Solar Response and Long‐Term Trend of Midlatitude Mesopause Region Temperature Based on 28 Years (1990–2017) of Na Lidar Observations

We present midlatitude solar response and linear trend from Colorado State University/Utah State University Na lidar nocturnal temperature observations between 1990 and 2017. Along with the nightly mean temperatures (_Ngt), we also use the corresponding 2‐hr means centered at midnight (_2MN), resulting in vertical trend profiles similar in shapes as those previously published. The 28‐year trend from _Ngt (_2MN) data set starts from a small warming at 85 km, to cooling at 87 (88) km, reaching a maximum of 1.85 ± 0.53 (1.09 ± 0.74) at 92 (93) km and turns positive again at 102 (100) km. The 6‐month winter trend is much cooler than the 4‐month summer trend with comparable solar response varying around 5 ± 1 K/100 SFU throughout the profile (85–105 km) with higher summer values. We explore the observed summer/winter trend difference in terms of observed gravity wave heat flux heating rate at a nearby station and the long‐term trend of gravity wave variance at a midlatitude. Between 89 and 100 km, the lidar trends are within the error bars of the Leibniz Middle Atmosphere (LIMA) summer trends (1979–2013), which are nearly identical to the lidar‐Ngt trend. We address the need of long data set for reliable analysis on trend, the extent of trend uncertainty due to possible tidal bias, the effect of a Pinatubo/episodic function, and the impact of stratospheric ozone recovery. Plain Language Summary Anthropogenic greenhouse gases, primarily CO2, alter the Earth's climate, causing warming in the troposphere and the Earth/ocean surface, and cooling in the middle and upper atmosphere. Although determination of the long‐term temperature trend in the important mesopause region, about 100 km from Earth surface where the annual mean temperature is lowest, from ground is very challenging, it has been measured using fluorescence lidar observations. Based on 28‐year (1990–2017) data sets of nightly means and 2‐hr means centered at midnight, the maximum cooling rates in a midlatitude mesopause region are 1.85 K/decade and 1.09 K/decade, respectively, with the latter thought to be relatively free from contamination by atmospheric tides. The winter trend is much cooler than the summer trend. We found good agreement in the summer trend with model prediction and the summer/winter difference correlate with the associated gravity wave heat flux convergence. Key Points We deduced long‐term solar response and linear trend from 28 years lidar observed midlatitude mesopause region noctu