The Russell-McPherron Effect: The Geometric Reason Aurora Peaks Around the Equinoxes

If you look at historical geomagnetic activity data across the calendar year, two peaks stand out: one in March and April, one in September and October. This pattern — repeated year after year regardless of where the solar cycle sits — is not a coincidence. It reflects a geometric relationship between Earth's orbital position at the equinoxes and the orientation of the interplanetary magnetic field, identified in 1973 by researchers C.T. Russell and R.L. McPherron. The effect they described is now a standard part of aurora trip planning for anyone thinking beyond the next few days.

What the Russell-McPherron Effect Is

The Russell-McPherron effect is a geometric mechanism that makes southward Bz — the key driver of geomagnetic activity — statistically more probable at the spring and fall equinoxes. Here's why: the interplanetary magnetic field carried by the solar wind has a typical orientation that is influenced by the sun's rotation and the spiral structure of the solar magnetic field. Earth's magnetic dipole axis is tilted relative to both its orbital plane and the solar wind flow direction. At the equinoxes, the geometry of Earth's orientation relative to the incoming solar wind is such that the component of the interplanetary magnetic field that projects onto Earth's magnetic axis — the component that drives magnetic reconnection at the magnetopause — is more likely to be southward than at the solstices.

What helped me picture it: think of two bar magnets being brought together. The angle at which you hold them determines whether they attract or repel. Earth and the solar wind are in a geometry that changes slowly through the year as Earth orbits the sun. At the equinoxes, the angle is more favorable for the solar wind's magnetic field to connect with Earth's field in a way that drives energy transfer and aurora. At the solstices, the geometry is less favorable on average. The Russell-McPherron effect is essentially an annual change in the angle at which the solar wind presents itself to Earth's magnetosphere.

What the Russell-McPherron Effect Means for Aurora Travelers

The equinox aurora peaks are a well-established statistical pattern — not a guarantee for any specific night, but a genuine elevation in the probability of active geomagnetic conditions during March-April and September-October. This pattern holds across solar cycles, which means it operates on top of whatever the solar cycle is doing. Near solar maximum, the equinox peaks are amplified by the higher baseline of solar activity. Near solar minimum, they remain visible as elevated probability windows even when overall activity is reduced.

For aurora travelers with flexibility in when they travel, aligning a trip with an equinox window — particularly combined with a predicted coronal hole passage from the 27-day outlook and a dark moon phase — represents the highest-probability combination available for planning purposes. The spring equinox window (roughly mid-March through mid-April) and fall equinox window (mid-September through mid-October) are consistently among the most active aurora periods of any year. For more on how equinox timing interacts with seasonal viewing conditions in Alaska, see our guide on the best time to see the northern lights in Alaska. Our Northern Lights Tour in Fairbanks runs during the winter viewing season when darkness is sufficient — and the shoulder periods around equinox timing offer some of the most statistically favorable conditions of the year.

What the Russell-McPherron Effect Means for Photographers

For aurora photographers planning dedicated trips, the Russell-McPherron effect is one of the more reliable inputs available for long-range date selection. A trip timed to fall within an equinox window has a statistically higher probability of encountering elevated Kp conditions than an equivalent trip in December or June — which translates to a higher probability of the intense, multi-colored, fast-moving aurora that produces the most compelling images.

The effect also interacts with the 27-day coronal hole cycle. When a known active coronal hole is predicted to be Earth-facing during an equinox window, the combination of the Russell-McPherron geometry and the coronal hole stream represents one of the better-defined elevated-probability windows available in aurora forecasting. Photographers who track both the equinox calendar and the solar rotation cycle can identify these overlap periods months in advance — even if the specific night-by-night detail only becomes clear within the 3-day forecast window. For more on solar cycle and equinox interactions, see our overview of solar cycles and the northern lights.

Return to the full Northern Lights Glossary to continue through the Forecasting and Observation Tools section.