How they can drive dynamos

We know that all information of planetary magnetic fields based on indirect measurements which are mostly obtained from the space missions. We also make Earth’s dynamo process as reference to understand other planet magnetic fields, although for the Earth itself there is no certain theory how it can generate a dynamo at least 3.5 Ga (Stevenson, 2003).

a. Mercury

This planet is a quite small with a diameter only 4880 km with less geological activities. Even though we have already made exploration to this planet via Mariner 10 which mapped 40 % of it over 3 flybys (Thomas, 2006 ), we still lack of detail information about its magnetic fields. The only interesting feature of the magnetic field in this planet is when the probe caught a sudden spam which was supposed as energized particles (Stern, 2007). This make the explanation of the permanent magnetism and dynamos theory more complicated because of the magnitude of its magnetic field (Stevenson, 1987). However, since Mercury is predicted to have a liquid outer core then it is likely to drive a self sustain dynamo (Stevenson, 2003).

The magnetic field in Mercury is very weak and not strong enough to trap many particles (Stern, 2007). This can be explained by considering physical parameters of Mercury to calculate the Elsasser number as an indicator of the field strength (Fearn and Roberts, 2007). So, Stevenson et al. (1987) suggested that the possible explanation regarding Mercury’s energy sources is through the thermoelectric effect.

b. Venus

Although this planet is probably to have a liquid outer core, it does not have a dynamo at present. Some people believe that there is no existence a dynamo in Venus because it has slow rotation (Levy, 1995). However, Stevenson (2003) states that the rotation of Venus is good enough to drive a dynamo. So, for Venus there is no certain theory that can successfully explain why it does not have a dynamo, logically if it has a liquid core to do mantle convection like Earth’s, it should have a dynamo. There are some possibilities regarding this case for example the liquid core of Venus can not afford to make convection because there is no inner core (Stevenson et al., 1983).

c. Earth

The dynamo theory may be the best theory that can explain how the Earth generates a self sustaining dynamo. This theory states that Earth’s inner core condition is sufficient for convection to drive a self sustaining dynamo. Since the Earth’s inner core is the fluid iron which has a significant role to drive the geodynamo, its rotation leads to dynamo effect. This convection process in the outer core of the Earth can generate an effective current loop which gives contribution to the magnetic dipole type magnetic field of the Earth (Nave, 2005).

In addition, There are some contributions of energy sources in running the geodynamo namely: radioactivity contribution, latent heat, specific heat, gravitational contribution and adiabatic contribution (Nimmo , Brodholt & Gubbins, 2003). However, it still can not afford to explain how it can drive a dynamo at least 3.5 Ga. We know that inner core had not formed at that time (Labrosse, Poirier & Mouel, 2001) and paleomagnetic evidences suggest that the Earth’s magnetic field has existed at 3.5 Ga. So, what kind of energy sources that maintained to run the geodynamo if there was no inner core (Jones, 2007). Some people believe (e.g Nimmo et al., 2003) that the radioactive energy and a cooling process in the liquid core produce adequate energy to drive the geodynamo. They modelled the thermal history of core and mantle by using the entropy and a convection scheme to generate the geodynamo as function of time. They found that the Earth’s core must contain a few hundred ppm potassium to obtain a good model. This is quite helpful in explaining why the Earth can maintain the geodynamo even though it did not have inner core.

d. Mars

This planet is predicted to have a dynamo in the past from the evidence of strong magnetism on Mars’s ancient crust. There are some speculations why the magnetic field disappeared from Mars (Stevenson, 2003).

1. There was a decreasing core cooling that caused lack of heat convection.
2. The mantle and core stopped cooling which shut down convection and the dynamo.
3. The core of Mars had completely frozen so that it could not afford to maintain a dynamo.

The origin of magnetization on Mars itself is still uncertain. One possibility scientific explanation is that the Mars’s magnetic field was driven by a volcanic mechanism (Stevenson, 2001).

e. Jupiter and Saturn

The structures of these planets can be divided into three homogenous regions: a small icy and rocky core, a fluid metallic H and He and an insulator region which composes primarily of molecular H₂ and He. The dynamo in these planets is believed from convecting metallic H and He (Merrill et al, 1998).

