260 research outputs found
Do quasi-regular structures really exist in the solar photosphere? I. Observational evidence
Two series of solar-granulation images -- the La Palma series of 5 June 1993
and the SOHO MDI series of 17--18 January 1997 -- are analysed both
qualitatively and quantitatively. New evidence is presented for the existence
of long-lived, quasi-regular structures (first reported by Getling and Brandt
(2002)), which no longer appear unusual in images averaged over 1--2-h time
intervals. Such structures appear as families of light and dark concentric
rings or families of light and dark parallel strips (``ridges'' and
``trenches'' in the brightness distributions). In some cases, rings are
combined with radial ``spokes'' and can thus form ``web'' patterns. The
characteristic width of a ridge or trench is somewhat larger than the typical
size of granules. Running-average movies constructed from the series of images
are used to seek such structures. An algorithm is developed to obtain, for
automatically selected centres, the radial distributions of the azimuthally
averaged intensity, which highlight the concentric-ring patterns. We also
present a time-averaged granulation image processed with a software package
intended for the detection of geological structures in aerospace images. A
technique of running-average-based correlations between the brightness
variations at various points of the granular field is developed and indications
are found for a dynamical link between the emergence and sinking of hot and
cool parcels of the solar plasma. In particular, such a correlation analysis
confirms our suggestion that granules -- overheated blobs -- may repeatedly
emerge on the solar surface. Based on our study, the critical remarks by Rast
(2002) on the original paper by Getling and Brandt (2002) can be dismissed.Comment: 21 page, 8 figures; accepted by "Solar Physics
Fixed-delay Interferometry for Doppler Extra-solar Planet Detection
We present a new technique based on fixed-delay interferometry for high
throughput, high precision and multi-object Doppler radial velocity (RV)
surveys for extra-solar planets. The Doppler measurements are conducted through
monitoring the stellar fringe phase shifts of the interferometer. High Doppler
sensitivity is achieved through optimizing the optical delay in the
interferometer and reducing photon noise by measuring multiple fringes over a
broadband. This broadband operation is performed through coupling the
interferometer with a low to medium resolution post-disperser. The total
Doppler sensitivity of this approach is, in theory, independent of dispersing
power of the post-disperser, which allows development of new generation RV
machines with much reduced size, high stability and low cost compared to
echelles. This technique has the potential to improve RV survey efficiency by
2-3 orders of magnitude over cross-dispersed echelle spectroscopy approach to
allow a full sky RV survey for planets once the instrument is operated as a
multi-object instrument and optimized for high throughput.
The simple interferometer response potentially allows this technique to be
operated at other wavelengths independent of popular iodine reference sources
to search for planets around early type stars, white dwarfs, and M, L and T
dwarfs for the first time.Comment: Accepted to ApJ Letters for publication, 11 pages, 2 figure
Numerical simulations of surface convection in a late M-dwarf
Based on detailed 2D and 3D numerical radiation-hydrodynamics (RHD)
simulations of time-dependent compressible convection, we have studied the
dynamics and thermal structure of the convective surface layers of a
prototypical late-type M-dwarf (Teff~2800K log(g)=5.0, solar chemical
composition). The RHD models predict stellar granulation qualitatively similar
to the familiar solar pattern. Quantitatively, the granular cells show a
convective turn-over time scale of ~100s, and a horizontal scale of 80km; the
relative intensity contrast of the granular pattern amounts to 1.1%, and
root-mean-square vertical velocities reach 240m/s at maximum. Deviations from
radiative equilibrium in the higher, formally convectively stable atmospheric
layers are found to be insignificant allowing a reliable modeling of the
atmosphere with 1D standard model atmospheres. A mixing-length parameter of
alpha=2.1 provides the best representation of the average thermal structure of
the RHD model atmosphere while alternative values are found when fitting the
asymptotic entropy encountered in deeper layers of the stellar envelope
alpha=1.5, or when matching the vertical velocity field alpha=3.5. The close
correspondence between RHD and standard model atmospheres implies that
presently existing discrepancies between observed and predicted stellar colors
in the M-dwarf regime cannot be traced back to an inadequate treatment of
convection in the 1D standard models. The RHD models predict a modest extension
of the convectively mixed region beyond the formal Schwarzschild stability
boundary which provides hints for the distribution of dust grains in cooler
(brown dwarf) atmospheres.Comment: 19 pages, 16 figures, accepted for publication in A&
Lead‐Free Semiconductors: Soft Chemistry, Dimensionality Control, and Manganese‐Doping of Germanium Halide Perovskites
Lead halide perovskites have drawn enormous interest due to their exceptional photovoltaic and optoelectronic properties. However, the heavy metal lead is harmful to humans and the environment resulting in a need for strategies to replace this toxic element. Herein, we report a facile aqueous synthesis of CsGeX3 (X=I, Br) perovskite nanocrystals with size control achieved by varying the concentration of a cysteammonium halide ligand. We observe a variety of morphologies including pyramidal, hexagonal, and spheroidal. CsGeX3 nanocrystals undergo a lattice expansion due to partial replacement of Cs+ with larger cysteNH3+ cations into their lattice. We successfully dope Mn2+into the CsGeX3 lattice for the first time with incorporation of up to 29% in bulk and 16% in nano samples. XRD peak shifts and EPR hyperfine splitting strongly indicate that Mn2+ is doped into the lattice. Our results introduce a new member to the lead‐free halide perovskite family and set the fundamental stage for their use in optoelectronic devices
Spectroscopic analysis of interaction between an EIT wave and a coronal upflow region
We report a spectroscopic analysis of an EIT wave event that occurred in
active region 11081 on 2010 June 12 and was associated with an M2.0 class
flare. The wave propagated near circularly. The south-eastern part of the wave
front passed over an upflow region nearby a magnetic bipole. Using EIS raster
