Coronal jets appear to be small-scale versions of larger eruptions, with the eruptive process that results in a minifilament eruption that produces a coronal-jet spire and a JBP corresponding to large-scale eruptions that result in filament eruptions and typical solar flares (Sterling et al., 2015). That is, just as “typical” solar filaments erupt (in what we are here calling “large-scale eruptions”) to make long-observed “typical” solar flares and that sometimes expel coronal mass ejections into the heliosphere, coronal-jets appear to be made by a minifilament eruption (a scaled-down version of a large-scale filament eruption) that leaves in its wake a JBP (a scaled-down version of a typical solar flare), and to result in material and magnetic disturbances that flow out along a spire and that sometimes flow into the heliosphere.

If coronal jets are indeed a scaled-down version of larger “standard flare model” solar eruptions, then we would expect other aspects of the smaller-scale eruptions that cause coronal jets and JBPs to have counterparts in the larger-scale eruptions that cause CMEs. Here we discuss examining large-scale eruptions based on what has been found in coronal jets.

A characteristic of coronal jets is the anemone magnetic setup, similar to that shown in Figure 2A. There are many examples of flares occurring from anemone active regions (Asai et al., 2009; Lugaz et al., 2011; Kumar and Manoharan, 2013; Devi et al., 2020). Joshi et al. (2017) showed that the setup for a large-scale eruption matched that of the coronal-jet minifilament-eruption picture, and that the dynamic motions of the eruption matched closely that of an erupting minifilament producing a coronal jet. A similar schematic was in fact drawn to explain a series of recurring solar eruptions much earlier (Sterling and Moore, 2001a, Sterling and Moore, 2001b; Sterling et al. 2001; these schematics in fact helped inspire the Figure 2 schematic of Sterling et al., 2015). These setups show that the same type of magnetic setup appears to be capable of generating similar solar expulsions both on the coronal-jet size scale, and on the size scale of typical solar eruptions. Whether a coronal jet results or a CME results depends on how much of the erupting minifilament/flux-rope lobe remains after the external reconnection in Figures 2B, C. If the flux rope is robust enough to survive that reconnection (that is, if only the outer portion of the flux-rope lobe is eroded away by external reconnection), then the remaining lobe and flux rope can escape to form a CME that carries a magnetic flux rope in its core region. If on the other hand the external reconnection totally reconnects the flux rope, so that the field lines that were previously closed in a flux rope all become open, then the feature becomes a coronal jet instead of a CME.

An anemone setup appears to be necessary for coronal-jet formation, and formation of coronal jets in such a setup is supported by numerical simulations (Wyper et al., 2017, 2018a; b; Wyper et al., 2019; Doyle et al., 2019). But large-scale eruptions also occur outside of an anemone setup, and so we might ask whether the eruptions of minifilaments that cause coronal jets might also have similarities to larger-scale eruptions, independent of whether those larger-scale eruptions occur in an anemone configuration. One possible such similarity is in the manner in which the minifilament eruptions and the large-scale eruptions are triggered to erupt. We have seen above that coronal-jet-producing erupting minifilaments are apparently often built-up and triggered to erupt by magnetic-flux cancelation. What about larger-scale eruptions?

To investigate whether large-scale eruptions are built up and triggered to erupt in a manner similar to coronal-jet-producing minifilament eruptions, Sterling et al. (2018) studied how large-scale eruptions evolve toward eruption. In the case of coronal jets, the magnetic elements taking part in the cancelation typically converge toward each other over the hours prior to the eruption onset, as discussed in several papers (Panesar et al., 2016b, 2017; 2018a; Sterling et al., 2017), and as exemplified by Figure 3C. This time period is short enough for those elements to have relatively little interaction with surrounding flux elements. In contrast, large-scale eruptions often occur in active regions that develop for many days, or even weeks, prior to expelling an eruption (complex regions, such as delta regions, can evolve faster than this, but the objective of Sterling et al., 2018 was to compare with more standard eruptions). Thus, in order to follow the region from the time of flux emergence through to the time of the eruption, it was necessary to look at regions that were small enough for this evolution to occur during a single disk passage of the region. Sterling et al. (2018) presented two examples of this class. In both cases the active regions were comparatively small bipolar active regions (total flux in each ≲ 10²¹ Mx). Also in both cases the eruptions occurred about five days after emergence, and those eruptions produced CMEs observed in coronagraphs. One of the regions remained almost completely isolated from any surrounding substantial flux over this period. And the second (shown in Figure 4) was largely isolated from surrounding flux, although one of its polarities did have some interaction with nearby pre-exisiting opposite-polarity flux.

