arXiv subjects

Sarah Jaeggli

Publications and source records attributed to Sarah Jaeggli.

8 recordsLinked to original sources

arXiv2026

DKIST Unveils Four-Lobed Stokes U Profiles in a Magnetically Complex Sunspot Penumbra

We present DKIST spectropolarimetric observations of a sunspot penumbra revealing spatially coherent four-lobed Stokes U profiles that persist after polarimetric crosstalk correction. The observed Stokes vectors are well reproduced by stratified inversions containing gradients in velocity and magnetic parameters, including line-of-sight variation of the magnetic azimuth. Forward synthesis experiments in which the magnetic azimuth is artificially held constant with optical depth substantially modify the Stokes U morphology while leaving Stokes I, Q, and V comparatively unchanged, indicating that azimuthal variation is an important contributor to the observed linear polarisation signatures within the tested one-component atmospheres. We explored degeneracies using both two-component atmospheres and one-component constant-azimuth models: the former generally converged toward pathological solutions, while the latter provided a plausible degenerate explanation for much of the observed four-lobed Stokes U morphology. A supervised machine-learning classifier applied to Stokes V profiles shows that the associated magnetic complexity is spatially confined to the penumbra-light-bridge boundary where the four-lobed Stokes U profiles and reverse-polarity fields are observed. These results identify four-lobed Stokes U profiles as a potential spectropolarimetric signature of complex line-of-sight magnetic structuring in sunspot penumbrae, providing new observational constraints on fine-scale penumbral magnetic topology.

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arXiv2021

Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE): II. Flares and Eruptions

Current state-of-the-art spectrographs cannot resolve the fundamental spatial (sub-arcseconds) and temporal scales (less than a few tens of seconds) of the coronal dynamics of solar flares and eruptive phenomena. The highest resolution coronal data to date are based on imaging, which is blind to many of the processes that drive coronal energetics and dynamics. As shown by IRIS for the low solar atmosphere, we need high-resolution spectroscopic measurements with simultaneous imaging to understand the dominant processes. In this paper: (1) we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne observatory to fill this observational gap by providing high-cadence (<20 s), sub-arcsecond resolution spectroscopic rasters over an active region size of the solar transition region and corona; (2) using advanced numerical models, we demonstrate the unique diagnostic capabilities of MUSE for exploring solar coronal dynamics, and for constraining and discriminating models of solar flares and eruptions; (3) we discuss the key contributions MUSE would make in addressing the science objectives of the Next Generation Solar Physics Mission (NGSPM), and how MUSE, the high-throughput EUV Solar Telescope (EUVST) and the Daniel K Inouye Solar Telescope (and other ground-based observatories) can operate as a distributed implementation of the NGSPM. This is a companion paper to De Pontieu et al. (2021; arXiv:2106.15584), which focuses on investigating coronal heating with MUSE.

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arXiv2020

Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)

The Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities which will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the Daniel K. Inouye Solar Telescope hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute.

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arXiv2015

Internetwork chromospheric bright grains observed with IRIS

The Interface Region Imaging Spectrograph (IRIS) reveals small-scale rapid brightenings in the form of bright grains all over coronal holes and the quiet sun. These bright grains are seen with the IRIS 1330 Å, 1400 Å and 2796 Å slit-jaw filters. We combine coordinated observations with IRIS and from the ground with the Swedish 1-m Solar Telescope (SST) which allows us to have chromospheric (Ca II 8542 Å, Ca II H 3968 Å, Hα, and Mg II k 2796 Å), and transition region (C II 1334 Å, Si IV 1402) spectral imaging, and single-wavelength Stokes maps in Fe I 6302 Åat high spatial (0.33"), temporal and spectral resolution. We conclude that the IRIS slit-jaw grains are the counterpart of so-called acoustic grains, i.e., resulting from chromospheric acoustic waves in a non-magnetic environment. We compare slit-jaw images with spectra from the IRIS spectrograph. We conclude that the grain intensity in the 2796 Å slit-jaw filter comes from both the Mg II k core and wings. The signal in the C II and Si IV lines is too weak to explain the presence of grains in the 1300 and 1400 Å slit-jaw images and we conclude that the grain signal in these passbands comes mostly from the continuum. Even though weak, the characteristic shock signatures of acoustic grains can often be detected in IRIS C II spectra. For some grains, spectral signature can be found in IRIS Si IV. This suggests that upward propagating acoustic waves sometimes reach all the way up to the transition region.

