18 research outputs found
A comparative study of resistivity models for simulations of magnetic reconnection in the solar atmosphere
Magnetic reconnection is a fundamental mechanism in astrophysics. A common
challenge in mimicking this process numerically in particular for the Sun is
that the solar electrical resistivity is small compared to the diffusive
effects caused by the discrete nature of codes. We aim to study different
anomalous resistivity models and their respective effects on simulations
related to magnetic reconnection in the Sun. We used the Bifrost code to
perform a 2D numerical reconnection experiment in the corona that is driven by
converging opposite polarities at the solar surface. This experiment was run
with three different commonly used resistivity models: 1) the hyper-diffusion
model originally implemented in Bifrost, 2) a resistivity proportional to the
current density, and 3) a resistivity proportional to the square of the
electron drift velocity. The study was complemented with a 1D experiment of a
Harris current sheet with the same resistivity models. The 2D experiment shows
that the three resistivity models are capable of producing results in
satisfactory agreement with each other in terms of the current sheet length,
inflow velocity, and Poynting influx. Even though Petschek-like reconnection
occurred with the current density-proportional resistivity while the other two
cases mainly followed plasmoid-mediated reconnection, the large-scale evolution
of thermodynamical quantities such as temperature and density are quite similar
between the three cases. For the 1D experiment, some recalibration of the
diffusion parameters is needed to obtain comparable results. Specifically the
hyper-diffusion and the drift velocity-dependent resistivity model needed only
minor adjustments, while the current density-proportional model needed a
rescaling of several orders of magnitude.Comment: 15 pages, 8 figures, 1 movi
Two-dimensional Radiative Magnetohydrodynamic Simulations of Partial Ionization in the Chromosphere. II. Dynamics and Energetics of the Low Solar Atmosphere
We investigate the effects of interactions between ions and neutrals on the
chromosphere and overlying corona using 2.5D radiative MHD simulations with the
Bifrost code. We have extended the code capabilities implementing ion-neutral
interaction effects using the Generalized Ohm's Law, i.e., we include the Hall
term and the ambipolar diffusion (Pedersen dissipation) in the induction
equation. Our models span from the upper convection zone to the corona, with
the photosphere, chromosphere and transition region partially ionized. Our
simulations reveal that the interactions between ionized particles and neutral
particles have important consequences for the magneto-thermodynamics of these
modeled layers: 1) ambipolar diffusion increases the temperature in the
chromosphere; 2) sporadically the horizontal magnetic field in the photosphere
is diffused into the chromosphere due to the large ambipolar diffusion; 3)
ambipolar diffusion concentrates electrical currents leading to more violent
jets and reconnection processes, resulting in 3a) the formation of longer and
faster spicules, 3b) heating of plasma during the spicule evolution, and 3c)
decoupling of the plasma and magnetic field in spicules. Our results indicate
that ambipolar diffusion is a critical ingredient for understanding the
magneto-thermo-dynamic properties in the chromosphere and transition region.
The numerical simulations have been made publicly available, similar to
previous Bifrost simulations. This will allow the community to study realistic
numerical simulations with a wider range of magnetic field configurations and
physics modules than previously possible.Comment: 13 figures. Accepted to be published in Ap
The chromosphere underneath a Coronal Bright Point
Coronal Bright Points (CBPs) are sets of small-scale coronal loops,
connecting opposite magnetic polarities, primarily characterized by their
enhanced extreme-ultraviolet (EUV) and X-ray emission. Being ubiquitous, they
are thought to play an important role in heating the solar corona. We aim at
characterizing the barely-explored chromosphere underneath CBPs, focusing on
the related spicular activity and on the effects of small-scale magnetic flux
emergence on CBPs. We used high-resolution observations of a CBP in H
and Fe I 617.3 nm from the Swedish 1-m Solar Telescope (SST) in coordination
with the Solar Dynamics Observatory (SDO). This work presents the first
high-resolution observation of spicules imaged in H. The spicules were
automatically detected using advanced image processing techniques, which were
applied to the Dopplergrams derived from H. Here we report their
abundant occurrence close to the CBP ``footpoints", and find that the
orientation of such spicules is aligned along the EUV loops, indicating that
they constitute a fundamental part of the whole CBP magnetic structure.
