223 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
A Statistical Test of Uniformity in Solar Cycle Indices
Several indices are used to characterize the solar activity cycle. Key among these are: the International Sunspot Number, the Group Sunspot Number, Sunspot Area, and 10.7 cm Radio Flux. A valuable aspect of these indices is the length of the record -- many decades and many (different) 11-year cycles. However, this valuable length-of-record attribute has an inherent problem in that it requires many different observers and observing systems. This can lead to non-uniformity in the datasets and subsequent erroneous conclusions about solar cycle behavior. The sunspot numbers are obtained by counting sunspot groups and individual sunspots on a daily basis. This suggests that the day-to-day and month-to-month variations in these numbers should follow Poisson Statistics and be proportional to the square-root of the sunspot numbers themselves. Examining the historical records of these indices indicates that this is indeed the case - even with Sunspot Area and 10.7 cm Radio Flux. The ratios of the RMS variations to the square-root of the indices themselves are relatively constant with little variation over the phase of each solar cycle or from small to large solar cycles. There are, however, important step-like changes in these ratios associated with changes in observer and/or observer system. Here we show how these variations can be used to construct more uniform datasets
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