Evolution of the solar Lyman-alpha profile line Print E-mail

Lyman-alpha line is one of the most prominent features in the UV part of the solar spectrum. It allows us to estimate the magnitude of radiation pressure, which is a force that photons from the Sun exert on hydrogen atoms. Radiation pressure is, next to the gravitational force, the main factor that determines the trajectories of neutral hydrogen and deuterium atoms inside the heliosphere.

The accurate knowledge of this parameter allows us to calculate the density of interstellar neutral hydrogen inside the heliosphere, and consequently to investigate the derivative populations of H atoms important for heliospheric physics, including pickup ions and energetic neutral atoms.

figure1Radiation pressure depends on solar activity, which is constantly changing. Even though variations related to the cycle of solar activity have been observed for hundreds of years, the mechanisms responsible for solar activity are still not fully understood. Every attempts of predicting solar activity are like weather forecasting – the main trend agrees with the predictions, but the details may turn out different. One of the indicators of the solar cycle activity, namely the total solar irradiance, is shown in Figure 1.


Previous models of the Lyman-alpha line profile were based on just a few observations and therefore they were not able to reproduce the evolution related to solar afigure2ctivity with a sufficient accuracy. A team of scientists from Laboratory of Solar System Physics and Astrophysics, CBK PAN, led by Dr. Izabela Kowalska-Leszczyńska decided to take that challenge and apply a new approach to the modeling of the evolution of the solar Lyman-alpha line profile. This new approach was possible thanks to new observations from the SOHO satellite that have been available since 2015. Based on these observations, scientists from CBK PAN developed an analytical formula composed of tree parts. Each of them describes a different feature of the profile line, shown in Figure 2 (the main line shape – blue line, the self-reversal in the center – red line, and a small slope of the whole line with respect to the vertical axis – green line).

figure3It turned out that by using that function the observations taken over several years (covering almost a full cycle of solar activity) could be reproduced with a very good accuracy. The example is shown in Figure 3, where two observed profiles (dots) are compared with fitted functions (lines). Furthermore, it was found that parameters of function shown in Figure 2 (e.g., the widths of the component functions, the depth of the self-reversal, the shifts of the profile component relative to each other) can be expressed as linear combinations of the total solar irradiance in the Lyman-alpha line. The evolution of the latter quantity strongly depends of the level of solar activity. All parameters and linear fits as a function of the total solar irradiance in the Lyman-alpha line are shown in Figure 4. Results of this analysis have been presented in a paper published in The Astrophysical Journal (link).figure4

The radiation pressure is very important for analyzing the interstellar hydrogen that is inflowing to our heliosphere, as well as for many derivative populations of particles used to diagnose different aspects of the heliosphere. The radiation pressure obtained from the new model is different from the one that was used so far (particularly during periods of low solar activity). Therefore, as a continuation of this study, scientists from CBK PAN are analyzing the consequences of the new model on different aspects of heliospheric physics.

Dr. Izabela Kowalska-Leszczyńska

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