Study Of Observational Properties Of Coronal Mass Ejections On The Statistical Results Of CME Source Locations

Coronal mass ejections (CMEs) are the most violent eruptive activity in the solar atmosphere and believed to be the major source of interplanetary and geomagnetic disturbances. Observations of white-light coronagraphs are widely used to study the propagation and evolution of CMEs, and thus how to properly understand CMEs viewed in white-light coronagraphs is crucial to many relative researches in solar and space physics. Besides, active regions (ARs) store a lot of free magnetic energy and are considered as the most efficient producer of CMEs. A thorough study on the relationship between CMEs and ARs is important to learn the mechanism of the CME trigger and initiation.In order to avoid the human-induced bias in the process of data selection, we try to keep the integrity of samples of CMEs and ARs in our study. We identify the source locations of all the 1078 CMEs listed in CDAW CME catalog during 1997-1998 and find that, except 231 CMEs whose source locations can not be identified due to poor data, there are 288 (34%) CMEs with location identified on the frontside solar disk, 234 (28%) CMEs appearing above solar limb, and 325 (38%) CMEs without evident eruptive signatures in the field of view of EIT 195A. Meanwhile, we identify ARs during 1997-1998 based on synoptic magnetic charts of MDI onboard SOHO by using an automated method developed by Wang and Zhang in 2008 and obtain a total of 108 MDI ARs. Combined with the information of CME source locations, we find that about 53% of ARs can produce CMEs (CME-producing ARs), particularly, about 14% of ARs are CME-rich ARs (ARs producing at least 3 CMEs); and that there is about 63% of CMEs originating from ARs (AR-related CMEs) while others occurring outside of ARs (AR-unrelated CMEs).Based on the above data, we analyze observational properties of LASCO CMEs and their relationship with ARs, including distributions of CME source locations, some scientific issues related to the CME brightness, deflections of CMEs in the inner corona, and the relationship between CMEs and ARs. Many interesting results are obtained as following.1. Distributions of CME source locationsThe distribution of CME source locations in latitude manifests a clear bimodal appearance with two most probable peaks in±(15°-30°), which is consistent with the location of active region belt. No CMEs came from polar regions (outside of ±75°). About 56% of detected CMEs occurred near the solar limb. The average apparent speed of CMEs is 435 km s-1, and there is no evident difference between the apparent speeds of on-disk and limb CMEs. According to the analysis of limb CMEs, the average angular width of CMEs is about 59°, and about 65% CMEs have a width from 30°to 90°. The statistical result shows that the average angular width of on-disk CMEs is twice wider than that of limb CMEs, which suggests a significant projection effect.2. Some scientific issues related to the CME brightness(1) About 32% of frontside CMEs can not be recognized by SOHO, which may be a natural explanation of high rate of missing alert of geomagnetic storms.(2) The brightness of a white-light CME at any heliocentric distance is roughly positively correlated with its speed, which suggests that the enhanced brightness in coronagraphs is contributed by not only a CME but also the ambient compressed solar wind plasma, and the CME mass derived from the brightness in white-light coronagraphs is probably overestimated.(3) Both projection effect and violent eruption are the major causes of halo CMEs, but for limb halo CMEs, the latter should be the primary one; overall, there is about 25% of halo CMEs stronger than the average level of CMEs.3. Deflections of CMEs in the inner coronaStatistics show that most CMEs deflected towards equator near the solar minimum, and according to the different properties in CME deflection, these deflection behaviors can be classified into three types:the asymmetrical expansion, the non-radial ejection and the deflected propagation.4. The relationship between CMEs and ARs(1) The CME productivity of ARs is strongly related to the AR complexity, but not related to its phase. All the average values of the total area, total magnetic flux, total length and number of polarity inversion lines for CME-producing ARs are almost twice as large as those for CME-less AR. For CME-rich ARs, the average values are even larger.(2) The waiting time of homologous related CMEs is about 8 hours that may probably reflect the time scale of a CME triggering another in the same AR or an AR accumulating free magnetic energy.(3) An AR can produce at most one strong CME (faster than 800 km s-1) within a time interval of 15 hours. (4) There is no evident correlation between CME apparent parameters, such as speed, acceleration and width, with whether CMEs originate from ARs or not. Besides, the statistical results show that there is no evident dependence of CME apparent speed on the AR parameters, however, the CME width manifests a weak correlation with the AR parameters (correlation coefficient is 0.45), and the area and magnetic flux are two most important factors.