1,505 research outputs found
Milne-Eddington inversions of the He I 10830 {\AA} Stokes profiles: Influence of the Paschen-Back effect
The Paschen-Back effect influences the Zeeman sublevels of the He I multiplet
at 10830 {\AA}, leading to changes in strength and in position of the Zeeman
components of these lines. We illustrate the relevance of this effect using
synthetic Stokes profiles of the He I 10830 {\AA} multiplet lines and
investigate its influence on the inversion of polarimetric data. We invert data
obtained with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum
Tower Telescope (VTT). We compare the results of inversions based on synthetic
profiles calculated with and without the Paschen-Back effect being included. We
find that when taking into account the incomplete Paschen-Back effect, on
average 16% higher field strength values are obtained. We also show that this
effect is not the main cause for the area asymmetry exhibited by many He I
10830 Stokes V-profiles. This points to the importance of velocity and magnetic
field gradients over the formation height range of these lines.Comment: Accepted for publication in A&A on Jun 12th 200
Temporal relation between quiet-Sun transverse fields and the strong flows detected by IMaX/SUNRISE
Localized strongly Doppler-shifted Stokes V signals were detected by
IMaX/SUNRISE. These signals are related to newly emerged magnetic loops that
are observed as linear polarization features. We aim to set constraints on the
physical nature and causes of these highly Doppler-shifted signals. In
particular, the temporal relation between the appearance of transverse fields
and the strong Doppler shifts is analyzed in some detail. We calculated the
time difference between the appearance of the strong flows and the linear
polarization. We also obtained the distances from the center of various
features to the nearest neutral lines and whether they overlap or not. These
distances were compared with those obtained from randomly distributed points on
observed magnetograms. Various cases of strong flows are described in some
detail. The linear polarization signals precede the appearance of the strong
flows by on average 84+-11 seconds. The strongly Doppler-shifted signals are
closer (0.19") to magnetic neutral lines than randomly distributed points
(0.5"). Eighty percent of the strongly Doppler-shifted signals are close to a
neutral line that is located between the emerging field and pre-existing
fields. That the remaining 20% do not show a close-by pre-existing field could
be explained by a lack of sensitivity or an unfavorable geometry of the
pre-existing field, for instance, a canopy-like structure. Transverse fields
occurred before the observation of the strong Doppler shifts. The process is
most naturally explained as the emergence of a granular-scale loop that first
gives rise to the linear polarization signals, interacts with pre-existing
fields (generating new neutral line configurations), and produces the observed
strong flows. This explanation is indicative of frequent small-scale
reconnection events in the quiet Sun.Comment: 11 pages, 8 figure
Solar total and spectral irradiance reconstruction over the last 9000 years
Changes in solar irradiance and in its spectral distribution are among the
main natural drivers of the climate on Earth. However, irradiance measurements
are only available for less than four decades, while assessment of solar
influence on Earth requires much longer records. The aim of this work is to
provide the most up-to-date physics-based reconstruction of the solar total and
spectral irradiance (TSI/SSI) over the last nine millennia. The concentrations
of the cosmogenic isotopes 14C and 10Be in natural archives have been converted
to decadally averaged sunspot numbers through a chain of physics-based models.
TSI and SSI are reconstructed with an updated SATIRE model. Reconstructions are
carried out for each isotope record separately, as well as for their composite.
We present the first ever SSI reconstruction over the last 9000 years from the
individual 14C and 10Be records as well as from their newest composite. The
reconstruction employs physics-based models to describe the involved processes
at each step of the procedure. Irradiance reconstructions based on two
different cosmogenic isotope records, those of 14C and 10Be, agree well with
each other in their long-term trends despite their different geochemical paths
in the atmosphere of Earth. Over the last 9000 years, the reconstructed secular
variability in TSI is of the order of 0.11%, or 1.5 W/m2. After the Maunder
minimum, the reconstruction from the cosmogenic isotopes is consistent with
that from the direct sunspot number observation. Furthermore, over the
nineteenth century, the agreement of irradiance reconstructions using isotope
records with the reconstruction from the sunspot number by Chatzistergos et al.
