Hi-C 2.1 Observations of Jetlet-like Events at Edges of Solar Magnetic Network Lanes

Solar jet-like features are narrow upward streams of plasma ubiquitously observed in the solar atmosphere (Shibata et al. 1992; Sterling 2000; Innes et al. 2011; Raouafi et al. 2016). Coronal jets appear in coronal holes, active regions, and quiet regions (Shibata et al. 1992; Nisticò et al. 2010; Pucci et al. 2013; Panesar et al. 2016a; Sterling et al. 2016) and are often observed in extreme ultraviolet (EUV) and X-ray images (Shimojo et al. 1996; Alexander & Fletcher 1999; Cirtain et al. 2007; Savcheva et al. 2007; Huang et al. 2012; Moore et al. 2018).

A long-standing widely held idea for the production of jets of all sizes is that the jet outflow is magnetically driven by a burst of reconnection of emerging closed magnetic field with far-reaching ambient magnetic field, with the burst of reconnection occurring suddenly at the current sheet between them when the current sheet has been sufficiently built up by the emergence of the closed field (e.g., Yokoyama & Shibata 1995; Shibata & Magara 2011). In contrast to this picture however, high-resolution and high-cadence observations from the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) show that coronal jets are often driven by the eruption of a minifilament (Sterling et al. 2015; also see Hong et al. 2011; Shen et al. 2012; Adams et al. 2014; Young & Muglach 2014). The eruption drives the jet outflow via reconnection with the far-reaching ambient field and, via internal reconnection of the legs of the minifilament-carrying erupting field, produces a jet bright point (JBP) centered on the neutral line where the minifilament was rooted prior to the eruption. Using SDO/Helioseismic and Magnetic Imager (HMI) magnetograms, Panesar et al. (2016b, 2017, 2018a) and McGlasson et al. (2019) found that flux cancelation usually builds the sheared/twisted magnetic field in and around the pre-jet minifilament and triggers it to erupt.

In addition to coronal jets, smaller-scale network jets (similar in form to coronal jets but 3–4 times smaller), named jetlets by Raouafi & Stenborg (2014), occur at the edges of lanes of the magnetic network (Panesar et al. 2018b). They have been seen to originate at canceling neutral lines and to show base brightenings during jet onset (Panesar et al. 2018b). Panesar et al. (2018b) found that jetlets are about three times smaller in base width (<5000 km) than typical coronal jets (∼18,000 km). Therefore, jetlets are plausibly small-scale versions of both larger coronal jets and the still-larger CME-producing eruptions (Sterling et al. 2018). However, we cannot rule out that there might be some jets of various sizes that are not driven by the eruption of a flux-cancelation-built-and-triggered minifilament flux rope. Instead, some might be driven by emerging closed magnetic field via reconnection with ambient far-reaching field, as has long been proposed and modeled (Shibata et al. 1992; Yokoyama & Shibata 1995; Shimojo & Shibata 2000; Shibata & Magara 2011).

Here, we present even-higher-resolution EUV observations of still-smaller jet-like events observed by NASA's High-resolution Coronal Imager 2.1 (Hi-C 2.1; hereafter "Hi-C") on a sounding rocket. Hi-C's images reveal fine-scale evolving structures that have not been discerned before at Hi-C's observing wavelength. During the five minutes of observations of Hi-C, we identified six jet-like fine-scale events (Table 1). To judge whether they are still-smaller versions of IRIS jetlets and coronal jets, we investigate the magnetic setting of these six jet-like events, examine their physical properties using the Hi-C data, and compare their properties with those of IRIS jetlets.

Hi-C was launched on a sounding rocket on 2018 May 29, and observed an active region (AR; AR 12712) in 172 Å emission (including the Fe ix/Fe x spectral lines) for about 5 minutes (18:56:26–19:01:43UT). The Hi-C images have a pixel size of 0129 and have 4.4 s cadence (Rachmeler et al. 2019). We identified six fine-scale jet-like events (Figure 1(a)) in the AR's outskirts, in the northwest of the Hi-C field-of-view (FOV). Here we study their structure, evolution, and magnetic settings in detail.

