Solar ultraviolet variability during the TIMED mission

Abstract

The Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) satellite was launched in December 2001 near solar cycle 23 maximum. The solar activity remained at a high level during 2002 and has since declined to moderate activity. As part of the TIMED mission objective to study the energetics of the upper atmosphere, the Solar EUV Experiment (SEE) aboard TIMED measures the solar extreme ultraviolet (EUV) energy input. The SEE instrument is measuring the solar UV irradiance with a spectral resolution of 0.4 nm between 27 and 194 nm and with 7–10 nm resolution shortward of 27 nm. The solar UV irradiance varies on all time scales, seconds to years, and this variation is very dependent on wavelength. During the TIMED mission, the SEE instrument has observed over 200 flares that last from minutes to hours, over 30 solar rotations that have a period of about 27 days, and maximum to moderate conditions during the current 11-year solar cycle. The coronal emissions, such as the Fe XVI 33.5 nm emission and X-rays, vary the most, with variations of a factor of 10 for the larger flares, a factor of two for solar rotation, and a factor of four during the TIMED mission (2 years). The transition region and upper chromospheric emissions, such as the H I 121.6 nm and He II 30.4 nm emissions, vary less, with variations of a factor of 1.2 for solar rotation and a factor of 2 during the TIMED mission. The lower chromospheric and photospheric emissions vary even less. The variations of the solar UV irradiance shortward of 194 nm will be discussed in the context of the TIMED mission.