The arrival of fall does more than change the calendar. Around the September equinox, Earth enters one of the two times of year when the geometry of our planet’s magnetic field makes it easier for energy from the Sun to get into Earth’s upper atmosphere. That can mean more opportunities for auroras, including occasional displays much farther south than the usual aurora zone.
The equinox does not switch on the northern lights. Instead, it changes the geometry of the Earth-Sun magnetic connection in a way that solar activity can take advantage of it. This is why aurora watchers pay particular attention to the weeks around March and September: the Sun supplies the energy, but around the equinox, Earth’s magnetic field is be positioned to make better use of it.
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Known as the Russell-McPherron effect for geophysicists Christopher Russell and Robert McPherron who first described how Earth's orientation toward the sun enables the mostly northward and the Sun's mostly southward magnetic fields to partially cancel each other out enabling more of the charged particles that light up the upper atmosphere in a solar storm to stream in.
Scientists also call these temporary openings "equinox cracks". Aurora lovers call it "aurora season".
The magnetic connection
The aurora begins with the Sun. The solar wind is a continuous stream of charged particles flowing outward from the Sun, carrying the Sun’s magnetic field with it. When that solar wind reaches Earth, our planet’s magnetic field normally acts as a shield, deflecting much of the incoming material around the planet.
But the shield is not equally effective in every circumstance. When the magnetic field carried by the solar wind points in the right direction relative to Earth’s magnetic field, the two fields can reconnect. That process allows energy from the solar wind to enter Earth’s magnetosphere and ultimately drive particles toward the upper atmosphere. When those particles collide with oxygen and nitrogen high above Earth, the gases release some of that energy as light. That is the aurora.
The Russell-McPherron effect is essentially a quirk of the geometry that makes that magnetic connection more favorable around the equinoxes.
It does not mean the Sun suddenly becomes more active when fall arrives. An event on the Sun's surface such as a coronal mass ejection would still be needed to provide the boost of energy hurtled toward the Earth for any aurora to be visible anywhere.
But, if that were to happen, the conditions could be right the extra energy needed to push aurora visibility southward. But that's a really big if.
The Sun has been relatively quiet with few sunspots observed in recent weeks as we are on the down slope of activity after a very active solar maximum in recent years. As a result, the geomagnetic activity forecast from NOAA's Space Weather Prediction center is similarly quiet early this week, with no geomagnetic storms expected.
