Effect of cyclone Nilofar on mesospheric wave dynamics as inferred from optical nightglow observations from Mount Abu, India

Abstract

Mesospheric nightglow intensities at three emissions (O2(0–1), OH(6–2) bands, and Na(589.3 nm)) from a low‐latitude location, Gurushikhar, Mount Abu (24.6°N, 72.8°E), in India, showed similar wave features on 26 October 2014 with a common periodicity of around 4 h. A convective activity due to the cyclone Nilofar, which had developed in the Arabian Sea during 25–31 October 2014, was found to be the source as this too showed a gravity wave period coherent with that of the mesospheric emissions on the 26th. The periodicities at the source region were obtained using outgoing longwave radiation fluxes (derived from Kalpana‐1 satellite) which were used as a tracer of tropospheric activity. Cyclone Nilofar had two centers located at a distance of 1103 and 1665 km from the observational station. From the phase offset in time between residuals of O2 and OH emission intensities and the observed common periodicity the vertical phase speed and wavelength have been found to be 1.13 ms−1 and 16.47 km. From the wavelet analyses it is seen that the travel time of the wave from the convection region to O2 emission height was around 8.1 h. From these observations the horizontal phase speed and wavelength of the wave in the mesosphere were calculated to be 37.8 ms−1 and 553 km. These results thus provide not only unambiguous evidence on the vertical coupling of atmospheres engendered by the tropical cyclone Nilofar but also the characteristics of waves that exist during such cyclonic events.

On Deriving Gravity Wave Characteristics in the Daytime Upper Atmosphere Using Radio Technique

“…The true height variation corresponding to a fixed transmission frequency (i.e., a particular value of ionospheric electron density) with time is known as an isoelectron density contour. The presence of common periodicity and downward phase propagation in the time variation of optical emissions from different altitudes provided an unambiguous signature of GW propagation from lower to higher altitudes in the mesosphere-lower thermosphere region (Singh & Pallamraju, 2016). It may be noted that there exists a clear time difference in the modulations of ionospheric heights which are earlier at greater transmission frequencies (and higher altitudes) as compared to the smaller ones (lower altitudes).…”

“…While periodicities of planetary scale oscillations are of the order of several days and spread over lengths of several thousands of kilometers, the oscillations of diurnal scales of periodicities of 24 hr and its subharmonics (12, 8, and 6 hr) have scale lengths of several thousands of kilometers. GWs are known to be generated due to changes in orography (e.g., Alexander, 1996), convective activity (e.g., Singh & Pallamraju, 2016), presence of sharp longitudinal gradients, as in the case of solar terminator (e.g., Forbes et al, 2008), and wind shears (Pallamraju et al, 2014;Pramitha et al, 2015). Sometimes this forcing is intertwined with the planetary scale oscillations in the Earth's atmosphere.…”

“…After the existence of GWs in the atmosphere was postulated by Hines (1960), there have been several experiments using optical, radio, and magnetic measurements from ground, balloon, and spaceborne instruments to characterize them. GWs are known to be generated due to changes in orography (e.g., Alexander, 1996), convective activity (e.g., Singh & Pallamraju, 2016), presence of sharp longitudinal gradients, as in the case of solar terminator (e.g., Forbes et al, 2008), and wind shears (Pallamraju et al, 2014;Pramitha et al, 2015). Further, auroral processes in the high latitudes (Hocke & Schlegel, 1996) and equatorial electrojet in the low and equatorial latitudes (Raghavarao et al, 1988) are known to generate GWs.…”

Multispectral and Multi‐instrument Observation of TIDs Following the Total Solar Eclipse of 21 August 2017

“…Wavelet analysis has been used by previous studies to identify wave characteristics of AGWs and TIDs (e.g., Kim & Chang, 2018;Singh & Pallamraju, 2016). Singh and Pallamraju (2016) studied the vertical propagation of AGWs due to a cyclone by performing wavelet analysis on optical emission brightnesses originating at different altitudes. Kim and Chang (2018) used wavelet analysis to study the variation in the geomagnetic field induced by eclipses.…”

Electrodynamic influence on the diurnal behaviour of neutral daytime airglow emissions

“…The mesospheric (OH, O 2 band) and lower thermospheric (OI 557.7 nm) emission variability has been used to derive information on the mesospheric temperatures (e.g. Taylor et al, 1995;Singh and Pallamraju, 2015), atmospheric gravity waves (Shiokawa et al, 2009;Singh and Pallamraju, 2016), tides, and planetary-scale waves (Nakamura et al, 1998).…”