Studies Of Coronal Mass Ejection And Its Space Weather Effect

Coronal Mass Ejection is a large-scale eruption of plasma and magnetic fields from the Sun. It is believed to be the main source of solar energetic particle events and intense geomagnetic storms. So, the study of CME and its space weather is an important area in space physics. To further study the CME and its space effect can help us understanding the space effect much more and improve the prediction level of solar energetic particles and geomagnetic storm. Based on the observation of CME and its relative events, the following three aspects are studied:1. Propagation and Evolution of CMEFirst, we studied the source region of the earth-encountered front-side halo CMEs. It is found that longitude distribution of them not only asymmetry but also depends on the EFHCMEs' transit speed from the Sun to 1AU. The faster the EFHCMEs are, the more westward does their distribution shift, and as a whole, the distribution shifts to the west. Combining the observational results and a simple kinetic analysis, we believe that such E-W asymmetry appearing in the source longitude distribution is due to the deflection of CMEs' propagation in the interplanetary medium. Under the effect of the Parker spiral magnetic field, a fast CME will be blocked by the backgroud solar wind ahead and deflected to the east, whereas a slow CME will be pushed by the following backgroud solar wind and deflected to the west. The deflection angle may be estimated according to the CMEs' transit speed by using a kinetic model.Then, five major CMEs originating from NOAA active region (AR) 808 during the period of 2005 September 7-13 have been analyzed. During this period, the AR 808 rotated from the east limb to near solar meridian. The solar and interplanetary observations suggest that the second and third CMEs, originating from E67°and E47°respectively, encountering the Earth, while the first CME originating from E77°missed the earth, and the last two CMEs, although originating from E39°and E10°respectively, probably only grazed the Earth. Based on the ice cream cone mode[Xue et al., 2005a] and CME deflection model, we find that the CME span angle and the deflection are important for the probability of encountering Earth. The large span angles allowed the middle two CMEs to hit the CME, even though their source location were not close to the solar center meridian. The significant deflection made the first CME totally miss the earth though it also had wide span angle. The deflection made the first CME totaly miss the Earth even though it originated close to the disk center. We suggest that, in order to effectively predict whether a CME will encounter the Earth, the factors of the CME source location, the span angle and the interplanetary deflection should all be taken into account.By analyzed all the front-side CME in 1997-1998 observed by SOHO, it is found that large fraction (132/162, 82%) of CMEs deflected to equator in meridian plane, and almost all the CME except 1 originated from high latitude deflected to equator. The mean deflection angle for all events is～16°and the peak of the continuously distribution appears at the range of 10°-15°. Furthermore, a case study of the CME deflection in meridian plane at near solar space show that such deflection may influenced by backgroud coronal magnetic filed.The propagation and evolution of a west limb CME event from Sun to interplanetary medium was studied by analyzing the STEREO/SECCHI data. The variation of speed, acceleration, angle width, center speed and expand speed of this event are studied. We also study the propagation of its associated prominence.2. Study of solar energetic particle (SEP) eventsGradual solar energetic particle (SEP) events are thought to be produced by shocks, which are usually driven by fast CMEs. The strength and magnetic field configuration of the shock are considered the two most important factors for shock acceleration.Coronal shocks are important structures, but there are no direct observations ofthem in solar and space physics. The strength of shocks plays a key role in shock-relatedphenomena, such as radio bursts and solar energetic particle (SEP) generation. Thispaper presents an improved method of calculating Alfvén speed and shock strength nearthe Sun. This method is based on using as many observations as possible, rather thanone-dimensional global models. Two events, a relatively slow CME on 2001 September15 and a very fast CME on 2000 June 15, are selected to illustrate the calculationprocess. The calculation results suggest that the slow CME drove a strong shock, withMach number of 3.43-4.18, while the fast CME drove a relatively weak shock, withMach number of 1.90 - 3.21. This is consistent with the radio observations, which finda stronger and longer decameter-hectometric (DH) type II radio burst during the firstevent, and a short DH type II radio burst during the second event. In particular, thecalculation results explain the observational fact that the slow CME produced a majorsolar energetic particle (SEP) event, while the fast CME did not. Through a comparisonof the two events, the importance of shock strength in predicting SEP events is addressed.Theoretically, strength and magnetic field configuration of the shock should be unfavorable for producing SEPs in or near coronal holes (CHs). Meanwhile, CMEs and CHs could impact each other. Thus, to answer the question whether CHs have real effects on the intensities of SEP events produced by CMEs, a statistical study is performed. First, a brightness gradient method is developed to determine CH boundaries. Using this method, CHs can be well identified, eliminating any personal bias. Then 56 front-side fast halo CMEs originating from the western hemisphere during 1997 - 2003 are investigated as well as their associated large CHs. It is found that neither CH proximity nor CH relative location manifests any evident effect on the proton peak fluxes of SEP events. The analysis reveals that almost all of the statistical results are significant at no more than one standard deviation,σ. Our results are consistent with the previous conclusion suggested by Kahler that SEP events can be produced in fast solar wind regions and there is no requirement for those associated CMEs to be significantly faster.The interplanetary structures would influence the SEP propagation. We also analyzed the behavior of SEPs in a shock-magnetic cloud interacting complex structure observed by the ACE spacecraft on 2001 November 5, in which a strong shock propagated in a preceding magnetic cloud (MC). It is found that an extraordinary SEP enhancement appeared at the high energy≥10MeV, and extended over and only over the entire period of the shock-MC structure passing through the spacecraft. Such SEP behavior is much different from the usual picture that the SEPs are depressed in MCs. A comparison of this event with other top SEP events (2000 Bastille event and 2003 Halloween event) is made, which shows that such an enhancement leads the shock-MC complex structure to be the producer of the largest SEP event since the solar cycle 23rd. Our analysis suggests that the relatively isolated magnetic field configuration of MCs combined with an embedded strong shock could significantly enhance the SEPs, which are accelerated by the shock and restricted in the MC. Further, we find that the SEP enhancement at lower energies not only happened within the shock-MC structure, but also after it. It is probably due to the presence of a following MC-like structure. This is consistent with the picture that SEP fluxes could be enhanced in the magnetic topology between two MCs, which was proposed based on numerical simulations by Kallenrode and Oliver [2001b].3. Study of geomagnetic stormFirst, we studied 105 geomagnetic storm with a Dst peak value≤-50 nT during1998-2001 to examine the influence of the interplanetary parameters -((VB_z)|-) and its duration At. A new criteria of interplanetary parameters causing geomagnetic storms is found. For moderate storms with Dst_min≤-50 nT, the threshold values are (B_s|-) > 3nT, - ((VB_z)|-)≥1 mV/m andΔt≥1 hour; for intense storms with Dst_min≤-100nT, the threshold values are (B_s|-)≥6 nT, -((VB_z)|-)≥3 mV/m andΔt≥2 hours. It isfound that -((VB_z)|-) is much important than At in creating storms, a stronger- ((VB_z)|-) can produce a more intense storm whereas a long duration can not. An simple empiricalformula: Dst_min = -19.01 -8.43(-((VB_z)|-))^1.09(Δt)^0.30 (nT) with the correlation coefficient of 0.95 is found. From the formula, one can conclude that a compressed southwardmagnetic fields have a more intense geoeffectiveness. We also identify 33 large -((VB_z)|-) intervals with -((VB_z)|-) > 5 mV/m andΔt > 3 hours in the same study interval, and find that they all caused intense storm ( Dst_min≤- 100nT ) and 8/9 of the great storm ( Dst_min≤-200nT ) were due to interplanetary compressed structures.Second, two similar major coronal mass ejections (CMEs) occurring on October 28 and November 18, 2003 are reported. Through the comparison of the two CMEs as well as their interplanetary responses, two primary space weather effects of them, i.e., solar energetic particle (SEP) events and large geomagnetic storms, are studied. The associated solar activities of both CMEs involved at least one large flare, a preceding minor fast CME and an eruption of filament. An extremely intense gradual SEP event was produced by the former CME, but no major SEP event appeared after the latter. However, they both caused a great geomagnetic storm; and the storm created by the latter CME was slightly larger than the former. By analyzing observations of the two CMEs, their associated activities and the corresponding interplanetary magnetic clouds (MCs), the reasons why the two similar major CMEs caused different consequences in the geo-space are addressed. The difference between the two CMEs with respect to SEP events is due to the evident different release rate of energy, and the similarity and difference in geomagnetic storms are related to the MC orientations and the paths along which the Earth intersects the MCs.