D. Gobbi

Image Measurements of Short Period Gravity Waves at Equatorial Latitudes

A high-performance, all-sky imaging system has been used to obtain novel data on the morphology and dynamics of short-period (<1 hour) gravity waves at equatorial latitudes. Gravity waves imaged in the upper mesosphere and lower thermosphere were recorded in three nightglow emissions, the near-infrared OH emission, and the visible wavelength OI (557.7 nm) and Na (589.2 nm) emissions spanning the altitude range ∼80–100 km. The measurements were made from Alcantara, Brazil (2.3°S, 44.5°W), during the period August-October 1994 as part of the NASA/Instituto Nacional de Pesquisas Espaciais “Guara campaign”. Over 50 wave events were imaged from which a statistical study of the characteristics of equatorial gravity waves has been performed. The data were found to divide naturally into two groups. The first group corresponded to extensive, freely propagating (or ducted) gravity waves with observed periods ranging from 3.7 to 36.6 min, while the second group consisted of waves of a much smaller scale and transient nature. The later group exhibited a bimodal distribution for the observed periods at 5.18±0.26 min and 4.32±0.15 min, close to the local Brunt-Vaisala period and the acoustic cutoff period, respectively. In comparison, the larger-scale waves exhibited a clear tendency for their horizontal wavelengths to increase almost linearly with observed period. This trend was particularly well defined around the equinox and can be represented by a power-law relationship of the form λh=(3.1±0.5)τob1.06±0.10, where λh is measured in kilometers and τob in minutes. This result is in very good agreement with previous radar and passive optical measurements but differs significantly from the relationship λh ∝ τ1.5ob inferred from recent lidar studies. The larger-scale waves were also found to exhibit strong anisotropy in their propagation headings with the dominant direction of motion toward the-NE-ENE suggesting a preponderance for wave generation over the South American continent.

An investigation of gravity wave activity in the low‐latitude upper mesosphere: Propagation direction and wind filtering

horizontal phase speeds of up to � 80 m s � 1 . The large-scale ‘‘band’’ wave patterns (horizontal wavelength between 10 and 60 km) exhibited a clear seasonal dependence on the horizontal propagation direction, propagating toward the southeast during the summer months and toward the northwest during the winter. The direction of propagation was observed to change abruptly around the equinox period in mid March and at the end of September. Using a numerical simulation of gravity wave propagation in a seasonally variable climatological wind field, we have determined that the observed anisotropy in the wave propagation directions can be attributed to a strong filtering of the waves in the middle atmosphere by stratospheric winds. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 3332 Meteorology and Atmospheric Dynamics: Mesospheric dynamics; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: airglow, winds, gravity waves, wind filtering, imager

Ionospheric irregularity zonal velocities over Cachoeira Paulista

Abstract We have studied the zonal drift velocity of nighttime ionospheric irregularities from Cachoeira Paulista (22.41°S,45°W, dip latitude −17.43°), a station under the Equatorial Anomaly, from December 1998 to February 1999 using L1 band GPS receivers and OI 630 nm all-sky images. The average decimetric solar flux index for this period of increasing solar activity was about 145 and magnetically quiet days with ΣKp 55 m in the magnetic east–west direction and probed small scale plasma structures (scale size about 400 m ) at altitudes near 350 km . The zonal irregularity drift velocities measured by this technique were eastward with values of about 160 m / s at 20 LT, about 140 m / s around midnight, and decreased further in the post-midnight sector. The variability of these drifts decreased significantly after midnight. The zonal velocities of large scale plasma structure were obtained using OI 630 nm all-sky images from a region located about 24.1°S and 45°W at a nominal height of 250 km which corresponds to the bubble projection along the magnetic field lines to 350 km over Cachoeira Paulista. These all-sky imager derived zonal drifts are also eastward, but have magnitudes smaller than the spaced GPS eastward drifts, particularly in the pre-midnight sector. We will discuss these two drift measurement techniques and the interpretation of our results.