The interaction between Jupiter’s magnetic field and its larger satellites results in an interesting feature. For example, the interaction between its ionosphere and the relative motion between Io (one of the Jupiter’s satellites) and Jupiter’s magnetosphere generates huge currents flowing around them. This also gives effect to the Jupiter’s radio emission propagating to the Earth as up and down signals (Stern, 2007).

f. Uranus and Neptune

The structure of these planets are very similar and they also have similar the magnetic field inclination by around 60^◦ to their rotation axes (Stern, 2007). This evidence indicates that the rotation has an important role to play in generating the dynamo on these planets (Merrill et al, 1998). Nevertheless, the magnetic fields in these planets are very unusual, in general the other planets have dipolar fields while these planet have large non dipolar fields. One of the possible explanations of this case is that their sources are far away from the center of the planet and they are not relatively symmetric. This lead to magnetic fields with large deviation of the dipole axis to the rotation axes (Merrill et al, 1998). Lack of data about these planets makes the explanation of unusual Uranus’ and Neptune’s magnetic field still in the scope of speculation theories.

Conclusions

We have already known the existence of the planetary magnetic fields which mostly come from space missions, but the limitation of data and observation of them make scientists just suggest speculation theories about how the planetary magnetic fields formed and how they could be drove. Even we still do not understand or have a certain theory that can explain how the earth maintained the magnetic field before its inner core formed. Einstein himself states that the Earth’s magnetic field is one of the most important unsolved problems in physics (Baker, 1999).

There is no doubt that the planetary magnetic fields are an interesting subject to be acquired. Human beings, who are thirsty on sciences, always try to broad their horizon to find a new thing in the universe. So, to accommodate our curiosity on the planetary magnetic fields at least we need more observations and data of the planets and computer simulations so that we can solve unanswered questions that we met in studying this subject. James Cameroon says that “ We stand on the edge of glorious new age of exploration. The future is ready and willing – if we are” (Burke, 2008).

References

Baker, O. (1999, 13 November). Scientists eye whirlpool in Earth’s core. Sciences News, 156, P. 310. Retrieved April 6, 2008, from http://www.sciencenews.org/pages/sn_arc99/11_13_99/fob5.htm

Burke, J.D. (2008 ) Why we explore. Retrieved April 6, 2008, from http://planetary.org/explore/why.html

Fearn, D. & Roberts, P. (2007) The geodynamo. In E. Dormy and A.M. Soward (Eds.), Mathematical Aspects of Natural Dynamos (pp. 201-255). Boca Raton : CRC Press

Fowler, M. (1997). Historical beginnings of theories of electricity and magnetism. Retrieved April 6, 2008, from http://galileo.phys.virginia.edu/classes/109N/more_stuff/E&M_Hist.html

Jones, C.(2007). Planetary dynamos. In E. Dormy and A.M. Soward (Eds.), Mathematical Aspects of Natural Dynamos (pp. 257-280). Boca Raton : CRC Press.

Labrosse, S., Poirier, J.P. & Mouel, J.L. (2001). The age of the inner core. Earth and Planetary Science Letters 190, 111-123.

Levy E.H. (1995). Planetary dynamos. Earth, Moon and Planet 67, 143-160.

Nave, R. (2005). Magnetic field of the earth. Retrieved April 6, 2008, from http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magearth.html

Nimmo, F., Price, G.D., Brodholt, J. & Gubbins, D. (2004). The influence of potassium on core and geodynamo evolution. Geophysics Journal International 156, 363-376.

Merrill, R.T., McElhinny, M.W. & McFadden, P.L. (1998). The magnetic field of the Earth. New York: Academic press.

Stern, D.P. (2007, August 26). Planetary magnetism. Retrieved April 6, 2008, from http://www-spof.gsfc.nasa.gov/earthmag/planetmg.htm

Stevenson, D.J. (2003). Planetary magnetic fields. Earth and Planetary Science Letters 208, 1-11.

Stevenson, D.J. (1987). Mercury’s magnetic field: a thermoelectric dynamo?. Earth and Planetary Science Letters 82, 114-120.

Stevenson, D.J. (2001). Mars’ core and magnetism. Nature 412. 214-219.

Stevenson, D.J., Spohn, T. & Schubert, G. (1983). Magnetism and thermal evolution of the terrestrial planets. Icarus 54, 466-489.

Thomas, C. (2006). Mercury’s technical data. Retrieved 5 April, 2008, from http://www.solarspace.co.uk/Mercury/mercury.php

U.S. researchers said they were able to measure small changes in the density of fractures along the San Andreas fault as much as 10 1/2 hours before an earthquake, a step toward the long-sought goal of predicting potentially devastating quakes.

The team has so far monitored only two events over a two-month period, so a great deal more work will be necessary before their findings can be verified and understood. But the results suggest for the first time that it may be possible to forecast quakes hours before they occur — giving residents time to prepare or, if necessary, evacuate.

