Studies On Geoeffectiveness Of Coronal Mass Ejections And Near-Earth Space Environment

Coronal mass ejections (CMEs) are considered by many space scientists as the main source of space weather. Understanding the relationship between geoeffective CMEs and their interplanetary counterparts (ICMEs), and predicting whether a halo CME can encounter the Earth are the important topics in space weather research. On the other hand, the near-Earth space (20-200km) as the ending response for space weather, is highly relative with our everyday living and has become an key region in solar-terrestrial studies. In this thesis, we emphasize on the source and the end of space weather, the following two aspects are studied observationally and theoretically:1. CMEs and their geoeffectivenessFirstly, we set up a 3D geometric model for CMEs using an Ice-Cream Cone to analyze the geometrical and kinematical properties of CMEs. Assuming in early phase, CMEs propagate with near constant speed and angular width, some useful properties of CMEs, such as the radial speed (v), the angular width (α) and the location at the heliosphere, can be obtained. This model is improved by (1) using an ice-cream cone to show the near real configuration of a CME, (2) determining the radial speed via fitting the projected speeds calculated from the height-time relation in different azimuthal angles, (3) not only applying to halo CMEs, but also applying to non-halo CMEs.Secondly, We analyze five major CMEs originating from NOAA active region (AR) 808 during the period of September 7 to 13, 2005, when the AR 808 rotated from the east limb to near solar meridian. Several factors that affect the probability of the CMEs' encounter with the Earth are demonstrated. The solar and interplanetary observations suggest that the 2nd and 3rd CMEs, originating from E67°and E47°respectively, encountered the Earth, while the 1st 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 our ice-cream cone model and CME deflection model, we find that the CME span angle and deflection are important for the probability of encountering Earth. The large span angles allowed the middle two CMEs hit the Earth, though their source locations were not close to the solar central meridian. The significant deflection made the first CME totally miss the Earth though it also had wide span angle. The deflection may also have made the last CME nearly miss the Earth 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.Thirdly, we try to illustrate what kind of CMEs and their interplanetary counterparts are the most geoeffective and the mechanism of causing such notable geoeffec-tiveness by analyzing the solar and interplanetary causes of 8 great geomagnetic storms (Dst≤-200nT) during the solar maximum (2000-2001). We find the magnetic clouds (MCs) play an important role in causing great geomagnetic storms, at the same time, the result also shows that the notable characteristic among the causal mechanism is compression of the southward magnetic fields. Six of eight great geomagnetic storms were associated with the compression, which can be classified into (1) the compression between ICMEs (2)the compression between ICMEs and interplanetary medium. It suggests that the compressed magnetic field would be more geoeffective. Half of all great storms were related to successive halo CMEs, most of which originated from the same active region. The interactions between successive halo CMEs usually can lead to greater geoeffectiveness by enhancing their southward field Bs interval either in the sheath region of the ejecta or within MCs. The types of them included: the compression between the fast speed transient flow and the slow speed background flow, the multiple MCs, besides shock compression. Further, the linear fit of the Dst versus (—(VB_z)|─)^α(Δt)^βgives the weights of—(VB_z)|─andΔt asα= 2.51 andβ= 0.75 respectively. This may suggest that the compression mechanism, with associated intense B_S, rather than duration, is the main factor in causing a great geomagnetic storm.2. Observations of the near-Earth space and studies of atmospheric tidesFirstly, we introduce a newly installed Mie-Rayleigh-Na fluorescence lidar system in USTC at Hefei. This lidar system employs dual-wavelength laser at 532nm and 589nm and has three transmitting and receiving channels for detecting sodium density at 80-110km, atmospheric density and temperature at 30-70km, and aerosol extinction coefficient below 30km. We illustrate the typical techniques of the lidar system in detail and summarize the lidar equations for back-scatter detecting lidars. In the past one year after the lidar was set up, we have carried on routine observations of sodium layer over Hefei, the characteristics of the shape, nocturnal variations and seasonal variations of sodium layer are given in this part. The results show that during the night, there are evident wave activities in sodium layer and sporadic sodium layers often occur. Sodium abundance reaches maximum value 6.014×10⁹cm^-2 during December approximately 5 times larger than the June minimum value 1.126×10⁹cm^-2. The abundance shows rather broader minimum values throughout the summer months, the centroid height has no evident seasonal variations, but the RMS width illustrates semi-annual variations.Secondly, we use Canonical Correlation Analysis (CCA) method to investigate the mesosphere and low thermosphere (MLT) diurnal tidal winds during the year 2002 ob- served by a newly installed meteor radar at Wuhan (306°N, 114.4°E). In general, 6 effective diurnal tidal pairs of patterns are obtained, which represent over 90% total variances of the origin data set. These patterns are expected to correspond to the atmospheric oscillations of diurnal frequency band excitated or modulated by different sources, namely, the seasonal variations and the modulations by semi-annual-like variations, solar 27-day rotation and the planetary wave oscillations. Among all of the patterns, the 1st pattern is the most notable which represents 40% of total variances and the amplitudes of the 1st pattern show maximum values in spring and autumn as well as sudden phase-transit near equinox month, which is in line with the results obtained from traditional harmonic analysis. The vertical wavelengths (～30km) suggest the classic tidal mode S(1,1) is dominant, and the preceding phases (～ 5 — 6h) of the meridional components of the diurnal tidal wind show right rotating circular polarization maybe the main characteristic in tidal wind. For the semi-diurnal tides, 4(3) effective semi-diurnal tidal pairs of patterns are obtained, representing ～2/3 total variances. They also correspond to the seasonal variations, the modulations by the planetary wave oscillations or the solar 27-day activity. However, no semi-annual variations is found in semi-annual canonical correlation patterns. The 1st pattern is still the most notable. Its amplitudes show maximum values in spring and autumn, and the vertical wavelengths are longer in summer and shorter in winter. The vertical wavelengths of the higher order patterns (～ 50km) suggest the classic semi-tidal mode S(2,4)/S(2,5) is dominant.