S U M M A R YThe Earth's crust is magnetized down to the Curie-temperature depth at about 10 to 50 km. This limited depth extent of the crustal magnetization is discernible in the power spectra of magnetic maps of South Africa and Central Asia. At short wavelengths, the power increases as rapidly towards longer wavelengths as expected for a self-similar magnetized crust with unlimited depth extent. Above wavelengths of about 100 km the power starts increasing less rapidly, indicating the absence of deep-seated sources. To quantify this effect we derive the theoretical power spectrum due to a slab carved out of a self-similar magnetization distribution. This model power spectrum matches the power spectra of South Africa and Central Asia for a self-similarity parameter of p = 4 and Curie temperature depths of 15 to 20 km.

We experimentally demonstrate a notably enhanced acceleration of protons to high energy by relatively modest ultrashort laser pulses and structured dynamical plasma targets. Realized by special deposition of snow targets on sapphire substrates and using carefully planned prepulses, high proton yields emitted in a narrow solid angle with energy above 21 MeV were detected from a 5 TW laser. Our simulations predict that using the proposed scheme protons can be accelerated to energies above 150 MeV by 100 TW laser systems.

We report on the first generation of 5.5-7.5 MeV protons by a moderate-intensity short-pulse laser (∼5×10(17)  W/cm(2), 40 fsec) interacting with frozen H(2)O nanometer-size structure droplets (snow nanowires) deposited on a sapphire substrate. In this setup, the laser intensity is locally enhanced by the snow nanowire, leading to high spatial gradients. Accordingly, the nanoplasma is subject to enhanced ponderomotive potential, and confined charge separation is obtained. Electrostatic fields of extremely high intensities are produced over the short scale length, and protons are accelerated to MeV-level energies.

Recent theoretical, computational, and experimental work carried out at the Naval Research Laboratory on the propagation of ultra-short laser pulses in air is presented. Fully time-dependent, three-dimensional, nonlinear equations describing the propagation of laser pulses in air under the influence of diffraction, group velocity dispersion, Kerr nonlinearity, stimulated Raman scattering, ionization, and plasma wakefield excitation are presented and analyzed. The propagation code, HELCAP [P. Sprangle, J. R. Peñano, and B. Hafizi, Phys. Rev. E 66, 046418 (2002)], is used to simulate the propagation of laser pulses in air under the influence of the physical processes mentioned above. Simulations of laser filamentation together with experimental measurements are used to confirm that the filamentation process is dependent on pulse duration. An equilibrium configuration for optical and plasma filaments in air is derived and the dynamic guiding and spectral broadening of a laser pulse is modeled. The effect of atmospheric turbulence on nonlinear self-focusing is demonstrated. Simulations of a recent electromagnetic pulse (EMP) generation experiment are also presented and the efficiency of EMP generation is determined and found to be extremely small.

The theory of electromagnetically induced transparency ͑EIT͒ in a plasma ͓S. E. Harris, Phys. Rev. Lett. 77, 5357 ͑1996͔͒ is examined in the context of an infinite system. A new dispersion relation is derived which accounts for relativistic effects in an overdense plasma. Several branches of the dispersion relation are plotted and discussed. Particle simulations are used to confirm the findings.