Following the traditional naming of “ eruptive flare ” and “ confined flares ” but not implying a causal relationship between flare and coronal mass ejection ( CME ) , we refer to the two kinds of large energetic phenomena occurring in the solar atmosphere as eruptive event and confined event , respectively : the former type refers to flares with associated CMEs , while the later type refers to flares without associated CMEs .
We find that about 90 % of X-class flares , the highest class in flare intensity size , are eruptive , but the rest 10 % confined .
To probe the question why the largest energy release in the solar corona could be either eruptive or confined , we have made a comparative study by carefully investigating 4 X-class events in each of the two types with a focus on the differences in their magnetic properties .
Both sets of events are selected to have very similar intensity ( X1.0 to X3.6 ) and duration ( rise time less than 13 minutes and decaying time less than 9 minutes ) in soft X-ray observations , in order to reduce the bias of flare size on CME occurrence .
We find no difference in the total magnetic flux of the photospheric source regions for the two sets of events .
However , we find that the occurrence of eruption ( or confinement ) is sensitive to the displacement of the location of the energy release , which is defined as the distance between the flare site and the flux-weighted magnetic center of the source active region .
The displacement is 6 to 17 Mm for confined events , but is as large as 22 to 37 Mm for eruptive events , compared to the typical size of about 70 Mm for active regions studied .
In other words , confined events occur closer to the magnetic center while the eruptive events tend to occur closer to the edge of active regions .
Further , we have used potential-field source-surface model ( PFSS ) to infer the 3-D coronal magnetic field above source active regions .
For each event , we calculate the coronal flux ratio of low corona ( < 1.1 R _ { \odot } ) to high corona ( \geq 1.1 R _ { \odot } ) .
We find that the confined events have a lower coronal flux ratio ( < 5.7 ) , while the eruptive events have a higher flux ratio ( > 7.1 ) .
These results imply that a stronger overlying arcade field may prevent energy release in the low corona from being eruptive , resulting in flares without CMEs .
A CME is more likely to occur if the overlying arcade field is weaker .