| dc.description.abstract | In this paper, we use the plasma density and velocity data from the IPEI payload onboard the ROCSAT-1 satellite in 2000 to statistically analyze the occurrence percentage of Fast Rising Bubble (FRB) with upward velocity > 400 m/s. We group the FRB events under different Kp conditions and examine the dependencies of the percentage occurrence on season, geographical location, local time, and scale size of the FRB structures. We also study the spectral characteristics of four cases of FRB event by using the high-resolution plasma data (1024 Hz) from IPEI.
According to the analysis results, the occurrence percentage of FRB is higher in the spring and fall seasons and also near the Atlantic Ocean longitude sector. The percentage of observing small-scale structure is increased as the rising speed of the bubble increases. The rising speed of the bubbles is not strongly affect the seasonal, local time, and geographical distributions of the FRB events, but is related to the geomagnetic disturbances. During the quiet time, the FRB events occur more often between 18:00 LT and 21:00 LT, and the locations of the FRB events are near the geomagnetic equator. During the storm time, the occurrence percentage of FRB increases in post midnight hours, and the presence of FRB is not restricted in the geomagnetic equator region. We have examined some FRB events during the storm time. The results show some interesting properties that the plasma density of FRB is highly variable and dominated by small-scale structures with high rising speed ( 1000 m/s) and that the FRB widely spreads in the space. Those properties are probably caused by the perturbed eastward electric fields during the storm time.
According to the spectral analysis of four FRB events, we found that most of the spectral power density (PSD) of the ion density decreases with increasing wave number, which is consistent with the power law ( ). The statistical result shows that the average spectral indexes (n1) of the middle-scale (1 km ~ 100 m) density wave are -1.87 and -1.62, respectively, for the quiet time and for the storm time, while for the small-scale (< 100m) density wave the indexes (n2) are -5.16 in the quiet time and -4.66 in the storm time.
The characteristics of the FRB velocity spectrum are quite different from the density wave. Specifically, during the storm time, we find that the spectrum of the FRB with speed greater than 2000 m/s does not obey the power law and the small-scale waves dominate the spectral structures. The mechanisms of these spectral features are an interesting subject for future study. | en_US |