High-resolution imaging and near-infrared spectroscopy of penumbral decay

Abstract

Combining high-resolution spectropolarimetric and imaging data is key to understanding the decay process of sunspots as it allows us scrutinizing the velocity and magnetic fields of sunspots and their surroundings. Active region NOAA 12597 was observed on 24/09/2016 with the 1.5-m GREGOR solar telescope using high-spatial resolution imaging as well as imaging spectroscopy and near-infrared (NIR) spectropolarimetry. Horizontal proper motions were estimated with LCT, whereas LOS velocities were computed with spectral line fitting methods. The magnetic field properties were inferred with the SIR code for the Si I and Ca I NIR lines. At the time of the GREGOR observations, the leading sunspot had two light-bridges indicating the onset of its decay. One of the light-bridges disappeared, and an elongated, dark umbral core at its edge appeared in a decaying penumbral sector facing the newly emerging flux. The flow and magnetic field properties of this penumbral sector exhibited weak Evershed flow, moat flow, and horizontal magnetic field. The penumbral gap adjacent to the elongated umbral core and the penumbra in that penumbral sector displayed LOS velocities similar to granulation. The separating polarities of a new flux system interacted with the leading and central part of the already established active region. As a consequence, the leading spot rotated 55-degree in clockwise direction over 12 hours. In the high-resolution observations of a decaying sunspot, the penumbral filaments facing flux emergence site contained a darkened area resembling an umbral core filled with umbral dots. This umbral core had velocity and magnetic field properties similar to the sunspot umbra. This implies that the horizontal magnetic fields in the decaying penumbra became vertical as observed in flare-induced rapid penumbral decay, but on a very different time-scale.

arXiv comment14 pages, 11 figures, Accepted to be published in Astronomy and Astrophysics

References in corpus (25)

• The Solar Optical Telescope for the Hinode Mission: An Overview
• Response of the Photospheric Magnetic Field to the X2.2 Flare on 2011 February 15
• Making Sense of Sunspot Decay I: Parabolic Decay Law and Gnevyshev-Waldmeier Relation
• The IAG solar flux atlas: Accurate wavelengths and absolute convective blueshift in standard solar spectra
• Numerical simulations of sunspot decay: On the penumbra -- Evershed flow -- moat flow connection
• Evaluating local correlation tracking using CO5BOLD simulations of solar granulation
• The formation of sunspot penumbra. I. Magnetic field properties
• Evershed clouds as precursors of moving magnetic features around sunspots
• The GREGOR Fabry-Pérot Interferometer
• Horizontal flow fields observed in Hinode G-band images. I. Methods
• Flow Field Evolution of a Decaying Sunspot
• Horizontal flow fields observed in Hinode G-band images II. Flow fields in the final stages of sunspot decay
• Magnetic Flux Loss and Flux Transport in a Decaying Active Region
• Spectropolarimetry of a decaying sunspot penumbra
• Statistical Study on the Nature of Solar Flux Emergence
• A distinct magnetic property of the inner penumbral boundary
• A distinct magnetic property of the inner penumbral boundary. II. Formation of a penumbra at the expense of a pore
• On the formation of a stable penumbra in a region of flux emergence in the Sun
• Rapid Transition of Uncombed Penumbrae to Faculae during Large Flares
• Horizontal flows concurrent with an X2.2 flare in active region NOAA 11158
• High-Cadence Imaging and Imaging Spectroscopy at the GREGOR Solar Telescope - A Collaborative Research Environment for High-Resolution Solar Physics
• Spectropolarimetric observations of an arch filament system with the GREGOR solar telescope
• sTools - a data reduction pipeline for the GREGOR Fabry-Pérot Interferometer and the High-resolution Fast Imager at the GREGOR solar telescope
• Fitting peculiar spectral profiles in He I 10830 Å absorption features
• A retrospective of the GREGOR solar telescope in scientific literature

Cited by in corpus (12)

• The problem of the height dependence of magnetic fields in sunspots
• High-Cadence Imaging and Imaging Spectroscopy at the GREGOR Solar Telescope - A Collaborative Research Environment for High-Resolution Solar Physics
• Properties of the Umbral Filament Observed in Active Region NOAA 12529
• Restoring process of sunspot penumbra
• Penumbral decay observed in active region NOAA 12585
• No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots
• Magnetized supersonic downflows in the chromosphere. A statistical study using the He I 10830 Å lines
• The formation and decay of sunspot penumbra in Active Region NOAA 12673
• The Decay of Two Adjacent Sunspots Associated with Moving Magnetic Features
• High-resolution observations of two pores with the integral field unit (IFU) of the GREGOR Infrared Spectrograph (GRIS)
• The Decay Process of an α-configuration Sunspot
• Observations of the formation of a proto-spot in a pre-existing field environment