The impact of gravity waves rising from convection in the lower atmosphere on the generation and nonlinear evolution of equatorial bubble
TL;DR: In this article, a nonlinear evolution of equatorial F-region plasma bubbles under varying ambient ionospheric conditions and gravity wave seeding perturbations in the bottom-side F-layer is studied.
Abstract: . The nonlinear evolution of equatorial F-region plasma bubbles under varying ambient ionospheric conditions and gravity wave seeding perturbations in the bottomside F-layer is studied. To do so, the gravity wave propagation from the convective source region in the lower atmosphere to the thermosphere is simulated using a model of gravity wave propagation in a compressible atmosphere. The wind perturbation associated with this gravity wave is taken as a seeding perturbation in the bottomside F-region to excite collisional-interchange instability. A nonlinear model of collisional-interchange instability (CII) is implemented to study the influences of gravity wave seeding on plasma bubble formation and development. Based on observations during the SpreadFEx campaign, two events are selected for detailed studies. Results of these simulations suggest that gravity waves can play a key role in plasma bubble seeding, but that they are also neither necessary nor certain to do so. Large gravity wave perturbations can result in deep plasma bubbles when ionospheric conditions are not conducive by themselves; conversely weaker gravity wave perturbations can trigger significant bubble events when ionospheric conditions are more favorable. But weak gravity wave perturbations in less favorable environments cannot, by themselves, lead to strong plasma bubble responses.
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TL;DR: In this paper, the role of gravity waves in the instability initiation leading to equatorial spread F development was investigated using ground-based experiments conducted during the 2005 SpreadFEx campaign in Brazil.
Abstract: . The data from ground based experiments conducted during the 2005 SpreadFEx campaign in Brazil are used, with the help of theoretical model calculations, to investigate the precursor conditions, and especially, the role of gravity waves, in the instability initiation leading to equatorial spread F development. Data from a digisonde and a 30 MHz coherent back-scatter radar operated at an equatorial site, Sao Luis (dip angle: 2.7°) and from a digisonde operated at another equatorial site (dip angle: −11.5°) are analyzed during selected days representative of differing precursor conditions of the evening prereversal vertical drift, F layer bottom-side density gradients and density perturbations due to gravity waves. It is found that radar irregularity plumes indicative of topside bubbles, can be generated for precursor vertical drift velocities exceeding 30 m/s even when the precursor GW induced density oscillations are marginally detectable by the digisonde. For drift velocities ≤20 m/s the presence of precursor gravity waves of detectable intensity is found to be a necessary condition for spread F instability initiation. Theoretical model calculations show that the zonal polarization electric field in an instability development, even as judged from its linear growth phase, can be significantly enhanced under the action of perturbation winds from gravity waves. Comparison of the observational results with the theoretical model calculations provides evidence for gravity wave seeding of equatorial spread F.
206 citations
Cites background or methods or result from "The impact of gravity waves rising ..."
...The divergence-free current density condition∇·δJ=0 implies∇·δE=0 (Kherani et al., 2009c)....
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...This aspect can be examined only with a nonlinear simulation which is presented in companion paper (Kherani et al., 2009a)....
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...The nonlinear process of the instability growth is discussed in a companion paper by Kherani et al. (2009a)....
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...In Appendix A, governing equations for a zonal polarization fieldδEx excited by RTI is derived using hydromagnetic equations, detailed derivation is presented in a recent submis- sion (Kherani et al, 2009b)....
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...The nonlinear process of the instability growth is discussed in a companion paper by Kherani et al. (2009a). For Case 1a (conditions of 23 October) in Fig....
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TL;DR: In this article, two airglow CCD imagers, located at Cariri (7.4° S, 36.5° W, geomag. 11° S) and near Brasilia (14.8°S, 47.6°W, geOMag. 10°S) were operated simultaneously and measured the equatorial ionospheric bubbles and their time evolution by monitoring the airglove OI 6300 intensity depletions.