Already teams in Japan and China are gearing up to test the new approach on faults in those countries, and the U.S. team is planning a much longer test to better understand the results, said seismologist Fenglin Niu of Rice University in Texas, who led the study, reported Thursday in the journal Nature.

“It’s very encouraging, but we definitely need more experiments,” he said.

It has been known for decades that the velocity of seismic waves through rock varies with the stress applied to the rock, presumably caused by the opening and closing of micro-cracks in the rock.

Researchers have tried several times before to exploit these changes for predictive purposes, but the instruments have been inadequate. In particular, the devices used to generate the seismic waves have not displayed sufficient repeatability to give accurate results.

Niu and his colleagues at the Carnegie Institution of Washington and the Lawrence Berkeley National Laboratory in California used a new piezoelectric source that expands when an electric current is applied and can be reproduced, creating a seismic wave, and a highly sensitive accelerometer to detect the resulting wave. The time required for a seismic wave to travel from the source to the accelerometer is a measure of the density of the rock through which it passes.

The two devices were placed in bore holes about 30 feet apart on the San Andreas fault in Parkfield, where quakes occur frequently. The devices were about half a mile underground adjacent to the fault zone.

The team operated the devices for two one-month periods in 2005 and 2006. The devices were so sensitive that they could detect changes in the barometric pressure at the surface: As the air pressure increased, the weight of the air would close micro-fractures, reducing the time for a seismic wave to travel between the source and detector by about three microseconds.

The team observed two larger changes during the monitoring periods. One occurred about 10 1/2 hours before a magnitude 3 quake on the fault, and a smaller change occurred about two hours before a magnitude 1 quake. The changes persisted until after the quakes had occurred, and the team believes that they were an indication of stress building up before the temblors.

“We hope the changes could be used to predict earthquakes, but that’s only two data points,” Niu said. “We need a much better understanding of the relationship between the timing [of the stress changes] and the size” of a quake.

Introduction

The solar system can be defined as the sun with all the planets, satellites and asteroids that move around it. The bodies in the solar system which have radius greater than 1000 km are likely to have dynamos that can generate magnetic fields (Jones, 2007). These bodies are the gas giant planets: Jupiter and Saturn, the ice rich planets Uranus and Neptune and the terrestrial planets, which contain iron rich cores, Mercury, Venus, Earth and Mars. Before explaining the solar system’ bodies one by one how they can drive their self-sustaining dynamos, first let we see the dynamo theory explaining that the magnetic field can be obtained through the conversion of mechanical energy (Merril, McElhinny & McFadden, 1998). From the Navier-Stokes equation we know that the Coriolis force has an important role to play in solving the dynamo problem. Therefore, the planets which have slow rotation may be difficult to drive a dynamo.

Moreover, the convection processes are also very important, the planets that have only gases as their main constituent may be much easier in producing magnetic field than those which have solid cores. Recent studies (Stevenson, 2003) suggest that some planets have dynamos namely Earth, Jupiter, Saturn, Uranus, Neptune and Mercury although in this planet the magnetic field is very weak, while Mars has strong remanent magnetism in the Martian crust suggest that in the past Mars had a dynamo. Even though Venus is predicted to have a liquid outer core but there is no dynamo.

Observation of planetary magnetic fields

Many observations to extract information about the planets with their magnetic fields have been conducted since forty years ago through the space exploration or even in ancient time for the Earth. The first magnetic phenomena that was observed in the Earth is conducted by the Greek philosopher Thales of Miletus in the sixth century B.C. although the explanation of this was in animistic term (Merril et al., 1998). He stated that loadstone has a soul or life to attract irons (Fowler, 1997). Advanced discovery around Earth’s magnetic field was observed 2000 years ago by ancient Chinese which found the magnetic compass and followed by the discovery of declination (the angle between Earth magnetic and north direction in geographic map. The inclination itself was discovered in Europe in the 16^th (Fearn and Roberts, 2007). Further a scientific statement that the earth acts as a giant magnet given by William Gilbert in his famous book “De Magnete” in 1600 (Fowler, 1997).

The second observation was Jupiter’s magnetic field through its radio emission tracked by two radio-astronomers Ken Franklin and Bernie Burke in 1955. This discovery successfully opened a new window to explore planetary magnetic fields. After the discovery of the Earth’s magnetic belt, Frank Drake discovered that radio emission from Jupiter is caused by electrons trapped in a strong magnetic field. Drake’s statement then was supported and strengthened by data from Pioneer 10 spacecraft that found a huge magnetic field in Jupiter. An interesting invention related with Jupiter’s radio emission observed by Voyager 1 is the interaction between Jupiter and its satellite Io causing the signal of radio emission which is received at the Earth to up and down (Stern, 2007).

