The Structure of Coronal Mass Ejections Recorded by the K-Coronagraph at Mauna Loa Solar Observatory

On 1971 September 29, the first orbiting white-light coronagraph (Koomen et al. 1975) was launched on Orbiting Solar Observatory Number 7 (Follett et al. 1974), and the coronagraph recorded the first images of a coronal mass ejection (CME) on 1971 December 14 (Tousey 1973). From that time onwards, CMEs have been a subject of intense investigation in solar physics (Chen 2011; Webb & Howard 2012). Theoretically, CMEs originate from eruption of magnetic flux ropes (MFRs), which can form prior to (Low 2001; Patsourakos et al. 2013; Song et al. 2015a; Kliem et al. 2021) and during (Song et al. 2014; Ouyang et al. 2015; Wang et al. 2017; Jiang et al. 2021) solar eruptions.

In general, hot channels and coronal cavities are regarded as the proxies of MFRs in active regions (Zhang et al. 2012; Song et al. 2015a) and quiet-Sun regions (Wang & Stenborg 2010; Chen et al. 2018), respectively. The MFR can provide support to the prominence against gravity (e.g., Yan et al. 2016). Therefore, CMEs are usually associated with eruptions of hot channels (high-temperature ejecta; Zhang et al. 2012; Cheng et al. 2013; Song et al. 2015b), coronal cavities (middle-temperature ejecta; Gibson et al. 2006; Forland et al. 2013; Song et al. 2022), and/or prominences (low-temperature ejecta; Gopalswamy et al. 2003; Li et al. 2012; Song et al. 2018; Zhou et al. 2023a, 2023b) observationally.

The white-light coronagraph on the Solar Maximum Mission satellite recorded a CME with three-part structure on 1980 August 5, i.e., a bright core within a dark cavity surrounded by a bright loop front (Illing & Hundhausen 1985) that included the low coronal observations of the CME from Mauna Loa MK3 coronameter. Since then, the three-part structure has become the prototypical structure of CMEs (e.g., Howard 2006) though only ∼30% of CMEs exhibit the three-part feature in the high corona, e.g., 2–6 R_⊙. In several decades, it has become widely accepted that the bright front originates from the plasma pileup along the MFR boundary, the cavity represents the MFR, and the bright core corresponds to the prominence (e.g., Vourlidas et al. 2013).

However, recent studies pointed out that the traditional opinion is questionable, because some three-part CMEs are not associated with prominence eruptions at all (Howard et al. 2017; Song et al. 2017, 2019b; Wang et al. 2022; Song et al. 2023). Based on dual-viewpoint and seamless observations from the inner to outer corona, a new explanation on the three-part nature has been proposed, in which the bright frontal loop is formed due to the compression as the magnetic loops are successively pushed to stretch up by the underlying MFR (Low 2001; Chen 2009), the core can correspond to the MFR and/or prominence, and the dark cavity between the CME front and the MFR is a low-density zone with a sheared magnetic field (Song et al. 2017, 2019a). Recent observations clearly demonstrated that both hot channels (Song et al. 2023) and coronal cavities (Song et al. 2022) evolved into the bright core of three-part CMEs.

The new explanation also points out that CMEs can lose the three-part feature gradually when propagating outwards, because the dark cavity vanishes due to the MFR expansion and growth through magnetic reconnections, and/or because the bright core fades away due to the prominence expansion and drainage (Song et al. 2023). This answers why only a portion of CMEs have the prototypical structure in previous survey studies (e.g., Vourlidas et al. 2013). As mentioned, CMEs result from MFR eruption; thus the new explanation predicts that all normal CMEs possess the three-part structure in the low corona, i.e., in the early eruption stage. Here the normal CMEs do not include the narrow ones with angular width less than 30° for two factors: (1) the narrow events might be jets, instead of CMEs with small angular width; (2) to observe the structure of narrow CMEs, coronagraphs with higher spatial resolution are necessary.

To examine whether all normal CMEs have the three-part structure in the early eruption stage, we conduct a survey study based on observations of the coronal solar magnetism observatory (COSMO) K-coronagraph (K-COR) from 2013 September to 2022 November. The paper is organized as follows. Section 2 introduces the related instruments and methods. The observations and results are displayed in Section 3, which is followed by a summary and discussion in the final section.

The Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al. 2012) takes images of the Sun through seven EUV channels. The AIA has a field of view (FOV) of 1.3 R_⊙, a spatial resolution of 12 and a cadence of 12 s. Here we use the 131 Å (Fe xxi, ∼10 MK) and 304 Å (He ii, ∼0.05 MK) to display the hot channel and prominence, respectively.

The K-COR is one of three proposed instruments in the COSMO facility suite (Tomczyk et al. 2016) located at the Mauna Loa Solar Observatory (MLSO), and records the coronal polarization brightness (pB) in the passband of 7200–7500 Å, which is formed by Thomson scattering of photospheric light from free electrons (Hayes et al. 2001). The FOV of K-COR is 1.05–3 R_⊙ with a pixel size of 55 and a nominal cadence of 15 s. The Large Angle and Spectrometric Coronagraph (LASCO; Brueckner et al. 1995) on board the Solar and Heliospheric Observatory (SOHO; Domingo et al. 1995) comprises of three telescopes (C1, C2, and C3), each of which has an increasingly large FOV. Here the C2 (FOV: 2–6 R_⊙) is adopted to observe the CME structure in the outer corona.

