Magnetic field fluctuations across the Earth’s bow shock

Abstract

Abstract. We present a statistical analysis of 132 dayside (LT 0700-1700) bow shock crossings of the AMPTE/IRM spacecraft. We perform a superposed epoch analysis of low frequency, magnetic power spectra some minutes upstream and downstream of the bow shock. The events are devicded into categories depending on the angle θ Bn between bow shock normal and interplanetary magnetic field, and on plasma-β. In the foreshock upstream of the quasiparallel bow shock, the power of the magnetic fluctuations is roughly 1 order of magnitude larger (δB ∼ 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 shock; 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 contrary, downstream of the quasi-perpendicular shock, the magnetic wave activity is considerably higher than upstream. Downstream of the quasi-perpendicular low-β bow shock, we find a dominance of the left-hand polarized component at frequencies just below the ion-cyclotron frequency, with amplitudes of about 3 nT. These waves are identified as ioncyclotron waves, which grow in a low-β regime due to the proton temperature anisotropy. We find a strong correlation of this anisotropy with the intensity of the left-hand polarized component. Downstream of some nearly perpendicular (θ Bn ≈ 90 • ) high-β crossings, mirror waves are identified. However, there are also cases where the conditions for mirror modes are met downstream of the nearly perpendicular shock, but no mirror waves are observed.

Solar wind vs magnetosheath turbulence and Alfvén vortices

“…A similar spectral maximum was observed during a flux transfer event on the magnetopause (Rezeau et al, 1993). This spectral knee has also been observed downstream of quasi-perpendicular bowshocks (Czaykowska et al, 2001) when the upstream solar wind beta was β<0.5. It was interpreted as a signature of AIC waves.…”

“…In both regions the anisotropy in wave vector distributions, k ⊥ >k , is observed (Bieber et al, 1996;Horbury et al, 2005;Sahraoui et al, 2006;Mangeney et al, 2006;Osman and Horbury, 2007). In both regions the turbu- lent spectrum has a break at the vicinity of the ion characteristic scales (Leamon et al, 1998;Czaykowska et al, 2001).…”

“…1a, one can see that the high frequency part of the spectrum, within the range [2.0, 12.5] Hz, follows a well defined power law f −4 . This power law is rather far from the mean spectrum ∼f −2.6 determined in (Czaykowska et al, 2001), which we have discussed in Sect. 1.…”

“…In the statistical study (Czaykowska et al, 2001), the authors consider not only frequencies below f ci , but also the high-frequency part f >f ci . Regardless of the fact that the mean fluctuation level downstream of a quasi-parallel shock is higher than downstream of a quasi-perpendicular shock, it was shown that both spectra have a break at the vicinity of f ci and that the high-frequency parts of the spectra f >f ci follow the same power-law ∼f −2.6 .…”

“…In a statistical study (Czaykowska et al, 2001), 132 samples of magnetosheath fluctuations are analyzed just downstream of quasi-parallel and quasi-perpendicular bowshocks. In this study, 4-min time periods centered at 3, 5 and 7 min from the shock crossing have been analyzed.…”