Coronal Mass Ejections: Models and Their Coronal Mass Ejections: Models and Their Observational Basis Observational Basis

(P.F. Chen. 2011.Living Rev. Solar Phys.)(P.F. Chen. 2011.Living Rev. Solar Phys.)

张英智 张英智 2011.05.052011.05.05

中国科学院空间科学与应用研究中心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空间天气学国家重点实验室

ContentsContents

1. Observational Features1. Observational Features 2. Theoretical Models2. Theoretical Models 3. Debates3. Debates 4. Summary4. Summary

1. Observational Featu1. Observational Featuresres

Morphology and massMorphology and mass Angular widthAngular width Occurrence rateOccurrence rate Velocity and energyVelocity and energy Association with flares and filament Association with flares and filament

eruptionseruptions

Observational FeaturesObservational Features

Narrow CMEs : the angular width less than 10 degreeNarrow CMEs : the angular width less than 10 degreeNormal CMEs : the othersNormal CMEs : the others

2 .Theoretical 2 .Theoretical ModelsModels

2.1 Basic principles2.1 Basic principles the typical energy density of possible energy the typical energy density of possible energy

sources is shown in Table 1 (Forbes, 2000).sources is shown in Table 1 (Forbes, 2000).

Except some slow CMEs which might be accelExcept some slow CMEs which might be accelerated by the ambient solar wind, it is well esterated by the ambient solar wind, it is well established that many CMEs are due to the rapid ablished that many CMEs are due to the rapid release of magnetic energy in the corona.release of magnetic energy in the corona.

2.2 Global 2.2 Global picturepicture

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

2.3 Progenitor2.3 Progenitor

It is always interesting for researchers to It is always interesting for researchers to know what kind of structures have the poknow what kind of structures have the potential to erupt as a CME.tential to erupt as a CME.

For narrow CMEs, the progenitor is the oFor narrow CMEs, the progenitor is the open magnetic field, usually the coronal hpen magnetic field, usually the coronal hole.ole.

For normal CMEs, the progenitor should For normal CMEs, the progenitor should be a strongly twisted or sheared magnetibe a strongly twisted or sheared magnetic structure, which has stored a lot of nonc structure, which has stored a lot of nonpotential energy.potential energy.

Progenitor—flux roProgenitor—flux ropepe

Regarding the CME progenitor, i.e., the strRegarding the CME progenitor, i.e., the strongly sheared and /or twisted core field reongly sheared and /or twisted core field restrained by the overlying envelop field, twstrained by the overlying envelop field, two issues are worthy to be clarified by futuro issues are worthy to be clarified by future MHD numerical simulations:e MHD numerical simulations:

(1) The helical flux rope– twisted field lines (1) The helical flux rope– twisted field lines winding many times.winding many times.

(2) The SXR sigmoids– SXR signature (2) The SXR sigmoids– SXR signature

2.4 Triggering 2.4 Triggering mechanismsmechanisms

1. Tether-cutting or flux cancellation 1. Tether-cutting or flux cancellation mechanismmechanism

2. Shearing motions.2. Shearing motions.

3. Magnetic breakout model3. Magnetic breakout model

4. Emerging flux triggering mechanism4. Emerging flux triggering mechanism

5. Flux injection triggering mechanism5. Flux injection triggering mechanism

6. Instability and catastrophe-related 6. Instability and catastrophe-related triggering mechanismstriggering mechanisms

7. Hybrid mechanisms7. Hybrid mechanisms

8. Other mechanisms8. Other mechanisms

2.4.1 Tether-cutti2.4.1 Tether-cutting ng

flux cancellation mechanisflux cancellation mechanismm

2.4.2 Magnetic breakout 2.4.2 Magnetic breakout modelmodel

2.4.3 Emerging flux triggering 2.4.3 Emerging flux triggering mechanismmechanism

Emerging flux triggering mechanEmerging flux triggering mechanismism

2.4.4 Flux injection triggering 2.4.4 Flux injection triggering mechanismmechanism

2.4.5 Instability and catastrophe-related 2.4.5 Instability and catastrophe-related triggering mechanismstriggering mechanisms

2.4.6 Hybrid 2.4.6 Hybrid mechanismsmechanisms

2.4.7 Other 2.4.7 Other mechanismsmechanisms

Mass drainage or Sympathetic effect or Solar windMass drainage or Sympathetic effect or Solar wind

3. Debates3. Debates

1.1. Is magnetic reconnection necessary?Is magnetic reconnection necessary?2.2. Should fast and slow CMEs be attribuShould fast and slow CMEs be attribu

ted to different models?ted to different models?3.3. Nature and the driving source of “EINature and the driving source of “EI

T waves”T waves”4.4. What is the nature of CMEs?What is the nature of CMEs?5.5. Are halo CMEs special? Are halo CMEs special?

4. 4. SummarySummary Morphologically, CMEs can be distinguMorphologically, CMEs can be distingu

ished as narrow (jet-like) and normal (lished as narrow (jet-like) and normal (loop-like) CMEs. oop-like) CMEs.

The physics of narrow CMEs is quite cleThe physics of narrow CMEs is quite clear, i.e., they correspond to the outflow ar, i.e., they correspond to the outflow as emerging flux or a coronal loop recoas emerging flux or a coronal loop reconnects with the open magnetic field.nnects with the open magnetic field.

Summary – normal CMSummary – normal CMEsEs (1) Classification: velocity (slow and fast) (1) Classification: velocity (slow and fast)

evolution (impulsive and evolution (impulsive and gradual)gradual)

(2) Progenitors: The pre-CME structure might (2) Progenitors: The pre-CME structure might be strongly sheared or weakly twisted be strongly sheared or weakly twisted magnetic field that is restrained by less-magnetic field that is restrained by less-sheared envelope field, and flux rope is not sheared envelope field, and flux rope is not necessarily required in the progenitor.necessarily required in the progenitor.

(3) Triggering mechanisms: It is generally (3) Triggering mechanisms: It is generally believed that magnetic free energy has been believed that magnetic free energy has been stored in the CME progenitors, and the stored in the CME progenitors, and the triggering process, which can be ideal or triggering process, which can be ideal or resistive, does not supply much energy to the resistive, does not supply much energy to the eruptions.eruptions.

Summary – normal CMSummary – normal CMEsEs (4) Eruption: current sheet is formed below the co(4) Eruption: current sheet is formed below the co

re and then reconnection is excited in the current re and then reconnection is excited in the current sheet.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:Finally, some comments for the future research:(1) In order to understand and predict the CME initi(1) In order to understand and predict the CME initi

ation, the internal cause, e.g., the magnetic nonpoation, the internal cause, e.g., the magnetic nonpotentiality, and external cause, e.g., the emerging fltentiality, and external cause, e.g., the emerging flux , should be considered together. ux , should be considered together.

(2) Little attention was paid to the nature of the CME (2) Little attention was paid to the nature of the CME frontal loop, which is not fully understood yet. frontal loop, which is not fully understood yet.

(3) It might be of great importance to predict how m(3) It might be of great importance to predict how much energy and magnetic helicity can be released uch energy and magnetic helicity can be released from a source region if it erupts as a CME.from a source region if it erupts as a CME.