Reconnection nanojets - small, fast, bursty plasma ejections transverse to coronal loop field lines, discovered as the first directly observable signature of reconnection-driven nanoflares - pose a coherent unresolved problem: what physical conditions govern their occurrence rates, propagation direction, and energetics, and how much do they contribute to heating the solar corona? Hi-C 2.1 observations of active region AR 12712 (Patel & Pant 2022) reported ten nanojets, six of which propagated outward from the loop, several of them fast and long-lived - in tension with magnetohydrodynamic modelling that predicts a strong inward preference - and left three linked issues open: statistically robust population statistics beyond ten-event samples, conversion of observed kinematics into net energy budgets and heating rates, and a physical explanation of the observed directional statistics. As of 2026 these questions remain open: the largest new samples (~120-300 events) all come from eruptive contexts rather than quiescent active-region loops, per-event kinetic energies now reach ~1e27 erg but carry order-of-magnitude density uncertainties, and competing explanations for the directional asymmetry (loop-curvature tension versus interaction with clumpy cool plasma) have not been reconciled with observed direction statistics.

Background

The solar corona - the Sun's outer atmosphere - is heated to millions of kelvin, far hotter than the visible surface below, and identifying the energy source of this heating is one of the classic unsolved problems of solar physics. A long-standing proposal is the nanoflare picture: the corona is heated by a myriad of tiny, impulsive bursts of magnetic energy released by magnetic reconnection, the fundamental plasma process in which misaligned magnetic field lines break and rejoin, converting stored magnetic energy into heat and kinetic energy. In coronal loops (magnetic flux tubes anchored in the solar surface), footpoint motions braid the field, and neighboring misaligned strands can reconnect at small angles - so-called component, or small-angle, reconnection.

Individual nanoflares were long thought to be observationally unresolvable because each releases too little energy. This changed with the discovery of nanojets (Antolin et al. 2021, Nature Astronomy): when two misaligned field lines reconnect at a small angle, the newly connected, strongly curved field line is accelerated sideways by magnetic tension - a slingshot effect - producing a small, fast, bursty jet-like ejection transverse to the loop axis. Observed nanojets are typically ~1500 km long and ~500 km wide, last less than ~15 s, and move at 150-250 km s−1^{-1}, perpendicular to the guide field, the dominant background magnetic field along the loop axis. Because they are a direct observable consequence of small-angle reconnection, nanojets are regarded as a telltale signature of reconnection-based nanoflares and hence a potential key to the coronal heating problem. The relevant instruments are: Hi-C 2.1, an extreme-ultraviolet (EUV) imager flown on a suborbital sounding-rocket flight of a few minutes, providing the highest-resolution coronal images (~0.13 arcsec, roughly 100 km on the Sun, at 4.4 s cadence); SDO/AIA, the full-disk EUV imager on the Solar Dynamics Observatory; SDO/HMI, the companion photospheric magnetograph (an instrument measuring the solar-surface magnetic field; in the source study the average magnetic flux at the nanojet bases was at the instrument noise level, below ~17 G); IRIS, an imaging spectrograph observing the transition region, the thin layer between the cooler solar atmosphere and the corona; and Solar Orbiter/EUI-HRI, the High Resolution Imager of the Extreme Ultraviolet Imager on board the Solar Orbiter spacecraft, currently operating with resolution and cadence comparable to Hi-C but with long, continuous observation windows.

Before Patel & Pant (2022), the observational basis consisted of one discovery event (a cluster of nanojets in a braided loop, predominantly inward-directed relative to the loop curvature) and a multi-structure survey (Sukarmadji et al. 2022) which found nanojet kinetic and thermal energies within the nanoflare range, a variety of propagation directions, and dynamic instabilities (Kelvin-Helmholtz and Rayleigh-Taylor) as plausible triggers. On the theory side, Pagano et al. (2021) modeled reconnection in curved loop geometry and found that the post-reconnection inward-directed magnetic tension is stronger than the outward-directed one by up to three orders of magnitude, with parameter regimes in which no outward jet is produced at observable scales - predicting that inward nanojets should be more frequent and more energetic than outward ones.

Against this backdrop, Patel & Pant (2022) inspected Hi-C 2.1 and AIA observations of active region AR 12712 (an active region is a patch of the solar surface with concentrated strong magnetic field, here hosting quiescent coronal loops) and identified ten nanojets: four directed inward and six directed outward with respect to the loop curvature. The outward jets were faster (up to more than 200 km s−1^{-1}, versus 40-60 km s−1^{-1} for the inward ones), longer (up to ~4500 km versus ~2800 km), and longer-lived (mean ~42 s versus ~24 s) - the opposite of the expected behavior, contradicting the prediction that outward jets should be shorter, slower, and rarer. The jet bases sat in a mostly quiet region where the average photospheric magnetic flux measured by HMI was at the instrument noise level (below ~17 G), so the photospheric driver of these events could not be identified; weaker or more complex drivers, including small-scale flux cancellation (the convergence and mutual disappearance of opposite-polarity magnetic flux at the solar surface) below HMI detectability, could not be ruled out. Several jets were visible in multiple AIA passbands (304, 171, 131, and 193 angstrom), indicating a multi-thermal nature spanning the upper transition region and the corona. The authors concluded that outward nanojets may not be uncommon, and explicitly called for larger high-cadence, high-resolution samples (Solar Orbiter/EUI-HRI, IRIS, future Hi-C-class instruments) to build population statistics, and for spectroscopic diagnostics (from the planned Multi-slit Solar Explorer, MUSE, and Solar-C/EUVST missions, EUVST being the Extreme-Ultraviolet Spectroscopic Telescope planned for the Solar-C spacecraft) to convert observed kinematics and intensities into heating rates.

Problem Statement

Determine the physical nature of reconnection nanojets as a population and thereby establish their role in coronal heating. This single objective, raised in its modern form by the Hi-C 2.1 observations of Patel & Pant (2022), comprises three jointly linked questions - occurrence statistics constrain the energy budget, and the heating contribution cannot be assessed without correcting direction-dependent detection biases:

(1) Population statistics. Build a statistically robust census of nanojets - occurrence rates and property distributions (direction relative to loop curvature, length, width, speed, lifetime, energy) - in quiescent active-region coronal loops (the environment of the source study), beyond the ten-event sample of Patel & Pant (2022) and beyond the event-specific eruptive samples available to date.

(2) Energetics and heating contribution. Convert observed nanojet kinematics and multi-thermal intensities into net energy budgets and heating rates, quantifying whether the nanojet population contributes significantly to coronal heating in active-region loops.

(3) Direction physics. Determine the physical conditions that set the direction, energetics, and occurrence frequency of nanojets - in particular, reconcile the observed fast, long-lived, apparently common outward jets (six of ten events in the source study) with MHD predictions of a strong inward preference (Pagano et al. 2021), deciding between candidate mechanisms such as loop-curvature tension asymmetry and interaction with clumpy cool plasma.

References

  1. Patel, Ritesh; Pant, Vaibhav (2022). Hi-C 2.1 Observations of Reconnection Nanojets. The Astrophysical Journal 938, 122. DOI: 10.3847/1538-4357/ac92e5. https://doi.org/10.3847/1538-4357/ac92e5 (arXiv:2209.06449)
  2. Antolin, Patrick; Pagano, Paolo; Testa, Paola; Petralia, Antonino; Reale, Fabio (2021). Reconnection nanojets in the solar corona. Nature Astronomy 5, 54-62. DOI: 10.1038/s41550-020-1199-8. https://doi.org/10.1038/s41550-020-1199-8
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