Interplanetary Shocks between 0.3 and 1.0 au: Helios 1 and 2 Observations

Abstract

The Helios 1 (H1) and Helios 2 (H2) spacecraft measured the solar winds at a distance between ∼0.3 and 1.0 au from the Sun. With increasing heliocentric distance ( r _h ), the plasma speed is found to increase at ∼34–40 km s ^−1 au ^−1 and the density exhibits a sharper fall ( ${{r}_{{\rm{h}}}}^{-2}$ ) compared to the magnetic field magnitude ( ${{r}_{{\rm{h}}}}^{-1.5}$ ) and the temperature ( ${{r}_{{\rm{h}}}}^{-0.8}$ ). Using all available solar wind plasma and magnetic field measurements, we identified 68 and 39 fast interplanetary shocks encountered by H1 and H2, respectively. The overwhelming majority (85%) of the shocks are found to be driven by interplanetary coronal mass ejections (ICMEs). While the two spacecraft encountered more than 73 solar wind high-speed streams (HSSs), only ∼22% had shocks at the boundaries of corotating interaction regions (CIRs) formed by the HSSs. All of the ICME shocks were found to be fast forward (FF) shocks; only four of the CIR shocks were fast reverse shocks. Among all ICME FF shocks (CIR FF shocks), 60% (75%) are quasi-perpendicular with shock normal angles ( θ _Bn ) ≥ 45° relative to the upstream ambient magnetic field, and 40% (25%) are quasi-parallel ( θ _Bn < 45°). No radial dependences were found in FF shock normal angle and speed. The FF shock Mach number ( M _ms ), magnetic field, and plasma compression ratios are found to increase with increasing r _h at the rates of 0.72, 0.89, and 0.98 au ^−1 , respectively. On average, ICME FF shocks are found to be considerably faster (∼20%) and stronger (with ∼28% higher M _ms ) than CIR FF shocks.

Keywords

• Interplanetary shocks
• Solar coronal mass ejections
• Corotating streams
• Interplanetary discontinuities
• Interplanetary medium