40 research outputs found
Dynamics of multi-cored magnetic structures in the quiet Sun
We report on the dynamical interaction of quiet-Sun magnetic fields and
granular convection in the solar photosphere as seen by \textsc{Sunrise}. We
use high spatial resolution (0\farcs 15--0\farcs 18) and temporal cadence (33
s) spectropolarimetric Imaging Magnetograph eXperiment data, together with
simultaneous CN and Ca\,\textsc{ii}\,H filtergrams from \textsc{Sunrise} Filter
Imager. We apply the SIR inversion code to the polarimetric data in order to
infer the line of sight velocity and vector magnetic field in the photosphere.
The analysis reveals bundles of individual flux tubes evolving as a single
entity during the entire 23 minute data set. The group shares a common canopy
in the upper photospheric layers, while the individual tubes continually
intensify, fragment and merge in the same way that chains of bright points in
photometric observations have been reported to do. The evolution of the tube
cores are driven by the local granular convection flows. They intensify when
they are "compressed" by surrounding granules and split when they are
"squeezed" between two moving granules. The resulting fragments are usually
later regrouped in intergranular lanes by the granular flows. The continual
intensification, fragmentation and coalescence of flux results in magnetic
field oscillations of the global entity. From the observations we conclude that
the magnetic field oscillations first reported by \citet{2011ApJ...730L..37M}
correspond to the forcing by granular motions and not to characteristic
oscillatory modes of thin flux tubes.Comment: 12 pages, 7 figures. Accepted in ApJ. Animation 1 can be downloaded
from: http://spg.iaa.es/download
The history of a quiet-Sun magnetic element revealed by IMaX/SUNRISE
Isolated flux tubes are considered to be fundamental magnetic building blocks
of the solar photosphere. Their formation is usually attributed to the
concentration of magnetic field to kG strengths by the convective collapse
mechanism. However, the small size of the magnetic elements in quiet-Sun areas
has prevented this scenario from being studied in fully resolved structures.
Here we report on the formation and subsequent evolution of one such
photospheric magnetic flux tube, observed in the quiet Sun with unprecedented
spatial resolution (0\farcs 15 - 0\farcs 18) and high temporal cadence (33 s).
The observations were acquired by the Imaging Magnetograph Experiment (IMaX)
aboard the \textsc{Sunrise} balloon-borne solar observatory. The equipartition
field strength magnetic element is the result of the merging of several same
polarity magnetic flux patches, including a footpoint of a previously emerged
loop. The magnetic structure is then further intensified to kG field strengths
by convective collapse. The fine structure found within the flux concentration
reveals that the scenario is more complex than can be described by a thin flux
tube model with bright points and downflow plumes being established near the
edges of the kG magnetic feature. We also observe a daisy-like alignment of
surrounding granules and a long-lived inflow towards the magnetic feature.
After a subsequent weakening process, the field is again intensified to kG
strengths. The area of the magnetic feature is seen to change in anti-phase
with the field strength, while the brightness of the bright points and the
speed of the downflows varies in phase. We also find a relation between the
brightness of the bright point and the presence of upflows within it.Comment: 13 pages. Accepted in ApJ. Animation 1 can be viewed and downloaded
from: http://spg.iaa.es/downloads.as
Small-scale Dynamo in Cool Stars: I. Changes in stratification and near-surface convection for main-sequence spectral types
Some of the small-scale solar magnetic flux can be attributed to a
small-scale dynamo (SSD) operating in the near-surface convection. The SSD
fields have consequences for solar granular convection, basal flux, as well as
chromospheric heating. A similar SSD mechanism is expected to be active in the
near-surface convection of other cool main-sequence stars, but this has never
been investigated. We aim to investigate changes in stratification and
convection due to inclusion of SSD fields for F3V, G2V, K0V and M0V spectral
types in the near-surface convection. 3D magnetohydrodynamic (MHD) models of
the four stellar boxes, covering the subsurface convection zone up to the lower
photosphere in a small cartesian box, are studied using the \textit{MURaM}
radiative-MHD simulation code. The SSD runs are compared against reference
hydrodynamic runs. An SSD is found to efficiently produce magnetic field with
energies ranging between 5\% to 80\% of the plasma kinetic energy at different
depths. This ratio tends to be larger for larger . The
relative change in density and gas pressure stratification for the deeper
convective layers due to SSD magnetic fields is negligible, except for the
F-star. For the F-star, there is a substantial reduction in convective
velocities due to Lorentz force feedback from magnetic fields, which, in turn,
reduces the turbulent pressure. SSD in near-surface convection for cool
main-sequence stars introduces small but significant changes in thermodynamic
stratification (especially for the F-star) due to reduction in convective
velocities.Comment: 10 pages, 7 figures, accepted for publication in A&
An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853
The coupling of the electromagnetic field directly with gravitational gauge
fields leads to new physical effects that can be tested using astronomical
data. Here we consider a particular case for closer scrutiny, a specific
nonminimal coupling of torsion to electromagnetism, which enters into a
metric-affine geometry of space-time. We show that under the assumption of this
nonminimal coupling, spacetime is birefringent in the presence of such a
gravitational field. This leads to the depolarization of light emitted from
extended astrophysical sources. We use polarimetric data of the magnetic white
dwarf to set strong constraints on the essential coupling
constant for this effect, giving k^2 \lsim (19 {m})^2 .Comment: Statements about Moffat's NGT modified. Accepted for publication in
Phys.Rev.
