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

The Helios 1 (H1) and Helios 2 (H2) spacecraft measured the solar winds at a distance between similar to 0.3 and 1.0 au from the Sun. With increasing heliocentric distance (r(h)), the plasma speed is found to increase at similar to 34-40 km s(-1) au(-1) and the density exhibits a sharper fall (r(h)(-2)) compared to the magnetic field magnitude (r(h)(-1.5)) and the temperature (r(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 similar to 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 (theta(Bn))>= 45 degrees relative to the upstream ambient magnetic field, and 40% (25%) are quasi-parallel (theta(Bn) < 45 degrees). 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 rh 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 (similar to 20%) and stronger (with similar to 28% higher M-ms) than CIR FF shocks.