As noted, CHs can be long-lived structures of open magnetic field lines in the solar corona along which the fast solar wind propagates and escapes to interplanetary space over several solar rotations, forming HSSs. HSSs shape the solar wind in interplanetary space, and their interaction with upstream slow solar wind flows generates the SIRs of compressed plasma and magnetic fields in the solar wind. The relevance of SIRs to space weather phenomena is multifaceted. Repeating/recurring SIRs, or CIRs, can significantly affect the evolution/propagation of coronal mass ejections (CMEs) in interplanetary space due to the different interactions between CMEs and fast vs. slow solar wind backgrounds as well as the more-complex HSS and/or SIR-type structures that CMEs can also interact with and sometimes deflect as a result of such interaction.
The pair of shock waves forming at the leading and trailing edges of SIRs/CIRs often trigger phenomena of particle acceleration to quasi-relativistic speeds, generating gradual solar energetic particle (SEP) events (and lower-energy Energetic Particles (EPs)). The high plasma pressure and magnetic fields associated to SIRs/CIRs also trigger recurrent strong geomagnetic storms, making HSSs/SIRs/CIRs major drivers of strong space weather disturbances at Earth and elsewhere in the heliosphere. Improving our current understanding of the origins and evolution/interaction of HSSs and thus the predictions of the HSSs/SIRs/CIRs arrival times, properties, and impact at Earth and other locations in the heliosphere is a critical component to improve predictions of space weather phenomena including the effects on CMEs/SEPs.
Present background heliospheric modelling requires improvements for the overall benefit of our understanding of the heliospheric structures as well as for the prediction of CME transits and impacts throughout the heliosphere and for the prediction of the evolution and impacts of HSSs/SIRs/CIRs.