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arXiv2014

The 2014 March 29 X-flare: sub-arcsecond resolution observations of Fe XXI 1354.1

The Interface Region Imaging Spectrometer (IRIS) is the first solar instrument to observe $\sim 10$ MK plasma at subarcsecond spatial resolution through imaging spectroscopy of the Fe XXI $λ$1354.1 forbidden line. IRIS observations of the X1 class flare that occurred on 2014 March 29 at 17:48 UT reveal Fe XXI emission from both the flare ribbons and the post-flare loop arcade. Fe XXI appears at all of the chromospheric ribbon sites, although typically with a delay of one raster (75 seconds) and sometimes offset by up to 1$^{\prime\prime}$. 100--200 km s$^{-1}$ blue-shifts are found at the brightest ribbons, suggesting hot plasma upflow into the corona. The Fe XXI ribbon emission is compact with a spatial extent of $< 2^{\prime\prime}$, and can extend beyond the chromospheric ribbon locations. Examples are found of both decreasing and increasing blue-shift in the direction away from the ribbon locations, and blue-shifts were present for at least 6 minutes after the flare peak. The post-flare loop arcade, seen in Atmospheric Imaging Assembly (AIA) 131 Å filtergram images that are dominated by Fe XXI, exhibited bright loop-tops with an asymmetric intensity distribution. The sizes of the loop-tops are resolved by IRIS at $\ge 1^{\prime\prime}$, and line widths in the loop-tops are not broader than in the loop-legs suggesting the loop-tops are not sites of enhanced turbulence. Line-of-sight speeds in the loop arcade are typically $<10$ km s$^{-1}$, and mean non-thermal motions fall from 43 km s$^{-1}$ at the flare peak to 26 km s$^{-1}$ six minutes later. If the average velocity in the loop arcade is assumed to be at rest, then it implies a new reference wavelength for the Fe XXI line of $1354.106\pm 0.023$ Å.

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arXiv2014

Evidence of Non-Thermal Particles in Coronal Loops Heated Impulsively by Nanoflares

The physical processes causing energy exchange between the Sun's hot corona and its cool lower atmosphere remain poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (about 20 to 60 seconds) of intensity and velocity on small spatial scales at the footpoints of hot dynamic coronal loops. The observations are consistent with numerical simulations of heating by beams of non-thermal electrons, which are generated in small impulsive heating events called "coronal nanoflares". The accelerated electrons deposit a sizable fraction of their energy in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the nonflaring corona.

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arXiv2014

Molecular absorption in transition region spectral lines

Aims: We present observations from the Interface Region Imaging Spectrograph (IRIS) of absorption features from a multitude of cool atomic and molecular lines within the profiles of Si IV transition region lines. Many of these spectral lines have not previously been detected in solar spectra. Methods: We examined spectra taken from deep exposures of plage on 12 October 2013. We observed unique absorption spectra over a magnetic element which is bright in transition region line emission and the ultraviolet continuum. We compared the absorption spectra with emission spectra that is likely related to fluorescence. Results: The absorption features require a population of sub-5000 K plasma to exist above the transition region. This peculiar stratification is an extreme deviation from the canonical structure of the chromosphere-corona boundary . The cool material is not associated with a filament or discernible coronal rain. This suggests that molecules may form in the upper solar atmosphere on small spatial scales and introduces a new complexity into our understanding of solar thermal structure. It lends credence to previous numerical studies that found evidence for elevated pockets of cool gas in the chromosphere.

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arXiv2011

Spectropolarimetry of Chromospheric Magnetic and Velocity Structure Above Active Regions

Active regions often host large-scale gas flows in the chromosphere presumably directed along curved magnetic field lines. Spectropolarimetric observations of these flows are critical to understanding the nature and evolution of their anchoring magnetic structure. We discuss recent work with the Facility Infrared Spectropolarimeter (FIRS) located at the Dunn Solar Telescope in New Mexico to achieve high resolution imaging-spectropolarimetry of the Fe I lines at 630 nm, the Si I line at 1082.7 nm, and the He I triplet at 1083 nm. We present maps of the photospheric and chromospheric magnetic field vector above a sunspot as well as discuss characteristics of surrounding chromospheric flow structures.

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