Temperature of eruptions helps to understand solar plasma – 04/10/2023 – Science

The rotation of the Sun produces changes in its magnetic field. And that means that, every 11 years approximately, our star enters a phase of intense activity. Eruptions on the surface of the Sun (solar flares, in English) throw away large amounts of particles and release high levels of radiation.

During eruptions, the release of energy heats the chromosphere, causing almost complete ionization of the atomic hydrogen present in this region. But since the plasma is very dense, the hydrogen recombination rate is also high. As a result, a recurrent process of hydrogen ionization and recombination is established, producing a characteristic type of radiation emission, in the ultraviolet range, called “Lyman Continuum” (LyC). The name is a tribute to the American physicist Theodore Lyman IV (1874-1954).

Theoretical descriptions suggest that the so-called “color temperature” of the Lyman Continuum would be associated with the temperature of the plasma that originated the eruption. Thus, the color temperature could be used as a resource to determine the plasma temperature during solar storms.

A new study has simulated emissions from dozens of different eruptions. And it confirmed the association between the color temperature of the Lyman Spectrum (LyC) and the temperature of the plasma in the region where the emission originates. It also confirmed that the region reaches a local thermodynamic equilibrium between the plasma and the photons that make up the LyC. An article about it was published in The Astrophysical Journal: “Formation of the Lyman Continuum during Solar Flares”.

The study had the support of FAPESP and the participation of Brazilian Paulo José de Aguiar Simões, a professor at the School of Engineering at the Mackenzie Presbyterian University and a researcher at the Mackenzie Center for Radio Astronomy and Astrophysics. “We showed that the Lyman Continuum is greatly intensified during solar flares. And that the analysis of the LyC spectrum can really be used for plasma diagnosis”, says Simões.

The simulations corroborated an important observational result obtained at the Solar Dynamics Observatory by Argentine astronomer Marcos Machado. This showed that the color temperature, which in calm periods is around 9,000 kelvins, rises, in flares, to the range of 12,000 to 16,000 kelvins. The article in which he communicated this result, and which also had the collaboration of Simões, was the last published by Machado. An international reference in studies of the Sun, the Argentine astronomer died in 2018, during the revision of the text.

solar dynamics

It is worth remembering here a little of what is known about solar structure and dynamics. The enormous amount of energy that provides the Earth with light and heat is mainly generated by the conversion of hydrogen into helium. Such a nuclear fusion process takes place inside the star, but this vast region is inaccessible to direct observation, because light does not pass through the “surface” of the Sun. “What we can observe directly is located from the surface outwards. And the first layer, which extends to an altitude of about 500 kilometers, is called the photosphere. Its temperature is around 5,800 kelvins. It is in this region that the spots appear solar panels, in places where the magnetic fields emerging from the interior inhibit convection, keeping the plasma cooler —which produces the dark appearance of the spots”, informs Simões.

Above the photosphere, the chromosphere extends for another 2,000 kilometers or so. “In this layer, the temperature increases, reaching more than ten thousand kelvins, and the plasma density decreases. Due to these characteristics, atomic hydrogen is partially ionized, with protons and electrons separated”, teaches the researcher.

At the top of the chromosphere, in a thin transition layer, the temperature rises sharply, past 1 million kelvins, and the plasma density drops by many orders of magnitude. This sudden heating in the passage from the chromosphere to the corona is a counterintuitive phenomenon, as one would expect a decrease in temperature with increasing distance from the source. “We still don’t have an explanation for this. Several proposals were presented by solar physicists, but none was unreservedly accepted by the community”, points out Simões.

The corona extends towards the interplanetary medium, without a new defined transition region. In it, the influence of magnetic fields is remarkable, structuring the plasma, especially in the so-called active regions, easily identified in ultraviolet images, such as the one reproduced at the beginning of this report. It is in these active regions that solar flares occur.

“In these solar storms, the energy accumulated in the coronal magnetic fields is suddenly released, heating the plasma and accelerating the particles. The electrons, because they have a smaller mass, can be accelerated to up to 30% of the speed of light. A part of these particles , which travel along magnetic field lines of force, is released into the interplanetary medium. Another part follows the opposite path, from the corona to the chromosphere—where it undergoes collisions in the high-density plasma and transfers its energy to the medium. of energy heats up the local plasma, causing ionization of atoms. The dynamics of ionization and recombination give rise to the Lyman Continuum”, details the researcher.

Solar activity peaks at approximately 11-year intervals. During periods of high activity, the effects on Earth are quite clear: greater occurrence of aurora borealis; blackouts in radio communications; increased flicker effect on GPS signals; increased drag force on satellites, reducing their speed and, consequently, the altitude of their orbits. The set of these phenomena, together with the physical properties of the interplanetary medium close to Earth, is called space weather.

“In addition to the fundamental knowledge they provide, studies of the physics of solar storms also contribute to improving our ability to predict space weather. These studies walk on two legs: direct observations and simulations based on computational models. Observational data in the different ranges of the electromagnetic spectrum allow us to better understand the evolution of solar storms and the physical properties of the plasma involved in the event. Computational models, such as the ones we use in the study in question, are used to test hypotheses and verify interpretations of the observations, since they give us access to quantities that cannot be directly obtained from the analysis of observational data”, summarizes Simões.

The article “Formation of the Lyman Continuum during Solar Flares” is freely accessible here.