It had taken 100 hours.
Now the proton, part of a supersonic stream of plasma ejected from the Sun, was nearing the boundary of Earth’s magnetic field. Normally it would there be slowed at the bow shock front and then drift subsonically through the magnetosheath to eventually leave the planet far behind.
Not this time. The was a hole in Earth's magnetosphere created by a rearrangement and reconnection of the opposed Sun and Earth magnetic fields. The proton slips through the defences into Earth’s domain. It does not reach Earth. Even if it could it was, like most solar wind particles, too slow and without enough energy to give any noticeable aurora. Instead it is swept in magnetically trapped plasma around to Earth’s dark side. There it is trapped in a million kilometre long bottle, the plasma sheath.
The imprisonment is not for long. The stretched and particle packed plasma sheath undergoes its own magnetic rearrangement and reconnection. Some contents are flung away from Earth. The proton is one of a mass accelerated sharply Earthwards. Now it has the energy to be noticed. Descending and spiraling giddily downwards around the ever clustering magnetic field lines it forms, with other protons and electrons, a ring around each pole. Then it is inside the tenuous outer atmosphere, ever slowing as it loses energy in collisions. Its journey is at an end but it is not without record. In its wake are oxygen atoms electronically excited by its passing. Some lose the hard won energy ignominiously in yet more collisions. A few have time to radiate away their energy as photons – the pure green light of the aurora.