A Millennium Scale Sunspot Number Reconstruction: Evidence For an Unusually Active Sun Since the 1940's
The extension of the sunspot number series backward in time is of
considerable interest for dynamo theory, solar, stellar, and climate research.
We have used records of the Be-10 concentration in polar ice to reconstruct the
average sunspot activity level for the period between the year 850 to the pr
esent. Our method uses physical models for processes connecting the Be-10
concentration with the sunspot number. The reconstruction shows reliably that
the period of high solar activity during the last 60 years is unique throughout
the past 1150 years. This nearly triples the time interval for which such a
statement could be made previouslyComment: 4 pages, LaTeX, revtex4 macros; Phys. Rev. Let., in pres
The influence of metallicity on stellar differential rotation and magnetic activity
Observations of Sun-like stars over the last half-century have improved our
understanding of how magnetic dynamos, like that responsible for the 11-year
solar cycle, change with rotation, mass and age. Here we show for the first
time how metallicity can affect a stellar dynamo. Using the most complete set
of observations of a stellar cycle ever obtained for a Sun-like star, we show
how the solar analog HD 173701 exhibits solar-like differential rotation and a
7.4-year activity cycle. While the duration of the cycle is comparable to that
generated by the solar dynamo, the amplitude of the brightness variability is
substantially stronger. The only significant difference between HD 173701 and
the Sun is its metallicity, which is twice the solar value. Therefore, this
provides a unique opportunity to study the effect of the higher metallicity on
the dynamo acting in this star and to obtain a comprehensive understanding of
the physical mechanisms responsible for the observed photometric variability.
The observations can be explained by the higher metallicity of the star, which
is predicted to foster a deeper outer convection zone and a higher facular
contrast, resulting in stronger variability.Comment: Submitted to Ap
LEMUR: Large European Module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission
Understanding the solar outer atmosphere requires concerted, simultaneous
solar observations from the visible to the vacuum ultraviolet (VUV) and soft
X-rays, at high spatial resolution (between 0.1" and 0.3"), at high temporal
resolution (on the order of 10 s, i.e., the time scale of chromospheric
dynamics), with a wide temperature coverage (0.01 MK to 20 MK, from the
chromosphere to the flaring corona), and the capability of measuring magnetic
fields through spectropolarimetry at visible and near-infrared wavelengths.
Simultaneous spectroscopic measurements sampling the entire temperature range
are particularly important.
These requirements are fulfilled by the Japanese Solar-C mission (Plan B),
composed of a spacecraft in a geosynchronous orbit with a payload providing a
significant improvement of imaging and spectropolarimetric capabilities in the
UV, visible, and near-infrared with respect to what is available today and
foreseen in the near future.
The Large European Module for solar Ultraviolet Research (LEMUR), described
in this paper, is a large VUV telescope feeding a scientific payload of
high-resolution imaging spectrographs and cameras. LEMUR consists of two major
components: a VUV solar telescope with a 30 cm diameter mirror and a focal
length of 3.6 m, and a focal-plane package composed of VUV spectrometers
covering six carefully chosen wavelength ranges between 17 and 127 nm. The
LEMUR slit covers 280" on the Sun with 0.14" per pixel sampling. In addition,
LEMUR is capable of measuring mass flows velocities (line shifts) down to 2
km/s or better.
LEMUR has been proposed to ESA as the European contribution to the Solar C
mission.Comment: 35 pages, 14 figures. To appear on Experimental Astronom
Achievements of Hinode in the first eleven years
Hinode is Japan’s third solar mission following Hinotori (1981–1982) and Yohkoh (1991–2001): it was launched on 2006 September 22 and is in operation currently. Hinode carries three instruments: the Solar Optical Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These instruments were built under international collaboration with the National Aeronautics and Space Administration and the UK Science and Technology Facilities Council, and its operation has been contributed to by the European Space Agency and the Norwegian Space Center. After describing the satellite operations and giving a performance evaluation of the three instruments, reviews are presented on major scientific discoveries by Hinode in the first eleven years (one solar cycle long) of its operation. This review article concludes with future prospects for solar physics research based on the achievements of Hinode