Observational Test for a Random Sweeping Model in Solar Wind Turbulence.

Evidence of frequency broadening at ion kinetic scales due to large-scale eddies and waves is found in solar wind turbulence by a test for a random sweeping model using the magnetic energy spectrum in the frequency vs wave number domain in the comoving frame of the flow obtained from multispacecraft observations. The statistical analysis of the frequency vs wave number spectra without using Taylor's hypothesis shows Gaussian frequency broadening around nearly zero frequencies that increases for larger wave numbers and non-Gaussian tails at higher frequencies. Comparison of the observed frequency broadening with a random sweeping model derived from hydrodynamic turbulence reveals similarities with respect to the Gaussian shape. The standard deviation of the broadening scales with ∼k^{1.6±0.2} and differs from the hydrodynamic turbulence model that predicts ∼k^{2/3}. We interpret this stronger increasing broadening as a consequence of the more diverse large scale structures (eddies and waves) in plasma turbulence and the accompanied more complex sweeping. Consequently, an identification and association of waves with normal modes based on their dispersion relation only, in particular at ion kinetic scales and below, is not possible in solar wind turbulence.

Parametric decay of oblique Alfvén waves in two-dimensional hybrid simulations.

Certain types of plasma waves are known to become parametrically unstable under specific plasma conditions, in which the pump wave will decay into several daughter waves with different wavenumbers and frequencies. In the past, the related plasma instabilities have been treated analytically for various parameter regimes and by use of various numerical methods, yet the oblique propagation with respect to the background magnetic field has rarely been dealt with in two dimensions, mainly because of the high computational demand. Here we present a hybrid-simulation study of the parametric decay of a moderately oblique Alfvén wave having elliptical polarization. It is found that such a compressive wave can decay into waves with higher and lower wavenumbers than the pump.

Little or no solar wind enters Venus' atmosphere at solar minimum.

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

Cosmic ray impact on extrasolar earth-like planets in close-in habitable zones.

Because of their different origins, cosmic rays can be subdivided into galactic cosmic rays and solar/stellar cosmic rays. The flux of cosmic rays to planetary surfaces is mainly determined by two planetary parameters: the atmospheric density and the strength of the internal magnetic moment. If a planet exhibits an extended magnetosphere, its surface will be protected from high-energy cosmic ray particles. We show that close-in extrasolar planets in the habitable zone of M stars are synchronously rotating with their host star because of the tidal interaction. For gravitationally locked planets the rotation period is equal to the orbital period, which is much longer than the rotation period expected for planets not subject to tidal locking. This results in a relatively small magnetic moment. We found that an Earth-like extrasolar planet, tidally locked in an orbit of 0.2 AU around an M star of 0.5 solar masses, has a rotation rate of 2% of that of the Earth. This results in a magnetic moment of less than 15% of the Earth's current magnetic moment. Therefore, close-in extrasolar planets seem not to be protected by extended Earth-like magnetospheres, and cosmic rays can reach almost the whole surface area of the upper atmosphere. Primary cosmic ray particles that interact with the atmosphere generate secondary energetic particles, a so-called cosmic ray shower. Some of the secondary particles can reach the surface of terrestrial planets when the surface pressure of the atmosphere is on the order of 1 bar or less. We propose that, depending on atmospheric pressure, biological systems on the surface of Earth-like extrasolar planets at close-in orbital distances can be strongly influenced by secondary cosmic rays.