Intense geomagnetic storms were recorded this weekend, as a result of solar flares detected by the NASA Solar Observatory, the North American space agency. In addition to producing northern lights and southern lights, these explosions have the potential to disrupt communications, electrical power transmission, navigation, and radio and satellite operations.
Phenomena even more intense than those recently occurring on the Sun were studied in not-so-distant stars (Kepler-411 and Kepler-396) by researchers from the Mackenzie Radio Astronomy and Astrophysics Center, Mackenzie Presbyterian University, in Brazil, and the School of Physics and Astronomy, from the University of Glasgow, Scotland. An article about this was published in the Monthly Notices of the Royal Astronomical Society.
“Just as solar explosions have an impact on Earth, the superexplosions that were the focus of this study can affect the atmosphere of exoplanets and impact, among other factors, the conditions for the formation or destruction of eventual microbiological life on these planets”, Paulo explains to Agência Fapesp. Simões, professor at Universidade Presbiteriana Mackenzie and first author of the article.
Despite their main purpose being the search for exoplanets, telescopes such as the Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (Tess) have provided a vast amount of data on stellar explosions, detected with excellent filter photometry. broadband in the visible light range.
As the stars are very far away, they are seen through telescopes only as bright points. And the phenomena interpreted as explosions are sudden increases in luminosity at these points.
There is also a lack of data in other bands of the electromagnetic spectrum. And most studies on these events focus on the issue of radiated energy: superexplosions (superflares), with energies 100 to 10 thousand times greater than those of the most energetic solar explosions, have been found. The question is which model best explains these very high energy levels.
There are two main models in comparison. The most adopted treats superexplosion radiation as the emission of a black body at a temperature of 10 thousand Kelvin. The other associates the phenomenon with a process of ionization and recombination of hydrogen atoms. The study in question analyzed both models. The group received support from FAPESP through three projects (18/04055-8, 21/02120-0 and 22/15700-7).
“Given the known energy transfer processes in flares, we argue that the hydrogen recombination model is physically more plausible than the blackbody model for explaining the origin of broadband optical emission,” says Simões.
The researchers compared 37 events from the Kepler-411 star system and five events from the Kepler-396 star, using both radiation mechanisms. “We found that the estimates for the total explosion energy based on the hydrogen recombination model are about an order of magnitude smaller than the values obtained from blackbody radiation. And they adapt better to known processes”, says Simões.
These processes are described from solar explosions. Despite many differences, solar flares continue to fuel the models on which interpretations of stellar explosions are based. After all, there is a vast amount of information accumulated about solar flares, which were recorded, for the first time, independently, by two English astronomers, Richard Carrington and Richard Hodgson, on September 1, 1859.
“Since that time, solar flares have been observed as an intense glow lasting from seconds to hours, in different wavelengths: radio, visible light, ultraviolet and X-ray. These flares are one of the most energetic phenomena in our Solar System and can affect satellite operations, radio communications, power transmission lines, navigation systems and GPS operation, to name a few examples”, informs Alexandre Araújo, doctoral student at the Mackenzie Radio Astronomy and Astrophysics Center, professor at the Municipal Secretariat of Education of São Paulo and co-author of the article.
Solar flares occur in active regions, associated with intense magnetic fields. The energy accumulated in the magnetic fields of the solar corona, the outermost part of the Sun, is released suddenly, heating the plasma and accelerating particles, such as electrons and protons.
“Because they have a lower mass, electrons can be accelerated to considerable fractions of the speed of light – typically up to 30%, but sometimes reaching greater values. The accelerated particles travel along magnetic field lines: some are thrown out into interplanetary space, while another part travels in the opposite direction, towards the chromosphere, located below the corona, where it collides with the high-density plasma and transfers its energy to the environment. The excess energy heats the local plasma, causing ionization and excitation of the atoms and, consequently, the production of radiation, which we detect with telescopes on the ground and in space”, describes Simões.
Since the 1960s, numerous observational and theoretical studies have attempted to explain the generation of excess visible light caused by explosions, but there is still no definitive solution. These studies gave rise to the two main alternatives already mentioned: (1) the model of blackbody radiation caused by heating in the photosphere, a layer located below the chromosphere; (2) radiation by hydrogen recombination in the solar chromosphere itself. It is worth explaining that recombination occurs when hydrogen protons and electrons, separated by the ionization process, come back together, forming atoms.
“The limitation of the first case can be summarized as an energy transport issue: none of the energy transport mechanisms normally accepted for solar flares has the capacity to deliver the necessary energy to the photosphere to cause plasma heating in a way that explains the observations ”, argues Simões.
And Araújo adds: “Calculations made in the 1970s – later confirmed by computer simulations – show that the majority of electrons accelerated in solar explosions cannot cross the solar chromosphere, reaching the photosphere. Thus, the blackbody model to explain the production of white light in solar flares is incompatible with the main accepted energy transport process for solar flares.”
The researchers regret that the hydrogen recombination radiation model, which is more physically consistent, cannot yet be confirmed through observations. Your article provides, in any case, reinforcement for the use of this model, which has been neglected in most studies.
The article “Hydrogen recombination continuum as the radiative model for stellar optical flares” can be accessed here.
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