Intermediate-scale Irregularities Research Articles

Effect of Magnetic Storm Related Thermospheric Changes on the Evolution of Equatorial Plasma Bubbles

AbstractPast efforts to predict scintillations on VHF and L‐band radio signals recorded at equatorial and low‐latitude stations have been mostly based on a theoretical linear growth rate of Rayleigh‐Taylor instability on the bottomside of the post‐sunset equatorial F layer, which is responsible for the generation of an equatorial plasma bubble (EPB). However, it is the maximum height that an EPB reaches above the dip equator and development of intermediate scale irregularities within the EPB, in its nonlinear phase of evolution that determines the latitudinal distribution of scintillations. Amplitude scintillations recorded by a network of VHF and L‐band receivers on a quiet day, 13 March 2015 and on 20 March 2015, a few days after the 17 March 2015 magnetic storm, show that latitudinal extent of scintillations caused by EPB irregularities is lesser on 20 March than on 13 March. Geomagnetic and ionosonde data from an equatorial station, and vertical total electron content distributions obtained from Global Navigation Satellite Systems observations, indicate that the equatorial ionospheric conditions are approximately same on these 2 days. Simulation of thermospheric conditions for these 2 days is carried out using the Coupled Thermosphere, Ionosphere, Plasmasphere, and Electrodynamics model. It is found that thermospheric atomic oxygen density is enhanced in the aftermath of the major magnetic storm of 17 March 2015, resulting in enhanced ion‐neutral collision frequencies over the dip equator on 20 March. This limits the height to which an EPB rises over the dip equator on this day, and thus impacts the latitudinal distribution of scintillations.

Ionospheric irregularities at low latitude using VHF scintillations during extreme low solar activity period (2008–2010)

In the present study, we have used 250 MHz radio signal radiated by geostationary satellite UFO-02 to study the occurrence characteristics of very high frequency scintillations associated with ionospheric irregularities during recent extreme low solar activity period from 2008 to 2010 at low latitude Indian station Varanasi (Geomag. latitude = 14°55′N, long. = 154°E, Dip angle = 37.3°, Sub-ionospheric dip = 34°). The impact of this recent extreme low solar activity period on ionosphere is investigated. It is observed that the scintillation occurrence is low having maximum percentage occurrence during pre-midnight periods. With increasing interest in understanding the behavior of ionospheric irregularities, an effort has been made to examine also the influence of solar and magnetic activity over the occurrence of scintillations. During the extreme low solar activity years the scintillation occurrences do not vary linearly with the sunspot number. The inhibition and generation of irregularities during enhanced magnetic activity period are explained by considering changes in the electric field. The spectral analysis provide spectral index for irregularities which varied between −1.5 and −8 and characteristic length of irregularities varied between 400 and 1200 m which confirms that 250 MHz scintillations observed over Varanasi were associated with intermediate scale irregularities.

Development of intermediate scale structure near the peak of the F region within an equatorial plasma bubble

AbstractScintillation observations are used to study the evolution of intermediate scale (~100 m–few kilometers) irregularities through growth of the Rayleigh‐Taylor (R‐T) instability on the bottom side of the post‐sunset equatorial F region during magnetically quiet periods. Amplitude scintillations on a VHF signal from a geostationary satellite, recorded by spaced receivers at an equatorial station, are used to compute as a function of local time: (1) the coherence scale length for spatial variations of intensity in the ground scintillation pattern, which is linked with the spectrum of the intermediate scale irregularities near the peak of the equatorial F region that contribute the most to the observed scintillations; and (2) the “random velocity”, which accounts for the de‐correlation of the spaced receiver signals. The relationship between the coherence scale length and the random velocity for saturated scintillations at different local times suggests that (1) the random velocity is linked with fluctuations in the drift velocity of the irregularities caused by the perturbation electric fields associated with the R‐T instability rather than structural changes in the intermediate scale irregularities, (2) the spectrum of intermediate scale irregularities in the equatorial F peak region tends to be shallowest after the decay of the perturbation electric fields associated with the R‐T instability, and (3) evolution of intermediate‐scale irregularity spectrum in the equatorial plasma bubble near the equatorial F region peak depends on season and solar flux. These have implications for observation of low‐latitude L‐band scintillations.

