88 research outputs found
Microjets in the penumbra of a sunspot
Penumbral Microjets (PMJs) are short-lived jets found in the penumbra of
sunspots, first observed in wide-band Ca H-line observations as localized
brightenings, and are thought to be caused by magnetic reconnection. Earlier
work on PMJs has been focused on smaller samples of by-eye selected events and
case studies. It is our goal to present an automated study of a large sample of
PMJs to place the basic statistics of PMJs on a sure footing and to study the
PMJ Ca II 8542 Angstrom spectral profile in detail. High spatial resolution and
spectrally well-sampled observations in the Ca II 8542 Angstrom line obtained
from the Swedish 1-m Solar Telescope (SST) are reduced by a Principle Component
Analysis and subsequently used in the automated detection of PMJs using the
simple learning algorithm k-Nearest Neighbour. PMJ detections were verified
with co-temporal Ca H-line observations. A total of 453 tracked PMJ events were
found, or 4253 PMJs detections tallied over all timeframes and a detection rate
of 21 events per timestep. From these, an average length, width and lifetime of
640 km, 210 km and 90 s were obtained. The average PMJ Ca II 8542 Angstrom line
profile is characterized by enhanced inner wings, often in the form of one or
two distinct peaks, and a brighter line core as compared to the quiet Sun
average. Average blue and red peak positions were determined at -10.4 km/s and
+10.2 km/s offsets from the Ca II 8542 Angstrom line core. We found several
clusters of PMJ hotspots within the sunspot penumbra, where PMJ events occur in
the same general area repeatedly over time. Our results indicate smaller
average PMJs sizes and longer lifetimes compared to previously published
values, but with statistics still in the same orders of magnitude. The
investigation and analysis of the PMJ line profiles strengthen the proposed
heating of PMJs to transition region temperatures.Comment: Figures 1, 2, 3, 4 and 11 exhibited artifacts in some pdf-readers,
and have been replotted with new graphical settings to remedy this. Apart
from slight changes in sizing and fonts, the figures are the same. The arXiv
abstract has had tex-syntax removed for better readabilit
Penumbral micro-jets at high spatial and temporal resolution
Sunspot observations in chromospheric spectral lines have revealed the
existence of short-lived linear bright transients, commonly referred to as
penumbral micro-jets (PMJs). Details on the origin and physical nature of PMJs
are to large extend still unkown. We aim to characterize the dynamical nature
of PMJs to provide guidance for future modelling efforts. We analyze high
spatial (0.1 arcsec) and temporal resolution (1 s) Ca II H filtergram (0.1 nm
bandwidth) observations of a sunspot obtained on two consecutive days with the
Swedish 1-m Solar Telescope. We find that PMJs appear to be the rapid
brightening of an already existing (faint) fibril. The rapid brightening is the
fast increase (typically less than 10 s) in intensity over significant length
(several 100s of km) of the existing fibril. For most PMJs, we cannot identify
a clear root or source from where the brightening appears to originate. After
the fast onset, about half of the PMJs have a top that is moving with an
apparent velocity between 5 and 14 km/s, most of them upwards. For the other
PMJs, there is no significant motion of the top. For about a third of the PMJs
we observe a splitting into two parallel and co-evolving linear features during
the later phases of the lifetime of the PMJ. We conclude that mass flows can
play only limited role in the onset phase of PMJs and that it is more likely
that we see the effect of a fast heating front.Comment: Accepted for publication in Astronomy & Astrophysics. Movies are
available at http://folk.uio.no/rouppe/pmj_highcadence
A multi-diagnostic spectral analysis of penumbral microjets
Penumbral microjets (PMJs) are short-lived, jet-like objects found in the
penumbra of sunspots. They were first discovered in chromospheric lines and
have later also been shown to exhibit signals in transition region (TR) lines.
