arXiv subjects

Udo Schühle

Publications and source records attributed to Udo Schühle.

14 recordsLinked to original sources

arXiv2024

Investigating coronal loop morphology and dynamics from two vantage points

Coronal loops serve as the fundamental building blocks of the solar corona. Therefore, comprehending their properties is essential in unraveling the dynamics of the Sun's upper atmosphere. In this study, we conduct a comparative analysis of the morphology and dynamics of a coronal loop observed from two different spacecraft: the High Resolution Imager (HRI$_{EUV}$) of the Extreme Ultraviolet Imager aboard the Solar Orbiter and the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory. These spacecraft were separated by 43$^{\circ}$ during this observation. The main findings of this study are: (1) The observed loop exhibits similar widths in both the HRI$_{EUV}$ and AIA data, suggesting that the cross-sectional shape of the loop is circular; (2) The loop maintains a uniform width along its entire length, supporting the notion that coronal loops do not exhibit expansion; (3) Notably, the loop undergoes unconventional dynamics, including thread separation and abrupt downward movement. Intriguingly, these dynamic features also appear similar in data from both spacecraft. Although based on observation of a single loop, these results raise questions about the validity of the coronal veil hypothesis and underscore the intricate and diverse nature of complexity within coronal loops.

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arXiv2023

Evolution of dynamic fibrils from the cooler chromosphere to the hotter corona

Dynamic fibrils (DFs) are commonly observed chromospheric features in solar active regions. Recent observations from the Extreme Ultraviolet Imager (EUI) aboard the Solar Orbiter have revealed unambiguous signatures of DFs at the coronal base, in extreme ultraviolet (EUV) emission. However, it remains unclear if the DFs detected in the EUV are linked to their chromospheric counterparts. Simultaneous detection of DFs from chromospheric to coronal temperatures could provide important information on their thermal structuring and evolution through the solar atmosphere. In this paper, we address this question by using coordinated EUV observations from the Atmospheric Imaging Assembly (AIA), Interface Region Imaging Spectrograph (IRIS), and EUI to establish a one-to-one correspondence between chromospheric and transition region DFs (observed by IRIS) with their coronal counterparts (observed by EUI and AIA). Our analysis confirms a close correspondence between DFs observed at different atmospheric layers, and reveals that DFs can reach temperatures of about 1.5 million Kelvin, typical of the coronal base in active regions. Furthermore, intensity evolution of these DFs, as measured by tracking them over time, reveals a shock-driven scenario in which plasma piles up near the tips of these DFs and, subsequently, these tips appear as bright blobs in coronal images. These findings provide information on the thermal structuring of DFs and their evolution and impact through the solar atmosphere.

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arXiv2023

Coronal Heating Rate in the Slow Solar Wind

This Letter reports the first observational estimate of the heating rate in the slowly expanding solar corona. The analysis exploits the simultaneous remote and local observations of the same coronal plasma volume with the Solar Orbiter/Metis and the Parker Solar Probe instruments, respectively, and relies on the basic solar wind magnetohydrodynamic equations. As expected, energy losses are a minor fraction of the solar wind energy flux, since most of the energy dissipation that feeds the heating and acceleration of the coronal flow occurs much closer to the Sun than the heights probed in the present study, which range from 6.3 to 13.3 solar radii. The energy deposited to the supersonic wind is then used to explain the observed slight residual wind acceleration and to maintain the plasma in a non-adiabatic state. As derived in the Wentzel-Kramers-Brillouin limit, the present energy transfer rate estimates provide a lower limit, which can be very useful in refining the turbulence-based modeling of coronal heating and subsequent solar wind acceleration.

