Northern lights over the Lomnicky stit peak. How was it seen by the neutron monitor
Aurora, more commonly the northern lights (aurora borealis) or the southern lights (aurora australis), is a natural phenomenon that occurs when high energy particles of cosmic rays penetrate to the upper atmosphere (thermosphere/exosphere). Depending on their residual energy they excite or ionise the molecules of air. The aurora itself then originates as a result of de-excitation and de-ionization of the air molecules, and its intensity depends on the flux of particles that overcome the Earth’s magnetic field shield.
As we mentioned previously, the Earth’s magnetic field acts as a shield against cosmic rays which are composed almost only from high energy charged particles. Its shielding capability is weaker near poles and reaches maximum values in equator regions. That is the reason why we can commonly observe the aurora in the very northern and southern Earth’s regions and why the aurora is rare in regions located in Earth’s mid-latitudes.
Nevertheless, there exist Sun events which can create disturbances in the Earth’s magnetic field and weaken the shielding capability in mid latitude and equator regions.
One of the most important events related to aurora forming is the coronal mass ejection or CME. CME is a significant ejection of the solar plasma from the Sun's corona. Merely stated, when such an ejection impacts the Earth's magnetosphere, the shielding ability of Earth’s magnetic field against cosmic rays decreases and allows high energy protons and electrons to enter the upper atmosphere sometimes even in the mid latitude regions.
One of the European countries, which lie in the mid latitude regions, is Slovakia. Northern lights on Slovakian sky are very rare but they do occur. This seemed to happen also during the early evening on 05 November 2023 when many people across Slovakia were watching the sky in dumb amazement. North eastern part of the sky above Slovakia and the Lomnický štít peak observatory just turned into purple pink colour.
One might assume that it was the aurora borealis. The statement is supported by the optical celestial observations of that evening, nevertheless due to the mid latitude location of the region we should confirm this assumption with proof about the CME from the Sun. Therefore are there any other proofs that could confirm this assumption? Fortunately, yes. CME can be observed in many ways - via dedicated satellites orbiting the Earth, solar telescope or by one very interesting instrument - the standard neutron monitor.
There exists a global network composed of more than 50 standard neutron monitors (data of most of them are available at https://www.nmdb.eu/) and one of these instruments is located also the Lomnický štít peak observatory and operated by the Institute of Experimental Physics SAS.
As the name already implies, the standard neutron monitor is an instrument for measurement of secondary neutrons that originate in the Earth’s atmosphere as a result of nuclear reactions between primary charged protons and air molecules in the atmosphere. In fact, it is one of many products of interactions of cosmic rays that undergo primary cosmic rays in the atmosphere. One should note that the standard neutron monitor at the Lomnický štít peak is of type 8NM64, works continuously from December 1981 and it is not a small and lightweight instrument, e.g. it has block shape with dimensions of 4m x 2.4m x 0.5m and contains approximately 12 metric tons of lead.
So let’s have a look at events from 05 November 2023 again. The flux of secondary neutrons as measured by the neutron monitor at the Lomnický štít peak is illustrated in the figure below. As can be seen, we observe an increase in secondary neutron rates which starts approximately at 10:00 Am and reaches its maximum at noon. This secondary neutron peak results from interaction of the most energetic part of CME particles that enter the Earth’s atmosphere. The aurora was not visible just because simply the daylight was more intense than the aurora itself. Later, in the evening, the CME particles did not have sufficient energy to increase the secondary neutron flux in the atmosphere, so we do not observe a similar peak in the evening hours. Nevertheless, due to known typical time and energy spectrum profiles of the CME we can state that the northern lights in the evening were the consequence of the CME event captured by the neutron monitor at noon.
So finally, we may say that on 05 November 2023, we did not observe only the aurora borealis above Lomnický štít peak and Slovakia sky, but we observed also a increase of secondary neutron flux as a result of massive CME from the sun.
Next time, we will familiarise you with another similar atmospheric phenomenon named S.T.E.V.E. and discovered just only in 2016. This effect is similar to the aurora but it is not aurora. Until then enjoy the beauty of CME that painted the Slovakia sky on 05 November 2023.
On video (in the loop) CMEs before 05 November 2023 as seen by SDO / AIA and SOHO / LASCO Coronagraph.
Text: Ján Kubančák a spolupracovníci, Ústav experimentálnej fyziky SAV, v. v. i.
Foto: ÚEF SAV, v. v. i., Vladimír Šifra a NMDB
Video, source: https://helioviewer.org/