Both regions displayed similar evolution. Figure 4 shows the evolution of one of these regions. The boxed region in (a) shows a bipolar active region that is still emerging in this frame. In panel (b) the emergence is continuing, with centroids of the main positive-polarity (white) and negative-polarity (black) patches further separated from their central neutral line than they were in (a). By the time of (c) however, they are no longer separating, and some of the opposite-polarity portions of the region have converged toward the central neutral line. Panel (d) shows a time-distance map of this region, analogous to that in Figure 3C. This shows that the polarities initially separate for about one day following their initial emergence. Their mutual directions then reverse, and the polarities start to converge. The orange line shows on this plot the time of the CME-producing eruption; this did not occur until after the polarities had converged on each other, and were undergoing flux cancelation along their central neutral line. There were no CME-producing eruptions from this region prior to this time. SDO/AIA observations show that the bright centroid of the resulting GOES C-class flare was on that neutral line. Thus, similar to the situation with coronal jets, a flux rope eruption occurs along a cancelation neutral line. In this case, the region evolved for about four days with essentially no activity, and then had an eruption only after that cancelation started taking place.

A second region examined in Sterling et al. (2018), which also was substantially isolated magnetically from surrounding structures, began with flux emergence, underwent flux-polarity separation, and then had flux convergence and apparent cancelation along its central neutral line among some portion of its two opposite-polarity patches. This resulted in a eruption that produced a GOES B-class flare on the region’s central neutral line (although in this case a second, weaker, eruption also occurred on a neutral line formed from one of the emerging polarities and a pre-existing opposite polarity patch), and in the expulsion of a CME.

Chintzoglou et al. (2019), studying more complex magnetic situations involving multiple active regions, also found that eruptions occurred at flux-cancelation neutral lines.

Returning to the discussion of the size scale of the erupting minifilaments that can cause coronal jets, Moore et al. (2022) examined the evolution of 10 bipolar ephemeral active regions (BEARs) in a manner similar to Sterling et al. (2018), but where they tracked their regions from emergence to disappearance. These 10 regions produced 43 small-scale eruptions in total, and all of these eruptions occurred at a neutral line in which apparent flux cancelation was taking place. This again supports that the physics that causes eruptions on the coronal-jet size scale is essentially the same as that which causes large-scale eruptions that produce typical solar flares and CMEs.

These observations strongly support that flux cancelation is often essential in the magnetic build-up and triggering of both smaller-scale eruptions that cause coronal jets, and larger-scale eruptions that cause flares and CMEs. This is fully consistent with the mechanism for the build-up of the non-potential energy required for eruption via flux cancelation, as suggested by van Ballegooijen and Martens (1989). Observations of these processes occurring on faster time scales in the smaller-size-scale jets, however, helps to elucidate strategies for investigating the processes in the more-slowly developing larger active regions.

From the preceding discussions, we have presented evidence that the same basic process leading to eruptions occurs on two size scales–that of large-scale eruptions and that of coronal jets. Sterling and Moore (2016) considered a possible extended relationship to smaller size scales, by plotting the size of a typical filament or filament-like structure that erupts on one axis, and, on the other axis, a measure of the number of the respective eruptive events occurring at any given time on the Sun. Their motivation was to see whether a substantial number of similar features might occur on a spicule size scale, assuming that the coronal-jet mechanism continues to scale downward to sufficiently small sizes. If so, then it might be that at least some spicules (and perhaps many or most spicules) result from erupting filament-like features (erupting flux ropes) of that size scale. (See §3.4 for a summary of spicule properties).

The larger of the two size scales of eruptive events that have observed filament eruptions are the “typical” filament eruptions that have solar flares occurring beneath them, and–in the case of ejective eruptions–expulsion of a CME. Sterling and Moore (2016) took a typical eruptive filament size to be 70,000 km, with an appropriate scatter based on observed values for a large number of filaments by Bernasconi et al. (2005). For the number of large-scale eruptions, they took observed CME rates of from less than one to a few per day (Yashiro et al., 2004; Chen, 2011). For coronal jets, they estimated corresponding numbers for the size of erupting minifilaments based on measurements in Sterling et al. (2015), which on average was just over 5,000 km for the erupting-minifilament lengths. For the frequency of eruptions they extrapolated rates given in Savcheva et al. (2007), which yields about a few hundred per day over the entire Sun.

