arXiv subjects

Nils Janitzek

Publications and source records attributed to Nils Janitzek.

5 recordsLinked to original sources

arXiv2026

Solar Orbiter observations of solar energetic electron events associated with hard microflares

Generally, large solar flares accelerate electrons to high energies more efficiently than microflares. However, some microflares, known as hard microflares (HMFs), also produce high-energy electrons, as indicated by their flat hard X-ray (HXR) spectra. These events are typically associated with footpoints located in or at the edge of sunspots. The mechanisms behind this efficient acceleration, and their connection to solar energetic electrons (SEEs), remain unclear. We compare, for the first time, HXR spectra of HMFs with in-situ electron spectra of associated SEEs using Solar Orbiter STIX and EPD observations. This provides insight into acceleration processes and the transport of high-energy electrons into interplanetary space. We identify eight HMFs observed jointly by Solar Orbiter and Earth-based instruments that are associated with SEEs, confirmed through timing and magnetic connectivity analysis. Each event is studied using HXR spectroscopy, SEE velocity-dispersion analysis, and in-situ electron spectral analysis. Seven of eight events show consistent timing between flare HXR emission and inferred electron injection, as well as good agreement with magnetic connectivity estimates. The known correlation between HXR photon and in-situ electron spectral indices extends to HMFs, which occupy the hard end of the distribution, even compared to larger flares. We conclude that HMFs produce prompt SEEs with hard spectra, demonstrating efficient electron acceleration without requiring large flare energy release. Their magnetic configuration, involving open field lines from the sunspot, suggests they may be an important contributor to filling the heliosphere with energetic particles.

astro-ph.SR↗

arXiv2026

The Non-Eruptive Reconfiguration of a Quiescent Filament After a Nearby Active Region Emergence

The unpredictability of solar filament eruptions presents major challenges for forecasting space weather, as such eruptions frequently drive coronal mass ejections (CMEs) that impact the heliosphere. While nearby flux emergence is often linked to their destabilisation, the specific characteristics of both the emerging flux and the filament that determine whether an eruption occurs remain unclear. We report observations of a quiescent filament that did not erupt following the nearby emergence of active region NOAA 13270 and a subsequent C-class flare in April 2023. Our analysis combines multi-viewpoint extreme ultraviolet (EUV) imaging and X-ray imaging with EUV spectroscopy, radio imaging and measurements of, and extrapolations from, the photospheric magnetic field. We identify the formation of a coronal null point and fan-spine topology at the interface between the active region and filament which exhibited persistent slow reconnection, indicated by chromospheric brightenings, persistent radio emission, and plasma upflows. Our results indicate that ongoing reconnection and jets can relieve magnetic stress and enable filament stability, even when under strong perturbation. We suggest that the orientation of emerging flux relative to the ambient field is a critical parameter in filament evolution, and provide observational constraints for models of filament stability and eruption.

astro-ph.SR↗

arXiv2025

Active region upflows in various coronal structures and their coupling to the lower atmosphere

Plasma upflows with a Doppler shift exceeding -10 km/s at active region (AR) boundaries are considered potential sources of the nascent slow solar wind. We investigate the driving mechanisms of a pair of coronal upflow regions on the western and eastern peripheries of an AR, which have different magnetic topologies and surroundings. It is aimed to explore how these upflows couple to the lower atmosphere. Using observations of the Fe XII 19.51 nm line from Hinode, we identified two upflow regions at the western and eastern boundaries of a decaying AR. Context images for the two regions were obtained by the High Resolution Imager (HRI) telescope of the Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter mission. Other instruments on Solar Orbiter and other observatories provide diagnostics to the lower atmosphere. Potential Field Source Surface (PFSS) extrapolations were used to examine the magnetic field configuration associated with the AR upflows. The eastern upflow region, located over the AR moss, displays small-scale dynamic fibril structures, whereas the western region hosts fan-like loops. We found blueshifted Ne VIII emission at the eastern site, in contrast to redshifted Ne VIII profiles in the west. Magnetic field extrapolations reveal a pseudostreamer topology connecting both these regions. Moreover, low transition-region lines show systematically reduced redshift below the eastern footpoint. The observations support the scenario in which both upflows are driven by pressure imbalances created by coronal reconnection, leading to a continuous upflow above approximately 0.6 MK (i.e., Ne VIII line formation temperature). Meanwhile, mass flows in the lower transition region beneath the eastern upflow region appear to respond passively to the pressure-driven coronal upflows.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2014

A Space Weather Mission Concept: Observatories of the Solar Corona and Active Regions (OSCAR)

Coronal Mass Ejections (CMEs) and Corotating Interaction Regions (CIRs) are major sources of magnetic storms on Earth and are therefore considered to be the most dangerous space weather events. The Observatories of Solar Corona and Active Regions (OSCAR) mission is designed to identify the 3D structure of coronal loops and to study the trigger mechanisms of CMEs in solar Active Regions (ARs) as well as their evolution and propagation processes in the inner heliosphere. It also aims to provide monitoring and forecasting of geo-effective CMEs and CIRs. OSCAR would contribute to significant advancements in the field of solar physics, improvements of the current CME prediction models, and provide data for reliable space weather forecasting. These objectives are achieved by utilising two spacecraft with identical instrumentation, located at a heliocentric orbital distance of 1~AU from the Sun. The spacecraft will be separated by an angle of 68$^{\circ}$ to provide optimum stereoscopic view of the solar corona. We study the feasibility of such a mission and propose a preliminary design for OSCAR.

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