observations for this region, we studied the properties of plasma dynamics in
the wave front, as well as the interaction between the wave and the upflow
region. We found a weak blueshift for the Fe XII {\lambda}195.12 and Fe XIII
{\lambda}202.04 lines in the wave front. The local velocity along the solar
surface, which is deduced from the line of sight velocity in the wave front and
the projection effect, is much lower than the typical propagation speed of the
wave. A more interesting finding is that the upflow and non-thermal velocities
in the upflow region are suddenly diminished after the transit of the wave
front. This implies a significant change of magnetic field orientation when the
wave passed. As the lines in the upflow region are redirected, the velocity
along the line of sight is diminished as a result. We suggest that this
scenario is more in accordance with what was proposed in the field-line
stretching model of EIT waves.Comment: 13 pages, 7 figures, accepted for publication in Ap
Multiwavelength Observations of Small-Scale Reconnection Events triggered by Magnetic Flux Emergence in the Solar Atmosphere
The interaction between emerging magnetic flux and the pre-existing ambient
field has become a "hot" topic for both numerical simulations and
high-resolution observations of the solar atmosphere. The appearance of
brightenings and surges during episodes of flux emergence is believed to be a
signature of magnetic reconnection processes. We present an analysis of a
small-scale flux emergence event in NOAA 10971, observed simultaneously with
the Swedish 1-m Solar Telescope on La Palma and the \emph{Hinode} satellite
during a joint campaign in September 2007. Extremely high-resolution G-band,
H, and \ion{Ca}{2} H filtergrams, \ion{Fe}{1} and \ion{Na}{1}
magnetograms, EUV raster scans, and X-ray images show that the emerging region
was associated with chromospheric, transition region and coronal brightenings,
as well as with chromospheric surges. We suggest that these features were
caused by magnetic reconnection at low altitude in the atmosphere. To support
this idea, we perform potential and linear force-free field extrapolations
using the FROMAGE service. The extrapolations show that the emergence site is
cospatial with a 3D null point, from which a spine originates. This magnetic
configuration and the overall orientation of the field lines above the emerging
flux region are compatible with the structures observed in the different
atmospheric layers, and remain stable against variations of the force-free
field parameter. Our analysis supports the predictions of recent 3D numerical
simulations that energetic phenomena may result from the interaction between
emerging flux and the pre-existing chromospheric and coronal field.Comment: In press for Ap
Space-Time Distribution of G-Band and Ca II H-Line Intensity Oscillations in Hinode/SOT-FG Observations
We study the space-time distributions of intensity fluctuations in 2 - 3 hour
sequences of multi-spectral, high-resolution, high-cadence broad-band
filtergram images (BFI) made by the SOT-FG system aboard the Hinode spacecraft.
In the frequency range 5.5 < f < 8.0 mHz both G-band and Ca II H-line
oscillations are suppressed in the presence of magnetic fields, but the
suppression disappears for f > 10 mHz. By looking at G-band frequencies above
10 mHz we find that the oscillatory power, both at these frequencies and at
lower frequencies too, lies in a mesh pattern with cell scale 2 - 3 Mm, clearly
larger than normal granulation, and with correlation times on the order of
hours. The mesh pattern lies in the dark lanes between stable cells found in
time-integrated G-band intensity images. It also underlies part of the bright
pattern in time-integrated H-line emission. This discovery may reflect
dynamical constraints on the sizes of rising granular convection cells together
with the turbulence created in strong intercellular downflows.Comment: 24 pages, 15 figure
Automated Detection and Tracking of Solar Magnetic Bright Points
Magnetic Bright Points (MBPs) in the internetwork are among the smallest
objects in the solar photosphere and appear bright against the ambient
environment. An algorithm is presented that can be used for the automated
detection of the MBPs in the spatial and temporal domains. The algorithm works
by mapping the lanes through intensity thresholding. A compass search, combined
with a study of the intensity gradient across the detected objects, allows the
disentanglement of MBPs from bright pixels within the granules. Object growing
is implemented to account for any pixels that might have been removed when
mapping the lanes. The images are stabilized by locating long-lived objects
that may have been missed due to variable light levels and seeing quality.
Tests of the algorithm employing data taken with the Swedish Solar Telescope
(SST), reveal that ~90% of MBPs within a 75"x 75" field of view are detected
Disintegration of Magnetic Flux in Decaying Sunspots as Observed with the Hinode SOT
Continuous observations of sunspot penumbrae with the Solar Optical Telescope
aboard \textit{Hinode} clearly show that the outer boundary of the penumbra
fluctuates around its averaged position. The penumbral outer boundary moves
inward when granules appear in the outer penumbra. We discover that such
granules appear one after another while moving magnetic features (MMFs) are
separating from the penumbral ``spines'' (penumbral features that have stronger
and more vertical fields than those of their surroundings). These granules that
appear in the outer penumbra often merge with bright features inside the
penumbra that move with the spines as they elongate toward the moat region.
This suggests that convective motions around the penumbral outer boundary are
related to the disintegration of magnetic flux in the sunspot. We also find
that dark penumbral filaments frequently elongate into the moat region in the
vicinity of MMFs that detach from penumbral spines. Such elongating dark
penumbral filaments correspond to nearly horizontal fields extending from the
penumbra. Pairs of MMFs with positive and negative polarities are sometimes
observed along the elongating dark penumbral filaments. This strongly supports
the notion that such elongating dark penumbral filaments have magnetic fields
with a ``sea serpent''-like structure. Evershed flows, which are associated
with the penumbral horizontal fields, may be related to the detachment of the
MMFs from the penumbral spines, as well as to the formation of the MMFs along
the dark penumbral filaments that elongate into the moat region.Comment: Accepted for publication in Ap
- …