Recent satellite observations have revealed the largest breach yet seen in the magnetic field that protects Earth from most of the sun's violent blasts, researchers reported Tuesday. The discovery was made last summer by Themis, a fleet of five small NASA satellites. Scientists have long known that the Earth's magnetic field, which guards against severe space weather, is similar to a drafty old house that sometimes lets in violent eruptions of charged particles from the sun. Such a breach can cause brilliant auroras or disrupt satellite and ground communications. Observations from Themis show the Earth's magnetic field occasionally develops two cracks, allowing solar wind - a stream of charged particles spewing from the sun at 1 million mph (1.6 million kph) - to penetrate the Earth's upper atmosphere. Last summer, Themis calculated a layer of solar particles to be at least 4,000 miles (6,400 kilometers) thick in the outermost part of the Earth's magnetosphere, the largest tear of the protective shield found so far.