Longtime aurora watchers know that Earth’s two equinoxes – late March and late September – mark the most colorful times of the year. Aurora hunters say the best time to look up into the night sky for these beautiful sights is during the equinoxes.
Science supports their wisdom. Data show (will open in a new tab) that the aurora peaks around the two equinoxes, and on the other hand, the auroras decrease between June and December, the two solstices. The sun, of course, has nothing to do with the rotation of the earth. Therefore, scientists have long tried to understand what links geomagnetic storms and the resulting auroras to the calendar.
Their most common responses point to the alignment of the Earth’s magnetic field. Although the Earth’s magnetic poles do not line up with its geographic poles, they are still tilted with respect to the Sun. Twice a year, on the equinoxes, the Earth’s orbit brings this tilted field into an ideal position to receive the charged particles that cause the auroras.
On the subject: Northern Lights (Northern Lights): what is it and how to see it
Read more: What is an equinox?
Scientists don’t agree with the full color picture of how auroras form, but they do believe that aurorae are caused by the solar wind and its ‘gusts’ such as solar flares and coronal mass ejections. Charged particles rush from the Sun and bathe the Earth, whose magnetic field attracts them to high latitudes. These high-energy particles slam into the atoms of Earth’s upper atmosphere and excite them, creating bright images that cascade across the sky.
Auroras are just one aspect of the storms that these particles cause when they blow over the Earth. The so-called geomagnetic storms increase in strength and number twice a year, indeed, on the days of the equinoxes. According to (will open in a new tab) Strong magnetic storms are almost twice as common in March as in June or July, according to the British Geological Survey.
In 1973, geophysicists Christopher Russell and Robert McFerron proposed (will open in a new tab) which would be the most acceptable explanation for why the Earth experiences a lot of magnetic activity at this time of the year. Today, scientists call this the Russell-McPherron effect.
Russell and McPherron determined that the answers lay in how the respective magnetic fields of the Sun and Earth meet each other. The tilt of the Earth’s magnetic field means they are largely offset. When the solar wind passes through the Earth, the disjunction deflects most of it away from the planet.
They looked at what scientists call the azimuthal component of the field: the direction that, from the Earth’s point of view, goes up and down through the planet’s poles. When the Earth approaches the equinox in its orbit, the azimuthal component of the Earth coincides with the sun.
An illustration showing how the Earth’s axial tilt determines the seasons. (Image credit: Photon Illustration/Stocktrek Images)
By itself, such an alignment would not expose the Earth to the solar wind. However, the two magnetic fields end up pointing in opposite directions. The result is determined by the same physics that causes the opposite ends of the two bar magnets to align. During the equinoxes, more solar wind passes through, resulting in stronger geomagnetic activity—in a broader sense, brighter auroras.
The Russell-McPherron effect is the most popular explanation among scientists, but it may not be the only reason. It is also known that on the days of the equinoxes, the Earth’s magnetic poles fall at right angles to the direction of the solar wind flow, making the solar wind more powerful. Scientists call this the “equinox effect.”
After all, many scientists still don’t know what causes the auroras. They are not sure what exactly is going on between the solar wind and the Earth’s magnetic field to cause them.
Meanwhile, beautiful, unpredictable auroral light shows continue to stream across the sky.
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