The rotation of the Sun causes changes in its magnetic field, which reverses approximately every 11 years, causing a phase of intense activity. Solar flares – huge eruptions from the surface of the Sun lasting minutes or hours – emit intense particle emissions and high levels of electromagnetic radiation. The release of energy during solar flares heats up the chromosphere, causing an almost complete ionization of the atomic hydrogen present in the region.

The chromosphere is a thin layer of plasma located at least 2000 km above the visible surface of the Sun (photosphere) and below the corona (upper layers of the Sun’s atmosphere). The plasma is very dense and the hydrogen recombines at a very high rate, resulting in a repetitive process of hydrogen ionization and recombination that produces a characteristic type of radiation in the ultraviolet known as the Lyman continuum (LyC) in memory of the American. physicist Theodore Lyman IV (1874-1954).

Theoretical descriptions suggest that the “color temperature” of LyC may be related to the temperature of the plasma causing the flare, and therefore the color temperature may be used to determine the temperature of the plasma during solar storms.

The new study simulated emissions from dozens of different solar flares and confirmed the relationship between the color temperature of LyC and the plasma temperature in the region from which the flare originated. This also confirms the existence of local thermodynamic equilibrium in the region between plasma and photons in LyC. An article about the study was published in Astrophysical journal.

The penultimate author of the article is Paulo José de Aguiar Simões, professor at Mackenzie Presbyterian University School of Engineering (EE-UPM) in São Paulo State, Brazil. “We show that LyC intensity increases significantly during solar flares and that Lyman spectrum analysis can indeed be used to diagnose plasma,” said Simoes, who is also a researcher at the Mackenzie Center for Radio Astronomy and Astrophysics (CRAAM).

The simulations confirmed an important result from the Solar Dynamics Laboratory by Argentinean astronomer Marcos Machado, showing that the color temperature, which is in the region of 9000 Kelvin (K) during quiet periods, rises to 12,000–16,000 K during flares.

V article in which he reported this result and was also co-authored by Simões, which was the last one published by Machado. A world-famous solar expert, he died in 2018 while the paper was under peer review.

solar dynamics

Here it is worth recalling a little of what is known about the structure and dynamics of the Sun. The vast amount of energy that provides the Earth with light and heat is mainly generated by converting hydrogen into helium in the process of nuclear fusion, which takes place deep inside the star. This vast region is not directly visible because the light does not cross the “surface” of the Sun, which is actually the photosphere.

“We can directly observe the region above the surface. The first layer, extending to an altitude of about 500 km, is the photosphere with a temperature of about 5800 K. It is here that we see sunspots, in places where the magnetic field of the field emanating from the sun interferes with convection and keeps the plasma relatively cool, creating dark areas, which we call sunspots,” Simones explained.

Above the photosphere, the chromosphere extends for about 2000 km. “The temperature of this layer is higher, exceeding 10,000 K, and the plasma is less dense. Due to these characteristics, atomic hydrogen is partially ionized, separating protons and electrons,” he said.

In a thin transition layer in the upper part of the chromosphere, the temperature rises sharply above 1 million K, and the plasma density drops by many orders of magnitude. This sudden heating during the transition from the chromosphere to the corona is a phenomenon contrary to common sense; it would be reasonable to expect the temperature to drop as the distance from the source increases.

“We don’t have an explanation yet. Solar physicists have submitted various proposals, but none of them has been accepted by the community without reservations,” Simones said.

The corona extends towards the interplanetary medium, without a clear transition region. The magnetic fields of the Sun have a strong effect on the corona, structuring the plasma, especially in active regions that are easily identified in ultraviolet images. Solar flares occur in these active regions.

“In these solar storms, energy stored in coronal magnetic fields is released abruptly, heating the plasma and accelerating particles. Electrons that have less mass can be accelerated up to 30% of the speed of light. These particles, moving along magnetic lines of force, are ejected into the interplanetary medium. Others move in the opposite direction, from the corona to the chromosphere, where they collide with dense plasma and transfer their energy to the medium. heats the local plasma, causing the ionization of atoms. The dynamics of ionization and recombination gives rise to the Lyman continuum,” Simoes said.

Solar bursts occur approximately every 11 years. During periods of intense activity, the effects on the Earth are significant, including more frequent aurora borealis, radio outages, increased scintillation effects on GPS signals, and increased drag on satellites, reducing their speed and hence their altitude. orbits. These phenomena and the physical properties of the near-Earth interplanetary medium are known as space weather.

“In addition to the fundamental knowledge they provide, solar flare physics research also improves our ability to predict space weather. These studies are going in two directions: direct observation and modeling based on computational models. Observational data in various ranges of the electromagnetic radiation spectrum allows us to better understand the evolution of solar flares and the physical properties of the plasma involved in these events. Computational models such as those used in our study serve to test hypotheses and test interpretations of observations because they give us access to quantities that cannot be obtained directly from analysis of observational data,” Simoins said.