Spectroscopic Observations of a Current Sheet in a Solar Flare (1801.03631v1)

Abstract: Current sheet is believed to be the region of energy dissipation via magnetic reconnection in solar flares. However, its properties, for example, the dynamic process, have not been fully understood. Here we report a current sheet in a solar flare (SOL2017-09-10T16:06) that was clearly observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory as well as the EUV Imaging Spectrometer on Hinode. The high-resolution imaging and spectroscopic observations show that the current sheet is mainly visible in high temperature (>10 MK) passbands, particularly in the Fe XXIV 192.03 line with a formation temperature of ~18 MK. The hot Fe XXIV 192.03 line exhibits very large nonthermal velocities up to 200 km/s in the current sheet, suggesting that turbulent motions exist there. The largest turbulent velocity occurs at the edge of the current sheet, with some offset with the strongest line intensity. At the central part of the current sheet, the turbulent velocity is negatively correlated with the line intensity. From the line emission and turbulent features we obtain a thickness in the range of 7--11 Mm for the current sheet. These results suggest that the current sheet has internal fine and dynamic structures that may help the magnetic reconnection within it proceeds efficiently.

Spectroscopic Observations of a Current Sheet in a Solar Flare: An Analytical Review

This paper by Li et al. presents a detailed spectroscopic paper of a current sheet within a solar flare, specifically focusing on the solar event SOL2017-09-10T16:06. The paper employs high-resolution imaging and spectroscopic data from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) and the Extreme Ultraviolet Imaging Spectrometer (EIS). The primary objective is to assess the properties of the current sheet, with an emphasis on its dynamic process and internal structures that facilitate magnetic reconnection.

Observational Findings

The current sheet observed in this paper is characterized by high temperatures exceeding 10 MK, prominently captured in the 192.03 Å spectral line associated with a formation temperature of approximately 18 MK. This spectral line is particularly significant due to its visibility at high temperatures, allowing for comprehensive observations of the current sheet's behavior. The spectroscopic data reveals substantial nonthermal velocities, reaching up to 200 km s⁻¹, indicative of turbulent motions likely caused by plasma instabilities or Petschek-type reconnection processes within the current sheet. Intriguingly, these velocities appear most prominent at the edges of the current sheet rather than at its center.

Through the analysis, a negative correlation between nonthermal velocity and line intensity was detected at the central part of the current sheet. These observations suggest the presence of fine structures within the current sheet, contributing variably to turbulent motion and radiative properties.

Thickness Measurement

A key focus of the paper is the measurement of the current sheet's thickness, derived from both line emission features and nonthermal velocity distributions. The analysis yields a thickness range of 7–11 Mm, considerably larger than theoretical predictions but consistent with prior observational findings. This disparity highlights the potential impacts of turbulent processes and magnetic islands on the sheet's structural dimensions.

Theoretical and Practical Implications

The findings from this research provide critical insights into the dynamics of current sheets, particularly regarding the role of fine-scale turbulence and structures. The presence of strong nonthermal motions aligns with concepts of enhanced magnetic reconnection efficiency facilitated by turbulence. These observations offer valuable constraints for theoretical models of magnetic reconnection and solar flare dynamics, potentially guiding advancements in numerical simulations.

Future Directions

The paper suggests several avenues for future research. There is a need for further observational studies to confirm whether the observed relationships between turbulence, emission intensity, and dynamic structures hold consistently across different solar events. Additionally, integrating these observational insights into existing and emerging models of magnetic reconnection in solar eruptions could improve our understanding of energy dissipation mechanisms in such astrophysical phenomena.

In summary, this paper by Li et al. contributes significant empirical evidence to the field of solar physics, advancing our understanding of the properties and dynamics of current sheets within solar flares, with implications for both theoretical frameworks and predictive modeling of solar activities.