Their origin and manner of evolution is not yet settled. We perform a
comprehensive analysis of PMJs through the use of spectral diagnostics that
span from photospheric to TR temperatures to constrain PMJ properties. We
employed high-spatial-resolution Swedish 1-m Solar Telescope observations in
the Ca II 8542 Angstrom and H-alpha lines, IRIS slit-jaw images, and IRIS
spectral observations in the Mg II h & k lines, the Mg II 2798.75 Angstrom &
2798.82 Angstrom triplet blend, the C II 1334 Angstrom & 1335 Angstrom lines,
and the Si IV 1394 Angstrom & 1403 Angstrom lines. We derived a wide range of
spectral diagnostics from these and investigated other secondary phenomena
associated with PMJs. We find that PMJs exhibit varying degrees of signal in
all of our studied spectral lines. We find low or negligible Doppler velocities
and velocity gradients throughout our diagnostics and all layers of the solar
atmosphere associated with these. Dark features in the inner wings of H-alpha
and Ca II 8542 Angstrom imply that PMJs form along pre-existing fibril
structures. We find evidence for upper photospheric heating in a subset of PMJs
through emission in the wings of the Mg II triplet lines. There is little
evidence for ubiquitous twisting motion in PMJs. There is no marked difference
in onset-times for PMJ brightenings in different spectral lines. PMJs most
likely exhibit only very modest mass-motions, contrary to earlier suggestions.
We posit that PMJs form at upper photospheric or chromospheric heights at
pre-existing fibril structures.Comment: 26 pages, 18 figures. Accepted for publication in Astronomy &
Astrophysics. This version rectifies bad formatting caused by an oversized
figure. In the previous version this caused the figure itself and subsequent
figures to be misplaced in the text. No other changes beside figure sizing
and placement was made. This altered the given page coun
Short dynamic fibrils in sunspot chromospheres
Sunspot chromospheres display vigorous oscillatory signature when observed in
chromospheric diagnostics like the strong Ca II lines and H-alpha. New
high-resolution sunspot observations from the Swedish 1-m Solar Telescope show
the ubiquitous presence of small-scale periodic jet-like features that move up
and down. This phenomenon has not been described before. Their typical width is
about 0.3 arcsec and they display clear parabolic trajectories in space-time
diagrams. The maximum extension of the top of the jets is lowest in the umbra,
a few 100 km, and progressively longer further away from the umbra in the
penumbra, with the longest more than 1000 km. These jets resemble dynamic
fibrils found in plage regions but at smaller extensions. LTE inversion of
spectro-polarimetric Ca II 8542 observations enabled for a comparison of the
magnetic field inclination and the properties of these short jets. We find that
the most extended of these jets also have longer periods and tend to be located
in regions with more horizontal magnetic fields. This is a direct observational
confirmation of the mechanism of long-period waves propagating along inclined
magnetic fields into the solar chromosphere. This mechanism was identified
earlier as the driver of dynamic fibrils in plage, part of the mottles in quiet
Sun, and type I spicules at the limb. The sunspot dynamic fibrils that we
report here represent a new class of manifestation of this mechanism. They are
not the same as the transient penumbral and umbral micro-jets reported earlier.Comment: animations of figures can be found at
http://folk.uio.no/rouppe/dfsunspot
Statistical properties of the Disk Counterparts of Type II Spicules from simultaneous observations of RBEs in Ca II 8542 and H{\alpha}
Spicules were recently found to exist as two types when a new class of
so-called type II spicules was discovered at the solar limb with Hinode. The
type II spicules have been linked with on-disk observations of Rapid
Blue-shifted Excursions (RBEs) in the Ha and Ca 8542 lines. Here we analyze
observations optimized for the detection of RBEs in both Ha and Ca 8542
simultaneously at a high temporal cadence taken with CRISP at the SST. This
study used a high-quality time sequence for RBEs at different blue-shifts and
employed an automated detection routine to detect a large number of RBEs in
order to expand on the statistics of RBEs. We find that the number of detected
RBEs is dependent on the Doppler velocity of the images on which the search is
performed. Detection of RBEs at lower velocities increases the estimated number
of RBEs to the same order of magnitude expected from limb spicules. This shows
that RBEs and type II spicules are exponents of the same phenomenon. We provide
evidence that Ca 8542 RBEs are connected to Ha RBEs and are located closer to
the network regions with the Ha RBEs being the continuation, and show that RBEs
have an average lifetime of 83.9 s when observed in both spectral lines with
