Near InfraRed Imaging Spectrograph (NIRIS) for ground-based mesospheric OH(6-2) and $$\hbox {O}_{2}$$ O 2 (0-1) intensity and temperature measurements

Abstract

This paper describes the development of a new Near InfraRed Imaging Spectrograph (NIRIS) which is capable of simultaneous measurements of OH(6-2) Meinel and O 2 (0-1) atmospheric band nightglow emission intensities. In this spectrographic technique, rotational line ratios are obtained to derive temperatures corresponding to the emission altitudes of 87 and 94 km. NIRIS has been commissioned for continuous operation from optical aeronomy observatory, Gurushikhar, Mount Abu (24.6 • N, 72.8 • E) since January 2013. NIRIS uses a diffraction grating of 1200 lines mm −1 and 1024×1024 pixels thermoelectrically cooled CCD camera and has a large field-of-view (FOV) of 80 • along the slit orientation. The data analysis methodology adopted for the derivation of mesospheric temperatures is also described in detail. The observed NIRIS temperatures show good correspondence with satellite (SABER) derived temperatures and exhibit both tidal and gravity waves (GW) like features. From the time taken for phase propagation in the emission intensities between these two altitudes, vertical phase speed of gravity waves, c z , is calculated and along with the coherent GW time period 'τ ', the vertical wavelength, λ z , is obtained. Using large FOV observations from NIRIS, the meridional wavelengths, λ y , are also calculated. We have used one year of data to study the possible cause(s) for the occurrences of mesospheric temperature inversions (MTIs). From the statistics obtained for 234 nights, it appears that in situ chemical heating is mainly responsible for the observed MTIs than the vertical propagation of the waves. Thus, this paper describes a novel near infrared imaging spectrograph, its working principle, data analysis method for deriving OH and O 2 emission intensities and the corresponding rotational temperatures at these altitudes, derivation of gravity wave parameters (τ , c z , λ z , and λ y), and results on the statistical study of MTIs that exist in the earth's mesospheric altitudes.

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

“…The neutral air temperatures near mesopause region are represented using rotational temperatures based on the assumption that they are at local thermodynamical equilibrium with the ambient gas temperatures. The intensity information of such thermalized distribution of OH and O 2 molecules has been used to derive their kinetic temperatures (Singh & Pallamraju, ). T (OH) have been derived from the ratio of P 1 (2) and P 1 (4) rotational line intensities centered at 840.0‐ and 846.5‐nm wavelengths of the OH(6‐2) Meinel band emission, whereas T(O 2 ) is derived using O 2 (0‐1) atmospheric band by taking ratio of PP and PQ pairs of rotational lines centered at 866.0‐ and 868.0‐nm wavelengths.…”

“…T (OH) have been derived from the ratio of P 1 (2) and P 1 (4) rotational line intensities centered at 840.0‐ and 846.5‐nm wavelengths of the OH(6‐2) Meinel band emission, whereas T(O 2 ) is derived using O 2 (0‐1) atmospheric band by taking ratio of PP and PQ pairs of rotational lines centered at 866.0‐ and 868.0‐nm wavelengths. A detailed description of NIRIS and procedure adopted to derive nocturnal intensity and temperature variations using rotational line ratios of O 2 and OH band emissions have been presented earlier (Singh & Pallamraju, , ). NIRIS provides a spectral image for every 5 min of integration.…”

“…In this work, T(O 2 ) and T (OH) for all 745 nights have been derived using procedure as described in our earlier work (Singh & Pallamraju, ). Nocturnal mean T(O 2 ) and T (OH) show large variability throughout the observational period of 1 January 2013 to 18 May 2017.…”

“…These kinds of variations in the nightglow emission intensities at mesospheric altitudes are seen commonly and are known to be due to the propagation of GWs. As can be seen from Figures a and d the O 2 and OH intensity variations need not necessarily show in‐phase variations and it primarily depends on the GW vertical wavelengths and angles of propagation for the upward propagating waves (Singh & Pallamraju, ).…”

“…Nightglow emission intensities and temperatures are important parameters that have been used for investigating dynamics of the MLT region. For carrying out measurement of these parameters a new grating‐based spectrograph, namely, Near‐Infrared Imaging Spectrograph (NIRIS), has been developed in‐house (Singh & Pallamraju, ). NIRIS simultaneously provides the spectral images of the O 2 (0‐1) atmospheric band and OH(6‐2) Meinel band nightglow emissions from which height‐weighted mesospheric temperatures at the two corresponding altitudes (94 and 87 km) are obtained.…”