One study suggests that the “color temperature” of the rays is linked to the temperature of the plasma from solar flares
The rotation of the sun creates changes in its magnetic field. And that means the star enters a period of intense activity every about 11 years. Eruptions on the solar surface (solar flares in English) blow off large amounts of particles and release high levels of radiation.
During eruptions, the release of energy heats the chromosphere, resulting in almost complete ionization of the atomic hydrogen present in that region. However, since the plasma is very dense, the hydrogen recombination rate is also high.
As a result, a recurring process of hydrogen ionization and recombination is established, producing a characteristic type of radiation emission in the ultraviolet range called LyC (Lyman Continuum). The name is a tribute to the American physicist Theodore Lyman IV (1874-1954).
Theoretical descriptions suggest that the call “color temperature” of the Lyman continuum have been linked to the temperature of the plasma that started the eruption. Thus, color temperature could be used as a resource for determining plasma temperature during solar storms.
A new study has simulated emissions from dozens of different eruptions. And it confirmed the relationship between the color temperature of the Lyman continuum and the temperature of the plasma in the region where the emission originated.
It also confirmed that the region reaches local thermodynamic equilibrium between the plasma and the photons that make up the LyC. An article about it was published in the magazine The Astrophysical Journal with the title “Formation of the Lyman Continuum during solar flares”.
The study was supported by FAPESP (São Paulo State Research Support Foundation) and the participation of Brazilian Paulo José de Aguiar Simões, Professor at the Mackenzie Presbyterian University School of Engineering and researcher at the Mackenzie Center for Radio Astronomy and Astrophysics.
“We show that the Lyman continuum is greatly intensified during solar flares. And this LyC spectrum analysis can really be used for plasma diagnosis.” says Simões.
The simulations confirmed an important observation that Solar Dynamics Observatory by Argentine astronomer Marcos Machado.
Machado showed that the color temperature, which is around 9,000 kelvins (8,726.85 ºC) in calm conditions, rises to the range of 12,000 to 16,000 kelvins (11,726.85 ºC to 15,726.85 ºC) in flares. The article in which he communicated this result, and in which Simões also contributed, was the last published by Machado. The Argentine astronomer, an international reference for solar studies, died in 2018 while the text was being revised.
solar dynamics
It is worth recalling a little of what is known about solar structure and dynamics here. The enormous amount of energy that provides the earth with light and heat comes mainly from 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 it “Surface” from the sun.
“What we can observe directly is from the surface outward. And the 1st layer, which extends to about 500 kilometers altitude, is called the photosphere. Its temperature is of the order of 5,800 Kelvin [5.526,85 ºC]. It is in this region that sunspots form, where the magnetic fields emanating from the interior inhibit convection and keep the plasma cooler – producing the dark appearance of the sunspots.” informs Simões.
Above the photosphere, the chromosphere extends about another 2,000 kilometers. “In this layer, the temperature rises to over ten thousand Kelvin and the plasma density decreases. Because of these properties, atomic hydrogen is partially ionized, with separated protons and electrons. explains the researcher.
At the top of the chromosphere, in a thin transition layer, the temperature increases sharply, over 1 million Kelvin (999,726.85 °C), and the plasma density decreases by many orders of magnitude.
This sudden warming at the transition from the chromosphere to the corona is a counterintuitive phenomenon since one would expect the temperature to decrease with increasing distance from the source. “We don’t have an explanation for that yet. Several suggestions have been made by solar physicists, but none have been unreservedly accepted by the community.” emphasizes Simões.
The corona extends towards the interplanetary medium without a newly defined transition region. In it, the influence of magnetic fields structuring the plasma is remarkable, especially in the so-called active regions, easily discernible in ultraviolet images like the one reproduced at the beginning of this report. Solar flares occur in these active regions.
“During these solar storms, the energy stored in the coronal magnetic fields is suddenly released, heating the plasma and accelerating the particles. Due to their lower mass, electrons can be accelerated to up to 30% of the speed of light. Some of these particles, moving along the lines of force of the magnetic field, are ejected into the interplanetary medium.” says the researcher.
“Another part goes the opposite way from the corona to the chromosphere – where it collides in the high-density plasma and transfers its energy to the medium. This excess energy heats the local plasma, causing atoms to be ionized. The dynamics of ionization and recombination lead to the Lyman continuum.” complete.
Solar activity peaks at roughly 11-year intervals. During times of high activity, the effects on Earth are quite clear: increased occurrence of Aurora Borealis; power failures in radio communications; increased flicker effect on GPS signals; increased drag on satellites, reducing their speed and consequently the altitude of their orbits.
The set of these phenomena along with the physical properties of the near-Earth interplanetary medium is referred to as “Space Weather”.
“In addition to the fundamental insights they provide, studies of the physics of solar storms also help improve our ability to predict space weather. These studies stand on two legs: direct observations and simulations based on computational models.
“Observational data in the different bands 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. Computer models like the ones we used in the study in question are used to test hypotheses and to check interpretations of observations, as they give us access to quantities that cannot be obtained directly from analysis of observational data.” sums up Simões.
The item “ Formation of the Lyman Continuum during solar flares” can be accessed at this link.
With information from the agency FAPESP