Interplanetary Coronal Mass Ejections (ICMEs), the interplanetary counterparts of the Coronal Mass Ejections (CMEs), are considered to be the main source of geomagnetic storms, especially the intense geomagnetic storms [1–9]. By studing the intense geomagnetic storms from 1995 to 2014, Shen et al. [9] reported that 87% of intense geomagnetic storms with Dst _min ≤ −100nT were caused by ICMEs. This result is consistent with the results obtained by Zhang et al. [3] and Kilpua et al. [10]. From another point of view, Shen et al. [9] found that 20% of ICMEs caused intense geomagnetic storms. This might be caused by the fact that ICMEs are the main sources of the southward component of the interplanetary magnetic field (B _s , the absolute value of the southward component of the magnetic field). When the southward magnetic field reaches the Earth, it will interact with the magnetosphere and then cause geomagnetic storms [11,12]. Thus, the intensities of B _s carried by ICMEs are important parameters in space weather studies and forecasting. Based on the ICMEs catalogue developed by the University of Science and Technology of China group [13], which is abbreviated as USTC ICME catalogue hereafter, the mean value of the B _s in the ICME (B _s,mean ) varied from 0 to 35.39 nT with the mean value for all ICMEs of 4.40 nT.

According to the previous work, the intensities of geomagnetic storms caused by ICMEs are strongly associated with the intensities of B _s embedded with ICMEs ([9,14–16], and references therein). ICMEs with higher B _s are more likely to cause intense magnetic storms [17,18]. Thus, the origin of the intense B _s is an important question. Based on the literature, the interaction between multiple CMEs would enhance the magnetic field in the ICMEs [9,15,17–19]. In addition, as a special type of multiple ICMEs interaction, the interaction between previous ICMEs and following shocks could also enhance the B _s in the ICMEs significantly [9,20–24]. Shen et al. [18] found that in the shock-ICME interaction event of 6–11 September 2017, the shock compression enhanced B _s inside the ICME from 10 to 30 nT. In their following work, Xu et al. [21] analyzed 18 moderate to intense geomagnetic storms caused by shock-ICME interaction events, showing that the shock compression can enhance the intensities of B _s in ICME by a factor of 2. But, questions still remained. For example, is it possible for the isolated ICME to carry extremely strong B _s ? If so, why? Are there any other possible explanations for the abnormally strong B _s in isolated ICMEs?

In this work, we discuss the possible origins of the extreme intense B _s in the ICMEs based on the typical events anaylsis. In Section 2–5, we will introduce some typical events with extremely high B _s in the ICMEs. In the last section, we will present some conclusions and discussions.

An ICME with extremely high B _s is the 2003 November 20 event. This event caused the strongest geomagnetic storms in the past 30 years with the peak value of Dst index of −422 nT [25–27]. As indicated in Figure 1, an obvious fast forward shock (red vertical line) was detected by Wind spacecraft at 08:37 UT on November 20. Approximately 3.5 h later, an obvious ICME reached the Earth and lasted about 16 h, as shown by the gray shadow region in Figure 1. The ICME exhibits characteristics of magnetic clouds including a rise in magnetic field strength, a smooth rotation in magnetic field vector, and a drop in proton temperature and plasma beta. No additional ICME was detected by Wind spacecraft in 6 h preceding or after this ICME, indicating that this ICME was an isolated ICME event. It indicates that the isolated ICME events also have the potential to carry extremely intense B _s . The possible reasons are: 1) this ICME has a southward axial magnetic field with a high inclination (−73^◦) to the ecliptic plane [25], allowing a long duration and intense Bs to be embedded. 2) this ICME may have interacted with another narrow CME, which erupted an hour earlier than this CME from the same solar source region [27]. During propagation, the preceding narrow CME may be cannibalised by the subsequent large CME, or its propagating direction was deflected away from the Sun-Earth line during the interaction and thus not recorded by the WIND spacecraft.

Figure 2 shows an example of multiple ICME interaction event: the 2001 March 31 event. This event has been widely discussed by different authors [15,28]. As shown in Figure 2, an ICME (shown as the first gray shadow) was detected from 06:18 UT to 12:22 UT on March 31. During this period, the interplanetary observations show obvious ICME signatures with enhanced magnetic field strength, smoothly rotated magnetic field vector, low proton temperature, and low plasma beta. This ICME drove a shock ahead of it (red vertical line), featured by sudden and simultaneous enhancements of the magnetic field, solar wind speed, proton density, and temperature. B _s in this ICME was extremely strong with the mean value of 35.39 nT and maximum value of 48.44 nT. This is the ICME event with the strongest B _s recorded near the Earth from 1995 to 2020 based on the USTC ICME catalogue [13]. After the arrival of the extremely strong Bs carried by this ICME, there was an intense geomagnetic storm with Dst _min of -387 nT at 09:00 UT. About 1 h later, another ICME crossed the Earth from 13:30 UT to 21:22 UT on March 31. This ICME also carried intense B _s with the mean value of 25.36 nT and the maximum value of 35.38 nT. This is the sixth strongest ICME in B _s from 1995 to 2020. Between the two ICMEs, an interaction region exhibited with less regular magnetic field direction, lower magnetic field intensity, higher proton density, higher temperature, and higher plasma β compared with the values in ICMEs [15]. Those signatures are consistent with the magnetic reconnection between the two ICMEs, which may reduce the magnetic field strength and heat the plasma temperature [29]. The abnormally high plasma temperature in the first ICME and the first half of the second ICME suggested a strong interaction and compression between them. The velocity at the rear of the second ICME has obviously increased from 600–800 kms ⁻¹. It might be caused by another ejecta, which was detected by the Wind spacecraft on April 1 at 05:26UT. The interval between the second ICME and the following ejecta exceeds 6 h. The following ejecta is not likely to interact with the second ICME adequately, according to the interaction criterion from Shen et al. [9]. Therefore, the interaction between CMEs was primarily responsible for the extraordinarily intense B _s in these two ICMEs.

