characteristics of sunspots differ sharply from properties of the environment, which sustains the interest of scientists in their nature and in clarifying their role in the general processes occurring on the Sun. The number of publications on this topic have been increasing rapidly, thereby enriching and refining our knowledge about the processes under study [Alissandrakis et al, 1988;Lites, 1988;Settele et al, 2001;Bogdan, Judge, 2006;Khomenko, Collados, 2008;Solov'ev, Kirichek, 2008Botha et al al., 2011;Zhugzhda, Sych, 2014;Felipe et al, 2014;Zhao et al,2016;Belov et al, 2021]. As a result, interpretation of some observed phenomena has changed.…”

Simultaneous Spectropolarimetric Observations in the Hα and Ca ii 8662 Å Lines of an Active Region

“…Just to mention a few recent studies, the 3D structure of the magnetic field of sunspots from the photosphere to the middle chromosphere has been studied by Murabito et al (2019), Joshi et al (2016Joshi et al ( , 2017, and Schad et al (2015). Magnetic field variation in umbral flashes has been studied by many authors in recent literature (for example, Houston et al 2018;Joshi & de la Cruz Rodríguez 2018;Bose et al 2019;Felipe et al 2023;French et al 2023). Magnetic topologies of inverse Evershed flow have also been reported in the literature (Beck & Choudhary 2019;Prasad et al 2022).…”