the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jan 2026
"Corotating Interaction Region (CIR)", "Interaction Region", "Stream Interaction Region (SIR)", which term should be used?
Abstract. We discuss the history of quasiperiodic ~27-day recurrent geomagnetic activity and the origin of the names "interaction region", "corotating interaction region" and "stream interaction region". The latter three names have an identical meaning. We recommend the most commonly used name "corotating interaction region" or CIR for sole usage in the literature to avoid confusion.
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Status: open (until 13 Mar 2026)
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RC1: 'Comment on egusphere-2026-5', Anonymous Referee #1, 30 Jan 2026
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AC1: 'Reply on RC1', Rajkumar Hajra, 18 Feb 2026
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Reply to comments by Anonymous Referee #1:
I understand the authors’ concern on the terminology confusions, as some authors use these two terms interchangeably (Waugh and Jardine, 2025). However, I’m not convinced with the authors’ current narratives. According to the modern studies, CIRs (coronating interacting regions) are used as subsets of SIRs (stream interacting regions). Allen et al. (2020) stated, “After a complete solar rotation, the SIR is classified as a corotating interaction region (CIR)”. Richardson (2018) uses “the general term “stream interaction region”, while being aware that some authors (e.g., Jian et al. 2006) use this term to distinguish a stream that is observed on only one solar rotation from a “corotating” interaction region that is seen on more than one rotation.” If the scientific community stops using the term of SIR, how would we call the CIRs that did not persist as long as one solar rotation? It’s straightforward to understand CIRs as a subset of SIRs.
Reply: Yes, there is confusion in the literature of the meaning of the terms CIR and SIR. We wish to try to clear this up by reviewing the history of the physics and the corresponding names.
Belcher and Davis (1971) clearly showed a schematic of the formation of an “interactive region” or IR. This had nothing to do with whether it would return 27 days later or not. Thus, we feel that Jian et al. (2006) made a mistake in not reviewing the literature carefully and realizing that an IR showed the dynamics of the interplanetary interaction between a fast solar wind stream and a slow solar wind stream. There was no need for the new term Stream Interaction Region or SIR. This unfortunately cascaded into Richardson (2018) escalating things and calling all IRs as SIRs. There is a tangential discontinuity within IRs, CIRs and SIRs (we claim that they are all the same thing) that is called the Stream Interface or SI. Now some people are mistaking an SIR with an SI (Region). The confusion can become far worse if things are not clarified.
Perhaps Smith and Wolfe (1976) made a mistake in calling IRs, Corotating Interaction Regions or CIRs? But clearly Smith and Wolfe (1976) were not insisting that the structures returned 27 days later, they just noticed that in the Pioneers 10 and 11 data that they often did return, a new feature that was not emphasized in Belcher and Davis (1971). The first sentence of the Smith and Wolfe (1976) abstract says: “interaction regions between adjacent solar wind streams have been identified between 1 and 5 AU using Pioneer 10 and 11 magnetic field and plasma measurements”. They clearly called them “interaction regions”. Smith was a collaborator with Belcher and Davis (JPL and Caltech) and they wrote many joint publications together. One of us (BTT) was there at the time of the naming of CIRs with Smith and Wolfe and knew the reasons for the naming of CIRs.
Going back to your last sentence “If the scientific community stops using the term of SIR, how would we call CIRs that did not persist as long as one solar rotation? It’s straightforward to understand CIRs as a subset of SIRs.” We suggest using the long-established (and only term until 2006) CIR and still used continuously now. Please remember that the term “corotating” in the acronym CIR was used to indicate the shape of the structure, not that it returns 27 days later.
Although all of these above points were made in our original Comment, because of your note, we have now emphasized them a bit more.
Citation: https://doi.org/10.5194/egusphere-2026-5-AC1
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AC1: 'Reply on RC1', Rajkumar Hajra, 18 Feb 2026
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RC2: 'Comment on egusphere-2026-5', Anonymous Referee #2, 09 Feb 2026
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The present manuscript comes back on the long-term persisting debate about whether should we say Co-rotating Interaction Region (CIR) or Stream Interaction (SIR) when talking about the overtake of a slow solar wind by a higher speed solar wind stream. Modern convention being to consider CIRs to be a subset of SIRs that persists for several rotations. Retracing the historical consideration of such events is a valuable point of this article. Neverheless, although the authors' recommandation to stick with the term CIR is understandable, the reasons they advance should be further detailed before considering the acceptance of their manuscript.
For instance:
- l.109-114: Although the events that do not recur 27days later do have a poor recurrence, I am not sure they have a poor occurrence in the existing stream interaction region catalogs (Jian et al. (2006), Chi et al. (2018), Nguyen et al. (2025)). Additionally, even if this naming includes so-called "transient and possibly localized stream interactions", these events still share common in-situ patterns when observed at L1 resulting in possible impact for the near-Earth environment. A possible distinction that could also be discussed would stand in the origin of such events, linking their occurrence with coronal hole. The actual denomination however appear as the most inclusive regarding the diversity of such events.
- l.120-132: The event presented in this paragraph is interseting but rather concerns the confusion made between ICMEs and SIRs. Thus, the interest for the debate raised by this manuscript is unclear.
- l.138-139: The authors' point about the possible misinterpretation of the concept of tangential discontinuity is quite interesting and could be a good argument in favor of their opinion. It is however barely mentioned and expanded in the paper.
