Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.

The transport of solar energetic charged particles along the interplanetary magnetic field in the ecliptic plane of the sun can be described roughly by a one-dimensional diffusion equation. Large-scale spatial variations of the guide magnetic field can be taken into account by adding an additional term to the diffusion equation that includes the effect of adiabatic focusing. We solve this equation analytically by assuming a point-like particle injection in time and space and a spatial power-law dependence for the focusing length and the spatial diffusion coefficient. We infer the intensity-and anisotropy-time profiles of solar energetic particles from this solution. Through these the influence of different assumptions for the diffusion parameters can be seen in a mathematically closed form. The comparison of calculated and measured intensity-and anisotropy-time profiles, which are a powerful diagnostic tool for interplanetary particle transport, gives information about the large-scale spatial dependence of the focusing length and the diffusion coefficient. For an exceptionally large solar energetic particle event, which did occur on 2001 April 15, we fit the 27−512 keV electron intensities and anisotropies observed by the Wind spacecraft using the theoretically derived profiles. We find a linear spatial dependence of the mean free path along the guiding magnetic field. We also find the mean free path to be energy independent, which supports the theory of "velocity-dependent diffusion". This means that the intensity profiles for the discussed energies exhibit the same shape if they are plotted against the traveled distance and not against the time. In this case the profiles differ only in their maximum values and we can determine the energy spectra of the solar flare electrons out of the scaling factor we need to fit the data. The derived spectra exhibits a power-law dependence ∝E −3 kin in an energy range from ∼50 keV to ∼500 keV.

The anisotropy-time profile of solar particle events provides a powerful diagnostics tool to the interplanetary plasma scattering parameters of energetic charged particles. In the weak focusing limit of the transport of solar particles in axisymmetric MHD turbulence, the particle anisotropy consists of two contributions, the streaming and the Compton-Getting contribution, resulting from the parallel spatial gradient and the momentum gradient of the isotropic part of the particles' phase space density, respectively. These gradients can be calculated from the appropriate solution to the timedependent focused transport equation of solar particles. For the illustrative case of the solution of the one-dimensional time-dependent focused transport equation with a constant focusing length and a point-like instantaneous injection of particles the streaming and Compton-Getting contributions to the anisotropy-time profile are analytically calculated in MHD turbulence consisting of isospectral undamped slab Alfven waves for equal magnetic helicity. The Compton-Getting contribution scales proportional to the ratio of interplanetary Alfven speed to solar particle speed, and therefore is much smaller than the streaming contribution for the observed mildly relativistic solar particles. After vanishing anisotropy values at times t < tM the streaming anisotropy suddenly attains its maximum value A S,max = 3 2 + (p) 2L at t M = t 0 + (z z 0)/v. At later times the streaming anisotropy decreases (t t 0) 1 approaching the asymptotic finite value ((p)/2L) for t t 0 , positive or negative, depending on the sign of the focusing length L. The new analytical form of the streaming anisotropy provides an excellent fit to the observed anisotropy profiles from the easter solar particle event of 2001 April 15 for 1.3 GeV protons, but does not well reproduce the anisotropies of 510 keV electrons.

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