caused by a strong geomagnetic storm

The Italian skies have once again become tinged with red: the evening of April 19, 2024about 9.30pm Red Northern Lights were recorded by several webcams pointing north in Veneto along the Alpine and pre-Alpine arcs reaching up to Emilia Romagna in the Apennines. There were not many sightings, also thanks to the bad weather in many of the areas from which the astronomical phenomenon could have been visible. The cause of the auroras was one category G3 geomagnetic stormi.e. “strong” according to the official classification.

It is rare to observe this type of phenomenon in our latitudes, but in recent months their frequency has intensified due to the fact that the Sun is approaching the peak of its magnetic activity (which could happen later this year). For example, there have been sightings in September 2023 it’s at November 2023: there has been a lot of talk about the latter not only because of the intensity of the phenomenon but also because in part the sightings were not due to auroras but to a visually very similar phenomenon called HRH (Stable Auroral Red arcor “stable red auroral arc”).

The Northern Lights seen in Italy were produced by acoronal mass ejection, that is, a violent jet of plasma composed of charged particles at high speed coming from our star. This expulsion reached the Earth, disturbing its magnetosphere and therefore causing, around 9pm Italian time, a geomagnetic storm G3. These are storms that can cause malfunctions in the electrical grid – especially at high latitudes – and GPS systems.

The Earth’s magnetic field is an effective “shield” that protects us from energetic particles arriving from space, but the arrival of the coronal ejection has weakened the Earth’s magnetosphere, allowing the material expelled from the Sun to reach the atmosphere, where the charged particles they interacted with theoxygen present, which in response emits visible light: this is how auroras are formed.

But if the auroras are generally greensas the one visible in Italy has never been red? The answer lies in the intensity of the geomagnetic storm. Modest disturbances in Earth’s magnetism allow solar particles to interact only with oxygen molecules at approximately 100km altitude, which emit green light as a reaction. Stronger storms, however, also result in the interaction of particles with individual oxygen atoms in the upper atmosphere, more than 400 km high, which instead emit red light. The Northern Lights at relatively low latitudes (such as those of Northern Italy) are typically red because you are seeing very high auroras in the distance that are actually developing at higher latitudes. Particularly high red auroras can also be seen from thousands of kilometers awayand in fact at the same time as the sightings in Italy the Northern Lights were occurring in Scandinavia.

This is confirmed by the fact that the geomagnetic storm was not intense enough to produce the aurora borealis at the latitudes of Northern Italy. To measure geomagnetic disturbance, the so-called is used index Kpwhich ranges from 0 to 9. To see the auroras at latitude 45° you need an extreme Kp, between 8 and 9, while during the aurora of April 19th the Kp index was equal to 7.

This is the value that was also recorded in November 2023, when together with the actual auroras in Italy they were also sighted HRH, phenomena visually very similar to the red Northern Lights but generated by a different physical mechanism. It is therefore legitimate to ask whether both phenomena were responsible for the red lights seen in the Italian skies on the evening of April 19, as they were in November. At the moment we do not have certain data, so everything is possible and nothing can be ruled out, but given the presence of auroras in Scandinavia and given the relatively low intensity of the phenomenon compared to those of November, it is possible that it was “only” the Northern Lights. However, we will await official updates in this regard.

And now what will happen? The geomagnetic storm that gave birth to the auroras it’s shutting down and a situation is expected in the next few hours relative calmalthough new coronal mass ejections from the Sun could reach us Sunday 21 April and therefore trigger new geomagnetic storms.