the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The Origins of a Near-Ecliptic Merged Interaction Region as a Magnetic-Cloud like Structure Embedded in a Co-rotating Interaction Region
Abstract. Using remote-sensing and in-situ observations across multiple spacecraft with complimentary methods of analysis, we investigate a Magnetic Cloud Like-structure (MCL) observed in-situ on 3–4 July 2007 near the ecliptic at OMNI, STEREO-A and -B (all within 15° longitude of Earth). The MCL is entrained in a Corotating Interaction Region (CIR) originating in the Northern heliospheric sector, to create a Merged Interaction Region (MIR). This event allows the comparison of MIR observations at different longitudes showing differences in size, formation of sheath, presence of forward and reverse waves and small-scale structuring, demonstrating the progression of the interaction between the CIR and MCL from West to East. In order to explore its origins further, we compare the MIR with the (Interplanetary) Coronal Mass Ejection (ICME/CME) studied in Maunder et al. (2022) in the mid-latitudes at Ulysses containing a Magnetic Cloud (MC) and present a comprehensive discussion of the challenges posed by observing and relating transients not in alignment, across different latitudes and longitudes, and in different solar wind environments. As the CME propagates almost directly towards Ulysses, we find through fitting and modelling that its flanks could also potentially skim the near-ecliptic spacecraft. Length-scale analysis appears to be consistent with this configuration. However, local expansion velocities of the MCL/MC indicate compression near the ecliptic and expansion at Ulysses and the magnetic flux rope orientations and helicities at the different latitudes oppose each other. The CIR likely causes more compression and re-aligns the transient axis orientation near the ecliptic while a High Speed Stream (HSS) from the Southern sector propagates directly into the back of the ICME/MC near the mid-latitude. Opposing signs of helicity could provide indications of flux added in the first stages of CME evolution or magnetic reconnection with the Heliospheric Current Sheet (HCS). These observations and analyses demonstrate the continued challenge of modelling and fitting the propagation of transients embedded in complex solar wind environments. We note some of the caveats and limitations in the methods and highlight the use of multi-spacecraft analysis to disentangle the origin and formation of ICME substructures from the solar wind and other transients.
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RC1: 'Comment on angeo-2023-39', Anonymous Referee #1, 02 Feb 2024
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AC3: 'Reply on RC1', Claire Foullon, 29 May 2024
We appreciate the interest in the paper, but we understand the referee had some questions. We believe those can be addressed in a revised version.
The proposed revised version will follow the referee’s last suggestion in point 8 to not include the Ulysses and CCMC modelling (section 3, pp.15-23, subsections 3.1, 3.3, 3.4 and 3.5, Ulysses aspects in 3.2), keeping the near-ecliptic length-scale analysis of 3.2 only.
Thus, the revised paper will concentrate on the connections between the near-ecliptic in-situ observations. We will change the title to be “Longitudinally Spaced Observations of Magnetic Cloud Like Structure Embedded in a Co-rotating Interaction Region”. The abstract and some of the texts in the Introduction and Conclusion will also be shortened to focus on the comparison of observations at different longitudes near the ecliptic, which demonstrates the progression of the interaction between a Corotating Interaction Region and a Magnetic Cloud Like-structure from West to East.
As noted by the referee, the word ‘origins’ in the title was confusing. It was not necessarily to be understood as ‘solar origins’, while in Section 3 of the manuscript, we introduced the word ‘origins’ to mean solar origins. This is what may have been a misunderstanding. In the title we wanted to emphasise the interpretation of the observations with the word ‘The origins of’ in the title attached to the rest of the sentence containing the interpretation starting with ‘as’ from the analysis conducted in the paper. To avoid such confusion and with most of the section 3 removed, the new proposed title will be more appropriate.
Most of the points made by the referee (1,2,3,5 and 6) correspond to aspects of Section 3, and despite the interest in exploring this difficult event and the section 3 inviting discussions, we accept the limitations and are removing these parts.
We thus address the remaining relevant aspects in points 4, 7 and 8, not pertaining to Section 3.
