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Studying the solar cycle

Solar cycle changes in the solar corona as viewed by Atmospheric Imaging Assembly (AIA) on the Solar Dynamic Observatory in extreme ultraviolet (EUV) toward the end of the latest solar minimum activity period in May 2010 (left half) and during the current solar maximum period in December 2014 (right half). The three-color red, green and blue image channels are composed of observations made in three AIA channels: 171, 193, and 211 Å, respectively, corresponding to their most dominant emission lines of Fe IX, Fe XII, and Fe XIV with formation temperatures of ~0.7, 1.2, and 2.0 MK.

Morgan et al. Sci. Adv. 2017;3: e1602056

Scientists have analyzed thousands of measurements from a NASA mission observing the Sun since 2010 in order to learn more about the solar corona, the outermost part of the Sun’s atmosphere. Studying the solar corona can help understand what drives the solar cycle, how this cycle affects light and heat output by the Sun, and how cycle-related changes may affect Earth.

Past studies of the solar corona have been largely limited to particular regions or much smaller datasets. Huw Morgan and colleagues looked at NASA observations of extreme ultraviolet, or EUV, irradiance from mid-2010 to 2017. This covers most of one approximately 11-year solar cycle in which the corona fluctuates from solar minimum to solar maximum, when sunspots are more pronounced due to intense magnetic activity.

The NASA mission observed both active regions – the shorter-lived, hot, high-mass areas of enhanced magnetic field activity associated with sunspots, as well as the regions that form the less active “quiet Sun.” Morgan and colleagues estimated contributions of the quiet corona and active regions to EUV irradiance.

In a paper published in Science Advances, Morgan et al. found that throughout the solar cycle, the quiet corona dominates contributions to EUV irradiance while active regions create high variability. Overall, the main emission measure (a quantity related to the distribution of plasma in the corona, and temperature) of the quiet corona and active regions remained “remarkably constant.” Importantly, the authors note that sunspots alone cannot predict EUV irradiance, highlighting the need for continued measurement of the Sun.

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