In order to compare with spicule occurrence rates, Sterling and Moore (2016) converted the occurrence rates for the large-scale eruptions and coronal-jet-size-scale minifilament eruptions to the expected number of events occurring on the Sun at any random given time. The motivation for these units was to utilize the historical studies of spicule counts, which were sometimes expressed as the total number of spicules on the Sun seen at a given time. Those resulting values vary substantially (Athay, 1959; Lynch et al., 1973), but overall they are roughly in the neighborhood of 10⁶ spicules on the Sun at a given time (Judge and Carlsson, 2010 estimate about a factor of ten higher; see Sterling et al., 2020a).

In order to complete the comparison with the larger erupting features, a value for the typical size of the erupting filament-like flux rope that would produce a spicule is required. No such cool-material eruptions have been convincingly observed to date, and therefore spicules being formed by the coronal-jet-producing minifilament-eruption-type mechanism is wholly speculative at this point. But because coronal-jet-spire widths are similar to the measured lengths of the erupting minifilaments that produce them, by analogy, Sterling and Moore (2016) hypothesized that there might be erupting microfilaments (erupting micro flux ropes) of lengths comparable to the width of spicules (a few hundred km), that produces some spicules.

Figure 5 shows the resulting plot. When a linear extension is made from the large-scale eruptions through the coronal-jet-sized eruptions, and then extended down to the size scale of the putative erupting microfilaments, the ordinate’s value for their occurrence rate falls near the lower end of the estimated spicule-number counts. This implies that at least some portion of spicules might be scaled-down versions of coronal jets, formed by eruptions of microfilaments, or it could be that multiple spicules are produced by a single such eruption.

We next consider from an observational standpoint the suggestion that the coronal-jet-producing mechanism operates on size scales smaller than that of coronal jets.

Raouafi and Stenborg (2014) studied long and narrow transient features of a scale smaller than coronal jets, using SDO/AIA images and HMI magentograms. They called these features jetlets, due to their similarity to coronal jets, except having a smaller size (both width and length). Jetlets are smaller than coronal jets, but larger than chromospheric spicules. Raouafi and Stenborg (2014) found the jetlets to occur at the base of coronal plumes, and they suggest that they (along with transient base brightenings) are the result of “quasi-random cancellations of fragmented and diffuse minority magnetic polarity.” Therefore, these features seem to be smaller versions of coronal jets. (“Plumes” are long and narrow features–first noticed during total eclipses–extending out to several solar radii in polar regions. Compared to jet-like features, plumes are long-lasting, persisting for ∼day. See, e.g., Poletto, 2015).

Panesar et al. (2018b) examined jetlets with IRIS UV and AIA EUV images, and HMI magnetograms. They studied ten jetlets, and found them to have lengths ∼27,000 km, spire widths ∼3,000 km, base size of ∼4,000 km and speed ∼70 km s⁻¹. They argued that jetlets are a more general solar feature than presented in Raouafi and Stenborg (2014), occurring at the edges of chromospheric network both inside and outside of plumes. In agreement with Raouafi and Stenborg (2014), they found that magnetic flux cancelation was the likely cause of the jetlets, just as it often leads to coronal jets. Furthermore, their jetlets were accompanied by brightenings analogous to the JBP seen at the base of coronal jets. Based on these and other characteristics, Panesar et al. (2018b) concluded that the jetlets are likely scaled-down version of coronal jets, and that they are consistent with the erupting-minifilament scenario for their production. They did not, however, observe a clear indication of the existence of an actual cool-material erupting minifilament at the base of their jetlets.

Panesar et al. (2019) extended these studies to even smaller-sized jetlets, using EUV 172 Å (Fe ix/Fe x) images from the Hi-C 2.1 rocket flight. This instrument had pixels of width 0″. 129, and cadence of 4.4 s; see Rachmeler et al. (2019) for details. Six events were identified from the data from Hi-C 2.1’s 5-min flight. As with the Panesar et al. (2018b) jetlet study, these events also occurred at the edges of network cells. On average they had spire lengths of ∼9,000 km, widths of 600 km, and speeds of ∼60 km s⁻¹. At least four of these events seemed consistent with being small-scale coronal jets following the erupting-minifilament mechanism, although once again there were no direct observations of erupting cool-material minifilaments.

Spicules are chromospheric features that are extremely common, have lengths ∼5,000–10,000 km, widths of a few hundred km, lifetimes of a few minutes, and cluster around the magnetic network (e.g., Beckers, 1968, 1972). Although many ideas exist, their generation mechanism is still unknown; see discussions in various review works (e.g., Beckers, 1968, 1972; Sterling, 2000; Tsiropoula et al., 2012; Hinode Review Team et al., 2019; Sterling, 2021).