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arXiv2023

Slow solar wind sources. High-resolution observations with a quadrature view

The origin of the slow solar wind is still an open issue. One possibility that has been suggested is that upflows at the edge of an active region can contribute to the slow solar wind. We aim to explain how the plasma upflows are generated, which mechanisms are responsible for them, and what the upflow region topology looks like. We investigated an upflow region using imaging data with the unprecedented temporal (3s) and spatial (2 pixels = 236km) resolution that were obtained on 30 March 2022 with the 174Å of the Extreme-Ultraviolet Imager (EUI)/High Resolution Imager (HRI) on board Solar Orbiter. During this time, the EUI and Earth-orbiting satellites (Solar Dynamics Observatory, Hinode, and the Interface Region Imaging Spectrograph, IRIS) were located in quadrature (92 degrees), which provides a stereoscopic view with high resolution. We used the Hinode/EIS (Fe XII) spectroscopic data to find coronal upflow regions in the active region. The IRIS slit-jaw imager provides a high-resolution view of the transition region and chromosphere. For the first time, we have data that provide a quadrature view of a coronal upflow region with high spatial resolution. We found extended loops rooted in a coronal upflow region. Plasma upflows at the footpoints of extended loops determined spectroscopically through the Doppler shift are similar to the apparent upward motions seen through imaging in quadrature. The dynamics of small-scale structures in the upflow region can be used to identify two mechanisms of the plasma upflow: Mechanism I is reconnection of the hot coronal loops with open magnetic field lines in the solar corona, and mechanism II is reconnection of the small chromospheric loops with open magnetic field lines in the chromosphere or transition region. We identified the locations in which mechanisms I and II work.

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arXiv2022

Signatures of dynamic fibrils at the coronal base: Observations from Solar Orbiter/EUI

The solar chromosphere hosts a wide variety of transients, including dynamic fibrils (DFs) that are characterised as elongated, jet-like features seen in active regions, often through H$α$ diagnostics. So far, these features have been difficult to identify in coronal images primarily due to their small size and the lower spatial resolution of the current EUV imagers. Here we present the first unambiguous signatures of DFs in coronal EUV data using high-resolution images from the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter. Using the data acquired with the 174~Å High Resolution Imager (HRI$_{EUV}$) of EUI, we find many bright dot-like features (of size 0.3-0.5 Mm) that move up and down (often repeatedly) in the core of an active region. In a space-time map, these features produce parabolic tracks akin to the chromospheric observations of DFs. Properties such as their speeds (14 km~s$^{-1}$), lifetime (332~s), deceleration (82 m~s$^{-2}$) and lengths (1293~km) are also reminiscent of the chromospheric DFs. The EUI data strongly suggest that these EUV bright dots are basically the hot tips (of the cooler chromospheric DFs) that could not be identified unambiguously before because of a lack of spatial resolution.

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arXiv2022

What drives decayless kink oscillations in active region coronal loops on the Sun?

We study here the phenomena of decayless kink oscillations in a system of active region (AR) coronal loops. Using high resolution observations from two different instruments, namely the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, we follow these AR loops for an hour each on three consecutive days. Our results show significantly more resolved decayless waves in the higher-resolution EUI data compared with the AIA data. Furthermore, the same system of loops exhibits many of these decayless oscillations on Day-2, while on Day-3, we detect very few oscillations and on Day-1, we find none at all. Analysis of photospheric magnetic field data reveals that at most times, these loops were rooted in sunspots, where supergranular flows are generally absent. This suggests that supergranular flows, which are often invoked as drivers of decayless waves, are not necessarily driving such oscillations in our observations. Similarly, our findings also cast doubt on other possible drivers of these waves, such as a transient driver or mode conversion of longitudinal waves near the loop footpoints. In conclusion, through our analysis we find that none of the commonly suspected sources proposed to drive decayless oscillations in active region loops seems to be operating in this event and hence, the search for that elusive wave driver needs to continue.

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arXiv2022

The Lyman-$α$ Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter

The hydrogen Lyman-$α$ (H {\sc i} Ly$α$) emission during solar flares has rarely been studied in spatially resolved images and its physical origin has not been fully understood. In this paper, we present novel Ly$α$ images for a C1.4 solar flare (SOL2021-08-20T22:00) from the Extreme Ultraviolet Imager aboard Solar Orbiter, together with multi-waveband and multi-perspective observations from the Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics Observatory spacecraft. It is found that the Ly$α$ emission has a good temporal correlation with the thermal emissions at 1--8 Å and 5--7 keV, indicating that the flaring Ly$α$ is mainly produced by a thermal process in this small event. However, nonthermal electrons play a minor role in generating Ly$α$ at flare ribbons during the rise phase of the flare, as revealed by the hard X-ray imaging and spectral fitting. Besides originating from flare ribbons, the Ly$α$ emission can come from flare loops, likely caused by plasma heating and also cooling that happen in different flare phases. It is also found that the Ly$α$ emission shows fairly similar features with the He {\sc ii} 304 Å emission in light curve and spatio-temporal variation along with small differences. These observational results improve our understanding of the Ly$α$ emission in solar flares and also provide some insights for investigating the Ly$α$ emission in stellar flares.