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We have coordinated data from the Interface Region Imaging Spectrograph (IRIS; De Pontieu et al. 2014), and AIA (Lemen et al. 2012) and HMI (Scherrer et al. 2012) from SDO. In AIA images, our six Hi-C events are also seen in 304 and 171 Å; they are barely visible in 193 and 211 Å and invisible in other AIA channels. Here we present only AIA 171 Å images because they show best the Hi-C 172 Å events.

Out of the six jet-like events, only one (event 1 of Table 1) was observed in IRIS slit-jaw images (SJIs), because of limited overlap of the IRIS FOV and the Hi-C FOV. The IRIS spectral slit did not cover any of these events. We use only Si iv SJIs because the jet-like events are best seen in this bandpass. AIA images, Si iv SJIs, and HMI magnetograms were co-aligned with the Hi-C data. We estimate the co-alignment to be within about 1''. To enhance the visibility of weak minority-polarity flux near the network lanes, we summed two magnetograms at each time step, the one taken at that step and the subsequent one taken 45 s later.

3.1. Overview

3.2. Jetlet-like Events

3.2.1. Jetlet-like Event 1

Figures 2(a)–(f) and (g)–(l) show the IRIS SJIs and Hi-C 172 Å images, respectively of jetlet-like event 1. Plasma starts to move upwards along a pre-existing faint loop at about 18:59:15UT (see the Hi-C animation Figure 2), and appears as a bright and thin spire along the loop over 19:00:20–19:00:51 UT (Figure 2). At the same time the spire becomes prominent in IRIS Si SJIs (see the IRIS animation in Figure 2 as well as Figure 2(a)–(f)). IRIS SJIs show two separate strands in the spire (green arrows in Figures 2(e) and (f)). The decrease in separation of the two strands from 19:00:30 UT to 19:00:43 UT is a possible but not decisive signature of twisting/spinning of the spire. After 19:01:52 UT, the spire starts to disappear and the loop structure persists (see the Hi-C animation in Figure 2). We do not see, rising from the base as the event begins, any dark feature that might be taken to be a tiny erupting filament; that is, no erupting "minifilament/microfilament" is discernible in these data.

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Figure 2(m)–(r) displays the line-of-sight (LOS) photospheric magnetic flux at the base. The jetlet-like event is rooted at the edge of the negative-polarity network flux lane, between the (majority-polarity) network flux and a smaller weak minority-polarity (positive) flux patch (red arrows in Figures 2(m), (n)). We followed these fluxes and observed discernible flux cancelation of the minority-polarity flux patch at the neutral line. To examine the magnetic field evolution quantitatively, we measured the minority-polarity flux patch of the base region that is bounded by the yellow box region of Figure 2(n). Figure 4(a) shows a decrease in that positive flux between 18:57:37 and 19:00:37 UT, presumably from flux cancelation that may have triggered the eruption of the jetlet-like event 1, as in larger jetlets and coronal jets examined by Panesar et al. (2016b, 2017, 2018a, 2018b) and by McGlasson et al. (2019). This event is similar to IRIS jetlets (Panesar et al. 2018b) in that the jetlets also occurred at the edges of network flux lanes at canceling neutral lines. Unlike the Panesar et al. (2018b) jetlets, this jetlet-like event does not show any base brightening (corresponding to the JBP in coronal jets/jetlets) at the canceling neutral line, either in the Hi-C or in the IRIS images.

3.2.2. Jetlet-like Event 3

Figure 3 and the online animations of the AIA, Hi-C, and HMI magnetogram images show the evolution of jetlet-like event 3. Hi-C images do not cover the beginning phase of this jetlet-like event, therefore we supplement Hi-C with AIA 171 Å images. There are two jetlet-like events from the same network edge: the first one, which ended prior to the start of Hi-C coverage, starts at 18:52:21UT (not listed in Table 1), and the second one starts at 18:54:09UT. Sequential coronal jets have also been observed to erupt multiple times from the same neutral line during ongoing flux cancelation there (Panesar et al. 2017; Sterling et al. 2017). Here, we present only the second one (jetlet-like event 3), the one partly covered by Hi-C (although both are visible in the animated AIA images in Figure 3). In both of these two jetlet-like events, a faint spire appears next to a small bright loop (green arrows in Figure 3). This small loop brightens as the spire shoots out.

