arXiv subjects

David H. Hathaway

Publications and source records attributed to David H. Hathaway.

24 records · Page 2Linked to original sources

arXiv2012

Supergranules as Probes of Solar Convection Zone Dynamics

Supergranules are convection cells seen at the Sun's surface as a space filling pattern of horizontal flows. While typical supergranules have diameters of about 35 Mm, they exhibit a broad spectrum of sizes from ~10 Mm to ~100 Mm. Here we show that supergranules of different sizes can be used to probe the rotation rate in the Sun's outer convection zone. We find that the equatorial rotation rate as a function of depth as measured by global helioseismology matches the equatorial rotation as a function of wavelength for the supergranules. This suggests that supergranules are advected by flows at depths equal to their wavelengths and thus can be used to probe flows at those depths. The supergranule rotation profiles show that the surface shear layer, through which the rotation rate increases inward, extends to depths of ~50 Mm and to latitudes of at least 70 degrees. Typical supergranules are well observed at high latitudes and have a range of sizes that extend to greater depths than those typically available for measuring subsurface flows with local helioseismology. These characteristics indicate that probing the solar convection zone dynamics with supergranules can complement the results of helioseismology.

astro-ph.SR↗

arXiv2011

The Sun's Shallow Meridional Circulation

The Sun's global meridional circulation is evident as a slow poleward flow at its surface. This flow is observed to carry magnetic elements poleward - producing the Sun's polar magnetic fields as a key part of the 11-year sunspot cycle. Current theories for the sunspot cycle assume that this surface flow is part of a circulation which sinks inward at the poles and turns equatorward at depths below 100 Mm. Here we use the advection of the Sun's convection cells by the meridional flow to map the flow velocity in latitude and depth. Our measurements show the largest cells clearly moving equatorward at depths below 35 Mm - the base of the Sun's surface shear layer. This surprisingly shallow return flow indicates the need for substantial revisions to solar/stellar dynamo theory.

astro-ph.SR↗

arXiv2011

Variations in the axisymmetric transport of magnetic elements on the Sun: 1996-2010

We measure the axisymmetric transport of magnetic flux on the Sun by cross-correlating narrow strips of data from line-of-sight magnetograms obtained at a 96-minute cadence by the MDI instrument on the ESA/NASA SOHO spacecraft and then averaging the flow measurements over each synodic rotation of the Sun. Our measurements indicate that the axisymmetric flows vary systematically over the solar cycle. The differential rotation is weaker at maximum than at minimum. The meridional flow is faster at minimum and slower at maximum. The meridional flow speed on the approach to the Cycle 23/24 minimum was substantially faster than it was at the Cycle 22/23 minimum. The average latitudinal profile is largely a simple sinusoid that extends to the poles and peaks at about $35\degr$ latitude. As the cycle progresses a pattern of in-flows toward the sunspot zones develops and moves equatorward in step with the sunspot zones. These in-flows are accompanied by the torsional oscillations. This association is consistent with the effects of the Coriolis force acting on the in-flows. The equatorward motions associated with these in-flows are identified as the source of the decrease in net poleward flow at cycle maxima. We also find polar counter-cells (equatorward flow at high latitudes) in the south from 1996 to 2000 and in the north from 2002 to 2010. We show that these measurements of the flows are not affected by the non-axisymmetric diffusive motions produced by supergranulation.

astro-ph.SR↗

arXiv2010

The Advection of Supergranules by the Sun's Axisymmetric Flows

We show that the motions of supergranules are consistent with a model in which they are simply advected by the axisymmetric flows in the Sun's surface shear layer. We produce a 10-day series of simulated Doppler images at a 15-minute cadence that reproduces most spatial and temporal characteristics seen in the SOHO/MDI Doppler data. Our simulated data have a spectrum of cellular flows with just two components -- a granule component that peaks at spherical wavenumbers of about 4000 and a supergranule component that peaks at wavenumbers of about 110. We include the advection of these cellular components by the axisymmetric flows -- differential rotation and meridional flow -- whose variations with latitude and depth (wavenumber) are consistent with observations. We mimic the evolution of the cellular pattern by introducing random variations to the phases of the spectral components at rates that reproduce the levels of cross-correlation as functions of time and latitude. Our simulated data do not include any wave-like characteristics for the supergranules yet can reproduce the rotation characteristics previously attributed to wave-like behavior. We find rotation rates which appear faster than the actual rotation rates and attribute this to the projection effects. We find that the measured meridional flow does accurately represent the actual flow and that the observations indicate poleward flow to $65\degr-70\degr$ latitude with equatorward counter cells in the polar regions.

astro-ph.SR↗

arXiv2010

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.

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arXiv2010

The Advection of Supergranules by Large-Scale Flows

We produce a 10-day series of simulated Doppler images at a 15-minute cadence that reproduces the spatial and temporal characteristics seen in the SOHO/MDI Doppler data. Our simulated data contains a spectrum of cellular flows with but two necessary components --- a granule component that peaks at wavenumbers of about 4000 and a supergranule component that peaks at wavenumbers of about 110. We include the advection of these cellular components by a differential rotation profile that depends on latitude and wavenumber (depth). We further mimic the evolution of the cellular pattern by introducing random variations to the amplitudes and phases of the spectral components at rates that reproduce the level of cross-correlation as a function of time and latitude. Our simulated data do not include any wave-like characteristics for the supergranules yet can accurately reproduce the rotation characteristics previously attributed to wave-like characteristics.

astro-ph.SR↗