2,453 research outputs found
Does the current minimum validate (or invalidate) cycle prediction methods?
This deep, extended solar minimum and the slow start to Cycle 24 strongly
suggest that Cycle 24 will be a small cycle. A wide array of solar cycle
prediction techniques have been applied to predicting the amplitude of Cycle 24
with widely different results. Current conditions and new observations indicate
that some highly regarded techniques now appear to have doubtful utility.
Geomagnetic precursors have been reliable in the past and can be tested with 12
cycles of data. Of the three primary geomagnetic precursors only one (the
minimum level of geomagnetic activity) suggests a small cycle. The Sun's polar
field strength has also been used to successfully predict the last three
cycles. The current weak polar fields are indicative of a small cycle. For the
first time, dynamo models have been used to predict the size of a solar cycle
but with opposite predictions depending on the model and the data assimilation.
However, new measurements of the surface meridional flow indicate that the flow
was substantially faster on the approach to Cycle 24 minimum than at Cycle 23
minimum. In both dynamo predictions a faster meridional flow should have given
a shorter cycle 23 with stronger polar fields. This suggests that these dynamo
models are not yet ready for solar cycle prediction.Comment: SOHO 23 Workshop (SOHO-23: Understanding a Peculiar Solar Minimum,
Northeast Harbor, ME, USA, 2009 September 21-25) Invited Paper, 8 pages, 9
figures
Convective forcing of global circulations on the Jovian planets
Examples of convection in rotating layers are presented to illustrate how convection can drive global circulations on the Jovian planets. For rapid rotation the convective motions become largely two-dimensional and produce Reynold stresses which drive large scale flows. The initial tendency is to produce a prograde equatorial jet and a meridional circulation which is directed toward the poles in the surface layers. Fully nonlinear numerical simulations for the slowly rotating solar convection zone show that the meridional circulation does not reach the poles. Instead a multicellular meridional circulation is produced which has a downward flowing branch in the mid-latitudes. For more rapidly rotating objects such as Jupiter and Saturn this meridional circulation may consist of a larger number of cells. Axisymmetric convective models then show that prograde jets form at the downflow latitudes. A nonlinear numerical simulation of convection in a prograde jet is presented to illustrate the interactions which occur between convection and these jets. Without rotation the convection removes energy and momentum from the jet. With rotation the convection feeds energy and momentum into the jet
Predicting the Sun's Polar Magnetic Fields with a Surface Flux Transport Model
The Sun's polar magnetic fields are directly related to solar cycle
variability. The strength of the polar fields at the start (minimum) of a cycle
determine the subsequent amplitude of that cycle. In addition, the polar field
reversals at cycle maximum alter the propagation of galactic cosmic rays
throughout the heliosphere in fundamental ways. We describe a surface magnetic
flux transport model that advects the magnetic flux emerging in active regions
(sunspots) using detailed observations of the near-surface flows that transport
the magnetic elements. These flows include the axisymmetric differential
rotation and meridional flow and the non-axisymmetric cellular convective flows
(supergranules) all of which vary in time in the model as indicated by direct
observations. We use this model with data assimilated from full-disk
magnetograms to produce full surface maps of the Sun's magnetic field at
15-minute intervals from 1996 May to 2013 July (all of sunspot cycle 23 and the
rise to maximum of cycle 24). We tested the predictability of this model using
these maps as initial conditions, but with daily sunspot area data used to give
the sources of new magnetic flux. We find that the strength of the polar fields
at cycle minimum and the polar field reversals at cycle maximum can be reliably
predicted up to three years in advance. We include a prediction for the cycle
24 polar field reversal.Comment: 12 pages, 9 figures, ApJ accepte
Measurements of the Sun's High Latitude Meridional Circulation
The meridional circulation at high latitudes is crucial to the build-up and
reversal of the Sun's polar magnetic fields. Here we characterize the
axisymmetric flows by applying a magnetic feature cross-correlation procedure
to high resolution magnetograms obtained by the Helioseismic and Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We focus on
Carrington Rotations 2096-2107 (April 2010 to March 2011) - the overlap
interval between HMI and the Michelson Doppler Investigation (MDI). HMI
magnetograms averaged over 720 seconds are first mapped into heliographic
coordinates. Strips from these maps are then cross-correlated to determine the
distances in latitude and longitude that the magnetic element pattern has
moved, thus providing meridional flow and differential rotation velocities for
each rotation of the Sun. Flow velocities were averaged for the overlap
interval and compared to results obtained from MDI data. This comparison
indicates that these HMI images are rotated counter-clockwise by 0.075 degrees
with respect to the Sun's rotation axis. The profiles indicate that HMI data
can be used to reliably measure these axisymmetric flow velocities to at least
within 5 degrees of the poles. Unlike the noisier MDI measurements, no evidence
of a meridional flow counter-cell is seen in either hemisphere with the HMI
measurements: poleward flow continues all the way to the poles. Slight
North-South asymmetries are observed in the meridional flow. These asymmetries
should contribute to the observed asymmetries in the polar fields and the
timing of their reversals.Comment: 6 pages, 3 color figures, accepted for publication in The
Astrophysical Journal Lette
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