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For the second of these two jetlet-like events, the base starts to brighten at ∼18:54:09UT (see the animated AIA images in Figure 3). The brightening sits on the neutral line between a larger majority-polarity (negative) flux clump and smaller and weaker minority-polarity (positive) flux grains (Figures 3(a) and (m)–(p)). This base brightening (pink arrows in Figure 3) appears to be a miniature version of the JBPs (Sterling et al. 2015; Panesar et al. 2016b, 2018b) that occur at the canceling neutral line at the base of IRIS jetlets and coronal jets. However, this jetlet-like event does not show an erupting minifilament. Later, starting at 18:54:33UT (Figures 3(b)–(i)), a faint spire extends up into the corona with an average speed of 115 ± 20 km s⁻¹. Simultaneously, some additional loop-like brightening appears next to the spire (green arrows in Figure 3) and extends to the neighboring positive-polarity flux patch (Figures 3(a) and (m)). The spire fades away after 18:56:56 UT, in both AIA and IRIS images.

Figure 4(b) shows the positive flux plot versus time. The positive flux starts to decrease at ∼18:51:37 UT and the jetlet-like event starts at ∼18:54 UT. One can see in Figures 3(m)–(r) that the positive (and negative) flux decreases at the base of the jetlet-like event.

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3.2.3. Jetlet-like Events 5 and 6

Events 5 and 6 are different from the other four events. They are shorter in length and narrower in width (Table 1). The average of the observed widths (375 ± 75 km), lengths (5500 ± 700 km), and speeds (32 ± 20 km) are similar to spicule widths (≤400 km), lengths (3000–6000 km), and speeds (10–100 km s⁻¹) of De Pontieu et al. (2007), Pereira et al. (2012), and Tian et al. (2014). The spires of these two events appear as a dark structure adjacent to a bright strand (Figures 5(a)–(d) and the online animation of the Hi-C images) and are rooted at the edge of a clump of network-lane flux (Figures 5(g), (h) and the online animation of the HMI magnetogram images). However, we find no definite signature of minority-polarity flux in these events.

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At 18:59:02 UT (Hi-C animation in Figure 5), a small brightening appears at the base of jetlet-like event 5. After the start of the brightening the spire rises with a speed of 15 ± 5 km s⁻¹, with a bright tip at its front (Figure 5(a)–(d)). In jetlet-like event 6, base brightening appears at 18:59:37 UT (Hi-C animation in Figure 5). The spire extends out with a speed of 50 ± 10 km s⁻¹. In both events, a bright component appears, to the left of the dark part of the spire (Figure 5(b)), and grows along with the dark part. However, we are not certain whether there is a direct connection between the bright and dark strands. Another possibility is that these dark structures are EUV absorption components of common chromospheric jets, with surrounding brightenings due to hot tips of chromospheric jets, and other transition-region brightenings (De Pontieu et al. 2011; Skogsrud et al. 2015). Moreover, the brightenings of these two events appear different from the other four events, appearing adjacent to the dark features and lasting for a relatively short time. Whether this is due to an intrinsic difference among our events, or due to resolution and sensitivity limitations of Hi-C, cannot be determined from this data set alone.

Using high-resolution (∼150 km), high-cadence (4.4 s) Hi-C 2.1 images, we examined the evolution of six small-scale jetlet-like events along with their magnetic setting. We find that (i) the Hi-C jetlet-like events are rooted at edges of magnetic network lanes similar to IRIS jetlets (Panesar et al. 2018b); and (ii) jetlet-like events (1–4) stem from sites of flux cancelation between merging majority-polarity and minority-polarity flux patches, evocative of coronal jets; and (iii) jetlet-like events 3, 4, 5, and 6 show brightenings at their bases, reminiscent of the base brightenings in coronal jets. As described in Section 3.2.3, because of the variable quality of the data the base brightenings in jetlet-like events (5, 6) are not as clearly visible as in jetlet-like events (3, 4). With the above cautions regarding the base brightening of jetlet-like events 5 and 6, overall these results are consistent with (although only marginally so for events 5 and 6) our Hi-C events being smaller versions of IRIS jetlets and coronal jets.

