344 research outputs found
Recovering Solar Toroidal Field Dynamics From Sunspot Location Patterns
We analyze both Kitt Peak magnetogram data and MDI continuum intensity
sunspot data to search for the following solar toroidal band properties: width
in latitude and the existence of a tipping instability (longitudinal m=1 mode)
for any time during the solar cycle. To determine the extent which we can
recover the toroidal field dynamics, we forward model artificially generated
sunspot distributions from subsurface toroidal fields we assigned certain
properties. We analyzed two sunspot distribution parameters using MDI and model
data: the average latitudinal separation of sunspot pairs as a function of
longitudinal separation, and the number of sunspot pairs creating a given angle
with respect to the E-W direction. A toroidal band of 10 degrees width with a
constant tipping of 5 degrees best fits MDI data early in the solar cycle. A
toroidal band of 20 degrees width with a tipping amplitude decreasing in time
from 5 to 0 degrees best fits MDI data late in the solar cycle. Model data
generated by untipped toroidal bands cannot fit MDI high latitude data and can
fit only one parameter at low latitudes. Tipped toroidal bands satisfy chi
squared criteria at both high and low latitudes. We conclude this is evidence
to reject the null hypothesis - that toroidal bands in the solar tachocline do
not experience a tipping instability - in favor of the hypothesis that the
toroidal band experiences an m=1 tipping instability. Our finding that the band
widens from ~10 degrees early in the solar cycle to ~20 degrees late in the
solar cycle may be explained in theory by magnetic drag spreading the toroidal
band due to altered flow along the tipped field lines.Comment: This paper is accepted to Astrophysical Journal, September 2005 issu
Concentration of toroidal magnetic field in the solar tachocline by eta-quenching
We show that if the turbulent magnetic diffusivity used in solar dynamos is
assumed to be 'quenched' by increasing toroidal fields, much larger amplitude
and more concentrated toroidal fields can be induced by differential rotation
from an assumed poloidal field than if there is no quenching. This
amplification and concentration mechanism is weakened and bounded by j x B
feedbacks on the differential rotation. Nevertheless, it is strong enough to
contribute to the creation of ~100 kG toroidal fields near the base of the
convection zone, perhaps in conjunction with the 'exploding flux tube' process.
Such high fields are necessary for sunspots to occur in low solar latitudes.Comment: 8 pages, 6 figures, added references, corrected typos, accepted by
Ap
Indirect measurement of the mean meridional circulation in the southern hemisphere
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1964.Includes bibliographical references (leaves 48-49).by Peter Augustus Gilman.M.S
Hydromagnetic model for the solar general circulation.
Massachusetts Institute of Technology. Dept. of Meteorology. Thesis. 1966. Ph.D.Bibliography: p. 197-202.Ph.D
Physical Origin of Differences among various Measures of Solar Meridional Circulation
We show that systematic differences between surface Doppler and magnetic
element tracking measures of solar meridional flow can be explained by the
effects of surface turbulent magnetic diffusion. Feature-tracking speeds are
lower than plasma speeds in low and mid-latitudes, because magnetic diffusion
opposes poleward plasma flow in low-latitudes whereas it adds to plasma flow at
high latitudes. Flux transport dynamo models must input plasma flow; the
model-outputs yield estimates of the surface magnetic feature tracking speed.
We demonstrate that the differences between plasma speed and magnetic pattern
speed in a flux-transport dynamo are consistent with the observed difference
between these speeds.Comment: To appear in Ap
Solar Multi-Scale Convection and Rotation Gradients Studied in Shallow Spherical Shells
The differential rotation of the sun, as deduced from helioseismology,
exhibits a prominent radial shear layer near the top of the convection zone
wherein negative radial gradients of angular velocity are evident in the low-
and mid-latitude regions spanning the outer 5% of the solar radius.
Supergranulation and related scales of turbulent convection are likely to play
a significant role in the maintenance of such radial gradients, and may
influence dynamics on a global scale in ways that are not yet understood. To
investigate such dynamics, we have constructed a series of three-dimensional
numerical simulations of turbulent compressible convection within spherical
shells, dealing with shallow domains to make such modeling computationally
tractable. These simulations are the first models of solar convection in a
spherical geometry that can explicitly resolve both the largest dynamical
scales of the system (of order the solar radius) as well as smaller-scale
convective overturning motions comparable in size to solar supergranulation
(20--40 Mm). We find that convection within these simulations spans a large
range of horizontal scales, and that the radial angular velocity gradient in
these models is typically negative, especially in low- and mid-latitude
regions. Analyses of the angular momentum transport indicates that such
gradients are maintained by Reynolds stresses associated with the convection,
transporting angular momentum inward to balance the outward transport achieved
by viscous diffusion and large-scale flows in the meridional plane. We suggest
that similar mechanisms associated with smaller-scale convection in the sun may
contribute to the maintenance of the observed radial shear layer located
immediately below the solar photosphere.Comment: 45 pages, 17 figures, ApJ in press. A preprint of paper with hi-res
figures can be found at
http://www-lcd.colorado.edu/~derosa/modelling/modelling.htm
Theory of Solar Meridional Circulation at High Latitudes
We build a hydrodynamical model for computing and understanding the Sun's
large-scale high latitude flows, including Coriolis forces, turbulent diffusion
of momentum and gyroscopic pumping. Side boundaries of the spherical 'polar
cap', our computational domain, are located at latitudes .
Implementing observed low latitude flows as side boundary conditions, we solve
the flow equations for a cartesian analog of the polar cap. The key parameter
that determines whether there are nodes in the high latitude meridional flow is
, in which is the interior rotation
rate, n the radial wavenumber of the meridional flow, the depth of the
convection zone and the turbulent viscosity. The smaller the
(larger turbulent viscosity), the fewer the number of nodes in high latitudes.
For all latitudes within the polar cap, we find three nodes for
, two for , and one or none for
or higher. For near our model exhibits 'node
merging': as the meridional flow speed is increased, two nodes cancel each
other, leaving no nodes. On the other hand, for fixed flow speed at the
boundary, as is increased the poleward most node migrates to the pole and
disappears, ultimately for high enough leaving no nodes. These results
suggest that primary poleward surface meridional flow can extend from
to the pole either by node-merging or by node migration and
disappearance.Comment: Accepted in Ap
The Solar Benchmark: Rotational Modulation of the Sun Reconstructed from Archival Sunspot Records
We use archival daily spot coverage measurements from Howard et al. (1984) to
study the rotational modulation of the Sun as though it were a distant star. A
quasi-periodic Gaussian process measures the solar rotation period
days, and activity cycle period years. We attempt to search for evidence of differential
rotation in variations of the apparent rotation period throughout the activity
cycle and do not detect a clear signal of differential rotation, consistent
with the null results of the hare-and-hounds exercise of Aigrain et al. (2015).
The full reconstructed solar light curve is available online.Comment: Accepted for publication in MNRA
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