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M - Factary


Magnetic Field


A region of force that is generated by a permanent magnet or by electric currents. The Sun's large-scale magnetic field, like that of the Earth, has a north and a south pole linked by lines of magnetic force. Many smaller magnetic fields exist all over the Sun's surface. Sunspots tend to appear in pairs which have positive and negative magnetic poles.


Find out about the Sun's magnetic field in the magnetic Sun section


Magnetic Field Lines


Image showing magnetic linesImaginary lines that indicate the strength and direction of a magnetic field. The orientation of the line and an arrow show the direction of the field. The lines are drawn closer together where the field is stronger. They do not exist as physical things, but were introduced as a convenient way of visualising a magnetic field.


You will often find statements like 'the electrons spiral around the magnetic field lines' - what actually happens is that the direction of the magnetic field determines the path of an electron and the centre of the electron's spiral path then defines what we could call the magnetic field line. The movement of charged particles like electrons 'across' the directions indicated by the field lines is very difficult, which makes it much easier to calculate how charged particles will travel in a magnetic field.


Isobars on a weather forecaster's map link locations with equal atmospheric pressure. This gives you a 'picture' of how the pressure changes over the country and gives an indication of the winds that can be expected. In the same way, drawing field lines can give you an idea of the magnetic conditions around a magnet.


BrainiacA couple of little activities you might like to try are:


1.Suspend a permanent bar magnet from a long piece of very fine thread and let it swing freely. Every time you do this it will always point the same way. One end will always point north (called the north seeking pole) and the other end will always point south (called the south seeking pole, naturally). A good way of making a compass!


2.Try drawing your own magnetic field lines. Take a permanent bar magnet and a small plotting compass. Place the magnet in the middle of a piece of plain white paper and draw around it. Put the plotting compass at one end of magnet and put a dot on the paper where it points. Then put the tail of the compass on the dot and put another dot where the compass points. Keep doing this, then join the dots when you think you have enough. Try putting the compass on the other side of the magnet and do the same thing again. What happens when you do it at the ends of the magnet? What shape do the field lines make?


Magnetic reconnection


Magnetic reconnection is the breaking and rearrangement of the magnetic field in a plasma. It is one of the most fundamental processes of plasma physics and has important relevance to fusion research, as well as to the physics of the Earth's magnetosphere and solar flares. It may also play a key role in heating the plasma in the solar atmosphere, or solar corona.


Find out more about the magnetosphere in the Sun - Earth connection and find out more about solar flares in our solar explosions section.





The ancient Greeks and Chinese knew about 'magical' rocks that could attract pieces of iron. These rocks were often known as 'lodestone' and were probably created when chunks of iron were struck by lightning. This discovery was originally attributed to the ancient Greeks living near to the city of Magnesia, hence the term 'magnetism'. A steel needle stroked with such a 'lodestone' became 'magnetic' as well. Around 1000 AD the Chinese found that such a needle, when freely suspended, pointed north-south.Chinese south facing compass


The compass, as it became known, soon spread to Europe. Columbus used it when he crossed the Atlantic ocean. It only deviated slightly from exact north. Around 1600 AD, William Gilbert, physician to Queen Elizabeth I of England, proposed an explanation of why a compass worked. He proposed that the Earth itself was a giant magnet.


Most planets and stars also have magnetic fields which play a very important role.




A map showing the strength of the magnetic field in different locations.


The black and white patches on this solar image from the SOHO MDI instrument represent magnetic fields of opposite polarities.




A magnetometer is an instrument for measuring the strength and direction of a magnetic field. It is an electric coil through which the magnetic field passes. By measuring the variation in the current passing through the coil, the magnetometer determines the strength of the magnetic field. Magnetometers on spacecraft can measure magnetic fields as weak as one-millionth of the strength of the Earth's magnetic field.




Region around a planet in which the planet's magnetic field is stronger than the magnetic field carried by the solar wind.




The shape taken up by the Earth's magnetosphere as it is swept along by the solar wind. The tail forms in the direction opposite to the Sun as in this diagram:



Find out more about the magnetosphere in the Sun - Earth connection




Normally the word 'magnitude' refers to the size, quality or importance of something but in astronomy it has a particular meaning in referring to the brightness of astronomical objects. The brightest stars have a magnitude of about 1 and the faintest stars seen with the naked eye have a magnitude of about 6.



In about 120 BCE, the Greek astronomer Hipparchus devised a system of recording the brightness of stars which is still in use today. He catalogued about 1000 stars visible to the unaided eye into six categories according to their apparent brightness. Each category was assigned a unit of apparent magnitude (probably just meaning 'importance').


magnitude 1 brightest
magnitude 2
magnitude 3
magnitude 4
magnitude 5
magnitude 6 faintest


In 1854, the British astronomer N.R. Pogson calculated that the human eye is capable of seeing stars that differ in brightness by about a factor of 100 so that 1st magnitude stars are about 100 times brighter than 6th magnitudes ones.


