ESS200C

ESS200C

Planetary Ionospheres

Lecture 16

Interactions with the Moon

•The Moon has no significantatmosphere and no ionosphere.

•The lunar crust is weakly magnetizedand can deflect the solar wind onlyover limited regions of the surface andonly when these regions are near theflanks.

•Zeroth order interaction is solar windabsorption and closure behind themoon.

•There is a small iron core in the moon,about 400 km in radius.

•This core excludes the magnetotailmagnetic field when the Moon entersthe lobes.

•This effect can be used to measurethe size of the core.

Hybrid Simulations of SolarWind Interaction

IMF in plane of simulation

Hybrid codes allow for kinetic ions, ambipolar electricfields, beaming instabilities

ARTEMIS

Ion Pickup on the Moon

•Ions formed from stationary (inthe lunar frame) atoms andmolecules will be acceleratedby the solar wind electric field.

•Their motion will be a cycloidaldrift (convection plus gyration).

•Different masses have differentgyro radii, and ions producedat one place will reach theMoon’s surface in quitedifferent places.

•Sensitive way of finding lunaroutgassing.

Solar Wind Interaction with a Bodywith an Atmosphere

•The sunlight partially ionizes the dayside atmosphere. Some of this flows tonight 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 SolarWind and Ionosphere

•The solar wind exerts dynamic pressure (ρu2) plus some magnetic andthermal pressure.

•The ionosphere exerts thermal pressure force against the solar wind at theionopause.

•The pressure at the peak of the ionosphere is generally greater than that ofthe solar wind.

•If the standoff distance is well above the collisional region, then themagnetic field will not diffuse into the ionosphere.

Interaction with the Exosphere

•The Venus exosphere has a hothydrogen and a hot oxygencomponent.

•The hot oxygen is produced by thedissociative recombination of O2+

e + O2+ → O* + O*

•Hot oxygen is produced with a varietyof directions and sharing of energy.

•Some atoms are shot upwards to 4000km and ionized in the solar wind.

•These atoms can be ionized byphotoionization, impact ionization andcharge exchange.

•These ions will drift downstream in acycloidal pattern.

Properties of the Ionosphere

•The Venus and Marsionospheres aresimilar but notidentical.

•Venus has higherdensities of atomicoxygen ions.

•At both planets, theion density at highaltitudes is less thanexpected.

Venus Dayside Structure

Weakly Magnetized Ionosphere

•If the gyro-frequency is much lowerthan the collision frequency, ions andelectrons move in the direction of theelectric field or opposite to it. This willproduce a current.

•For typical Venus ionosphericparameters,

•≈ 3x10-7(n/in) S/m (assuming ions areO2+)

•≈ 3x10-3 S/m

•Skin depth

 ≈ 5 1/2 km

Magnetization of the Ionosphere

•The Venus ionosphere recombines at low altitudes. In steady state, theremust be a downward velocity to bring the ions down to where they canrecombine.

•When the ionosphere is at high altitudes, the ionosphere acts as a perfectconductor excluding the magnetic field but flux bundles can sink.

•If the ionopause is pushed downward, diffusion can become fast enough tocreate 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 terminatorfrom day to night and supply the nightionosphere.

•The flow in the ionosphere can transportmagnetic flux bundles both downward andtoward the night side.

•The flux bundles become twisted as theyconvect.

•Some bundles become force-free so that thetwist in the field balances the magneticpressure gradient.

•In a force free rope, the current is parallel tothe magnetic field.

•If j=αB and α is constant, this is called aTaylor state, and the field components (axialand azimuthal) are described by Besselfunctions.

Draped Magnetic Fields

•The plasma closest to theobstacle slows down, but theplasma farther from theobstacle on the same field linekeeps going. This stretches themagnetic field lines.

•At low altitude, the field linesare quite horizontal.

•So-called reverse drapingoccurs on the nightside whenthe field lines remain at lowaltitude.

•The interaction at Titan issimilar to that at Venus eventhough the flow at Titan issubsonic.

Nightside Maps – 30 kHz

Nightside Maps – 100 Hz

Acceleration of Plasma in Tail

•The slowed solar wind plasma and anyplanetary plasma that is picked up isaccelerated by the magnetic forces inthe tail (pressure gradient andcurvature).

•From the average magnetic fieldpattern in the tail, the current can becalculated.

•The force is derivable from jxB.

•Velocity profile can be derived frommapping solar wind electic field intotail.

•The velocity profile and jxB can beused to determine mass density ≈1.6x10-21 kg/m3 ≈ 1 proton/cm3.

Alfven Wings

•When the magnetic field is strongso that the flow is sub-Alfvenic,the field lines bend but do notstrongly drape.

•If the flow across the polar capbecomes very slow perhaps dueto intense mass loading, then theflux tube is essentially frozen tothe moon (e.g. Io) and the otherflux tubes have to move aroundthe Alfven wing field lines rootedto the moon.

•Once the massloaded field linescomplete their traversal of thepolar cap, the field lines attempt tostraighten up. This process can bequite slow and a long trail of bentfield can follow the moondownstream.

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

Cometary Interactions

•When comets approach the Sun,they heat up and outgas.

•The expanding gas decreases indensity and is lost throughphotoionization

where u is the outflow velocity andτ is the photoionization time scale.

•The incoming solar wind picks upthe ions and carries them to andpast the comet.

•The comet can produce a ‘small’field-free region and a largedraped field region around it.

Cometary Interaction

•A numerical simulation of thisprocess shows that the streamlines of the flow do not becomevery diverted but flow almoststraight through the mass-loading region.

•The field lines become drapedand eventually straighten fardownstream.

•Comets also produce lots ofdust. This dust is charged andcan interact with the solarwind.

CME Driven Tail Disruption

Comet Encke taildisruption

Imaged by ConnectionCoronal and HeliosphericInvestigation (SECCHI)Heliospheric Imager-1(HI-1) aboard STEREO