Origin and Structures of Solar Eruptions I: Magnetic Flux Rope (Invited Review) (1705.08198v2)

Abstract: Coronal mass ejections (CMEs) and solar flares are the large-scale and most energetic eruptive phenomena in our solar system and able to release a large quantity of plasma and magnetic flux from the solar atmosphere into the solar wind. When these high-speed magnetized plasmas along with the energetic particles arrive at the Earth, they may interact with the magnetosphere and ionosphere, and seriously affect the safety of human high-tech activities in outer space. The travel time of a CME to 1 AU is about 1-3 days, while energetic particles from the eruptions arrive even earlier. An efficient forecast of these phenomena therefore requires a clear detection of CMEs/flares at the stage as early as possible. To estimate the possibility of an eruption leading to a CME/flare, we need to elucidate some fundamental but elusive processes including in particular the origin and structures of CMEs/flares. Understanding these processes can not only improve the prediction of the occurrence of CMEs/flares and their effects on geospace and the heliosphere but also help understand the mass ejections and flares on other solar-type stars. The main purpose of this review is to address the origin and early structures of CMEs/flares, from multi-wavelength observational perspective. First of all, we start with the ongoing debate of whether the pre-eruptive configuration, i.e., a helical magnetic flux rope (MFR), of CMEs/flares exists before the eruption and then emphatically introduce observational manifestations of the MFR. Secondly, we elaborate on the possible formation mechanisms of the MFR through distinct ways. Thirdly, we discuss the initiation of the MFR and associated dynamics during its evolution toward the CME/flare. Finally, we come to some conclusions and put forward some prospects in the future.

Overview of "Origin and Structures of Solar Eruptions I: Magnetic Flux Rope"

The invited review by X. Cheng, Y. Guo, and M. D. Ding, titled "Origin and Structures of Solar Eruptions I: Magnetic Flux Rope," offers a comprehensive examination of the enigmatic processes leading to solar eruptions, focusing on the magnetic flux rope (MFR) structure. This work explores coronal mass ejections (CMEs) and solar flares, the most significant solar phenomena, which release plasma and magnetic flux, potentially impacting technological systems on Earth.

Key Findings

• Pre-eruptive Configuration:

The review discusses the prevailing models of pre-eruptive structures, chiefly emphasizing the MFR's role. The structure, often debated as a pre-existing element, is identified through manifestations such as filaments, sigmoids, coronal cavities, and hot channels. The existence of the MFR prior to eruption is supported by observational evidence.

• Formation Mechanisms:
  • Bodily Emergence: This involves the MFR emerging from below the photosphere, although fully emerging structures are rare.
  • Reconnection Processes: Magnetic reconnection in the corona before or during eruptions is argued as a more likely formation mechanism, driven by shearing and converging motions.
• Initiation and Dynamics:

The paper categorizes CME/flare initiation into reconnection-based models and ideal MHD instabilities. It emphasizes the role of torus instability, with analysis pointing to its critical role in MFR initiation when decay indices exceed specific thresholds.

• 3D Dynamics and Observations:

The review integrates 3D observational analyses, highlighting the CME formation process driven by hot channel dynamics. The paper also associates flare ribbon morphology with the 3D magnetic structure and underlines the importance of high-cadence, high-resolution observations for understanding these phenomena.

Implications and Future Directions

The research has profound implications for space weather forecasting and understanding stellar activities on solar-type stars. By clarifying the MFR's formation and initiation processes, this paper advances the predictive modeling of solar eruptions. Future research should enhance MHD simulations to reproduce observed solar phenomena comprehensively, addressing gaps like the differentiation between stable and unstable MFRs. Additionally, exploring the broader 3D dynamics during eruptions through advanced simulations and observing their implications on particle acceleration could yield significant insights.

The fusion of observational prowess and theoretical modeling promised in this review paves the way for more precise forecasting methods, potentially mitigating the adverse effects of solar activities on Earth-based technologies. As the understanding of solar magnetic structures deepens, so too will our ability to comprehend and predict the space weather phenomena stemming from our closest star.