Cyclic and Long-Term Variation of Sunspot Magnetic Fields
AIR FORCE RESEARCH LAB KIRTLAND AFB NM SPACE VEHICLES DIRECTORATE
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Measurements from the Mount Wilson Observatory MWO were used to study the long-term variations of sunspot field strengths from 1920 to 1958. Following a modified approach similar to that presented in Pevtsov et al. Astrophys. J. Lett. 742, L36, 2011, we selected the sunspot with the strongest measured field strength for each observing week and computed monthly averages of these weekly maximum field strengths. The data show the solar cycle variation of the peak field strengths with an amplitude of about 500 700 gauss G, but no statistically significant long-term trends. Next, we used the sunspot observations from the Royal Greenwich Observatory RGO to establish a relationship between the sunspot areas and the sunspot field strengths for cycles 15 19. This relationship was used to create a proxy of the peak magnetic field strength based on sunspot areas from the RGO and the USAFNOAA network for the period from 1874 to early 2012. Over this interval, the magnetic field proxy shows a clear solar cycle variation with an amplitude of 500 700 G and a weaker long-term trend. From 1874 to around 1920, the mean value of magnetic field proxy increases by about 300 350 G, and, following a broad maximum in 1920 1960, it decreases by about 300 G.