530 research outputs found
On-Orbit Performance of the Helioseismic and Magnetic Imager Instrument onboard the Solar Dynamics Observatory
The Helioseismic and Magnetic Imager (HMI) instrument is a major component of
NASA's Solar Dynamics Observatory (SDO) spacecraft. Since beginning normal
science operations on 1 May 2010, HMI has operated with remarkable continuity,
e.g. during the more than five years of the SDO prime mission that ended 30
September 2015, HMI collected 98.4% of all possible 45-second velocity maps;
minimizing gaps in these full-disk Dopplergrams is crucial for helioseismology.
HMI velocity, intensity, and magnetic-field measurements are used in numerous
investigations, so understanding the quality of the data is important. We
describe the calibration measurements used to track HMI performance and detail
trends in important instrument parameters during the mission. Regular
calibration sequences provide information used to improve and update the HMI
data calibration. The set-point temperature of the instrument front window and
optical bench is adjusted regularly to maintain instrument focus, and changes
in the temperature-control scheme have been made to improve stability in the
observable quantities. The exposure time has been changed to compensate for a
15% decrease in instrument throughput. Measurements of the performance of the
shutter and tuning mechanisms show that they are aging as expected and continue
to perform according to specification. Parameters of the tunable-optical-filter
elements are regularly adjusted to account for drifts in the central
wavelength. Frequent measurements of changing CCD-camera characteristics, such
as gain and flat field, are used to calibrate the observations. Infrequent
expected events, such as eclipses, transits, and spacecraft off-points,
interrupt regular instrument operations and provide the opportunity to perform
additional calibration. Onboard instrument anomalies are rare and seem to occur
quite uniformly in time. The instrument continues to perform very well.Comment: 50 pages, 18 figures, 20 table
Observation and Modeling of the Solar-Cycle Variation of the Meridional Flow
We present independent observations of the solar-cycle variation of flows
near the solar surface and at a depth of about 60 Mm, in the latitude range
. We show that the time-varying components of the meridional flow
at these two depths have opposite sign, while the time-varying components of
the zonal flow are in phase. This is in agreement with previous results. We
then investigate whether the observations are consistent with a theoretical
model of solar-cycle dependent meridional circulation based on a flux-transport
dynamo combined with a geostrophic flow caused by increased radiative loss in
the active region belt (the only existing quantitative model). We find that the
model and the data are in qualitative agreement, although the amplitude of the
solar-cycle variation of the meridional flow at 60 Mm is underestimated by the
model.Comment: To be published in Solar Physcis Topical Issue "Helioseismology,
Asteroseismology, and MHD Connections
Does the Babcock--Leighton Mechanism Operate on the Sun?
The contribution of the Babcock-Leighton mechanism to the generation of the
Sun's poloidal magnetic field is estimated from sunspot data for three solar
cycles. Comparison of the derived quantities with the A-index of the
large-scale magnetic field suggests a positive answer to the question posed in
the title of this paper.Comment: 5 pages, 2 figures, to apper in Astronomy Letter
Rotation profiles of solar-like stars with magnetic fields
The aim of this work is to investigate rotation profile of solar-like stars
with magnetic fields. A diffusion coefficient of magnetic angular momentum
transport is deduced. Rotating stellar models with different mass are computed
under the effect of the coefficient. Then rotation profiles are obtained from
the theoretical stellar models. The total angular momentum of solar model with
only hydrodynamic instabilities is about 13 times larger than that of the Sun
at the age of the Sun, and this model can not reproduce quasi-solid rotation in
the radiative region. However, not only can the solar model with magnetic
fields reproduce an almost uniform rotation in the radiative region, but its
total angular momentum is consistent with helioseismic result at the level of 3
at the age of the Sun. The rotation of solar-like stars with magnetic
fields is almost uniform in the radiative region. But there is an obvious
transition region of angular velocity between the convective core and the
radiative region of models with 1.2 - 1.5 , where angular velocity
has a sharp radial change, which is different from the rotation profile of the
Sun and massive stars with magnetic fields. Moreover the changes of the angular
velocity in the transition region increase with the increasing in the age and
mass.Comment: Accepted for publication in ChjA
The Radial Distribution of Magnetic Helicity in the Solar Convective Zone: Observations and Dynamo Theory
We continue our attempt to connect observational data on current helicity in
solar active regions with solar dynamo models. In addition to our previous
results about temporal and latitudinal distributions of current helicity
(Kleeorin et al. 2003), we argue that some information concerning the radial
profile of the current helicity averaged over time and latitude can be
extracted from the available observations. The main feature of this
distribution can be presented as follows. Both shallow and deep active regions
demonstrate a clear dominance of one sign of current helicity in a given
hemisphere during the whole cycle. Broadly speaking, current helicity has
opposite polarities in the Northern and Southern hemispheres, although there
are some active regions that violate this polarity rule. The relative number of
active regions violating the polarity rule is significantly higher for deeper
active regions. A separation of active regions into `shallow', `middle' and
`deep' is made by comparing their rotation rate and the helioseismic rotation
law. We use a version of Parker's dynamo model in two spatial dimensions, that
employs a nonlinearity based on magnetic helicity conservation arguments. The
predictions of this model about the radial distribution of solar current
helicity appear to be in remarkable agreement with the available observational
data; in particular the relative volume occupied by the current helicity of
"wrong" sign grows significantly with the depth.Comment: 12 pages, 8 Postscript figures, uses mn2e.cl
On maximum likelihood estimation of the concentration parameter of von Mises–Fisher distributions
A precise asteroseismic age and radius for the evolved Sun-like star KIC 11026764
