Influence of solar cycle and chemistry on tropical (10°N-15°N) mesopause variabilities

Abstract

Monthly averaged zonal mean temperature and ozone volume mixing ratios obtained from "Sounding of the Atmosphere using Broadband Emission Radiometry" instrument on board "Thermosphere Ionosphere Mesosphere Energetics and Dynamics" satellite for the years 2002-2012 are used to study the seasonal and solar cycle variabilities of tropical (10°N-15°N) mesopause structure. The mesopause temperature and ozone mixing ratios are positively correlated with solar cycle due to changes in CO2 and O, respectively. Although the seasonal variation in mesopause temperatures is quite small, the mesopause altitudes are comparatively higher (~99-100-km) in April and lower (~95-km) in September. The factors controlling the tropical mesopause structure are investigated by taking the mesopause variability for the year 2011 as a case study as it resembles the long-term mean seasonal variation. It is found that the radiative cooling due to 15-μm CO2 infrared emissions is the only cooling mechanism in the mesopause region. The net heating rates obtained from (i) solar heating by O2 and O3, (ii) chemical heating due to seven major exothermic reactions, (iii) O3 long-wave radiative heating, and (iv) CO2 cooling are smaller (~20-K/d) in April and larger (~85-K/d) in September at lower thermosphere (~99-101-km). The downward heat conduction from the lower thermosphere forces the mesopause to lower heights in September, although no downward heat conduction is observed in April. Key Points Tropical mesopause temperature and ozone correlate positively with solar cycle Mesopause height is slightly lower in June-September than in November-April Heating due to O3 and radiative cooling due to CO2 determine mesopause height