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

Abstract

The Visible Spectro-Polarimeter (ViSP) of the NSF Daniel K. Inouye Solar Telescope (DKIST) collected its Science Verification data on May 7-8, 2021. 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 average temporal cadence of 7.76 seconds, for a 38.8 minute duration. The slit moves southward across the plane-of-the-sky at 3.83 km/s. The spectropolarimetric scans exhibit prominent oscillatory 'ridge' structures which lie nearly perpendicular to the direction of slit motion (north to south). These ridges are visible in maps of line intensity, central wavelength, line width, and both linear and circular polarizations. Contemporaneous Atmospheric Imaging Assembly observations indicate these ridges are purely temporal in character and 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 enable us to estimate the shock Mach numbers (= 2), propagation speeds (= 9 km/s), and their impact on longitudinal magnetic field (delta B = 50 G), gas pressure, and temperature (delta T/T = 0.1) of the subshocks over 30 seconds. We also find evidence for rarefaction waves situated between neighboring wave-train shocks. The Ca-II 854 nm line width is steady throughout the umbral flash except for a sharp 1.5 km/s dip immediately before, and comparable spike immediately after, the passage of the shock front. This zig-zag in line width is centered on the subshock and extends over 0.4".

11 pages, 5 figures, accepted for publication to ApJ Letters (February 2023)

References in corpus (13)

• The Visible Spectro-Polarimeter of the Daniel K. Inouye Solar Telescope
• Spectropolarimetric observations of the Ca II 8498 A and 8542 A lines in the quiet Sun
• Synthetic observations of wave propagation in a sunspot umbra
• High Resolution Observations of Chromospheric Jets in Sunspot Umbra
• Two-fluid numerical simulations of solar spicules
• Initial Observations of Sunspot Oscillations Excited by Solar Flare
• Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes
• Chromospheric Plasma Ejections in a Light Bridge of a Sunspot
• High-frequency wave power observed in the solar chromosphere with IBIS and ALMA
• Stability of two-fluid partially-ionised slow-mode shock fronts
• Spatial Distribution of Origin of Umbral Waves in a Sunspot Umbra
• SUMER observations of the inverse Evershed effect in the transition region above a sunspot
• Limitations of the Ca II 8542 Å line for the determination of magnetic field oscillations

Cited by in corpus (1)

• Simultaneous spectropolarimetric observations in the H$\alpha$ and Ca II 8662 Å lines of an active region