The primary science goal of the Kepler Mission is to provide a census of
exoplanets in the solar neighborhood, including the identification and
characterization of habitable Earth-like planets. The asteroseismic
capabilities of the mission are being used to determine precise radii and ages
for the target stars from their solar-like oscillations. Chaplin et al. (2010)
published observations of three bright G-type stars, which were monitored
during the first 33.5 days of science operations. One of these stars, the
subgiant KIC 11026764, exhibits a characteristic pattern of oscillation
frequencies suggesting that it has evolved significantly. We have derived
asteroseismic estimates of the properties of KIC 11026764 from Kepler
photometry combined with ground-based spectroscopic data. We present the
results of detailed modeling for this star, employing a variety of independent
codes and analyses that attempt to match the asteroseismic and spectroscopic
constraints simultaneously. We determine both the radius and the age of KIC
11026764 with a precision near 1%, and an accuracy near 2% for the radius and
15% for the age. Continued observations of this star promise to reveal
additional oscillation frequencies that will further improve the determination
of its fundamental properties.Comment: 16 pages, 6 figures, 4 tables, ApJ in pres
Reconstruction of Solar Subsurfaces by Local Helioseismology
Local helioseismology has opened new frontiers in our quest for understanding
of the internal dynamics and dynamo on the Sun. Local helioseismology
reconstructs subsurface structures and flows by extracting coherent signals of
acoustic waves traveling through the interior and carrying information about
subsurface perturbations and flows, from stochastic oscillations observed on
the surface. The initial analysis of the subsurface flow maps reconstructed
from the 5 years of SDO/HMI data by time-distance helioseismology reveals the
great potential for studying and understanding of the dynamics of the quiet Sun
and active regions, and the evolution with the solar cycle. In particular, our
results show that the emergence and evolution of active regions are accompanied
by multi-scale flow patterns, and that the meridional flows display the
North-South asymmetry closely correlating with the magnetic activity. The
latitudinal variations of the meridional circulation speed, which are probably
related to the large-scale converging flows, are mostly confined in shallow
subsurface layers. Therefore, these variations do not necessarily affect the
magnetic flux transport. The North-South asymmetry is also pronounced in the
variations of the differential rotation ("torsional oscillations"). The
calculations of a proxy of the subsurface kinetic helicity density show that
the helicity does not vary during the solar cycle, and that supergranulation is
a likely source of the near-surface helicity.Comment: 17 pages, 10 figures, in "Cartography of the Sun and the Stars",
Editors: Rozelot, Jean-Pierre, Neiner, Corali
Lithium side effects and toxicity: prevalence and management strategies
Despite its virtually universal acceptance as the gold standard in treating bipolar disorder, prescription rates for lithium have been decreasing recently. Although this observation is multifactorial, one obvious potential contributor is the side effect and toxicity burden associated with lithium. Additionally, side effect concerns assuredly play some role in lithium nonadherence. This paper summarizes the knowledge base on side effects and toxicity and suggests optimal management of these problems. Thirst and excessive urination, nausea and diarrhea and tremor are rather common side effects that are typically no more than annoying even though they are rather prevalent. A simple set of management strategies that involve the timing of the lithium dose, minimizing lithium levels within the therapeutic range and, in some situations, the prescription of side effect antidotes will minimize the side effect burden for patients. In contrast, weight gain and cognitive impairment from lithium tend to be more distressing to patients, more difficult to manage and more likely to be associated with lithium nonadherence. Lithium has adverse effects on the kidneys, thyroid gland and parathyroid glands, necessitating monitoring of these organ functions through periodic blood tests. In most cases, lithium-associated renal effects are relatively mild. A small but measurable percentage of lithium-treated patients will show progressive renal impairment. Infrequently, lithium will need to be discontinued because of the progressive renal insufficiency. Lithium-induced hypothyroidism is relatively common but easily diagnosed and treated. Hyperparathyroidism from lithium is a relatively more recently recognized phenomenon
Structure and Rotation of the Solar Interior: Initial Results from the MDI Medium-L Program
The medium-l program of the Michelson Doppler Imager instrument on board SOHO provides continuous observations of oscillation modes of angular degree, l, from 0 to approximately 300. The data for the program are partly processed on board because only about 3% of MDI observations can be transmitted continuously to the ground. The on-board data processing, the main component of which is Gaussian-weighted binning, has been optimized to reduce the negative influence of spatial aliasing of the high-degree oscillation modes. The data processing is completed in a data analysis pipeline at the SOI Stanford Support Center to determine the mean multiplet frequencies and splitting coefficients. The initial results show that the noise in the medium-l oscillation power spectrum is substantially lower than in ground-based measurements. This enables us to detect lower amplitude modes and, thus, to extend the range of measured mode frequencies. This is important for inferring the Sun's internal structure and rotation. The MDI observations also reveal the asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in the upper convective boundary layer. The sound-speed profile inferred from the mean frequencies gives evidence for a sharp variation at the edge of the energy-generating core. The results also confirm the previous finding by the GONG (Gough et al., 1996) that, in a thin layer just beneath the convection zone, helium appears to be less abundant than predicted by theory. Inverting the multiplet frequency splittings from MDI, we detect significant rotational shear in this thin layer. This layer is likely to be the place where the solar dynamo operates. In order to understand how the Sun works, it is extremely important to observe the evolution of this transition layer throughout the 11-year activity cycle
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