Another widely known mechanism that can cause extremely intense B _s is the interaction between shocks and ICMEs. This type of interaction event is identified when the shock driven by the subsequent ICME with a higher velocity propagates into the preceding ICME [20]. Based on the recent works, there are two types of shock-ICME interaction events, i.e., shock-ICME interaction event, and shock passing through preceding ICME event.

To make a further analysis, we analyzed all the ICMEs events with B _s,mean ≥ 10nT during the period from 1995 to 2020 using the USTC ICME catalogue. Due to the lack of Venus Express observations for most events, we simply divided all the ICME events observed by Wind into three groups: isolated ICMEs (I-ICMEs), multiple ICMEs (M-ICMEs), and shock-ICME interaction events (S-ICMEs), using the same criteria from Shen et al. [9]. When the time interval between two ICMEs is less than 6 h, the two or three ICMEs are grouped together as an M-ICMEs event. It is characterised as an S-ICME when in-situ observations identify a shock generated by a subsequent ICME propagating into the preceding ICME or passing through the preceding ICME.

During this period, there are 48 ICME groups with B _s,mean ≥ 10nT and 8 ICMEs with B _s,mean ≥ 20nT. Table 1 shows the numbers of such events originated in different types of ICME events. It is found that 39.6% (20 in 48) of B _s,mean ≥ 10nT events and 50% (4 in 8) of B _s,mean ≥ 20nT events are associated with shock-ICME events. Meanwhile, we also present the percentage of different types of ICMEs with B _s,mean ≥ 10nT and B _s,mean ≥ 20nT. It is found that the possibilities of S-ICMEs causing intense B _s are much higher than that of other groups. Meanwhile, I-ICMEs are much less likely to have intense B _s . These results suggest that the interaction between multiple CMEs, especially the interaction between shock and CME is the main cause of intense B _s in the interplanetary space. It is consistent with the prior results from Shen et al. [9,18] and Xu et al. [21], that the interaction between multiple CMEs has a greater chance of generating severe geomagnetic storms.

In this work, we studied the origin of extremely intense B _s based on the typical events analysis and statistical analysis. In the analysis of the typical events, four events with extremely high B _s were studied in detail. Extremely intense B _s have been discovered to come from a variety of ICME types, including I-ICMEs, M-ICMEs, S-ICMEs with shocks inside ICMEs, and S-ICMEs with shocks passed through ICMEs. Furthermore, we studied all the ICME events with B _s,mean ≥ 10nT from 1995 to 2020, and found that the interaction events (i.e., M-ICMEs, and S-ICMEs) can result in extremely high B _s with a higher probability. The possible reason is the compression. For M-ICMEs, the compression between two ICMEs can enhance the magnetic field intensity. While, for S-ICMEs, the shock compression the magnetic field inside the previous ICME might be the main mechanism. When we compare the two interaction ICME types, we find that approximately 35.7% of S-ICMEs events had B _s,mean ≥ 10nT, which is significantly higher than that of M-ICMEs (12.1%). It makes sense because the shock can intensify the pre-condition B _s by a factor of 3–6 [35]. The intensity of B _s embedded in the preceding ICME can be amplified when the shock driven by the subsequent ICME propagated into the preceding one. Because the intensity of B _s is a key parameter for intense geomagnetic storms, these findings confirm the previous results that ICME interaction events are more likely to cause intense geomagnetic storms [9,17].

It should be noted that, we reported a special type of ICMEs with extremely intense B _s : S-ICMEs with shocks passing through ICMEs. This type of S-ICME observed at 1AU appears to be an I-ICME. As a result, it is difficult to distinguish this type in the statistical analysis, only based on the interplanetary observations near the Earth. It is a plausible assumption that some I-ICMEs detected by Wind spacecraft may not be the real isolated ICME events. In addition, Lugaz et al. [17] reported another possibility that some multiple-ICMEs interaction events could also exhibit I-ICME signature [19,36]. These results suggest an overestimate of the possibility of I-ICMEs in carrying extremely intense B _s . The direct and multi-point observations of CME’s propagation in interplanetary space from Solar Ring Mission [37] will be valuable in advancing our understanding of such phenomena.