Short-period Atmospheric Gravity Waves Research Articles

Observation of Very Short Period Atmospheric Gravity Waves in the Lower Ionosphere Using Very Low Frequency Waves

AbstractWe present observations of very short period (<5 min) wavy fluctuations (WFs) in the lower ionosphere (75–85 km) on the night of 21 January 2016, using subionospherically propagating very low frequency signals. Four out of six transmitter signals recorded by the very low frequency/low frequency receiver at the Pisgah Astronomical Research Institute showed WFs simultaneously. However, their time of occurrence and intensities were different. Power spectral analysis indicates a period of ~3–4 mins, largely associated with the two regions of strong convective and lightning activity in the lower troposphere. Background wind (at ~15 and 80 km) direction shows westward propagation, suggesting important role of the convective storm located east of Pisgah Astronomical Research Institute. The GPS total electron content analysis also divulges similar WFs at upper ionospheric altitude (~300 km) on the GPS total electron content stations around the two storms. The observational analysis suggests atmospheric gravity waves from the convective regions propagate upward up into the ionospheric altitudes. Hence, the present study reinforces the strong coupling of troposphere and ionosphere through the convectively generated very short period waves.

On the use of Very Low Frequency transmitter data for remote sensing of atmospheric gravity and planetary waves

Continuous ground-based monitoring of Very Low Frequency (VLF) transmitter signals is an efficient remote sensing tool for studying of the lower ionosphere (60–90km). Here, we present the use of VLF radio data to study short-period (∼min–hrs) atmospheric gravity waves and long-period (∼days) planetary waves. We analyse VLF data from several receiving stations obtained by ICSP-VLF network during the total solar eclipse of July, 2009 to show the existence of short-period atmospheric gravity waves. We find dominant wave periods range from 10min to 1h around the time of maximum eclipse phase which could be associated with atmospheric gravity waves excited due to the eclipse. We also analyse VLF amplitude data of 2007 received at ICSP, Kolkata from VTX (18.2kHz) transmitter for planetary wave-type oscillations in the mesosphere–lower ionosphere system. Fourier and wavelet analysis show presence of periodic structures with periodicity in the range of 5–27days. We compare VLF planetary spectrum with spectrum obtained from total column density of Ozone and mesospheric average temperature data which may indicate vertical coupling between the stratosphere and ionosphere in winter to early spring time.

Mesosphere summer echoes observed with VHF and MF radars at Wakkanai, Japan (45.4°N)

Mesosphere summer echoes (MSE) were observed during 2000–2001 and in 2009 with a 46.5-MHz VHF radar at Wakkanai, Japan (45.4°N). MSE at VHF (VHF-MSE) are active at 80–90km altitudes in the daytime mainly during June–July. Echo layer thickness ranges from a few to about 5km, and layer altitude and echo intensity are often modulated by short-period atmospheric gravity waves while the layer descends gradually at 0.2–0.8m/s with time, maybe due to tides. Detailed analyses of three VHF-MSE events show that meridional winds at the MSE altitudes over Wakkanai observed with a collocated 1.96-MHz MF radar are equatorward, indicating that cold ice particles and/or irregularities are advected southward from higher latitudes. It is found that VHF-MSE are often accompanied by simultaneous MSE at MF (MF-MSE) from a common volume, suggesting that plasma turbulence and/or short-scale electron density structures are responsible for the generation of VHF- and MF-MSE.

Multilayered structures in the ionosphereF2region and impulse-like increase of the nightglow red 630 nm line intensity as a result of influence of shear excited atmospheric vortical perturbations

[1] It is shown that the formation of multilayered structures in the midlatitude ionosphere F2 layer can be caused by atmospheric vortical perturbation (shear wave) evolving in the meridional wind with linear zonal shear and can be accompanied by an impulse-like increase (with duration about 1–3 h) in the red 630 nm line total nightglow intensity. The shear wave can produce an enhancement of the northward wind (or decrease in the southward one), which can cause a downward motion of the F2 layer peak height resulting in the impulse-like increase in the red line intensity. An important increase in the red line intensity occurs until the appearance of the secondary peak below the F2 layer main peak close to the luminous layer. The observed impulse-like increase of the red line intensity followed by oscillations characteristic of short-period atmospheric gravity waves (AGWs) is considered as a possible result of transformation of shear excited three-dimensional vortical perturbation (shear wave) into AGW. The possibility of the appearance of the multilayered structure in the ionosphere F2 region and the impulse-like increase in the red line intensity under the influence of shear excited waves with vertical wave number kz ≠ 0 is demonstrated analytically and numerically.