As mentioned in §3.2, erupting microfilament flux ropes have not been observed in spicules. They may, however, be present, but hard to observe for a variety of reasons. Similarly, a bright point that corresponds to a JBP has not been convincingly observed at the base of spicules. These details are not consistent with an erupting-microfilament mechanism for spicules. Nonetheless, these absences are not definitive evidence that these features do not exist in spicules; they may exist but be hard to detect, as discussed in Sterling et al. (2020a).

Moreover, there are several observations that are consistent with a microfilament-eruption mechanism for spicules. One of these is the observation of mixed polarity elements at the base of many spicules. Fresh evidence for this is presented in Samanta et al. (2019), obtained using state-of-the-art ground-based observations in the pre-DIKIST era. This work presents evidence that spicules result from dynamic activity at the base of spicules, which could be due to flux cancelation and/or emergence. As discussed above, there is extensive evidence that many coronal jets result from flux-cancelation episodes. Ideas for coronal-jet production from flux emergence has also been presented (Yokoyama and Shibata, 1995).

Spicules also display characteristics of spinning motions (Pasachoff et al., 1968; De Pontieu et al., 2012; Sterling et al., 2020b). We have noted that the minifilament-eruption idea offers an explanation for the spinning of coronal jets, when an erupting twisted minifilament transfers its twist to the coronal-jet-spire’s coronal field via external reconnection. Thus the same mechanism acting on speculative erupting microfilaments might explain this spicule spinning as magnetic untwisting.

There remains, however, the possibility that spicules are driven by any of a number of other suggested mechanisms (see the above-cited reviews), and many of these ideas cannot yet be ruled out. Spicule-sized features that work via the coronal-jet-production mechanism may instead drive other features of that size, such as the UV network jets described by Tian et al. (2014) (some of which are UV observable in EUV, at least in AIA 171 Å images; Tian et al., 2014), or the “chromospheric anemone jets” observed in active regions, or similar jet-like features observed in plages (De Pontieu et al., 2004; Sterling et al., 2020a).

As pointed out in §2, coronal jets in polar coronal holes such that the spires are generally seen in the SDO/AIA channels of 304, 171, 193, and/or 211 Å. Among these, 171, 193, and 211 are all coronal lines. It would be best to include all of these channels, as sometimes the spire tends to be better seen in one than the other. Nonetheless, if it is visible in one of these three channels, it is usually at least detectable in the other two, based on observations of 41 coronal jets in Sterling et al. (2015) and Sterling and Moore (2023). Therefore, as a minimum, one of these three channels should be included in a minimal mission. The 304 Å channel shows a mixture of what might be called “upper”-chromospheric and transition region plasmas. It sometimes shows features of coronal jets detected in SXRs that are not apparent in the other three cool-coronal (171, 193, 211 Å) channels (Sterling and Moore, 2023), and therefore that channel would be essential to include in a SEIM mission.

Brighter coronal jets, such as those occurring at the periphery of active regions, are often visible in all AIA EUV channels. Therefore, an instrument designed to see coronal-jet-like features in coronal holes (and quiet Sun) well would also be able to see coronal jets and similar features in active regions.

A channel that shows photospheric emissions, such as AIA’s 1600 Å channel, would be essential to facilitate comparisons with other instruments such as DKIST. This channel also shows ribbon-like flare emission at the base of some active-region jets (Sterling et al., 2016).

Given these considerations, a minimal wavelength-coverage package for a SEIM instrument could be 304, 171 or 193, 94, and 1600 Å.

In active regions, some strands of erupting minifilaments are substantially thinner than the width of erupting minifilaments in coronal holes or quiet regions, having widths of ≲ 2 ^″ (Sterling et al., 2016). Hi-C, either in its original incarnation (Kobayashi et al., 2014) or the Hi-C2.1 version discussed above (§3.3), would be able to resolve many of these, and in general sees features at the limit of or beyond what is readily detectable in AIA (e.g., Brooks et al., 2013; Tiwari et al., 2016; Panesar et al., 2019; Tiwari et al., 2019; Sterling et al., 2020a). Based on this, we are confident that a resolution comparable to that of Hi-C (0″. 1 pixels) will be adequate for revolutionary breakthroughs in the study of coronal jets and similar-sized phenomena.

For coronal-jet studies, the 12-s cadence of AIA has been adequate. Smaller-scale jet-like features can have lifetimes shorter than the ∼tens of minutes of coronal jets, including ∼one minute for small jetlets (Panesar et al., 2018b) and UV network jets. Therefore a faster-than-AIA time cadence comparable to that of Hi-C, about 5 s, would be preferred in order to sample these objects well.