Abstract: . During the Spread F Experiment campaign, under NASA Living with a Star (LWS) program, carried out in the South American Magnetic Equator region from 22 September to 8 November 2005, two airglow CCD imagers, located at Cariri (7.4° S, 36.5° W, geomag. 11° S) and near Brasilia (14.8° S, 47.6° W, geomag. 10° S) were operated simultaneously and measured the equatorial ionospheric bubbles and their time evolution by monitoring the airglow OI 6300 intensity depletions. Simultaneous observation of the mesospheric OH wave structures made it possible to investigate the relationship between the bubble formation in the ionosphere and the gravity wave activity at around 90 km. On the evening of 30 September 2005, comb-like OI 6300 depletions with a distance of ~130 km between the adjacent ones were observed. During the same period, a mesospheric gravity wave with a horizontal wavelength of ~130 km was observed. From the 17 nights of observation during the campaign period, there was a good correlation between the OI 6300 depletion distances and the gravity wave horizontal wavelengths in the mesosphere with a statistically significant level, suggesting a direct contribution of the mesospheric gravity wave to plasma bubble seeding in the equatorial ionosphere.
124 citations
Cites background from "The impact of gravity waves rising ..."
...Abdu et al. (2009) and Kherani et al. (2009) have provided some cases showing evidence for direct GW seeding of the RTI and spread F generation as diagnosed by the 30 MHz radar and the Digisonde at S̃ao Lúıs....
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TL;DR: In this paper, a comprehensive analysis and discussion of the processes of ESF development, suppression or disruption under different phases of a storm activity sequence is presented, and the consequences for ESF occurrence from undershielding and over-shielding penetration electric fields as well as from the disturbance winds and wind dynamo electric field occurring in different local time sectors of the night, as also the irregularity dynamics and longitude extension.
121 citations
TL;DR: A major goal of the National Space Weather Program, and of C/NOFS, is predicting these storms, analogous to thunderstorms in the lower atmosphere due to their adverse effects on communication and navigation signals.
Abstract: [1] Equatorial spread F (ESF) was discovered almost a century ago using the first radio wave instrument designed to study the upper atmosphere: the ionosonde. The name came from the appearance of reflections from the normally smooth ionosphere, which were spread over the altitude frequency coordinates used by the instrument. Attempts to understand this phenomenon in any depth activated such tools as radars and in situ probes such as rockets and satellites in the 1960s. Over the next 15 years, these tools expanded our experimental understanding enormously, and new nonlinear theoretical methods developed in the late 1970s, which led to proposing a name revision from ESF to convective ionospheric storms. Interest in these phenomena continues, but a new, practical aspect has developed from the associated turbulence effects on communications (transionosphere) and navigation (GPS). The first satellite to specifically investigate this problem and the associated goal of predicting occurrences is under the umbrella of the Communications/Navigation Outage Forecast System (C/NOFS). In contemplating the successful first years of the C/NOFS program, reviewing the state of the art in our knowledge of convective ionospheric storms seems appropriate. We also present some initial results of this satellite program. A major goal of the National Space Weather Program, and of C/NOFS, is predicting these storms, analogous to thunderstorms in the lower atmosphere due to their adverse effects on communication and navigation signals. Although ambitious, predictive capability is a noble and important goal in the current technological age and is potentially within our reach during the coming decade.
98 citations
Cites background from "The impact of gravity waves rising ..."
...Figure 9 shows a downward phase progression of the vertical velocity over Jicamarca, which can only be due to an electric field associated with a gravity wave [Abdu et al., 2009; Kherani et al., 2009]....
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TL;DR: In this paper, a seeding hypothesis for the equatorial spread F (ESF) was proposed, based on the discovery that a direct link exists between regions of deep convective activity in the troposphere, where atmospheric gravity waves (GWs) are spawned, and the occurrence frequency of ESF during solstices.
Abstract: [1] A comprehensive explanation for the complex climatology of the so-called equatorial spread F (ESF) has eluded researchers for more than 70 years. Recently, however, a seeding hypothesis has been proposed, which appears to provide the final major piece of this puzzle. The hypothesis is based on the discovery that a direct link exists between regions of deep convective activity in the troposphere, where atmospheric gravity waves (GWs) are spawned, and the occurrence frequency of ESF during solstices. The objective here is to answer two questions that may impede the general acceptance of this hypothesis. We first show why seed plasma perturbations should develop from GW-driven neutral-wind perturbations, but only when the GW source region is located very close to the magnetic dip equator. We then reexamine this relationship using a data set on GW source regions that is better matched (in time and longitudinal coverage), than that used previously, to the data set on ESF activity used by Tsunoda (2010a). We conclude that seeding is indeed playing an important role in the development of ESF.
91 citations
References
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TL;DR: In this paper, the authors present the first study of the three-dimensional nonlinear evolution of an equatorial spread-F bubble, using numerical simulation techniques, and find that finite parallel conductivity effects slow down both the linear and nonlinear bubble evolution compared to the two-dimensional evolution.
