Transcript Planetary_Ionospheres_Lecture16

ESS200C
Planetary Ionospheres
Lecture 16
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Interactions with the Moon
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The Moon has no significant
atmosphere and no ionosphere.
•
The lunar crust is weakly magnetized
and can deflect the solar wind only
over limited regions of the surface and
only when these regions are near the
flanks.
•
Zeroth order interaction is solar wind
absorption and closure behind the
moon.
•
There is a small iron core in the moon,
about 400 km in radius.
•
This core excludes the magnetotail
magnetic field when the Moon enters
the lobes.
•
This effect can be used to measure
the size of the core.
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Hybrid Simulations of Solar
Wind Interaction
IMF in plane of simulation
Hybrid codes allow for kinetic ions, ambipolar electric
fields, beaming instabilities
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ARTEMIS
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Ion Pickup on the Moon
• Ions formed from stationary (in
the lunar frame) atoms and
molecules will be accelerated
by the solar wind electric field.
• Their motion will be a cycloidal
drift (convection plus gyration).
• Different masses have different
gyro radii, and ions produced
at one place will reach the
Moon’s surface in quite
different places.
• Sensitive way of finding lunar
outgassing.
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Solar Wind Interaction with a Body
with an Atmosphere
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The sunlight partially ionizes the dayside atmosphere. Some of this flows to
night side.
The solar wind is absorbed by the planetary atmosphere.
If the solar wind is magnetized, it cannot immediately enter the ionosphere,
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so the planet becomes an obstacle to the solar wind flow.
Pioneer Venus Wave Maps
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Pressure Balance between Solar
Wind and Ionosphere
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The solar wind exerts dynamic pressure (ρu2) plus some magnetic and
thermal pressure.
The ionosphere exerts thermal pressure force against the solar wind at the
ionopause.
The pressure at the peak of the ionosphere is generally greater than that of
the solar wind.
If the standoff distance is well above the collisional region, then the
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magnetic field will not diffuse into the ionosphere.
Interaction with the Exosphere
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The Venus exosphere has a hot
hydrogen and a hot oxygen
component.
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The hot oxygen is produced by the
dissociative recombination of O2+
e + O2+ → O* + O*
•
Hot oxygen is produced with a variety
of directions and sharing of energy.
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Some atoms are shot upwards to 4000
km and ionized in the solar wind.
•
These atoms can be ionized by
photoionization, impact ionization and
charge exchange.
•
These ions will drift downstream in a
cycloidal pattern.
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Properties of the Ionosphere
• The Venus and Mars
ionospheres are
similar but not
identical.
• Venus has higher
densities of atomic
oxygen ions.
• At both planets, the
ion density at high
altitudes is less than
expected.
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Venus Dayside Structure
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Weakly Magnetized Ionosphere
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If the gyro-frequency is much lower
than the collision frequency, ions and
electrons move in the direction of the
electric field or opposite to it. This will
produce a current.
For typical Venus ionospheric
parameters,
  02pi /in
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≈ 
3x10-7(n/in) S/m (assuming ions are
O2+)
≈ 3x10-3 S/m
Skin depth

 ≈ 5 1/2 km
1/2
 1/20 

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Magnetization of the Ionosphere
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The Venus ionosphere recombines at low altitudes. In steady state, there
must be a downward velocity to bring the ions down to where they can
recombine.
When the ionosphere is at high altitudes, the ionosphere acts as a perfect
conductor excluding the magnetic field but flux bundles can sink.
If the ionopause is pushed downward, diffusion can become fast enough to
create a conveyor belt of magnetic flux that magnetizes the ionosphere.
In steady state,
B
 B 
0
D
 ( Bu h )
t
h h h
where
D
me (en  ei )
ne e 2 0
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Flows and Flux Ropes
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In a 1-D treatment, flow is either up or down,
but in 3-D, flow can go over the terminator
from day to night and supply the night
ionosphere.
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The flow in the ionosphere can transport
magnetic flux bundles both downward and
toward the night side.
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The flux bundles become twisted as they
convect.
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Some bundles become force-free so that the
twist in the field balances the magnetic
pressure gradient.
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In a force free rope, the current is parallel to
the magnetic field.
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If j=αB and α is constant, this is called a
Taylor state, and the field components (axial
and azimuthal) are described by Bessel
functions.
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Draped Magnetic Fields
• The plasma closest to the
obstacle slows down, but the
plasma farther from the
obstacle on the same field line
keeps going. This stretches the
magnetic field lines.
• At low altitude, the field lines
are quite horizontal.
• So-called reverse draping
occurs on the nightside when
the field lines remain at low
altitude.
• The interaction at Titan is
similar to that at Venus even
though the flow at Titan is
subsonic.
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Nightside Maps – 30 kHz
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Nightside Maps – 100 Hz
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Acceleration of Plasma in Tail
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The slowed solar wind plasma and any
planetary plasma that is picked up is
accelerated by the magnetic forces in
the tail (pressure gradient and
curvature).
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From the average magnetic field
pattern in the tail, the current can be
calculated.
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The force is derivable from jxB.
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Velocity profile can be derived from
mapping solar wind electic field into
tail.
The velocity profile and jxB can be
used to determine mass density ≈
1.6x10-21 kg/m3 ≈ 1 proton/cm3.
1540
Vx (X) 
11.530.68X
 Vx (X) [km/sec]


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Alfven Wings
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When the magnetic field is strong
so that the flow is sub-Alfvenic,
the field lines bend but do not
strongly drape.
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If the flow across the polar cap
becomes very slow perhaps due
to intense mass loading, then the
flux tube is essentially frozen to
the moon (e.g. Io) and the other
flux tubes have to move around
the Alfven wing field lines rooted
to the moon.
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Once the massloaded field lines
complete their traversal of the
polar cap, the field lines attempt to
straighten up. This process can be
quite slow and a long trail of bent
field can follow the moon
downstream.
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–Jovian auroral oval and aurorae associated with
Jupiter’s interaction with Io, Europa and Ganymede.
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Cometary Interactions
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When comets approach the Sun,
they heat up and outgas.
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The expanding gas decreases in
density and is lost through
photoionization
Qo
r
Q  2 exp( )
R
u
where u is the outflow velocity and
τ is the photoionization time scale.
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The incoming solar wind picks up
the ions and carries them to and
past the comet.
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The comet can produce a ‘small’
field-free region and a large
draped field region around it.
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Cometary Interaction
• A numerical simulation of this
process shows that the stream
lines of the flow do not become
very diverted but flow almost
straight through the massloading region.
• The field lines become draped
and eventually straighten far
downstream.
• Comets also produce lots of
dust. This dust is charged and
can interact with the solar
wind.
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CME Driven Tail Disruption
Comet Encke tail
disruption
Imaged by Connection
Coronal and Heliospheric
Investigation (SECCHI)
Heliospheric Imager-1
(HI-1) aboard STEREO
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