A FOV of about 6′ × 6′, would be acceptable for an initial mission. This would be slightly larger than the 4′.4 × 4′.4 Hi-C2.1 FOV (Rachmeler et al., 2019). SDO/AIA’s detector is a circle of diameter 41′ (Lemen et al., 2012), and so our proposed detector would have a FOV of about one-sixth that of AIA’s. Thus we could obtain our goal of 0″. 1 with a FOV one-sixth that of AIA’s, by using an AIA-sized detector (4096 × 4096 pixels²). Advances in technology might make it feasible to improve upon this, allowing for increased FOV and/or higher resolution, but these minimal criteria would allow for substantial advancements in our understanding of coronal jets and jet-like features. Figure 6 shows a sample image from XRT with a FOV similar to that being discussed (that image’s FOV is only slightly larger, at 6′.67 × 6′.67), with much of the northern polar coronal hole and several X-ray coronal jets visible.

Ideally, a mission carrying a SEIM instrument would have extended, uninterrupted views of the Sun. Accordingly, a Sun-synchronous orbit, such as that of Hinode (Kosugi et al., 2007) or IRIS (De Pontieu et al., 2014) would be appropriate, allowing for ∼nine mouths of uninterrupted viewing, and ∼three months with orbits that include spacecraft nights while still allowing ∼one hour of solar observing per orbit. Longer periods of uninterrupted viewing would be possible from L1 or a similar location, but that might be more appropriate for a more extensive followup mission.

An imaging-only mission plan would be of limited value for advancing the science of jet-like features. As a minimum, systematic corresponding line-of-sight magnetograms would be essential to complement these observations. This could be included on the same spacecraft, in which case a magnetogram FOV comparable to that of the EUV instrument would be acceptable. Alternatively, it would be possible to use synoptic full-disk magnetograms from elsewhere if appropriate ones are available; for example, it would be fully acceptable to rely on magnetograms from SDO/HMI or a similar instrument on a different satellite that is operational at the time of a SEIM mission. In either case, the time cadence should be about ∼1 min, comparable to that of SDO/HMI (45 s). Spatial resolution of HMI’s level would be the minimum desired, but would be adequate for an initial mission. For special programs, coordinated observations with DKIST or other ground-based instruments (such as BBSO; e.g., Samanta et al., 2019) would allow for much higher-resolution magnetograms. It will also be extremely valuable to have spectroscopic observations at UV and/or EUV, or even SXR, wavelengths, to obtain diagnostic information on the observed objects. These spectra should have sufficient spatial and spectral resolution and high-enough cadence to address questions such as whether jetlet, UV network jet, and even spicule-sized objects routinely display characteristics of jets, such as spinning motion of their spires.

We know that coronal jets are common in polar coronal holes, and they are observed in on-disk coronal holes also. Therefore, a basic minimal-maintenance plan would be to observe (with tracking) an on-disk coronal hole when one is available. This would allow for coordination with line-of-sight magentograms. A second low-latitude target would be active regions. The frequency of typical coronal jets from active regions is not yet known, but they are not uncommon. Even in the absence of such coronal jets, there are smaller-scale penumbral jets that are ubiquitous in active regions, and it would be desirable to have high-resolution, high-cadence observations of other active-region activity, and of course it would be highly desirable for the instrument to observe large-scale eruptions from active regions. In the absence of on-disk coronal holes and active regions (or if there are only active regions showing essentially no substantial activity), one of the two polar regions (preferably one with a prominent coronal hole) would be the standard default target.

The instrument proposed here could act as a proving ground for a more elaborate mission that features a full-disk FOV and wider wavelength coverage. This would be analogous to how the TRACE mission (Handy et al., 1999) preceded AIA on SDO. It would be fully appropriate for such a more-extensive mission to operate from L1 or similar location, with uninterrupted solar viewing. Such an instrument would ideally be accompanied by a complementary magnetograph, and perhaps other instruments, on the same spacecraft.

The Appendix (in Supplementary Material) provides a summary of properties of coronal jets and jet-like features that will be either observed directly with SEIM, or to which SEIM will provide valuable supplementary observations, for refining our understanding of all of these features.

Even in the simplest form however, the SEIM instrument suggested above would be far more than just a “solar-jets telescope.” As we have argued above, solar jets can be viewed as a proxy for one of the many types of possibly similar solar features, on both larger and smaller size scales, that could be observed and studied in detail with such as instrument. Therefore, SEIM would provide a new, high-resolution, high-time-cadence window into an understanding of fundamental explosive phenomena that occur on multiple size scales in the lower solar atmosphere, and that possibly power the heliosphere as well.