10 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
Dvou-dimenzionalní spektropolarimetrie sluneční skvrny
Matematicko-fyzikální fakultaFaculty of Mathematics and Physic
3D view of transient horizontal magnetic fields in the photosphere
We infer the 3D magnetic structure of a transient horizontal magnetic field
(THMF) during its evolution through the photosphere using SIRGAUS inversion
code. The SIRGAUS code is a modified version of SIR (Stokes Inversion based on
Response function), and allows for retrieval of information on the magnetic and
thermodynamic parameters of the flux tube embedded in the atmosphere from the
observed Stokes profiles. Spectro-polarimetric observations of the quiet Sun at
the disk center were performed with the Solar Optical Telescope (SOT) on board
Hinode with Fe I 630.2 nm lines. Using repetitive scans with a cadence of 130
s, we first detect the horizontal field that appears inside a granule, near its
edge. On the second scan, vertical fields with positive and negative polarities
appear at both ends of the horizontal field. Then, the horizontal field
disappears leaving the bipolar vertical magnetic fields. The results from the
inversion of the Stokes spectra clearly point to the existence of a flux tube
with magnetic field strength of G rising through the line forming
layer of the Fe I 630.2 nm lines. The flux tube is located at around
at =0 s and around
at =130 s. At =260 s the horizontal part is already above
the line forming region of the analyzed lines. The observed Doppler velocity is
maximally 3 km s, consistent with the upward motion of the structure as
retrieved from the SIRGAUS code. The vertical size of the tube is smaller than
the thickness of the line forming layer. The THMF has a clear
-shaped-loop structure with the apex located near the edge of a
granular cell. The magnetic flux carried by this THMF is estimated to be
Mx.Comment: 35 pages, 9 figures, Accepted for publication in Ap
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
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 European Solar Telescope
The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Telescope Heliographique pour l'etude du Magnetisme et des Instabilites Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems.ISSN:0004-6361ISSN:1432-074