140 research outputs found
H-alpha features with hot onsets. II. A contrail fibril
The solar chromosphere observed in H-alpha consists mostly of narrow fibrils.
The longest typically originate in network or plage and arch far over adjacent
internetwork. We use data from multiple telescopes to analyze one well-observed
example in a quiet area. It resulted from the earlier passage of an
accelerating disturbance in which the gas was heated to high temperature as in
the spicule-II phenomenon. After this passage a dark H-Halpha fibril appeared
as a contrail. We use Saha-Boltzmann extinction estimation to gauge the onset
and subsequent visibilities in various diagnostics and conclude that such
H-alpha fibrils can indeed be contrail phenomena, not indicative of the
thermodynamic and magnetic environment when they are observed but of more
dynamic happenings before. They do not connect across internetwork cells but
represent launch tracks of heating events and chart magnetic field during
launch, not at present.Comment: Accepted for Astronomy & Astrophysic
Formation of H in the solar atmosphere
Aims. We aim to understand how Hepsilon is formed in the quiet Sun. In
particular, we consider the particular physical mechanism that sets its source
function and extinction, how it is formed in different solar structures, and
why it is sometimes observed in emission. Methods. We used a 3D radiative
magnetohydrodynamic (MHD) simulation that accounts for non-equilibrium hydrogen
ionization, run with the Bifrost code. To synthesize Hepsilon and Ca II H
spectra, we made use of the RH code, which was modified to take into account
the non-equilibrium hydrogen ionization. To determine the dominant terms in the
H source function, we adopted a multi-level description of the
source function. Using synthetic spectra and simulation, we studied the
contribution function to the relative line absorption or emission and compared
it with atmospheric quantities at different locations. Results. Our multi-level
source function description suggests that the H source function is
dominated by interlocking, with the dominant interlocking transition being
through the ground level, populating the upper level of H via the
Lyman series. This makes the H source function partly sensitive to
temperature. The H extinction is set by Lyman-. In some
cases, this temperature dependence gives rise to H emission,
indicating heating. High-resolution observations reveal that H is
not just a weak absorption line. Regions with H in emission are
especially interesting to detect small-scale heating events in the lower solar
atmosphere, such as Ellerman bombs. Thus, H can be an important new
diagnostic tool for studies of heating in the solar atmosphere, augmenting the
diagnostic potential of Ca II H when observed simultaneousl
Accelerated particle beams in a 3D simulation of the quiet Sun. Lower atmospheric spectral diagnostics
Nanoflare heating through small-scale magnetic reconnection events is one of
the prime candidates to explain heating of the solar corona. However, direct
signatures of nanoflares are difficult to determine, and unambiguous
observational evidence is still lacking. Numerical models that include
accelerated electrons and can reproduce flaring conditions are essential in
understanding how low-energetic events act as a heating mechanism of the
corona, and how such events are able to produce signatures in the spectral
lines that can be detected through observations. We investigate the effects of
accelerated electrons in synthetic spectra from a 3D radiative
magnetohydrodynamics simulation to better understand small-scale heating events
and their impact on the solar atmosphere. We synthesised the chromospheric Ca
II and Mg II lines and the transition region Si IV resonance lines from a quiet
Sun numerical simulation that includes accelerated electrons. We calculated the
contribution function to the intensity to better understand how the lines are
formed, and what factors are contributing to the detailed shape of the spectral
profiles. The synthetic spectra are highly affected by variations in
temperature and vertical velocity. Beam heating exceeds conductive heating at
the heights where the spectral lines form, indicating that the electrons should
contribute to the heating of the lower atmosphere and hence affect the line
profiles. However, we find that it is difficult to determine specific
signatures from the non-thermal electrons due to the complexity of the
atmospheric response to the heating in combination with the relatively low
energy output (~1e21 erg/s). Even so, our results contribute to understanding
small-scale heating events in the solar atmosphere, and give further guidance
to future observations
Signatures of ubiquitous magnetic reconnection in the deep atmosphere of sunspot penumbrae
Ellerman bombs are regions with enhanced Balmer line wing emission and mark
magnetic reconnection in the deep solar atmosphere in active regions and quiet
Sun. They are often found in regions where opposite magnetic polarities are in
close proximity. Recent high resolution observations suggest that Ellerman
bombs are more prevalent than thought before. We aim to determine the
occurrence of Ellerman bombs in the penumbra of sunspots. We analyze high
spatial resolution observations of sunspots in the Balmer H-alpha and H-beta
lines as well as auxiliary continuum channels obtained with the Swedish 1-m
Solar Telescope and apply the k-means clustering technique to systematically
detect and characterize Ellerman Bombs. Features with all the defining
characteristics of Ellerman bombs are found in large numbers over the entire
penumbra. The true prevalence of these events is only fully appreciated in the
H-beta line due to highest spatial resolution and lower chromospheric opacity.
