R. Komm

Meridional Circulation Variability from Large‐Aperture Ring‐Diagram Analysis of Global Oscillation Network Group and Michelson Doppler Imager Data

Ring-diagram analysis, a local helioseismology technique, has proven to be very useful for studying solar subsurface velocity flows down to a depth of about 0.97 R☉. The depth range is determined by the modes used in this type of analysis, and thus depends on the size of the area analyzed. Extending the area allows us to detect lower spherical harmonic degree (l) modes which, at a constant frequency, penetrate deeper in the Sun. However, there is a compromise between the size of the area and the validity of the plane-wave approximation used by the technique. We present the results of applying the ring diagrams to 30° diameter areas over the solar surface in an attempt to reach deeper into the solar interior. Meridional flows for 25 consecutive Carrington rotations (1985-2009) are derived by applying this technique to Global Oscillation Network Group (GONG) and Michelson Doppler Imager (MDI) data. This covers a time span of almost 2 yr, starting at the beginning of 2002. The amplitude of the meridional flow shows a variation of the order of 5 m s-1 during this period. Our results indicate that the flows increase toward the interior of the Sun for the depth range studied. We find a 1 yr periodicity in the appearance of an equatorward meridional cell at high latitudes that coincides with maximum values of the solar inclination toward the Earth (B0 angle).

SOLAR SUBSURFACE FLUID DYNAMICS DESCRIPTORS DERIVED FROM GLOBAL OSCILLATION NETWORK GROUP AND MICHELSON DOPPLER IMAGER DATA

We analyze Global Oscillation Network Group (GONG) and Michelson Doppler Imager (MDI) observations obtained during Carrington rotation 1988 (2002 March 30-April 26) with a ring-diagram technique in order to measure the zonal and meridional flow components in the upper solar convection zone. We derive daily flow maps over a range of depths up to 16 Mm on a spatial grid of 75 in latitude and longitude covering ±60° in latitude and central meridian distance and combine them to make synoptic flow maps. We begin exploring the dynamics of the near-surface layers and the interaction between flows and magnetic flux by deriving fluid dynamics descriptors such as divergence and vorticity from these flow maps. Using these descriptors, we derive the vertical velocity component and the kinetic helicity density. For this particular Carrington rotation, we find that the vertical velocity component is anticorrelated with the unsigned magnetic flux. Strong downflows are more likely associated with locations of strong magnetic activity. The vertical vorticity is positive in the northern hemisphere and negative in the southern hemisphere. At locations of magnetic activity, we find an excess vorticity of the same sign as that introduced by differential rotation. The vertical gradient of the zonal flow is mainly negative except within 2 Mm of the surface at latitudes poleward of about 20°. The zonal-flow gradient appears to be related to the unsigned magnetic flux in the sense that locations of strong activity are also locations of large negative gradients. The vertical gradient of the meridional flow changes sign near about 7 Mm, marking a clear distinction between near-surface and deeper layers. GONG and MDI data show very similar results. Differences occur mainly at high latitudes, especially in the northern hemisphere, where MDI data show a counter cell in the meridional flow that is not present in the corresponding GONG data.

Localizing the Solar Cycle Frequency Shifts in Global p-Modes

The 6.5 yr span of observations from the Global Oscillation Network Group and the Michelson Doppler Imager aboard the Solar and Heliospheric Observatory allows a detailed study of the solar cycle-related frequency shifts at the level of central frequencies and a-coefficients from individual multiplets and even of individual modes within a multiplet. We analyze such data and show that the shifts at all levels of averaging are consistent with the hypothesis that the global p-mode frequency shifts are closely related to the surface magnetic field distribution. Furthermore, the evolution of the surface magnetic flux distribution can be reconstructed by an inversion technique operating on the shifts within individual (n, l) multiplets.

A NOTE ON THE TORSIONAL OSCILLATION AT SOLAR MINIMUM

We examine the evolution of the zonal flow pattern in the upper solar convection zone during the current extended solar minimum, and compare it with that during the previous minimum. The results suggest that a configuration matching that at the previous minimum was reached during 2008, but that the flow band corresponding to the new cycle has been moving more slowly toward the equator than was observed in the previous cycle, resulting in a gradual increase in the apparent length of the cycle during the 2007-2008 period. The current position of the lower-latitude fast-rotating belt corresponds to that seen around the onset of activity in the previous cycle.

Subsurface Meridional Circulation in the Active Belts

Temporal variations of the subsurface meridional flow with the solar cycle have been reported by several authors. The measurements are typically averaged over periods of time during which surface magnetic activity existed in the regions where the velocities are calculated. The present work examines the possible contamination of these measurements due to the extra velocity fields associated with active regions plus the uncertainties in the data obtained where strong magnetic fields are present. We perform a systematic analysis of more than five years of GONG data and compare meridional flows obtained by ring-diagram analysis before and after removing the areas of strong magnetic field. The overall trend of increased amplitude of the meridional flow towards solar minimum remains after removal of large areas associated with surface activity. We also find residual circulation toward the active belts that persists even after the removal of the surface magnetic activity, suggesting the existence of a global pattern or longitudinally-located organized flows.

