Superposed epoch analysis of corotating interaction regions at 0.3 and 1.0 AU: A comparative study

Abstract

Applying the superposed epoch analysis technique to 16 and to 31 well‐defined, nonshock‐associated stream‐stream interaction regions observed by the Helios spacecraft in the distance ranges 0.3 to 0.4 AU and 0.9 to 1.0 AU, respectively, we obtain the average azimuthal variation in the solar wind density, velocity and temperature, in the magnetic field strength, and in the total proton plasma plus magnetic field pressure across CIRs at these two radial distances separately. For the radial evolution of these interaction regions we find by comparison: (1) due to compressional and rarefactional effects the amplitudes of all parameters in question taken along the leading as well as along the trailing part of the CIR are steadily increasing with the most pronounced increase in the pressure; (2) at the same time even the leading portion of the velocity profile steepens; (3) simultaneously, the positions in azimuth of the overall maximum values of the solar wind density and temperature, of the magnetic field strength and of the plasma plus magnetic field pressure are getting steadily lined up in longitude; (4) at the same time the leading portions of all profiles are steepening into discontinuous, shocklike structures. Thus, this analysis provides observational evidence for the following results obtained earlier from numerical simulation studies: Stream steepening does occur within 1 AU, and the probability of corotating shocks to form is, on average, much higher beyond than at or within 1 AU.