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Why does southward-directed interplanetary magnetic field increase the chance of geomagnetic storms?
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Ionosphere and magnetosphere
The layers of Earth's atmosphere, with a yellow line showing the air temperature at various heights.
A southward-directed interplanetary magnetic field (IMF) increases the likelihood of geomagnetic storms because it is antiparallel to Earth’s magnetic field, which facilitates the process of magnetic reconnection.12
When the IMF turns southward, it connects with Earth’s geomagnetic field lines at the dayside magnetopause. This reconnection allows solar wind energy to enter the magnetosphere much more efficiently than through other processes like viscous interaction. The consequences of this enhanced coupling include:12
- Increased Convection: Magnetospheric convection is significantly strengthened, causing charged particles to drift closer to Earth and gain more energy from the dawn-to-dusk electric field.2
- Ring Current Growth: The energized particles contribute to the intensification of the nightside ring current, which reduces the magnetic field at Earth’s surface, characterizing the main phase of a geomagnetic storm.2
- Substorm Activity: The process often triggers geomagnetic substorms, which involve the injection of energetic particles into the inner magnetosphere and result in intense auroral displays.23
Essentially, the southward orientation acts as a "gateway" that allows the solar wind to transfer mass, momentum, and energy into the magnetosphere, driving the dynamic reconfigurations that constitute a storm.23