19 research outputs found
Dark structures in sunspot light bridges
We present unprecedented high-resolution TiO images and Fe I 1565 nm
spectropolarimetric data of two light bridges taken by the 1.6-m Goode Solar
Telescope at Big Bear Solar Observatory. In the first light bridge (LB1), we
find striking knot-like dark structures within the central dark lane. Many dark
knots show migration away from the penumbra along the light bridge. The sizes,
intensity depressions and apparent speeds of their proper motion along the
light bridges of 33 dark knots identified from the TiO images are mainly in the
ranges of 80200~km, 30\%50\%, and 0.31.2~km~s,
respectively. In the second light bridge (LB2), a faint central dark lane and
striking transverse intergranular lanes were observed. These intergranular
lanes have sizes and intensity depressions comparable to those of the dark
knots in LB1, and also migrate away from the penumbra at similar speeds. Our
observations reveal that LB2 is made up of a chain of evolving convection
cells, as indicated by patches of blue shift surrounded by narrow lanes of red
shift. The central dark lane generally corresponds to blueshifts, supporting
the previous suggestion of central dark lanes being the top parts of convection
upflows. In contrast, the intergranular lanes are associated with redshifts and
located at two sides of each convection cell. The magnetic fields are stronger
in intergranular lanes than in the central dark lane. These results suggest
that these intergranular lanes are manifestations of convergent convective
downflows in the light bridge. We also provide evidence that the dark knots
observed in LB1 may have a similar origin.Comment: 6 figure
Inferring Line-of-Sight Velocities and Doppler Widths from Stokes Profiles of GST/NIRIS Using Stacked Deep Neural Networks
Obtaining high-quality magnetic and velocity fields through Stokes inversion
is crucial in solar physics. In this paper, we present a new deep learning
method, named Stacked Deep Neural Networks (SDNN), for inferring line-of-sight
(LOS) velocities and Doppler widths from Stokes profiles collected by the Near
InfraRed Imaging Spectropolarimeter (NIRIS) on the 1.6 m Goode Solar Telescope
(GST) at the Big Bear Solar Observatory (BBSO). The training data of SDNN is
prepared by a Milne-Eddington (ME) inversion code used by BBSO. We
quantitatively assess SDNN, comparing its inversion results with those obtained
by the ME inversion code and related machine learning (ML) algorithms such as
multiple support vector regression, multilayer perceptrons and a pixel-level
convolutional neural network. Major findings from our experimental study are
summarized as follows. First, the SDNN-inferred LOS velocities are highly
correlated to the ME-calculated ones with the Pearson product-moment
correlation coefficient being close to 0.9 on average. Second, SDNN is faster,
while producing smoother and cleaner LOS velocity and Doppler width maps, than
the ME inversion code. Third, the maps produced by SDNN are closer to ME's maps
than those from the related ML algorithms, demonstrating the better learning
capability of SDNN than the ML algorithms. Finally, comparison between the
inversion results of ME and SDNN based on GST/NIRIS and those from the
Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory in
flare-prolific active region NOAA 12673 is presented. We also discuss
extensions of SDNN for inferring vector magnetic fields with empirical
evaluation.Comment: 16 pages, 8 figure
Photospheric Signatures of Granular-scale Flux Emergence and Cancellation at the Penumbral Boundary
We studied flux emergence events of sub-granular scale in a solar active
region. New Solar Telescope (NST) of Big Bear Solar Observatory made it
possible to clearly observe the photospheric signature of flux emergence with
very high spatial (0".11 at 7057{\AA}) and temporal (15 s) resolution. From TiO
observations with the pixel scale of 0".0375, we found several elongated
granule-like features (GLFs) stretching from the penumbral filaments of a
sunspot at a relatively high speed of over 4 km s-1. After a slender arched
darkening appeared at a tip of a penumbral filament, a bright point (BP)
developed and quickly moved away from the filament forming and stretching a
GLF. The size of a GLF was approximately 0.5" wide and 3" long. The moving BP
encountered nearby structures after several minutes of stretching, and a
well-defined elongated shape of a GLF faded away. Magnetograms from SDO/HMI and
NST/IRIM revealed that those GLFs are photospheric indicators of small-scale
flux emergence, and their disappearance is related to magnetic cancellation.
From two well-observed events, we describe detailed development of the
sub-structures of GLFs, and different cancellation processes that each of the
two GLFs underwent.Comment: Accepted for publication in The Astrophysical Journa