944 research outputs found
Dynamic fibrils in H-alpha and C IV
Aim: To study the interaction of the solar chromosphere with the transition
region, in particular active-region jets in the transition region and their
relation to chromospheric fibrils. Methods: We carefully align image sequences
taken simultaneously in C IV with the Transition Region and Coronal Explorer
and in H-alpha with the Swedish 1-m Solar Telescope. We examine the temporal
evolution of "dynamic fibrils", i.e., individual short-lived active-region
chromospheric jet-like features in H-alpha. Results: All dynamic fibrils appear
as absorption features in H-alpha that progress from the blue to the red wing
through the line, and often show recurrent behavior. Some of them, but not all,
appear also as bright features in C IV which develop at or just beyond the apex
of the H-alpha darkening. They tend to best resemble the H-alpha fibril at +700
mA half a minute earlier. Conclusions: Dynamic chromospheric fibrils observed
in H-alpha regularly correspond to transition-region jets observed in the
ultraviolet. This correspondence suggests that some plasma associated with
dynamic fibrils is heated to transition-region temperatures.Comment: 8 pages, 8 figure
Ubiquitous High Speed Transition Region and Coronal Upflows in the Quiet Sun
We study the line profiles of a range of transition region (TR) emission
lines observed in typical quiet Sun regions. In magnetic network regions, the
Si IV 1402\AA{}, C IV 1548\AA{}, N V 1238\AA{}, O VI 1031\AA{}, and Ne VIII
770\AA{} spectral lines show significant asymmetry in the blue wing of the
emission line profiles. We interpret these high-velocity upflows in the lower
and upper TR as the quiet Sun equivalent of the recently discovered upflows in
the low corona above plage regions (Hara et al., 2008). The latter have been
shown to be directly associated with high-velocity chromospheric spicules that
are (partially) heated to coronal temperatures and play a significant role in
supplying the active region corona with hot plasma (DePontieu et al., 2009}. We
show that a similar process likely dominates the quiet Sun network. We provide
a new interpretation of the observed quiet Sun TR emission in terms of the
relentless mass transport between the chromosphere and corona - a mixture of
emission from dynamic episodic heating and mass injection into the corona as
well as that from the previously filled, slowly cooling, coronal plasma.
Analysis of the observed upflow component shows that it carries enough hot
plasma to play a significant role in the energy and mass balance of the quiet
corona. We determine the temperature dependence of the upflow velocities to
constrain the acceleration and heating mechanism that drives these upflows. We
also show that the temporal characteristics of these upflows suggest an
episodic driver that sometimes leads to quasi-periodic signals. We suggest that
at least some of the quasi-periodicities observed with coronal imagers and
spectrographs that have previously been interpreted as propagating
magnetoacoustic waves, may instead be caused by these upflows.Comment: 10 pages, 15 figures. In press ApJ. Higher resolution figures, and
movies supporting them, can be found at
http://download.hao.ucar.edu/pub/mscott/papers/QS
Chromospheric jets around the edges of sunspots
Aims. Evidence is beginning to be put forward that demonstrates the role of the chromosphere in supplying energy and mass to the corona. We aim to asses the role of chromospheric jets in active region dynamics.
Methods. Using a combination of the Hinode/SOT Ca II H and TRACE 1550 Å and 1600 Å filters we examine chromospheric jets situated at the edge of a sunspot.
Results. Analysis reveals a near continuous series of jets, that raise chromospheric material into the low corona above a sunspot. The jets have average rise speeds of 30 km/s
and a range of 10−100 km/s. Enhanced emission observed at the jets leading edge suggests the formation of a shock front. Increased emission in TRACE bandpasses above the sunspot and the disappearance of the jets from the Ca II filter suggests that some of the chromospheric jet material is at least heated to ∼ 0.1 MK. The evidence suggests that the jets could be a mechanism which provides a steady, low-level heating for active region features
Quasi-periodic Propagating Signals in the Solar Corona: The Signature of Magnetoacoustic Waves or High-Velocity Upflows?