We find evidence both consistent with and not consistent with these events being scaled-down versions of coronal jets and jetlets. The Hi-C jetlet-like events (1–4), on average, are at least three times smaller in spire length (10,000 ± 3000 km) and five times smaller in spire width (650 ± 70 km) than the spire length (27,000 ± 8000 km) and spire width (3200 ± 200 km) of IRIS jetlets (Panesar et al. 2018b). The average speeds of the Hi-C jetlet-like events (70 ± 30 km s⁻¹) are similar to the speeds of IRIS jetlets (75 ± 40 km s⁻¹) and of coronal jets (100 ± 30 km s⁻¹; Panesar et al. 2016b, 2018a). These measured values are additional evidence that the Hi-C jetlet-like events are scaled-down versions of larger jetlets and coronal jets.

Jetlet-like events (1–4) occur at sites of discernible flux cancelation. In jetlet-like events 3 and 4, we also observe brightenings at the canceling neutral line. This is consistent with flux cancelation at the neutral line building and triggering these jetlet-like eruptions (and the jetlet-like eruption preceding jetlet-like event 3).

The evidence that our Hi-C jetlet-like events are not consistent with coronal jets (and also perhaps jetlets) includes that we do not find any evidence of an erupting minifilament in the Hi-C events. Also, not all of our Hi-C events include brightenings that would correspond to the JBP of coronal jets. One possibility is that these Hi-C events are merely too small to resolve possible minifilament counterparts and the brightenings are simply be too weak for Hi-C to detect. We plan to analyze similar small-scale events using IRIS spectra to find out if there is any cool material in jetlet-like events.

Alternatively, the Hi-C events might instead be different from coronal jets and jetlets (also see Section 3.2.3). They might, for example, be driven by some other mechanism(s), such as (i) evaporation flow in which a jet can shoot out by the gas pressure gradient during the eruption, (ii) magnetic pressure force driving a jet as a result of magnetic reconnection between a twisted loop and ambient coronal field, (iii) or a jet may result from magnetic tension in the interchange-reconnection outflow when the current sheet for that reconnection is made by field emergence rather than by eruption of the minifilament-carrying field (e.g., Shibata et al. 1996; Shibata 1999; Shimojo & Shibata 2000).

More studies are required to differentiate between these (and other) possible reasons for the inconsistencies between our Hi-C events on the one hand, and coronal jets and jetlets on the other hand.

The Hi-C 172 Å jetlet-like events (5, 6) show lengths, widths, and speeds in the range of chromospheric spicule lengths (3000–6000 km), widths (300–400 km), and speeds (15–110 km s⁻¹) observed by De Pontieu et al. (2007), Pereira et al. (2012), and Skogsrud et al. (2015). Both of these events occur at the edges of the network lanes, as do chromospheric spicules. This suggests that some chromospheric spicule events might have a transition-region coronal temperature component that is detectable in Fe ix/Fe x emission by Hi-C, in agreement with AIA EUV spicule observations presented by De Pontieu et al. (2011). It is possible that small-scale minority flux was present in the base of these events, at the edge of the network lane, but was not detectable by HMI. Higher spatial resolution, higher magnetic sensitivity data from future telescopes (e.g., the Daniel K. Inouye Solar Telescope (DKIST)) will further clarify the magnetic setting of such small-scale events.

However, the presence of minority polarity at the base of these types of jets is not unique to the minifilament eruption model for jets. In fact, an alternative promising mechanism for spicules (Martínez-Sykora et al. 2017) also involves the presence of opposite polarity flux at the footpoint of strong flux concentrations and associated spicules. In their model, a sudden release of magnetic tension by ambipolar diffusion impulsively drives small-scale jet outflows from edges of the magnetic network.

