arXiv subjects

Daniel Müller

Publications and source records attributed to Daniel Müller.

At least 19 recordsLinked to original sources

arXiv2026

Active nests at solar maximum: How nested flux emergence dominated flaring activity and structured the heliosphere

Multi-viewpoint observations from near-Earth satellites and ESA's Solar Orbiter enable a global view of the Sun. This continuous coverage is essential for characterising active nests. We investigated how the formation of active nests during the maximum of solar cycle 25 affected the Sun's flaring activity and large-scale magnetic field. Active nests were identified from their persistent activity in Carrington coordinates. For each nest, we monitored the evolution of their photospheric magnetic field, extreme ultraviolet emission, and associated solar flares. Their impact on the solar corona was explored through potential field source surface extrapolations and coronagraph observations. These results were then compared with in-situ measurements of solar wind speed and magnetic field polarity. During 156 days of global monitoring in 2024, nearly 80% of all solar flares originated from three active nests in the southern hemisphere. Two of the nests converged over several months, colliding with a burst of activity in October 2024. The third nest was linked to the emergence of NOAA active region AR13664 in May 2024. Magnetically complex NOAA active regions were more prevalent within active nests. During 2024, 28% of the 147 nested active regions were complex, compared to just 16% of the 181 regions elsewhere. In January 2025, two active nests emerged from the collision site, and a new active nest appeared in the north. This reinforced a tilted dipole topology throughout 2025. Active nests modulated global solar activity in 2024 and shaped the solar corona and wind in 2025. Nearly continuous tracking was used to isolate their flaring and flux emergence rates. Active nest migration was driven by the emergence of magnetic flux, implying a coherent sub-surface origin. By anchoring the coronal magnetic field, active nests created more favourable source regions for solar wind connectivity studies.

astro-ph.SR↗

arXiv2026

Transient Stability of Offshore Energy Hubs

Offshore energy hubs (OEHs) use grid-forming modular multilevel converters (MMCs) to enable large-scale offshore wind integration and multi-terminal HVDC operation. In HVDC-connected offshore wind farms and OEHs, the offshore grid-forming HVDC converters absorb active power from an offshore AC grid supplied by the wind farms and convert it to DC power for transmission to the onshore grid. Converter current limiting under different fault types in this setting is an understudied topic in the literature, which mostly focuses on power-injecting converters. This paper proposes a unified current-limiting strategy that combines a variable virtual impedance (VVI), based on a smooth threshold function, with a novel virtual-power (VP) mechanism derived from the power dissipated in the virtual resistance. The VVI ensures current limitation during fault-induced overcurrents while preserving voltage-source behavior, whereas the VP mechanism adds a compensating power term into the synchronization loop, enabling automatic power redistribution among converters. P-delta analysis further shows that a more resistive VVI can improve the transient stability of power-absorbing converters, while the proposed VP mechanism further enlarges the stability margin. EMT simulations validate that the combined VVI-VP strategy limits fault currents, maintains synchronism during severe faults, and achieves coordinated post-fault power sharing in fully converter-based OEHs.

eess.SY↗

arXiv2026

A note on splitting solutions in $4+1$ dimensional quadratic gravity

In the present paper we consider anisotropic cosmological vacuum solutions in (4+1) dimensional general quadratic gravity. In particular, we present a solution with 3 equal and 1 different Hubble parameters, and study its stability. We show that for a certain range of coupling constants this solution is stable. This means that initially totally anisotropic 4-dim Universe can evolve naturally to a product of 3-dim isotropic subspace and 1-dim space. By numerical integration of equations of motion we construct bassin of attraction of this solution which covers part of the initial conditions space with non-zero measure.

gr-qc↗

arXiv2026

Robust Wada Boundaries and Entropy Scaling in pp-Wave Spacetimes

We study the dynamics of the geodesics of pp-wave spacetimes with polynomial profiles, which are dynamically equivalent to the motion of a classical particle in a two-dimensional harmonic polynomial potential. We demonstrate that the Wada property of the escape basins is robust under variation of the polynomial degree, i.e., the basin boundaries remain maximally intermingled as the number of escape channels increases. We further provide a quantitative characterization of the degree of dynamical uncertainty by computing the basin entropy $S_{b}$ and the boundary basin entropy $S_{bb}$. We find that these measures increase monotonically with the polynomial degree, indicating enhanced unpredictability of the final state of the system. We also show that $S_{bb}$ is greater than $\ln(2)$ for $n>3$, and this confirms that the basin boundaries are fractal.

