Hello Fifth Graders from Deer Crossing Elementary!
The aurora borealis (northern lights) and aurora australis (southern lights) occur as the result of a complicated interaction between the solar wind and the Earth's magnetic field or magnetosphere. On the day side of the Earth, the solar wind compresses the Earth's magnetic field. On the night side of the Earth, the solar wind stretches the Earth's magnetic field out into a long tail, which scientists call the magnetotail. The transfer of solar wind energy from the day side magnetosphere to the night side is very important to the formation of the aurora.
The sequence of processes that result in spectacular auroral displays with ribbons of light that wiggle and
swirl across the sky is called a
"substorm." A typical auroral substorm has three phases: growth, expansion, and recovery.
During the growth phase of a substorm, solar wind energy is transported
from the dayside magnetosphere and stored on the nightside in the
magnetotail through the processes of magnetic reconnection and convection. In magnetic reconnection, the solar wind magnetic field can briefly merge with the Earth's magnetic field to allow solar wind particles and energy to enter the magnetosphere. When a magnetic field line in the Earth's magnetosphere merges with a solar wind magnetic field line, it gets blown away by the solar wind (or convected) towards the night side magnetosphere. This deposits a lot of magnetic energy in the magnetotail. The growth phase of an average substorm might last 1-2
hours.
The expansion phase of a substorm starts when the energy stored
in the magnetotail is explosively released. This is the phase when the
most brilliant auroral displays occur, and it typically lasts about 30
minutes, although it can be longer.
During the recovery phase, the
aurora fade and the magnetosphere becomes quiet again. I'm not quite
sure how long a typical recovery phase takes - sometimes this phase
lasts only a few minutes, other times it is much longer.
This entire
cycle only takes 2-4 hours, so it can repeat several times a night. However, unlike eruptions of the Old Faithful Geyser in Yellowstone National Park, substorms do not occur on a regular, predictable schedule. The length of a substorm and the auroral display depends upon solar wind conditions and how much
solar wind energy has been stored in the Earth's magnetotail. There is
no specific pattern to the occurrence of substorms that repeats
regularly every night.
However, during the spring and fall, substorms are more likely to occur due to the alignment of the Earth's magnetic axis relative to the solar wind magnetic field. The solar wind magnetic field is like an extension of the Sun's magnetic field out into interplanetary space. Although the solar wind magnetic field direction can vary on time scales of just a few minutes, there are also larger scale magnetic field structures in the solar wind that vary much more slowly. These larger scale structures are what causes the seasonal variations in the occurrence of aurora because during the spring and fall, they create conditions that are more favorable for magnetic reconnection. You can learn more about the seasonal variations here: http://science.nasa.gov/h...sep_auroraseason.htm
Of course, during certain times of the 11 year solar cycle, you are also more likely to see aurora. During solar maximum, solar activity like coronal mass ejections (CMEs) and solar flares are more common. These types of disturbances are even more effective at causing geomagnetic storms and substorms than the more gentle solar wind variations we see all of the time.
Kris