Solar coronal jets — transient, collimated EUV/X-ray ejections occurring hundreds of times per day in all coronal environments — are accepted to be driven by magnetic reconnection, but which magnetic history triggers them, and where and in what topology the reconnection proceeds, remain unresolved for on-disk jets. The open question is: through on-disk observations, determine (i) whether magnetic bipoles are really emerging through the photosphere and causing the jet activity in on-disk jets whose morphology matches the emerging-flux MHD models — resolving the limb-location limitation of the emergence-shaped events observed so far — or whether the trigger is flux cancellation and minifilament eruption at the neutral line, above all for active-region jets, where the cancellation/minifilament paradigm is least confirmed; and (ii) the reconnection-site height for on-disk jets and the evolution from bald-patch to X-point (null) geometry, which is difficult to derive on the disk. Later literature statistically establishes cancellation triggering only for quiet-Sun and coronal-hole jets (85% of 60 events), leaving both questions open as of 2026.

Background

Solar coronal jets are transient, collimated plasma ejections, typically a few thousand to tens of thousands of kilometers long and lasting minutes, that occur in all coronal environments — coronal holes, the quiet Sun, and active regions — probably several hundred times per day. They are seen as hot spires in extreme-ultraviolet (EUV) and X-ray images, frequently accompanied by cooler, darker chromospheric ejections called surges, and they are widely used as small-scale laboratories for magnetic reconnection, the process by which stressed magnetic fields reconfigure their line connectivity and release energy. Observationally, jets divide into "standard" jets (narrow inverted-Y spire with a bright base) and "blowout" jets (broader, eruptive, often twisted), a dichotomy established from X-ray observations (Moore et al. 2010, doi:10.1088/0004-637X/720/1/757).

Two competing trigger scenarios structure the field (reviewed in Raouafi et al. 2016, doi:10.1007/s11214-016-0260-5). In the flux-emergence scenario, new magnetic bipoles rise through the photosphere from the solar interior and reconnect with pre-existing ambient coronal field; three-dimensional magnetohydrodynamic (MHD) simulations of this process predict, beneath the jet, a characteristic two-chamber ("double-vault") structure — one chamber of emerged cool loops shrinking as reconnection proceeds, one of hot reconnected loops growing — topped by a flattened current sheet with plasmoids, together with a fast hot jet and a slower cool surge. In the flux-cancellation (minifilament-eruption) scenario, opposite-polarity magnetic patches converge and cancel at a magnetic neutral line (flux cancellation: photospheric flux of one polarity disappears as it meets and annihilates opposite polarity), building a twisted flux rope — a magnetic field wound around a common axis, often visible in chromospheric lines as a cool, small-scale filament (a "minifilament") — that eventually erupts and reconnects with open or overlying field (Sterling et al. 2015, doi:10.1038/nature14556; Panesar et al. 2018, doi:10.3847/1538-4357/aaa3e9).

Jets are often helical or rotating ("untwisting" jets). Rotation is understood as the transfer of magnetic twist — the winding of field lines about the axis, a form of magnetic helicity — from closed field to newly opened field at the reconnection site, with torsional Alfvén waves (transverse magnetic waves that also twist the field) carrying the twist along the jet spire.

The reconnection site itself can have different topological geometries with very different heights and energy-release depths. A magnetic null point is a location where the magnetic field strength vanishes; field lines in projection cross in an X shape (an "X-point" current sheet), and in three dimensions nulls sit in fan-spine topologies that are a standard jet geometry in the corona. A bald patch, by contrast, is a stretch of the polarity inversion line, inside a closed-field region, where the horizontal field is tangent to the photosphere; reconnection at a bald patch occurs at very low atmospheric height. Whether a given jet reconnects at a bald patch, at a coronal null, or both in sequence determines where the energy is released and what the associated chromospheric and transition-region diagnostics should look like.

The relevant instruments are the Solar Dynamics Observatory (SDO), whose Atmospheric Imaging Assembly (AIA) images the full EUV disk and whose Helioseismic and Magnetic Imager (HMI) provides line-of-sight and vector magnetograms (maps of the full photospheric magnetic-field vector, at roughly 12-minute cadence for vectors); the Interface Region Imaging Spectrograph (IRIS), a UV imaging spectrograph sampling chromospheric and transition-region lines such as Mg II, C II, and Si IV with high spatial resolution; the Swedish 1-m Solar Telescope (SST), a high-resolution ground-based telescope; and the Solar Orbiter and Parker Solar Probe missions, whose off-Sun-Earth-line and near-Sun vantage points enable stereoscopic and in-situ constraints.

The multi-instrument case studies of solar jets on record span the trigger scenarios: recurrent limb jets from active region NOAA 12644 whose double-vault structure and cool surges closely match the flux-emergence simulations (Joshi et al. 2020, doi:10.1051/0004-6361/202037806); a jet associated with a narrow coronal mass ejection (a large-scale eruption of magnetized plasma from the corona into interplanetary space); a twisted jet in which photospheric motions extended a nearby non-eruptive flux rope into an X-point current sheet, transferring its twist into the jet without erupting (Joshi et al. 2020, doi:10.1051/0004-6361/202038562); and an IRIS spectroscopic study of a white-light miniflare/jet in which a cloud-model analysis (an inversion treating the flow as an optically thick cloud projected against the background) found a 290 km s−1^{-1} cool-plasma flow and documented a bald patch transforming into an X-point current sheet (Joshi et al. 2021, doi:10.1051/0004-6361/202039229). Because the key emergence-shaped events occurred at the limb, magnetograms could not confirm whether magnetic bipoles were actually emerging; and each mechanism was demonstrated for a single event. These case studies carry corresponding per-event limitations and outlooks: for the limb events, "there is no possibility of ascertaining whether magnetic bipoles are really emerging at the photosphere and causing the jet activity"; the twist transfer from a remote non-eruptive flux rope was demonstrated for one event, with the outlook that spectral analysis of the IRIS C II, Si IV, and Mg II lines "will provide a precise calibration of magnetic reconnection height"; and for events occurring on the disk the altitude of reconnection is difficult to derive.

Problem Statement

Establish, through observations of on-disk solar coronal jets, the two open issues above as two jointly supporting facets of one objective:

(i) Magnetic confirmation of the trigger. For on-disk jets whose morphology matches the emerging-flux MHD models (the double-vault structure, hot jet with cool surge), determine from photospheric (vector) magnetograms whether magnetic bipoles are really emerging through the photosphere and causing the jet activity, or whether the trigger is magnetic flux cancellation and minifilament/flux-rope eruption at a converging neutral line. This resolves the limb-location limitation of the emergence-shaped events observed so far, whose magnetic driver could not be confirmed. Existing statistics establish cancellation triggering only for quiet-Sun and coronal-hole jets (85% of 60 events); the emergence channel has no magnetic confirmation on disk, and it remains discussed in the literature whether jets are due to flux emerging, cancelling, or both — above all for active-region jets, where the cancellation/minifilament paradigm is least confirmed.

(ii) Reconnection-site height and topology. Constrain, for on-disk jets, the altitude of the magnetic reconnection site and the evolution of its magnetic topology — in particular whether and how reconnection progresses from a photosphere-tangent bald-patch current sheet to a coronal X-point/null-point current sheet.

The twist transfer from a remote non-eruptive flux rope, demonstrated for a single event, is background rather than a facet of this problem. The per-event radiative-hydrodynamic interpretation of fast cloud-model flows and Si IV opacity effects, and the quantitative contribution of jets to coronal heating and solar-wind acceleration, are outside the scope of this task.

References

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