57 research outputs found
Chromospheric heating by acoustic waves compared to radiative cooling
Acoustic and magnetoacoustic waves are among the possible candidate
mechanisms that heat the upper layers of solar atmosphere. A weak chromospheric
plage near a large solar pore NOAA 11005 was observed on October 15, 2008 in
the lines Fe I 617.3 nm and Ca II 853.2 nm with the Interferometric
Bidimemsional Spectrometer (IBIS) attached to the Dunn Solar Telescope.
Analyzing the Ca II observations with spatial and temporal resolutions of 0.4"
and 52 s, the energy deposited by acoustic waves is compared with that released
by radiative losses. The deposited acoustic flux is estimated from power
spectra of Doppler oscillations measured in the Ca II line core. The radiative
losses are calculated using a grid of seven 1D hydrostatic semi-empirical model
atmospheres. The comparison shows that the spatial correlation of maps of
radiative losses and acoustic flux is 72 %. In quiet chromosphere, the
contribution of acoustic energy flux to radiative losses is small, only of
about 15 %. In active areas with photospheric magnetic field strength between
300 G and 1300 G and inclination of 20-60 degrees, the contribution increases
from 23 % (chromospheric network) to 54 % (a plage). However, these values have
to be considered as lower limits and it might be possible that the acoustic
energy flux is the main contributor to the heating of bright chromospheric
network and plages.Comment: 9 pages, 10 figures. Accepted for publication in The Astrophysical
Journa
Properties of the inner penumbral boundary and temporal evolution of a decaying sunspot
It was empirically determined that the umbra-penumbra boundaries of stable
sunspots are characterized by a constant value of the vertical magnetic field.
We analyzed the evolution of the photospheric magnetic field properties of a
decaying sunspot belonging to NOAA 11277 between August 28 - September 3, 2011.
The observations were acquired with the spectropolarimeter on-board of the
Hinode satellite. We aim to proof the validity of the constant vertical
magnetic-field boundary between the umbra and penumbra in decaying sunspots. A
spectral-line inversion technique was used to infer the magnetic field vector
from the full-Stokes profiles. In total, eight maps were inverted and the
variation of the magnetic properties in time were quantified using linear or
quadratic fits. We found a linear decay of the umbral vertical magnetic field,
magnetic flux, and area. The penumbra showed a linear increase of the vertical
magnetic field and a sharp decay of the magnetic flux. In addition, the
penumbral area quadratically decayed. The vertical component of the magnetic
field is weaker on the umbra-penumbra boundary of the studied decaying sunspot
compared to stable sunspots. Its value seem to be steadily decreasing during
the decay phase. Moreover, at any time of the shown sunspot decay, the inner
penumbra boundary does not match with a constant value of the vertical magnetic
field, contrary to what was seen in stable sunspots. During the decaying phase
of the studied sunspot, the umbra does not have a sufficiently strong vertical
component of the magnetic field and is thus unstable and prone to be
disintegrated by convection or magnetic diffusion. No constant value of the
vertical magnetic field was found for the inner penumbral boundary.Comment: Accepted for publication in Astronomy & Astrophysics, 6 pages, 7
figure
Dynamics of the solar atmosphere above a pore with a light bridge
Context: Solar pores are small sunspots lacking a penumbra that have a
prevailing vertical magnetic field component. They can include light bridges at
places with locally reduced magnetic field. Like sunspots, they exhibit a wide
range of oscillatory phenomena.
Aims: A large isolated pore with a light bridge (NOAA 11005) is studied to
obtain characteristics of a chromospheric filamentary structure around the
pore, to analyse oscillations and waves in and around the pore, and to
understand the structure and brightness of the light bridge.
Methods: Spectral imaging observations in the line Ca II 854.2 nm and
complementary spectropolarimetry in Fe I lines, obtained with the DST/IBIS
spectrometer and HINODE/SOT spectropolarimeter, were used to measure
photospheric and chromospheric velocity fields, oscillations, waves, the
magnetic field in the photosphere, and acoustic energy flux and radiative
losses in the chromosphere.
Results: The chromospheric filamentary structure around the pore has all
important characteristics of a superpenumbra: it shows an inverse Evershed
effect and running waves, and has a similar morphology and oscillation
character. The granular structure of the light bridge in the upper photosphere
can be explained by radiative heating. Acoustic waves leaking up from the
photosphere along the inclined magnetic field in the light bridge transfer
enough energy flux to balance the total radiative losses of the light-bridge
chromosphere.
