StarMariner.net Magnetospheres

Space probes where sent to visit the outer solar planets to monitor magnetic fields and other electromagnetic properties. As a bonus the on board devices detected a phenomenon now known to be the solar wind . Solar wind is Ionised gas that interacts with the Suns magnetic field to form plasma, the fourth state of a matter. The five states of matter being Solid,Liquid Gas ,Plasma and B.E.C.'s .
The solar wind spirals outward from the sun sweeping across into the path of each planet in our solar system and beyond. When it interacts with a planets magnetic fields, physical reactions occur that help us humans back on Earth determine the chemical composition of its atmosphere, the speed of rotation, its magnetic dipole and more.
With each new discovery new devices are engineered to detect these properties with higher accuracy, but we have to reverse engineer how the planets magnetic fields are made, so as to understand how there behaviour would possibly affect us back on Earth.

The dynamo theory is the most accepted theory of how planetary magnetic fields are produced as of 2007 a.d. The following parameters must exist, for a magnetic field to be generated.

It must have an electrically conductive interior.
Jupiter and Saturn have a layer of Metallic Liquid Hydrogen with an iron nickel alloy core.
Uranus and Neptune have a layer of compressed Methane /Ammonia Ice with a silicate gabbro core.

The Internal material must be hot enough to create convectional flows.
This transports the electrical charge around the planet, for the Jovian planets the heat is possibly created by gravitational pulling and primordial heat still trapped from the accretion of matter during the planets creation

The interior must be rotating fast enough to build up a powerful electromagnetic force.
This leads to the production of a magnetic field. The difference in rotation speeds between the fast spinning core and slower inner layer twists the electrically conductive layer; any existing external magnetic field energy that enters the layer is stored. When the Interior heats up, the electromagnetic energy formed by convection flows add to that formed by rotation and a self-sustained magnetic field is produced.

The distribution of magnetic fields on Earth, Jupiter and Saturn show that magnetic field strength is most intense around the Polar Regions and is distributed symmetrically on both hemispheres with the magnetic dipole axis in line with the rotation axis.

However Uranus and Neptune are asymmetrical with the strongest fields mainly concentrated on one hemisphere and the magnetic dipole offset between the poles and the equator.
The equatorial magnetic field strength of 0.23 Gauss for Uranus and 0.24 Gauss for Neptune is much weaker than Earths 0.305 Gauss. On the other hand, Jupiter and Saturn have very intense magnetic fields. So why are Jupiter and Saturn’s magnetic fields strong, whilst Uranus and Neptune’s are weak? Well lets look at the 4 Jovian outer planets , we can compare there theoretical composition,size, distance from the Sun ,electromagnetic properties and other parameters to determine why they are so different and yet so similiar!

Jupiter has the most powerful magnetic fields of any planet in our solar system, yet it has a relatively small core of iron nickel alloy; this alone would not be powerful enough to generate its 4.28 Gauss magnetic field. It is plausible that within Jupiter’s atmosphere is a thick layer of “liquid metallic hydrogen”, this coupled with a. very fast rotation period, produces very large amounts of electromagnetic energy and so a large magnetic field. The magnetic field is so intense and wide, that an volumetric area of magnetic influence, known as a magnetosphere, is larger than the Sun and it influences Earths magnetic field, adding billions of watts of electrical energy to it .The influence has also been detected far beyond the orbit of Saturn, over 650 million miles past Jupiter.

Gases and dust mainly from the moons IO and Ganymede mix with the inner regions of Jupiter's magnetosphere to make plasma. Data received from the Voyager 1 and 2 probes in the 1970’s indicated that the plasma is at 300 to 400 Million Kelvin, making it the hottest place in the solar system. The layer of plasma is very thin and widely spread, yet very strong due to very high pressures acting upon it.

The magnetosphere acts like a shield for the planet, holding back particles from the solar wind. The magnetic field pulls in the particles, the two entities mix with plasma, the particles then accelerate and shoot back into space. Some of the charged particles from the solar wind pass through the weaker points of the magnetosphere at the planets poles, captured by the planets magnetic field lines, spiralling around them creating more plasma. In an excited form the plasma along with charged particles in the atmosphere changes to a higher energy state. The increase in energy releases Photons; these are seen as an aurora of spectral light in the visible and ultraviolet wavelengths. The wavelengths of electromagnetic radiation are then analysed to determine which chemical elements are present in the upper atmosphere. Observations show the aurora with a red glow, which is a sign of atomic hydrogen in Jupiter's atmosphere.