For these reason, I would recommand the authors to strengthen the arguments in favor of their opinion beofre considering the acceptance of this paper.
Citation: https://doi.org/10.5194/egusphere-2026-5-RC2 -
AC2: 'Reply on RC2', Rajkumar Hajra, 18 Feb 2026
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Reply to comments by Anonymous Referee #2:
The present manuscript comes back on the long-term persisting debate about whether should we say Co-rotating Interaction Region (CIR) or Stream Interaction (SIR) when talking about the overtake of a slow solar wind by a higher speed solar wind stream. Modern convention being to consider CIRs to be a subset of SIRs that persists for several rotations. Retracing the historical consideration of such events is a valuable point of this article. Nevertheless, although the authors’ recommendation to stick with the term CIR is understandable, the reasons they advance should be further detailed before considering the acceptance of their manuscript.
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Reply: Thank you for your general comments above. In the paper, we have documented the history of the interplanetary phenomena and the namings used to show that errors in the literature were made. These errors were compounded and has led to a state where there is not only confusion of the meanings of the terms, but that we can envision even more confusion developing in the future.
Based on your last sentence, we will revise our Comment to give a stronger argument why the term SIR should be dropped from usage. More details will be given below.
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For instance:
- 109-114: Although the events that do not recur 27days later do have a poor recurrence, I am not sure they have a poor occurrence in the existing stream interaction region catalogs (Jian et al. (2006), Chi et al. (2018), Nguyen et al. (2025)). Additionally, even if this naming includes so-called “transient and possibly localized stream interactions”, these events still share common in-situ patterns when observed at L1 resulting in possible impact for the near-Earth environment. A possible distinction that could also be discussed would stand in the origin of such events, linking their occurrence with coronal hole. The actual denomination however appears as the most inclusive regarding the diversity of such events.
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- Reply: This discussion is focused on naming interplanetary phenomena based on their origins, interactions, and evolution. The effects on the near-Earth environment should not be a consideration. Given this, one has to ask the question what are “transient and possibly localized stream interactions”? This phrase is incredibly vague and meaningless. We know of coronal mass ejection (CME) releases from the Sun and high speed-slow speed solar wind stream interactions causing CIRs. CMEs are a “transient phenomenon” and CIRs are due to “local stream interactions”. These two phenomena have been well described and documented in the literature. What else is there? Perhaps nothing? Thus, a category of “transient and possibly localized stream interactions” is not a useful categorization.
- On the other hand, if something different from CMEs and CIRs are found, scientists should identify them and give them new names. We believe that there is no new interplanetary phenomenon that we know of in “transient and possibly localized stream interactions”.
- We have added the Chi et al. (2018) and Nguyen et al. (2025) references to the revised version of the Comment.
- Some of the above points will be added to the Comment.
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- 120-132: The event presented in this paragraph is interesting but rather concerns the confusion made between ICMEs and SIRs. Thus, the interest for the debate raised by this manuscript is unclear.
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- Reply: Because the event is believed to be associated with a coronal hole and a high-speed stream, the authors have claimed it to be a CIR. Of course, the dynamics causing the unusually high speed of the event and the unusual internal magnetic features make it different than a common CIR. The authors would claim it as a subset of CIRs. Since this is the first one of its type to have been identified, it perhaps is too early to give a name to this subset?
- But if a name would be given to this type of event, it could possibly be “Super CIRs or SCIRs”. The name would be given for the extremely fast speed/strong shocks or the constant Bz magnetic field strength feature, not the extremely intense geomagnetic activity that it caused (see above point). Other SCIRs could be detected where the IMF Bz is northward and geomagnetic quiet will result. We know that for CMEs/magnetic clouds, this happens. Sometimes there is strong southward Bz and sometimes strong northward Bz. However, both cases are CMEs/magnetic clouds. One would say that the southward and northward IMF Bz are subsets of CMEs/magnetic clouds. Likewise for CME magnetic clouds, there are also loops and filaments which are subsets of CMEs. They are identified by their interplanetary/solar features and not by the geomagnetic activity that they cause or do not cause.
- This will be clarified in the revised Comment.
- Because of your comment, we have removed the discussion of high-speed streams plus references in the paper.
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- 138-139: The authors’ point about the possible misinterpretation of the concept of tangential discontinuity is quite interesting and could be a good argument in favour of their opinion. It is however barely mentioned and expanded in the paper.
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- Reply: Thank you. We will strengthen our arguments in the revised Comment.
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For these reasons, I would recommend the authors to strengthen the arguments in favour of their opinion before considering the acceptance of this paper.
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- Reply: Thank you for your constructive comments.
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Citation: https://doi.org/10.5194/egusphere-2026-5-AC2
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I understand the authors’ concern on the terminology confusions, as some authors use these two terms interchangeably (Waugh and Jardine, 2025). However, I’m not convinced with the authors’ current narratives. According to the modern studies, CIRs (coronating interacting regions) are used as subsets of SIRs (stream interacting regions). Allen et al. (2020) stated, “After a complete solar rotation, the SIR is classified as a corotating interaction region (CIR)”. Richardson (2018) uses “the general term “stream interaction region”, while being aware that some authors (e.g., Jian et al. 2006) use this term to distinguish a stream that is observed on only one solar rotation from a “corotating” interaction region that is seen on more than one rotation.” If the scientific community stops using the term of SIR, how would we call the CIRs that did not persist as long as one solar rotation? It’s straightforward to understand CIRs as a subset of SIRs.