Conclusions: The presence of a penumbra is not a necessary condition for the
formation of a superpenumbra. The light bridge is heated by radiation in the
photosphere and by acoustic waves in the chromosphere.Comment: 14 pages, 14 figures, 3 tables, accepted for publication in
Astrononomy & Astrophysic
The formation and disintegration of magnetic bright points observed by Sunrise/IMaX
The evolution of the physical parameters of magnetic bright points (MBPs)
located in the quiet Sun (mainly in the interwork) during their lifetime is
studied. First we concentrate on the detailed description of the magnetic field
evolution of three MBPs. This reveals that individual features follow
different, generally complex, and rather dynamic scenarios of evolution. Next
we apply statistical methods on roughly 200 observed MBP evolutionary tracks.
MBPs are found to be formed by the strengthening of an equipartition field
patch, which initially exhibits a moderate downflow. During the evolution,
strong downdrafts with an average velocity of 2.4 km/s set in. These flows,
taken together with the concurrent strengthening of the field, suggest that we
are witnessing the occurrence of convective collapses in these features,
although only 30% of them reach kG field strengths. This fraction might turn
out to be larger when the new 4 m class solar telescopes are operational as
observations of MBPs with current state of the art instrumentation could still
be suffering from resolution limitations. Finally, when the bright point
disappears (although the magnetic field often continues to exist) the magnetic
field strength has dropped to the equipartition level and is generally somewhat
weaker than at the beginning of the MBP's evolution. Noteworthy is that in
about 10% of the cases we observe in the vicinity of the downflows small-scale
strong (exceeding 2 km/s) intergranular upflows related spatially and
temporally to these downflows.Comment: 19 pages, 13 figures; final version published in "The Astrophysical
Journal
Relation between magnetic field inclination and apparent motion of penumbral grains
Context. Bright heads of penumbral filaments, penumbral grains (PGs), show
apparent horizontal motions inwards, towards the umbra, or outwards, away from
the umbra.
Aims. We aim to prove statistically whether the direction of PGs' apparent
motion is related to the inclination of the surrounding magnetic field.
Methods. We use spectropolarimetric observations of five sunspot penumbrae to
compare magnetic inclinations inside PGs with those in their surroundings. The
data are taken by three observatories: Hinode satellite, Swedish Solar
Telescope, and GREGOR solar telescope. The direction of PGs' motion is
determined by feature tracking. The atmospheric conditions in PGs and their
surroundings, including the magnetic field information, are retrieved by means
of height-stratified spectropolarimetric inversions.
Results. On a sample of 444 inward- and 269 outward-moving PGs we show that
43% of the inward-moving PGs have magnetic inclination larger by than the inclination in their surroundings and 51% of the
outward-moving PGs have the inclination smaller by than
the surrounding one. The opposite relation of inclinations is observed at only
one-fifth of the inward- and outward-moving PGs.
Conclusions. Rising hot plasma in PGs surrounded by a less inclined magnetic
field may adapt its trajectory to be more vertical, causing an inward apparent
motion of PGs. Oppositely, it may be dragged by a more horizontal surrounding
magnetic field such that an outward apparent motion is observed.Comment: 7 pages, 4 figures, 2 table
The properties of penumbral microjets inclination
We investigate the dependence of penumbral microjets inclination on the
position within penumbra. The high cadence observations taken on 10 November
2006 with the Hinode satellite through the \ion{Ca}{ii} H and G--band filters
were analysed to determine the inclination of penumbral microjets. The results
were then compared with the inclination of the magnetic field determined
through the inversion of the spectropolarimetric observations of the same
region. The penumbral microjet inclination is increasing towards the outer edge
of the penumbra. The results suggest that the penumbral microjet follows the
opening magnetic field lines of a vertical flux tube that creates the sunspot.Comment: 4 pages, 4 figures, A&A Letter in pres
The Evershed Flow and the Brightness of the Penumbra
The Evershed flow is a systematic motion of gas that occurs in the penumbra
of all sunspots. Discovered in 1909, it still lacks a satisfactory explanation.
