Plasma and magnetic field behavior across the magnetosheath near local noon

Abstract

Using Active Magnetospheric Particle Tracer Explorers/Ion Release Module data from 10 magnetosheath crossings in the 11–13 LT sector, we have investigated the behavior of the proton temperature and the proton temperature anisotropy between the bow shock and the magnetopause. The proton anisotropy increases toward the magnetopause. Comparison with earlier predictions, however, shows that this increase is much more modest than was predicted by assuming the double adiabatic equations. It is shown that neither of the two double‐adiabatic equations holds separately. We conclude that energy is transferred between the perpendicular and the parallel component of the proton temperature. It is well established that instabilities such as the mirror and the ion‐cyclotron instability show such a behavior and lead to a decrease in the temperature anisotropy. Our data show that in the magnetosheath proper, the threshold for the onset of the mirror instability is always marginally met. We show that we can give an empirical relation between the temperature anisotropy and β⊥, as long as β⊥ is not less than 1.

Effect of He++ ions on the propagation of low‐frequency magnetohydrodynamic waves in the magnetosheath

“…= 3/2, g k = 2 they describe an ultrarelativistic state [Tsikarishvili and Rogava, 1994;Chou and Hau, 2004]. The double-polytropic laws have also been applied to observational data Hill et al, 1995;Farrugia et al, 2000;Farrugia et al, 2001] which indicates that most of the space plasmas are neither adiabatic nor isothermal and the double-polytropic laws with appropriate indices may provide a suitable scheme for closure. Further, it has been shown [Hau, 1996] that complicated nonideal MHD effects can be simulated by appropriate values of the two polytropic exponents of these laws.…”

“…Observational data Hill et al, 1995;Farrugia et al, 2000;Farrugia et al, 2001] indicate that the CGL laws are unsuitable in the magnetosphere. The double-polytropic laws provide better description of the magnetospheric plasma because one can use polytropic indices from observed plasma and magnetic-field data.…”

“…These authors modeled the magnetosheath plasma by taking 96% protons and 4% He ++ ions. Hill et al [1995] studied the effect of various percentages (0 -9%) of He ++ on the mirror mode and concluded that about 5% of He ++ would be necessary to marginally stabilize the mirror mode.…”

Jump conditions for pressure anisotropy and comparison with the Earth's bow shock

“…Therefore, experimental studies of the thermal behaviour of the magnetosheath, the region between the bow shock and the magnetopause, might be taken into account. Using data from the AMPTE/IRM spacecraft, Hill et al (1995) have shown that the double adiabatic equations do not hold in the magnetosheath. Moreover, the thermal behaviour of the magnetosheath is studied by Phan et al (1996) using WIND spacecraft data.…”

“…Since the Rankine Hugoniot equations can only be applied to regions close to the shock wave, the use of these plasma instabilities, which are identified by spacecraft missions (e.g. Hill et al, 1995;Phan et al, 1996), gives some boundaries for the pressure anisotropy for either side of the shock. Different pressure anisotropy rates in the solar wind were examined by Biernat et al (2000), Kiendl et al (2000) and Vogl et al (2000).…”