The Solar Wind / Geomagnetic Activity Plot is a comparison of the solar wind speed near Earth and the geomagnetic activity that it generates. Low solar wind speeds are approximately 300 kilometers per second (about 650,000 miles per hour), but when a solar event blasts out material into space or a fast stream from the sun intercepts Earth it can rise to more than 2,500 kilometers per second. Because the sun is 150,000,000 kilometers (about 92,000,000 miles) from Earth, it takes from 17 hours to 7 days for the disturbance to reach Earth.

When the disturbance, in the form of increased velocity and changing IMF, reaches Earth it increases the electrical currents in the Earth's magnetic field, just like an electrical generator. These currents flow throughout the magnetic field, including into the atmosphere above the poles, generating the aurora. The higher the solar wind velocity, the greater the electrical currents in the magnetic field and the greater the magnetic disturbance. Thus, the magnetic disturbance is a measure of the strength and extent of the auroral activity. Kp is the measure of the disturbance. It ranges from 0 to 9. A value of 0 corresponds to the aurora over Point Barrow, Alaska. A value of 9 corresponds to aurora over Texas. The aurora reaches the Canadian border around Kp = 4 (yellow). 0 to 3 is green and 5 to 9 is red in the colored bars. For more on the relationship between aurora and magnetic activity go to http://sec.noaa.gov/Aurora/index.html.

Because the sources of disturbances on the sun tend to repeat, especially during low activity periods, we have arranged the plots according to the 28 day solar rotation period. These are called Carrington rotations in honor of the astronomer who first noticed the relationship between disturbances on the sun and magnetic activity on Earth. Because the activity tends to repeat, and we have included a 28 day solar wind velocity prediction, you can plan your auroral viewing periods a month or so in advance.