Spatio-temporal analysis across multiple channels indicates that there are
coronal propagating disturbances associated with the studied spicules,
producing transient EUV intensity variations of the individual CBP loops. Two
small-scale flux emergence episodes appearing below the CBP were analyzed; one
of them leading to quiet-sun Ellerman bombs and enhancing the nearby spicular
activity. This paper presents unique evidence of the tight coupling between the
lower and upper atmosphere of a CBP, thus helping to unravel the dynamic
phenomena underneath CBPs and their impact on the latter.Comment: Accepted for publication in the Astrophysical Journal. 16 pages, 11
figures. Animations embedde
Probing the physics of the solar atmosphere with the Multi-slit Solar Explorer (MUSE). I. Coronal heating
Funding: I.D.M. has received support from the UK Science and Technology Facilities Council (Consolidated grant ST/K000950/1), the European Union Horizon 2020 research and innovation program (grant agreement No. 647214), and the Research Council of Norway through its Centres of Excellence scheme, project number 262622.The Multi-slit Solar Explorer (MUSE) is a proposed mission composed of a multislit extreme ultraviolet (EUV) spectrograph (in three spectral bands around 171 Å, 284 Å, and 108 Å) and an EUV context imager (in two passbands around 195 Å and 304 Å). MUSE will provide unprecedented spectral and imaging diagnostics of the solar corona at high spatial (≤0.″5) and temporal resolution (down to ∼0.5 s for sit-and-stare observations), thanks to its innovative multislit design. By obtaining spectra in four bright EUV lines (Fe ix 171 Å, Fe xv 284 Å, Fe xix–Fe xxi 108 Å) covering a wide range of transition regions and coronal temperatures along 37 slits simultaneously, MUSE will, for the first time, “freeze” (at a cadence as short as 10 s) with a spectroscopic raster the evolution of the dynamic coronal plasma over a wide range of scales: from the spatial scales on which energy is released (≤0.″5) to the large-scale (∼170″ × 170″) atmospheric response. We use numerical modeling to showcase how MUSE will constrain the properties of the solar atmosphere on spatiotemporal scales (≤0.″5, ≤20 s) and the large field of view on which state-of-the-art models of the physical processes that drive coronal heating, flares, and coronal mass ejections (CMEs) make distinguishing and testable predictions. We describe the synergy between MUSE, the single-slit, high-resolution Solar-C EUVST spectrograph, and ground-based observatories (DKIST and others), and the critical role MUSE plays because of the multiscale nature of the physical processes involved. In this first paper, we focus on coronal heating mechanisms. An accompanying paper focuses on flares and CMEs.Publisher PDFPeer reviewe
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 (subarcseconds) and temporal (less than a few tens of seconds) scales 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 the Interface Region Imaging Spectrograph 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), subarcsecond-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 Extreme Ultraviolet Solar Telescope, 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., which focuses on investigating coronal heating with MUSE
A 2D Model for Coronal Bright Points: Association with Spicules, UV Bursts, Surges, and EUV Coronal Jets
Coronal bright points (CBPs) are ubiquitous structures in the solar atmosphere composed of hot small-scale loops observed in extreme-ultraviolet (EUV) or X-rays in the quiet Sun and coronal holes. They are key elements to understanding the heating of the corona; nonetheless, basic questions regarding their heating mechanisms, the chromosphere underneath, or the effects of flux emergence in these structures remain open. We have used the Bifrost code to carry out a 2D experiment in which a coronal-hole magnetic null-point configuration evolves perturbed by realistic granulation. To compare with observations, synthetic SDO/AIA, Solar Orbiter EUI-HRI, and IRIS images have been computed. The experiment shows the self-consistent creation of a CBP through the action of stochastic granular motions alone, mediated by magnetic reconnection in the corona. The reconnection is intermittent and oscillatory, and it leads to coronal and transition-region temperature loops that are identifiable in our EUV/UV observables. During the CBP lifetime, convergence and cancellation at the surface of its underlying opposite polarities takes place. The chromosphere below the CBP shows a number of peculiar features concerning its density and the spicules in it. The final stage of the CBP is eruptive: Magnetic flux emergence at the granular scale disrupts the CBP topology, leading to different ejections, such as UV bursts, surges, and EUV coronal jets. Apart from explaining observed CBP features, our results pave the way for further studies combining simulations and coordinated observations in different atmospheric layers
: Danse et astronomie
Compte-rendu de collaboration Art-ScienceEn résidence à l'Observatoire de Lille en 2018, Julie Magri invite à découvrir sa recherche chorégraphique. Danseuse de flamenco, elle réinterprète l’ordre du chaos à partir des études et modélisations du nuage de Oort développées par le laboratoire. ChaoOmète est un court-métrage de 10 minutes suivi d’un spectacle de danse d’environ 45 minutes
Judaism and the Love of Reason
Context. Magnetic flux emergence from the solar interior has been shown to be a key mechanism for unleashing a wide variety of phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized.
Aims. We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as ambipolar diffusion, on the dynamics and thermodynamics of the flux emergence process.
Methods. Using the radiation-magnetohydrodynamic Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H2 molecules, and the ambipolar diffusion term of the generalized Ohm’s law.
Results. Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the ambipolar diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H2 molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H2 molecule formation to the thermal energy during the flux emergence process. We find that the ambipolar diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of ambipolar diffusion in the atmosphere
Case study of multi-temperature coronal jets for emerging flux MHD models
Context. Hot coronal jets are a basic observed feature of the solar atmosphere whose physical origin is still actively debated.
Aims. We study six recurrent jets that occurred in active region NOAA 12644 on April 4, 2017. They are observed in all the hot filters of AIA as well as cool surges in IRIS slit–jaw high spatial and temporal resolution images.
Methods. The AIA filters allow us to study the temperature and the emission measure of the jets using the filter ratio method. We studied the pre-jet phases by analysing the intensity oscillations at the base of the jets with the wavelet technique.
Results. A fine co-alignment of the AIA and IRIS data shows that the jets are initiated at the top of a canopy-like double-chambered structure with cool emission on one and hot emission on the other side. The hot jets are collimated in the hot temperature filters, have high velocities (around 250 km s−1) and are accompanied by cool surges and ejected kernels that both move at about 45 km s−1. In the pre-phase of the jets, we find quasi-periodic intensity oscillations at their base that are in phase with small ejections; they have a period of between 2 and 6 min, and are reminiscent of acoustic or magnetohydrodynamic waves.
Conclusions. This series of jets and surges provides a good case study for testing the 2D and 3D magnetohydrodynamic emerging flux models. The double-chambered structure that is found in the observations corresponds to the regions with cold and hot loops that are in the models below the current sheet that contains the reconnection site. The cool surge with kernels is comparable with the cool ejection and plasmoids that naturally appears in the models