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ACE News #75 - Sep 22, 2003

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Merged Interaction Regions (MIRs) at 1 AU

Interaction Regions are regions at transient and corotating flows in the solar wind where the magnetic field strength and pressure are high. Merged Interaction Regions (MIRs) are produced by interactions among two or more flows and the merging of the interaction regions of each of these flows. MIRs are dominant structures in the outer heliosphere but they are not common at 1 AU. Nevertheless, MIRs at 1 AU are important, because by virtue of their large size and strong magnetic fields they can produce major geomagnetic storms and strongly influence energetic particles and cosmic rays.

Corotating streams originate in coronal holes at the Sun. From maps of the coronal holes on the Sun one predicts the arrival at 1 AU of two corotating streams (CRS-1 and CRS-2) driving two corotating interaction regions, CIR-1 and CIR-2, from March 17 - 23, 2002. These two streams and interaction regions were observed (see plot of the magnetic field strength, B, and solar wind velocity, V in the above-left Figure), but they were just part of a complicated stream profile related to interactions among transient flows, and they were related to two MIRs that were the dominant features in the solar wind near 1 AU at that time. CIR-1 was located between MIR-1 and MIR-2, and CIR-2 was a small part of MIR-2, bounded in the front by a stream interface (SI) and in the rear by a possible reverse shock (RS?).

Using ACE data from MAG, SWEPAM, and SWICS to identify the nature of the flows and the solar data to identify their origin, one can determine how the MIRs formed by interactions among several flows, including complex ejecta (CE), magnetic clouds, shocks (S), and corotating streams (see above-right diagrams).

Space weather forecasts will have to be able to predict the formation of MIRs and the strength and direction of their magnetic fields. This is not possible with current observations, models and theories, but insight gained from the analysis of ACE data is an important step in this process.

Submitted by L. Burlaga and the ACE MAG Team.

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