87 research outputs found
Investigating the reliability of coronal emission measure distribution diagnostics using 3D radiative MHD simulations
Determining the temperature distribution of coronal plasmas can provide
stringent constraints on coronal heating. Current observations with the Extreme
ultraviolet Imaging Spectrograph onboard Hinode and the Atmospheric Imaging
Assembly onboard the Solar Dynamics Observatory provide diagnostics of the
emission measure distribution (EMD) of the coronal plasma.
Here we test the reliability of temperature diagnostics using 3D radiative
MHD simulations. We produce synthetic observables from the models, and apply
the Monte Carlo Markov chain EMD diagnostic. By comparing the derived EMDs with
the "true" distributions from the model we assess the limitations of the
diagnostics, as a function of the plasma parameters and of the signal-to-noise
of the data.
We find that EMDs derived from EIS synthetic data reproduce some general
characteristics of the true distributions, but usually show differences from
the true EMDs that are much larger than the estimated uncertainties suggest,
especially when structures with significantly different density overlap along
the line-of-sight. When using AIA synthetic data the derived EMDs reproduce the
true EMDs much less accurately, especially for broad EMDs. The differences
between the two instruments are due to the: (1) smaller number of constraints
provided by AIA data, (2) broad temperature response function of the AIA
channels which provide looser constraints to the temperature distribution.
Our results suggest that EMDs derived from current observatories may often
show significant discrepancies from the true EMDs, rendering their
interpretation fraught with uncertainty. These inherent limitations to the
method should be carefully considered when using these distributions to
constrain coronal heating.Comment: Accepted for publication on The Astrophysical Journal. 25 pages, 29
figures. Paper version with full resolution images and appendixes can be
found at: http://folk.uio.no/bdp/papers/3dEMD_ptesta.pd
A Statistical Study of IRIS Observational Signatures of Nanoflares and Non-thermal Particles
Nanoflares are regarded as one of the major mechanisms of magnetic energy
release and coronal heating in the solar outer atmosphere. We conduct a
statistical study on the response of the chromosphere and transition region to
nanoflares, as observed by the Interface Region Imaging Spectrograph (IRIS), by
using an algorithm for the automatic detection of these events. The initial
atmospheric response to these small heating events is observed, with IRIS, as
transient brightening at the footpoints of coronal loops heated to high
temperatures (>4 MK). For four active regions, observed over 143 hours, we
detected 1082 footpoint brightenings under the IRIS slit, and for those we
extracted physical parameters from the IRIS Mg II and Si IV spectra that are
formed in the chromosphere and transition region, respectively. We investigate
the distribution of the spectral parameters, and the relationship between the
parameters, also comparing them with predictions from RADYN numerical
simulations of nanoflare-heated loops. We find that these events, and the
presence of non-thermal particles, tend to be more frequent in flare productive
active regions, and where the hot Atmospheric Imaging Assembly 94 \AA\ emission
is higher. We find evidence for highly dynamic motions characterized by strong
Si IV non-thermal velocity (not dependent on the heliocentric x coordinate,
i.e., on the angle between the magnetic field and the line-of-sight) and
asymmetric Mg II spectra. These findings provide tight new constraints on the
properties of nanoflares, and non-thermal particles, in active regions, and
their effects on the lower atmosphere.Comment: 18 pages, 8 figures, Accepted to Ap
Forward modeling of emission in SDO/AIA passbands from dynamic 3D simulations
It is typically assumed that emission in the passbands of the Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is
dominated by single or several strong lines from ions that under equilibrium
conditions are formed in a narrow range of temperatures. However, most SDO/AIA
channels also contain contributions from lines of ions that have formation
temperatures that are significantly different from the "dominant" ion(s). We
investigate the importance of these lines by forward modeling the emission in
the SDO/AIA channels with 3D radiative MHD simulations of a model that spans
the upper layer of the convection zone to the low corona. The model is highly
dynamic. In addition, we pump a steadily increasing magnetic flux into the
corona, in order to increase the coronal temperature through the dissipation of
magnetic stresses. As a consequence, the model covers different ranges of
coronal temperatures as time progresses. The model covers coronal temperatures
that are representative of plasma conditions in coronal holes and quiet sun.
The 131, 171, and 304 \AA{} AIA passbands are found to be least influenced by
the so-called "non-dominant" ions, and the emission observed in these channels
comes mostly from plasma at temperatures near the formation temperature of the
dominant ion(s). On the other hand, the other channels are strongly influenced
by the non-dominant ions, and therefore significant emission in these channels
comes from plasma at temperatures that are different from the "canonical"
values. We have also studied the influence of non-dominant ions on the AIA
passbands when different element abundances are assumed (photospheric and
coronal), and when the effects of the electron density on the contribution
function are taken into account.Comment: 48 pages, 14 figures, accepted to be publish in Ap
The Multi-slit Approach to Coronal Spectroscopy with the Multi-slit Solar Explorer (MUSE)
The Multi-slit Solar Explorer (MUSE) is a proposed mission aimed at understanding the physical mechanisms driving the heating of the solar corona and the eruptions that are at the foundation of space weather. MUSE contains two instruments, a multi-slit EUV spectrograph and a context imager. It will simultaneously obtain EUV spectra (along 37 slits) and context images with the highest resolution in space (0.33-0.4 arcsec) and time (1-4 s) ever achieved for the transition region and corona. The MUSE science investigation will exploit major advances in numerical modeling, and observe at the spatial and temporal scales on which competing models make testable and distinguishable predictions, thereby leading to a breakthrough in our understanding of coronal heating and the drivers of space weather. By obtaining spectra in 4 bright EUV lines (Fe IX 171A, Fe XV 284A, Fe XIX-XXI 108A) covering a wide range of transition region and coronal temperatures along 37 slits simultaneously, MUSE will be able to "freeze" the evolution of the dynamic coronal plasma. We describe MUSE's multi-slit approach and show that the optimization of the design minimizes the impact of spectral lines from neighboring slits, generally allowing line parameters to be accurately determined. We also describe a Spectral Disambiguation Code to resolve multi-slit ambiguity in locations where secondary lines are bright. We use simulations of the corona and eruptions to perform validation tests and show that the multi-slit disambiguation approach allows accurate determination of MUSE observables in locations where significant multi-slit contamination occurs
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.Comment: Published in Science on October 17:
http://www.sciencemag.org/content/346/6207/1255724 . 26 pages, 10 figures.