Since the discovery of quasi-periodic propagating oscillations with periods
of order three to ten minutes in coronal loops with TRACE and EIT (later with
EUVI and EIS), they have been almost universally interpreted as evidence for
propagating slow-mode magnetoacoustic (MA) waves in the low-beta coronal
environment. We show that this interpretation is not unique. We focus instead
on the ubiquitous faint upflows, associated with blue asymmetries of spectral
line profiles in footpoint regions of coronal loops, and as faint disturbances
propagating along coronal loops in EUV/XR imaging timeseries. The two scenarios
are difficult to differentiate using only imaging data, but careful analysis of
spectral line profiles indicates that faint upflows are likely responsible for
some of the observed quasi-periodic oscillatory signals in the corona. We show
that EIS measurements of intensity and velocity oscillations in coronal lines
(previously interpreted as direct evidence for propagating waves) are actually
accompanied by significant oscillations in the line width that are driven by a
quasi-periodically varying component of emission in the blue wing of the line.
The faint blue-shifted emission component quasi-periodically modulates the peak
intensity and line-centroid of a single Gaussian fit to the profile with the
same small amplitudes (respectively a few percent of background intensity, and
a few km/s) used to infer the presence of MA waves. Our results indicate that a
significant fraction of the quasi-periodicities observed with coronal imagers
and spectrographs, previously interpreted as propagating MA waves, are caused
by these upflows. The different physical cause for coronal oscillations would
significantly impact the prospects of successful coronal seismology using
propagating disturbances in coronal loops.Comment: To appear Astrophysical Journal. 14 pages, 13 color figures, 4
movies. High resolution figures and online supporting movies are available at
http://tinyurl.com/29s7c4
On The Doppler Velocity of Emission Line Profiles Formed in the "Coronal Contraflow" that is the Chromosphere-Corona Mass Cycle
This analysis begins to explore the complex chromosphere-corona mass cycle
using a blend of imaging and spectroscopic diagnostics. Single Gaussian fits to
hot emission line profiles (formed above 1MK) at the base of coronal loop
structures indicate material blue-shifts of 5-10km/s while cool emission line
profiles (formed below 1MK) yield red-shifts of a similar magnitude -
indicating, to zeroth order, that a temperature-dependent bifurcating flow
exists on coronal structures. Image sequences of the same region reveal weakly
emitting upward propagating disturbances in both hot and cool emission with
apparent speeds of 50-150km/s. Spectroscopic observations indicate that these
propagating disturbances produce a weak emission component in the blue wing at
commensurate speed, but that they contribute only a few percent to the
(ensemble) emission line profile in a single spatio-temporal resolution
element. Subsequent analysis of imaging data shows material "draining" slowly
(~10km/s) out of the corona, but only in the cooler passbands. We interpret the
draining as the return-flow of coronal material at the end of the complex
chromosphere-corona mass cycle. Further, we suggest that the efficient
radiative cooling of the draining material produces a significant contribution
to the red wing of cool emission lines that is ultimately responsible for their
systematic red-shift as derived from a single Gaussian fit when compared to
those formed in hotter (conductively dominated) domains. The presence of
counter-streaming flows complicates the line profiles, their interpretation,
and asymmetry diagnoses, but allows a different physical picture of the lower
corona to develop.Comment: 7 pages, 5 color figures. Accepted to Appear Ap
Dynamic Ly alpha jets
The solar chromosphere and transition region are highly structured and
complex regimes. A recent breakthrough has been the identification of dynamic
fibrils observed in H alpha as caused by field-aligned magnetoacoustic shocks.
We seek to find whether such dynamic fibrils are also observed in Ly alpha. We
used a brief sequence of four high-resolution Ly alpha images of the solar limb
taken by the Very high Angular resolution ULtraviolet Telescope (VAULT), which
displays many extending and retracting Ly alpha jets. We measured their top
trajectories and fitted parabolas to the 30 best-defined ones. Most jet tops
move supersonically. Half of them decelerate, sometimes superballistically, the
others accelerate. This bifurcation may arise from incomplete sampling of
recurrent jets. The similarities between dynamic Ly alpha jets and H alpha
fibrils suggest that the magnetoacoustic shocks causing dynamic H alpha fibrils
also affect dynamic Ly alpha jets.Comment: 5 pages, 7 figures; changed title and content; accepted in Astronomy
and Astrophysics; eps figures in full resolution are available at
http://www.astro.sk/~koza/publications/vault/figs
What do iris observations of Mg II k tell us about the solar plage chromosphere?
