35 research outputs found
Anomalous flows in a sunspot penumbra
High-resolution spectropolarimetric observations of active region NOAA 11271
were obtained with the spectro-polarimeter on board Hinode to analyze the
properties of an anomalous flow in the photosphere in a sunspot penumbra. We
detect a blue-shifted feature that appeared on the limb-side penumbra of a
sunspot and that was present intermittently during the next two hours. It
exhibited a maximum blue-shift of 1.6 km/s, an area of 5.2 arcsec^2, and an
uninterrupted lifetime of 1 hr. The blue-shifted feature, when present, lies
parallel to red-shifts. Both blue and red shifts flank a highly
inclined/horizontal magnetic structure that is radially oriented in the
penumbra. The low-cadence SP maps reveal changes in size, radial position in
the penumbra and line-of-sight velocity of the blue-shifted feature, from one
scan to the other. There was an increase of nearly 500 G in the field strength
and a marginal reduction in the field inclination of about 10 deg with the
onset of the blue-shifts. In the chromosphere, intense, arc-shaped brightenings
were observed close to the location of the blue-shifts, that extend from the
edge of the umbral core to the penumbra-quiet Sun boundary. The strongest and
largest brightenings were observed about 30 min after the strongest blue-shifts
were detected at the photosphere. The close spatial proximity of the two
phenomenon strongly suggests a causal relationship. The blue-shifted feature
represents plasma motion that could be related to a magnetic structure that
rises in the solar atmosphere and subsequently reconnects with the ambient
chromospheric magnetic field of the sunspot or an inverse Evershed flow, which
would be unique in the photosphere. This transient phenomena is presumably
related to the dynamic stability of the sunspot because the corresponding
umbral core separated two days later at the location of the blue-shifts and
fragmented subsequently.Comment: Accepted for publication in A&A: 8 pages, 8 figure
Formation of a penumbra in a decaying sunspot
Context : Penumbrae are an important characteristic of sunspots, whose
formation is intricately related to the nature of sub-photospheric magnetic
fields. Aims : We study the formation of a penumbra in a decaying sunspot and
compare its properties with those seen during the development of a proto-spot.
Methods : High-resolution spectropolarimetric observations of active region
NOAA 11283 were obtained from the spectro-polarimeter on board Hinode. These
were complemented with full-disk filtergrams of continuum intensity,
line-of-sight magnetograms, and dopplergrams from the Helioseismic and Magnetic
Imager at high cadence. Results : The formation of a penumbra in the decaying
sunspot occurs after the coalescence of the sunspot with a magnetic
fragment/pore, which initially formed in the quiet Sun close to an emerging
flux region. At first, a smaller set of penumbral filaments develop near the
location of the merger with very bright penumbral grains with intensities of
1.2 I_QS, upflows of 4 km/s, and a lifetime of 10 hr. During the decay of these
filaments, a larger segment of a penumbra forms at the location of the
coalescence. These new filaments are characterized by nearly supersonic
downflows of 6.5 km/s that change to a regular Evershed flow nearly 3 hr later.
Conclusions : The coalescence of the pore with the decaying sunspot provided
sufficient magnetic flux for the penumbra to form in the sunspot. The emerging
flux region could have played a decisive role in this process because the
formation occurred at the location of the merger and not on the opposite side
of the sunspot.Comment: Accepted for publication in A&A Letters, 6 pages, 4 figure
Properties of Umbral Dots from Stray Light Corrected Hinode Filtergrams
High resolution blue continuum filtergrams from Hinode are employed to study
the umbral fine structure of a regular unipolar sunspot. The removal of
scattered light from the images increases the rms contrast by a factor of 1.45
on average. Improvement in image contrast renders identification of short
filamentary structures resembling penumbrae that are well separated from the
umbra-penumbra boundary and comprise bright filaments/grains flanking dark
filaments. Such fine structures were recently detected from ground based
telescopes and have now been observed with Hinode. A multi-level tracking
algorithm was used to identify umbral dots in both the uncorrected and
corrected images and to track them in time. The distribution of the values
describing the photometric and geometric properties of umbral dots are more
easily affected by the presence of stray light while it is less severe in the
case of kinematic properties. Statistically, umbral dots exhibit a peak
intensity, effective diameter, lifetime, horizontal speed and a trajectory
length of 0.29 I_QS, 272 km, 8.4 min, 0.45 km/s and 221 km respectively. The 2
hr 20 min time sequence depicts several locations where umbral dots tend to
appear and disappear repeatedly with various time intervals. The correction for
scattered light in the Hinode filtergrams facilitates photometry of umbral fine
structure which can be related to results obtained from larger telescopes and
numerical simulations.Comment: Accepted for publication in ApJ : 10 pages, 10 figures, 3 table
A study of the propagation of magnetoacoustic waves in small-scale magnetic fields using solar photospheric and chromospheric Dopplergrams: HMI/SDO and MAST observations
In this work, we present a study of the propagation of low-frequency
magneto-acoustic waves into the solar chromosphere within small-scale inclined
magnetic fields over a quiet-magnetic network region utilizing
near-simultaneous photospheric and chromospheric Dopplergrams obtained from the
HMI instrument onboard SDO spacecraft and the Multi-Application Solar Telescope
(MAST) operational at the Udaipur Solar Observatory, respectively. Acoustic
waves are stochastically excited inside the convection zone of the Sun and
intermittently interact with the background magnetic fields resulting into
episodic signals. In order to detect these episodic signals, we apply the
wavelet transform technique to the photospheric and chromospheric velocity
oscillations in magnetic network regions. The wavelet power spectrum over
photospheric and chromospheric velocity signals show a one-to-one
correspondence between the presence of power in the 2.5-4 mHz band. Further, we
notice that power in the 2.5-4 mHz band is not consistently present in the
chromospheric wavelet power spectrum despite its presence in the photospheric
wavelet power spectrum. This indicates that leakage of photospheric
oscillations (2.5-4 mHz band) into the higher atmosphere is not a continuous
process. The average phase and coherence spectra estimated from these
photospheric and chromospheric velocity oscillations illustrate the propagation
of photospheric oscillations (2.5-4 mHz) into the solar chromosphere along the
inclined magnetic fields. Additionally, chromospheric power maps estimated from
the MAST Dopplergrams also show the presence of high-frequency acoustic halos
around relatively high magnetic concentrations, depicting the refraction of
high-frequency fast mode waves around vA ~ vs layer in the solar atmosphere.Comment: 16 pages, 7 figures, Accepted for Publication in Journal of
Atmospheric and Solar-Terrestrial Physics (Special Issue of STP-15