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

## 6. Flux of -meteoroids

We determine the flux of -meteoroids according to the following equation:

where represents the time interval of the measurement, A is the effective area of the detector in this time interval and N stands for the number of particles detected within the interval .

Fig. 8 shows the flux of -meteoroids derived from the impact data. The error given for the flux rate is calculated assuming a Gaussian distribution for the number of impacts. The error in the heliocentric distance presents the range where impact data were considered to derive the respective flux rate. The flux rates are between at 1.3 AU and at 2.8 AU whereas a value of about is derived for solar ecliptic latitudes between and . The derived flux of -meteoroids at 1 AU is in agreement with the study of Berg & Grün (1973) which is based on Pioneer data. Our result is also comparable to the interplanetary flux model derived by Grün et al. (1985). The flux of -meteoroids given in the interplanetary flux model for masses from to g reaches a value of about at 1 AU (The other flux values in the literature for Helios 1 and HEOS 2 refer to the apex flux for larger mass values).

 Fig. 8. The flux of -meteoroids that is derived from the impact data presented as a function of the solar distance. The observed flux indicates a decrease to larger solar distances.

The data indicate a decrease of the detected flux of -meteoroids at larger solar distances. This can be explained as a result of the radial direction of the flux. A radial direction of the velocity would lead to a -dependency of the flux rate for the case of constant velocities (r: solar distance). However, the velocity changes with solar distance r and hence the radial dependence of the flux is calculated for different perihelion distances, -values and eccentricities of the parent body. Fig. 9 shows the variation of the flux rate with the solar distance r. It is normalized to the flux rate near 1 AU. The derived decrease of the flux rate with solar distance r is in agreement with the experimental results. Also the higher eccentricity of the orbit of the parent body leads to lower escape velocities and hence -meteoroids with low -values. To produce a similar flux rate assuming higher perihelion distances it requires higher -values compared to the circular orbit. Between 2 and 2.5 AU the enhancement in the flux is probably based on the combined detection of -meteoroids on prograde and retrograde orbits while the decrease in the data for larger solar distances may indicate a lack of -meteoroids in retrograde orbits as will be discussed in the next section.

 Fig. 9. Flux rates for different orbital parameters (here: eccentricity of the parent body , perihelion distance AU) are calculated in relation to the flux rate near 1 AU (small dashed line: , dashed-dotted line: , dashed-dotted-dotted line: , big dashed line: ).

© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998