Small-scale impulsive EUV emission enhancements along network loops

Abstract

Network loops are a common feature in the quiet Sun. The physical processes sustaining their energy budget is still under discussion. We rely on a multi-instrumental (Solar Orbiter/EUI, Solar Orbiter/PHI, IRIS) observation of a six hours quiet Sun region to measure the dynamics and the possible magnetic drivers of impulsive EUV emission enhancements along network loops. We report the detection of small-scale impulsive EUV emission enhancements with EUI/HRIEUV in three network loops. We selected four EUV emission enhancements to measure their plane-of-sky velocities in HRIEUV; their Doppler velocities in the line (log Si iv T = 4.8) with IRIS ; their possible relation to small-scale flux emergence and fluctuation in one of the loop footpoint. The plane-of-sky velocities of the four EUV emission enhancements have a component that seem to appear almost instantaneously along the loop (more than 220 km/s) ; and two of them had a co-temporal component with a PoS velocity of up to , starting near one of the loop footpoint. In one case, we measured with IRIS a co-temporal intensity increase in the line associated with Doppler velocities down to and up to along the line of sight. Finally, we found evidence of small-scale (8E16 Mx) mixed polarity field emergence and fluctuation near one of the loop footpoint. We concluded that the fast component on the plane-of-sky are consistent with a thermal transfer or supersonic plasma flows, while the slower component is consistent with plasma flows. A possible physical origin for these EUV emission enhancements would be magnetic reconnection driven by either photospheric motion of the loop footpoints or by the reconnection of the loop with small-scale magnetic bipoles.

Figures (18)

• Figure 1: Observations of network loop bundles. HRIEUV (a), PHI-HRT (b), and IRIS SJI 1400 (c) images showing the region surrounding the network loops of interest. The images are those closest in time. The blue rectangle shows thed FOV of the IRIS rasters. The time displayed for SJI 1400 has been corrected to take the difference of 354s in light time travel from the Sun to Earth and the Sun to Solar Orbiter into account. The white rectangle shows the FOV of Fig. \ref{['fig:results:loop1_2_3_fov']}a. The yellow arrow (a) highlights the loop bundle of interest seen in HRIEUV, and the red and blue arrows indicate the locations in the network in which it is assumed to be anchored.
• Figure 2: Location of the slits for the three loop bundles. HRIEUV images with a FOV centered around (a) loop bundle 1 at 04:15:06 UT, (b) loop bundle 2 at 07:15:06 UT, (c) and loop bundle 3 at 04:05:06 UT. The $B_\mathrm{los}/\mu$ values measured by PHI-HRT closest in time are shown as dark orange ($B_\mathrm{los}/\mu>$ 50), light orange ($30\gauss > B_\mathrm{los}/\mu \geq 50\gauss$), light blue ($-30\gauss > B_\mathrm{los}/\mu > -50\gauss$), and dark blue contours ($-50\gauss > B_\mathrm{los}/\mu$). The slits along which the time-distance maps are computed are shown as dashed white lines. The slits are named L1, L2, and L3 for loop bundles 1, 2, and 3, respectively. The yellow bar in the bottom right corner of each panel represents 10Mm and is shown as a reference. The associated movie for L1 is available online.
• Figure 3: Time-distance map along slit L1 covering network loop bundle 1 (Fig. \ref{['fig:results:loop1_2_3_fov']}a). In this figure only, the signal is detrended for visualization purposes. The white arrows indicate the four EUV emission enhancements investigated in Sect. \ref{['sec:results:apparent_v']}. The numbers 1 to 4 are the labels of the slits over which the apparent velocity of these EUV emission enhancements are measured (Fig. \ref{['fig:results:all_slits_fov_oneimage']}).
• Figure 4: HRIEUV images zoomed-in to the right footpoint of loop bundle 1, showing (a) slit L1.1 at 04:13:26 UT, (b) slits L1.2 and L1.3 at 04:33:06 UT, and (c) slit L1.4 at 04:50:36 UT. The slits are displayed as either white or blue dotted lines. The blue arrows above slit 3 highlight am apparent bi-directional motion of intensity peaks. The associated movies are available online.
• Figure 5: Time-distance maps (a and c) along slits L1.1 and L1.2 (Figs. \ref{['fig:results:all_slits_fov_oneimage']}a and \ref{['fig:results:all_slits_fov_oneimage']}b). The number of each slit is shown as a white circle in the bottom-right side. The intensity motions of interest are the events $E_1$ to $E_4$. The red crosses show the times of the intensity peaks associated with the events at different locations. The red line represents the computed PoS velocity on the time-distance map for each event. Panels (b) and (d) show four examples of light curves at distinct locations (noted $P_1$ to $P_4$) at the times of $E_1$ and $E_4$, respectively. The vertical dotted lines represent the time intervals that include the intensity peak. A Gaussian function (in blue) is fitted to estimate the central time of the intensity peak (see Appendix \ref{['sec:annex:measurement_apparent_v']} for more details). The events $E_1$, $E_2$ and $E_4$ seem to appear almost instantaneously along the slit, within the limitations due to the HRIEUV temporal resolution. In that case, we only show the highest resolvable PoS velocity as a lower limit. The intensity values are in DN/s.