Since 1611, humans have recorded the comings and goings of black spots on the sun. The number of sunspots can change from cycle to cycle and according to scientists’ monitoring the sun by space-based instruments, 2008 saw the longest and weakest solar minimum.According to space weather scientist Bruce Tsurutani at NASA’s Jet Propulsion Lab in Pasadena, Calif.,the solar minimum is defined by sunspot number and based on that, 2008 was identified as the period of solar minimum. But the geomagnetic effects on Earth reached their minimum quite some time later in 2009. Geomagnetic effects basically amount to any magnetic changes on Earth due to the sun, and they’re measured by magnetometer readings on the surface of the Earth.
The scientists believe that three things help determine how much energy from the sun is transferred to Earth’s magnetosphere from the solar wind, the speed of the solar wind, the strength of the magnetic field outside Earth’s bounds (known as the interplanetary magnetic field) and which direction it is pointing, since a large southward component is necessary to connect successfully to Earth’s magnetosphere and transfer energy.