Onset Of Equatorial Spread Research Articles

Investigation onFlayer height rise and equatorial spreadFonset time: Signature of standing large-scale wave

Equatorial spread F observations have been categorized into three categories based on ionograms recorded over Sriharikota. First category comprised cases where the onset of equatorial spread F (ESF) was concurrent with the peak h′F time. Second and third categories comprised cases where the onset of ESF happened with a delay of 30 min and more than 30 min, respectively, with reference to the peak h′F time. Average peak h′F in the first category was more than 35 km higher than that in the second and third categories. Also, the peak vertical (upward) plasma drift was higher in the first category. Assuming the genesis of F region irregularity to have happened at or before the time of F layer attaining the peak height, late onset of ESF indicates the genesis of irregularities to have happened westward of Sriharikota. The fact that the peak h′F values were remarkably different in the three categories indicates a zonal variation of eastward electric field and postsunset height rise of F layer. The relative magnitude of the F layer height rise in the three different categories over Sriharikota has also been found to be significantly different than that over Thumba, an equatorial (magnetic) station located ~360 km westward of Sriharikota longitude. This scenario points toward the existence of a large-scale zonal standing wave in the F layer and its important role in F region instability process. Results presented in the manuscript have been discussed in the light of current understanding on the large-scale wave structure.

Numerical simulation of equatorial plasma bubbles over Cachimbo: COPEX campaign

The problem of day-to-day variability in onset of equatorial spread F (ESF) is addressed using data from the 2002 COPEX observational campaign in Brazil and numerical modeling. The observational results show that for values of virtual height of the F layer base less than 355km at around 18:35 LT, and for the prereversal peak enhancement of the vertical plasma drift (Vp) less than 30m/s, the spread-F (ESF) was absent on four nights over Cachimbo (9.5°S, 54.8°W, dip latitude=−2.1°). In this work we analyze the geophysical conditions for the generation of the irregularities by comparing the nights with and without the ESF. In the comparison a numerical code is used to simulate plasma irregularity development in an extended altitude range from the bottom of the equatorial F layer. The code uses the flux corrected transport method with Boris–Book’s flux limiter for the spatial integration and a predictor–corrector method for the direct time integration of the continuity equation for O+ and the SOR (Successive-Over-Relaxation) method for electric potential equation. The code is tested with different evening eastward electric fields (or vertical drifts Vp<30m/s and Vp>30m/s) in order to study the influence of the prereversal enhancement in the zonal electric field on plasma bubble formation and development. The code also takes into account the zonal wind, the vertical electric field and the collision frequency of ions with neutrals and the amplitude of initial perturbation. The simulation shows a good agreement with the observational results of the ESF. The results of the code suggest that the instability can grow at the F layer bottomside by the Rayleigh–Taylor mechanism only when the Vp>30m/s. In the analyzed cases we have considered the competition of other geophysical parameters in the generation of plasma structures.

Coherent backscatter radar imaging in Brazil: large-scale waves in the bottomside F-region at the onset of equatorial spread &amp;lt;I&amp;gt;F&amp;lt;/I&amp;gt;

Abstract. The 30 MHz coherent backscatter radar located at the equatorial observatory in São Luís, Brazil (2.59° S, 44.21° W, −2.35° dip lat) has been upgraded to perform coherent backscatter radar imaging. The wide field-of-view of this radar makes it well suited for radar imaging studies of ionospheric irregularities. Radar imaging observations were made in support to the spread F Experiment (SpreadFEx) campaign. This paper describes the system and imaging technique and presents results from a bottom-type layer that preceded fully-developed radar plumes on 25 October 2005. The radar imaging technique was able to resolve decakilometric structures within the bottom-type layer. These structures indicate the presence of large-scale waves (~35 km) in the bottomside F-region with phases that are alternately stable and unstable to wind-driven gradient drift instabilities. The observations suggest that these waves can also cause the initial perturbation necessary to initiate the Generalized Rayleigh-Taylor instability leading to spread F. The electrodynamic conditions and the scale length of the bottom-type layer structures suggest that the waves were generated by the collisional shear instability. These results indicate that monitoring bottom-type layers may provide helpful diagnostics for spread F forecasting.

The early phase of spread F development studied in situ with impedance and Langmuir probes

The postsunset equatorial ionosphere is studied with rocket‐borne plasma diagnostic instruments within the Dynamics of the Equatorial Ionosphere Over Shriharikota Range (DEOS) campaign. Data from an impedance probe and a Langmuir probe allow for a critical comparison between the instruments, with ionosonde data, and with the International Reference Ionosphere model. During flight F05, which was launched before the onset of equatorial spread F (ESF), a generally smooth density profile was found. Density depletions (bubbles) are only found when the rocket crosses the lower edge of the F layer during the downleg of the trajectory. In flight F06, launched shortly after onset of ESF, a large‐scale depletion is found on the upleg of the trajectory below 360‐km altitude, where a transition to an unperturbed density profile occurs. Impedance probe and Langmuir probe data show close agreement of the density depletions and their fine structure with a resolution of 100 m. The morphology of the plasma depletion is compared with models and previous investigations. Electron temperature measurements with Langmuir probes show that the temperature in the depleted region is lower than that in the ambient plasma.

In situ measurements of winds, electric fields and electron densities at the onset of equatorial spread- F

The results of barium-strontium(Ba-Sr) releases and Langmuir probe measurements at the onset time of equatorial spread- F by two RH-560 rockets launched from SHAR on 16 February 1982 are discussed. Winds and electric fields are measured by Ba-Sr blob releases at heights of 139, 182, 225 and 294 km. The horizontal winds reveal the presence of vertical shears in both the zonal and meridional components located around 150 and 160 km, with minimum shear strengths of 1.7 and 1.9 m s −1 km −1. respectively. The largest zonal wind observed was 112±3 m s −1 at 225 km. Significant magnitudes (10–40 m s −1) of vertical winds were also measured at altitudes of 182, 225 and 294 km, which are below the altitude of the F-layer. Electron density irregularities were also observed in a localized height region at 240–300 km by a Langmuir probe flown by the second RH-560 rocket. The causative mechanisms of the observed density irregularities are discussed and comparison between the various possible destabilizing mechanisms are made.