Sub-Rayleigh optical vortex coronagraphy.

We introduce a new optical vortex coronagraph (OVC) method to determine the angular distance between two sources when the separation is sub-Rayleigh. We have found a direct relationship between the position of the minima and the source angular separation. A priori knowledge about the location of the two sources is not required. The superresolution capabilities of an OVC, equipped with an ℓ = 2 N-step spiral phase plate in its optical path, were investigated numerically. The results of these investigations show that a fraction of the light, increasing with N, from the secondary source can be detected with a sub-Rayleigh resolution of at least 0.1 λ/D.

Stimulated Raman and Brillouin backscattering of collimated beams carrying orbital angular momentum.

We study theoretically the exchange of angular momentum between electromagnetic and electrostatic waves in a plasma, due to the stimulated Raman and Brillouin backscattering processes. Angular momentum states for plasmon and phonon fields are introduced for the first time. We demonstrate that these states can be excited by nonlinear wave mixing, associated with the scattering processes. This could be relevant for plasma diagnostics, both in laboratory and in space. Nonlinearly coupled paraxial equations and instability growth rates are derived.

Unprecedentedly strong and narrow electromagnetic emissions stimulated by high-frequency radio waves in the ionosphere.

Experimental results of secondary electromagnetic radiation, stimulated by high-frequency radio waves irradiating the ionosphere, are reported. We have observed emission peaks, shifted in frequency up to a few tens of Hertz from radio waves transmitted at several megahertz. These emission peaks are by far the strongest spectral features of secondary radiation that have been reported. The emissions are attributed to stimulated Brillouin scattering, long predicted but hitherto never unambiguously identified in high-frequency ionospheric interaction experiments. The experiments were performed at the High-Frequency Active Auroral Research Program (HAARP), Alaska, USA.

Utilization of photon orbital angular momentum in the low-frequency radio domain.

We show numerically that vector antenna arrays can generate radio beams that exhibit spin and orbital angular momentum characteristics similar to those of helical Laguerre-Gauss laser beams in paraxial optics. For low frequencies (< or = 1 GHz), digital techniques can be used to coherently measure the instantaneous, local field vectors and to manipulate them in software. This enables new types of experiments that go beyond what is possible in optics. It allows information-rich radio astronomy and paves the way for novel wireless communication concepts.

Prospects for lunar satellite detection of radio pulses from ultrahigh energy neutrinos interacting with the moon.

The Moon provides a huge effective detector volume for ultrahigh energy cosmic neutrinos, which generate coherent radio pulses in the lunar surface layer due to the Askaryan effect. In light of presently considered lunar missions, we propose radio measurements from a Moon-orbiting satellite. First systematic Monte Carlo simulations demonstrate the detectability of Askaryan pulses from neutrinos with energies above 10(20) eV at the very low fluxes predicted in different scenarios.

Competition between Langmuir and upper-hybrid turbulence in a high-frequency-pumped ionosphere.

We show how the secondary escaping radiation, also known as stimulated electromagnetic emission (SEE), from the ionosphere irradiated by a high-intensity radio beam, can be used to study both reflection altitude ponderomotive parametric instabilities and upper-hybrid altitude thermal parametric instabilities. This has allowed us to observe the transfer of energy from smaller to higher sideband frequency offsets and to identify a new transient SEE feature.

Slow magnetosonic solitons detected by the cluster spacecraft.

Experimental evidence is provided for the existence of slow-mode magnetosonic solitons in the col-lisionless plasma at the magnetopause boundary layer. The solitons were detected by the fleet of Cluster spacecraft at the dusk flank of the magnetosphere as magnetic field depressions (up to 85%) accom-panied with enhancement of the plasma density and temperature by a factor of 2. The solitons propagate 250 km/s with respect to the satellites and have perpendicular size of 1000-2000 km, which is a few ion inertial scale lengths. The comparison with numerical solutions of a theoretical model shows quantitative agreement between the model and observations.