634 Encyclopedia of the Solar System
FIGURE 13 Charging processes of meteoroids in interplanetary
space. UV radiation releases photoelectrons, electrons, and ions
from the solar wind plasma, and they are collected; the impact of
energetic particle radiation releases secondary electrons.
of very high solar wind densities does the electron flux to the
particle dominate and the particle gets charged negatively.
The final charging state is reached when all currents to and
from the meteoroid cancel. The timescale for charging is
seconds to hours depending on the size of the particle; small
particles charge slower. Electric charges on dust particles
in interplanetary space have been measured by theCassini
Cosmic Dust Analyzer. These measurements indicate a dust
potential of+5 V. In the dense plasma of the inner Satur-
nian magnetosphere, dust particles at−2 V potential have
been found.
The outward-streaming (away from the Sun) solar wind
carries a magnetic field away from the Sun. Due to the ro-
tation of the Sun (at a period of 25.7 days), magnetic field
lines are drawn in a spiral, like water from a lawn sprinkler.
The polarity of the magnetic field can be positive or negative
depending on the polarity at the base of the field line in the
solar corona, which varies spatially and temporally. For an
observer or a meteoroid in interplanetary space, the mag-
netic field sweeps outward at the speed of the solar wind
(400 to 600 km/s). [SeeTheSun.] In the magnetic refer-
ence frame, the meteoroid moves inward at about the same
speed because its orbital speed is comparatively small. The
Lorentz forceon a charged dust particle near the ecliptic
plane is mostly either upward or downward depending on
the polarity of the magnetic field. Near the ecliptic plane,
the polarity of the magnetic field changes at periods (days
to weeks) that are much faster than the orbital period of
an interplanetary dust particle, and the net effect of the
Lorentz force on micrometer-sized particles is small. Only
secular effects on the bigger zodiacal particles are expected
to occur, which could have an effect on the symmetry plane
of the zodiacal cloud close to the Sun. For nanometer-sized
particles, like the ones that have been found in the dust
streams, the Lorentz force dominates all other forces, and
as a result the particles gyrate about the magnetic field lines
and are eventually convected with the solar wind out of the
solar system.
The overall polarity of the solar magnetic field changes
with the solar cycle of 11 years. For one solar cycle, posi-
tive magnetic polarity prevails away from the ecliptic in the
northern hemisphere and negative polarity in the south-
ern hemisphere. Submicrometer-sized interstellar particles
that enter the solar system are deflected either toward the
ecliptic plane or away from it depending on the overall po-
larity of the magnetic field. Interstellar particles entering
the heliosphere from one direction at a speed of 26 km/s
need about 20 years (two solar cycles) to get close to the Sun.
Therefore, trajectories of small interstellar grains (0.1μm
in radius) are strongly diverted: In some regions of space,
their density is strongly increased; in others, they are de-
pleted. At the time of the initialUlyssesandGalileomea-
surements (1992 to 1996), the overall solar magnetic field
had changed to the unfavorable configuration; therefore,
only big (micrometer-sized) interstellar particles reached
the positions ofUlyssesandGalileo. By 2003, the mag-
netic field had changed to the focusing configuration and
the interstellar dust flux had recovered. [SeeTheSolar
Wind.]3.5 Evolution of Dust in Interplanetary Space
Forces acting on interplanetary particles are compared in
Table 4. The force from solar gravity depends on the mass
of the particle; therefore, it depends on the size asFG∼s^3.
Radiation pressure depends on the cross section of the par-
ticle, henceFR∼s^2. The electric charge on a dust grain de-
pends on the size directly, as does the Lorentz forceFL∼s.
Therefore, these latter forces become more dominating at
smaller dust sizes. At a size comparable to the wavelength of
visible light (s∼0.5μm), radiation pressure is dominating
gravity, and below that size the Lorentz force dominates the
particles’ dynamics. Though gravity is attractive to the Sun,
radiation pressure is repulsive. The net effect of solar wind
interactions on small particles is that they are convected out
of the solar system.
Besides energy-conserving forces, there are also dissipa-
tive forces: the Poynting–Robertson effect and the ion drag
from the solar wind. They cause a loss of orbital energy
and force particles to slowly spiral to the Sun, where they
eventually evaporate. These atoms and molecules become
ionized and are flushed out of the solar system by the solar
wind.
Figure 14 shows the flow of meteoritic matter through
the solar system as a function of the meteoroid size. There
is a constant input of mass from comets and asteroids. From
the intensity enhancement of zodiacal light toward the Sun,
it was deduced that, inside 1 AU, significant amounts of
mass have to be injected by short-period comets into the
zodiacal cloud. While comets shed their debris over a large