15 research outputs found
The small-scale solar surface dynamo
The existence of a turbulent small-scale solar surface dynamo is likely,
considering existing numerical and laboratory experiments, as well as
comparisons of a small-scale dynamo in MURaM simulations with Hinode
observations. We find the observed peaked probability distribution function
(PDF) from Stokes-V magnetograms is consistent with a monotonic PDF of the
actual vertical field strength. The cancellation function of the vertical flux
density from a Hinode SP observation is found to follow a self-similar power
law over two decades in length scales down to the ~200 km resolution limit.
This provides observational evidence that the scales of magnetic structuring in
the photosphere extend at least down to 20 km. From the power law, we determine
a lower bound for the true quiet-Sun mean vertical unsigned flux density of ~43
G, consistent with our numerically-based estimates that 80% or more of the
vertical unsigned flux should be invisible to Stokes-V observations at a
resolution of 200 km owing to cancellation. Our estimates significantly reduce
the order-of-magnitude discrepancy between Zeeman- and Hanle-based estimates.Comment: Proceedings of the Second Hinode Science Meeting, ASP Series 2009. 8
pages, 4 figure
Vigorous convection in a sunspot granular light bridge
Light bridges are the most prominent manifestation of convection in sunspots.
The brightest representatives are granular light bridges composed of features
that appear to be similar to granules. An in-depth study of the convective
motions, temperature stratification, and magnetic field vector in and around
light bridge granules is presented with the aim of identifying similarities and
differences to typical quiet-Sun granules. Spectropolarimetric data from the
Hinode Solar Optical Telescope were analyzed using a spatially coupled
inversion technique to retrieve the stratified atmospheric parameters of light
bridge and quiet-Sun granules. Central hot upflows surrounded by cooler fast
downflows reaching 10 km/s clearly establish the convective nature of the light
bridge granules. The inner part of these granules in the near surface layers is
field free and is covered by a cusp-like magnetic field configuration. We
observe hints of field reversals at the location of the fast downflows. The
quiet-Sun granules in the vicinity of the sunspot are covered by a low-lying
canopy field extending radially outward from the spot. The similarities between
quiet-Sun and light bridge granules point to the deep anchoring of granular
light bridges in the underlying convection zone. The fast, supersonic downflows
are most likely a result of a combination of invigorated convection in the
light bridge granule due to radiative cooling into the neighboring umbra and
the fact that we sample deeper layers, since the downflows are immediately
adjacent to the slanted walls of the Wilson depression.Comment: 10 pages, 11 figure
On chromospheric heating during flux emergence in the solar atmosphere
Context. The radiative losses in the solar chromosphere vary from
4~kW~m in the quiet Sun, to 20~kW~m in active regions. The
mechanisms that transport non-thermal energy to and deposit it in the
chromosphere are still not understood. Aims. We aim to investigate the
atmospheric structure and heating of the solar chromosphere in an emerging flux
region. Methods. We use observations taken with the CHROMIS and CRISP
instruments on the Swedish 1-m Solar Telescope in the Ca II K, Ca II 854.2 nm,
H, and Fe I 630.1 nm and 630.2 nm lines. We analyse the various line
profiles and in addition perform multi-line, multi-species, non-Local
Thermodynamic Equilibrium (non-LTE) inversions to estimate the spatial and
temporal variation of the chromospheric structure. Results. We investigate
which spectral features of Ca II K contribute to the frequency-integrated Ca II
K brightness, which we use as a tracer of chromospheric radiative losses. The
majority of the radiative losses are not associated with localized high-Ca II
K-brightness events, but instead with a more gentle, spatially extended, and
persistent heating. The frequency-integrated Ca II K brightness correlates
strongly with the total linear polarization in the Ca II 854.2 nm line, while
the Ca II K profile shapes indicate that the bulk of the radiative losses occur
in the lower chromosphere.
Non-LTE inversions indicate a transition from heating concentrated around
photospheric magnetic elements below to a more
space-filling and time-persistent heating above . The
inferred gas temperature at correlates strongly with
the total linear polarization in the Ca II 854.2 nm line, suggesting that that
the heating rate correlates with the strength of the horizontal magnetic field
in the low chromosphere.Comment: Accepted for publication by A&
Chromospheric condensations and magnetic field in a C3.6-class flare studied via He I D3 spectro-polarimetry
Context. Magnetic reconnection during flares takes place in the corona but a
substantial part of flare energy is deposited in the chromosphere. However,
high-resolution spectro-polarimetric chromospheric observations of flares are
very rare. The most used observables are Ca II 8542 {\AA} and He I 10830 {\AA}.
