Magnetic reconnection, a basic course of in astrophysical atmosphere plasma is believed to facilitate the discharge of vitality saved within the magnetic subject. Nevertheless, the place the magnetic reconnections happen, how and the place the launched magnetic vitality is transported, and the way it’s transformed to different kinds stay unclear.
Right here, we report well-connected observational signatures of magnetic reconnection, plasma heating, and electron acceleration noticed through the post-impulsive gradual section of the X8.2-class eruptive photo voltaic flare on 2017 September 10 (see a group of related papers of the celebrated limb flare on ADS). The mixed white mild and EUV imaging observations permit us to determine the timing and placement of a number of intermittent reconnection occasions by monitoring bi-directional plasma outflows in an especially lengthy plasma sheet. The arrivals of the plasma downflows on the looptop area of the flare arcade correlate with plasma heating occasions that manifest as impulsive X-ray bursts. Within the meantime, nonthermal microwave bursts, obtained by Expanded Owens Valley Solar Array (EOVSA; Gary et al. 2018), are detected within the loopleg area, which haven’t any response in arduous X-rays. Such a series of reconnection-associated observational signatures affords a brand new view of the vitality launch and conversion processes with a degree of readability not beforehand achieved.
Impulsive Microwave/X-Ray Bursts within the Publish-impulsive Part
Throughout the post-impulsive section of the flare, the EOVSA total-power (full-disk built-in) microwave dynamic spectrum is featured by a number of broadband bursts (Determine 1(b)). These bursts have an impulsive look within the dynamic spectrum and lightweight curves. These bursts have a mean recurrence interval of ∼5.6 minutes. The person microwave bursts correlate with weak X-ray bursts at 6–100 keV noticed by each RHESSI and Fermi/GBM (Determine 1(c)).
EOVSA multi-frequency photographs (Determine 1(a)) present that the general morphology of the evolving microwave supply is per the form and orientation of the EUV flare arcade. There seem like two distinct sources: one coincides with the looptop HXR supply, whereas the opposite is within the northern leg of the flare arcade (proper aspect of the diagram). We notice that microwave emission is weak or absent within the southern leg of the flare arcade (proper aspect within the diagram).
The time-series of microwave photographs reveals that the impulsive part of the microwave emission in every burst is especially from the loopleg supply. In these time-sequence photographs, the loopleg supply exhibits a big variation in depth throughout every burst. In distinction, the looptop supply seems comparatively steady with more-minor variations in morphology and depth.
Determine 1 — Microwave bursts noticed by EOVSA. (a) Picture sequences of the microwave emission at 30 spectral home windows overlaid on SDO/AIA 131 Å photographs round 17:35 UT. The time is indicated by the vertical line in (b-c). (b) EOVSA total-power dynamic spectrum of the post-impulsive section in 2.5–18 GHz. Overplotted is the EOVSA 5 GHz mild curve after eradicating the slowly various background. (c) Detrended mild curves of RHESSI 6–50 keV and Fermi 25–50 keV X-ray counts, and detrended GOES 1–8 Å flux.
Reconnection within the late section: bidirectional outflows vs. Microwave/X-Ray Bursts
Shortly after the eruption of the coronal mass ejection, a large-scale reconnection present sheet (RCS; see our different latest paper Chen et al. 2020 and references therein) appeared in white and EUV photographs (Fig 2(a)-(b)). Multitudes of plasma outflows are current within the RCS throughout totally different phases of the occasion for an prolonged time frame. We discover many recurring pairs of bi-directional plasma outflows that propagate concurrently alongside the RCS (Determine 2(d)). The upward-moving EUV outflows prolong properly into the MLSO/Okay-cor subject of view in white mild to no less than 1100 Mm (or 1.6 R⊙) above the photo voltaic floor (pink shaded area in Determine 2(c)). The downward-moving EUV outflows appear to terminate on the looptop area (blue shaded area in Determine 2(c)). Every pair of bi-directional outflows seems to diverge from a discrete web site at various heights within the plasma sheet. We attribute the diverging location of every bi-directional outflow pair as the location of a person magnetic reconnection occasion (or reconnection “X” level). Most of those recognized reconnection websites are positioned at d ≈ 50–180 Mm above the limb (Determine 2(c)), which is only one%–3% of the whole size of the plasma sheet (~10 R⊙) throughout that interval.
The downflows fade away as they merge into the tip of the cusp-shaped flare arcade, the place a mess of gradual, downward-contracting loops are current. We illustrate the timing of the impulsive microwave bursts in accordance with the noticed EUV plasma downflows in Determine 2(e,f). We overlay the EOVSA 5 GHz microwave mild curve (from Determine 1(b)) on the time–distance plots close to the separatrix area the place the downflow motions seem to “terminate”. The arrival of most EUV plasma downflows on the separatrix area is instantly adopted by a microwave burst. This correlation in each area and time is a robust indication for a causal connection between the plasma downflows arriving on the looptop and the looks of microwave-emitting nonthermal electrons within the flare arcade.
