BIRKELAND (FIELD-ALIGNED) CURRENTS Magnetometers carried on satellites have detected persistent magnetic perturbations over the auroral zones which can only be interpreted as resulting from currents flowing into and out of the ionosphere. The following figure shows the average locations of these currents as determined from measurements on the TRIAD satellite. The solar wind/magnetosphere interaction provides energy and momentum to the magnetosphere system; the magnetospheric circulation is determined by redistributing its plasma and fields in a way that allows for dissipation of this energy. This dissipation occurs in the form of: • energizing particles which give up their • dispelling blobs of plasma out energy to the neutral atmosphere; the magnetotail; • developing a current system capable of • transferring momentum dissipating energy through ohmic losses; to the neutral atmosphere. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Region 1 and Region 2 Current Systems R2 R2 R2 R1 R1 R2 Quiet Active Current flow is also consistent with the requirement for dissipation of the energy deposited into the magnetosphere by the solar wind; ohmic dissipation of currents in the ionosphere is one way of doing this. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Region-1 Current System The magnetic field lines are highly conducting, and so it is natural that the magnetosphere seeks some closure of current through an ionospheric route. In fact, the so-called Region 1 currents are necessary if we are to require the polar ionosphere to convect with the magnetic field lines: MHD Dynamo Region 1 Configuration ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Ionosphere Closure Currents Pedersen Currents The electric field impressed upon the ionosphere by the solar wind-magnetosphere dynamo moves the ions and electrons in the ionosphere in an ExB direction. Below about 180km the ions experience neutral collisions and move nearly parallel to E. The ion current in the direction of E is divergent ( ,here "H" refers to "horizontal“) and cannot close in the ionosphere and so there must be some current linkage with the magnetosphere -- This ion current is called the Pedersen current whose current carriers are ions ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Ionosphere Closure Currents Hall Currents At altitudes above about 90km, electrons move with the field lines in a quasi-horizontal plane in the ExB direction. This flow differs from the ion motion creating a current that is essentially divergence-free. (Here "e" refers to "electrons"; the above is zero since ). These currents are called Hall currents whose current carriers are electrons. Together, the Hall and Pedersen currents form the ionospheric current system. (see following figure) ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Field-Aligned and Closure Currents in the Ionosphere Horizontal View Vertical View ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Region-2 Current System The Region-2 current system maps to the equatorial plane of the magnetosphere at distances of L~7-10 and the currents are supplied by an excess distribution of charge ASEN 5335 Aerospace Environments -- Magnetospheres

Region-2 Current System

ASEN 5335 Aerospace Environments -- Magnetospheres The origin of the Region 2 currents is connected with a longitudinal asymmetry in the ring current induced by pressure gradients maintained by the dawn to dusk transpolar electric field. This results in a decrease in the ring current as it flows from midnight --> dusk --> noon; it must therefore discharge current (into the conducting ionosphere) on the dusk side. decrease increase Similarly the ring current increases as it flows from noon --> dawn --> midnight; it must therefore draw current from the ionosphere in the dawn sector. ASEN 5335 Aerospace Environments -- Magnetospheres

The Substorm Current Wedge McPherron et al. (1973)

between the plasma sheet The flow of current through the ionospheric circuit is consistent with the collapse of the tail at the neutral point; the latter can only occur if the cross-tail current is substantially reduced, as it is when being diverted along the field lines into the ionosphere. Region 1 currents flow between the plasma sheet and the ionosphere Concurrent with the flow of Region-1 currents into the ionosphere at the onset of a substorm, currents and electric fields spread throughout the conducting ionosphere; even at equatorial latitudes the signatures of “penetration” electric fields are seen. About ~1 hour after storm onset, the Region-2 currents intensify in the ionosphere and set up a counter-potential field, shielding the lower latitudes from electro-dynamic coupling from high latitudes. ASEN 5335 Aerospace Environments -- Magnetospheres