(2017) is better than that with the reconstruction from the WDC-SILSO series
(Clette et al. 2014), with a lower chi-square-value
Solar Irradiance Variability is Caused by the Magnetic Activity on the Solar Surface
The variation in the radiative output of the Sun, described in terms of solar
irradiance, is important to climatology. A common assumption is that solar
irradiance variability is driven by its surface magnetism. Verifying this
assumption has, however, been hampered by the fact that models of solar
irradiance variability based on solar surface magnetism have to be calibrated
to observed variability. Making use of realistic three-dimensional
magnetohydrodynamic simulations of the solar atmosphere and state-of-the-art
solar magnetograms from the Solar Dynamics Observatory, we present a model of
total solar irradiance (TSI) that does not require any such calibration. In
doing so, the modeled irradiance variability is entirely independent of the
observational record. (The absolute level is calibrated to the TSI record from
the Total Irradiance Monitor.) The model replicates 95% of the observed
variability between April 2010 and July 2016, leaving little scope for
alternative drivers of solar irradiance variability at least over the time
scales examined (days to years).Comment: Supplementary Materials;
https://journals.aps.org/prl/supplemental/10.1103/PhysRevLett.119.091102/supplementary_material_170801.pd
The small-scale structure of photospheric convection retrieved by a deconvolution technique applied to Hinode/SP data
Solar granules are bright patterns surrounded by dark channels called
intergranular lanes in the solar photosphere and are a manifestation of
overshooting convection. Observational studies generally find stronger upflows
in granules and weaker downflows in intergranular lanes. This trend is,
however, inconsistent with the results of numerical simulations in which
downflows are stronger than upflows through the joint action of gravitational
acceleration/deceleration and pressure gradients. One cause of this discrepancy
is the image degradation caused by optical distortion and light diffraction and
scattering that takes place in an imaging instrument. We apply a deconvolution
technique to Hinode/SP data in an attempt to recover the original solar scene.
Our results show a significant enhancement in both, the convective upflows and
downflows, but particularly for the latter. After deconvolution, the up- and
downflows reach maximum amplitudes of -3.0 km/s and +3.0 km/s at an average
geometrical height of roughly 50 km, respectively. We found that the velocity
distributions after deconvolution match those derived from numerical
simulations. After deconvolution the net LOS velocity averaged over the whole
FOV lies close to zero as expected in a rough sense from mass balance.Comment: 32 pages, 13 figures, accepted for publication in Ap
Evidence for polar jets as precursors of polar plume formation
Observations from the Hinode/XRT telescope and STEREO/SECCHI/EUVI are
utilized to study polar coronal jets and plumes. The study focuses on the
temporal evolution of both structures and their relationship. The data sample,
spanning April 7-8 2007, shows that over 90% of the 28 observed jet events are
associated with polar plumes. EUV images (STEREO/SECCHI) show plume haze rising
from the location of approximately 70% of the polar X-ray (Hinode/XRT) and EUV
jets, with the plume haze appearing minutes to hours after the jet was
observed. The remaining jets occurred in areas where plume material previously
existed causing a brightness enhancement of the latter after the jet event.
Short-lived, jet-like events and small transient bright points are seen (one at
a time) at different locations within the base of pre-existing long-lived
plumes. X-ray images also show instances (at least two events) of
collimated-thin jets rapidly evolving into significantly wider plume-like
structures that are followed by the delayed appearance of plume haze in the
EUV. These observations provide evidence that X-ray jets are precursors of
polar plumes, and in some cases cause brightenings of plumes. Possible
mechanisms to explain the observed jet and plume relationship are discussed.Comment: 10 pages, 4 figures, accepted as APJ Lette
A comparison of measured and simulated solar network contrast
Long-term trends in the solar spectral irradiance are important to determine
the impact on Earth's climate. These long-term changes are thought to be caused
mainly by changes in the surface area covered by small-scale magnetic elements.
