This picture shows the two different tails of a comet - the dust tail and the ion tail.
Click on image for full size
The Comet Tail
A comet generally has two tails, not one. Some pictures accurately show this phenomena. One tail is due to the comet's dust particles, the other is due to ionized gas from the comet coma
Dust particles form the first tail. The only force which affects these small particles in space is corpuscular radiation from the Sun itself. This radiation pushes the tiny dust particles in the direction from which they came, the way wind can blow rain droplets at an angle. Cometary dust particles can be thought of as billowing up from the comet's surface. Thus, this comet tail generally points back along the comet trajectory.
Ions (electrically charged particles), which first come from the nucleus as (neutral) gaseous particles, are swept into the second comet tail by interaction with the Interplanetary Magnetic Field (IMF). The IMF, or magnetic field of the sun, which is present everywhere in interplanetary space, sweeps past the comet nucleus and carries the ions with it to form the tail. Because of this special interaction with the IMF, this tail always points directly away from the Sun.
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