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, a gap-free version of the seasonal and longitudinal 0 (s/l) variations of P(sub EFI), the equatorial F region irregularity (EFI) occurrence probability, based on data from the AE-E spacecraft is presented.
Abstract: We present a new gap-free version of the seasonal and longitudinal 0 (s/l) variations of P(sub EFI), the equatorial F region irregularity (EFI) occurrence probability, based on data from the AE-E spacecraft. The agreement of this and three earlier partial P(sub EFI) patterns verifies all four. We reinterpret another earlier gap-ridden pattern, that of D(bar)(sub RSF), a topside ionogram index of average darkening by range spread F. We compare it with P(sub EFI) and, using ionosonde radio science considerations, we conclude that D(bar)(sub RSF) = P(sub EFI) times a factor depending on the average number of topside plasma bubbles visible to the ionosonde. The s/l variations of D(baar)(sub RSF) thus imply s/l variations in the average spacing of bubbles, whose seeds have an occurrence probability pattern P(sub seed). For discussion we assume P(sub EFI) = P(sub inst)P(sub seed) is the pattern of F region instability. The P(sub EFI) pattern, which is by definition independent of seed and/or bubble spacing, is far too complex to be explained by the dominant paradigm, that of changes in P(sub inst) by simple changes in the F region altitude and/or north-south asymmetry. We examine evidence behind this dominance, and find it unconvincing. Both the asymmetry and sunset-node/altitude hypotheses of 1984 and 1985, respectively, seem to be partly based on misunderstood data, and their features appear displaced in time and space from those of our repeatable P(sub EFI) pattern. In contrast, if P(sub seed) variations influence the P(sub EFI) pattern and depend on thermospheric gravity waves from tropospheric convection near the dip equator, then the seasonal maxima (minima) Of P(sub EFI) could be explained, since they all occur above relatively warm (cold) surface features, where convection is maximal (minimal). Also, the hypothesis of the dominance of the P(sub seed) term could explain an unusual December/January P(sub EFI) maximum in the deep, wide, normal Pacific minimum in the one data set obtained in El Nino years. Based on the experiments we consider, we predict that the s/l variations Of P(sub seed) will be found to be similar to those of P(sub EFI) and largely to explain them. Finally, we find reasons, based on the similarity of the D(sub RSF) variations to s/l patterns of the average scintillation index, for not using, as is commonly done, such scintillation patterns as substitutes for P(sub EFI) or P(sub inst) patterns.
193 citations
"The impact of gravity waves rising ..." refers background in this paper
...More evidence, based on in-situ measurements, as to the role of GW in the spread F has been provided by Singh et al. (1997) and McClure et al. (1998)....
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TL;DR: In this article, four different two-dimensional (perpendicular to the ambient magnetic field) plasma fluid-type numerical simulations following the nonlinear evolution of the collisional Rayleigh-Taylor instability in the nighttime equatorial F region ionosphere have been performed.
Abstract: Four different two-dimensional (perpendicular to the ambient magnetic field) plasma fluid-type numerical simulations following the nonlinear evolution of the collisional Rayleigh-Taylor instability in the nighttime equatorial F region ionosphere have been performed. Realistic altitude dependent ion-neutral collision frequencies, recombination rates, and ambient electron density profiles were used. In three cases (ESF 0, 1, 3) the electron density profile was kept constant, with a minimum bottomside background electron density gradient scale length L ∼ 10 km, but the altitude of the F peak was changed, with F peak altitudes at 340, 350, and 430 km. All cases resulted in bottomside growth of the instability (spread F) with dramatically different time scales for development. Plasma density depletions were produced on the bottomside with rise velocities, produced by nonlinear polarization E × B forces, of 2.5, 12, and 160 m/s and percentage depletions of 16, 40, and 85, respectively. In one case, ESF 0, the bubble did not rise to the topside, but in ESF 1 and 3, topside irregularities were produced by the bubbles (where linear theory predicts no irregularities). In these three cases, spread F could be described from weak to strong. In the fourth case (ESF 2) the altitude of the F peak was 350 km, but the minimum L on the bottomside was changed to 5 km. This resulted in a bubble rise velocity of ∼23 m/s and a 60% depletion with strong bottomside and moderate topside spread F and a time scale for development between ESF 1 and 3. Two other cases, ESF 0′ and 0″ with peaks at 330 and 300 km, respectively, and bottomside L ∼ 10 km, were investigated via linear theory. These cases resulted in extremely weak bottomside spread F and no spread F (entire bottomside linearly stable), respectively. These simulations show that under appropriate conditions, the collisional Rayleigh-Taylor instability causes linear growth on the bottomside of the F region. This causes the formation of plasma density depletions (bubbles) which rise to the topside (under appropriate conditions) F region by polarization E × B motion. High altitude of the F peak, small bottomside electron density gradient scale lengths, and large percentage depletions yield large vertical bubble rise velocities, with the first two conditions favoring bottomside linear growth of the instability. The numerical simulation results are in good agreement with rocket and satellite in situ measurements and radar backscatter measurements, including some of the recent results from the August 1977 coordinated ground-based measurement campaign conducted by Defense Nuclear Agency at Kwajalein.
