the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jul 2026
“Corotating Interaction Regions (CIRs)”, “Interaction Regions (IRs)” and “Stream Interaction Regions (SIRs)”, which term should be used?
Abstract. We discuss the early history of quasiperiodic ~27-day recurrent geomagnetic activity starting with Maunder (1904, 1905), Chree (1913) and Bartels (1932, 1934), and the Bartels term “M-regions”. We show the iconic “interaction region (IR)” schematic of Belcher and Davis (1971) and the further development of Smith and Wolfe (1976) and the term “corotating interaction region (CIR)”. We quote the Jian et al. (2006) definition of a “stream interaction region (SIR)”. We disagree with Jian et al. (2006) on the use of the term (SIR) to indicate “transient and possibly localized stream interactions” with “poor recurrence” (Gosling et al., 2001). We feel that this description is too vague for use in scientific studies. We suggest, instead identifying the specific known interplanetary phenomena: interplanetary coronal mass ejection (ICME) sheaths, ICMEs (loops, magnetic clouds, filaments), CIRs, high-speed streams (HSSs) and slow streams. All of these various interplanetary phenomena have different solar and interplanetary origins and different plasma and magnetic field properties. The different interplanetary phenomena have been shown to have different geomagnetic effectivenesses. In keeping with this theme of naming specific interplanetary phenomenon, we introduce the term “Super CIR (SCIR)”, which describes a CIR associated with magnetic reconnection at the edge of a solar coronal hole with an embedded coronal jet. SCIRs are a new form of a “transient event” and can be identified by exceptionally strong internal magnetic fields and bounded by both forward and reverse shocks. The SCIR on 6–7 April 2000 caused an exceptionally strong SYM-H = –319 nT superstorm, a first detected/reported event of its kind.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-3506', Anonymous Referee #1, 28 Aug 2026
- AC1: 'Reply on RC1', Rajkumar Hajra, 30 Sep 2026
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RC2: 'Comment on egusphere-2026-3506', Anonymous Referee #2, 07 Sep 2026
The authors present a manuscript discussing the origin and terminology of Stream Interaction Regions (SIRs) versus Corrotating Interaction Regions (CIRs) and attempt to introduce a new term for "special/super" CIRs: SCIRs. I believe the discussion regarding SIRs and CIRs is important due to the existing confusion around their definitions in the community, and a clarification is definitely needed. However, the introduction of a new term does not aid this goal. With some revisions, I believe this manuscript would make a valuable contribution to the community.
Major Comments:
- General Comment: I agree that the terms SIR and CIR are poorly defined and understood in the community, which leads to confusion. This warrants a community-wide discussion to establish common terminology, and this paper serves as a good starting point in that regard. However, the manuscript currently focuses heavily on favoring CIR over SIR. The authors should explore the possibility that SIR might actually be the better and more accurate term to adopt, as it provides a clearer physical description of the structure (i.e., emphasizing the stream interaction itself) and helps clear up confusion surrounding what is meant by "corotation." Crucially, relying on historical precedence for CIR is not, on its own, a valid scientific argument for its continued use.
- On the proposed "SCIR" term: While bringing up the SIR vs. CIR discussion is important and the arguments against using "SIR" have merit, the rationale behind introducing the new term "SCIR" remains unclear. I do not see any clear benefit to this definition; rather than resolving confusion, adding another loosely defined term for interplanetary structures will likely amplify it. Furthermore, in their case study event the authors claim that "This event is believed to have a very different solar source than typical CIRs," which implies that the high-speed stream (HSS) does not originate from a coronal hole, a claim for which no evidence is provided. It is far more likely that this event represents an interaction between known transients rather than a distinct, newly categorized phenomena.
- Scope and Focus: The authors should keep the primary focus of this short paper on the SIR/CIR terminology. They may mention that complicated edge cases exist (which is to be expected when dealing with such complex space plasma systems), but they should refrain from introducing yet another term.
Minor Comments:
- Paragraph starting at Line 50: The term "coronal hole" was originally coined around 1956 by Max Waldmeier, who associated M-regions with areas of reduced density in coronagraph images. Furthermore, Krieger et al. were not the sole group to revisit and reintroduce the term to the community in the 1970s, but rather one of several key contributors. In a historical context, all pioneering authors should be given proper credit.
Citation: https://doi.org/10.5194/egusphere-2026-3506-RC2 - AC2: 'Reply on RC2', Rajkumar Hajra, 30 Sep 2026
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The manuscript by Tsurutani et al. gives a very detailed historic review of the naming of distinct solar wind structures, such as high-speed streams, stream interfaces, corotating interaction regions, etc. Indeed, there is some confusion and, I agree, CIR is frequently wrongly used in the literature. Hence, this paper is extremely helpful to clarify the term.
However, while I think the historic review is needed for giving to the community more clarity about the terms SI, SIR, CIR, and HSS, I do not see the merit of introducing one extra category of “super-CIRs”. First, only a single event is reported which might be something rare (some future detections are discussed, but statistics is clearly missing). Similar authors, namely Meng et al. (2019), concluded that “No superstorms in this study were induced by corotating interaction regions.”
The single event presented in Tsurutani et al. (2024) concluded: “ We hypothesize that a shrinking coronal hole and magnetic reconnection caused the formation and release of the jet.” Hence, there is not a full analysis done for these conclusions.
*) E.g., no cross-check with EUV or white-light data was done; the jet could be as well a very narrow CME such as frequently observed streamer blowouts, which are strongly related to the edges of coronal holes. Edges of coronal holes are also strongly related to magnetic sector boundaries (see e.g., papers by Schwenn and Marsch in the 90s). We also know of small-scale CMEs within coronal holes as there are many closed loops (e.g., such as coronal bright points observed in SXR and EUV), which may undergo interchange reconnection and produce mini-CMEs. I just would like to make the point, that one should not forget the link to the solar perspective.
*) Moreover, the backmapping of the in-situ detected structure to exactly the edge of a contracting coronal hole, might also be debatable. Detecting edges of coronal holes is a subject on its own, with plenty of papers in the recent years concluding uncertainties. Tsurutani et al. (2024) used manually detected coronal holes. Extrapolations from magnetic field data are in a similar range uncertain. We are talking about uncertainties in the order of several degrees. We also know that coronal holes evolve, not very rapidly, but steadily. The entire evolution is related to interchange reconnection and small-scale ejecta – think of the numerous coronal blobs detected by Sheeley et al. – again, related to the magnetic sector boundaries and, hence, edges of coronal hole. We might ask, why we haven’t seen such cases as reported in Tsurutani et al. (2024) not in many more cases?
In that respect, some unexpected strong geomagnetic activities might also be due to the crossing of the heliospheric current sheet carrying high density structures propagating together or interacting with CMEs – which I would call a merged interaction region (MIR): see paper by Burlaga et al. (2003). I would suggest checking the paper by Burlaga et al (2023) for the merged interaction regions and add also that term to the manuscript. It is useful to have that category covered as well, since these types of interacting structures are closely related to what is discussed in the manuscript.
Concluding: I really enjoyed reading the paper from the historic perspective and agree with the authors; I suggest adding the term and explanation of MIRs to the manuscript; before introducing yet another category, one might show statistics first to see whether a new category is necessary.