The difference between each magnitude is then a constant factor of 2.512 which is the 5th root of 100, that is 2.512 x 2.512 x 2.512 x 2.512 x 2.512 = 100. This led to the formal definition of the magnitude scale in which a difference of 5 magnitudes represents exactly a ratio of 100 in brightness.


Today, we can measure magnitudes to a precision of 0.01 or better and the largest telescopes can detect objects down to magnitude 25 or fainter. Since the magnitude numbers get smaller with increasing brightness the brightest objects can have zero or negative magnitudes! That's the reason Sirius has a magnitude of -1.46 and the Sun a magnitude of -26.72.


The formula to switch between brightness and magnitude is:


Difference in magnitude = 2.5 * logarithm (ratio of brightness)




Ratio in brightness = 10(0.4 x difference in magnitude)


This table shows how much fainter than magnitude zero other magnitude levels are.


Magnitude Number of times fainter than 0th magnitude:
































Michelson Doppler Imager/Solar Oscillations Investigation. Helioseismology instrument aboard SOHO which analyses the vibrational modes of the Sun. Also measures the Sun's magnetic field in the photosphere.




Prefix to indicate one million of something.

One million bytes, or a megabyte, is rather a small computer memory size these days. How many days is a megasecond? There are 3,600 x 24 = 86,400 seconds in a day. So 1,000,000 seconds is 1,000,000/86,400, which is about 11.6 days.


Metre (m)


The standard unit of length in the SI system. Originally it was defined as 10-7 (or 0.0000001) of the distance from the pole to the equator on Earth. Then it was more accurately defined as the distance between two marks on a standard metal (platinum-iridium) bar kept in Paris. The word metre comes from the Greek word for measure, 'metron'


Between 1892 and 1940 nine determinations of the metre bar in terms of the wavelength of light were made in various laboratories, and in 1960 the average of these nine results became the basis of the new definition of the metre as "the length equal to 1,650,763.73 wavelengths in a vacuum of the radiation of krypton-86."


However - Since 1983, the metre has been internationally defined as the length of the path travelled by light in a vacuum during a time interval of 1/ 299 792 458 of a second. This definition is a major improvement on the original one as the speed of light is regarded to be a constant of nature making it ideal as a length standard.


By UK Act of Parliament (1963) 1 yard is defined as 0.9144 metres.




The Soviet manned space station which was finally destroyed by forcing its re-entry into the atmosphere in March 2001. A detailed log of events was kept by Anatoly Zak.


23 March 2001:

After fifteen triumphant years in orbit, the Mir space station ended its record-breaking mission with a flawless re-entry into the Earth atmosphere. Mir's final manoeuvers started in the early hours of March 23, as mission control in Korolev sent the command to the Progress M1-5 cargo ship docked to the station to fire its eight docking and attitude control thrusters at 03:31:59 Moscow Time. The burn, which lasted 1293.8 seconds, sent the station into 219.2 by 188.2-kilometer elliptical orbit.


One orbit later, or at 05:00:24 Moscow Time, the Progress fired its thrusters again for 24 minutes, leaving Mir at 216 by 158-kilometree orbit, compared to planned 218.5 by 158.9-kilometre orbit. After two passive rotations around the planet, the last burn involving both attitude control thrusters and main engine on the Progress M1-5 started at 08:07:36 Moscow Time.


The final engine cut-off was scheduled at 08:27:03 Moscow Time, however, looking at the latest data from orbit, the ground controllers left engines burning as Mir left the range of ground control stations at 08:31:13 Moscow Time. The Progress engines were expected to keep firing until they consume all the propellant on board, pushing Mir into a slightly steeper re-entry trajectory.


According to pre-re-entry calculations, the station would plunge into the dense atmosphere below 100 kilometres at 08:44:04 Moscow Time.


The initial disintegration of the outpost was expected at 08:52:34 Moscow Time at the altitude of 80 kilometres and splash down of the debris in the Pacific Ocean at 09:00:13 Moscow Time.


At 08:45 Moscow Time, the mission control in Korolev announced that US radar station at Atoll Kwajalein confirmed Mir's descent along the projected path.


At 08:54 the flight commentator in mission control reported that the complex descended to 68 kilometres, at 08:55 to 63.8 kilometres, at 08:55 to 60 kilometres and at 08:56 to 39 kilometres.


At 08:57 the mission control in Korolev declared Mir's re-entry successfully completed in the designated area of the ocean with coordinates 40 degrees South and 160 degrees West.


According to mission control it was orbit number 86,331 for the station, which during its more than 15 years in service was visited by 104 people. "The space station Mir became the first truly international space station. The chapter in the history of space exploration has ended and its significance yet to be comprehended by the humanity," the commentator said.


lWater MoeculeMolecule


See also, atom.


A building block of matter made up of more than one atom. Water (H2O) is a molecule made up of two atoms of hydrogen and one atom of oxygen. A carbon dioxide molecule (CO2) has two oxygen atoms joined to a single carbon atom.


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