Other observations to detect existence of planetary magnetic fields were done through the space missions. In 1974 Mariner 10 was successfully discovered magnetic field in Mercury and this made most scientists surprised because Mercury is a small planet without geological activities (Stevenson, 1987). Early Data about Venus’s magnetic dipole moment were interpreted from U.S. and USSR spacecraft in 1976 by Russel (Stevenson, Spohn & Schubert, 1983). The Viking in 1979 detected a small permanent field in Mars by using potential analyzer (Stevenson et al.,1983), at the same time two US spacecraft Voyager 1 and 2 reached Jupiter and continued to Saturn and Voyager 2 traveled to Uranus and reached to Neptune in 1989 (Jones, 2007). continue to part 2 click here

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Source: http://astronomyonline.org

The magnetic field of the Earth consists of magnetic fields coming from several sources both internal and external source. The external magnetic field of the Earth is mainly from the interaction between the solar wind and geomagnetic field. The solar wind can be described as the movement of particles particularly H⁺, Helium and electron which are traveling from the Sun to the Earth and interacting with the geomagnetic field. This interaction leads to pressure to the geomagnetic field creating a cavity called the magnetosphere.

The solar wind can force the geomagnetic field out of the way at the distances larger than 10 R_E, closer to the earth the solar wind is deflected and it flows around the geomagnetic field. This interaction results in the outer boundary of the magnetosphere called as the magnetopause. On the night side it also forms a comet shape area called geomagnetictail or magnetotail which has diameter around 40 R_E. Another boundary called the bow shock forms in front of the dayside of the magnetopause as result of the motion of the Earth in the solar wind. The region between the magnetopause and the bow shock is called the magnetosheath which has a significant role in reducing speed of the solar wind particles to sub-magnetosonic speeds relative to the Earth. The cleft or polar cusp is the region in which some penetration of the solar wind is allowed to enter the ionosphere causing the slight kink to the magnetopause. In the central part of the magneto tail, when the field lines of different direction are located in very short distance causing they eliminate each other, there is a narrow region with zero field called the neutral sheet.

The plasmasphere region in the magnetosphere protected from the interference of the solar wind leads to ring currents related with the motion of ions in which the positive and negative ions move in opposite directions on the closed field lines. Out side the boundary of plasmaphere called plasmapause, the plasma is strongly influenced by the solar wind’s interaction. A few very energetic particle of the solar wind can also be able to penetrate into magnetosphere forming what is well known as the radiation belt or Van Allen Belt as result of the motion of the particle in the geomagnetic field experiencing an electromagnetic force which forces them to move around the field lines and send back and fore from pole to pole. Significant numbers of very energetic particles of the solar wind are also able to penetrate into magnetosphere under special condition i.e. the large increases of the solar wind’s intensity due to solar flares. They can travel and penetrate to the Ionosphere causing the magnetic storms and aurora.

There is evident from the magnetic observatory record that the geomagnetic field has a regular daily variation in which the solar daily variation and lunar daily variation plays a significant contribution in this daily variation. The solar daily variation, S, has a period 24 h while the lunar daily variation has period near 25 hour. During 24 h period, the additional field, D or the disturbance variation, can present and affect the regular daily variation. The solar quiet day variations, S_q, is a term where the geomagnetic field has a daily variation without extreme variations and it depends on the latitude and the local time. The solar disturbance daily variation, S_D which is part of D, also present in the days with small disturbance or normal days. So, all of them can be formulated by using their relation with the difference between the total field and the average field (∆F) as

D = ∆F-S_q-L

The geomagnetic field sometimes shows the evident of the irregular disturbances on the magnetogram. The interaction of the solar wind with the geomagnetic field is the main factor in creating the irregular disturbances. When the intensity of these disturbances is very large, they are called a magnetic storm. The magnetic storm consists of three phases : the initial, main and recovery phases. The initial phase can be recognized as a sudden change called a sudden commencement. During the main phase, the horizontal component of the geomagnetic field experiences reduction in its intensity. The recovery phase is process in which the horizontal component increases significantly towards the auroral zone.

References

Merrill, R.T., McElhinny, M.W. & McFadden, P.L. (1998). The magnetic field of the Earth. New York: Academic press.

Parkinson, W.D. (1983). Introduction to Geomagnetism. Edinburgh: Scottish Academic press