Aims. We aim to study the chromosphere during a C3.6 class flare via
spectro-polarimetric observations of the He I D3 line. Methods. We present the
first SST/CRISP spectro-polarimetric observations of He I D3. We analyze the
data using the inversion code HAZEL, and estimate the LOS velocity and the
magnetic field vector. Results. Strong He I D3 emission at the flare
footpoints, as well as strong He I D3 absorption profiles tracing the flaring
loops are observed during the flare. The He I D3 traveling emission kernels at
the flare footpoints exhibit strong chromospheric condensations of up to
60 km/s at their leading edge. Our observations suggest that such condensations
result in shocking the deep chromosphere, causing broad and modestly
blueshifted He I D3 profiles indicating subsequent upflows. A strong and rather
vertical magnetic field of up to 2500 G is measured in the flare
footpoints, confirming that the He I D3 line is likely formed in the deep
chromosphere at those locations. We provide chromospheric line-of-sight
velocity and magnetic field maps obtained via He I D3 inversions. We propose a
fan-spine configuration as the flare magnetic field topology. Conclusions. The
He I D3 line is an excellent diagnostic to study the chromosphere during
flares. The impact of strong condensations on the deep chromosphere has been
observed. Detailed maps of the flare dynamics and the magnetic field are
obtained.Comment: Accepted for publication in A&
Physical properties of bright Ca II K fibrils in the solar chromosphere
Broad-band images of the solar chromosphere in the Ca II H&K line cores
around active regions are covered with fine bright elongated structures called
bright fibrils. The mechanisms that form these structures and cause them to
appear bright are still unknown. We aim to investigate the physical properties,
such as temperature, line-of-sight velocity, and microturbulence, in the
atmosphere that produces bright fibrils and to compare those to the properties
of their surrounding atmosphere. We used simultaneous observations of a plage
region in Fe I 6301-2 \r{A}, Ca II 8542 \r{A}, Ca II K, and H acquired
by the CRISP and CHROMIS instruments on the Swedish 1-m Solar Telescope. We
manually selected a sample of 282 Ca II K bright fibrils. We compared the
appearance of the fibrils in our sample to the Ca II 8542 \r{A} and H
data. We performed non-local thermodynamic equilibrium (non-LTE) inversions
using the inversion code STiC on the Fe I 6301-2 \r{A}, Ca II 8542 \r{A}, Ca II
K lines to infer the physical properties of the atmosphere. The line profiles
in bright fibrils have a higher intensity in their K peaks compared to
profiles formed in the surrounding atmosphere. The inversion results show that
the atmosphere in fibrils is on average ~K hotter at an optical depth
log compared to their surroundings. The line-of-sight velocity
at chromospheric heights in the fibrils does not show any preference towards
upflows or downflows. The microturbulence in the fibrils is on average 0.5 km
s higher compared to their surroundings. Our results suggest that the
fibrils have a limited extent in height, and they should be viewed as hot
threads pervading the chromosphere
Three-dimensional modeling of chromospheric spectral lines in a simulated active region
Because of the complex physics that governs the formation of chromospheric
lines, interpretation of solar chromospheric observations is difficult. The
origin and characteristics of many chromospheric features are, because of this,
unresolved. We focus here on studying two prominent features: long fibrils and
flare ribbons. To model them, we use a 3D MHD simulation of an active region
which self-consistently reproduces both of them. We model the H, Mg II
k, Ca II K, and Ca II 8542 {\AA} lines using the 3D non-LTE radiative transfer
code Multi3D.
This simulation reproduces long fibrils that span between the
opposite-polarity sunspots and go up to 4 Mm in height. They can be traced in
all lines due to density corrugation. Opposite to previous studies, H,
Mg II h&k, and Ca II H&K, are formed at similar height in this model. Magnetic
field lines are aligned with the H fibrils, but the latter holds to a
lesser extent for the Ca II 8542 {\AA} line.
The simulation shows structures in the H line core that look like
flare ribbons. The emission in the ribbons is caused by a dense chromosphere
and a transition region at high column mass. The ribbons are visible in all
chromospheric lines, but least prominent in Ca II 8542 {\AA} line. In some
pixels, broad asymmetric profiles with a single emission peak are produced,
similar to the profiles observed in flare ribbons. They are caused by a deep
onset of the chromospheric temperature rise and large velocity gradients.
The simulation produces long fibrils similar to what is seen in observations.
It also produces structures similar to flare ribbons despite the lack of
non-thermal electrons in the simulation. The latter suggests that thermal
conduction might be a significant agent in transporting flare energy to the
chromosphere in addition to non-thermal electrons.Comment: 17 pages, 15 figures, accepted for publication by A&
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