The aurora’s highest intensity is at the Polar Regions, where the magnetic field lines leap from one point to another over the equator. The magnetic influence from IO, Ganymede and Europe leaves visible trails on Jupiters aurora where the moons have passed over head. Magnetometers on board the Galileo space probe detected a gap in Jupiter's magnetosphere; this was also the magnetic influence of Io and Ganymede. Its thought these moons could be making there own magnetic fields, generated from an Iron sulphide core. However a current sheet of highly charged plasma that could be material coming from the atmosphere of Io and influenced by Jupiter’s magnetic field may be hiding the truth as to where Io’s magnetic field originates.
Radio wave observations of Jupiter established that there was a region of trapped electrons within the magnetic field. The radiation known as synchrotron emission is common amongst astronomical objects such as pulsars, and not seen in other planets. With observations from the Pioneer 10 and 11 space probes, the magnetic dipole of Jupiter was found to be at 9.6° relative to its equatorial rotation.

Saturn has auroras too! an ultraviolet image spectrograph taken by Cassini showed auroras around both poles of Saturn which lasted for at least an hour. Spectral analysis of ovals on the aurora showed signs of atomic Hydrogen at high altitudes similar to Jupiter’s.
Saturn's magnetic field at the equator is 0.22 Gauss, much weaker than Jupiter's, but is dominant enough to make a magnetosphere. The magnetic field is probably produced in a similar way to Jupiter's, however Saturn's thick atmosphere may also contain liquid metallic helium as well as hydrogen.

Saturn has an inner and outer magnetosphere giving it the best-protected surface of the Jovian planets.
The inner magnetosphere is produced on the Sun side from the fast rotation of the planet creating a current sheet of charged particles (plasma sheet) that co-rotates with the planet. The internal magnetic field lines travelling from north to south poles have both ends attached to the planet and do not reach the ring system. External field lines move beyond the rings on the night side of the planet, they become broken with one end attached to the planet and the other end open to interplanetary space. These open field lines emit from around the polar caps, charged particles and solar wind are locked in with them and forced to travel in a convectional manner over the planet, away from the direction of the Sun, they are then pulled in beyond the rings where they join up, to produce a long tubular magnetotail.
The magnetosphere may be responsible for some strange spoke like shadows noticed on the rings of Saturn in 2005. Saturn's magnetic dipole is less than 1º offset from its axis of rotation. One possible explanation for the tight alignment is that the current sheet and the influence of charged particles from its moons and dust within the rings have stabilize it

We have little understanding of the two outer Jovian planets. Up to now only one space probe has visited them, Voyager 2. As it passed by Uranus in 1986, it measured radio emissions coming from the planet. The radio waves where analysed and it was discovered that the magnetic field is strong, yet offset from the axis of rotation by 58.6°. This lead to the assumption that the planets core may be offset too .At first the Magnetic offset was linked to the unusual 90° tilt of the planet as one of its poles face the Earth, possibly caused by a catastrophic impact by another body, but it has been subsequently discovered that Neptune has a similar magnetic offset. Whats unusual is that Uranus has North and South poles on the opposite sides to that expected, it might therefore be undergoing a reversale of magnetic poles . A similar event is thought to have occurred in Earths past, but never observed before, so we might be seeing the process in action .

It is still not clear what chemical process generates the magnetic fields.The core of the planet is assumed to be silicate gabbro (many minerals) or could even be made of a metallic iced rock substance derived from high compressed ammonia / methane. There is not enough Metallic liquid hydrogen in the layers of Uransus to form a magnetic field the strength of Jupiter and Saturn’s but a magnetosphere does exist .The magnetosphere has the same basic attributes of Jupiter and Saturn, with a bow shock facing the Sun, and a long magnetotail trailing past the dark side.
The magnetosphere is wide enough to cover all the moons in the ring system of Uranus. However the Magnetosphere is irregular and changes dependening on the rotation axis of the planet compared to its magnetic axis, this leads to the magnetotail forming a helix shape rather than the expected tubular one .
The few charged particles trapped in the same region as the rings are swept from the magnetosphere to create plasmas capable of forming a weak aurora. The magnetic field is also weak and only a small plasmasphere exists. A current sheet was detected linking Uranus and one of its moons Miranda, suggesting that Miranda is the source of magnetic influence producing the atmospheric aurora on Uranus.

