20 Years of GONG: p Mode Parameters and the Solar Cycle
Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany
Main category
Natural Sciences (Astrophysics and Astrononmy)
The properties of solar acoustic oscillations are known to vary with the solar cycle. With 20 years of continuous observations by GONG, we are now in the position to carry out a comparison of the variation of the p mode parameters between the last two solar cycles. We present results of our analyses of the widths, amplitudes, areas (width $\times$ amplitude), and background amplitudes of solar p modes, as well as the corresponding physical quantities for modes of harmonic degree $10\leq l \leq 150$ of the global solar oscillations. We investigate the variation of these parameters for different ranges of mode frequency and harmonic degree. The fractional change in mode widths is largest for modes in the frequency range 2400-3300 $\mu$Hz, regardless of harmonic degree, with an amplitude of $\sim 10\%$ between solar minimum and maximum. The fractional change in mode amplitude exhibits a peak-to-peak amplitude of $\sim 30\%$ over solar cycle 23. Also, we find hysteresis in mode amplitudes over cycle 23 for some combinations of mode frequencies and harmonic degree. The physical quantities which correspond to the mode parameters (mean square velocity power, total mode energy, mode energy supply rate) are investigated as a function of time for different ranges of harmonic degree. We find that the mode energy supply rate decreased by $\sim 20\%$ over the last 20 years.
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