Astron. Astrophys. 341, 296-303 (1999)

5. -meteoroids inside and outside the ecliptic

Looking at the set of detected interplanetary particles we try to obtain some direct indication from the data in order to predict -meteoroids. This is done with the help of the impact rate as well as with the help of the distribution of the rotation angle of the spacecraft at the time of the impact. (For the definition of rotation angle see Grün et al, 1993). Fig. 4 shows the impact rate averaged over 50 days together with the error bars based on the Gaussian statistics whereas the solid line represents the smoothing curve. During the first weeks of measurements within the ecliptic we see a high impact rate which may be due to the detection of -meteoroids. In the out-of-ecliptic part of the orbit a slightly increasing impact rate can be observed in the second part of the year 1995 shortly after the large increase of impact rate during the ecliptic passage, an indication that -meteoroids are detected in addition to the usual flux rate. There is no obvious increase of the flux rate in the southern hemisphere.

Fig. 4. The impact rate is shown against the time of impact. The rate is averaged over 50 days. The error bars are calculated with the gaussian statistics. The solid line shows a smoothed curve through the data.

On the basis of the maximum deviation from the solar direction together with the minimum speed condition described before we can identify 25 -meteoroids in the first part of the orbit within the ecliptic, 4 -meteoroids in the southern interval and 19 -meteoroids in the interval of the orbit north from the ecliptic plane.

The first time span can be compared with results from Baguhl (1993). He assumed the mass of particles and the rotation angle of the detector at the time of impact as a criterion for -meteoroids. Particles identified as -meteoroids in this part of the mission are in a small mass interval from to g compared to the total mass interval of the detected particles between and g. This means that -meteoroids are detected in a mass range where particles can have high -values. Both the distributions of the particle mass as well as the distribution of the detector rotation angle for the impacts that we identified as -meteoroids confirm the criteria that Baguhl used in his analysis.

In Fig. 5 the mass distribution of the -meteoroids identified within the ecliptic as well as in the out-of-ecliptic part is presented in comparison to the mass distribution of the interstellar and interplanetary component. In contrast to the interstellar component there is no big difference between the distribution of the -meteoroids and the mass distribution of the remaining interplanetary particles. A more careful study of this difference shows that the mass distribution of these -meteoroids is slighty shifted to smaller particles. The mass values of the particles detected in the out-of-ecliptic part of the mission tend to be higher. While the number of -meteoroids within the ecliptic and within the northern polar passage corresponds nearly to 25 per cent of the detected number of the interplanetary particles at this point of the orbit, the -meteoroids amount only to about 5% of the detected particles within the southern hemisphere. For the latter part the mass values of -meteoroids are slightly higher than in the other mentioned parts.

Fig. 5. The mass distribution of -meteoroids is given in comparison to the mass distribution of the interstellar and interplanetary component (solid line: -meteoroids, dotted line: interstellar component, dashed-dotted line: interplanetary component).

Together with the velocity and the position of the spacecraft we calculate the orbital elements especially the perihelion distances of detected -meteoroids in order to find their place of origin. We assume that the particles reach the detector perpendicularly, i.e. the impact velocity vector is antiparallel to the sensor axis. The uncertainty in the speed determination mentioned above leads to an error area shown in Fig. 6, where the perihelion distances are presented within the ecliptic as well as for the out-of-ecliptic part. Regarding the part within the ecliptic we can conclude the origin of detected -meteoroids to be inside 0.5 AU around the Sun.

Fig. 6. The perihelion distances of detected -meteoroids are calculated under the assumption that the particles reach the detector perpendicularly. The uncertainty in the speed determination leads to an error area. Left side: perihelion distance for -meteoroids detected within the ecliptic; right side: perihelion distance for -meteoroids detected in the out-of-ecliptic part.

For a better determination of the orbital parameters, we, a priori, assume that the particles that were classified as -meteoroids impact only the part of the opening cone, which turns to the Sun. Based on this restricted detection area we determine the perihelion distances of orbits. The resulting distribution for the averaged perihelion distances including the uncertainty in the speed determination shows values smaller than about 0.5 AU, as shown in Fig. 7. However due to the large number of assumptions the distribution should be seen rather as an indication than as a clear experimental result.

Fig. 7. The distribution of the corrected perihelion distances for the out-of-ecliptic part is given including the uncertainty in the speed determination. This correction assumes that particles hit only the part of the opening cone, which turns to the Sun.

© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998
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