ROCSAT observations of topside ionospheric undulations and irregularities at low to middle latitudes

[1] Starting from 0415 UT on 8 January 2000, ROCSAT-1 observed intermediate-scale (0.1 to 50 km) ion density irregularities in conjunction with mesoscale (∼50 to 1000 km) density and flow undulations in a total of eight consecutive orbits from low to middle latitudes in the northern hemisphere of nightside topside ionosphere. The mesoscale undulation indicates in-phase variation between the outward as well as westward flow and the density enhancement. The induced electric fields derived from the flow perturbations point along the northeast–southwest direction. The locations of the disturbances mapped from the onset of each mesoscale undulation event observed in eight consecutive ROCSAT orbits form a frontal structure that is aligned from northwest to southeast. These disturbances cover a large geographic area from 120° to 300° in longitude and from latitude 20° up to the ROCSAT turn-around latitude at 35° in the nightside ionosphere. Since the first onset of mesoscale undulations is preceded by a sudden enhancement of the background flow in the southeast direction, which can be induced by a southwest-pointed electric field, the cause of mesoscale undulations is identified as the Perkins instability. On the other hand, it is difficult to explain the occurrence of intermediate-scale irregularities that either appear alone in time and space, or coexist with mesoscale undulations. The signature of intermediate-scale irregularities, in addition to the density fluctuations, is the existence of a dominant zonal flow fluctuation that is completely different from the characteristics of mesoscale undulations. One possible explanation for the existence of intermediate-scale irregularities is from the mapping of irregularity structures from E region or lower F region. However, theoretical model for the production and mapping process of intermediate-scale irregularity is not available at the moment. As the in situ observation of midlatitude irregularities is quite rare, we hope that the unique and distinctive properties of midlatitude irregularities, at mesoscale and intermediate scale, presented here will stimulate future theoretical study of midlatitude irregularities.

On the origin and spatial extent of high‐latitude F region irregularities

Further evidence is presented that structured soft‐electron precipitation is an important source of large‐scale (λ ≳ 10 km ) F region ionization irregularities in the high‐latitude ionosphere. We show that large amplitude, 20‐km to 80‐km structured plasma exists in both the dayside and nightside auroral oval. In the latter case, the structured F layer plasma has been observed to convect into and through the field of view of the Chatanika radar. Here, and in a companion paper, we hypothesize that this soft electron precipitation is the primary source of high‐latitude structure and that convection acts to distribute the irregular plasma throughout the polar ionosphere. Indeed, at the observed production scale (20–80 km), irregularities can easily convect throughout the high‐latitude region with negligible decay from classical or anomalous diffusion, including the effects of a conducting E region. Our convection/decay model also explains the following observed features of published high‐latitude irregularity data: (1) The steep gradient in irregularity intensity at the equatorward edge of the nighttime magnetospheric convection zone. (2) The existence of irregularities throughout the polar cap. (3) The reduction of irregularity intensity in the central polar cap. (4) The dawn‐dusk asymmetry in the equatorward boundary of the high‐latitude irregularity zone. On the other hand, if classical theory is applied to a 100‐m scale structure, diffusion should rapidly limit the irregularities to an area within a few degrees of the production zone, a prediction not upheld by experimental data. Thus, plasma instabilities must operate on the large‐scale structures to produce the observed intermediate scale (100 m < λ < 10 km) power law irregularity spectra. Calculations of the instability growth rate in its generalized form, which includes field‐aligned currents (the current convective instability), shows that growth should occur. The universal drift instability should then operate on these intermediate scale features. We suggest that this latter instability maintains the shorter scale plasma structure at the expense of a more rapid decay of the intermediate scale irregularities in a cascade‐like process. Enhanced, turbulent diffusion can reduce significantly the lifetime of the intermediate scale irregularities but not of the larger structures, which can therefore still transit the polar cap.