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