10 100 Impact Velocity [km/s]
Figure 2. Velocity and mass of detected particles. Filled marks are impacts in 1998 (circum-terrestrial phase) and open marks denote impacts in 1999 (cruise phase). The detected velocity range is 2 to 100 km s"1 and the mass range is 3xl0"19 to 10"12 kg, which correspond well to MDC-HITEN .
velocity, would be explained by interstellar particles since the MDC aperture avoids particles from the solar direction such as beta meteoroids. The direction of interstellar particles (Figure 3) should correspond to the relative direction of the solar system against the ambient gas cloud with relative velocity as high as 26 km s'1 . Among the high-velocity particles, two should correspond to the typical interstellar particles. One example of dust impact is shown in Figure 1(b). Here, high velocity is characterized by a sharp rise of charge signals. Moreover, a couple of high-velocity particles have neither Keplerian or interstellar gas directions. They might be ascribed to unknown interstellar sources. As seen in Figure 3, when NOZOMI is around -x axis, i.e., around the vernal equinox, the relative velocity between NOZOMI and the interstellar flow is higher than 50 km s"1. This would be the reason why MDC detected more high-velocity particles around this season. A change of detection rate in 1999 (15 in the first half and 8 in the latter half) is probably due to the change of MDC aperture direction, since NOZOMI's spin axis is directed always towards the Earth for the telemetry.
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