Solar Wind Activity Dependence of the Occurrence of Field-Line Resonance at low Latitudes (L~1.3)

It is known that the field line resonance (FLR below) is caused by hydromagnetic waves in the magnetosphere. The fundamental field line eigenfrequency can be expressed by the magnetic field line length, the magnetic field intensity, and the plasma density at the magnetic field line. We can measure the fundamental field line eigenfrequency by ground-based observation. The field line length and the magnetic field intensity can be calculated from some magnetic field model (such as the IGRF model) of the magnetosphere. Then, it is possible that the plasma density at the magnetic field line is determined by these factors. The final aim of this study is to monitor and study time-dependent changes in the plasmaspheric plasma distributions by using ground magnetic field observations. For this purpose, we are working in the following three research phases. The first phase is to confirm the possibility of identifying FLR at low-latitudes (L~1.3). The second phase is to examine the correlation between FLR and solar wind parameters. The third phase is to estimate the plasma density from the FLR data, and monitor the density in a continuous manner. We are now in the third phase, and we report here the results of the first two phase. In the first phase, in order to investigate features of FLR close to the Earth, we installed three magnetometers in Japan at L~1.3 (at Kawatabi, Zaou, and Iitate), and started observing ULF geomagnetic pulsations. Each adjacent stations are separated in latitude by 50 to 100 km. The magnetic field data from these stations and Kakioka geomagnetic observatory, Japan, were analyzed by using the amplitude-ratio method and the cross-phase method. As a result, we identified FLR events whose frequency decreased with decreasing geomagnetic latitude; we infer that this feature was caused by heavy ion mass loading to low-L field lines. In the second phase, we studied the dependence of the occurrence probability of the above-identified FLR events on solar wind parameters: To investigate the dependence, we used every 20-min of ground data to judge if FLR took place in the 20-min interval. For the same intervals, we recorded maximum values of solar wind parameters such as the dynamic pressure and the IMF (Interplanetary Magnetic Field) intensity. We then studied the dependence of the FLR occurrence on each solar wind parameters (normalized by its background distribution). As a result, we found that the occurrence probability is large at very large values of the IMF intensity and the solar wind dynamic pressure (Psw below), which suggests the following scenario for the generation of FLRs at L~1.3: When the IMF intensity and Psw suddenly increase (in a step-like manner, as in sudden commencements), such a change in the solar wind hits the magnetopause and generates hydromagnetic waves at all frequencies, including the field line eigenfrequency at L~1.3; some of them travel in the magnetosphere to the Earth and cause FLRs. Acknowledgment. Solar wind parameters used in this study were obtained from the Coordinated Data Analysis Web (http://rumba.gsfc.nasa.gov/cdaweb/istp_public/).