We know that the flow is magnetized, often supersonic, and that it shows
conspicuous fine structure on spatial scales of 0.2"-0.3", but its origin
remains unclear. The hope is that a good observational understanding of the
relation between the flow and the penumbral magnetic field will help us
determine its nature. Here I review advances in the characterization of the
Evershed flow and sunspot magnetic fields from high-resolution spectroscopic
and spectropolarimetric measurements. Using this information as input for 2D
heat transfer simulations, it has been demonstrated that hot Evershed upflows
along nearly horizontal field lines are capable of explaining one of the most
intriguing aspects of sunspots: the surplus brightness of the penumbra relative
to the umbra. They also explain the existence of penumbral filaments with dark
cores. These results support the idea that the Evershed flow is largely
responsible for the transport of energy in the penumbra.Comment: 18 pages, to appear in "Magnetic Coupling between the Interior and
the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer, Heidelberg, 200
The formation of small-scale umbral brightenings in sunspot atmospheres
Sunspot atmospheres have been shown to be highly inhomogeneous hosting both quasi-stable and transient features, such as small-scale umbral brightenings (previously named "umbral micro-jets") and dark fibril-like events. Aims. We seek to understand the morphological properties and formation mechanisms of small-scale umbral brightenings (analogous to umbral micro-jets). In addition, we aim to understand whether links between these events and short dynamic fibrils, umbral flashes, and umbral dots can be established. Methods. A Swedish 1 m Solar Telescope (SST) filtergram time-series sampling the Ca ii H line and a CRisp Imaging Spectro-Polarimeter (SST/CRISP) full-Stokes 15-point Ca ii 8542 Å line scan dataset were used. The spatial resolutions of these datasets are close to 0.1′′ and 0.18′′ with cadences of 1.4 s and 29 s, respectively. These data allowed us to construct light-curves, plot line profiles, and to perform a weak-field approximation in order to infer the magnetic field strength. Results. The average lifetime and lengths of the 54 small-scale brightenings identified in the sunspot umbra are found to be 44.2 s (σ = 20 s) and 0.56′′ (σ = 0.14′′), respectively. The spatial positioning and morphological evolution of these events in Ca ii H filtergrams was investigated finding no evidence of parabolic or ballistic profiles nor a preference for co-spatial formation with umbral flashes. Line scans in Ca ii 8542 Å and the presence of Stokes V profile reversals provided evidence that these events could form in a similar manner to umbral flashes in the chromosphere (i.e. through the formation of shocks either due to the steepening of localised wavefronts or due to the impact of returning material from short dynamic fibrils, a scenario we find evidence for). The application of the weak-field approximation indicated that changes in the line-of-sight magnetic field were not responsible for the modifications to the line profile and suggested that thermodynamic effects are, in fact, the actual cause of the increased emission. Finally, a sub-set of small-scale brightenings were observed to form at the foot-points of short dynamic fibrils. Conclusions. The small-scale umbral brightenings studied here do not appear to be jet-like in nature. Instead they appear to be evidence of shock formation in the lower solar atmosphere. We found no correlation between the spatial locations where these events were observed and the occurrence of umbral dots and umbral flashes. These events have lifetimes and spectral signatures comparable to umbral flashes and are located at the footpoints of short dynamic fibrils, during or at the end of the red-shifted stage. It is possible that these features form due to the shocking of fibrilar material in the lower atmosphere upon its return under gravity
Magnetic field strength distribution of magnetic bright points inferred from filtergrams and spectro-polarimetric data
Small scale magnetic fields can be observed on the Sun in G-band filtergrams
as MBPs (magnetic bright points) or identified in spectro-polarimetric
measurements due to enhanced signals of Stokes profiles. These magnetic fields
and their dynamics play a crucial role in understanding the coronal heating
problem and also in surface dynamo models. MBPs can theoretically be described
to evolve out of a patch of a solar photospheric magnetic field with values
below the equipartition field strength by the so-called convective collapse
model. After the collapse, the magnetic field of MBPs reaches a higher stable
magnetic field level. The magnetic field strength distribution of small scale
magnetic fields as seen by MBPs is inferred. Furthermore, we want to test the
model of convective collapse and the theoretically predicted stable value of
about 1300 G. We used four different data sets of high-resolution Hinode/SOT
observations that were recorded simultaneously with the broadband filter device
(G-band, Ca II-H) and the spectro-polarimeter. To derive the magnetic field
strength distribution of these small scale features, the spectropolarimeter
(SP) data sets were treated by the Merlin inversion code. The four data sets
comprise different solar surface types: active regions (a sunspot group and a
region with pores), as well as quiet Sun. In all four cases the obtained
magnetic field strength distribution of MBPs is similar and shows peaks around
1300 G. This agrees well with the theoretical prediction of the convective
collapse model. The resulting magnetic field strength distribution can be
fitted in each case by a model consisting of log-normal components. The
important parameters, such as geometrical mean value and multiplicative
standard deviation, are similar in all data sets, only the relative weighting
of the components is different.Comment: 12 pages, 12 figures, final version to be published in Astronomy and
Astrophysic
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