Planetary Ionospheres Lecture 16 ESS200C Planetary Ionospheres Lecture 16

Interactions with the Moon 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.

Hybrid Simulations of Solar Wind Interaction IMF in plane of simulation Hybrid codes allow for kinetic ions, ambipolar electric fields, beaming instabilities

ARTEMIS

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.

Solar Wind Interaction with a Body with an Atmosphere 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, so the planet becomes an obstacle to the solar wind flow.

Pioneer Venus Wave Maps

Pressure Balance between Solar Wind and Ionosphere 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 magnetic field will not diffuse into the ionosphere.

Interaction with the Exosphere The Venus exosphere has a hot hydrogen and a hot oxygen component. 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. 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.

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.

Venus Dayside Structure

Weakly Magnetized Ionosphere 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, ≈ 3x10-7(n/in) S/m (assuming ions are O2+) ≈ 3x10-3 S/m Skin depth  ≈ 5 1/2 km

Magnetization of the Ionosphere 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,

Flows and Flux Ropes 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. The flow in the ionosphere can transport magnetic flux bundles both downward and toward the night side. The flux bundles become twisted as they convect. Some bundles become force-free so that the twist in the field balances the magnetic pressure gradient. In a force free rope, the current is parallel to the magnetic field. 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.

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.

Nightside Maps – 30 kHz

Nightside Maps – 100 Hz

Acceleration of Plasma in Tail 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). From the average magnetic field pattern in the tail, the current can be calculated. The force is derivable from jxB. 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.

Alfven Wings When the magnetic field is strong so that the flow is sub-Alfvenic, the field lines bend but do not strongly drape. 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. 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.

Jovian auroral oval and aurorae associated with Jupiter’s interaction with Io, Europa and Ganymede.

Cometary Interactions When comets approach the Sun, they heat up and outgas. The expanding gas decreases in density and is lost through photoionization where u is the outflow velocity and τ is the photoionization time scale. The incoming solar wind picks up the ions and carries them to and past the comet. The comet can produce a ‘small’ field-free region and a large draped field region around it.

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 mass-loading 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.

CME Driven Tail Disruption Comet Encke tail disruption Imaged by Connection Coronal and Heliospheric Investigation (SECCHI) Heliospheric Imager-1 (HI-1) aboard STEREO