66 research outputs found
EUV Sunspot Plumes Observed with SOHO
Bright EUV sunspot plumes have been observed in five out of nine sunspot
regions with the Coronal Diagnostic Spectrometer -- CDS on SOHO. In the other
four regions the brightest line emissions may appear inside the sunspot but are
mainly concentrated in small regions outside the sunspot areas. These results
are in contrast to those obtained during the Solar Maximum Mission, but are
compatible with the Skylab mission results. The present observations show that
sunspot plumes are formed in the upper part of the transition region, occur
both in magnetic unipolar-- and bipolar regions, and may extend from the umbra
into the penumbra.Comment: 8 pages, 3 figures, to be published in ApJ Letter
Continuous upflows and sporadic downflows observed in active regions
We present a study of the temporal evolution of coronal loops in active
regions and its implications for the dynamics in coronal loops. We analyzed
images of the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics
Observatory (SDO) at multiple temperatures to detect apparent motions in the
coronal loops. Quasi-periodic brightness fluctuations propagate upwards from
the loop footpoint in hot emission at 1MK, while sporadic downflows are seen in
cool emission below 1MK. The upward motion in hot emission increases just after
the cool downflows. The apparent propagating pattern suggests a hot upflow from
the loop footpoints, and is considered to supply hot plasma into the coronal
loop, but a wavelike phenomenon cannot be ruled out. Coronal condensation
occasionally happens in the coronal loop, and the cool material flows down to
the footpoint. Emission from cool plasma could have a significant contribution
to hot AIA channels in the event of coronal condensation.Comment: 5 pages, 6 figures, A&A in pres
SUMER observations of the inverse Evershed effect in the transition region above a sunspot
Aims. We analyse SUMER spectral scans of a large sunspot within active region
NOAA 10923, obtained on 14-15 November 2006, to determine the morphology and
dynamics of the sunspot atmosphere at different heights/temperatures. Methods:
The data analysed here consist of spectroheliograms in the continuum around
142.0 nm and in the Si iv 140.2 nm, O iii 70.3 nm, N iv 76.5 nm, and O iv 79.0
nm spectral lines. Gaussian-fitting of the observed profiles provides
line-of-sight velocity and Doppler-width maps. Results: The data show an
asymmetric downflow pattern compatible with the presence of the inverse
Evershed flow in a region within roughly twice the penumbral radius at
transition-region temperatures up to 0.18 MK. The motions, highly inhomogeneous
on small scales, seem to occur in a collar of radially directed filamentary
structures, with an average width less than the 1 Mm spatial resolution of
SUMER and characterised by different plasma speeds. Assuming that the flows are
directed along the field lines, we deduce that such field lines are inclined by
10 deg to 25 deg with respect to the solar surface.Comment: 5 pages, 3 figures. Text height reduced for better layout. No changes
in conten
Numerical simulation of the internal plasma dynamics of post-flare loops
We integrate the MHD ideal equations of a slender flux tube to simulate the
internal plasma dynamics of coronal post-flare loops. We study the onset and
evolution of the internal plasma instability to compare with observations and
to gain insight into physical processes and characteristic parameters
associated with flaring events. The numerical approach uses a finite-volume
Harten-Yee TVD scheme to integrate the 1D1/2 MHD equations specially designed
to capture supersonic flow discontinuities. We could reproduce the
observational sliding down and upwardly propagating of brightening features
along magnetic threads of an event occurred on October 1st, 2001. We show that
high--speed downflow perturbations, usually interpreted as slow magnetoacoustic
waves, could be better interpreted as slow magnetoacoustic shock waves. This
result was obtained considering adiabaticity in the energy balance equation.
However, a time--dependent forcing from the basis is needed to reproduce the
reiteration of the event which resembles observational patterns -commonly known
as quasi--periodic pulsations (QPPs)- which are related with large scale
characteristic longitudes of coherence. This result reinforces the
interpretation that the QPPs are a response to the pulsational flaring
activity.Comment: Accepted MNRAS, 10 pages, 14 figures, 1 tabl
A solar active region loop compared with a 2D MHD model
We analyzed a coronal loop observed with the Normal Incidence Spectrometer
(NIS), which is part of the Coronal Diagnostic Spectrometer (CDS) on board the
Solar and Heliospheric Observatory (SOHO). The measured Doppler shifts and
proper motions along the selected loop strongly indicate unidirectional flows.
