Merged interaction regions at 1 AU

We discuss the existence of large, complex merged interaction regions (MIRs) in the solar wind near Earth. MIRs can have configurations that cause more prolonged geomagnetic effects than a single flow structure. A MIR or successive MIRs can produce relatively long lasting Forbush decreases at 1 AU. We illustrate MIRs at 1 AU with two examples (MIR-1 and MIR-2) seen by WIND and ACE in the interval from 18 March through 29 March 2002. We determined the probable structure and origin of each in terms of interacting flows and shocks using in situ and solar observations, but we emphasize that there are uncertainties that cannot be resolved with these data alone. The MIRs were relatively large structures with radial extent approximate to2/3 and 3/4 AU, respectively. MIR-1 was formed by interactions related to at least two complex ejecta, a magnetic cloud, and two shocks. MIR-2 was related to a corotating stream, the heliospheric plasma sheet (HPS), two complex ejecta, a magnetic cloud and at least two shocks. A MIR can evolve significantly while it moves to 1 AU, and memory of the conditions near the Sun is lost in the process. Thus one cannot unambiguously determine the structure of a MIR and the manner in which it formed using observations from a single spacecraft at 1 AU. The magnetic field strength profiles in MIRs are not correlated with the speed and density profiles so that one cannot infer the magnetic field strength in MIRs from remote sensing observation that give density and speed information. It will be possible to better understand the dynamical processes leading to the formation of MIRs with remote sensing observations, but they cannot measure the magnetic fields in MIRs.