Magnetic field fluctuations across the Earth's bow shock

We present a statistical analysis of 132 dayside (LT 0700-1700) bow shock c rossings of the AMPTE/IRM spacecraft. We perform a superposed epoch analysi s of low frequency, magnetic power spectra some minutes upstream and downst ream of the bow shock. The events are devicded into categories depending on the angle BE, between bow shock normal and interplanetary magnetic field, and on plasma-beta. In the foreshock upstream of the quasi parallel bow sho ck, the power of the magnetic fluctuations is roughly I order of magnitude larger (deltaB similar to 4 nT for frequencies 0.01-0.04 Hz) than upstream of the quasi-perpendicular shock. There is no significant difference in the magnetic power spectra upstream and downstream of the quasi-parallel bow s hock; only at the shock itself, is the magnetic power enhanced by a factor of 4. This enhancement may be due to either an amplification of convecting upstream waves or to wave generation at the shock interface. On the contrar y, downstream of the quasi-perpendicular shock, the magnetic wave activity is considerably higher than upstream. Downstream of the quasi-perpendicular low-beta bow shock, we find a dominance of the left-hand polarized compone nt at frequencies just below the ion-cyclotron frequency, with amplitudes o f about 3 nT. These waves are: identified as ion-cyclotron waves, which gro w in a low-beta regime due to the proton temperature anisotropy. We find a strong correlation of this anisotropy with the intensity of the left-hand p olarized component. Downstream of some nearly perpendicular (theta (Bn) app roximate to 90 degrees) high-beta crossings, mirror waves are identified. H owever, there are also cases where the conditions for mirror modes are met downstream of the nearly perpendicular shock, but no mirror waves are obser ved.