Abstract: [1] Using numerical simulation techniques, we present the first study of the three-dimensional nonlinear evolution of an equatorial spread-F bubble. The background ionosphere used to initialize the bubble evolution is computed using a time-dependent first-principles equatorial plasma fountain model together with a prereversal enhancement vertical drift model. We find that finite parallel conductivity effects slow down both the linear and nonlinear bubble evolution compared to the two-dimensional evolution. In addition we find that bubble-like structures with extremely sharp density gradients can be generated off the equator at equatorial anomaly latitudes in agreement with recent observations.
89 citations
TL;DR: In this paper, numerical simulations of the generalized Rayleigh-Taylor instability are presented and the spectral properties of the nonlinear state are analyzed using one-dimensional power spectra calculated along spatial trajectories for selected angles to E(eff).
Abstract: Numerical simulations of the generalized Rayleigh-Taylor instability are presented. The model and simulations are applicable to bottomside and topside spread F, unstable barium cloud dynamics, and collisional interchange instability in general. The principal result is that the evolution of the instability tends to an anisotropic state consisting of nearly sinusoidal (quasi-periodic) variation along the effective electric field, and shocklike structures propagating perpendicular to E(eff) along the extrema of the quasi-periodic structures. The spectral properties of the nonlinear state are analyzed using one-dimensional power spectra calculated along spatial trajectories for selected angles to E(eff). In this way a direct comparison to in situ probe data can be made. The inherent anisotropy of the nonlinear state is reflected in major qualitative differences between the spectra taken parallel to and perpendicular to E(eff). The fundamental finding of the present work is that anisotropy in interchange dynamics is much greater than had been previously reported. This strong anisotropy can explain much of the spectral and spatial structural characteristics of both bottomside and topside spread F. In a companion paper a comparison of the simulation results to various in situ data sets is given.
83 citations
TL;DR: In this article, the authors study the Equatorial Spread F, by looking at its correlations with vertical winds and electric fields and conclude that the ESF extends beyond the height normally ascribed to it by linear theories.
Abstract: The authors study the Equatorial Spread F, by looking at its correlations with vertical winds and electric fields. Present theory relates the ESF to Rayleigh-Taylor instability mechanisms. Here the authors develop correlations between the spread of the ESF and the presence and activity of downward winds and electric fields. As a consequence of this study they conclude that the ESF extends beyond the height normally ascribed to it by linear theories. Daily variations in the vertical winds could be responsible for the variations seen in the ESF.
76 citations
"The impact of gravity waves rising ..." refers background in this paper
...It is known from earlier simulation studies that large upward velocity inside a depletion is an indication of nonlinear growth of CII (Sekar et al., 1994)....
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...These boundary conditions are the same as those chosen by Sekar et al. (1994) and sufficient to ensure the vanishing current density across the lower boundary provided that the ambient ionosphere is invariant at the boundary....
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...The seed perturbation used is either in the form of plasma density perturbation (Ossakow et al., 1979; Sekar et al., 1994; Kherani et al., 2005) or of wind perturbation, that subsequently cause density perturbation (Huang and Kelley, 1996)....
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TL;DR: The Spread F Experiment (or SpreadFEx) as mentioned in this paper performed from September to November 2005, with primary measurements in Brazil, was used to define the potential role of neutral atmosphere dynamics, specifically gravity wave motions propagating upward from the lower atmosphere, in seeding Rayleigh-Taylor instability and plasma bubbles extending to higher altitudes.
