shows how changes of the solar wind affect the tails of these dust and gas clumps.
Explanations to some keywords:
The cometary dust consists of silicates and organic material. The size of a dust particle varies from a thousandth of a millimeter to the size of a boulder. When the Earth crosses the orbit of a comet dust particles dive into the Earth's atmosphere. Larger particles flash up as meteors. High-flying aircraft can even collect dust of cometary origin.
Ultraviolet light is an electromagnetic radiation with wavelengths somewhat shorter than visible light. The Sun emits radiation in a broad range of wavelengths. Our eyes see only a small fraction of it. Infrared radiation with longer waves generates pleasant warmth on our skin. Ultraviolet radiation has shorter waves. It is therefore more energetic. For example, it generates the tan on our skin during a sunbath, but also sunburn and skin cancer. Fortunately, UV light is almost completely absorbed by the Earth's atmosphere.
Gas which consists to a large part of charged particles (ions and electrons) is called plasma. Almost all atoms and molecules on Earth retain all their electrons, since they are protected by our atmosphere and are exposed only to comfortable temperatures. But under the rough conditions present in outer space almost all particles lose their electrons and become electrically charged (ionization). Gas molecules after leaving the nucleus of a comet stay on average about ten days electrically neutral before they become ionized by the solar UV radiation. These ions form together with the protons and electrons from the solar wind the plasma tail of the comet.
The solar wind is a stream of charged particles originating in the solar corona. The Sun is surrounded by the corona which has a temperature of one to two million degrees. From this "overboiling" corona a continuous flux of protons and electrons emanates and flows into Space with a velocity of about 400 km/s. This so-called solar wind hits the Earth after a travel time of about four days and strongly affects the Earth's magnetic field. The solar wind picks up the cometary gas particles as soon as they become ionized and blows them off forming a long, straight plasma tail.
Solar flares are gigantic explosions on the solar surface caused by strong magnetic fields. Strong magnetic fields in sunspots are stretched and twisted by the motion of solar matter. In this way energy may be stored in the magnetic field. When a part of the field cancels the field lines can eventually rearrange. By such a 'reconnection' stored energy can be released in a violent event called flare. Such eruptions can last hours and the energy released can be the energy of billions of atomic bombs. Solar flares can disturb the radio communication on Earth and damage satellites in Earth's orbit.
If in a gas or a fluid a disturbance occurs (e.g. by an explosion) which is so strong, that equilibrium cannot be restored by "normal" processes like fluid flow or waves, a shock wave is generated. In such a shock wave, density, pressure and velocity change abruptly. An example from everyday life is the bang caused by planes flying at supersonic speeds. The ear notices the discontinuous change of air pressure in the shock wave as a bang.
The magnetic field dragged by the solar wind is caught by the comet. It is draped around the nucleus like a horseshoe magnet and squeezes the tail between the poles to a flat ribbon. This ribbon structure can be seen clearly, when the magnetic field turns. The tail follows the rotation which moves from front to back through the whole tail. In the picture of Comet Austin, you can see on the broad front part of the tail, while on the right you still see the narrow edge of the rear tail. The rays in the tail are generated by this geometrical effect.
Transcript of the movie text:
A comet can be regarded as a big dirty snowball consisting of ice and dust. When the comet comes close to the Sun the ice evaporates and forms a giant gas cloud. Hereby the dust that had been entrapped in the ice comes free and forms a broad curved dust tail, which gleams reddish in the sunlight. The gas molecules, however, electrically charged through the exposure to the solar ultraviolet radiation are carried away by the solar wind with its embeded magnetic fild. They form a long straight plasma tail which fluoresces bluish in the sunlight. In some cases the plasma tail becomes completely detached from the comet. This indicates that something dramatic must have happened within the solar wind or on the Sun itself. In this UV-picture of the Sun explosions, so called flares, appear as bright spots. These explosions propagate outwards through the solar wind in the form of shock waves. On a computer we have simulated the effect of such an interplanetary shock on the plasma tail of the comet. In this animation, dimmer areas of the plasma tail are shown in blue colours, bright ones in red. It is evident that a large part of the tail is torn apart. After several hours the solar wind returns to its normal state and a new tail is formed.
The solar wind is variable in space and time. A comet's plasma tail reacts sensitive to such varations and can serve as a measuring instrument, free of cost. Sometimes the plasma tail is completely severed from the comet. This points to a dramatic incident in the solar wind or on the sun itself.
Solar Wind 1
When the density of the solar wind abruptly rises, this is like a blow with a hammer, which strips off a great part of the comet's tail, like it is shown in this computer simulation.
Solar Wind 2
Depending on from which part of the sun the solar wind originates, two different types can be distinguished: The slow wind with typical speeds of 400km/s originates mainly near the solar equator. The fast wind with speeds up to 800 km/s, comes from regions near the solar poles. Near the ecliptic slow and fast winds alternate, often with sharp changes in density and velocity. The comet experiences several such transitions on its way through the inner solar system. In particular, the change from slow to fast wind leaves clear traces in the plasma tail.
Sometimes explosions occur on the sun's surface, which generate shock waves in the solar wind. We have simulated on a computer how such a shock wave acts on the plasma tail of a comet. In fact, a great part of the tail is torn away. When the solar wind returns to its normal state, after some hours a new tail is formed.