The outer solar atmosphere, the corona, is structured by strong magnetic fields. Where these fields are closed, often above sunspot groups, the confined solar atmosphere can suddenly and violently release bubbles or tongues of gas and magnetic fields called coronal mass ejections. A large CME can contain 10.0E16 grams (a billion tons) of matter that can be accelerated to several million miles per hour in a spectacular explosion. Solar material streaks out through the interplanetary medium, impacting any planets or spacecraft in its path. CMEs are sometimes associated with flares but usually occur independently. If a CME hits earth, it can cause a geomagnetic storm. The intensity of the storm will be affected by the orientation of the CME's magnetic field, its speed and density. A strong CME hit often causes Aurora displays at low latitudes.