Wrapped in a powerful magnetic field, CMEs consist of huge bubbles of energized gas from the sun's superheated corona (the solar atmosphere). The speed at which the CME travels into interplanetary space and the alighnment of its magnetic field can severely influence that CME's impact on Earth's magnetic field.
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Our planet has a global magnetic field called the magnetosphere, so should a CME hit the magnetosphere at just the right alignment, the CME's magnetic field can reconnect, causing intense magnetic disruption, injecting the magnetosphere with huge quantities of plasma from the sun. In this situation, the CME is said to be "geoeffective" and the resulting geomagnetic storm can be extreme.
Today's storm is so intense that it far overshadows anything that has come before it in our sun's current solar cycle. Approximately every 11 years, the sun waxes and wanes in magnetic activity, culminating in solar maximum, when the solar magnetic field is so stressed that flares and CMEs are commonplace. Although the sun is currently declining in activity from maximum that was predicted to have peaked in 2013, it goes to show that Solar Cycle 24 hasn't finished with us quite yet.