Coronal Mass Ejections: Models and Their Observational Basis (P.F. Chen Living Rev. Solar Phys.) 张英智 中国科学院空间科学与应用研究中心空间天气学国家重点实验室

Contents 1. Observational Features 1. Observational Features 2. Theoretical Models 2. Theoretical Models 3. Debates 3. Debates 4. Summary 4. Summary

1. Observational Features Morphology and mass Morphology and mass Angular width Angular width Occurrence rate Occurrence rate Velocity and energy Velocity and energy Association with flares and filament eruptions Association with flares and filament eruptions

Observational Features Narrow CMEs : the angular width less than 10 degree Normal CMEs : the others

2.Theoretical Models 2.1 Basic principles 2.1 Basic principles the typical energy density of possible energy sources is shown in Table 1 (Forbes, 2000). the typical energy density of possible energy sources is shown in Table 1 (Forbes, 2000). Except some slow CMEs which might be accelerated by the ambient solar wind, it is well established that many CMEs are due to the rapid release of magnetic energy in the corona. Except some slow CMEs which might be accelerated by the ambient solar wind, it is well established that many CMEs are due to the rapid release of magnetic energy in the corona.

2.2 Global picture The essential feature that can distinguish them is that narrow CMEs show an elongated jet-like shape, whereas normal CMEs present a closed (or convex-outward) loop. The essential feature that can distinguish them is that narrow CMEs show an elongated jet-like shape, whereas normal CMEs present a closed (or convex-outward) loop.

2.3 Progenitor It is always interesting for researchers to know what kind of structures have the potential to erupt as a CME. It is always interesting for researchers to know what kind of structures have the potential to erupt as a CME. For narrow CMEs, the progenitor is the open magnetic field, usually the coronal hole. For narrow CMEs, the progenitor is the open magnetic field, usually the coronal hole. For normal CMEs, the progenitor should be a strongly twisted or sheared magnetic structure, which has stored a lot of nonpotential energy. For normal CMEs, the progenitor should be a strongly twisted or sheared magnetic structure, which has stored a lot of nonpotential energy.

Progenitor — flux rope

Regarding the CME progenitor, i.e., the strongly sheared and /or twisted core field restrained by the overlying envelop field, two issues are worthy to be clarified by future MHD numerical simulations: Regarding the CME progenitor, i.e., the strongly sheared and /or twisted core field restrained by the overlying envelop field, two issues are worthy to be clarified by future MHD numerical simulations: (1) The helical flux rope – twisted field lines winding many times. (1) The helical flux rope – twisted field lines winding many times. (2) The SXR sigmoids – SXR signature (2) The SXR sigmoids – SXR signature

2.4 Triggering mechanisms 1. Tether-cutting or flux cancellation mechanism 1. Tether-cutting or flux cancellation mechanism 2. Shearing motions. 2. Shearing motions. 3. Magnetic breakout model 3. Magnetic breakout model 4. Emerging flux triggering mechanism 4. Emerging flux triggering mechanism 5. Flux injection triggering mechanism 5. Flux injection triggering mechanism 6. Instability and catastrophe-related triggering mechanisms 6. Instability and catastrophe-related triggering mechanisms 7. Hybrid mechanisms 7. Hybrid mechanisms 8. Other mechanisms 8. Other mechanisms

2.4.1 Tether-cutting

flux cancellation mechanism

2.4.2 Magnetic breakout model

2.4.3 Emerging flux triggering mechanism

Emerging flux triggering mechanism

2.4.4 Flux injection triggering mechanism

2.4.5 Instability and catastrophe-related triggering mechanisms

2.4.6 Hybrid mechanisms

2.4.7 Other mechanisms Mass drainage or Sympathetic effect or Solar wind

3. Debates 1. Is magnetic reconnection necessary? 2. Should fast and slow CMEs be attributed to different models? 3. Nature and the driving source of “ EIT waves ” 4. What is the nature of CMEs? 5. Are halo CMEs special?

4. Summary Morphologically, CMEs can be distinguished as narrow (jet-like) and normal (loop-like) CMEs. Morphologically, CMEs can be distinguished as narrow (jet-like) and normal (loop-like) CMEs. The physics of narrow CMEs is quite clear, i.e., they correspond to the outflow as emerging flux or a coronal loop reconnects with the open magnetic field. The physics of narrow CMEs is quite clear, i.e., they correspond to the outflow as emerging flux or a coronal loop reconnects with the open magnetic field.

Summary – normal CMEs (1) Classification: velocity (slow and fast) evolution (impulsive and gradual) (1) Classification: velocity (slow and fast) evolution (impulsive and gradual) (2) Progenitors: The pre-CME structure might be strongly sheared or weakly twisted magnetic field that is restrained by less-sheared envelope field, and flux rope is not necessarily required in the progenitor. (2) Progenitors: The pre-CME structure might be strongly sheared or weakly twisted magnetic field that is restrained by less-sheared envelope field, and flux rope is not necessarily required in the progenitor. (3) Triggering mechanisms: It is generally believed that magnetic free energy has been stored in the CME progenitors, and the triggering process, which can be ideal or resistive, does not supply much energy to the eruptions. (3) Triggering mechanisms: It is generally believed that magnetic free energy has been stored in the CME progenitors, and the triggering process, which can be ideal or resistive, does not supply much energy to the eruptions.

Summary – normal CMEs (4) Eruption: current sheet is formed below the core and then reconnection is excited in the current sheet. (4) Eruption: current sheet is formed below the core and then reconnection is excited in the current sheet. (5) Nature of the CME frontal loop? (5) Nature of the CME frontal loop? (6) Is magnetic reconnection necessary? (6) Is magnetic reconnection necessary? Finally, some comments for the future research: (1) In order to understand and predict the CME initiation, the internal cause, e.g., the magnetic nonpotentiality, and external cause, e.g., the emerging flux, should be considered together. (2) Little attention was paid to the nature of the CME frontal loop, which is not fully understood yet. (3) It might be of great importance to predict how much energy and magnetic helicity can be released from a source region if it erupts as a CME.