How Do Coronal Hole Storms Affect the Upper Atmosphere?
The solar cycle, often described as an increase and decrease of solar activity with a period of about 11 years, can strongly affect Earth's thermosphere and ionosphere. Although the longest direct record of solar activity is based on sunspot number, a more quantifiable parameter is solar irradiance at extreme ultraviolet (EUV) wavelengths, which varies by more than a factor of 3 over the sunspot cycle. To first order, upper atmospheric variation is a result of changes in ionizing fluxes at EUV wavelengths. As the solar cycle passes its EUV peak and approaches minimum, the number of solar active regions declines, leading to a reduction and then a near absence of coronal mass ejections (CMEs)—episodic events of high-energy bursts of solar plasma that cause geomagnetic storms at Earth. During the solar cycle's declining phase, coronal holes begin to occupy lower latitudes on the solar surface and fall in line with the ecliptic plane.
Related discussions occurred in September 2010 during the Boulder (Colorado) High Speed Stream and Solar Minimum Workshop at the National Center for Atmospheric Research (NCAR). NCAR is supported by the National Science Foundation. Research by Mannucci, Tsurutani, and Verkhoglyadova occurred at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under a contract with NASA. Research by Thayer was supported by NASA and the Air Force Office of Scientific Research Multidisciplinary University Research Initiative award FA9550-07-1-0565 (University of Colorado).