Understanding the Nature of Solar Plasma through Solar Flare Temperatures

The Sun’s rotation results in changes in its magnetic field, which flips completely every 11 years or so, ushering in a phase of intense activity. Solar flares — huge eruptions from the sun’s surface that last for minutes or hours — emit intense bursts of particles and high levels of electromagnetic radiation. The release of energy during solar flares heats the chromosphere, causing almost complete ionization of the atomic hydrogen present in the region.

The chromosphere is a thin layer of plasma located at least 2,000 km above the visible surface of the Sun (photosphere) and below the corona (upper atmosphere). The plasma is extremely dense, and hydrogen recombines at a very high rate, leading to a repeated process of ionization and recombination of hydrogen that produces a distinctive type of emission of radiation in the ultraviolet range known as Lyman Continuum (LyC) in the memory of Americans. Physicist Theodore Lehmann IV (1874-1954).

Theoretical descriptions indicate that the “color temperature” of LyC can be related to the temperature of the plasma that produces the glow, and thus the color temperature can be used to determine the temperature of the plasma during solar storms.

A new study simulated emissions from dozens of different solar flares and confirmed the correlation between LyC’s color temperature and the temperature of the plasma in the region from which the flare erupted. It also confirms that a local thermodynamic equilibrium occurs in the region between plasma and photons in LyC. An article about the study has been published in Astrophysical Journal.

The penultimate author of the article is Paulo José de Aguiar Simoes, Professor in the College of Engineering at Mackenzie Presbyterian University (EE-UPM) in the state of São Paulo, Brazil. “We’ve shown that LyC intensity increases dramatically during solar flares, and Lyman spectrum analysis can really be used to diagnose plasmas,” said Simoes, who is also a researcher at the McKinsey Center for Astronomy and Astrophysics (CRAAM).

The simulations confirmed an important result obtained by Argentine astronomer Marcos Machado at the Solar Dynamics Laboratory, which showed that the color temperature, which falls in the region of 9,000 K (Kelvin) during quiet periods, rises to 12,000-16,000 K during flares.

the condition in which he reported this finding and of which Simes was also a co-author, was the last published by Machado. A world-famous expert in the sun, he passed away in 2018 while the article was being peer-reviewed.

Solar dynamics

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

“We can observe the region just above the surface. The first layer, which extends up to an altitude of about 500 kilometers, is the photosphere, with a temperature of about 5,800 K, and here we see sunspots, in the places where the magnets are. The fields emanating from the sun prevent Convection keeps the plasma relatively cool, which results in darker regions we call sunspots.”

Above the photosphere, the chromosphere extends for about 2,000 km. “The temperature of this layer is higher, exceeding 10,000 K, and the plasma is less dense. Because of these properties, atomic hydrogen is partially ionized, keeping protons and electrons separate,” he said.

In a thin transition layer at the top of the chromosphere, the temperature rises sharply to over a million K, and the plasma density drops by several orders of magnitude. This sudden heating of the passage from the chromosphere to the corona is a counterintuitive phenomenon. It would be reasonable to expect the temperature to decrease with increasing distance from the source.

“We don’t have an explanation yet,” said Simoes. “Various proposals have been put forward by solar physicists, but none of them has been accepted without reservations by the community.”

The corona extends toward the interplanetary middle, without a well-defined transition region. The Sun’s magnetic fields exert a strong influence on the corona, building up plasma, especially in active regions that are easily identifiable in ultraviolet images. Solar flares occur in these active regions.

“In these solar storms, the energy accumulated in the coronal magnetic fields is abruptly released, heating the plasma and accelerating the particles. Electrons, which have less mass, can be accelerated to up to 30% of the speed of light. Some of these particles, which travel Along magnetic lines of force, they are ejected into the interplanetary medium. Others go in the opposite direction, from the corona to the chromosphere, where they collide with the denser plasma and transfer their energy to the medium. This excess energy heats the local plasma, causing ionization of the atoms. Ionization dynamics And recombination leads to Lyman persistence.”

Spurts of solar activity occur approximately every 11 years. During periods of intense activity, the effects on Earth are significant, including more displays of aurorae, interruptions in radio communications, increased scintillation effects on GPS signals, and increased drag on satellites, which reduces their speed and thus their altitude. orbits. These phenomena and physical properties of the near-Earth interplanetary medium are known as space weather.

“Beside the basic knowledge they provide, studies of the physics of solar flares also improve our ability to predict space weather. These studies proceed in two directions: direct observation and simulation based on computational models. Observational data in different bands of the electromagnetic spectrum enables us to understand the evolution of solar flares and the physical properties of the plasma involved in these events better. Computational models, such as the one used in our study, serve to test hypotheses and verify interpretations of observations because they allow us to access quantities that cannot be obtained directly from analysis of observational data,” said Simoes.