Focusing of HF radio-waves by ionospheric ducts

International audienceThis paper presents the first direct observations of HF focusing induced by natural and artificial ionospheric ducts along with a simple theoretical model. The experiments were conducted by injecting HF radio-waves using the Ionospheric Research Instrument of the High Frequency Active Auroral Research Program located in Gakona, Alaska and detecting them with instruments on the overflying French micro-satellite DEMETER. The latter observed a multiple frequency band structure, which is characteristic of a strong HF signal exceeding the detector's saturation level. Analysis of the O+ density measured by DEMETER along its orbit shows that the strong radio signal coincides with the presence of a “negative” duct in the ionosphere. “Negative” refers to the presence of a plasma density depletion with the peak depletion located near the center of the duct. Such ducts induce changes in the index of refraction leading to the focusing of HF waves in a manner equivalent to a “thick” plasma lens. Examination of the data along with a simple plasma lens model indicates the presence of focal node(s) in the vicinity of the overflying satellite. Two examples, one corresponding to focusing by a natural duct and one by an artificial one are presented

Formation of artificial ionospheric ducts

International audienceIt is well known that strong electron heating by a powerful HF-facility can lead to the formation of electron and ion density perturbations that stretch along the magnetic field line. Those density perturbations can serve as ducts for ELF waves, both of natural and artificial origin. This paper presents the first experimental evidence of plasma modifications associated with ion outflows due to HF heating. The experiments were conducted using the HAARP heater during times that the DEMETER satellite flying at an altitude of approximately 670 km was close to the magnetic zenith of HAARP. The DEMETER satellite has provided in situ measurements of the ion temperature and composition. The experimental results are compared with the numerical model of inter-hemispheric artificial ducts, and it is found that they are in qualitative agreement

25 Years of Self-Organized Criticality: Solar and Astrophysics

Shortly after the seminal paper “Self-Organized Criticality: An explanation of 1/fnoise” by Bak et al. (1987), the idea has been applied to solar physics, in “Avalanches and the Distribution of Solar Flares” by Lu and Hamilton (1991). In the following years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into the numerical SOC toy models, such as the discretization of magneto-hydrodynamics (MHD) processes. The novel applications stimulated also vigorous debates about the discrimination between SOC models, SOC-like, and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC studies from the last 25 years and highlight new trends, open questions, and future challenges, as discussed during two recent ISSI workshops on this theme.Fil: Aschwanden, Markus J.. Lockheed Martin Corporation; Estados UnidosFil: Crosby, Norma B.. Belgian Institute For Space Aeronomy; BélgicaFil: Dimitropoulou, Michaila. University Of Athens; GreciaFil: Georgoulis, Manolis K.. Academy Of Athens; GreciaFil: Hergarten, Stefan. Universitat Freiburg Im Breisgau; AlemaniaFil: McAteer, James. University Of New Mexico; Estados UnidosFil: Milovanov, Alexander V.. Max Planck Institute For The Physics Of Complex Systems; Alemania. Russian Academy Of Sciences. Space Research Institute; Rusia. Enea Centro Ricerche Frascati; ItaliaFil: Mineshige, Shin. Kyoto University; JapónFil: Morales, Laura Fernanda. Canadian Space Agency; Canadá. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Nishizuka, Naoto. Japan National Institute Of Information And Communications Technology; JapónFil: Pruessner, Gunnar. Imperial College London; Reino UnidoFil: Sanchez, Raul. Universidad Carlos Iii de Madrid. Instituto de Salud; EspañaFil: Sharma, A. Surja. University Of Maryland; Estados UnidosFil: Strugarek, Antoine. University Of Montreal; CanadáFil: Uritsky, Vadim. Nasa Goddard Space Flight Center; Estados Unido