Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 14/07/2012 14:30:01
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Wilf James asked the Naked Scientists:
I didn't learn about the Juice mission until sometime after the original podcast was made available.
I have listened to the "Space Boffins" part about the Juice mission several times in an attempt to discover more about Ganymede's magnetic field.
It would appear that those concerned with the mission still think of planetary magnetism as being internally generated. I am convinced that it can't be because such a hypothesis is in direct conflict with Lenz's Law.
Lenz's Law expressed in a formal way is not easily comprehended by those who have not built their careers on the study of electricity and magnetism. I therefore offer a less formal explanation of Lenz's Law here.
"The current in a conductor in a continuous circuit induced by a moving magnetic field will have a magnetic field of the opposite polarity to the magnetic field that induced the current in the first place."
In other words the magnetic field produced by an induced current will tend to cancel the magnetic field that induced the current.
I have read many supposed explanations about planetary magnetic fields that attempt to circumvent Lenz's Law. All of them fail because their authors have not comprehended the very basic laws concerning electricity and magnetism.
The FIRST thing that these authors ignore or forget is that magnetism is a PROPERTY of an electric current. An electric current by definition is a movement of electrons. A magnetic field is, in general, a static phenomenon. A static magnetic field generates no electric currents. However, if the magnetic field or a conductor within it is moved by mechanical energy supplied, a current is generated. Faraday proved years ago that only moving magnetic fields or conductors produce electric currents in conductors.
The conclusion from these basic principles is that a self-sustaining magnetic field around a planet can never be produced. If a magnetic field is present it must be generated by some external agency.
I offer my hypothesis based on what I have been able to discover about the planets in the Solar System.
The planets with significant magnetic fields all have two things in common.
1. They all have significant atmospheres.
2. They all rotate relatively quickly.
Mercury has virtually no atmosphere and doesn't rotate very quickly. It has a very weak magnetic field.
Venus has a lot of atmosphere and hardly rotates at all. It also has a very weak magnetic field.
Earth has plenty of atmosphere and its equatorial rotation speed is in excess of 1000 miles an hour.
Mars has hardly any atmosphere and doesn't rotate quite as fast as the Earth. Its equatorial speed is around half that of the Earth. If the equatorial speed and atmospheric density are compared with those of the Earth, the magnetic field strength of Mars is in the same proportion to the Earth's magnetic field.
Jupiter has an enormous atmosphere and a very high equatorial speed. It has a magnetic field to match.
Saturn, Uranus and Neptune have magnetic fields that relate to their equatorial speeds and their atmospheric densities.
There is one factor omitted from these calculations and that is the density of the solar wind at a planet's orbit. It declines as the inverse square law so that it is very much less dense at Neptune that it is at Jupiter.
The gravity of the planets also has some effect by attracting some of the particles in the solar wind to increase the local solar wind density in proportion.
The question now is: How do these factors affect the magnetic field around Ganymede?
Ganymede's rotational speed is apparently the same as its orbital speed around Jupiter. I cannot hazard a guess at the effect of Jupiter's gravity on the solar wind as seen by Ganymede. Estimation of the magnetic field strength for Mars in relation to Earth is a relatively simple matter based on rotation, atmosphere density and solar wind apparent density. The situation for Ganymede is very much more complicated. However, I am willing to bet that none of Ganymede's magnetic field is internally generated.
I think that the Juice mission should include devices to measure the density and radial speed of the solar wind in the vicinity of Jupiter and its moons. If the density of Ganymede's atmosphere can be obtained, these factors can be used to compare the measured strength of Ganymede's magnetic field with that which can be obtained by calculations.
The Earth's magnetic field agrees with known factors. The unneutralised proton density in the solar wind at the Earth's orbit is around 5 per cc. Protons from the solar wind dragged around the Earth by the Earth's atmosphere form a positive ring current around the Earth. The polarity of the Earth's magnetism is the same as would be produced by a positive ring current flowing eastwards around the Earth.
This email deals with only one aspect of what I call "Astronomer's Magnetism". In general, it seems that most, if not all astronomers think that planetary magnetism is internally generated and that magnetism elsewhere in the universe is "just there". Many astronomers ascribe phenomena to the effects of magnetic fields without apparently noticing that magnetic fields themselves are the effects of currents. As any electrical engineer will tell you, MAGNETISM IS EXCLUSIVELY A PROPERTY OF AN ELECTRIC CURRENT (if permanent magnets are ignored, and there aren't many natural permanent magnets in space.) It seems that these astronomers ignore the causes of the electric currents that produce the magnetism observed. The electric currents concerned are all in moving plasmas. Plasmas generally move under the influences of heat and gravity. The solar wind plasma is apparently projected outward faster than the speed of escape by a form a vulcanism from the sun.
Wilf James BSc.
Letchworth G. C.
Herts.
What do you think?