The images of space-borne LASCO/C2 have better contrast than those of the ground-based K-COR. The normalized radially graded filter (NRGF) is employed to increase the K-COR contrast, which flattens the steep brightness gradient of the corona (Morgan et al. 2006). In this paper, we examine the CME structure through the NRGF data of K-COR that are available online, ⁶ while for the C2 observations the original data are used.

The K-COR data are available since 2013 September 30, while the coronagraph is closed temporarily due to the volcanic eruption of Mauna Loa on 2022 November 27. Therefore, our survey covers an interval from 2013 to 2022, during which 369 CMEs (excluding the possible events) are identified. On the whole, more CMEs are recorded around solar maximum though the K-COR does not work 24 hr continuously. The basic information for each event, such as the date, time (universal Time, UT), and location (E—east, S—south, W—west, and N—north), is listed on the MLSO website. After inspecting the 369 CMEs combining observations of the AIA, K-COR, and C2, we find 71 events according to the criteria: (1) limb event, which requires that the source region centered within 30° of the solar limb for the front-side events. For the far-side events, a limb event requires that the suspended prominence (or hot channel) prior to the eruption or the coronal disturbance (or post eruption arcade) during the eruption can be observed with the AIA; (2) normal CME, i.e., angular width ≥30° in the C2 FOV; (3) K-COR caught the early eruption stage.

We first scrutinize the 71 CMEs one by one through the NRGF images of K-COR, and find that all of them have the three-part feature in the low corona, irrespective of their appearance in the C2 images, agreeing with the prediction of the new explanation on the three-part structure of CMEs. However, visual identification of the three-part feature is not entirely objective as no quantitative criteria. There exist six events that do not have the clear three-part feature in the K-COR images (see Table 1), and several or all of them might be identified as the non-three-part CMEs. Therefore, we suggest that 90%–100% of the 71 CMEs possess the three-part structure in the low corona.

Table 1 lists the information of the 71 CMEs. The first column is the sequential number. Columns (2)–(4) give the date, time, and location of each event, which are from the MLSO website. The asterisks in column (2) denotes the six ambiguous events in the K-COR images mentioned above. Combining the observations of K-COR and AIA, we identify the type of source region, i.e., active region (AR) or quiet-Sun region (QS), and the ejecta, i.e., hot channel (HC) or prominence (P) for each event. The source type and ejecta are listed in columns (5) and (6), respectively, where the "?" denotes that the source-region type or the ejecta are unsure, mainly because the events are located on the far side of the Sun. The subsequent four columns present CME information observed with LASCO, which are provided by the coordinated data analysis workshops (CDAW ⁷ ). Column (7) is the time of the CME's first appearance in the C2 FOV, and columns (8)–(10) are the central position angle (PA), linear velocity (LV), as well as angular width (AW) correspondingly. The last column tells whether the CME exhibits the three-part feature in the C2 FOV, with Y/N denoting yes/no. Note that the asterisks in the last column indicate the ambiguous events in the C2 images.

Table 1 shows that 49 and 22 CMEs originate from active regions (AR) and quiet-Sun regions (QS), respectively. For the ejecta type, there are 49 events associated with a prominence (P) eruption, and the rest are correlated with a hot-channel (HC) eruption. In the LASCO FOV, the linear velocities range from 95 to 1730 km s⁻¹, and their angular widths, from 30° to 360°. After scrutinizing the 71 CMEs one by one through the C2 images, we find that only 21 events (∼30%) possess the three-part structure, agreeing with previous statistical results based on C2 observations (e.g., Vourlidas et al. 2013). For the 50 non-three-part CMEs in the C2 images, seven are ambiguous and might be identified as the three-part events, which are denoted with asterisks in the last column as mentioned. Therefore, we suggest that 30%–40% of the 71 CMEs possess the three-part structure in the high corona. To demonstrate that a hot channel or prominence can lead to a three-part CME in its early eruption stage, and the three-part feature can sustain or disappear in the outer corona, we select four representative events and display them in Figures 1–4 sequentially.

Figure 1 displays the event occurring on 2014 October 14 (Event 10 in Table 1), which resulted from a hot-channel eruption in an active region located at the SE limb. Panel (a) shows the hot channel with the AIA 131 Å observation at 18:45:32 UT. Panel (b) presents the K-COR observation at 18:58:51 UT, and the three-part CME can be identified. The bright core is very obvious, and the bright front is delineated with the red-dashed line as we cannot discern the front clearly in the static image. The bright front and three-part structure can be distinguished clearly through the animation accompanying with Figure 1. The C2 image at 20:00:05 UT is presented in panel (c), and we can see that the CME keeps the typical three-part feature there. Note that the white circles in both panels (b) and (c) denote the solar limb.

