The cosmic ray energy spectrum between 10 and 10 eV

The energy spectrum of cosmic rays with primary energies between 10¹⁴ eV and 10¹⁶ eV has been studied with the CASA-MIA air shower array. The measured differential energy spectrum is a power law (dj/dE ∝ E^−y) with spectral indices γ of 2.66±0.02 below approximately 10¹⁵ eV and 3.00±0.05 above. A new method is used for measuring primary energy derived from ground-based data in a compositionally insensitive way. In contrast with some previous reports, the “knee” of the energy spectrum does not appear sharp, but rather a smooth transition over energies from 10¹⁵ eV to 3.0 × 10¹⁵ eV.     

Decaying stratified turbulence: comparison between a two-point closure EDQNM model and direct numerical simulations

A new statistical two-point closure EDQNM (Eddy Damped Quasi Normal Markovian) model for axisymmetric stratified turbulence is presented. This model takes into account the detailed anisotropic structure of the flow, including angular dependence, and its closure assumption may involve explicit effects of the stratification. In the present paper, a simplified closure assumption is used, in which these explicit effects are not taken into account. The model is tested against three-dimensional direct numerical simulations (DNS) of decaying homogeneous stratified turbulence. A very good agreement is found between DNS results and EDQNM predictions, when energy transfers between the different modes of motion are considered. However, transfers among wavenumbers are insufficiently damped by the EDQNM model, because of the simplified closure assumption, thus yielding a somewhat less satisfactory agreement. An interesting prediction of the EDQNM model is the existence of a ‘directional’ anisotropy at all scales of motion, to the smallest: the level of the total (kinetic + potential) energy density spectrum E(k,θ_k) varies with the angle θ_k that the wavenumber vector k makes with the vertical.     

DOT Tomography of the Solar Atmosphere VII. Chromospheric Response to Acoustic Events

We use synchronous movies from the Dutch Open Telescope sampling the G band, Ca?ii?H, and Hα with five-wavelength profile sampling to study the response of the chromosphere to acoustic events in the underlying photosphere. We first compare the visibility of the chromosphere in Ca?ii?H and Hα, demonstrate that studying the chromosphere requires Hα data, and summarize recent developments in understanding why this is so. We construct divergence and vorticity maps of the photospheric flow field from the G-band images and locate specific events through the appearance of bright Ca?ii?H grains. The reaction of the Hα chromosphere is diagnosed in terms of brightness and Doppler shift. We show and discuss three particular cases in detail: a regular acoustic grain marking shock excitation by granular dynamics, a persistent flasher, which probably marks magnetic-field concentration, and an exploding granule. All three appear to buffet overlying fibrils, most clearly in Dopplergrams. Although our diagnostic displays to dissect these phenomena are unprecedentedly comprehensive, adding even more information (photospheric Doppler tomography and magnetograms along with chromospheric imaging and Doppler mapping in the ultraviolet) is warranted.