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arXiv2022

Observation of Magnetic Switchback in the Solar Corona

Switchbacks are sudden, large radial deflections of the solar wind magnetic field, widely revealed in interplanetary space by the Parker Solar Probe. The switchbacks' formation mechanism and sources are still unresolved, although candidate mechanisms include Alfvénic turbulence, shear-driven Kelvin-Helmholtz instabilities, interchange reconnection, and geometrical effects related to the Parker spiral. This Letter presents observations from the Metis coronagraph onboard Solar Orbiter of a single large propagating S-shaped vortex, interpreted as first evidence of a switchback in the solar corona. It originated above an active region with the related loop system bounded by open-field regions to the East and West. Observations, modeling, and theory provide strong arguments in favor of the interchange reconnection origin of switchbacks. Metis measurements suggest that the initiation of the switchback may also be an indicator of the origin of slow solar wind.

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arXiv2022

A highly dynamic small-scale jet in a polar coronal hole

We present an observational study of the plasma dynamics at the base of a solar coronal jet, using high resolution extreme ultraviolet imaging data taken by the Extreme Ultraviolet Imager on board Solar Orbiter, and by the Atmospheric Imaging Assembly on board Solar Dynamics Observatory. We observed multiple plasma ejection events over a period of $\sim$1 hour from a dome-like base that is ca.~4 Mm wide and is embedded in a polar coronal hole. Within the dome below the jet spire, multiple plasma blobs with sizes around 1--2 Mm propagate upwards to dome apex with speeds of the order of the sound speed (ca.~120~km~s$^{-1}$ ). Upon reaching the apex, some of these blobs initiate flows with similar speeds towards the other footpoint of the dome. At the same time, high speed super-sonic outflows ($\sim$230~km~s$^{-1}$) are detected along the jet spire. These outflows as well as the intensity near the dome apex appear to be repetitive. Furthermore, during its evolution, the jet undergoes many complex morphological changes including transitions between the standard and blowout type eruption. These new observational results highlight the underlying complexity of the reconnection process that powers these jets and also provide insights into the plasma response when subjected to rapid energy injection.

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arXiv2021

Propagating brightenings in small loop-like structures in the quiet Sun corona: Observations from Solar Orbiter/EUI

Brightenings observed in the solar extreme-ultraviolet (EUV) images are generally interpreted as signatures of micro- or nanoflares occurring at the transition region or coronal temperatures. Recent observations with the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have revealed the smallest of such brightenings (termed campfires) in the quiet-Sun corona. Analyzing EUI 174 Å data at a resolution of about 400 km on the Sun with a cadence of 5 s from 30-May-2020, we report here a number of cases where these campfires exhibit propagating signatures along their apparent small (3-5 Mm) loop-like structures. Measured propagation speeds are generally between 25 km s$^{-1}$ and 60 km s$^{-1}$. These apparent motions would be slower than the local sound speed if the loop plasma is assumed to be at a million Kelvin. Furthermore, these brightenings exhibit non-trivial propagation characteristics such as bifurcation, merging, reflection and repeated plasma ejections. We suggest that these features are manifestations of the internal dynamics of these small-scale magnetic structures and could provide important insights into the dynamic response ($\sim$40 s) of the loop plasma to the heating events as well as into the locations of the heating events themselves.

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arXiv2021

Exploring the Solar Wind from its Source on the Corona into the Inner Heliosphere during the First Solar Orbiter - Parker Solar Probe Quadrature

This Letter addresses the first Solar Orbiter (SO) -- Parker Solar Probe (PSP) quadrature, occurring on January 18, 2021, to investigate the evolution of solar wind from the extended corona to the inner heliosphere. Assuming ballistic propagation, the same plasma volume observed remotely in corona at altitudes between 3.5 and 6.3 solar radii above the solar limb with the Metis coronagraph on SO can be tracked to PSP, orbiting at 0.1 au, thus allowing the local properties of the solar wind to be linked to the coronal source region from where it originated. Thanks to the close approach of PSP to the Sun and the simultaneous Metis observation of the solar corona, the flow-aligned magnetic field and the bulk kinetic energy flux density can be empirically inferred along the coronal current sheet with an unprecedented accuracy, allowing in particular estimation of the Alfvén radius at 8.7 solar radii during the time of this event. This is thus the very first study of the same solar wind plasma as it expands from the sub-Alfvénic solar corona to just above the Alfvén surface.

