The Northern Lights, or Aurora Borealis, are a natural wonder that captivates anyone fortunate enough to see them. These colorful displays in the night sky are most often observed in regions near the Arctic Circle, like Tromsø and Senja, but their visibility and intensity are influenced by more than just geographical location.
One of the most significant factors is the timing of the equinoxes in March and September. These periods mark a unique alignment in the Earth’s relationship with the sun, leading to a noticeable increase in auroral activity. But what exactly makes the equinoxes so special when it comes to the Northern Lights, and why do people plan their journeys around them? Let us get a bit scientific.
The Northern Lights are caused by charged particles from the sun colliding with atoms in Earth’s atmosphere. These particles, carried by the solar wind, are drawn toward the Earth’s poles by its magnetic field. When these solar particles hit the gases in our atmosphere—primarily oxygen and nitrogen—they create stunning light displays as the gasses cool down by emitting light after the collision.
The colors we see in the auroras are due to the type of gas and the altitude at which these collisions occur. Green is the most common color, resulting from oxygen at lower altitudes, while red, blue, and purple can also appear depending on various factors like the height of the lights in the sky.
The Russel-McPherron Effect
The equinoxes occur twice a year when the tilt of the Earth’s axis is such that day and night are nearly equal in length everywhere on the planet. However, the equinoxes are more than just a day of balance; they also create conditions that are particularly favorable for the Northern Lights. This is primarily due to something called the Russell-McPherron effect.
During the equinoxes, the orientation of the Earth’s magnetic field aligns in a way that enhances the interaction between the solar wind and the Earth’s magnetosphere. This effect, named after scientists Christopher Russell and Robert McPherron, explains why geomagnetic storms—those disturbances in Earth’s magnetic field that drive auroral activity—are more common around the equinoxes. The solar wind carries the sun’s magnetic field toward Earth, and during the equinoxes, this magnetic field can align oppositely with the Earth’s field. This opposite alignment allows for a more efficient transfer of energy, fueling more intense geomagnetic storms and, in turn, brighter and more widespread auroras.
Explained differently: The Bz component that you will find in some of the more advanced Aurora Apps, like Space Weather Live, is a crucial aspect of solar magnetic fields, which, like all magnetic fields, have both a north and south pole. These solar magnetic fields are carried to Earth by the solar wind. Solar physicists refer to the “north-south” component of the sun's magnetic field as the Bz component. Researchers Russell and McPherron discovered that this Bz component fluctuates throughout the year, reflecting the wobble in Earth’s axis. These fluctuations are most pronounced during the equinoxes.
Geomagnetic storms, which lead to auroras, are more likely to occur when the “north-south” component of the solar wind is oriented opposite to the “north-south” component of Earth’s magnetic field. This happens because, similar to how two oppositely oriented bar magnets attract each other, opposite Bz components also attract. This attraction opens up a gap in Earth’s magnetic field, allowing the solar wind to penetrate more easily toward Earth’s magnetic poles.
Geomagnetic Storms and Auroral Displays
The intensity of the Northern Lights largely depends on the strength of geomagnetic storms, which are measured using the Kp index, a scale from 0 to 9. A Kp index of 5 or higher signals a geomagnetic storm, with higher numbers indicating stronger storms and thus more intense auroras. During such storms, the Northern Lights can sometimes be seen at much lower latitudes than usual, occasionally reaching as far south as the northern United States or central Europe.
The equinoxes are prime times for these geomagnetic storms, thanks to the increased energy transfer enabled by the Russell-McPherron effect. This makes the weeks around the equinoxes some of the most reliable times of the year for aurora viewing.
The Equinoctial Effect
In addition to the Russell-McPherron effect, there’s another factor at play during the equinoxes that contributes to increased auroral activity: the equinoctial effect. This phenomenon occurs because the Earth’s magnetic poles are positioned at right angles to the solar wind twice a day during the equinoxes. This orientation enhances the solar wind’s ability to disturb the Earth’s magnetosphere, further boosting the chances of geomagnetic storms and strong auroras.
While not as significant as the Russell-McPherron effect, the equinoctial effect adds another layer of intensity to the auroras during these times. Combining these two effects creates a perfect storm of conditions that make the Northern Lights more frequent and vivid around the equinoxes.
Why the Equinoxes Matter for Aurora Watchers
If you’re keen on witnessing the Northern Lights, the equinoxes should be marked on your calendar. The enhanced geomagnetic activity during these times means that the chances of seeing the lights are higher, and the displays are often more dramatic. Unlike other times of the year when auroral activity can be hit or miss, the equinoxes offer a more reliable window for aurora viewing.
Additionally, the equinoxes occur during transitional seasons—spring and autumn—when the weather in aurora-prone regions like the Arctic is more temperate compared to the colder winter. This makes it a more comfortable time to travel and spend long nights outside watching the skies.
The Science Behind the Spectacle
Understanding the connection between the equinoxes and the Northern Lights adds depth to the experience of witnessing these natural light shows. It’s not just about standing under the night sky and watching colors dance above you; it’s about appreciating the intricate dance of celestial mechanics that makes such displays possible.
The increased geomagnetic activity during the equinoxes isn’t just a random occurrence; it’s a predictable and fascinating result of our planet’s unique position and orientation in space. The Earth’s magnetic field acts as a shield, protecting us from the solar wind, but it also creates the conditions for the Northern Lights when the solar particles break through this shield during geomagnetic storms.
Is it worth going when there are no Equinoxes?
The answer to that is a clear yes. The Equinoxes are periods with a theoretical increase in the Northern Lights' activity, but in the end, it comes down to a few more factors like the weather and activity of the sun. In the Tromsø area and on Senja there is a potential chance of seeing the lights daily from the beginning of September to the middle of April, and after guiding for more than 15 years and living here my whole life I can honestly say that we see wonderful lights even outside these periods. The advantages coming mid-September might be less people, the possibility to find a more budget-friendly hotel room, and as already mentioned the milder weather - but the chances are more or less the same through the whole season. November 2023 was the best month last season with sightings almost every day.
If you have questions go to our Facebook group Guide to Tromsø & Senja - here you will find experts and fellow travelers ready to give you their best in making your tour comfortable and unforgettable.
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