Ionospheric plasma bubble zonal drift: a methodology using OI 630 nm all-sky imaging systems

Abstract With the recent advances in all-sky imaging technology for nightglow emission studies, the F-region OI 630 nm emission has become an important tool for ionospheric/thermospheric coupling studies. At equatorial and low latitude regions, the all-sky imaging observations of the OI 630 nm emission show quasi north-south aligned intensity depletion bands, which are the optical signatures of large scale F-region plasma irregularities. By observing the motion of the intensity depleted bands it is possible to infer the ionospheric plasma zonal velocity of the depletion. The north-south aligned structures seen in the field of view of the all-sky imaging system corotate with the ionospheric plasma, so that by calculating the spatial displacements occurring during successive OI 630 nm emission images we can infer the ionospheric plasma drift velocity. However, the plasma bubbles have their own internal space-time dynamics leading to changes in their shape and dimensions and this may induce some errors in the calculated drift velocities. In this paper we take into account the space-time changes in the plasma bubbles in order to calculate the ionospheric plasma zonal drift velocities using the OI 630 nm nightglow emission.

Equatorial planetary wave signatures observed in mesospheric airglow emissions

AbstractTheuppermesosphericairglowemissions,OI557:7nm,NaD589:3nm,OH(6,2)andO 2 atmospheric(0,1)bandhavebeenmeasuredusingaground-basedmultichannelairglowphotometerintheequatorialregion,S˜aoJo˜aodoCariri(7 ◦ S,37 ◦ W),Brazil.Goodweatherconditionmadeitpossibletostudynocturnalanddaytodayvariationoftheemissionratescontinuouslyformorethan12dayspermonthfromJanuarytoDecember1998.Itisfoundthattherearedistinct2-and3.5-dayperiodoscillationsintheemissionrates,thosearemostprobablyduetoRossby-gravitywaveandKelvinwave,respectively.TheamplitudesofoscillationoftheKelvinwavearelargeinJuneandJuly,being40%forOI5577,23%forO 2 b(0,1)and26%forOH(6,2). c 2002ElsevierScienceLtd.Allrightsreserved. Keywords:Equatorialairglow;Planetarywaves 1.IntroductionIntheEarth’smiddleatmosphere,between20and90km,long period and large scale atmospheric waves are propa-gating along the longitudinal (east–west) direction. In themiddle to high latitude, they are called Planetary waves(Rossby waves), with the phase propagating towards thewest. In the equatorial zone, characteristics of the wavesaredierent.Matsuno(1966)predictedfromhismodelcal-culationthepresenceoftwowaves,onewestwardandtheothereastwardpropagating.Almostatthesametime,fromradiosondeobservation,YanaiandMaruyama(1966)iden-tiedawestwardpropagatingwave,withaperiodofaround5 days and horizontal scale of about 10;000km (wavenumber

Response of the airglow OH emission, temperature and mesopause wind to the atmospheric wave propagation over Shigaraki, Japan

Simultaneous observations of the night airglow OH (6, 2) band emission intensity and rotational temperature, by a sky scanning airglow spectrophotometer, and meteor winds, by a middle and upper atmosphere radar (MU radar), were carried out at Shigaraki (34.9°N, 136.1°E), Japan, from October 29 to November 11, 1994, as the first phase of a campaign, and from July 25 to July 31, 1995 as the second phase. Horizontal structures in the OH emission intensity and rotational temperature were monitored optically, together with the background wind and its wave induced fluctuations, measured by MU radar. Since the MU radar makes a direct measurement of the vertical wavelength, and the OH spectrophotometer makes a direct measurement of the horizontal wavelength, the two techniques are mutually complementary to determine intrinsic wave parameters. Gravity waves with intrinsic periods of 2 to 9 hours, horizontal wavelengths of 500 to 3000 km and vertical wavelengths of 12 to 75 km were identified. Between the two different observation techniques, there is a reasonable agreement in the inferred wave characteristics.

An unusual airglow wave event observed at Cachoeira Paulista 23° S

Abstract An all-sky CCD airglow imager has been used to obtain a wealth of data on the MLT airglow emissions from Cachoeira Paulista, Brazil (22.7° S, 45.0° W). Measurements of the NIR OH, OI(557.7 nm), O2(0,1) and the thermospheric OI(630.0 nm) emissions have been made since October 1998. In addition, simultaneous measurements of the airglow zenith intensities and rotational temperatures, and vertical wind structure between 80 and 100 km were made using a co-located multi-channel photometer and a meteor radar. On the night of July 13, 1999, the CCD images showed active wave events exhibiting a variety of short period wave structures. The activity started with ripple patterns over the whole sky, lasting for almost one hour. Later the patterns changed to extensive band-like structures that lasted for several hours. After local midnight a bore-like wave-front passed overhead propagating towards the North. When the front was in the zenith the image data showed an increase in both the OI (5577 nm) and O2 (0,1) emissions, whilst a decrease in the OH intensity was observed, which is opposite to that observed by Taylor, (1995a). The co-located photometer revealed an exceptionally large intensity (40%) and temperature (∼12%) perturbation throughout the night that appears to be associated with a large-scale gravity wave perturbation with significant vertical phase shift between the different emissions.

