7.3 GONG/MDI comparison

Once both GONG and MDI had been running for a few years, it became evident that the two projects were producing inferences of the interior rotation profile that were different in some significant details, particularly at high latitudes within the convection zone. Schou et al. (2002Jump To The Next Citation Point) carried out a careful comparison, taking data from three epochs at different phases of the solar cycle from each project and deriving rotational splittings or splitting coefficients from each, both with the usual algorithms and with those regularly used for the other project’s data, before using both RLS and OLA inversions. The results clearly showed that most of the discrepancies arose from the analysis pipelines rather from the data themselves. The “CA” peak-fitting algorithm used for the MDI data was able to extract modes from the GONG data to somewhat higher degrees and lower frequencies than the “AZ” algorithm could manage with either GONG or MDI input data. However, for both MDI and GONG data, the “CA” algorithm introduced an anomaly in the splitting coefficients centered at around 3.3 mHz, which in turn caused the inversion inferences to show a higher rotation rate deep in the convection zone at higher latitudes. Excluding these data brought the GONG and MDI data (analyzed with the “AZ” and “CA” pipelines respectively) into much better agreement, at the cost of somewhat degraded resolution. Restricting both data sets to the common mode set below 3 mHz reduced the discrepancies even farther, but did not remove the “jet” in the MDI data. Since the “jet” feature was only seen in the MDI data analyzed with the CA pipeline, however, the authors concluded that this feature was probably spurious.
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