Dynamic Pressure

Dynamic pressure is the force per unit area exerted by the solar wind as it flows into and compresses Earth's magnetosphere, and it depends on both how dense the solar wind is and how fast it is moving — specifically, it increases with density and with the square of speed, meaning a doubling of solar wind speed quadruples the dynamic pressure it delivers. This pressure is what physically pushes against Earth's magnetic field and determines where the magnetopause, the outer boundary of the magnetosphere, sits at any given moment; under calm conditions the magnetopause typically sits around 10 Earth radii away, but a sharp increase in dynamic pressure, such as from the arrival of a fast, dense coronal mass ejection, can compress it dramatically closer to Earth, sometimes to within geostationary satellite orbit. A sudden spike in dynamic pressure produces an effect called a sudden impulse or sudden storm commencement, an abrupt jump in Earth's surface magnetic field that ground-based magnetometers can detect within seconds, often serving as the very first sign that a geomagnetic storm is beginning. Because of this, real-time dynamic pressure measurements from solar wind monitoring spacecraft are one of the most closely watched numbers by space weather forecasters trying to anticipate exactly when a storm's effects will actually begin.