Divergence and Vorticity of Solar Subsurface Flows Derived from Ring‐Diagram Analysis of MDI and GONG Data

We measure the relation between divergence and vorticity of subsurface horizontal flows as a function of unsigned surface magnetic flux. Observations from the Michelson Doppler Imager (MDI) Dynamics Program and Global Oscillation Network Group (GONG) have been analyzed with a standard ring-diagram technique to measure subsurface horizontal flows from the surface to a depth of about 16 Mm. We study residual horizontal flows after subtracting large-scale trends (low-order polynomial fits in latitude) from the measured velocities. On average, quiet regions are characterized by weakly divergent horizontal flows and small anticyclonic vorticity (clockwise in the northern hemisphere), while locations of high activity show convergent horizontal flows combined with cyclonic vorticity (counterclockwise in the northern hemisphere). Divergence and vorticity of horizontal flows are anticorrelated (correlated) in the northern (southern) hemisphere. This is especially noticeable at greater depth, where the relation between divergence and vorticity of horizontal flows is nearly linear. These trends show a slight reversal at the highest levels of magnetic flux; the vorticity amplitude decreases at the highest flux levels, while the divergence changes sign at depths greater than about 10 Mm. The product of divergence and vorticity of the horizontal flows, a proxy of the vertical contribution to the kinetic helicity density, is on average negative (positive) in the northern (southern) hemisphere. The helicity proxy values are greater at locations of high magnetic activity than at quiet locations.

MERIDIONAL CIRCULATION DURING THE EXTENDED SOLAR MINIMUM: ANOTHER COMPONENT OF THE TORSIONAL OSCILLATION?

We show here a component of the meridional circulation developing at medium-high latitudes (40°-50°) before the new solar cycle starts. Like the torsional oscillation of the zonal flows, this extra circulation seems to precede the onset of magnetic activity at the solar surface and moves slowly toward lower latitudes. However, the behavior of this component differs from that of the torsional oscillation regarding location and convergence toward the equator at the end of the cycle. The observation of this component before the magnetic regions appear at the solar surface has only been possible due to the prolonged solar minimum. The results could settle the discussion as to whether the extra component of the meridional circulation around the activity belts, which has been known for some time, is or is not an effect of material motions around the active regions.

RING ANALYSIS OF SOLAR SUBSURFACE FLOWS AND THEIR RELATION TO SURFACE MAGNETIC ACTIVITY

We measure the horizontal flows in the outer 2% of the Sun by analyzing 14 consecutive Carrington rotations of Global Oscillation Network Group (GONG) Doppler images and two of Michelson Doppler Imager (MDI) Dynamics Program data with the ring-diagram technique. The zonal and meridional flows show no variation with activity at low to medium activity levels (below 71 G). At active region locations, the zonal flow increases with increasing activity; active regions rotate faster than their quieter surroundings. The meridional flow at active region locations is more equatorward than on average at depths less than about 10 Mm; the flow converges toward the mean latitude of activity. At depths greater than about 10 Mm, some active region locations show poleward and others equatorward motions indicating strong outflows from active regions. The estimated vertical flow decreases with increasing activity levels except at active region locations at depths greater than about 10 Mm; active regions show downflows near the surface and upflows at depths greater than about 10 Mm. The velocity errors increase somewhat with increasing activity at flux levels below 71 G, but they increase rapidly up to about 2 times the median error at higher flux values. This increase occurs at all depths. The flows averaged over all longitudes show the patterns expected from solar cycle variations. The quiet and the intermediate activity subsets show the same flow pattern, while the active region subset shows a mixture of solar cycle flow pattern and local flow behavior.

Temporal Variation of Angular Momentum in the Solar Convection Zone

We derive the angular momentum as a function of radius and time with the help of the rotation rates resulting from inversions of helioseismic data obtained from the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI) and the density distribution from a model of the Sun. The base of the convection zone can be identified as a local maximum in the relative angular momentum after subtracting the contribution of the solid-body rotation. The angular momentum as a function of radius shows the strongest temporal variation near the tachocline. This variation extends into the lower convection zone and into the radiative interior and is related to the 1.3 yr periodicity found in the equatorial rotation rate of the tachocline. In the upper convection zone, we find a small systematic variation of the angular momentum that is related to torsional oscillations. The angular momentum integrated from the surface to a lower limit in the upper convection zone provides a hint that the torsional oscillation pattern extends deep into the convection zone. This is supported by other quantities such as the coefficients of a fit of Legendre polynomials to the rotation rates as a function of latitude. The temporal variation of the coefficient of P4, indicative of torsional oscillations, suggests that the signature of these flows in the inversion results extend to about r ≈ 0.83 R☉. With the lower limit of integration placed in the middle or lower convection zone, the angular momentum fluctuates about the mean without apparent trend, i.e., the angular momentum is conserved within the measurement errors. However, when integrated over the layers slightly below the convection zone (0.60-0.71 R☉), the angular momentum shows the 1.3 yr period and hints at a long-term trend that might be related to the solar activity cycle.

The High-latitude Branch of the Solar Torsional Oscillation in the Rising Phase of Cycle 24

We use global heliseismic data from the Global Oscillation Network Group, the Michelson Doppler Imager on board the Solar and Heliospheric Observatory, and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, to examine the behavior, during the rising phase of Solar Cycle 24, of the migrating zonal flow pattern known as the torsional oscillation. Although the high-latitude part of the pattern appears to be absent in the new cycle when the flows are derived by subtracting a mean across a full solar cycle, it can be seen if we subtract the mean over a shorter period in the rising phase of each cycle, and these two mean rotation profiles differ significantly at high latitudes. This indicates that the underlying high-latitude rotation has changed; we speculate that this is in response to weaker polar fields, as suggested by a recent model.