Because the outward gradient of gas pressure in the hot corona is too high to be balanced by solar gravity, this outermost layer of the atmosphere expands into space. As Pioneer 10 left the known solar system, it was still detecting this solar wind. At the Earth's orbit the outward velocity of the solar wind is between 300 and 700 km/sec (185 and 435 mi/sec), but its density is between 1 and 10 particles/cm3. Thus the mass flux is only about 10 -13 solar masses per year. Still, the solar wind has observable effects on the Earth's upper atmosphere, such as the auroras seen at high latitudes.