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RADIO WAVE

Earth’s magnetic field plays a dominant role in the dynamics of electrons and ions in the E- and F- region of the ionosphere and hence in various plasma instability processes. Some of the most important processes are the Equatorial Spread F (ESF), Equatorial Electrojet (EEJ), Counter Equatorial Electrojet (CEEJ), and Equatorial Ionization anomaly (EIA) which generate plasma irregularities (fluctuations in electron density) with scale sizes ranging from few centimeters to hundreds of kilometer, through different plasma instability processes operating at different regions. Scattering of radio waves by these irregularities gives rise to practical problems in transionospheric communications. The present work focuses on the theory of generation of these irregularities due to growth of plasma instabilities, and the theory of scattering of radio waves by the irregularities, which gives rise to fluctuations or scintillations in the signal intensity and phase. The scintillation data is also used in conjunction with theory, to extract information about the irregularities. Recent major achievements include a theoretical calculation of the spatial correlation function of intensity scintillations under strong scattering conditions; development of a method to extract the coherence scale length from spaced receiver scintillation data; a new theory for the development of nighttime equatorial F region irregularities due to growth of an electromagnetic instability, where for the first time, geomagnetic field aligned currents connecting the equatorial F and E regions were introduced explicitly to estimate the time scale for discharging the equatorial plasma bubbles through the conjugate E regions; and estimation of the ‘age’ of irregularities, associated with the observed scintillations at a given time, which is critical in the determination of a cause-effect relationship between geomagnetic storms and generation of ionospheric irregularities.

The detailed studies of the equatorial Spread F and equatorial electrojet irregularities are being done using the ground based, colocated, 54.95 MHz, 18 MHz, 9 MHz radars and Ionosonde at Trivandrum (8.5° N, 76.9° E, dip. latitude 0.5° N), and the 53 MHz MST radar located at Gadanki (13.5°N, 79.2° E, dip latitude 6.5° N). The equatorial spread F (ESF) irregularities as observed by the 18 MHz radar have been used to characterize the morphology and the spectral parameters of the 8.3 m ESF irregularities which are found to be remarkably different from that observed so extensively at 3 m scale size elsewhere. Simultaneous measurements on the structure and dynamics of equatorial spread F irregularities using 18 MHz radar at the geomagnetic equator (Trivandrum) and 53 MHz radar at off geomagnetic equator (Gadanki), have been investigated in details. The irregularity structures and the velocities observed at the two locations seem to have some resemblance, but they differ in detail. The velocities and spectral widths observed using the 18 MHz radar are less than ±150 m s-1 and 100 m s-1 respectively whereas they exceed ± 200.

The formation of F region ionization irregularities over equator is mainly attributed to the Rayleigh-Taylor (RT) Instability in which an ionization perturbation forming on the bottomside of the ionospheric F layer is amplified to produce an upward moving depletion or bubble and subsequently developing into a spectrum of irregularities with scale sizes ranging from few centimeters to few tens of kilometers. These eventually map down along magnetic field lines to the north and south Appleton anomaly crests located about 12 to 15 degrees from the magnetic equator. Since India is in equatorial region where ionospheric irregularities are frequently observed at ground receiver in terms of scintillations in radio signals traversed through the ionized medium, positional accuracies are greatly degraded over these zones. Therefore, it is essential to understand the electrodynamical processes and role of earth’s magnetic field in generation of these irregularities and their effect on radio signal in the global scenario in general. The GPS signals obtained as a part of GAGAN (GPS And Geo Augmented Navigation) project at various locations in India are used to study and understand the latitudinal and longitudinal extent of the scintillations in L1 frequency and TEC variability. Some of the major phenomena occurring at Sun like solar flares, magnetic storms, sub-storms etc. and their relation to the generation/inhibition of these irregularities are studied as space weather point of view. The VHF scintillation observations obtained at Tirunelveli and MST radar observations of equatorial spread F irregularities obtained from Gadanki, a low latitude location in complement with GPS observations are also used.

Investigations on equatorial electrojet (EEJ) plasma irregularities made using newly established 9/18 MHz radars are in agreement with existing results obtained at Jicamarca/Trivandrum at generally higher frequency (50 MHz). Both type-1 and type-2 irregularities were regularly observed but occurrence of type-1 is less with lesser threshold. Observed type-1 spectra are mostly composite in nature. 8.3 m scale size EEJ irregularities show remarkable anisotropy in the scattering cross section in the E-W plane perpendicular to magnetic field in contrast to that known for 3m irregularities. East-west asymmetry for type-2 echoes has also been observed in all the spectral parameters. Doppler spectra are Gaussian in nature, less structured as compared to 3 m spectra. Spectral widths are typically in the range of 50-150 m s-1. Doppler spectra at 16.6 m EEJ irregularities are Gaussian in nature and less structured. Irregularity phase velocities and spectral widths are less as compared to 8.3 EEJ irregularities.

Work is underway to analyze the magnetic field fluctuations associated with equatorial plasma bubbles (EPBs) using CHAMP satellite data and space weather effect on it.

S. Sripathi, Bharati Kakad, Tulasi Ram and Mala Bagiya



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