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The smallest grains highest charge-to-mass ratio get filtered out and are carried around the heliosphere. More research is needed to get this clear. The ISD grains that do pass the heliopause may still be filtered out from the inner solar system by solar radiation pressure force and Lorentz forces.

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The effect of focusing and defocusing w. Landgraf modeled the secondary filtering and compared the results with the measured Ulysses data from — Here, we go one step further and investigate the effect of this filtering on the size distribution of the dust. We start with the simple case of solar radiation pressure and solar gravity alone Sect. In Sect. To demonstrate this effect, we evaluated the ISD density Eq.

In the incoming portion up to about —5 AU, the relative density for all sizes is about 1, i. The variation in the relative mass distribution is shown in Fig.

Well in front of the Sun the density variation as function of particle mass is small. However, it becomes significant around and behind the Sun. The enhancement of particles smaller than 10 kg will be reduced or enhanced by electromagnetic interactions see below.

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The Sun is at distance 0 AU. The densities correspond to the mean densities in a 1 AU column around the flow axis. The density lines for different particle masses are offset by a factor Density 0 was set to the bottom of the diagram at 0. The blue lines denote the plane of density level of 1. In Sects. Below we sketch only the most important facts of this interaction.

The outwardly streaming solar wind carries a magnetic field away from the Sun, causing the Lorentz force on the charged particle. The net effect of the Lorentz force on micron-sized particles is weak compared to gravity and radiation pressure forces. For sub-micron-sized interstellar particles, the Lorentz force becomes stronger and has a large effect on their trajectories. The overall polarity of the solar magnetic field changes with the solar cycle of 11 years Table 2, Paper I.

For one solar cycle positive magnetic polarity prevails in the northern and negative polarity in the southern solar hemisphere. In Paper I we modeled the interplanetary magnetic field and calculated the trajectories of interstellar grains. Sub-micron-sized interstellar particles that enter the solar system are either deflected toward the solar equatorial plane that is close to the ecliptic plane or away from it depending on the overall polarity of the magnetic field. In Fig. Each point in this diagram represents a particle of a specific material characterized by size, composition, and structure density or fluffiness.

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A more absorbing material e. Table 2. In Appendices B and C and Fig. In this paragraph we analyze how the size distribution of our reference particles solid thick line in Fig. For an initial reference size distribution we show the number density and the plain enhancement or reduction factor that can be applied to any size distribution we choose. The filtering at the heliopause is ignored. The crosses in Fig. Far upstream at —10 AU, the number density is reduced in the defocusing phase of the solar cycle for all grain sizes year top row of Fig.

During the defocusing phase, the smallest grains get filtered out e. Upstream —10 to —3 AU in Fig. Even if reduction factors of 10 or more are present for the smallest grains, they still outnumber the largest grains in the number density! The different colors indicate different epochs in the solar cycle.

The black straight line is the reference distribution range for which simulations were done. We apply the size filtering to three cases: to Saturn ca. In these cases we also demonstrate what variation due to the solar cycle can be expected. The left plot corresponds to the defocusing phase, and the right plot corresponds to the focusing phase of the solar cycle for the asteroid and Jupiter orbits. We applied the method of Sect.

Here, the flux variations in time and location in the solar system are correlated through the orbit of Saturn. We looked at the size distribution at Saturn during one orbit between and and chose eight positions along its orbit relative to the ISD flux: upstream, downstream, sidestreams, and four positions in between.

Figure 5 shows the orbit of Saturn in this period. The eight blue dots on the orbit show the eight positions for which the relative fluxes and size distributions are predicted Figs.

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The relative fluxes with respect to Saturn throughout the solar cycle are shown in Fig. The biggest particles 0. The enhanced fluxes factor 2 around are due to the motion of Saturn against the interstellar dust flow. Conversely the flux is reduced around when Saturn moves parallel to the interstellar flow. The flux enhancements of smaller interstellar grains around become even more pronounced for smaller grains 0. Sharp peaks in the small particle flux appear around when Saturn is in the upstream region of the interstellar dust flow. The modulation of the size-dependent flux ranges from total disappearance of some particle sizes to enhancements of up to factor This is visible around the year when only the biggest particles 0.

At favorable times to the mass flux will be highest not for the biggest particles as for the undisturbed reference distribution but for the mid-sized grains around 0. In the small grains contribute very clearly if they are not filtered out at the heliopause.

Depending on that filtering, a final number for the small grains can be given. In , , and later in the focusing phase, and in even in the onset of the defocusing phase , there are high fluxes for small grains, owing to mirroring and focusing upstream from the Sun. Therefore we applied the simulated dust flux to the case of Jupiter for two orbital periods from until Fig. Figure 8 shows the simulated time variation of the relative flux of interstellar grains of nominal composition between and at and relative to Jupiter.

This is because Jupiter is at the gravitational focusing downstream of the Sun. Between these peaks and gaps, there is a cyclic variation that doubles or halves the relative flux depending on whether Jupiter moves with or against the stream of ISD. The density increases slightly in front of the exclusion zone.

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As the grain size decreases, the influence of the solar cycle becomes more dominant. Filtering at the heliopause is not taken into account. The gravitational focusing in is clear for the largest particles, as is the strong influence of the solar cycle on the size distributions, especially for the smaller grains cf. The last part of the cruise phase is perfectly suited to some add-on science: the ISD flux is high due to the focusing phase of the solar cycle, and Jupiter is moving into the stream of ISD grains.

The gravitational focusing in and is clear for the largest particles. The influence of the solar cycle on the size distributions is visible, especially for the smaller grains cf.

Galileo was launched in and reached Jupiter in , which it orbited until During the orbital tour of the Galileo mission — the conditions for measuring interstellar dust were quite unfavorable because the solar cycle was in the defocusing condition and Jupiter moved approximately parallel with the interstellar dust flow, reducing both the relative speed and the flux of interstellar grains Fig. Besides, there were also pointing issues: the antenna was directed toward the earth, and the dust counter was thus pointing in the opposite direction from ISD stream, at least in the downstream part of the orbit.

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Besides this, the mission is planned for the focusing phase of the solar cycle, therefore JUICE is perfectly suited to doing some add-on dust science. The conditions are very favorable for measuring interstellar grains on top of Jupiter system dust. Figure 10 shows the derived size distributions of the ISD at Jupiter, at different epochs corresponding to eight positions of Jupiter along two subsequent orbits starting in cf. These distributions are — as in Sects. Very clear is that the mass and number density vary by large factors with the phase of the solar cycle and the orbital position of Jupiter.

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The size distribution for shows two points for the larger grains 0. During the defocusing periods the fluxes are at least a factor 3 lower. In this section we discuss the variation in the size-dependent ISD flux in the main asteroid belt.