Research On The Relationship Between Large SEP Events And"Twin-CME" Erupting From The Same Source Region

The relationsip between Solar Energetic Particle (SEP) events and Coronal Mass Ejection (CME) is an important problem in space weather study. The answer of this question is of help to establishing the prediction model of SEP events according to CME eruption and/or improving the accuracy of current SEP prediction model. This dissertation extanded the "twin CME" model to large SEP events, and used comprehensive analyses of representive SEP events and statistics of a large number of events, to present some observable evidances of the large SEP events triggered by "twin-CME" erupting from the same source region. We also analyzed the relationship between large SEP events and "twin-CME" eruptions, and gave some evidances of more effective particle acceleration process of the "twin-CME" than the "single-CME". The contents of this dissertation are summarized as the following:First, this dissertation analyzed extreme large SEP event and the first Ground Level Enhancement events (GLE) in Solar Cycle24with the help of Gradual Cylindral Shell (GCS) model, using CME observations by coronagraph instruments on board SOHO(LASCO) and Stereo-A/B(SECCHI) spacecrafts. The results showed that "twin-CME" erupting from the same source region were found in all these events. Furthermore, these two CMEs overlapped partially with each other in their propagating path, which were consistent with "twin-CME" scenario. In these events, we also checked the process of CMEs erupting from their source region in low corona, such as the association of type II radio bursts, the associated AIA multi-waveband images on board SDO apacecraft, HMI magnetogram and the magnetic field from the extrapolated Non-Linear Force Free Field (NLFFF) model, and confirmed that these preceding CMEs were not the early phase of the main CME in "twin-CME" observations.Second, we examined126fast CMEs having western hemisphere source regions in Salor Cycle23, which included59CMEs that led to large SEP events. The statistical results indicate that the peak flux intensities of these SEP events associated with "twin-CME" eruptions did not seem to be correlated with the speed of the associated fast CMEs, the class of associated flares, and the daily (24hr) average seed population prior to SEPs onset time. However, for single-CME events, the peak flux intensity of the SEPs and these three parameters had distinct positive correlations. This can be understood naturely within "twin-CME" scenario:the presence of preceding CMEs can previde enough seed population for the main fast CME shock acceleration, so there is no need to have a high CME speed and/or intense flare and/or the seed particles prior to event. The results also indicate that there is a strong correlation between "twin-CME" and the large SEP events, and suggest that the probability of a "twin-CME" event leading to large SEP events (such as the events with peak flux>10pfu at10MeV channel) is significantly higher than "single-CME" events.Third, we tried to answer several questions about SEP events:1). The choice of the time interval threshold between preceding CME and main CME, we decide this parameter by examining the tendency of the percentage of the SEP events caused by "twin-CME" eruptions among all SEP events by varying this time interval, and by examining how the probability of single fast CMEs failing to generate large SEP events as a function of this time interval. We chose the time threshhold between1hr to24hr with a bin of1hr. The time of-12hr turns out to be a best choice.2). Using the Velocity Dissipation Analysis (VDA) method, we found that the starting release heights of solar energetic particles from low corona is less than15Rsun. In these heights, the preceding CME can provide seed population more effectively for the main CME than a preceding flare, because the turbulence stimulated by the preceding CME shock can held more seed particles for a longer time ahead of the main fast CME.3). Our findings about type II radio bursts in this paper were shown that there is no indicative difference between SEP events and no SEP events, but the proportion of type II radio bursts associating CME with large SEP events is higher than that of the CMEs with no SEP events.4). We discussed the longitudinal dependence of time delay and enhancement slope of SEP flux profiles for different relative longtitudes comparing to the earth-well-connected footpoint. The more eastern relative longitudes comparing to the footpoints, the longer time delay of SEP onset from background to peak and the slower enhancement slope of SEP flux. This implies that the SEP events with eastern source region usually have weaker intensity and lower peak flux than western events.In conclusion, we analyzed the causal association between SEP events and CME eruptions through a comprehensive set of observations. We find that the "twin-CME" erupting consecutively from the same region usually leads to a large SEP events more effectively than a single fast CME. In the "twin-CME" scenario, the traditional variables (such as speed of associated fast CME, or associated flare class et al.) are not deciding factors for the resulting SEP intensity, but the presence of a preceding CME is. We also suggest that the reference value of "twin-CME" time interval between main CME and preceding CME can be set to12hr.