Neptune Is much like Uranus; both planets may have the same materials in the core and conductive inner layers. The magnetosphere is similar in size and both have a large magnetic tilt, Neptune’s being 47°.
As with Uranus, Neptune's magnetic dipole is offset and not near the planets centre as expected, but found somewhere within a layer of conductive liquid, nearer the surface.
The magnetic field strength of the planet varies at the surface, from 0.1 gauss in the northern hemisphere up to 1 gauss in the southern hemisphere. These magnetic observations also helped determine Neptune’s speed of rotation, which is now established at 16 hours, 7 minutes. Radio emissions where detected by Voyager 2, showing that one of Neptune's moons Triton, may have been gravitationally captured, causing variances in Neptune’s magnetic dipole tilt and plasma production.

Neptune's axis of rotation is a more conventional one than that of Uranus but just as odd. The magnetic dipole moves around 23? to 114? relative to the solar wind during Neptune’s day, its magnetosphere should therefore contain charged particles, but they appear to have been swept into the ring system and trapped there. As the number of charged particles is very low only 3 small plasmaspheres exist which are dependent on the planets magnetic rotation as it alignment with the oncoming solar wind.

Neptune's auroras are very weak, estimated to be about 50 million watts. The unstable magnetic field causes the aurora to occur over wide regions of the planet, not just near the planet's magnetic poles.

NASA has plans to make a Cassini class mission to orbit Neptune after 2013 called project Prometheus. It is hoped that this future mission will enhance what little we know of this most distant of gas giants.

References:

Higher Electrical Technology
Author: J.O.Bird
Page 251 to 270
ISBN 0-7506-0101-9

The New Solar System (4th Edition)
Edited by: J. Kelly Beatty, Carolyn Collins Peterson, Andrew Chaikin
Chapter 4, Magnetospheres.
ISBN 0- 521-64587-5

Universe (3rd Edition)
Author: William J Kaufmann III
Page 251 252 (Jupiter) [image information adapted], Page 309 (Uranus)
ISBN 0-7167-2094-9

Atlas of The Universe
Author; Patrick Moore
Page 97 (Missions to Jupiter) [image information adapted], Page 123 (Uranus)

University of IOWA (usa website) (RPWS)
Author: Don Gurnett
http://cassini.physics.uiowa.edu/space-audio/cassini/bow-shock/

Windows to the Universe (website)
Author: June 3, 2003 by the Windows Team Uranus
Boulder, CO: ©2000-04 University Corporation of Atmospheric Research (UCAR),
©1995-1999, 2000 The Regents of the University of Michigan
http://www.windows.ucar.edu/tour/link=/uranus/upper_atmosphere.html
Neptune
http://www.windows.ucar.edu/tour/link=/neptune/upper_atmosphere.html

SPACE.COM (website)
http://www.space.com/reference/brit/neptune/climate.html

NASA
Solar System Explorer
Text read
http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=166
Galileo Jupiter Mission
Text read
http://www2.jpl.nasa.gov/galileo/jupiter/magnetic_field.html
HST Image and Text (Jupiter)
http://photojournal.jpl.nasa.gov/catalog/PIA03155
Cassini Huygens Mission
Image and text
http://photojournal.jpl.nasa.gov/catalog/PIA06436
(Saturn)
http://saturn1.jpl.nasa.gov/multimedia/products/pdfs/chapter6.pdf
(Uranus)
http://voyager.jpl.nasa.gov/science/uranus_magnetosphere.html
(Uranus) Road map
http://www.astrobio.net/cgi-bin/h2p.cgi?sid=1337&ext=.pdf
(Neptune)
http://www.windows.ucar.edu/tour/link=/neptune/upper_atmosphere.html