Longtime aurora watchers will know that Earth’s two equinoxes (late March and late September) mark the most colorful times of the year. aurora hunters say that, to look at the night sky in search of these beautiful spectacles, the dates around the equinoxes are the best.
Science backs up their wisdom. The data show (opens in a new tab) that the auroras peak around the two equinoxes and, on the other hand, the auroras decline around June and December, the two solstices. He sun, of course, is not tied to the rotation of the Earth. So, scientists have long tried to understand what unites geomagnetic storms and the result auroras — to the calendar.
Their most common answers point to the alignment of earth’s magnetic field. Although Earth’s magnetic poles do not coincide with its geographic poles, they are still tilted with respect to the sun. Twice a year, around the equinoxes, the Earth’s orbit brings this tilted field into a prime position to receive the charged particles that cause the auroras.
Related: Northern lights (aurora borealis): what they are and how to see them
Read more: What is an equinox?
Scientists can’t agree on a full-color picture of how auroras form, but they’re sure auroras come from the solar wind and its ‘gusts’, like solar flares and coronal mass ejections. Charged particles fly away from the sun and wash over Earth, whose magnetic field pulls them toward high latitudes. These high energy particles collide and excite the carbon atoms. Earth’s upper atmospherecreating brilliant screens that cascade across the sky.
Auroras are just one aspect of the storms that these particles generate as they blow across Earth. So-called geomagnetic storms increase in strength and number twice a year, in fact around the equinoxes. According data (opens in a new tab) According to the British Geological Survey, on average, a sizeable magnetic storm occurs on almost twice as many days in March as it does in June or July.
In 1973, geophysicists Christopher Russell and Robert McPherron proposed (opens in a new tab) which would become the most accepted explanation of why the Earth experiences more magnetic activity at these times of the year. Today, scientists call it the Russell-McPherron effect.
Russell and McPherron determined that the answers lie in how the respective magnetic fields of the sun and Earth meet. The tilt of Earth’s magnetic field means they are largely misaligned. As the solar wind reaches Earth, the disjunction deflects much of it away from the planet.
They looked at what scientists call the azimuthal component of the field: the direction that, from Earth’s perspective, rises and falls across the planet’s poles. As the Earth approaches the equinox in its orbit, the azimuthal component of the Earth aligns with that of the sun.
By itself, this alignment would not open Earth to the solar wind. However, the two magnetic fields end up pointing in opposite directions. The result is guided by physics similar to what makes the opposite ends of two bar magnets line up. Around the equinoxes, more solar wind passes through, resulting in stronger geomagnetic activity and, by extension, brighter auroras.
The Russell-McPherron effect is the most popular explanation among scientists, but it may not be the only cause. It is also known that, at the equinoxes, the Earth’s magnetic poles fall at right angles to the direction of the flow of the solar wind, making the solar wind more powerful. Scientists call this the “equinox effect.”
Ultimately, there’s still a lot scientists don’t know about what causes auroras. They’re not sure exactly what happens between the solar wind and Earth’s magnetic field to trigger them.
Meanwhile, the beautiful and unpredictable aurora light shows continue to streak across the sky.
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