Since the 1970s, helioseismology has provided several insights into the interior solar rotation: the approximately-rigid rotation of the radiative interior; the differential rotation throughout the convection zone; the thin tachocline; the extension of the surface torsional oscillation throughout the convection zone. More than once, these discoveries have overturned theoretical expectations, inspiring modelers to improve their calculations in an effort to reproduce the observed behavior. Because of the surprising nature of many of the findings, it has been important to have more than one source of observations, so that it is possible to distinguish between real solar features – especially the unexpected ones – and systematic error.
It may be that in the future solar cycle 23, with MDI and GONG operating in parallel, will be seen as a golden age of helioseismology. At the time of writing, we eagerly anticipate the launch of the Solar Dynamics Observatory [SDO] with its Helioseismic and Magnetic Imager [HMI], a successor to MDI that will provide near-continuous helioseismic observations at higher resolutions than ever before and may help in unraveling the relationships between active region flows, magnetic fields, and geoeffective solar activity as well as providing a continued watch on the longer-term variations in the solar velocity fields. Sadly, however, current plans call for both GONG and MDI to cease to collect data soon after the successful launch of SDO, which would leave HMI without any independent cross-checks, while on the low-degree front the BiSON network has recently lost its funding and there are no new dedicated low-degree space-based instruments currently scheduled.
There are still areas – such as the strength of the near-surface shear at high latitudes, the rotation of the inner core, and any inhomogeneities and changes in the tachocline – that remain unclear. Furthermore, a complete numerical model of the solar dynamo – vital for any long-term predictive capability – is still lacking, and helioseismic observations still have an important part to play in constraining such models as they develop.
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