- For context, we will include an EUV image (and/or possibly a Carrington map) of the Sun to show the relevant coronal holes. Travelling the distance of 1 AU with the High Speed Stream speed of about 600 km/s indicates that the relevant coronal hole plasma parcel for the in-situ observations will be seen 2.9 days earlier on 1st of July 2007.
- The in-situ measurements are consistent with one or more structures developing between spacecraft. We will make sure the text is not misleading as we did not intend to say that they are strictly the same structure. However, we will present more discussions and include relevant literature. The longitudinal extent of blobs is likely to vary according to their size. Table 5 in our manuscript shows the length-scales of the MC(L) at the different near-ecliptic spacecraft, which are 0.07 AU at OMNI, 0.1 AU at ST-B and 0.12 AU at ST-A. The Y-GSE distances between ST-A and Earth and Earth and ST-B are about 0.17 AU and 0.14 AU, respectively. Magnetic discontinuities in the HCS surroundings are more likely to be aligned with the HCS and to have similar longitudinal extents as the HCS, as observed by Foullon et al. 2009 Sol Phys 259, with ST-A and ST-B separated by nearly 0.8 AU in the Y GSE direction. We will also refer to more recent works by Sanchez-Diaz et al. 2017 ApJ 851 32, Sanchez-Diaz et al. 2017 ApJL 835 L7 and Lavraud et al. 2020 ApJL 894 L19.
- Fig. 6 shows the different structures already represented in the various spacecraft. The referee proposed a different launch date for the transients, with a speed of the transient of 194km/s. This speed is not consistent and rather low compared with the in-situ measurements of 330-444 km/s in the MCLs. Travelling the distance of 1AU with those speeds indicates possible formation/launch of the blob plasma at the Sun 3.9 to 5.3 days earlier on 29-30 June 2007. We will investigate adding an image of a blob around those dates in the HI data (similar but in place of Figure 8).
Citation: https://doi.org/10.5194/angeo-2023-39-AC3
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AC3: 'Reply on RC1', Claire Foullon, 29 May 2024
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RC2: 'Comment on angeo-2023-39', Anonymous Referee #2, 26 Feb 2024
This paper studies a magnetic cloud structure within a co-rotating Interaction Region (CIR) using data from multiple longitudinally separated spacecraft. The paper is very well written and clear. I have a few comments below.
Introduction: It would be interesting to briefly mention the observations showing that there seem to be continuous spread of MCs and MCLs from very small to large scale, e.g. Feng et al. 2007 (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2006JA011962)
Line 56: Add here reference to Burlaga et al. 2003 https://ui.adsabs.harvard.edu/abs/2003JGRA..108.1425B/abstract
Line 114-116: Here it is left a bit unclear whether the observed temperatures match the expected ones. I would rewrite this part
Figures 2-5:
- Is it necessary to show both OMNI and ACE data separately? Could you perhaps time shift ACE data to the magnetopause and add pitch angle spactrogram from ACE to OMNI data? I assume that OMNI data is taken from the Wind spacecraft as it seems that ACE lacks some plasma measurements at this time.
- Are FW1 and FW2 are developing forward shocks or something else? Discussion on them and FS could be clarified. Also FS could be marked in Figure 3, at least I don’t see obvious forward shock structure in the figure (at FW1 the density clearly drops in Fig. 3 so this cannot be forward shock). Or is FS marking Foward Sheath rather than Forward Shock? If so, accronym is a bit misleading.
Lines 137-139: Not sure about this statement. Isn’t FW2 after or at the MCL rear boundary for all spacecraft shown? I.e. if FW2 marks the developing wave at the front of a CIR in all cases the MCL would be compressed at the leading edge of the CIR?
I would emphasize in Section 2.2 that the fact that all spacecraft give negative helicity and same flux rope type to support that indeed the same structure was observed at all locations.
Lines 187 - 189: Is HPS structure that is often observed close to CIRs also expected to have high beta? I remember they have low temperature at least and can sometimes be mixed with ICMEs
In Sect. 3.3 would be interesting to know a bit about the CME association and the parameters used in the ENLIL run. Was it a clear event and were parameters based on single spacecraft evaluation that has uncertainties?
