After the G4 category geomagnetic storm (“severe”) that it caused Northern Lights also visible from Italy in the early hours of yesterday, the geomagnetic disturbance on Earth is still ongoing, currently as a storm category G1 (“minor”) with NOAA forecasting a possible worsening up to category G3 (“strong”) or G4 by today. Many are wondering if it will therefore be possible to admire the Northern Lights again this night, but there is only one truth: We don’t know and we can’t know.
Yesterday there was an enormous amount of online content announcing the arrival of new spectacular auroras in the night, which however they did not occur. The possibility actually existed: thecoronal mass ejection (CME, Coronal Mass Ejection) caused by X5.1 blasting which left the Sun on November 11th and reached our planet yesterday evening, with all the potential to produce auroral phenomena clearly visible in our country.
However it didn’t happen. Moved by the arrival of the powerful CME, many they leveraged desire of people to have new auroras to admire – those of November 12th were visible tbetween 3 and 5 in the morningtherefore at a very inconvenient time – but the truth is that the arrival of a CME on Earth, even a large one like yesterday’s, does not in any way guarantee the actual occurrence of the Northern Lights, and indeed our ability to predict the Northern Lights is extremely limited, except with very little notice.
The reason lies in the fact that the mechanism that generates auroras is very complex and depends on the simultaneous occurrence of different conditions. Simply put, polar auroras are formed when a substantial flow of charged particles coming from the Sun manages to reach the upper atmosphere, where it “collides” with atmospheric atoms or molecules, transferring energy to them which the atoms re-emit in the form of light, which is precisely the light of the auroras.
This is the simple conceptual explanation, but the exact dynamics of what happens in the upper atmosphere is very complicated and depends on a large number of variables. In short, to put it “down to earth” it’s not like stuff comes from the Sun and we automatically have auroras. What (did not) happen last night is the clear demonstration of precisely this.
First, the plasma arriving with the solar wind must be enough, and energetic enough, to significantly perturb the Earth’s magnetic field. This disturbance is measured with a parameter called Kp index: He determines how intense a geomagnetic storm is. With a Kp of 7 we have a geomagnetic storm of category G3 (therefore “strong”) and the possibility of spotting auroras in Northern Italy.
But be careful: it’s only a possibility, because other things have to happen at the same time. First the solar charged particles must reach the upper atmosphere, and this it only happens if the interplanetary magnetic field is “inclined” to let them in instead of diverting them away. What is the interplanetary magnetic field? It is essentially that part of the Sun’s magnetic field that extends into the Solar System “transported” by solar winds. We can interpret its component in the north-south direction, called Bzlike a door: if Bz is positive, the door is closed and the CMEs do not reach the atmosphere; if it is negative, and far enough from zero, the door is open, particles enter and the formation of polar auroras is possible.
Last night, for example, Bz was positive, the door was closed: no matter how powerful the CME was, since it couldn’t enter it couldn’t even cause auroras visible from Italy. Which in fact there were none.
There are other parameters at play and other conditions, but you understand the concept: the phenomenon of the formation of polar auroras is more complex than what is commonly told and its prediction is an extremely difficult art. Of course, we have satellites pointed towards the Sun that warn us when a CME starts, at what speed and allow us to understand if it is pointed towards the Earth. In this case entities like it Space Weather Prediction Center (SWPC) of NOAA can produce estimates of the arrival times of coronal ejections on our planet. The SWPC is naturally also equipped with complex numerical models to simulate what will happen when the CME arrives: it is on these that it bases its forecasts and the space weather bulletins issued by NOAA.
These models, however, depend on a large amount of details that cannot be known a priori, therefore model predictions are inherently uncertain. This is why there are satellites positioned between us and the Sun, in a gravitationally stable point called L1 located one and a half million kilometers from our planet, which collect as much data as possible on CMEs and the solar wind and immediately transmit them to the ground to produce more convincing predictions. At this point, however, we only have 30 minutes approximately before the solar plasma reaches the planet and produces – possibly – its effects. For this reason NOAA’s auroral forecast is 30-90 minutes. As you can understand, it is a very short time which makes anyone who states with certainty phrases like “Tonight we will have visible auroras from Italy” absolutely uncredible.
So, in conclusion, will we see the Northern Lights from Italy again tonight? Having reiterated all the above premises, i.e. that no one can say anything for sure, we can say that at the moment the possibility of having a Kp of at least 7 cannot be ruled out and that at the time this article is published Bz is positivetherefore it does not bode well for possible sightings of the Northern Lights, especially in our country.