The direct measurement of the contrast to determine the impact of these
small-scale magnetic elements is, however, limited to a few wavelengths, and
is, even for space instruments, affected by scattered light and instrument
defocus. In this work we calculate emergent intensities from 3-D simulations of
solar magneto-convection and validate the outcome by comparing with
observations from Hinode/SOT. In this manner we aim to construct the contrast
at wavelengths ranging from the NUV to the FIR.Comment: Proceedings paper, IAU XXVII, Symposium 264, 3 page
Reconstruction of spectral solar irradiance since 1700 from simulated magnetograms
We present a reconstruction of the spectral solar irradiance since 1700 using
the SATIRE-T2 (Spectral And Total Irradiance REconstructions for the Telescope
era version 2) model. This model uses as input magnetograms simulated with a
surface flux transport model fed with semi-synthetic records of emerging
sunspot groups. We used statistical relationships between the properties of
sunspot group emergence, such as the latitude, area, and tilt angle, and the
sunspot cycle strength and phase to produce semi-synthetic sunspot group
records starting in the year 1700. The semisynthetic records are fed into a
surface flux transport model to obtain daily simulated magnetograms that map
the distribution of the magnetic flux in active regions (sunspots and faculae)
and their decay products on the solar surface. The magnetic flux emerging in
ephemeral regions is accounted for separately based on the concept of extended
cycles whose length and amplitude are linked to those of the sunspot cycles
through the sunspot number. The magnetic flux in each surface component
(sunspots, faculae and network, and ephemeral regions) was used to compute the
spectral and total solar irradiance between the years 1700 and 2009. This
reconstruction is aimed at timescales of months or longer although the model
returns daily values. We found that SATIRE-T2, besides reproducing other
relevant observations such as the total magnetic flux, reconstructs the total
solar irradiance (TSI) on timescales of months or longer in good agreement with
the PMOD composite of observations, as well as with the reconstruction starting
in 1878 based on the RGO-SOON data. The model predicts an increase in the TSI
of 1.2[+0.2, -0.3] Wm-2 between 1700 and the present. The spectral irradiance
reconstruction is in good agreement with the UARS/SUSIM measurements as well as
the Lyman-alpha composite.Comment: 13 pages, 10 figure
Temporal evolution of arch filaments as seen in He I 10830 \r{A}
We study the evolution of an arch filament system (AFS) and of its individual
arch filaments to learn about the processes occurring in them. We observed the
AFS at the GREGOR solar telescope on Tenerife at high cadence with the very
fast spectroscopic mode of the GREGOR Infrared Spectrograph (GRIS) in the He I
10830 \AA\ spectral range. The He I triplet profiles were fitted with analytic
functions to infer line-of-sight (LOS) velocities to follow plasma motions
within the AFS. We tracked the temporal evolution of an individual arch
filament over its entire lifetime, as seen in the He I 10830 \AA\ triplet. The
arch filament expanded in height and extended in length from 13" to 21". The
lifetime of this arch filament is about 30 min. About 11 min after the arch
filament is seen in He I, the loop top starts to rise with an average Doppler
velocity of 6 km/s. Only two minutes later, plasma drains down with supersonic
velocities towards the footpoints reaching a peak velocity of up to 40 km/s in
the chromosphere. The temporal evolution of He I 10830 \AA\ profiles near the
leading pore showed almost ubiquitous dual red components of the He I triplet,
indicating strong downflows, along with material nearly at rest within the same
resolution element during the whole observing time. We followed the arch
filament as it carried plasma during its rise from the photosphere to the
corona. The material then drained toward the photosphere, reaching supersonic
velocities, along the legs of the arch filament. Our observational results
support theoretical AFS models and aids in improving future models.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 15
figures, 1 online movi
Quiet Sun magnetic fields from simultaneous inversions of visible and infrared spectropolarimetric observations
We study the quiet Sun magnetic fields using spectropolarimetric observations
of the infrared and visible Fe I lines at 6301.5, 6302.5, 15648 and 15653 A.
Magnetic field strengths and filling factors are inferred by the simultaneous
fit of the observed Stokes profiles under the MISMA hypothesis. The
observations cover an intra-network region at the solar disk center. We analyze
2280 Stokes profiles whose polarization signals are above noise in the two
spectral ranges, which correspond to 40% of the field of view. Most of these
profiles can be reproduced only with a model atmosphere including 3 magnetic
components with very different field strengths, which indicates the
co-existence of kG and sub-kG fields in our 1.5" resolution elements. We
measure an unsigned magnetic flux density of 9.6 G considering the full field
of view. Half of the pixels present magnetic fields with mixed polarities in
the resolution element. The fraction of mixed polarities increases as the
polarization weakens. We compute the probability density function of finding
each magnetic field strength. It has a significant contribution of kG field
strengths, which concentrates most of the observed magnetic flux and energy.
This kG contribution has a preferred magnetic polarity, while the polarity of
the weak fields is balanced.Comment: 16 pages and 14 figure
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