Russell–McPherron effect

The Russell–McPherron effect explains the semiannual variation of geomagnetic activity through the relative position of Earth's magnetic field to the solar wind. The theory was published by the geophysicists Christopher Russell and Robert McPherron in 1973.^[1]

As early as 1856 with a publication by Edward Sabine it is known that geomagnetic activity is higher near the spring and autumnal equinox than during other seasons.^[2] The higher occurence is especially noticeable with large sun storms, like the Carrington Event. Around the equinoxes, Earth’s axis lays perpendicular to the sun, whereas during the solistices it is tilted toward or away from the sun. Thanks to this geometry Earth’s magnetic field points north and can connect to the southward pointing field of the solar wind, through magnetic reconnection.^[3]

Other proposed factors that have a weaker influence than the Russell–McPherron effect, include the equinoctial effect and the axial effect.^[3][4] The equinoctial effect stems also from Earth’s tilt being near zero at the equinoxes and therfore also its magnetic poles. They fall nearly at right angles to the solar wind which leds to more open interactions with it.^[3] The axial effect originated with Aloysius Cortie in 1912 and explains the semiannual geomagnetic activity with changes in Earth’s heliographic latitude and is based on the 7° tilt of the solar equatorial plane with respect to the ecliptic plane.^[5]