1,819 research outputs found
Solar Dynamics, Rotation, Convection and Overshoot
We discuss recent observational, theoretical and modeling progress made in
understanding the Sun's internal dynamics, including its rotation, meridional
flow, convection and overshoot. Over the past few decades, substantial
theoretical and observational effort has gone into appreciating these aspects
of solar dynamics. A review of these observations, related helioseismic
methodology and inference and computational results in relation to these
problems is undertaken here.Comment: 31 pages, 10 figures, Space Science Review
On The Determination of MDI High-Degree Mode Frequencies
The characteristic of the solar acoustic spectrum is such that mode lifetimes
get shorter and spatial leaks get closer in frequency as the degree of a mode
increases for a given order. A direct consequence of this property is that
individual p-modes are only resolved at low and intermediate degrees, and that
at high degrees, individual modes blend into ridges. Once modes have blended
into ridges, the power distribution of the ridge defines the ridge central
frequency and it will mask the true underlying mode frequency. An accurate
model of the amplitude of the peaks that contribute to the ridge power
distribution is needed to recover the underlying mode frequency from fitting
the ridge.
We present the results of fitting high degree power ridges (up to l = 900)
computed from several two to three-month-long time-series of full-disk
observations taken with the Michelson Doppler Imager (MDI) on-board the Solar
and Heliospheric Observatory between 1996 and 1999.
We also present a detailed discussion of the modeling of the ridge power
distribution, and the contribution of the various observational and
instrumental effects on the spatial leakage, in the context of the MDI
instrument. We have constructed a physically motivated model (rather than some
ad hoc correction scheme) resulting in a methodology that can produce an
unbiased determination of high-degree modes, once the instrumental
characteristics are well understood.
Finally, we present changes in high degree mode parameters with epoch and
thus solar activity level and discuss their significance.Comment: 59 pages, 38 figures -- High-resolution version at
http://www-sgk.harvard.edu:1080/~sylvain/preprints/ -- Manuscript submitted
to Ap
Solar-cycle variation of the sound-speed asphericity from GONG and MDI data 1995-2000
We study the variation of the frequency splitting coefficients describing the
solar asphericity in both GONG and MDI data, and use these data to investigate
temporal sound-speed variations as a function of both depth and latitude during
the period from 1995-2000 and a little beyond. The temporal variations in even
splitting coefficients are found to be correlated to the corresponding
component of magnetic flux at the solar surface. We confirm that the
sound-speed variations associated with the surface magnetic field are
superficial. Temporally averaged results show a significant excess in sound
speed around 0.92 solar radii and latitude of 60 degrees.Comment: To be published in MNRAS, accepted July 200
Is the solar convection zone in strict thermal wind balance?
Context: The solar rotation profile is conical rather than cylindrical as one
could expect from classical rotating fluid dynamics (e.g. Taylor-Proudman
theorem). Thermal coupling to the tachocline, baroclinic effects and
latitudinal transport of heat have been advocated to explain this peculiar
state of rotation. Aims: To test the validity of thermal wind balance in the
solar convection zone using helioseismic inversions for both the angular
velocity and fluctuations in entropy and temperature. Methods: Entropy and
temperature fluctuations obtained from 3-D hydrodynamical numerical simulations
of the solar convection zone are compared with solar profiles obtained from
helioseismic inversions. Results: The temperature and entropy fluctuations in
3-D numerical simulations have smaller amplitude in the bulk of the solar
convection zone than those found from seismic inversions. Seismic inversion
find variations of temperature from about 1 K at the surface up to 100 K at the
base of the convection zone while in 3-D simulations they are of order 10 K
throughout the convection zone up to 0.96 . In 3-D simulations,
baroclinic effects are found to be important to tilt the isocontours of
away from a cylindrical profile in most of the convection zone helped
by Reynolds and viscous stresses at some locations. By contrast the baroclinic
effect inverted by helioseismology are much larger than what is required to
yield the observed angular velocity profile. Conclusion: The solar convection
does not appear to be in strict thermal wind balance, Reynolds stresses must
play a dominant role in setting not only the equatorial acceleration but also
the observed conical angular velocity profile.Comment: 8 pages, 6 figures (low resolution), Accepted by Astronomy and
Astrophysics - Affiliation: (1) AIM, CEA/DSM-CNRS-Univ. Paris Diderot,
IRFU/SAp, France & (2) LUTH, Observatoire de Paris, CNRS-Univ. Paris Diderot,
France ; (3) Tata Institute of Fundamental Research, India; (4) Centre for
Basic Sciences, University of Mumbai, Indi
Helioseismic and Magnetic Imager observations of linear polarization from a loop prominence system
White-light observations by the Solar Dynamics Observatory's Helioseismic and
Magnetic Imager of a loop-prominence system occurring in the aftermath of an
X-class flare on 2013 May 13 near the eastern solar limb show a linearly
polarized component, reaching up to 20% at an altitude of 33 Mm,
about the maximal amount expected if the emission were due solely to Thomson
scattering of photospheric light by the coronal material. The mass associated
with the polarized component was 8.210 g. At 15 Mm altitude, the
brightest part of the loop was 3(+/-0.5)% linearly polarized, only about 20% of
that expected from pure Thomson scattering, indicating the presence of an
additional unpolarized component at wavelengths near Fe I (617.33 nm), probably
thermal emission. We estimated the free electron density of the white-light
loop system to possibly be as high as 1.810 cm.Comment: 9 pages, 5 figure
Global-scale equatorial Rossby waves as an essential component of solar internal dynamics
The Sun's complex dynamics is controlled by buoyancy and rotation in the
convection zone and by magnetic forces in the atmosphere and corona. While
small-scale solar convection is well understood, the dynamics of large-scale
flows in the solar convection zone is not explained by theory or simulations.
Waves of vorticity due to the Coriolis force, known as Rossby waves, are
expected to remove energy out of convection at the largest scales. Here we
unambiguously detect and characterize retrograde-propagating vorticity waves in
the shallow subsurface layers of the Sun at angular wavenumbers below fifteen,
with the dispersion relation of textbook sectoral Rossby waves. The waves have
lifetimes of several months, well-defined mode frequencies below 200 nHz in a
co-rotating frame, and eigenfunctions of vorticity that peak at the equator.
Rossby waves have nearly as much vorticity as the convection at the same
scales, thus they are an essential component of solar dynamics. We find a
transition from turbulence-like to wave-like dynamics around the Rhines scale
of angular wavenumber of twenty; this might provide an explanation for the
puzzling deficit of kinetic energy at the largest spatial scales.Comment: This is the submitted version of the paper published in Nature
Astronomy. 23 pages, 8 figures, 1 tabl
A Coronal Jet Ejects from Sunspot Light Bridge
Chromospheric brighten and H surge are the evident and common
phenomena along sunspot light bridge. In this paper, a coronal jet ejects from
sunspot light bridge is presented. Using the data from the Solar Dynamics
Observatory (SDO) and Hinode satellites, it is confirmed that the jet has the
root near light bridge, this suggests that the jet may be a result of
reconnection between main sunspot and light bridge. Due to the processing of
jet ejects, the intensity and width of light bridge have some changes at some
extent. This also suggests that jet is related to the interaction between light
bridge and umbra, possibly magnetic reconnection or heat plasma trapped in
light bridge escaping and moving along field line.Comment: It has been accepted for publication in PAS
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