gr-qc↗

arXiv2025

First coordinated observations between Solar Orbiter and the Daniel K. Inouye Solar Telescope

Solar Orbiter and the Daniel K. Inouye Solar Telescope (DKIST) are two of the newest facilities available to the solar physics community. The first coordinated observations of the Sun by these two facilities occurred over the course of one week in October 2022. The returned data are open-access and will provide a valuable resource to researchers in the field. We provide an overview of the datasets collected by Solar Orbiter and DKIST through this coordination and discuss their scientific potential. Our aim is to demonstrate how these unique high-resolution coordinated observations, as well as similar observations obtained through subsequent campaigns, can help tackle important science questions in the field. Between 18 and 24 October 2022, Solar Orbiter and DKIST observed a decayed active region simultaneously. During this period, Solar Orbiter's separation angle with Earth decreased from 77{\deg} to 51{\deg}, enabling stereoscopic observations with DKIST. From Solar Orbiter, observations are provided by the Extreme Ultraviolet Imager (EUI), Polarimetric and Helioseismic Imager (PHI), and the Spectral Imaging of the Coronal Environment (SPICE) instruments. Meanwhile, DKIST observed using the Cryogenic Near Infrared Spectropolarimeter (CryoNIRSP), the Visible Broadband Imager (VBI), and the Visible Spectropolarimeter (ViSP). The first coordinated Solar Orbiter and DKIST campaign was a success, collecting observations multiple times over the week. We focus on three specific topics as representative examples, namely, coronal loop physics, the formation and evolution of the small-scale active region brightenings, and coronal rain dynamics. These open-access observations, and others like them, should help the solar physics community tackle key questions in the field. Such stereoscopic coordinated observations open up a new era in the analysis of the solar atmosphere.

astro-ph.SR↗

arXiv2025

28 Years of Sun-as-a-star Extreme Ultraviolet Light Curves from SOHO EIT

The Solar and Heliospheric Observatory (SOHO) Extreme-ultraviolet Imaging Telescope (EIT) has been taking images of the Solar disk and corona in four narrow EUV bandpasses (171\AA, 195\AA, 284\AA, and 304\AA) at a minimum cadence of once per day since early 1996. The time series of fully-calibrated EIT images now spans approximately 28 years, from early 1996 to early 2024, covering solar cycles 23, 24, and the beginning of cycle 25. We convert this extensive EIT image archive into a time series of `Sun-as-a-star' light curves in EIT's four bandpasses, providing a long-term record of solar EUV variability. These Sun-as-a-star light curves, available for download from https://doi.org/10.5281/zenodo.15474179, trace the Sun as if it were a distant point source, viewed from a fixed perspective. We find that our EUV light curves trace the $\sim$ 11-year solar activity cycle and the $\sim$ 27-day rotation period much better than comparable optical observations. In particular, we can accurately recover the solar rotation period from our 284\AA light curve for 26 out of 28 calendar years of EIT observations (93% of the time), compared to only 3 out of 29 calendar years (10% of the time) of the VIRGO total solar irradiance time series, which is dominated by optical light. Our EIT light curves, in conjunction with Sun-as-a-star light curves at optical wavelengths, will be valuable to those interested in inferring the EUV/UV character of stars with long optical light curves but no intensive UV observations, as well as to those interested in long-term records of solar and space weather.

