Statistic Study On Coronal Mass Ejections In Cycle 23

Coronal Mass Ejections (CMEs), the large-scale eruptions of plasma and magnetic fields from the Sun , are believed to be the main sources of the strong interplanetary disturbances that cause many moderate to intense geomagnetic storms . So, the study on CMEs is an important topic that relates directly to space environments. Since 1996 January, The Large Angle and Spectromet-ric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) mission has observed the most CMEs, providing us with a great opportunity to examine the properties of CMEs. Based on SOHO/LASCO observation, we study the statistical properties of CMEs. The major results are obtained as follows:1. We study the phase relation of the sunspot cycle and the coronal mass ejection events respectively at low (less than 50°) and high (larger than 60°) latitudes in detail. We find that the activity of CME events respectively at low and high latitudes all have the same phase as the sunspot activity. We also study the distribution of the CME events from 1996 January to 2005 October on the solar disk. We find that the CME events have no latitudinal drift within a CME cycle. The reason is suggested to be that the sunspots are the activities on the AR scale but the CMEs are that on the large-scale, and they are both the manifestation of magnetic structures destabilized on the corresponding scales.2.We analyze the distributions of speeds (v) determined for 10513 coronal mass ejections(CMEs) observed by the Large Angle and Spectrometric Corona-graph Experiment on board the Solar and Heliospheric Observatory(SOHO/LASCO) in cycle 23 from 1996 to 2005, respectively at low and high latitudes. The ln(υ) distributions of CME events respectively at high and low latitudes are both found to a good approximation they can be fitted with a normal distribution. The fitting curves of the ln(υ) distributions for CMEs respectively at high and low latitudes are almost identical, implying that there is statistically no physical distinction between CME events respectively at high and low latitudes, and fur- nishing evidence to support that CMEs are intrinsically associated with source magnetic structures on a large spatial scale, but CMEs at high latitudes, statistically, are apparently slower than those at low latitudes. Respectively at high and low latitudes, the speed distributions respectively for accelerating and decelerating CME events are nearly identical and also to a good approximation they can be both fitted with a normal distribution. It implies that, statistically, there is no physical distinction between accelerating and decelerating CME events, even if CMEs are divided into those respectively at high and low latitudes, augmenting the results obtained by Yurchyshyn et al.3.The latitudinal distribution of Coronal Mass Ejections in Cycle 23, from September 1996 to June 2006, has been analyzed. CMEs occur at all latitudes and are most common at low latitudes. This should furnish evidence to support that CMEs are associated with source magnetic structures on a large spatial scale, even with transequatorial source magnetic structures on a large spatial scale. The latitudinal distributions of CMEs respectively in the northern and southern hemispheres are no difference from the statistical point of view. Through calculating the actual probability of the hemispheric distribution of the activity of the CMEs, we find that a southern dominance of the activity of CMEs is shown to occur in cycles 23 from September 1996 to December 2005, which was predicted by Atac and Ozguc .The conclusion confirms that hypothesis about a 12 cycle periodicity is valid. The speed distributions respectively in the northern and southern hemispheres are to a good approximation they can be fitted with a single lognormal distribution. The speed distributions respectively in the northern and southern hemispheres are no difference from the statistical point of view. This finding implies that, statistically, there is no physical distinction between the CME events in the southern and northern hemispheres, and a same mechanism of a nonlinear nature is acting in both the CME events in the southern and northern hemispheres.