A study involving Brazilian researchers appears to have shown that the standard model currently used for solar flares is outdated. The data obtained through telescopic observation of the Sun could not be explained by any of the theories applied.
Solar flares are extremely intense events that occur in the Sun’s atmosphere and last from minutes to a few hours. According to the standard model, the energy that triggers such phenomena is transported by accelerated electrons that rush from the magnetic reconnection region in the corona to the chromosphere.
Through collisions, these electrons deposit energy in the chromosphere, causing heating and ionization of the plasma and intense radiation in several bands of the electromagnetic spectrum. The regions of energy deposition are called the “feet” of the explosion arcs and usually appear in magnetically connected pairs.
To test the validity of the standard model, a recent study compared results from computer simulations based on the model with observational data provided by the McMath-Pierce telescope during the SOL2014-09-24T17:50 eruption. The focus of the study was to measure the time lag in infrared (IR) radiation emission from two paired chromospheric sources.
The work was published in the newspaper Monthly Notices of the Royal Astronomical Society.
“We found an important difference between the data provided by telescopic observation and the behavior predicted by the model. In telescopic observation, the paired arc feet appear as two intensely luminous regions in the solar chromosphere. Since the incident electrons originate from the same region of the corona and follow similar trajectories, one would expect, based on the model, that the two spots would shine almost simultaneously in the chromosphere. However, this was not what the telescopic observation showed. There was a delay of 0.75 seconds between one brightness and the other,” says Paulo José de Aguiar Simões, professor at the School of Engineering at Mackenzie Presbyterian University, researcher at the Mackenzie Center for Radio Astronomy and Astrophysics and first author of the article.
A delay of 0.75 seconds may seem insignificant, but considering all possible geometric configurations, the researchers found that, based on the model, the maximum delay would be 0.42 seconds. The actual number is significantly higher.
“We used a sophisticated statistical technique to infer the time differences in the emissions from the arc feet, and the so-called Monte Carlo method to estimate the uncertainties of these values. In addition, the results were tested by electron transport simulations and radiative-hydrodynamic simulations. Using all these resources, we were able to construct different scenarios for the flight time of electrons from the corona to the chromosphere and the time of production of infrared radiation. All scenarios based on the simulations presented time differences much smaller than those obtained by telescopic observation,” Simões reported.
One of the scenarios tested was that of spiralization and magnetic trapping of electrons in the corona. “Using electron transport simulations, we explored scenarios of magnetic asymmetry between the arc feet. The expectation was that the greater the difference in magnetic field intensities between the arc feet, the greater the time delay in electron penetration into the chromosphere. This should also cause a greater discrepancy in the amount of electrons reaching the chromosphere, due to the magnetic trapping effect,” he said.
“However, the analysis of the observational X-ray data showed very similar intensities originating at the feet of the arches, indicating similar amounts of electron deposition in these regions. Therefore, this was not the cause of the delay observed in the emissions,” the researcher stated.
Radiative-hydrodynamic simulations also showed that ionization and recombination times in the chromosphere are too fast to explain the delay.
“We simulated the generation time of infrared emissions. In addition to calculating the transport of electrons to the chromosphere, we also calculated their energy deposition and the consequences they produce in the plasma: heating; expansion; ionization and recombination of hydrogen and helium atoms; radiation produced locally, which has the effect of releasing excess energy. Infrared radiation is produced as a result of the increase in electron density in the chromospheric medium, a consequence of the ionization of hydrogen, originally in a neutral state in the plasma,” he explained.
The results of the simulations showed that, with the penetration of accelerated electrons, ionization and generation of infrared emission are almost instantaneous. “And therefore, unable to explain the 0.75 second delay between the arc foot emissions,” added Simões.
In short, none of the processes simulated using the model were able to explain the observed data. Given this, the researchers’ conclusion was somewhat obvious: the standard model needs to be reformulated. This is how science works.
“The observed time delay between chromospheric sources challenges the standard model of energy transport by electron beams. The existence of a larger time delay suggests that other energy transport mechanisms may be at play. Mechanisms such as magnetosonic waves, conductive transport or other forms of energy transport may be necessary to explain the observed time delay. These additional mechanisms need to be considered for a complete understanding of solar flares,” the paper summarizes.
The study received support from FAPESP through two projects (13/24155-3 and 22/15700-7 ).
This content was originally published in New study questions the validity of the standard model of solar flares on the CNN Brasil website.
Source: CNN Brasil