astro-ph.SR↗

arXiv2025

Solar Orbiter's 2024 Major Flare Campaigns: An Overview

Solar Orbiter conducted a series of flare-optimised observing campaigns in 2024 utilising the Major Flare Solar Orbiter Observing Plan (SOOP). Dedicated observations were performed during two distinct perihelia intervals in March/April and October, during which over 22 flares were observed, ranging from B- to M-class. These campaigns leveraged high-resolution and high-cadence observations from the mission's remote-sensing suite, including the High-Resolution EUV Imager (EUI/HRI_EUV), the Spectrometer/Telescope for Imaging X-rays (STIX), the Spectral Imaging of the Coronal Environment (SPICE) spectrometer, and the High Resolution Telescope of the Polarimetric and Helioseismic Imager (PHI/HRT), as well as coordinated ground-based and Earth-orbiting observations. EUI/HRI_EUV operating in short-exposure modes, provided two-second-cadence, non-saturated EUV images, revealing structures and dynamics on scales not previously observed. Simultaneously, STIX captured hard X-ray imaging and spectroscopy of accelerated electrons, while SPICE acquired EUV slit spectroscopy to probe chromospheric and coronal responses. Together, these observations offer an unprecedented view of magnetic reconnection, energy release, particle acceleration, and plasma heating across a broad range of temperatures and spatial scales. These campaigns have generated a rich dataset that will be the subject of numerous future studies addressing Solar Orbiter's top-level science goal: "How do solar eruptions produce energetic particle radiation that fills the heliosphere?". This paper presents the scientific motivations, operational planning, and observational strategies behind the 2024 flare campaigns, along with initial insights into the observed flares. We also discuss lessons learned for optimizing future Solar Orbiter Major Flare campaigns and provide a resource for researchers aiming to utilize these unique observations.

astro-ph.SR↗

arXiv2025

Tilt in quadratic gravity II

We investigate a tilted fluid component on a Bianchi V geometry in the theories of General Relativity (GR) and Quadratic Gravity (QG). The main objective of this work is the study of how the properties of matter can modify the future evolution of the attractors and their consequences on the regions of initial conditions of the solutions. As is well known, QG contains the Ruzmaikina-Ruzmaikin (RR) solution. This solution describes the slow-roll regime of Starobinsky's inflationary model, which is currently the best one due to the excellent agreement with Cosmic Microwave Background Radiation (CMBR) data. In QG, we found universes that can be attracted to the RR solution or recollapse toward the isotropic singularity attractor. If the Equation of State (EoS) parameter is ultra-radiative w>1/3, the tilt variable increases both in RR and Milne for QG or GR, respectively. In both cases, the fluid expansion and acceleration diverge, while the vorticity initially increases and then decreases to zero.

gr-qc↗

arXiv2024

Tilt in quadratic gravity

In this work, tilted source solutions in both Einstein-Hilbert General Relativity (GR) and Quadratic Gravity (QG) for the anisotropic Bianchi V model are addressed. Since the excellent CMBR match of Starobinsky's inflation with Planck's team measurements data, QG has acquired a prominent status in the effective sense, for sufficiently strong gravity fields. The main interest is in the numeric time evolution to the past towards the singularity and the behavior of the kinematic variables, vorticity, acceleration, and the expansion of this source substance. In QG we found that for universes with higher and smaller matter densities fall into the Kasner or isotropic singularity attractors to the past, respectively. We also found that the Kasner singularity attractor to the past has always zero vorticity, for both GR and QG theories. While for QG the isotropic singularity attractor may have divergent vorticity. For the set of assumptions and conditions supposed in this work, the isotropic singularity attractor, favors QG as compared to GR. Only in QG we were able to find a geometric singularity with divergences in all of the kinematic variables of the substance, decreasing to finite values to the future, upon time reversing. That is, we obtained an initial kinematic singularity substance, that approaches a perfect fluid source.

gr-qc↗

arXiv2023

Initial conditions for Starobinsky Inflation with a positive spatial curvature

We have found numerically initial conditions in the $(R, H)$ plane leading to a successful Starobinsky inflation in $R+R^2$ gravity for a isotropic metrics with positive spatial curvature. Trajectories can reach inflation regime either directly or going through a bounce, and even recollapse followed by a bounce. Our numerical plots indicate that ``good" initial conditions exist even for big initial spatial curvature, however, we argue that such a trajectory must cross a region of rather big $R$ or $H$. This means that the range of viability of $R+R^2$ theory in the $(R,H)$ plane directly affect the question of viability of Starobinsky inflation for a positive spatial curvature isotropic Universe.