Movies are available at: http://www.lmsal.com/~ptesta/iris_science_mov
Coronal energy release by MHD avalanches II. EUV line emission from a multi-threaded coronal loop
Funding: GC, PP, and FR acknowledge support from ASI/INAF agreement n. 2022-29-HH.0. This work made use of the HPC system MEUSA, part of the Sistema Computazionale per l’Astrofisica Numerica (SCAN) of INAF-Osservatorio Astronomico di Palermo. JR and AWH acknowledge the financial support of Science and Technology Facilities Council through Consolidated Grant ST/W001195/1 to the University of St Andrews. PT was supported by contract 4105785828 (MUSE) to the Smithsonian Astrophysical Observatory, and by NASA grant 80NSSC20K1272x.Context. Magnetohydrodynamic (MHD) instabilities, such as the kink instability, can trigger the chaotic fragmentation of a twisted magnetic flux tube into small-scale current sheets that dissipate as aperiodic impulsive heating events. In turn, the instability could propagate as an avalanche to nearby flux tubes and lead to a nanoflare storm. Our previous work was devoted to related 3D MHD numerical modeling, which included a stratified atmosphere from the solar chromosphere to the corona, tapering magnetic field, and solar gravity for curved loops with the thermal structure modelled by plasma thermal conduction, along with optically thin radiation and anomalous resistivity for 50 Mm flux tubes. Aims. Using 3D MHD modeling, this work addresses predictions for the extreme-ultraviolet (EUV) imaging spectroscopy of such structure and evolution of a loop, with an average temperature of 2–2.5MK in the solar corona. We set a particular focus on the forthcoming MUSE mission, as derived from the 3D MHD modeling. Methods. From the output of the numerical simulations, we synthesized the intensities, Doppler shifts, and non-thermal line broadening in 3 EUV spectral lines in the MUSE passbands: Fe ix 171 Å, Fe xv 284 Å, and Fe xix 108 Å, emitted by ~1MK, ~2MK, and ~10MK plasma, respectively. These data were detectable by MUSE, according to the MUSE expected pixel size, temporal resolution, and temperature response functions. We provide maps showing dierent view angles (front and top) and realistic spectra. Finally, we discuss the relevant evolutionary processes from the perspective of possible observations. Results. We find that the MUSE observations might be able to detect the fine structure determined by tube fragmentation. In particular, the Fe ix line is mostly emitted at the loop footpoints, where we might be able to track the motions that drive the magnetic stressing and detect the upward motion of evaporating plasma from the chromosphere. In Fe xv, we might see the bulk of the loop with increasing intensity, with alternating filamentary Doppler and non-thermal components in the front view, along with more defined spots in the topward view. The Fe xix line is very faint within the chosen simulation parameters; thus, any transient brightening around the loop apex may possibly be emphasized by the folding of sheet-like structures, mainly at the boundary of unstable tubes. Conclusions. In conclusion, we show that coronal loop observations with MUSE can pinpoint some crucial features of MHD-modeled ignition processes, such as the related dynamics, helping to identify the heating processes.Peer reviewe
Is the High-resolution Coronal Imager Resolving Coronal Strands? Results from AR 12712
Following the success of the first mission, the High-Resolution Coronal Imager (Hi-C) was launched for a third time (Hi-C 2.1) on 2018 May 29 from the White Sands Missile Range, NM, USA. On this occasion, 329 s of 17.2 nm data of target active region AR 12712 were captured with a cadence of ≈4 s, and a plate scale of 0farcs129 pixel−1. Using data captured by Hi-C 2.1 and co-aligned observations from SDO/AIA 17.1 nm, we investigate the widths of 49 coronal strands. We search for evidence of substructure within the strands that is not detected by AIA, and further consider whether these strands are fully resolved by Hi-C 2.1. With the aid of multi-scale Gaussian normalization, strands from a region of low emission that can only be visualized against the contrast of the darker, underlying moss are studied. A comparison is made between these low-emission strands and those from regions of higher emission within the target active region. It is found that Hi-C 2.1 can resolve individual strands as small as ≈202 km, though the more typical strand widths seen are ≈513 km. For coronal strands within the region of low emission, the most likely width is significantly narrower than the high-emission strands at ≈388 km. This places the low-emission coronal strands beneath the resolving capabilities of SDO/AIA, highlighting the need for a permanent solar observatory with the resolving power of Hi-C
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
- …