166 citations
TL;DR: In this article, the authors show that the wave structures observed here that served as the initial seed ion density perturbations were caused by gravity waves, strengthening the view that gravity waves seed equatorial spread F irregularities.
Abstract: Some examples from the Atmosphere Explorer E data showing plasma bubble development from wavy ion density structures in the bottomside F layer are described. The wavy structures mostly had east-west wavelengths of 150-800 km, in one example it was about 3000 km. The ionization troughs in the wavy structures later broke up into either a multiple-bubble patch or a single bubble, depending upon whether, in the precursor wavy structure, shorter wavelengths were superimposed on the larger scale wavelengths. In the multiple bubble patches, intrabubble spacings vaned from 55 km to 140 km. In a fully developed equatorial spread F case, east-west wavelengths from 690 km down to about 0.5 km were present simultaneously. The spacings between bubble patches or between bubbles in a patch appear to be determined by the wavelengths present in the precursor wave structure. In some cases, deeper bubbles developed on the western edge of a bubble patch, suggesting an east-west asymmetry. Simultaneous horizontal neutral wind measurements showed wavelike perturbations that were closely associated with perturbations in the plasma horizontal drift velocity. We argue that the wave structures observed here that served as the initial seed ion density perturbations were caused by gravity waves, strengthening the view that gravity waves seed equatorial spread F irregularities.
160 citations
"The impact of gravity waves rising ..." refers background in this paper
...More evidence, based on in-situ measurements, as to the role of GW in the spread F has been provided by Singh et al. (1997) and McClure et al. (1998)....
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TL;DR: In this article, the authors analyzed a spectabular spread F event that for the first time demonstrates a layering which, the authors argue, is controlled by a gravity wave effect.
Abstract: Studies dating back more than 15 years have presented evidence that atmospheric gravity waves play a role in initiating nighttime equatorial F region instabilities. This paper analyzes a spectabular spread F event that for the first time demonstrates a layering which, the authors argue, is controlled by a gravity wave effect. The 50-km vertical wavelength of a gravity wave which they have found is related theoretically to a plasma layering irregularity that originated at low altitudes and then was convected, intact, to higher altitudes. Gravity waves also seem to have determined bottomside intermediate scale undulations, although this fact is not as clear in the data. The neutral wind dynamo effect yields wave number conditions on the gravity wave's ability to modulate the Rayleigh-Taylor instaiblity process. Finally, after evaluating the gravity wave dispersion relation and spatial resonance conditions, we estimate the properties of the seeding wave.
155 citations
"The impact of gravity waves rising ..." refers background in this paper
...Thereafter, many researchers have experimentally suggested that GWs propagating in the equatorial thermosphere play an important role in seeding the bubbles and spread F (Kelley, 1986; Hysell et al., 1990)....
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Cora1
TL;DR: In this paper, a broad range of gravity wave phase speeds, spatial scales, and intrinsic frequencies were considered to address all of the major gravity wave sources in the lower atmosphere potentially impacting the thermosphere.
Abstract: We previously considered various aspects of grav- ity wave penetration and effects at mesospheric and ther- mospheric altitudes, including propagation, viscous effects on wave structure, characteristics, and damping, local body forcing, responses to solar cycle temperature variations, and filtering by mean winds. Several of these efforts focused on gravity waves arising from deep convection or in situ body forcing accompanying wave dissipation. Here we generalize these results to a broad range of gravity wave phase speeds, spatial scales, and intrinsic frequencies in order to address all of the major gravity wave sources in the lower atmosphere potentially impacting the thermosphere. We show how pen- etration altitudes depend on gravity wave phase speed, hor- izontal and vertical wavelengths, and observed frequencies for a range of thermospheric temperatures spanning realistic solar conditions and winds spanning reasonable mean and tidal amplitudes. Our results emphasize that independent of gravity wave source, thermospheric temperature, and fil- tering conditions, those gravity waves that penetrate to the highest altitudes have increasing vertical wavelengths and decreasing intrinsic frequencies with increasing altitude. The spatial scales at the highest altitudes at which gravity wave perturbations are observed are inevitably horizontal wave- lengths of 150 to 1000 km and vertical wavelengths of 150 to 500 km or more, with the larger horizontal scales only becoming important for the stronger Doppler-shifting conditions. Observed and intrinsic periods are typically 10 to 60 min and 10 to 30 min, respectively, with the intrinsic periods shorter at the highest altitudes because of preferen- tial penetration of GWs that are up-shifted in frequency by thermospheric winds.
124 citations
"The impact of gravity waves rising ..." refers background or methods in this paper
...1a–b) fall in the same range as suggested by Fritts and Vadas (2008)....
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...A SpreadFEx companion paper by Fritts and Vadas (2008) employed the ray tracing methods of Vadas and Fritts (2004) to assess GW Doppler shifting by thermospheric winds, preferential penetration and dissipation, and selection of those GW periods and spatial scales achieving the highest altitudes for…...
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