NSO and the University of Sonora at Hermosillo, Mexico sponsored a two-part meeting on the status and prospects of research on the solar cycle. The first sessions were held in Tucson 28-30 March, the second sessions in Hermosillo 1-4 April. About 30 researchers attended each meeting, including visitors from Mexico, France, Spain, Holland, Germany, Austria and Russia.
The Tucson meeting was the first of several intended to engage the community in NSO's long-range study of the solar cycle, the Solar Variability Enterprise (SOLVE). A small number of invited talks followed by extended discussions and impromptu short contributions made for a lively and productive meeting. Exchanges between theorists and observers were especially revealing to both sides.
An important theme to emerge from the Tucson meeting was that numerical modelling of the internal circulation of the convection zone and solar dynamo continues to lag far behind the observations. The alpha mechanism for amplification of poloidal magnetic fields has been shown to be inadequate, and as yet no plausible mechanism has been proposed to replace it. Two approaches seem to be favored: numerical modelling with steadily improving treatment of the huge range of relevant scales, or parametrized solutions embodying the best intuition on the relevant physics. A physical model for internal rotation, compatible with observations, is perhaps the highest priority.
Helioseismology is producing exciting results, as expected. Early data from GONG provide new estimates of the magnitude of meridional flows, torsional oscillations, and, possibly, giant cells. Local helioseismology techniques are revealing the depth variation of horizontal flows in the convection zone, down to a few tens of megameters.
The reversal of polar magnetic fields, successfully modelled as an axisymmetric phenomenon in Cycle 22, appears to be far more complicated in Cycle 23. Polar coronal holes "migrated" in latitude as if guided by a gap in the polar crown of filaments--a further challenge to the theorists!
The Hermosillo meeting surveyed the rich variety of observations that bear on the solar cycle and that remain to be incorporated in an overall empirical model. We heard contributions on the cyclic variation of coronal holes, proxies for ultraviolet variability, the magnitude of polar magnetic fields, the tilt of bipolar active regions, and large-scale magnetic patterns, among others. Proceedings will be published on a short timescale.
We were impressed with the quality of research by our Mexican colleagues and grateful for the opportunity to visit with them; their standards of hospitality were exemplary. We look forward to future international cooperative meetings of this kind.
Jack Zirker