Interstellar Magnetic Fields and Cosmic Ray Electrons

The synchrotron emission at frequency $\nu$ is dominated by electrons of energy

$$E= 3 \,\left( {\nu \over {1\ GHz}} \right)^{1/2} {\rm GeV}$$ (14)

The intensity is proportional to the product of the magnetic field intensity and the density of cosmic ray electrons. The simulations described in show that PLANCK will be able to map the synchrotron emission accurately at Galactic latitudes below 30 degrees and to derive useful information even in the lowest brightness regions at high Galactic latitudes. The synchrotron emission as detected in the PLANCK frequency range, when combined with lower frequency data, will lead to new information on the spatial distribution of the spectrum of cosmic ray electrons in the energy range 10 to 20 GeV.

The high frequency data will have other major advantages over lower frequency maps obtained from the ground: a significantly higher angular resolution (typically 15 arcminute instead of 51 arcminute for the 408 MHz Haslam survey) and a homogeneous coverage of the whole sky. An important unresolved question of interstellar medium physics is the distribution of magnetic field with respect to the matter distribution. In recent years, interstellar matter has been found to be structured on all scales observed (see for example Falgarone 1992). The physics of the very small scale structure is still not understood. It is known that on large scales the magnetic field is correlated with matter, probably as a result of gravitational instability in a medium where gas and magnetic fields are well coupled. On very small scales where gravity is not important, it has been proposed that matter and magnetic fields could be anticorrelated (Tagger & Falgarone, 1995). It has been recently shown that the dust polarised emission (reconstructed from the available 3D Leiden-Dwingeloo HI maps) correlates to the large scale magnetic field in our galaxy (see Prunet et al. 1998). Thus, high sensitivity measurements of the polarised signal in the highest frequency channels of the HFI, where dust emission is expected to dominate, will provide detailed information about the spatial distribution of the magnetic field in the galaxy. The polarised galactic synchrotron emission is also expected to be correlated to the strength and direction of the galactic magnetic field, although the lack of obervational data difficults the modelling of such correlation.

[last update: 1 August 1999 by P. Fosalba]