We analyze observations from the Interface Region Imaging Spectrograph of the
Mg II k line, the Mg II UV subordinate lines, and the O I 135.6 nm line to
better understand the solar plage chromosphere. We also make comparisons with
observations from the Swedish 1 m Solar Telescope of the H{\alpha} line, the Ca
II 8542 line, and Solar Dynamics Observatory/Atmospheric Imaging Assembly
observations of the coronal 19.3 nm line. To understand the observed Mg II
profiles, we compare these observations to the results of numerical
experiments. The single-peaked or flat-topped Mg II k profiles found in plage
imply a transition region at a high column mass and a hot and dense
chromosphere of about 6500 K. This scenario is supported by the observed
large-scale correlation between moss brightness and filled-in profiles with
very little or absent self-reversal. The large wing width found in plage also
implies a hot and dense chromosphere with a steep chromospheric temperature
rise. The absence of emission in the Mg II subordinate lines constrain the
chromospheric temperature and the height of the temperature rise while the
width of the O I 135.6 nm line sets a limit to the non-thermal velocities to
around 7 km/s
Observations of a pulse driven cool polar jet by SDO/AIA
Context. We observe a solar jet at north polar coronal hole (NPCH) using SDO
AIA 304 {\deg}A image data on 3 August 2010. The jet rises obliquely above the
solar limb and then retraces its propagation path to fall back. Aims. We
numerically model this observed solar jet by implementing a realistic (VAL-C)
model of solar temperature. Methods. We solve two-dimensional ideal
magnetohydrodynamic equations numerically to simulate the observed solar jet.
We consider a localized velocity pulse that is essentially parallel to the
background magnetic field lines and initially launched at the top of the solar
photosphere. The pulse steepens into a shock at higher altitudes, which
triggers plasma perturbations that exhibit the observed features of the jet.
The typical direction of the pulse also clearly exhibits the leading front of
the moving jet. Results. Our numerical simulations reveal that a large
amplitude initial velocity pulse launched at the top of the solar photosphere
produces in general the observed properties of the jet, e.g., upward and
backward average velocities, height, width, life-time, and ballistic nature.
Conclusions. The close matching between the jet observations and numerical
simulations provides first strong evidence for the formation of this jet by a
single velocity pulse. The strong velocity pulse is most likely generated by
the low- atmospheric reconnection in the polar region which results in
triggering of the jet. The downflowing material of the jet most likely vanishes
in the next upcoming velocity pulses from lower solar atmosphere, and therefore
distinctly launched a single jet upward in the solar atmosphere is observed.Comment: 8 pages, 4 figures, A&
Spicule-like structures observed in 3D realistic MHD simulations
We analyze features that resemble type i spicules in two different 3D
numerical simulations in which we include horizontal magnetic flux emergence in
a computational domain spanning the upper layers of the convection zone to the
lower corona. The two simulations differ mainly in the preexisting ambient
magnetic field strength and in the properties of the inserted flux tube. We use
the Oslo Staggered Code (OSC) to solve the full MHD equations with non-grey and
non-LTE radiative transfer and thermal conduction along the magnetic field
lines. We find a multitude of features that show a spatiotemporal evolution
that is similar to that observed in type i spicules, which are characterized by
parabolic height vs. time profiles, and are dominated by rapid upward motion at
speeds of 10-30 km/s, followed by downward motion at similar velocities. We
measured the parameters of the parabolic profile of the spicules and find
similar correlations between the parameters as those found in observations. The
values for height (or length) and duration of the spicules found in the
simulations are more limited in range than those in the observations. The
spicules found in the simulation with higher preexisting ambient field have
shorter length and smaller velocities. From the simulations, it appears that
these kinds of spicules can, in principle, be driven by a variety of mechanisms
that include p-modes, collapsing granules, magnetic energy release in the
photosphere and lower chromosphere and convective buffeting of flux
concentrations.Comment: 31 pages, 9 figures. accepted the 23 of June in Ap
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