2,904 research outputs found
A distinct magnetic property of the inner penumbral boundary
A sunspot emanates from a growing pore or protospot. In order to trigger the
formation of a penumbra, large inclinations at the outskirts of the protospot
are necessary. The penumbra develops and establishes by colonising both umbral
areas and granulation. Evidence for a unique stable boundary value for the
vertical component of the magnetic field strength, ,
was found along the umbra-penumbra boundary of developed sunspots. We use
broadband G-band images and spectropolarimetric GFPI/VTT data to study the
evolution of and the vertical component of the magnetic field on a forming
umbra-penumbra boundary. For comparison with stable sunspots, we also analyse
the two maps observed by Hinode/SP on the same spot after the penumbra formed.
The vertical component of the magnetic field, , at the
umbra-penumbra boundary increases during penumbra formation owing to the
incursion of the penumbra into umbral areas. After 2.5 hours, the penumbra
reaches a stable state as shown by the GFPI data. At this stable stage, the
simultaneous Hinode/SP observations show a value comparable to
that of umbra-penumbra boundaries of fully fledged sunspots. We confirm that
the umbra-penumbra boundary, traditionally defined by an intensity threshold,
is also characterised by a distinct canonical magnetic property, namely by
. During the penumbra formation process, the inner
penumbra extends into regions where the umbra previously prevailed. Hence, in
areas where , the magneto-convection
mode operating in the umbra turns into a penumbral mode. Eventually, the inner
penumbra boundary settles at , which hints toward the
role of as inhibitor of the penumbral mode of
magneto-convection.Comment: Accepted as a Letter to A&A. Reproduced with permission from
Astronomy & Astrophysics, \copyright ES
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
The formation of sunspot penumbra. I. Magnetic field properties
We study the formation of a sunspot penumbra in the active region NOAA11024.
We simultaneously observed the Stokes parameters of the photospheric iron lines
at 1089.6 nm with the TIP and 617.3 nm with the GFPI spectropolarimeters along
with broad-band images using G-band and CaIIK filters at the German VTT. The
formation of the penumbra is intimately related to the inclined magnetic field.
Within 4.5 h observing time, the magnetic flux of the penumbra increases from
9.7E+20 to 18.2E+20 Mx, while the magnetic flux of the umbra remains constant
at about 3.8E+20 Mx. Magnetic flux in the immediate surroundings is
incorporated into the spot, and new flux is supplied via small flux patches
(SFPs), which on average have a flux of 2-3E+18 Mx. The spot's flux increase
rate of 4.2E+16 Mx/s corresponds to the merging of one SFP per minute. We also
find that during the formation of the spot penumbra: a) the maximum magnetic
field strength of the umbra does not change, b) the magnetic neutral line keeps
the same position relative to the umbra, c) the new flux arrives on the
emergence side of the spot while the penumbra forms on the opposite side, d)
the average LRF inclination of the light bridges decreases from 50 to 37 deg,
and e) as the penumbra develops, the mean magnetic field strength at the spot
border decreases from 1.0 to 0.8 kG. The SFPs associated with elongated
granules are the building blocks of structure formation in active regions.
During the sunspot formation, their contribution is comparable to the
coalescence of pores. A quiet environment in the surroundings is important for
penumbral formation. As remnants of trapped granulation between merging pores,
the light bridges are found to play a crucial role in the formation process.
They seem to channel the magnetic flux through the spot during its formation.
Light bridges are also the locations where the first penumbral filaments form.Comment: 14 pages, 12 figures, accepted by A&
Precursor of Sunspot Penumbral Formation discovered with Hinode SOT Observations
We present observations of a precursory signature that would be helpful for
understanding the formation process of sunspot penumbrae. The Hinode Solar
Optical Telescope successfully captured the entire evolution of a sunspot from
the pore to a large well-developed sunspot with penumbra in an emerging flux
region appeared in NOAA Active Region 11039. We found an annular zone (width
3"-5") surrounding the umbra (pore) in Ca II H images before the penumbra is
formed around the umbra. The penumbra was developed as if to fill the annular
zone. The annular zone shows weak magnetogram signals, meaning less magnetic
flux or highly inclined fields there. Pre-existing ambient magnetic field
islands were moved to be distributed at the outer edge of the annular zone and
did not come into the zone. There is no strong systematic flow patterns in the
zone, but we occasionally observed small magnetic flux patches streaming out.
The observations indicate that the annular zone is different from sunspot moat
flow region and that it represents the structure in the chromosphere. We
conclude that the annular zone reflects the formation of a magnetic canopy
overlying the region surrounding the umbra at the chromospheric level, much
before the formation of the penumbra at the photospheric level. The magnetic
field structure in the chromosphere needs to be considered in the formation
process of the penumbrae.Comment: 16 pages, 5 figures, accepted for ApJ Letter
The magnetic nature of umbra-penumbra boundary in sunspots
Sunspots are the longest-known manifestation of solar activity, and their
magnetic nature has been known for more than a century. Despite this, the
boundary between umbrae and penumbrae, the two fundamental sunspot regions, has
hitherto been solely defined by an intensity threshold. Here, we aim at
studying the magnetic nature of umbra-penumbra boundaries in sunspots of
different sizes, morphologies, evolutionary stages, and phases of the solar
cycle. We used a sample of 88 scans of the Hinode/SOT spectropolarimeter to
infer the magnetic field properties in at the umbral boundaries. We defined
these umbra-penumbra boundaries by an intensity threshold and performed a
statistical analysis of the magnetic field properties on these boundaries. We
statistically prove that the umbra-penumbra boundary in stable sunspots is
characterised by an invariant value of the vertical magnetic field component:
the vertical component of the magnetic field strength does not depend on the
umbra size, its morphology, and phase of the solar cycle. With the statistical
Bayesian inference, we find that the strength of the vertical magnetic field
component is, with a likelihood of 99\%, in the range of 1849-1885 G with the
most probable value of 1867 G. In contrast, the magnetic field strength and
inclination averaged along individual boundaries are found to be dependent on
the umbral size: the larger the umbra, the stronger and more horizontal the
magnetic field at its boundary. The umbra and penumbra of sunspots are
separated by a boundary that has hitherto been defined by an intensity
threshold. We now unveil the empirical law of the magnetic nature of the
umbra-penumbra boundary in stable sunspots: it is an invariant vertical
component of the magnetic field.Comment: accepted as A&A lette
Supersonic Downflows at the Umbra-Penumbra Boundary of Sunspots
High resolution spectropolarimetric observations of 3 sunspots taken with
Hinode demonstrate the existence of supersonic downflows at or close to the
umbra-penumbra boundary which have not been reported before. These downflows
are confined to large patches, usually encompassing bright penumbral filaments,
and have lifetimes of more than 14 hr. The presence of strong downflows in the
center-side penumbra near the umbra rules out an association with the Evershed
flow. Chromospheric filtergrams acquired close to the time of the
spectropolarimetric measurements show large, strong, and long-lived
brightenings in the neighborhood of the downflows. The photospheric intensity
also exhibit persistent brightenings comparable to the quiet Sun.
Interestingly, the orientation of the penumbral filaments at the site of the
downflows is similar to that resulting from the reconnection process described
by Ryutova et al. The existence of such downflows in the inner penumbra
represents a challenge for numerical models of sunspots because they have to
explain them in terms of physical processes likely affecting the chromosphere.Comment: Accepted for publication in Ap
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