The impact of gravity waves rising from convection in the lower atmosphere on the generation and nonlinear evolution of equatorial bubble

Gravity wave initiation of equatorial spread F/plasma bubble irregularities based on observational data from the SpreadFEx campaign

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.

Simultaneous observation of ionospheric plasma bubbles and mesospheric gravity waves during the SpreadFEx Campaign

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.

Equatorial spread F/plasma bubble irregularities under storm time disturbance electric fields

Abstract: Magnetosphere–ionosphere coupling is responsible for storm time disturbance electric field propagation to equatorial latitudes, by processes of direct penetration and disturbance wind dynamo New results have been forthcoming in recent years from satellite and ground based observations and modeling studies on the important characteristics of these electric fields as well their effects on the electrodynamics of the equatorial ionosphere and thermosphere, especially, in terms of their impact on the equatorial spread F (ESF) plasma bubble irregularity development conditions that is in focus here The disturbance zonal electric fields, when superimposed on equatorial evening pre-reversal enhancement electric field, PRE, can drastically modify the post-sunset, and night time, F layer heights, a basic control factor for the instability growth by Rayleigh–Taylor mechanism leading to plasma bubble development Based on published results and some new data we present here a comprehensive, but brief, analysis and discussion of the processes of ESF development, suppression or disruption under different phases of a storm activity sequence Consequences for ESF occurrence from under-shielding 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, etc, are highlighted in this paper Some outstanding problems for further research are also presented

Convective ionospheric storms: a review

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.

On equatorial spread F: Establishing a seeding hypothesis

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.

Fully multidimensional flux-corrected transport algorithms for fluids

Abstract: The theory of flux-corrected transport (FCT) developed by Boris and Book [J. Comput. Phys. 11 (1973) 38; 18 (1975) 248; 20 (1976) 397] is placed in a simple, generalized format, and a new algorithm for implementing the critical flux limiting stage' in multidimensions without resort to time splitting is presented. The new flux limiting algorithm allows the use of FCT techniques in multidimensional fluid problems for which time splitting would produce unacceptable numerical results, such as those involving incompressible or nearly incompressible flow fields. The “clipping” problem associated with the original one dimensional flux limiter is also eliminated or alleviated. Test results and applications to a two dimensional fluid plasma problem are presented.

Linear theory and modeling of the Rayleigh‐Taylor instability leading to the occurrence of equatorial spread F

Abstract: In a test of the generally accepted Rayleigh-Taylor (R-T) instability mechanism for equatorial spread F (ESF) a linear instability growth rate γ RT is derived following the formalism of Haerendel (preprint, 1973) which takes into account the variations of physical parameters along geomagnetic flux tubes. The resulting form of γ RT extends the results of previous work by including direct dependencies on transequatorial neutral winds, zonal electric fields, vertical and horizontal ionospheric density gradients, the presence of an E region, and chemical recombination. Realistic atmospheric and ionospheric density model inputs are used for the first time to make quantitative calculations of R-T growth rates for a range of geophysical conditions. The key result of this study is that time/altitude domains having positive calculated instability growth rates are found to coincide with observed time/altitude patterns of ESF occurrence over both a monthly and a yearly time frame. This success in being able to model the climatological occurrence of ESF lends support to the physical model adopted for the instability mechanism and opens up new avenues of research into ESF predictability on a night-to-night and even an hour-to-hour basis.

Gravity wave initiation of equatorial spread F: A case study

Abstract: Jicamarca radar backscatter maps were made during four consecutive nights in March 1979. Two of these maps displayed single towering plumes extending to nearly 1000-km altitude. On a third night, discussed in detail here, six plumes were generated in clear association with a nearly sinusoidal oscillation of the height of the bottomside of the F layer. The vertical amplitude of the oscillation was several hundred kilometers, and the period about 100 minutes. The plumes were generated either when the bottomside of the F layer was at the highest altitude or in the descending phase of the motion. Families of curves are presented which correspond to the solution of the dispersion relation for gravity waves capable of initiating the observed bottomside oscillations via the spatial resonance mechanism. We conclude that the solutions thus derived are reasonable and present a criterion for how well matched the gravity wave phase velocity and plasma drift have to be to produce a given perturbation in the ionization density. This criterion indicates that although initiation by a gravity wave seems likely, the gravity wave interaction cannot yield the large displacements observed without further amplification by the Rayleigh-Taylor instability. Finally, we show that the preferential generation of plumes during the descending phase of the F layer height oscillation can be explained by a generalized Rayleigh-Taylor instability operating on the distorted ionosphere with the destabilizing effects of gravity, a zonal electric field, and a zonal neutral wind included.

Horizontal and vertical propagation and dissipation of gravity waves in the thermosphere from lower atmospheric and thermospheric sources

Abstract: [1] The dissipation of high-frequency gravity waves (GWs) in the thermosphere is primarily due to kinematic viscosity and thermal diffusivity. Recently, an anelastic GW dispersion relation was derived which includes the damping effects of kinematic viscosity and thermal diffusivity in the thermosphere and which is valid before and during dissipation. Using a ray trace model which incorporates this new dispersion relation, we explore many GW properties that result from this dispersion relation for a wide range of thermospheric temperatures. We calculate the dissipation altitudes, horizontal distances traveled, times taken, and maximum vertical wavelengths prior to dissipation in the thermosphere for a wide range of upward-propagating GWs that originate in the lower atmosphere and at several altitudes in the thermosphere. We show that the vertical wavelengths of dissipating GWs, λz(zdiss), increases exponentially with altitude, although with a smaller slope for z > 200 km. We also show how the horizontal wavelength, λH, and wave period spectra change with altitude for dissipating GWs. We find that a new dissipation condition can predict our results for λz(zdiss) very well up to altitudes of ∼500 km. We also find that a GW spectrum excited from convection shifts to increasingly larger λz and λH with altitude in the thermosphere that are not characteristic of the initial convective scales. Additionally, a lower thermospheric shear shifts this spectrum to even larger λz, consistent with observations. Finally, we show that our results agree well with observations.

Nonlinear equatorial spread F

Abstract: A numerical simulation of the non-linear evolution of the collisional Rayleigh-Taylor instability using a set of equations appropriate for the equatorial F region ionosphere has been performed. Our results show that the irregularities produced by the instability grow on the bottomside of the F region peak, as predicted by linear theory, and then the irregularities nonlinearly bubble through to the topside, where linear theory predicts no irregularities. Fourier analysis of the irregularities show one dimensional power law power spectrum for both the vertical and horizontal directions.