Abstract
Building upon our previous work, in which we analyzed smoothed and subsampled velocity data from the Michelson Doppler Imager (MDI), we extend our analysis to unsmoothed, full-resolution MDI data. We also present results from the Helioseismic and Magnetic Imager (HMI), in both full resolution and processed to be a proxy for the low-resolution MDI data. We find that the systematic errors that we saw previously, namely peaks in both the high-latitude rotation rate and the normalized residuals of odd a\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$a$\\end{document}-coefficients, are almost entirely absent in the two full-resolution analyses. Furthermore, we find that both systematic errors seem to depend almost entirely on how the input images are apodized, rather than on resolution or smoothing. Using the full-resolution HMI data, we confirm our previous findings regarding the effect of using asymmetric profiles on mode parameters, and also find that they occasionally result in more stable fits. We also confirm our previous findings regarding discrepancies between 360-day and 72-day analyses. We further investigate a six-month period previously seen in f\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$f$\\end{document}-mode frequency shifts using the low-resolution datasets, this time accounting for solar-cycle dependence using magnetic-field data. Both HMI and MDI saw prominent six-month signals in the frequency shifts, but we were surprised to discover that the strongest signal at that frequency occurred in the mode coverage for the low-resolution proxy. Finally, a comparison of mode parameters from HMI and MDI shows that the frequencies and a\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$a$\\end{document}-coefficients agree closely, encouraging the concatenation of the two datasets.
Highlights
The effect of using asymmetric profiles on the mode parameters themselves is discussed in the context of the Helioseismic and Magnetic Imager (HMI) analysis
Investigating the polar jet (LS15), we found that it was clearly visible in inversions of the 1998 Dynamics Run alone, so we are able to compare all four analyses
This is not shown, we found that the mode parameters changed for the two apodizations
Summary
Designed to be the successor to the Michelson Doppler Imager (MDI: Scherrer et al, 1995) onboard the Solar and Heliospheric Observatory (SOHO), the Helioseismic and Magnetic Imager (HMI: Schou et al, 2012) was launched onboard the Solar Dynamics Observatory (SDO) in February 2010. HMI is equipped with a 4096 × 4096 pixel CCD and takes images with a spatial resolution of approximately 0.5 arscec per pixel, or about four times that of MDI. SDO is in geosynchronous orbit, whereas SOHO orbits the Sun–Earth L1 Lagrange point; partly for this reason, HMI is able to send down much more telemetry. HMI produces full-resolution dopplergrams at a cadence of 45 seconds. HMI observes the Fe I 6173 Å spectral line, so that it sees a slightly lower height in the solar atmosphere than MDI, which observed the Ni I 6768 Å line (Fleck, Couvidat, and Straus, 2011)
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