Abstract: . We provide here an overview of, and a summary of results arising from, an extensive experimental campaign (the Spread F Experiment, or SpreadFEx) performed from September to November 2005, with primary measurements in Brazil. The motivation was to define the potential role of neutral atmosphere dynamics, specifically gravity wave motions propagating upward from the lower atmosphere, in seeding Rayleigh-Taylor instability (RTI) and plasma bubbles extending to higher altitudes. Campaign measurements focused on the Brazilian sector and included ground-based optical, radar, digisonde, and GPS measurements at a number of fixed and temporary sites. Related data on convection and plasma bubble structures were also collected by GOES 12, and the GUVI instrument aboard the TIMED satellite. Initial results of our SpreadFEx analyses are described separately by Fritts et al. (2009). Further analyses of these data provide additional evidence of 1) gravity wave (GW) activity near the mesopause apparently linked to deep convection predominantly to the west of our measurement sites, 2) small-scale GWs largely confined to lower altitudes, 3) larger-scale GWs apparently penetrating to much higher altitudes, 4) substantial GW amplitudes implied by digisonde electron densities, and 5) apparent influences of these perturbations in the lower F-region on the formation of equatorial spread F, RTI, and plasma bubbles extending to much higher altitudes. Other efforts with SpreadFEx data have also yielded 6) the occurrence, locations, and scales of deep convection, 7) the spatial and temporal evolutions of plasma bubbles, 8) 2-D (height-resolved) structures in electron density fluctuations and equatorial spread F at lower altitudes and plasma bubbles above, and 9) the occurrence of substantial tidal perturbations to the large-scale wind and temperature fields extending to bottomside F-layer and higher altitudes. Collectively, our various SpreadFEx analyses suggest direct links between deep tropical convection and large GW perturbations at large spatial scales at the bottomside F-layer and their likely contributions to the excitation of RTI and plasma bubbles extending to much higher altitudes.
55 citations
"The impact of gravity waves rising ..." refers background or result in this paper
...The precise nature of the seed perturbations is one of the issues that has remained elusive and therefore constitutes one of the major focus of the Spread F Experiment (SpreadFEx) (Fritts et al., 2009, and reference therein)....
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...The campaign period was September to November 2005, during which spread F were observed on several nights (Fritts et al., 2009; Abdu et al., 2009)....
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...These theoretical estimates appear to be in good agreement with the typical GW periods observed during the SpreadFEx campaign that were found to be in the ∼15–60 min range near 300 km (Abdu et al., 2009; Fritts et al., 2009)....
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...Spread F activity was found to vary day-to-day depending on the ambient ionospheric conditions and GW characteristics (Fritts et al., 2009; Abdu et al., 2009)....
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TL;DR: In this article, the problem of day-to-day variability in the occurrence of equatorial spread F (ESF) is discussed, and it is shown that gravity waves over a wide range of amplitude and wavelength are a very effective seed mechanism for production of ESF.
Abstract: Production and evolution of plasma bubbles seeded by gravity waves in the equatorial F region are studied under different conditions using a computer simulation. The problem of day-to-day variability in the occurrence of equatorial spread F (ESF) is discussed. It is shown that gravity waves over a wide range of amplitude and wavelength are a very effective seed mechanism for production of ESF. However, there is the day-to-day variability in the occurrence of ESF even if gravity waves are omnipresent. The maximum height of the F peak arid the bottomside background electron density gradient can significantly influence production and rise of plasma bubbles. It is found also that the timing of the seed gravity wave is critical to ESF generation. If a gravity wave exists in the F region during the upward drifting of the F layer, the gravity wave can initiate the Rayleigh-Taylor instability and result in topside bubbles. In contrast, gravity-wave-induced perturbations in a descending ionosphere evolve into large-scale, wavelike structures but not bubbles. Gravity waves can propagate in any direction, but only the gravity waves propagating with an azimumal angle range of about 10° relative to the zonal direction are expected to be capable of generating ESF bubbles. These factors are all variable from day to day. Their variation must cause variation of resulting ESF. In addition, we show first that in agreement with linear analysis, a velocity shear in the plasma drift can generate plasma structures in the bottomside F layer. For reasonable ionospheric parameters, the shear-determined wavelength of the Rayleigh-Taylor instability is a few tens of kilometers. The plasma structures organized by velocity shear appear as bottomside spread F but cannot evolve into topside bubbles. Velocity shear provides a possible explanation of generation of bottomside spread F and secondary plumelike structures on the west wall of large-scale plasma upwelling.
53 citations
"The impact of gravity waves rising ..." refers background or result in this paper
...(8), it is evident that the growth rate modificationδγ depends onδWy which, in our study, is very small compared to its magnitude used in the study by Huang and Kelley (1996)....
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...It is important to mention that more general numerical studies have been performed in the past on the GW seeding mechanism (Huang et al., 1993; Huang and Kelley, 1996)....
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...This is in accord with the results found by Huang and Kelley (1996) though the time-difference in Fig....
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..., 2005) or of wind perturbation, that subsequently cause density perturbation (Huang and Kelley, 1996)....
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...In their numerical simulation of CII with GW as a seeding perturbation, Huang and Kelley (1996) have found that though the plasma bubbles may be produced in the all situations, the production of the plasma bubbles initiated by GWs takes a time much shorter than that resulting from two-dimensional…...
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