| Auroral variations with time |
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The dynamics of auroras are of special interest. The auroras can move more than 500 kilometres north or south in one hour, depending on conditions in the solar wind and the interplanetary magnetic field.
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Local movements also occur. The position and intensity of auroras have been shown to be a good indicator of the total energy transfer from the solar wind to the polar ionosphere. In the following we will list some characteristics variations of auroras. |
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| Diurnal variation |
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Both the occurrence and the intensity of auroras reach a maximum near magnetic midnight – which in Scandinavia is a couple of hours before local midnight. The activity is less significant in the morning hours than in the hours before midnight. Furthermore, auroras in the early morning are rather diffuse, whereas the evening auroras are more discrete in characteristics. |
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| The 27-day variations |
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The ~27-day rotation period of the Sun, is clearly noticeable in the occurrence frequency. The light is strong when active sunspots occur. Consequently, the probability for observing a strong auroral break out after 27 and 54 days is great. |
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| Seasonal variations |
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The occurrence of auroras is greatest in the spring and fall. In Scandinavia, the best months for auroral observations are October, February and March. |
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| Variations with the sunspot cycle |
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It has been well established that the auroral occurrence follows the sunspot cycle fairly closely. However, the statistical investigations indicate that the maximum occurrence is roughly one year after sunspot maximum. It even appears to be a period of 22 years – two sunspot periods. This may be due to the fact that the magnetic fields connected to the sunspots change orientation between two sunspot periods. |
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