We find that the penumbra hosts some of the highest Ellerman bomb densities,
only surpassed by the moat in the immediate surroundings of the sunspot. Some
penumbral Ellerman bombs show flame morphology and rapid dynamical evolution.
Many penumbral Ellerman bombs are fast moving with typical speed of 3.7 km/s
and sometimes more than 10 km/s. Many penumbral Ellerman bombs migrate from the
inner to the outer penumbra over hundreds of km and some continue moving beyond
the outer penumbral boundary into the moat. Many penumbral Ellerman bombs are
found in the vicinity of regions with opposite magnetic polarity. We conclude
that reconnection is a near continuous process in the low atmosphere of the
penumbra of sunspots as manifest in the form of penumbral Ellerman bombs. These
are so prevalent that they may be a major sink of sunspot magnetic energy.Comment: accepted for publication in A&A. Movies can be found at
https://www.mn.uio.no/astro/english/people/aca/rouppe/movies
Penumbral decay observed in active region NOAA 12585
The physical conditions leading the sunspot penumbra decay are poorly
understood so far. We investigate the photospheric magnetic and velocity
properties of a sunspot penumbra during the decay phase to advance the current
knowledge of the conditions leading to this process. A penumbral decay was
observed with the CRISP instrument at the Swedish 1m Solar Telescope on 2016
September 4 and 5 in active region NOAA 12585. During these days, full-Stokes
spectropolarimetric scans along the Fe I 630 nm line pair were acquired over
more than one hour. We inverted these observations with the VFISV code in order
to obtain the evolution of the magnetic and velocity properties. We complement
the study with data from instruments onboard the Solar Dynamics Observatory and
Hinode space missions. The studied penumbra disappears progressively in both
time and space. The magnetic flux evolution seems to be linked to the presence
of Moving Magnetic Features (MMFs). Decreasing Stokes V signals are observed.
Evershed flows and horizontal fields were detected even after the disappearance
of the penumbral sector. The analyzed penumbral decay seems to result from the
interaction between opposite polarity fields in type III MMFs and penumbra,
while the presence of overlying canopies rules the evolution in the different
penumbral sectors.Comment: 13 pages, 11 figures, accepted for publication in A&
Bombs and flares at the surface and lower atmosphere of the Sun
This research was supported by the Research Council of Norway and by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007–2013)/ERC Grant agreement no. 291058.A spectacular manifestation of solar activity is the appearance of transient brightenings in the far wings of the Hα line, known as Ellerman bombs (EBs). Recent observations obtained by the Interface Region Imaging Spectrograph have revealed another type of plasma "bombs" (UV bursts) with high temperatures of perhaps up to 8 × 104 K within the cooler lower solar atmosphere. Realistic numerical modeling showing such events is needed to explain their nature. Here, we report on 3D radiative magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. We find that ubiquitous reconnection between emerging bipolar magnetic fields can trigger EBs in the photosphere, UV bursts in the mid/low chromosphere and small (nano-/micro-) flares (106 K) in the upper chromosphere. These results provide new insights into the emergence and build up of the coronal magnetic field and the dynamics and heating of the solar surface and lower atmosphere.Publisher PDFPeer reviewe
Dissecting bombs and bursts: non-LTE inversions of low-atmosphere reconnection in SST and IRIS observations
Ellerman bombs and UV bursts are transient brightenings that are ubiquitously
observed in the lower atmospheres of active and emerging flux regions. Here we
present inversion results of SST/CRISP and CHROMIS, as well as IRIS data of
such transient events. Combining information from the Mg II h & k, Si IV and Ca
II 8542A and Ca II H & K lines, we aim to characterise their temperature and
velocity stratification, as well as their magnetic field configuration. We find
average temperature enhancements of a few thousand kelvin close to the
classical temperature minimum, but localised peak temperatures of up to
10,000-15,000 K from Ca II inversions. Including Mg II generally dampens these
temperature enhancements to below 8000 K, while Si IV requires temperatures in
excess of 10,000 K at low heights, but may also be reproduced with secondary
temperature enhancements of 35,000-60,000 K higher up. However, reproducing Si
IV comes at the expense of overestimating the Mg II emission. The line-of-sight
velocity maps show clear bi-directional jet signatures and strong correlation
with substructure in the intensity images, with slightly larger velocities
towards the observer than away. The magnetic field parameters show an
enhancement of the horizontal field co-located with the brightenings at similar
heights as the temperature increase. We are thus able to largely reproduce the
observational properties of Ellerman bombs with UV burst signature with
temperature stratifications peaking close to the classical temperature minimum.
Correctly modelling the Si IV emission in agreement with all other diagnostics
is, however, an outstanding issue. Accounting for resolution differences,
fitting localised temperature enhancements and/or performing spatially-coupled
inversions is likely necessary to obtain better agreement between all
considered diagnostics.Comment: Accepted for publication in Astronomy & Astrophysics. 24 pages, 17
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