The formation of sporadic E layers by a vortical perturbation excited in a horizontal wind shear flow

Abstract. The formation of the mid-latitude sporadic E layers (Es layers) by an atmospheric vortical perturbation excited in a horizontal shear flow (horizontal wind with a horizontal linear shear) is investigated. A three-dimensional atmospheric vortical perturbation (atmospheric shear waves), whose velocity vector is in the horizontal plane and has a vertical wavenumber kz≠0, can provide a vertical shear of the horizontal wind. The shear waves influence the vertical transport of heavy metallic ions and their convergence into thin and dense horizontal layers. The proposed mechanism takes into account the dynamical influence of the shear wave velocity in the horizontal wind on the vertical drift velocity of the ions. It also can explain the multi-layer structure of Es layers. The pattern of the multi-layer structure depends on the value of the shear-wave vertical wavelength, the ion-neutral collision frequency and the direction of the background horizontal wind. The modelling of formation of sporadic E layers with a single and a double peak is presented. Also, the importance of shear wave coupling with short-period atmospheric gravity waves (AGWs) on the variations of sporadic E layer ion density is examined and discussed.

Finland HF and Esrange MST radar observations of polar mesosphere summer echoes

Abstract. Peculiar near range echoes observed in summer with the SuperDARN HF radar in Finland are presented. The echoes were detected at four frequencies of 9, 11, 13 and 15 MHz at slant ranges of 105–250 km for about 100 min. Interferometer measurements indicate that the echoes are returned from 80–100 km altitudes with elevation angles of 20°–60°. Echo power (< 16 dB), Doppler velocity (between –30 and + 30 ms-1) and spectral width (< 60 ms-1) fluctuate with periods of several to 20 min, perhaps due to short–period atmospheric gravity waves. When the HF radar detected the echoes, a vertical incidence MST radar, located at Esrange in Sweden (650 km north of the HF radar site), observed polar mesosphere summer echoes (PMSE) at altitudes of 80–90 km. This fact suggests that the near range HF echoes are PMSE at HF band, although both radars did not probe a common volume. With increasing radar frequency, HF echo ranges are closer to the radar site and echo power becomes weaker. Possible mechanisms to explain these features are discussed.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; thermospheric dynamics; waves and tides; instruments and techniques)

On the possibility of in situ shear excitation of vortical perturbations and their coupling with short-period gravity waves by airglow and ionosphere observations

The essential part of in situ shear excited vortical perturbations (shear waves) at mesosphere–thermosphere heights is considered by use of airglow, from Abastumani (42.8°E, 41.8°N), and ionospheric, from Tbilisi (44.8°E 41.7°N), observations. One of the characteristic processes for excitation of vortical perturbations (shear waves) is a phenomenon of its possible transformation into short-period atmospheric gravity waves (GWs) with a frequency close to the Brunt–Väisälä frequency (BVf). More convenient condition for shear excitation of vortical perturbations and their transformation into short-period GWs at mesosphere–thermosphere heights appears on magnetically disturbed days and during the mid-latitude springtime transition. Nighttime observations of intensities of the hydroxyl OH(8-3) band, the oxygen green 557.7 and red 630.0 nm lines from Abastumani show a possible presence of shear excited vortical perturbations and an accumulation of wave energy by short-period GWs with frequencies close to BVf on magnetically disturbed days and during springtime transition. The amplitude amplification of short-period gravity waves is considered as a possible indicator of regional and planetary scale changes of the horizontal wind in the upper atmosphere.

Short-Period Atmospheric Gravity Waves: A Study of Their Dynamic and Synoptic Features

Abstract Atmospheric gravity wave records at Palisades, N.Y., during the two-mouth period November–December 1969 an investigated in the light of concurrent weather patterns and atmospheric soundings. Although it seems to be a characteristic of atmospheric gravity waves that no one mechanism affords a complete explanation for their behavior, it is shown that many of the waves seem to be generated by shear instability of the upper troposphere winds and that the pressure amplitudes of the waves are greatly increased by the presence of large static stability in the lower troposphere. A simple three-layer model is able to simulate these features. The synoptic applications of these observations are then investigated and it is shown that times of large-amplitude gravity waves often precede the onset of cyclonic precipitation by some 12–24 h while times of small-amplitude gravity waves usually indicate continuation of fair weather.