RESUMO
We present an interpretation of the time variability of the x-ray flux recently reported from a multiepoch campaign of 15 years of observations of the supernova remnant Cassiopeia A by Chandra. We show for the first time quantitatively that the [4.2-6] keV nonthermal flux increase up to 50% traces the growth of the magnetic field due to a vortical amplification mechanism at a reflection inward shock colliding with inner overdensities. The fast synchrotron cooling as compared with shock-acceleration time scale qualitatively supports the flux decrease.
RESUMO
I derive analytically the temporal dependence of the perpendicular transport coefficient of a charged particle in the three-dimensional anisotropic turbulence conjectured by Goldreich-Sridhar by implementing multispacecraft constraints on the turbulence power spectrum. The particle motion away from the turbulent local field line is assessed as gradient-curvature drift of the guiding center and compared with the magnetic field line random walk. At inertial scales much smaller than the turbulence outer scale, particles decorrelate from field lines in a free-streaming motion, with no diffusion. In the solar wind at 1 AU, for energy sufficiently small (<1 keV protons), the perpendicular average displacement due to field line tangling generally dominates over two decades of turbulent scales. However, for higher energies (≃25 MeV protons) within the range of multispacecraft measurements, the longitudinal spread originating from transport due to gradient-curvature drift reaches up to ≃10^{∘}-20^{∘}. This result highlights the role of perpendicular transport in the interpretation of interplanetary and interstellar data.