Analysing the Emission Measure Curves of the observed spectral lines, we
estimated that the temperature along the loop was about 380000 K. We adapted a
solution of the ideal MHD steady equations to our set of measurements. The
derived energy balance along the loop, as well as the advantages/disadvantages
of this MHD model for understanding the characteristics of solar coronal loops
are discussed.Comment: A&A in press, 10 pages, 6 figure
Dynamics of solar coronal loops II. Catastrophic cooling and high-speed downflows
This work addresses the problem of plasma condensation and ``catastrophic
cooling'' in solar coronal loops. We have carried out numerical calculations of
coronal loops and find several classes of time-dependent solutions (static,
periodic, irregular), depending on the spatial distribution of a temporally
constant energy deposition in the loop. Dynamic loops exhibit recurrent plasma
condensations, accompanied by high-speed downflows and transient brightenings
of transition region lines, in good agreement with features observed with
TRACE. Furthermore, these results also offer an explanation for the recent EIT
observations of De Groof et al. (2004) of moving bright blobs in large coronal
loops. In contrast to earlier models, we suggest that the process of
catastrophic cooling is not initiated by a drastic decrease of the total loop
heating but rather results from a loss of equilibrium at the loop apex as a
natural consequence of heating concentrated at the footpoints of the loop, but
constant in time.Comment: 13 pages, 15 figure
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
A study of velocity fields in the transition region of Epsilon Eri (K2 V)
Analyses of the widths and shifts of optically thin emission lines in the
ultraviolet spectrum of the active dwarf Epsilon Eri (K2 V) are presented. The
spectra were obtained using the Space Telescope Imaging Spectrograph on the
Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. The line
widths are used to find the non-thermal energy density and its variation with
temperature from the chromosphere to the upper transition region. The energy
fluxes that could be carried by Alfven and acoustic waves are investigated, to
test their possible roles in coronal heating. Acoustic waves do not appear to
be a viable means of coronal heating. There is, in principle, ample flux in
Alfven waves, but detailed calculations of wave propagation are required before
definite conclusions can be drawn about their viability. The high sensitivity
and spectral resolution of the above instruments have allowed two-component
Gaussian fits to be made to the profiles of the stronger transition region
lines. The broad and narrow components which result share some similarities
with those observed in the Sun, but in Epsilon Eri the broad component is
redshifted relative to the narrow component and contributes more to the total
line flux. The possible origins of the two components and the energy fluxes
implied are discussed. On balance our results support the conclusion of Wood,
Linsky & Ayres, that the narrow component is related to Alfven waves reaching
to the corona, but the origin of the broad component is not clear.Comment: 19 pages, 16 figures. Accepted for publication by MNRA
Modeling the Subsurface Structure of Sunspots
While sunspots are easily observed at the solar surface, determining their
subsurface structure is not trivial. There are two main hypotheses for the
subsurface structure of sunspots: the monolithic model and the cluster model.
Local helioseismology is the only means by which we can investigate
subphotospheric structure. However, as current linear inversion techniques do
not yet allow helioseismology to probe the internal structure with sufficient
confidence to distinguish between the monolith and cluster models, the
development of physically realistic sunspot models are a priority for
helioseismologists. This is because they are not only important indicators of
the variety of physical effects that may influence helioseismic inferences in
active regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In this paper,
we provide a critical review of the existing sunspot models and an overview of
numerical methods employed to model wave propagation through model sunspots. We
then carry out an helioseismic analysis of the sunspot in Active Region 9787
and address the serious inconsistencies uncovered by
\citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). We find
that this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model) and that
travel-time measurements are consistent with a horizontal outflow in the
surrounding moat.Comment: 73 pages, 19 figures, accepted by Solar Physic
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
This review paper outlines background information and covers recent advances
made via the analysis of spectra and images of prominence plasma and the
increased sophistication of non-LTE (ie when there is a departure from Local
Thermodynamic Equilibrium) radiative transfer models. We first describe the
spectral inversion techniques that have been used to infer the plasma
parameters important for the general properties of the prominence plasma in
both its cool core and the hotter prominence-corona transition region. We also
review studies devoted to the observation of bulk motions of the prominence
plasma and to the determination of prominence mass. However, a simple inversion
of spectroscopic data usually fails when the lines become optically thick at
certain wavelengths. Therefore, complex non-LTE models become necessary. We
thus present the basics of non-LTE radiative transfer theory and the associated
multi-level radiative transfer problems. The main results of one- and
two-dimensional models of the prominences and their fine-structures are
presented. We then discuss the energy balance in various prominence models.
Finally, we outline the outstanding observational and theoretical questions,
and the directions for future progress in our understanding of solar
prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a
better resolution in the published version. New version reflects minor
changes brought after proof editin
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