Ring-diagram analysis was the first local helioseismic technique to be developed It is the one that is easiest to exploit both in terms of processing speed and ease of interpretation. Since the launch of SOHO in 1996, regular synoptic ring-diagram analyses have been performed on all MDI data of sufficient continuity, generally those taken during the annual two- or three-month Dynamics Programs. The most important results to come out of that analysis to date are the discovery and characterization of flow concentrations around active regions, and the structure of the near-surface global meridional circulation pattern, including a very surprising turnover at depth in the northern hemisphere only during the peak years of the solar activity cycle. Those analyses have been performed on an over-sampled grid of 15° diameter regions sampled 24 times per Carrington rotation. Here we report an attempts to duplicate that analysis using both GONG+ data for the same observing periods and the newly developed GONG++ analysis pipeline (designated A here) in addition to the MDI data and the traditional MDI pipeline (C )
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The two analysis pipelines involve multiple independent steps, involving data selection and preprocessing; tracking; spectral cleaning, ring fitting, and inversion. Preprocessing is more or less tailored to the observational data, but from that point on data from either stream can, at least in principle, be run through a processing pipeline built from any combination of options in the two approaches.
In addition to the MDI and GONG+ data there is another independent data set of sufficient resolution, extent, and continuity to be suitable for synoptic ring-diagram analysis: the Doppler data from the magneto-optic filter on the 60-ft solar tower telescope at Mt. Wilson. These single-site data go back to the year 1988 and continue to the present. We have begun to integrate analysis of Mt. Wilson data into pipeline C and have produced some initial analysis results that appear promising. There are problems with the gap structure, however, that may require some modifications to the analysis program. In any case the data for the comparison period selected, CR 1988, are not yet available for processing at this time, so that comparison awaits a subsequent report.
GONG data must of course be merged as well as properly detrended. Pipeline A achieves this by scaling the individual site data to a common sensitivity level, first-differencing consecutive images, remapping individual site data to a common scale and location, and averaging together data for the same minute from multiple sites when more than one is available, typically 50 - 60% of the time during this month. In pipeline C the individual site data are scaled to a common sensitivity level, detrending by removing the effect of the known observer motion, and simply selecting data from one site when more than one is available, in this case the easternmost site.
Due to misunderstandings in time indexing (start vs. center of observing interval and use of TAI vs. GPS), the times selected for the processing intervals of MDI and GONG+ data differed by 60 seconds rather than near simultaneity when accounting for the 5-second light travel time difference.
For this comparison, we track at the locations used in the standard dense-pack sub-surface weather (SSW) analysis described in Haber et al. 2000. These are sampled in increments of 7°.5 in latitude and central-meridian longitude out to ±52°.5.
Turning to the mean flows averaged over a rotation (Figure 2), we see the same pattern of close agreement at low latitudes and increasing discrepancies at higher latitudes, and also at greater depths. In particular, the very intriguing counter cell in the meridional flow at depth in the northern hemisphere seen in the MDI data is not confimred by the GONG data, though there is indication of at least a slowing of the poleward rate. There appears to be a consistent offset in the mean zonal flows at all latitudes between the two data sources, even though the zonal structure matches quite well. This is equivalent to a discrepancy in tracking rates, which in turn is equivalent to first order to a plate scale discrepancy. The east-west pattern in the mean zonal flows is quite strange: MDI data show a previously-noted slowing toward the west limb, at least at the surface, which seems inexplicable except in terms of uncorrected image distortion. (It seems only to show up in the f-mode as evidenced by its disappearnce in the inverted results below about 5 Mm.) This effect is not present in the GONG data. On the contrary, there appears to be an acceleration near the east limb that is seen only at depth, i.e. presumably in the p-modes!
Results from processing of the two data streams through pipeline A are broadly similar, but there are a few noticeable differences (Figure 3) In particular, while the meridional flow structure inferred from the MDI data with both pipelines is similar, there are substantial differences in the high-northern-latitude structure inferred from the GONG data; however neither is in agreement with the MDI results. In general, there is somewhat less noise in the results from the C processing of the MDI data than the A processing of the same data. The fact that the derived zonal flows are in better agreement than the meridional flows at higher latitudes may be due to the differences in the mapping, since the A mapping is designed to minimize distortion in the east-west direction, while the C mapping is isotropic in its distortion pattern; but the differential distortions between the two mappings over such small regions are less than 10-3..
One very puzzling feature is that there is a marked south-east/north-west gradient on the projected solar disc in the inferred frequencies from the C pipeline applied to GONG data only (using both merges), as shown in Figure 4. We do not yet understand the source of this feature, although interpretation of positional metadata and distortion corrections in the mappings are certainly suspect. In any case, the inferred flows, resulting differential rather than absolute measurements in the ring power spectra, are insensitive to these mapping effects to first order.
RH and RK are partially supported by NASA Grant S-92698-F.
SB is partially supported by NASA grant NAG5-10912.