Reconnection nanojets associated with a prominence eruption observed with Solar Orbiter/EUI-HRI

TL;DR

This study uses Solar Orbiter/HRIEUV to quantify reconnection-driven nanojets during a prominence eruption, revealing ~120 jets (40 analyzed) with lengths of $~10^3$ km, widths of a few hundred kilometers, durations of several seconds, and instant velocities up to $\sim$600 km s$^{-1}$. The jets show higher speeds, energies, and occurrence during eruption, including clear acceleration/deceleration patterns, and pre-eruption clustering that suggests an avalanche-like sequence of small-scale reconnection events. The findings demonstrate nanojets in a fully ionised coronal environment, imply a connection between small-scale reconnection and large-scale eruption dynamics, and highlight the potential role of nanojets in coronal heating and flare initiation.

Abstract

Magnetic reconnection is a proposed mechanism for nanojets associated with coronal heating. We investigate the characteristics of reconnection-driven nanojets just before and during a prominence eruption using the High Resolution Imager (HRI) of the Extreme Ultraviolet imager (EUI) aboard Solar Orbiter during its perihelion on September 30, 2024. Extreme UV (EUV) images at unprecedented high spatial and temporal resolution from \hrieuv were analysed. The dimensions and propagation speeds of nanojets were estimated and used to estimate the kinetic energies. Nanojet activity was compared with GOES X-ray flux to assess its relation to flare evolution. The high spatial and temporal resolution in the EUV was found to be essential to fully capture the properties and numbers of reconnection nanojets. Approximately 120 nanojets were detected during the eruption, with 40 analysed in detail. Nanojets exhibited lengths of $200 - 5000$~km, widths of $200 - 500$~km and durations of $2-12$~s. Instant velocities ranged from 150 km~s$^{-1}- 600~$km~s$^{-1}$ with kinetic energies reaching $1.56\times10^{27}$~erg. These nanojets are faster, longer, more energetic and more numerous compared to previous studies. We also find clear signatures of acceleration and deceleration, reflecting magnetic tension release and reach of new equilibria. Reconnection events during the eruption were found to be more frequent and energetically intense. Pre-flare nanojet clustering indicates small-scale reconnection may precede large eruptive activity. These results suggest that nanojets also occur in fully ionised coronal plasma, playing a role in both quiescent and eruptive solar activity.

Figures (13)

• Figure 1: Location of Solar Orbiter (SOLO) and Solar Dynamic Observatory (SDO) with respect to Mercury (grey), Venus (gold) and Earth (blue) on September 30th 2024 at 23:30:00 UT along the elliptical plane. The line-of-sight (LOS) of SOLO and SDO are indicated in the sketch.
• Figure 2: (a) Coronal overview of the impulsive phase of the prominence eruption at 23:41:31 universal time (UT), observed by HRIEUV . The HRIEUV map of the eruption is displayed with the FOV of approximately 107 Mm$\times$71 Mm. The top right image is a zoom panel of the red rectangle showing the activation phase of the prominence. (b) The eruption is shown using the AIA 171 Å passband at time 23:41:00 UT with HRIEUV 's FOV overlaid. The top inlet in dark blue colour shows a zoomed image of the red rectangle.
• Figure 3: (a) A typical reconnection nanojet observed at 23:47:37 UT post activation of the prominence with a slit showing the direction of travel. (b) the perpendicular slit across the nanojet for the width to be measured. (c) Time-distance diagram from 23:47:26 UT to 23:47:53 UT illustrating the nanojet's evolution. The slope of the feature gives the bulk velocity. The cyan vertical dashed line shows the length used to calculate the instant velocity of the nanojet. (d) Intensity profile and the corresponding Gaussian fit along the slit shown in (b) for the calculation of the width.
• Figure 4: (a) The longest duration nanojet captured during the activation happening before eruption at 23:27:56 UT. (b) Nanojet with the shortest duration, observed at 23:29:35 UT. (c) Nanojet with the thinnest width, recorded at 23:25:11 UT. (d) Time–distance diagram corresponding to (a), with speed and duration indicated in the bottom right corner. (e) Time–distance diagram corresponding to (b). (f) Gaussian fit to find the shortest width corresponding with (c).
• Figure 5: (a) Cutout showing the nanojet with the thickest width at 23:26:14 UT. (b) bidirectional nanojet with the smallest length observed at 23:26:12 UT. (c) Longest nanojet, observed at 23:40:32 UT. (d) Intensity profile along the green dashed line in (a), with a Gaussian fit and FWHM displaying the width of the nanojet. Intensity map of (c) with characteristics shown in top left. (e) Shortest length calculated from intensity profile along with its corresponding bulk and instant velocity. (f) Data used to calculate the longest length corresponding to (e), displayed in the top left corner.