Doppler velocity ranges of 10-25 km/s in Ca 8542 and 30-50 km/s in Ha. In
addition, we determine the transverse motion of a much larger sample of RBEs
than previous studies and find that like type II spicules, RBEs undergo
significant transverse motions, 5-10 km/s. Finally, we find that the
intergranular jets discovered in BBSO are a subset of RBEs.Comment: Accepted for publication in the Astrophysical Journal, 15 pages, 10
figure
Unresolved fine-scale structure in solar coronal loop-tops
New and advanced space-based observing facilities continue to lower the
resolution limit and detect solar coronal loops in greater detail. We continue
to discover even finer sub-structures within coronal loop cross sections, in
order to understand the nature of the solar corona. Here, we push this lower
limit further to search for the finest coronal loop sub-structures, through
taking advantage of the resolving power of the Swedish 1- m Solar Telescope
(SST) / CRisp Imaging Spectro-Polarimeter (CRISP), together with
co-observations from the Solar Dynamics Observatory (SDO) / Atmospheric Image
Assembly (AIA). High resolution imaging of the chromospheric H-alpha 656.28 nm
spectral line core and wings can, under certain circumstances, allow one to
deduce the topology of the local magnetic environment of the solar atmosphere
where its observed. Here, we study post-flare coronal loops, which become
filled with evaporated chromosphere that rapidly condenses into chromospheric
clumps of plasma (detectable in H-alpha) known as a coronal rain, to
investigate their fine-scale structure. We identify, through analysis of three
datasets, large-scale catastrophic cooling in coronal loop-tops and the
existence of multi-thermal, multi-stranded sub-structures. Many cool strands
even extend fully-intact from loop-top to foot-point. We discover that coronal
loop fine-scale strands can appear bunched with as many as 8 parallel strands,
within an AIA coronal loop cross-section. The strand number density vs
cross-sectional width distribution, as detected by CRISP within AIA-defined
coronal loops, most-likely peaks at well below 100 km and currently 69% of the
sub-structure strands are statistically unresolved in AIA coronal loops.Comment: 10 pages, 10 figures, submitted to Ap
Are giant tornadoes the legs of solar prominences?
Observations in the 171 AA channel of the Atmospheric Imaging Assembly of the
space-borne Solar Dynamics Observatory show tornadoes-like features in the
atmosphere of the Sun. These giant tornadoes appear as dark, elongated and
apparently rotating structures in front of a brighter background. This
phenomenon is thought to be produced by rotating magnetic field structures that
extend throughout the atmosphere. We characterize giant tornadoes through a
statistical analysis of properties like spatial distribution, lifetimes, and
sizes. A total number of 201 giant tornadoes are detected in a period of 25
days, suggesting that on average about 30 events are present across the whole
Sun at a time close to solar maximum. Most tornadoes appear in groups and seem
to form the legs of prominences, thus serving as plasma sources/sinks.
Additional Halpha observations with the Swedish 1-m Solar Telescope imply that
giant tornadoes rotate as a structure although clearly exhibiting a thread-like
structure. We observe tornado groups that grow prior to the eruption of the
connected prominence. The rotation of the tornadoes may progressively twist the
magnetic structure of the prominence until it becomes unstable and erupts.
Finally, we investigate the potential relation of giant tornadoes to other
phenomena, which may also be produced by rotating magnetic field structures. A
comparison to cyclones, magnetic tornadoes and spicules implies that such
events are more abundant and short-lived the smaller they are. This comparison
might help to construct a power law for the effective atmospheric heating
contribution as function of spatial scale.Comment: 15 pages, 12 figures; ApJ, accepted versio
Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines
We characterize, for the first time, type-II spicules in Ca II K 3934\AA\
using the CHROMIS instrument at the Swedish 1-m Solar Telescope. We find that
their line formation is dominated by opacity shifts with the K minimum
best representing the velocity of the spicules. The K features are either
suppressed by the Doppler-shifted K or enhanced via an increased
contribution from the lower layers, leading to strongly enhanced but un-shifted
K peaks, with widening towards the line-core as consistent with
upper-layer opacity removal via Doppler-shift. We identify spicule spectra in
concurrent IRIS Mg II k 2796\AA\ observations with very similar properties.
Using our interpretation of spicule chromospheric line-formation, we produce
synthetic profiles that match observations.Comment: 10 pages, 8 figures, accepted for publication in Astronomy and
Astrophysics Letter
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