Figure 2 presents the CME occurring on 2016 January 1 (Event 25 in Table 1). This event is associated with a prominence eruption that can be observed in the AIA 304 Å image as shown in panel (a). Panel (b) displays the K-COR observation at 23:23:42 UT, in which the bright front and core are delineated with the red- and yellow-dashed lines, respectively, to display the three-part structure clearly. Please see the accompanied animation to examine the three-part feature continuously. This CME also exhibits the three-part feature in the C2 FOV as shown in panel (c), where the red-dashed line denotes the leading front.

The above two CMEs exhibit the three-part structure in both K-COR and C2 images. Next we will show two events that do not have the three-part feature in the C2 FOV. Figure 3 displays a CME induced by the hot-channel eruption as revealed with the AIA 131 Å observation in panel (a). This event occurred on 2021 May 7 (Event 45 in Table 1) and a previous study (Wang et al. 2022) has demonstrated that the hot channel evolved into the bright core in the K-COR image as shown in panel (b). The three-part feature is distinguishable directly in the static image; thus no dashed lines are a guide for the eye in this panel. Panel (c) shows the observation of C2 at 19:48:05 UT with the red-dashed line delineating the CME front. The CME lost its three-part feature due to the MFR expansion and growth as mentioned (Song et al. 2019a, 2023).

Figure 4 shows the CME occurring on 2021 October 10 (Event 53 in Table 1), which is associated with the prominence eruption from an active region located at the NW limb. Panel (a) illustrates the erupting prominence with the AIA 304 Å image at 22:51:29 UT. The K-COR observations present a typical three-part CME as shown in panel (b), where the red-dashed line depicts the CME front. Please see the accompanied animation to view the three-part feature in the K-COR images. The animation also demonstrates that the prominence did not erupt outward eventually. This leads to a non-three-part CME in the C2 image (Song et al. 2023) as presented in panel (c), where the red-dashed line delineates the CME front.

To verify the new explanation on the three-part structure of CMEs, which predicts that all normal CMEs should exhibit the three-part feature in their early eruption stage, we conducted a survey study on the CME structure with the observations of K-COR (FOV: 1.05–3 R_⊙) at MLSO. In total, 369 CMEs (excluding the possible events) are identified from 2013 September to 2022 November. Combining the observations of AIA, K-COR and LASCO/C2, we inspected the events one by one manually, and found 71 events according to the criteria: (1) limb event; (2) normal CME with angular width ≥30° in the C2 FOV (2–6 R_⊙); (3) K-COR caught the early eruption stage. The results showed that 90%–100% of the 71 events exhibit the three-part structure in the K-COR observations, basically agreeing with the prediction of the new explanation, and 30%–40% of the events have the three-part appearance in the C2 observations, consistent with previous survey studies (e.g., Vourlidas et al. 2013; Song et al. 2023). These suggest that CMEs can lose the three-part feature during their propagation outwards, and further support the new explanation on the nature of the three-part structure of CMEs (Song et al. 2022).

As mentioned, theoretical studies demonstrate that CMEs result from MFR eruption, and no physical mechanism can produce large-scale CMEs without involving an MFR. However, do all CMEs have an MFR structure near the Sun observationally? Our current survey study intends to answer "Yes" to this question, as 90%–100% of normal CMEs exhibit the three-part structure in their early eruption stage. We think that the several ambiguous events could exhibit the three-part feature if the observations were clearer. For the narrow CMEs (angular width <30°), we speculate that they can also exhibit the three-part feature when the spatial resolution of coronagraphs is high enough.

The CME and MFR are called ICME (interplanetary CME) and magnetic cloud (Burlaga et al. 1981), respectively, after they leave the corona. If the MFR structures are not destroyed during their propagation, all ICMEs should possess the magnetic cloud features near 1 au, such as enhanced magnetic-field intensity, large and smooth rotation of the magnetic-field direction, and low proton temperature or low plasma β (Zurbuchen & Richardson 2006; Wu & Lepping 2011; Song & Yao 2020). However, only about one-third of ICMEs have the magnetic cloud features near the Earth (Chi et al. 2016). From the statistical point of view, this might be a result of glancing cuts between the spacecraft and ICME, as ICMEs with magnetic cloud features have narrower sheath region compared to the noncloud ICMEs (Song et al. 2020). The analyses on the morphological structure of CMEs near the Sun and the geometric character of ICMEs near 1 au support that most (if not all) CMEs have the MFR structures.

We thank the anonymous referee for the comments and suggestions that helped to improve the original manuscript. We are grateful to Profs. Jie Zhang (GMU), Pengfei Chen (NJU), Yuandeng Shen (YNAO), and Mr. Zihao Yang (PKU) for their helpful discussions. This work is supported by the National Key R&D Program of China 2022YFF0503003 (2022YFF0503000), the NSFC grants U2031109, 11790303 (11790300), and 12073042. H.Q.S is also supported by the CAS grants XDA-17040507. The authors acknowledge the use of data from the SDO, MLSO, and SOHO, as well as the usage of the CDAW CME catalog generated by NASA and The Catholic University of America and the MLSO activity tables created by the MLSO team.