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arXiv2019

Metis: the Solar Orbiter visible light and ultraviolet coronal imager

Metis is the first solar coronagraph designed for a space mission capable of performing simultaneous imaging of the off-limb solar corona in both visible and UV light. The observations obtained with Metis aboard the Solar Orbiter ESA-NASA observatory will enable us to diagnose, with unprecedented temporal coverage and spatial resolution, the structures and dynamics of the full corona from 1.7 $R_\odot$ to about 9 $R_\odot$. Due to the uniqueness of the Solar Orbiter mission profile, Metis will be able to observe the solar corona from a close vantage point (down to 0.28 AU), achieving out-of-ecliptic views with the increase of the orbit inclination over time. Moreover, observations near perihelion, during the phase of lower rotational velocity of the solar surface relative to the spacecraft, will allow longer-term studies of the coronal features. Thanks to a novel occultation design and a combination of a UV interference coating of the mirrors and a spectral bandpass filter, Metis images the solar corona simultaneously in the visible light band, between 580 and 640 nm, and in the UV H I Lyman-α line at 121.6 nm. The coronal images in both the UV Lyman-α and polarised visible light are obtained at high spatial resolution with a spatial scale down to about 2000 km and 15000 km at perihelion, in the cases of the visible and UV light, respectively. A temporal resolution down to 1 second can be achieved when observing coronal fluctuations in visible light. The Metis measurements will allow for complete characterisation of the main physical parameters and dynamics of the electron and neutral hydrogen/proton plasma components of the corona in the region where the solar wind undergoes acceleration and where the onset and initial propagation of coronal mass ejections take place, thus significantly improving our understanding of the region connecting the Sun to the heliosphere.

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arXiv2013

The SUMER Data in the SOHO Archive

We have released an archive of all observational data of the VUV spectrometer Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on SOHO that has been acquired until now. The operational phase started with 'first light' observations on 27 January 1996 and will end in 2014. Future data will be added to the archive when they become available. The archive consists of a set of raw data (Level 0) and a set of data that are processed and calibrated to the best knowledge we have today (Level 1). This communication describes step by step the data acquisition and processing that has been applied in an automated manner to build the archive. It summarizes the expertise and insights into the scientific use of SUMER spectra that has accumulated over the years. It also indicates possibilities for further enhancement of the data quality. With this article we intend to convey our own understanding of the instrument performance to the scientific community and to introduce the new, standard-FITS-format database.

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arXiv2011

LEMUR: Large European Module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission

Understanding the solar outer atmosphere requires concerted, simultaneous solar observations from the visible to the vacuum ultraviolet (VUV) and soft X-rays, at high spatial resolution (between 0.1" and 0.3"), at high temporal resolution (on the order of 10 s, i.e., the time scale of chromospheric dynamics), with a wide temperature coverage (0.01 MK to 20 MK, from the chromosphere to the flaring corona), and the capability of measuring magnetic fields through spectropolarimetry at visible and near-infrared wavelengths. Simultaneous spectroscopic measurements sampling the entire temperature range are particularly important. These requirements are fulfilled by the Japanese Solar-C mission (Plan B), composed of a spacecraft in a geosynchronous orbit with a payload providing a significant improvement of imaging and spectropolarimetric capabilities in the UV, visible, and near-infrared with respect to what is available today and foreseen in the near future. The Large European Module for solar Ultraviolet Research (LEMUR), described in this paper, is a large VUV telescope feeding a scientific payload of high-resolution imaging spectrographs and cameras. LEMUR consists of two major components: a VUV solar telescope with a 30 cm diameter mirror and a focal length of 3.6 m, and a focal-plane package composed of VUV spectrometers covering six carefully chosen wavelength ranges between 17 and 127 nm. The LEMUR slit covers 280" on the Sun with 0.14" per pixel sampling. In addition, LEMUR is capable of measuring mass flows velocities (line shifts) down to 2 km/s or better. LEMUR has been proposed to ESA as the European contribution to the Solar C mission.

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