The temperature of solar flares helps understand the nature of solar plasma

May 16, 2023

(Nanowerk News) The Sun’s rotation produces changes in its magnetic field, which reverse completely every 11 years or so, triggering phases of intense activity. Solar flares – large eruptions from the surface of the Sun that last minutes or hours – emit intense bursts of particulate matter and high levels of electromagnetic radiation. The release of energy during solar flares heats up the chromosphere, causing almost complete ionization of the hydrogen atoms present in that region.

The chromosphere is a thin layer of plasma that lies at least 2,000 km above the Sun’s visible surface (photosphere) and below the corona (the Sun’s upper atmosphere). Plasma is very dense, and hydrogen recombines at very high rates, resulting in repeated processes of ionization and recombination of hydrogen that produce a characteristic type of emission of radiation in the ultraviolet band known as the Lyman Continuum (LyC) in American memory. physicist Theodore Lyman IV (1874-1954).

The theoretical description suggests that the “color temperature” of LyC can be related to the temperature of the plasma that generates flares, and therefore color temperature can be used to determine the temperature of the plasma during solar storms. A new study simulated dozens of solar flares and demonstrated that analysis of the spectra of the Lyman Continuum formed by hydrogen ionization and recombination can be used for the diagnosis of solar plasma (ultraviolet image of the Sun at a wavelength of 17.1 nanometers, highlighting the spectral lines emitted by ferrous iron atoms;( Image: Solar Dynamics Observatory/NASA)

A new study has simulated emission from dozens of different solar flares and confirmed the relationship between the color temperature of LyC and the temperature of the plasma in the region where the flares erupt. This also confirms that a local thermodynamic equilibrium occurs in the region between plasma and photons in LyC.

An article about this research was published in Astrophysics Journal (“Formation of the Lyman Continuum during Solar Flares”).

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

The simulation corroborates important results obtained at the Laboratory of Solar Dynamics by the Argentine astronomer Marcos Machado showing that the color temperature, which in calm periods is in the region of 9,000 Kelvin (K), rises to 12,000-16,000 K during flares. The article in which he reported these results and which was also written by Simões, was the last published by Machado. A world-renowned Sun expert, he died in 2018 while the article was under peer review.

Sun dynamics

Here it is necessary to recall a little about what is known about the structure and dynamics of the Sun. A large amount of the energy that gives Earth light and heat is mainly generated by the conversion of hydrogen to helium in the process of nuclear fusion that occurs deep within the star. This vast region cannot be observed directly because light does not traverse the “surface” of the Sun, which is actually the photosphere.

“We can observe the area above the surface directly. The first layer, which extends to an altitude of about 500 km, is the photosphere, with a temperature of about 5,800 K. This is where we see sunspots, where the magnetic field rising from the Sun inhibits convection and keeps the plasma relatively cool, resulting in the darker areas we see. call sunspots,” explains Simões.

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 this characteristic, the hydrogen atom is partially ionized, separating the protons and electrons,” he said.

In the thin transition layer at the top of the chromosphere, the temperature rises sharply above 1 million K, and the plasma density drops many times. This sudden heating on the way from the chromosphere to the corona is a counter-intuitive phenomenon; it is reasonable to expect the temperature to drop as the distance from the source increases.

“We have not received an explanation. Various proposals have been put forward by solar physicists, but none have been unconditionally accepted by society,” says Simões.

The corona extends toward the interplanetary medium, with no clear transition region. The Sun’s magnetic field exerts a strong influence on the corona, composing the plasma, especially in the active regions which are easily identifiable in ultraviolet images.

“In these solar storms, the energy accumulated in the coronal magnetic field is suddenly released, heating the plasma and accelerating the particles. Electrons, which have less mass, can be accelerated to 30% of the speed of light. Some of these particles, moving along the lines of magnetic force, were ejected into the interplanetary medium. Others go in the opposite direction, from the corona to the chromosphere, where they collide with high-density plasma and transfer their energy to the medium. This excess energy heats up the local plasma, causing ionization of the atoms. The dynamics of ionization and recombination gave rise to the Lyman Continuum,” said Simoes.

A spike in solar activity occurs approximately every 11 years. During periods of intense activity, the effects on Earth are enormous, including more displays of the aurora borealis, blackouts of radio communications, increased scintillation effects on GPS signals, and increased drag on satellites, reducing their speed and therefore altitude. orbit. These phenomena and physical properties of the near-Earth interplanetary medium are known as space weather.

“Besides the fundamental knowledge they provide, the study of the physics of solar flares also enhances our ability to predict space weather. These studies go both ways: direct observation, and simulation based on computational models. Observational data in various bands of the electromagnetic 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 verify the interpretation of observations because they give us access to quantities that cannot be obtained directly from the analysis of observational data, ”says Simões.

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