gr-qc↗

arXiv2022

Solar Orbiter and SDO Observations, and Bifrost MHD Simulations of Small-scale Coronal Jets

We report high-resolution, high-cadence observations of five small-scale coronal jets in an on-disk quiet Sun region observed with Solar Orbiter's EUI/\hri\ in 174 Å. We combine the \hri\ images with the EUV images of SDO/AIA and investigate magnetic setting of the jets using co-aligned line-of-sight magnetograms from SDO/HMI. The \hri\ jets are miniature versions of typical coronal jets as they show narrow collimated spires with a base brightening. Three out of five jets result from a detectable minifilament eruption following flux cancelation at the neutral line under the minifilament, analogous to coronal jets. To better understand the physics of jets, we also analyze five small-scale jets from a high-resolution Bifrost MHD simulation in synthetic \FeIX/\FeX\ emissions. The jets in the simulation reside above neutral lines and four out of five jets are triggered by magnetic flux cancelation. The temperature maps show the evidence of cool gas in the same four jets. Our simulation also shows the signatures of opposite Doppler shifts (of the order of $\pm$10s of \kms) in the jet spire, which is evidence of untwisting motion of the magnetic field in the jet spire. The average jet duration, spire length, base width, and speed in our observations (and in synthetic \FeIX/\FeX\ images) are 6.5$\pm$4.0 min (9.0$\pm$4.0 min), 6050$\pm$2900 km (6500$\pm$6500 km), 2200$\pm$850 km, (3900$\pm$2100 km), and 60$\pm$8 \kms\ (42$\pm$20 \kms), respectively. Our observation and simulation results provide a unified picture of small-scale solar coronal jets driven by magnetic reconnection accompanying flux cancelation. This picture also aligns well with the most recent reports of the formation and eruption mechanisms of larger coronal jets.

astro-ph.SR↗

arXiv2022

Plasma composition measurements in an active region from Solar Orbiter/SPICE and Hinode/EIS

A key goal of the Solar Orbiter mission is to connect elemental abundance measurements of the solar wind enveloping the spacecraft with EUV spectroscopic observations of their solar sources, but this is not an easy exercise. Observations from previous missions have revealed a highly complex picture of spatial and temporal variations of elemental abundances in the solar corona. We have used coordinated observations from Hinode and Solar Orbiter to attempt new abundance measurements with the SPICE (Spectral Imaging of the Coronal Environment) instrument, and benchmark them against standard analyses from EIS (EUV Imaging Spectrometer). We use observations of several solar features in AR 12781 taken from an Earth-facing view by EIS on 2020 November 10, and SPICE data obtained one week later on 2020 November 17; when the AR had rotated into the Solar Orbiter field-of-view. We identify a range of spectral lines that are useful for determining the transition region and low coronal temperature structure with SPICE, and demonstrate that SPICE measurements are able to differentiate between photospheric and coronal Mg/Ne abundances. The combination of SPICE and EIS is able to establish the atmospheric composition structure of a fan loop/outflow area at the active region edge. We also discuss the problem of resolving the degree of elemental fractionation with SPICE, which is more challenging without further constraints on the temperature structure, and comment on what that can tell us about the sources of the solar wind and solar energetic particles.

astro-ph.SR↗

arXiv2021

Refractory elements in the gas phase for comet 67P/Churyumov-Gerasimenko

Results. We found that gas-phase silicon was present throughout the Rosetta mission. Furthermore, the presence of sodium and iron atoms near the comet's perihelion confirms that sputtering cannot be the sole release process for refractory elements into the gas phase. Nickel was found to be below the detection limit. The search for parent species of any of the identified gas phase refractories has not been successful. Upper limits for a suite of possible fragment species (SiH, SiC, NaH, etc.) of larger parent and daughter species have been obtained. Furthermore, Si did not exhibit the same drop in signal as do common cometary gases when the spacecraft is pointed away from the nucleus. The combined results suggest that a direct release of elemental species from small grains on the surface of the nucleus or from small grains in the surrounding coma is a more likely explanation than the previous assumption of release via the dissociation of gaseous parent molecules.

