Bow shock perturbation of the upstream solar wind proton component

Abstract

A study of the perturbation of the upstream proton thermal component due to the proximity of the earth's bow shock has been made. When all proton spectra measured by the LASL plasma probe aboard the Imp 6 satellite between March 18, 1971, and May 30, 1972, are sorted according to either a magnetic field-bow shock unconnected or a connected geometry (parallel or antiparallel electron and proton heat flux vectors, respectively), the following results are determined. First, upstream locations on the dawn side of the magnetosphere are connected to the bow shock through the magnetic field B more often than they are on the dusk side, and, for all solar ecliptic longitudes, are connected more often closer to the bow shock. Second, the proton temperature Tp does not depend on whether the plasma is unconnected (U) or connected (C) to the bow shock through B, 〈Tp〉C = 7.34 ± 0.21 × 104 °K and 〈Tp〉U = 7.56 ± 0.14 × 104 °K, with ratio 〈Tp〉C/〈Tp〉U = 0.97 ± 0.03. Third, the thermal anisotropy T∥/T⊥ is less when it is connected to the bow shock, 〈T∥/T⊥〉C = 1.88 ± 0.03 and 〈T∥/T⊥〉U = 2.00 ± 0.02, with ratio 〈T∥/T⊥〉C/〈T∥/T⊥〉U = 0.94 ± 0.02. Fourth, the dimensionless third velocity moment qp is also less when it is connected to the bow shock, 〈qp〉C = 0.226 ± 0.007 and 〈qp〉U = 0.285 ± 0.006, with ratio 〈qp〉C/〈qp〉U = 0.79 ± 0.04. These changes are small in comparison with variations of the proton thermal state normally observed in the bow shock unperturbed plasma. These results indicate that the enhanced wave level characteristic of the perturbed plasma is sufficient to scatter yet not transfer measurable amounts of energy to the ambient proton component on time scales less than about 10³ s. Lastly, the fact that 〈qp〉C < 〈qp〉U is further evidence that interpenetrating proton streams are not generated by the earth's bow shock.