Current State of Knowledge of the Magnetospheres of Uranus and Neptune

28 July 2014

Abigail Rymer [email protected] What does a ‘normal’ magnetosphere look like?

. N-S dipole field perpendicular to SW . Solar wind driven circulation system . Well defined central tail plasmasheet . Loaded flux tubes lost via substorm activity . Trapped radiation on closed (dipolar) field lines close to the planet. …but then what is normal…. . Mercury  Not entirely at the mercy of the solar wind after all

. Jupiter . More like its own little solar system . Sub-storm activity maybe totally internally driven …but then what is normal…. Saturn

Krimigis et al., 2007 What is an ice giant (as distinct from a gas giant) planet? Consequenses for the magnetic field

Soderlund et al., 2012 Consequenses for the magnetic field

Herbert et al., 2009 Are Neptune and Uranus the same?

URANUS NEPTUNE Equatorial radius 25 559 km 24 622 km

Mass 14.5 ME 17.14 ME Sidereal spin period 17h12m36s (retrograde) 16h6m36s Obliquity (axial tilt to ecliptic) -97.77º 28.32 Semi-major axis 19.2 AU 30.1 AU Orbital period 84.3 Earth years 164.8 Earth years

Dipole moment 50 ME 25 ME Magnetic field Highly complex with a surface Highly complex with a surface field up to 110 000 nT field up to 60 000 nT Dipole tilt -59º -47

Natural satellites 27 (9 irregular) 14 (inc Triton at 14.4 RN) Spacecraft at Uranus and Neptune – Voyager 2 Voyager 2 observations - Uranus

Stone et al., 1986 Voyager 2 observations - Uranus

Voyager 2 made excellent plasma measurements with PLS, a Faraday cup type instrument.

Bridge et al., 1986 Cassini at Saturn – for analogy Investigating Magnetospheric Dynamics

After Green et al., 2004 Voyager 2 observations - Uranus

Cheng et al., 1991 “Shishi Odoshi” Effect Controls the Frequency of Filling and Flushing Saturn’s Magnetosphere

Cassini data have shown that Saturn’s magnetosphere fills with material from Enceladus’ jets and Saturn’s rings. Some of this mass becomes ionized by sunlight and migrates to Saturn’s night side where it stretches out the magnetotail and is shed from the system. With the mass unloaded, the magnetosphere elastically returns to its co-rotating flow around the planet. The period is estimated to take 8 to 31 hours. New research suggests that this cycle may speed up by hours when, for example, Enceladus is more active, or near the Saturnian solstice (when more of the rings are in sunlight) or near solar maximum (when the Sun is brightest). A “shishi odoshi” collects dripping water and when full, tips to empty its load. It returns to its resting state for the process to repeat. A faster flow of water means the bamboo tips more often.

‘Mass-unloaded’ state

Mass-loaded state

A greater rate of mass flow into Saturn’s magnetosphere will increase the frequency of mass unloading and restoration of the magnetosphere to its “refill” state. “Saturn’s Magnetospheric Refresh Rate” A.M. Rymer, D.G. Mitchell, T.W. Hill, E.A Kronberg, N. Krupp and C.M. Jackman, 2013. Geophysical Review Letters, 40, 2479- 2483. Plasma Circulation Mechanisms

. First proposed by Pontius and Hill [1989] to explain Voyager observations at Jupiter. . Introduced as a mechanism applicable to the Earth by Pontius and Wolf [1990]. . Observed to be a prolific feature of Saturn’s magnetosphere [e.g. Hill et al., 2005]

E0

after Pontius and Wolf, 1990

Angelopoulos et al., 1992 and Baumjohann et al. 1990 showed that at the Earth the apparent steady sunward convection of the plasma sheet could, in reality, be a superposition of bursty high speed flows with intermittent intervals of near stagnant plasma and that small bubbles could accomplish earthward mass, energy and flux transport comparable with that expected from “stead state” convection. Magnetospheric circulation at the outer planets. Voyager 2 observations - Uranus

Krimigis et al., 1986 Voyager 2 observations – Uranus

. Uranus at sostice during V2 flyby (1986) . M’spheric reconfigures from LHS to RHS in 9 hours

Bagenal, review 1992

. Simulations show a helical tail, or dual plasma sheet configuration.

Toth et al., 2004 Voyager 2 at Uranus – Summary (of Uranus at Solstice)

. Solar wind driven magnetosphere, [Selesnick and McNutt, 1987] . Strong dynamic injection phenomena [Mauk et al., 1987; Belcher et al., 1991] . Whistler/chorus plasma wave emission more intense than Voyager observed at any other planet [Kurth and Gurnett, 1991] . Radiation belt electrons as strong as those observed during supermagnetic storms at Earth [Mauk and Fox, 2010]

• And yet…

. Auroral emissions with the high powers and ordered (ringed) structures of the sort observed at Earth, Jupiter and Saturn were not observed at Uranus. [Herbert and Sandel, 1994; Herbert, 2009] . (Only Neptune displayed weaker aurora.) Voyager 2 observations – Neptune

Neptune’s magnetic configuration changed from LHS to RHS in ~ 8 hours

Bagenal, review 1992 Neptune plasma – Voyager 2 (Richardson et al., 1992)

. Plasma was observed to be super-corotational – this is unique to Neptune. Electrons – Neptune and Saturn, hot versus cold

Young et al., 2004 Nitrogen – Neptune and Saturn, Triton versus Titan

Nitrogen at Saturn Smith et al., private comm Planetary dipole fields… Summary and musings

. An ice giant is as different to Jupiter and Saturn as they are to terrestrial planets. . The difference is markedly manifest by a very complex magnetic field.  (Is the asymmetrical field a product of the mass/formation? Why the peculiar spin/tilt?) . We can’t study the consequences of the complex field geometry by studying either of Jup/Sat – or any other planets.  (How do we know that the majority of exoplanets are ice giants? Why are the majority of exoplanets ice giants?) . How the magnetosphere loads/unloads and how the radiation belts are formed in these peculiar magnetic environments is an enormous question that will have relevance to our understanding of radiation belt formation at Earth – an area that remains highly controversial. . Uranus and Neptune are very similar, but with fundamental differences due to the Uranus tilt and the Neptune captured Kuiper belt object, Triton. . It is difficult to say a mission to either Neptune over Uranus would be more informative from a magnetospheric prospective – either would provide ground breaking science – both would be complicated to interpret.