Mesospheric Temperature Inversions Observed in OH and O₂ Rotational Temperatures From Mount Abu (24.6°N, 72.8°E), India

Abstract

We present a detailed investigation of upper mesospheric temperature inversions (MTIs) based on around 4.5 years of data of O 2 and OH nightglow emission intensities (I(O 2 ) and I (OH)) and temperature (T(O 2 ) and T (OH)) corresponding to 94 and 87-km altitudes. These measurements were carried out using Near-Infrared Imaging Spectrograph from a low-latitude location, Mount Abu (24.6°N, 72.8°E), in India. A total of 745 nights of Near-Infrared Imaging Spectrograph observations is used, which showed the mean of T(O 2 ) and T (OH) to be 199.6 and 203.0 K. However, there are nights that showed T(O 2 ) greater than T (OH), which is considered as an indicator of upper MTIs. Among these nights, around 28% (209 out of 745) showed MTIs. It is found that 75% and 25% of MTIs occurred during premidnight and postmidnight hours. It was noted that maximum number of nights showed MTIs for a duration of around 4 hr followed by 3, 2, 5, and 6 hr. Investigation of causative mechanism for the upper MTIs revealed that although both wave dynamics and chemical heating by the exothermic reactions do work together, the in situ chemical heating process seems to be a more probable cause as compared to the vertical transport of energy from lower below. So far, such detailed statistics on MTIs does not exist in the published literature, and thus, the information presented in this work provides the necessary input for a greater understanding of the atmospheric temperature structure through modeling and simulation studies.Plain Language Summary Earth's mesosphere lower thermosphere (MLT) region (60-110 km) is very dynamic and least explored region. It responds to forcing from above due to solar influences and from below due to various upward propagating waves that are generated in the troposphere. Understanding the vertical coupling of the atmosphere under varying geophysical conditions is an emerging field of research. One of the methods to investigate dynamics of the MLT region is by measuring nightglow emission intensities and temperatures corresponding to the different airglow emission altitudes. We have investigated upper mesospheric temperature inversions (MTIs) that are the narrow thermal layers showing an inversion in the vertical temperature gradient from negative to positive. This work is possible due to around 4.5 years of high cadence ground-based observations of O2 and OH nightglow emission intensities and corresponding temperatures from, Gurushikhar, Mount Abu (24.6°N, 72.8°E) in India. Results on the statistics in terms of the percentage and duration of occurrences and possible formation mechanisms for the upper MTIs have been discussed. Such detailed investigations on the occurrence characteristics and possible causative mechanism of MTIs provides the necessary inputs for a greater understanding of the mesospheric temperature structure through theoretical and simulation studies.

Vertical Propagation Speeds of Gravity Waves in the Daytime as a Precursor to the Onset of the Equatorial Spread‐F

“…It is to be noted that gravity waves do not show vertical phase propagation every day in thermospheric altitudes. Different background factors (such as the wind, temperature, and neutral densities) affect their propagation activity (Laskar et al., 2015; Mandal et al., 2020; Pramitha et al., 2015; Singh & Pallamraju, 2016, 2018). In one of our earlier studies with data of 2 years, it was noted that vertical propagation in gravity waves exists in about 40% of the days over a low‐latitude location, Ahmedabad, India (Mandal & Pallamraju, 2020).…”

Upper stratosphere-mesosphere-lower thermosphere perturbations during the intense Arctic polar night jet in 2019-2020

“…With the usual vertical coupling of the stratosphere and the MLT region, when the polar stratosphere is disturbed by planetary waves destroying the polar jet, a corresponding wind reversal also occurs in the MLT, allowing the GW to break there. However, in the winter of 2019-2020, the situation seems the opposite, since fewer GWs propagate upward through the well-developed stratospheric polar vortex.Another mechanism is chemical heating in the MLT region due to exothermic reactions (Mlynczak & Solomon, 1993;Singh & Pallamraju, 2018). Odd oxygen and odd hydrogen reactions are the most important in the MLT.…”

Lower-thermosphere–ionosphere (LTI) quantities: current status of measuring techniques and models

“…Electron and ion precipitation ionise and dissociate neutrals through collisions (Sinnhuber et al, 2012). This has a direct effect in the atmospheric chemical composition via ion chemistry which leads to production of odd hydrogen (HO x ) and nitrogen (NO x ) (e.g.…”

“…Chemical energy is deposited in the LTI through the exothermic reactions typically involving oxygen (atomic and molecular) and ozone (e.g. Singh and Pallamraju, 2018). Neutral species, namely O 3 , H 2 O, CO 2 , OH, and aerosols, are believed to play a role both in the chemistry of the LTI and in the radiative balance of the mesosphere (Mlynczak, 2000).…”