Dynamical influence on the equatorial airglow observed from the South American sector

The upper atmospheric airglow emissions, OI 557.7 nm, NaD 589.3 nm, OH (9,4) and O2 atmospheric (0,1) bands and their rotational temperatures have been measured using ground-based multichannel airglow photometers, one located near the equator at Fortaleza (3.9 S, 38.4 W) and the other at lowmiddle latitude Cachoeira Paulista (22.7 S, 45.0 W). Monthly averaged nocturnal variations calculated from the data from 1987 to 1993 were used to study the influence of atmospheric dynamical processes on these emissions. Harmonic analysis revealed that the Cachoeira Paulista data are mainly represented by a 12 hour period oscillation. Phase differences between the different emissions reveal that the vertical phase velocity is about 4 km/h and the vertical wavelength about 50 km. Fortaleza data on the other hand showed longer period, 24 hours, slow vertical propagation velocity, 1.1 km/h, and short vertical wavelength, about 30 km. These facts lead us to conclude that the Cachoeira Paulista data are mainly controlled by the semidiurnal tidal oscillation and the Fortaleza data are controlled by the diurnal tide.

Simultaneous lidar observation of a sporadic sodium layer, a “wall” event in the OH and OI5577 airglow images and the meteor winds

Abstract Rare and sporadic events, caused by unusual conditions in the MLT region may occur such as the sporadic metal layers, observed by lidars, and the more recently detected sharp front in the nightglow emissions imaged with CCD all sky cameras. Few events of this latter kind have been reported in the literature and no simultaneous occurrence of both. In this work, we report the observation of a sporadic sodium layer measured by lidar in Sao Jose dos Campos, Brazil (22.5° S , 46° W ) on the night of July 13–14, 1999 which was followed by a rare “wall” event observed at Cachoeira Paulista (CP) (23° S , 45° W ) in the OH NIR and OI5577 by a CCD imager, and zenith tilting filter photometers. The wind field was also observed by a SkiYmet meteor radar system also operated at CP. At around 23:45 the sodium layer had an abrupt change in its shape. At the same time unusual situation also had occurred in the airglow data taken in CP both with the imager and with the zenith photometer. A sequence of all sky images for the OI 557.7 nm and OH NIR showed very clear lines dividing the sky into bright and dark areas, with the first propagating from the NE to SW at ∼70 m / s and the second propagating to NNE. Zenith photometer also showed a rapid decrease of the OH(6,2) intensity at around 23:45 and a very large increase in the rotational temperature of O2(0,1) (at ∼94 km ) from 23:00 to 24:00 (∼40 K ) . The meridional and zonal wind structure measured by the meteor radar in CP showed that from 21:00 to 01:00 a wind shear was present both in the meridional and zonal winds with the phase propagating from ∼97 to 80 km during this time which were very well correlated with the vertical displacement of the sporadic peak. Large-amplitude gravity wave was also detected. The temperature enhancement above 90 km could be responsible for the sodium density increase and also for the set up of the conditions for the airglow events.

Simultaneous measurements of airglow oh emissionand meteor wind by a scanning photometer and the muradar

Abstract Simultaneous observations of the night airglow OH (6,2) band emission intensity androtational temperature, by a sky scanning airglow spectrophotometer, and meteor winds, by amiddle and upper atmosphere radar (MU radar), were carried out at Shigaraki (34.9°N, 136.1°E), Japan, from 29 October to 11 November 1994, during the CRISTA/MAHRSIcampaign. Horizontal structures in the OH emission intensity and rotational temperature weremonitored optically, together with the background wind and its wave induced fluctuations,measured by MU radar. A total of 5 nights among the 7 nights of useful data showed wavelikeoscillations in both the emission intensity and temperature. Good agreement was found betweenindividual wave parameters—intrinsic period, phase velocity, horizontal and verticalwavelength—obtained from the radar and optical techniques. On some occasions a short period(∼3 h) gravity wave was observed to propagate, superposed on a long period (∼9 h) wave.