Magnetopause
Definition
The boundary surface where Earth's magnetic field meets the solar wind.
In plain language
This is the skin of Earth's magnetic bubble โ the front line where the solar wind presses in and, when conditions are right, where its energy first breaks through.
The science
The magnetopause is the pressure-balance boundary between magnetosheath plasma and the geomagnetic field, nominally ~10 Earth radii sunward, compressed inward by high dynamic pressure. Dayside reconnection occurs here under southward IMF.
Why it matters for aurora viewing
Every strong aurora night begins with the magnetopause being pressed and pried open โ density spikes and southward Bz do the prying.
See this in action on tonight's data.