Conclusions: Is it a likely scenario that helicity of different sign would be added to a flux rope upon eruption or that magnetic reconnection, which should conserve helicity, could lead to the opposite helicity? This topic could have a bit more discussion as having one ICME consisting of parts with opposite helicities would be a really intriguing finding.
Citation: https://doi.org/10.5194/angeo-2023-39-RC2 -
AC1: 'Reply on RC2', Claire Foullon, 29 May 2024
Publisher’s note: the content of this comment was removed on 30 May 2024 since the comment was posted by mistake.
Citation: https://doi.org/10.5194/angeo-2023-39-AC1 -
AC2: 'Reply on RC2', Claire Foullon, 29 May 2024
We appreciate the constructive comments, and we propose to address those in a revised version as follows.
- References to continuous distribution in size will be added as suggested.
- Line 56. The reference will be added as suggested.
- Lines 114-166. Thanks for this. The sentence needed correction. This will be replaced with: The proton temperature, in panel (f) is below the expected solar wind temperature Tex (over plotted in blue, Richardson and Cane, 1995; Lopez, 1987) for STEREO-A and OMNI, indicative of a MC, but matches approximately Tex for STEREO-B.
- Figures 2-5:
*The OMNI data is not obtained by a fixed time shift, as the time shift depends on the solar wind speed, thus the ACE pitch angle spectrogram cannot be easily shifted to the same timeframe as OMNI. This is why we provided the ACE complementary figure. Some ACE measurements are indeed missing but not all (this is also why we needed the OMNI data), and the OMNI data will combine both ACE and Wind data if they both exist. An alternative was to show Wind and ACE separately, but then we are missing the common timeframe and there will be gaps in data. We have explored this possibility, but we believe we have presented the data in the most efficient way given the circumstances.
* FW1 and FW2 have been labelled as Forward Waves (as opposed to Forward Shocks FS). At STEREO-A (Figure 3), they are pressure waves that will indeed develop into shocks but are not yet steepened enough. The term is introduced on line 156 in reference to an increase in pressure and speed and the caption of Figure 3 refers to a pressure wave. The referee is correct that this is not a shock wave, and perhaps the confusion comes from associating a wave with a shock, which is a particular steepened wave. - Lines 137-139. The referee is correct that the FW2 boundary coincides with the MCL rear boundary for all spacecraft. In the case of STEREO-A only, this also coincides with the steepened wave at the front of a CIR. We will add this sentence for clarity.
- Section 2.2. Thanks for this. This will be added.
- Lines 187-189. The heliospheric plasma sheet (HPS) is usually characterised by an increase in plasma density and beta. According to the Figures 2, 3 and 4, the HPS is likely to encompass all the structures that we see here, including MCL and CIR. We will add this to the texts. The temperature of the HPS is not generally high or low. Crooker et al. (2004) list various HPS with and without high temperature signatures. The high beta, high density, high temperature observations are consistent with reconnection exhausts. On that basis, they can contain magnetic field discontinuities and small flux ropes, which are too small to be listed as CMEs. Foullon et al. (2009) showed an example in their Figure 2, where there is a case of an accompanying decrease in temperature consistent with the slow solar wind stream of the equatorial regions. Magnetic field discontinuities interpreted as a pattern of interchange reconnection are found closer to the HCS and not in the region of the HPS with decreased temperature.
- Section 3.3. This section will be removed, in response to another referee.
- Conclusions. In response to another referee, we will not have comparison with the Ulysses data, but we appreciated the interest.
References:
Crooker, N. U., C.-L. Huang, S. M. Lamassa, D. E. Larson, S. W. Kahler, and H. E. Spence (2004), Heliospheric plasma sheets, J. Geophys. Res., 109, A03107, doi:10.1029/2003JA010170.
Foullon, C., Lavraud, B., Wardle, N.C. et al. The Apparent Layered Structure of the Heliospheric Current Sheet: Multi-Spacecraft Observations. Sol Phys 259, 389–416 (2009). https://doi.org/10.1007/s11207-009-9452-4
Citation: https://doi.org/10.5194/angeo-2023-39-AC2
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