Determine 2 — Magnetic reconnection within the large-scale vertical RCS above the post-flare arcade. (a) Composition of the SDO/AIA 131 Å, MLSO/Okay-cor, and SOHO/LASCO C2 and C3 white-light photographs, exhibiting the CME bubble and an extended plasma sheet connecting the core of CME and the underlying flare web site. (b) Detailed view of the decrease portion of the plasma sheet seen in EUV and white mild. The inexperienced dashed curved is used to derive the time–distance maps proven in panel (c), wherein upflows are seen to increase to no less than 1200 Mm (or ~1.7 R⊙) above the photo voltaic floor. (d) Successive SDO/AIA 131 Å background-detrended photographs that present bidirectional outflows diverging from a compact area. (e-f) Enlarged view of the time–distance plot for 2 chosen time intervals proven because the orange containers in panel (c). The tracks of outflows are denoted by blue and pink curves. The doable X and Y factors are denoted by crosses and circles. The orange strong line is the detrended EOVSA 5 GHz mild curve.
Dialogue and Conclusion
Our observational outcomes are per the usual CSHKP eruptive flare state of affairs for the post-impulsive section. A schematic image is proven in Determine 3. Sporadic magnetic reconnections happen at localized magnetic null factors (or X factors) within the RCS, creating pairs of extremely bent magnetic flux tubes. Plasma is ejected from the X factors each upward and downward alongside the RCS, leading to bidirectional plasma outflows.
The plasma outflows carry a good portion of the whole launched magnetic vitality within the type of electromagnetic Poynting flux, enthalpy flux, and kinetic vitality flux of the majority flows and turbulence (Fletcher & Hudson 2008). Arrival of the downward-propagating plasma outflows on the cusp area dissipates their vitality, leading to plasma heating by way of thermal conduction and/or adiabatic heating. If a fast-mode termination shock is established within the cusp area (which is maybe implicated by the presence of the secondary ALT X-ray supply close to the cusp tip), plasma heating would happen within the shock downstream area (Forbes 1986; Masuda et al. 1994). Such heated plasma is revealed by the thermal X-ray and microwave supply noticed on the loop high.
The cusp area might function the first plasma heating and electron acceleration web site. This argument is supported by the relative timing between the X-ray/microwave bursts and the magnetic reconnection occasions within the RCS—the prevalence of the X-ray/microwave bursts correlates with the arrival time of the plasma downflows on the cusp, however not the time of the magnetic reconnection occasions themselves. A simple interpretation is that the electrons chargeable for the microwave bursts are accelerated regionally on the looptop, the place freshly injected vitality is obtainable from the arrival of the plasma downflows.
Determine 3 — Animated schematic diagram of post-impulsive flare arcade and the large-scale RCS (tailored from Forbes & Acton 1996). (b) Schematic diagram of flare arcade and the RCS depicted in 3D. Discrete reconnection occasions happen at totally different occasions and heights inside the 3D RCS, seen because the noticed scattering of the reconnection websites seen edge-on. Schematic of the observational signatures together with the plasma sheet (with a finite width), EUV flare arcade, and microwave and X-ray sources is proven projected on the airplane of sky. The flare arcade is barely tilted with respect to the road of sight, which can account for the absence of the microwave supply within the southern (proper) aspect of the arcade. (c) Composite EOVSA 2.5-18 GHz, RHESSI 6-12 keV and 131 Å picture through the post-impulsive section, exhibiting the MW/X-ray emission within the post-flare arcade.
Based mostly on the latest paper: Sijie Yu, Bin Chen, Katharine Okay. Reeves, Dale E. Gary, Sophie Musset, Gregory D. Fleishman, Gelu M. Nita, and Lindsay Glesener (2020) “Magnetic Reconnection through the Publish-impulsive Part of a Lengthy-duration Photo voltaic Flare: Bidirectional Outflows as a Explanation for Microwave and X-Ray Bursts”, 2020, The Astrophysical Journal, 900, 17. DOI:https://doi.org/10.3847/1538-4357/aba8a6; Preprint:https://arxiv.org/abs/2007.10443; Private weblog: https://web.njit.edu/~sjyu/Res.Blog/20201006.html
Chen, B., Shen, C., Gary, D. et al. 2020, Nat. Astron, 10.1038/s41550-020-1147-7
Benz, A. O. 2017, LRSP, 14, 2
Fletcher, L., & Hudson, H. S. 2008, ApJ, 675, 1645
Forbes, T. G. 1986, ApJ, 305, 553
Forbes, T. G., & Acton, L. W. 1996, ApJ, 459, 330
Gary, D., Chen, B., Dennis, B., et al. ApJ, 863, 83
Masuda, S., Kosugi, T., Hara, H., et al. 1994, Natur, 371, 495