Properties of slow‐mode shocks in the distant (>200 RE) geomagnetic tail

Abstract

The two distant ISEE 3 geomagnetic tail passes have been examined to identify all slow‐mode shocks present in the data. We find a total of 86 events from 439 plasma sheet/lobe crossings, using five criteria based on relations between the upstream lobe and the downstream plasma sheet magnetic field and plasma measurements. The statistical results of slow‐mode shock parameters such as the angle between magnetic field and shock normal, θBn, Alfven Mach number along the normal direction, MAn, and electron beta, βe, are calculated and reported. On average, the magnetic field decreases by a factor ∼2.7, the electron density increases by ∼1.7, temperature increases by ∼1.8, and the plasma flow velocity increases by ∼ 3.8 across the shocks. The average upstream θBn is ∼76°, while the downstream angle is ∼ 50°. The shocks have an average MAn ∼ 0.87 along the normal direction, and an upstream βe ∼ 0.04. In the downstream plasma sheet region, the dominant plasma flow associated with the shocks is in the tailward direction with an average speed ∼ 585 km/s. Only a few cases of Earthward downstream plasma flow have been detected. The slow shocks have thicknesses, on average, of −5380 km (about 7 ion inertial lengths) and an average tilt angle of ∼ ±22.4° between the shock normal and z axis. Using the Petschek slow shock model, the average location of the neutral lines is located in a range of ±40 RE from observation sites. About half the slow mode shock events are detected during southward IMF intervals and half during northward intervals. There is a weak substorm dependence of slow mode shocks and plasmoids, a dependence which is most obvious when Bz > +2 nT and Bz < −2 nT intervals are intercompared. We see a substorm dependence for plasma sheet/lobe crossings which suggests that the deep tail become more dynamic during substorm intervals. We have also sought the existence of large wave trains downstream of slow shocks that have been theoretically predicted by Coroniti [1971] and simulation studies. No such wave trains were observed throughout the two ISEE 3 passes of the distant tail. However, we do detect some medium amplitude transverse waves in the shock ramp regions. The waves have frequencies and polarizations similar to the plasma sheet boundary layer waves reported by Tsurutani et al. [1985]. The waves present in the shock ramp are also right‐hand ion cyclotron waves in the plasma frame. We believe that these waves are generated by the ion beams flowing away from the magnetic merging regions.