Under Disturbed Conditions Magnetic Merging Occurs Near ~100 RE, and a Plasmoid is Ejected out the Magnetotail Neutral line location under quiescent conditions ~1000 RE Thinning and pinching of plasma sheet upon onset of intensified coupling expel plasmoid Return to pre-substorm conditions ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres VENUS AND MARS Venus and Mars have very weak magnetic fields, and the interaction of the solar wind with these planets is governed by different processes. without an IMF, the flow around the planet is similar to that of fluid flow around a sphere. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres With an IMF that is not steady (as in the solar wind), the interaction of the changing IMF with the conducting ionosphere generates currents that keep the field from penetrating through the body by generating a canceling field: (Faraday's law -- time changing magnetic field induces currents in a conductor). A steady IMF would eventually diffuse into the body (time constant depends on ionospheric conductivity) A bow shock is expected, since the supersonic solar wind plasma is diverted around the body. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres For Venus, pressure balance considerations similar to those for a magnetosphere apply, except with the ionospheric pressure replacing the magnetic pressure. An ionopause is formed when the solar wind pressure balances the ionospheric pressure. The subsolar ionopause of Venus is quite low -- about 350 km, as compared with about 1,000 km at the terminator. The bow shock is at about 2,000 km. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Ionospheric ions can gyrate around the flowing IMF lines, and thus be "picked up" by the solar wind. Ion pickup Integrated over the 4.5 billion year lifetime of the planet, this scavenging by the solar wind has probably had an impact on the evolution of the atmospheres of Mars and Venus. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres bow shock Solar wind O+, O2+ scavenging IMF ionopause NASA Mission: MAVEN – Mars Atmosphere and Volatile EvolutioN To study the loss processes of the Martian atmosphere Launch in late 2013 rV2 ~ npkTp crustal magnetism H+ scavenging Solar wind IMF ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres IMF A Scenario for the Loss of Water on Mars H+, O+, O2+, etc. scavenging photo- ionization  H+, O+, etc. photo- dissociation H2O + hn  H + OH H2 + O Vertical Transport (large-scale dynamics and diffusion) H2O Lower Atmosphere ASEN 5335 Aerospace Environments -- Magnetospheres

Solar Wind and IMF Interactions with Mars’ Ionosphere Courtesy NASA/Nagoya University ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres As shown below, this ion removal process becomes evident in comparisons of measured ionospheric profiles with those obtained from models which do not take into account this process. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Mercury Mercury has a significant magnetic field, but no atmosphere; interaction of the solar wind with Mercury forms a magnetosphere that shares many gross characteristics with that of earth. ASEN 5335 Aerospace Environments -- Magnetospheres

Schematic of Mercurian Magnetosphere and Anticipated Processes ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Launched August 2004 Orbit insertion March 2011 ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Jupiter and Io ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Deep within Jupiter's interior, at extreme pressures and temperatures, hydrogen changes from molecular liquid to a state called liquid metallic hydrogen, an excellent electrical conductor. The liquid metallic hydrogen and the planet's rapid rotation (9 hours 55 minutes) produce dynamo electric currents that generate Jupiter's magnetic field, which is more than 10 times stronger than that of Earth. Scales ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres NASA New Frontiers Mission: JUNO – To study Jupiter’s atmosphere and magnetosphere Launch – August 2011 Jupiter arrival – 2016 Previous Jupiter orbiting mission: GALILEO Launch – October 1989 Jupiter arrival – December 1995 Jupiter demise – September 2003 ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Europa Ganymede Io Callisto 1 ton / sec ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres The weak atmosphere of Io is maintained by continual volcanic eruptions. Neutral atoms in the cloud become ionized. As the magnetosphere rotates with Jupiter, it sweeps past Io, stripping away about a ton of matter per second and forming a torus—a doughnut-shaped ring around Jupiter The torus is predominantly composed of electrified oxygen and sulfur glowing in the ultraviolet. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Jupiter’s Io The reason for the larger value of Jupiter’s subsolar magnetopause is that Io provides a significant source of mass to the Jovian magnetosphere, which in turn accelerates this mass to high velocities because of the rapid rotation of the planet. The resulting centrifugal force and pressure of the plasma pushes out against the solar wind, inflating the magnetosphere and leading to a more distant standoff distance. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres For Jupiter, the actual subsolar magnetopause is found to be over 100 RJ , as opposed to the value of 45 RJ expected from pressure balance arguments. This is due to the effects of the plasma spewing from Io. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres As the heavy ions from Io migrate outward, their pressure inflates the magnetosphere to more than twice its expected size. Some of the more energetic ions fall into the atmosphere along the magnetic field to create Jupiter's auroras. Io Infrared The Io Aurora IR, UV spot images Text -> implications - energy does get to ionosphere - but not just at Io footprint energetic particles bombard atmosphere ‘wake’ emission extends halfway around Jupiter ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Clarke et al. Aurora Polar storms Io wake Main Oval Io footprint ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres NASA Cassini Mission to Saturn Launched in October 1997 and arrived in July 2004 Saturn Magnetosphere ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Additional Slides ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Io 300 km Amirani ASEN 5335 Aerospace Environments -- Magnetospheres

Io’s Volcanoes & Geysers Pilan Plume Prometheus Pilan 5 months apart Pele IR ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Nightside of Io - Visible Glowing Lava Plume Gas & Dust + Airglow ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Ground-based telescopic observations of scattered sunlight from neutral sodium atoms produced by charge exchange of (cold) torus ions Sodium Mendillo et al. ASEN 5335 Aerospace Environments -- Magnetospheres

ASEN 5335 Aerospace Environments -- Magnetospheres Cassini/MIMI observations of hot neutral sulfur from charge-exchange of radiation belt particles S* S+ + O -> O+ + S* Krimigis et al. ASEN 5335 Aerospace Environments -- Magnetospheres