astro-ph.EP↗

arXiv2021

Generality of Starobinsky and Higgs inflation in the Jordan frame

We revisit the problem of generality of Starobinsky and Higgs inflation. The known results obtained in the Einstein frame are generalized for the case of an arbitrary initial energy of the scalar field. These results are compared with the results obtained directly in the Jordan frame, which, to our knowledge, has not been thoroughly explored in the literature previously. We demonstrate that the qualitative picture of initial conditions zone in the $(ϕ, \dot ϕ)$ plane, which leads to sufficient amount of inflation, is quite similar for both the frames in the case of Higgs inflation. For Starobinsky inflation, the conformal transformation between the frames relates the geometrical variables in the Jordan frame with the properties of an effective scalar field in the Einstein frame. We show that the transformation $(H, R) \to (ϕ, \dot ϕ)$ is not regular everywhere, leading to some peculiarities in the zone of good initial conditions in the $(H, R)$ plane.

gr-qc↗

arXiv2021

On the order reduction

In this work we present an extension of the technique of the order reduction to higher perturbative approximations in an iterative fashion. The intention is also to analyze more carefully the conditions for the validity of the order reduction technique. With this in mind, a few simple situations in which the iterative order reduction converges analytically to the exact solutions are presented as examples. It is discovered that the order reduction as a perturbative iterative technique does not converge in the weak coupling limit as most of the known perturbative schemes, at least when applied to these examples. Also, considering these specific examples, the convergence of the order reduction occurs in strong coupling regimes. As a more realistic case, the order reduction is applied to Starobinsky's inflationary model is presented. It is verified that the method converges to the inflationary solution in the slow-roll regime.

gr-qc↗

arXiv2020

The divergence and curl in arbitrary basis

In this work, the divergence and curl operators are obtained using the coordinate free non rigid basis formulation of differential geometry. Although the authors have attempted to keep the presentation self-contained as much as possible, some previous exposure to the language of differential geometry may be helpful. In this sense the work is aimed to late undergraduate or beginners graduate students interested in mathematical physics. To illustrate the development, we graphically present the eleven coordinate systems in which the Laplace operator is separable. We detail the development of the basis and the connection for the cylindrical and paraboloidal coordinate systems. We also present in [1] codes both in Maxima and Maple for the spherical orthonormal basis, which serves as a working model for calculations in other situations of interest. Also in [1] the codes to obtain the coordinate surfaces are given.

physics.class-ph↗

arXiv2020

Decay and electromagnetic production of strongly coupled quarkonia in pNRQCD

We improve the pNRQCD approach to annihilation processes of heavy quarkonium and make first pNRQCD predictions for exclusive electromagnetic production of heavy quarkonium. We consider strongly coupled quarkonia, i.e., quarkonia that are not Coulombic bound states. Possible strongly coupled quarkonia include excited charmonium and bottomonium states. For these, pNRQCD provides expressions for the decay and exclusive electromagnetic production NRQCD matrix elements that depend on the wavefunctions at the origin and few universal gluon field correlators. We compute electromagnetic decay widths and exclusive production cross sections, and inclusive decay widths into light hadrons for $P$-wave quarkonia at relative order $v^2$ and leading order, respectively. We also compute the decay widths of $2S$ and $3S$ bottomonium states into lepton pairs and their ratios with the inclusive widths into light hadrons at relative order $v^2$.

hep-ph↗

arXiv2017

Anisotropic cosmological solutions in $R + R^2$ gravity

In this paper we investigate the past evolution of an anisotropic Bianchi I universe in $R+R^2$ gravity. Using the dynamical system approach we show that there exists a new two-parameters set of solutions that includes both an isotropic "false radiation" solution and an anisotropic generalized Kasner solution, which is stable. We derive the analytic behaviour of the shear from a specific property of $f(R)$ gravity and the analytic asymptotic form of the Ricci scalar when approaching the initial singularity. Finally we numerically checked our results.

gr-qc↗