Most often during spring and autumn.

The occurrence of aurorae is not conditioned by strong solar activity, the constant flow of solar wind particles is sufficient. Apart from the occasional strong eruptions, during which a cloud with a large number of the above-mentioned charged particles from the Sun is ruptured, about 1.2 billion tons of these particles are released from our star every second at a speed of 200-1200 km/s. The solar wind escapes faster from the so-called coronal holes and solar poles (about 700 km/s), and little slower from the normally closed magnetic field from the equatorial regions of the Sun (about 400 km/s). It is the faster one that can be another "trigger" for more pronounced aurorae.

"Long-term monitoring of aurorae has shown that regardless of the intensity of the solar wind or the eruptive activity of the Sun, stronger aurorae most often occur around the spring or autumn equinox, more precisely in March or September and October," describes Petr Horálek from the Institute of Physics in Opava, which, for this reason, went to Norway to watch the aurora borealis this week. The reason why the period of the spring and autumn equinoxes contribute to the observation of more frequent aurorae is the inclination of the Earth's axis (and thus the Earth's magnetic field) to the solar magnetic field in these periods. As Dr. Tony Phillips, a solar physicist at NASA and administrator of Spaceweather.com, explains, it is during the equinoxes that the mutual interference of polarity  in the solar wind and the Earth's magnetosphere field  becomes most effective, creating cracks that allow more charged particles to pass through. This phenomenon is known as Russell-McPherron according to two American physicists, Christopher Russell and Robert McPherron, who first described the weakening of the Earth's magnetic field during the equinoxes in 1973.

"It is important for the variable interplanetary magnetic field (IMF), through which particles flow from the Sun due to the solar wind, to be oriented in such a way that allows more effectively follow the lines of force of the Earth's magnetic field. Then charged particles can enter the Earth's atmosphere in larger quantities and conjure up the desired spectacle even without a strong solar flare, "adds Horálek, who for this reason went to Sweden and Norway to observe this phenomenon. Experts mark the orientation of the magnetic field with the index "Bz", and if the indicators on Internet monitors of aurora activity (such as SolarHam) or on online applications for "aurora hunters" show a negative value, the chances of aurora borealis increase rapidly. If the value is less than 20 (ie the indicators indicate Bz = -20 nT and less), there are chances to observe aurora borealis even in lower latitudes.

Soup of glow and moonlight

Even more recent, this year's studies show that the Earth can partially create an aurora borealis itself. The so-called diffuse aurora borealis, which with a little imagination resembles a mixed pea soup, are created by electron beams accelerated in an electric field in the Earth's magnetosphere. During this process, secondary electrons are released, which fall back into the Earth's atmosphere, creating a "soup" glow. The phenomenon has been a mystery for many years, and the THEMIS-ARTEMIS scientific satellite mission, which has been performing measurements in the Earth's magnetic field since 2007, has helped to decipher it.

Last but not least, our Moon also participate in creation of the beauties of aurorae: It helps to saturate the blue component in their shades, which are usually very faint and formed by ionized molecular nitrogen. When these ions are hit by the moonlight, they capture and re-emit photons of blue light coming from the moon. The moon itself does not shine, it scatters and reflects sunlight.

And what about the aurora borealis in the Czech Republic?         

"In the Czech Republic, aurorae are less common, but they are not completely unusual. For example, in 2003 one of the most significant in the last 50 years occurred and in our region it was observable all over the sky. A very bright one took place right on the revolutionary evening of November 17, 1989. The last time we could see a brighter glow was in March 2015. Usually, aurorae visible from the Czech Republic appear around the maximum of solar activity, which peaks once every 11 years, “notes Tomáš Gráf from the Institute of Physics in Opava.

The festive eruption of class X1 on Thursday, October 28, at 5:35 pm CET was supposed to cause strong aurora borealis visible in the Czech Republic as well, but in the end they were weaker. The most portion of the charged particles cloud missed our planet (it flew south under the Sun-Earth line), and therefore, unfortunately, the announced spectacle did not take place in the end. It is actually very difficult to predict such events, because we only observe the eruption from Earth itself and then the data from the satellites shortly before the arrival of the cloud of particles on our planet. For two days, during which such a cloud usually travels through interplanetary space to us, astronomers don't have sufficient amount of data to refine the prediction.

Solar activity rises, so we can expect more of such phenomena in the future. The nearest maximum of solar activity is expected around 2025. At that time, most of the eruptive phenomena related to local disturbances of the Sun's magnetic field, which are typical of the solar maximum, will take place on the Sun. These disorders are accompanied by the occurrence of darker areas on the Sun, so-called sunspots. The more spots, the more unstable areas, and therefore stronger eruptions aimed at the Earth, and the greater the chance of a strong aurora borealis. 

"However, the last solar cycles were weaker than those at the end of the last millennium, so there were fewer aurorae in the Czech Republic. However, the solar wind is constantly flowing from the Sun, and thanks to the Russell-McPherron effect, we now know that traveling to the Nordic countries around the vernal or autumnal equinoxes is practically a certainty to see them. Of course, if traveling is not possible, we will be happy to provide the experience of aurora borealis to visitors of our spherical projection in the Unisphere in the building of the Institute of Physics in Opava”, concludes Gráf.