478 research outputs found
Effect of the C-bridge length on the ultraviolet-resistance of oxycarbosilane low-k films
The ultra-violet (UV) and vacuum ultra-violet (VUV) resistance of bridging alkylene groups in organosilica films has been investigated. Similar to the Si-CH3 (methyl) bonds, the Si-CH2-Si (methylene) bonds are not affected by 5.6 eV irradiation. On the other hand, the concentration of the Si-CH2-CH2-Si (ethylene) groups decreases during such UV exposure. More significant difference in alkylene reduction is observed when the films are exposed to VUV (7.2 eV). The ethylene groups are depleted by more than 75% while only about 40% methylene and methyl groups loss is observed. The different sensitivity of bridging groups to VUV light should be taken into account during the development of curing and plasma etch processes of low-k materials based on periodic mesoporous organosilicas and oxycarbosilanes. The experimental results are qualitatively supported by ab-initio quantum-chemical calculations
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
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
Chromospheric emission from nanoflare heating in RADYN simulations
Heating signatures from small-scale magnetic reconnection events in the solar
atmosphere have proven to be difficult to detect through observations.
Numerical models that reproduce flaring conditions are essential in the
understanding of how nanoflares may act as a heating mechanism of the corona.
We study the effects of non-thermal electrons in synthetic spectra from 1D
hydrodynamic RADYN simulations of nanoflare heated loops to investigate the
diagnostic potential of chromospheric emission from small-scale events. The Mg
II h and k, Ca II H and K, Ca II 854.2 nm, H-alpha and H-beta chromospheric
lines were synthesised from various RADYN models of coronal loops subject to
electron beams of nanoflare energies. The contribution function to the line
intensity was computed to better understand how the atmospheric response to the
non-thermal electrons affects the formation of spectral lines and the detailed
shape of their spectral profiles. The spectral line signatures arising from the
electron beams highly depend on the density of the loop and the lower cutoff
energy of the electrons. Low-energy (5 keV) electrons deposit their energy in
the corona and transition region, producing strong plasma flows that cause both
redshifts and blueshifts of the chromospheric spectra. Higher-energy (10 and 15
keV) electrons deposit their energy in the lower transition region and
chromosphere, resulting in increased emission from local heating. Our results
indicate that effects from small-scale events can be observed with ground-based
telescopes, expanding the list of possible diagnostics for the presence and
properties of nanoflares
Wave Propagation and Jet Formation in the Chromosphere
We present the results of numerical simulations of wave propagation and jet
formation in solar atmosphere models with different magnetic field
configurations. The presence in the chromosphere of waves with periods longer
than the acoustic cutoff period has been ascribed to either strong inclined
magnetic fields, or changes in the radiative relaxation time. Our simulations
include a sophisticated treatment of radiative losses, as well as fields with
different strengths and inclinations. Using Fourier and wavelet analysis
techniques, we investigate the periodicity of the waves that travel through the
chromosphere. We find that the velocity signal is dominated by waves with
periods around 5 minutes in regions of strong, inclined field, including at the
edges of strong flux tubes where the field expands, whereas 3-minute waves
dominate in regions of weak or vertically oriented fields. Our results show
that the field inclination is very important for long-period wave propagation,
whereas variations in the radiative relaxation time have little effect.
Furthermore, we find that atmospheric conditions can vary significantly on
timescales of a few minutes, meaning that a Fourier analysis of wave
propagation can be misleading. Wavelet techniques take variations with time
into account and are more suitable analysis tools. Finally, we investigate the
properties of jets formed by the propagating waves once they reach the
transition region, and find systematic differences between the jets in inclined
field regions and those in vertical field regions, in agreement with
observations of dynamic fibrils.Comment: 27 pages, 29 figures; accepted for publication in Astrophysical
Journa
Intermittent reconnection and plasmoids in UV bursts in the low solar atmosphere
Magnetic reconnection is thought to drive a wide variety of dynamic phenomena
in the solar atmosphere. Yet the detailed physical mechanisms driving
reconnection are difficult to discern in the remote sensing observations that
are used to study the solar atmosphere. In this paper we exploit the
high-resolution instruments Interface Region Imaging Spectrograph (IRIS) and
the new CHROMIS Fabry-Perot instrument at the Swedish 1-m Solar Telescope (SST)
to identify the intermittency of magnetic reconnection and its association with
the formation of plasmoids in so-called UV bursts in the low solar atmosphere.
The Si IV 1403A UV burst spectra from the transition region show evidence of
highly broadened line profiles with often non-Gaussian and triangular shapes,
in addition to signatures of bidirectional flows. Such profiles had previously
been linked, in idealized numerical simulations, to magnetic reconnection
driven by the plasmoid instability. Simultaneous CHROMIS images in the
chromospheric Ca II K 3934A line now provide compelling evidence for the
presence of plasmoids, by revealing highly dynamic and rapidly moving
brightenings that are smaller than 0.2 arcsec and that evolve on timescales of
order seconds. Our interpretation of the observations is supported by detailed
comparisons with synthetic observables from advanced numerical simulations of
magnetic reconnection and associated plasmoids in the chromosphere. Our results
highlight how subarcsecond imaging spectroscopy sensitive to a wide range of
temperatures combined with advanced numerical simulations that are realistic
enough to compare with observations can directly reveal the small-scale
physical processes that drive the wide range of phenomena in the solar
atmosphere.Comment: Accepted for publication in Astrophysical Journal Letters. Movies are
available at http://folk.uio.no/rouppe/plasmoids_chromis
Magnetoacoustic shocks as driver of quiet Sun mottles
We present high spatial and high temporal resolution observations of the
quiet Sun in H-alpha obtained with the Swedish 1-m Solar Telescope on La Palma.
We observe that many mottles, jet-like features in the quiet Sun, display clear
up- and downward motions along their main axis. In addition, many mottles show
vigorous transverse displacements. Unique identification of the mottles
throughout their lifetime is much harder than for their active region
counterpart, dynamic fibrils. This is because many seem to lack a sharply
defined edge at their top, and significant fading often occurs throughout their
lifetime. For those mottles that can be reliably tracked, we find that the
mottle tops often undergo parabolic paths. We find a linear correlation between
the deceleration these mottles undergo and the maximum velocity they reach,
similar to what was found earlier for dynamic fibrils. Combined with an
analysis of oscillatory properties, we conclude that at least part of the quiet
Sun mottles are driven by magnetoacoustic shocks. In addition, the mixed
polarity environment and vigorous dynamics suggest that reconnection may play a
significant role in the formation of some quiet Sun jets.Comment: 12 pages, 4 figures. ApJ Letters, in pres
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