We cannot be certain, however, that we are observing true jetlet-like events. Another possibility is that we are just seeing selected chromospheric fibrils that show strongly in 171 Å just like some dynamic AR Hα fibrils show up in EUV (Berger et al. 1999; De Pontieu et al. 1999). There is strong observational (e.g., De Pontieu et al. 2003) and theoretical (e.g., Hansteen et al. 2006; Heggland et al. 2011) evidence that dynamic fibrils result from magnetic acoustic waves driven from photospheric motions. Such a mechanism is very different from that suspected of driving coronal jets. It is vital to have high-resolution instruments such as Hi-C in conjunction with Hα, Swedish Solar Telescope, and DKIST to differentiate between these disparate ideas for the small-scale features like our Hi-C jetlets. Detailed studies with Hi-C-like instruments in conjunction with high-quality Hα observations would be needed to distinguish with confidence between these possibilities.

Hi-C jetlet-like events (3, 4, 5, 6) show base brightenings during onset. However, the base brightenings of jetlet-like events (5, 6) are not as clear as the base brightenings of jetlet-like events (3, 4). Therefore, the causality of brightenings and the jet evolution are difficult to establish in these two events. The brightenings appear at the neutral line and/or network-lane edge and are analogs of the jet-base brightenings that are seen to occur at and near the cancelation neutral line during coronal-jet onset. If the Hi-C events are miniature versions of coronal jets then these jet-base-like base brightenings would result from both internal reconnection (Sterling et al. 2015) that occurs within the legs of an erupting magnetic arcade with the minifilament flux rope in its core and external reconnection of the erupting magnetic arcade with the encountered far-reaching field. The spire would result from the external reconnection driven by the eruption of the minifilament-carrying field. Several simulations of jet eruptions have shown that the jet spire outflow can be driven by the magnetic pressure in the magnetic twist that is transferred from the twisted closed field to the ambient far-reaching field by the interchange reconnection (Shibata & Uchida 1986; Shibata 1999; Wyper et al. 2018).

Jetlet-like events that occur at the footpoints of far-reaching loops (evidently due to flux cancelation) might contribute to coronal heating of the loops. However, a more detailed analysis is required to establish that eruptions from flux cancelation at the feet of coronal loops drive heating in the loops (e.g., Tiwari et al. 2014, 2017; Chitta et al. 2017; Tiwari et al. 2019).

Our observations provide evidence that some Hi-C jetlet-like eruptions are analogs of larger-scale coronal-jet minifilament eruptions and also still-larger-scale solar eruptions that make CMEs. If this is the case, then based on these four Hi-C jetlet-like events and previous observations of IRIS jetlets and coronal jets, the implication would be that flux cancelation may play a key role in the buildup and triggering of solar eruptions of a wide range of sizes, from as small as jetlets to as large as CME eruptions.

We thank an anonymous referee for constructive comments. N.K.P. acknowledges current support from NASAs SDO/AIA (NNG04EA00C) and previous support from NPP at the NASA/MSFC, administered by USRA under contract with NASA. A.C.S and R.L.M acknowledge the support from the NASA HGI program. S.K.T. gratefully acknowledges support by NASA contracts NNG09FA40C (IRIS), and NNM07AA01C (Hinode). We acknowledge the use of Hi-C 2.1, IRIS, and SDO data. AIA is an instrument on board the Solar Dynamics Observatory, a mission for NASAs Living With a Star program. We acknowledge the Hi-C 2.1 instrument team for making the second re-flight data available under NASA Heliophysics Technology and Instrument Development for Science (HTIDS) Low Cost Access to Space (LCAS) program). MSFC/NASA led the mission with partners including the SAO, the UCLan, and LSMAL. Hi-C 2.1 was launched out of the White Sands Missile Range on 2018 May 29. IRIS is a NASA small explorer mission developed and operated by LMSAL with mission operations executed at NASA Ames Research center and major contributions to downlink communications funded by ESA and the Norwegian Space Centre.