How well do we understand the resonant radiation pressure in the heliosphere? Print E-mail
Thursday, 22 November 2018 15:13

In the beginning of this year, scientists from the Laboratory for Solar System Physics and Astrophysics of the Space Research Centre, Polish Academy of Sciences (CBK PAN) developed a new model (see here) of the resonance radiation pressure acting on hydrogen atoms in the heliosphere.

This model is based on larger observational sample than the model used previously, and better reproduces effects related to changes in the solar activity. Therefore, it is expected to be able to better reproduce the force caused by radiation pressure, which together with the solar gravity force determines the trajectories of hydrogen atoms in the heliosphere. Now, while studying the consequences of using new model, the team of scientists from CBK PAN came to surprising conclusions.

Radiation pressure plays a very important role in determination of the interstellar hydrogen distribution in the heliosphere. Studies of the interstellar neutral hydrogen in the heliosphere result in better understanding of the interactions between the heliosphere and surrounding interstellar medium and, of the properties of that medium. In order to create a reliable model of the hydrogen distribution, it is very important to know how the force due to the resonance radiation pressure is changing in time and with distance from the Sun.

The hydrogen distribution can be analyzed based on observations of the heliospheric backscatter glow in the resonance spectral line Lyman-alpha. One of the instruments (GLOWS) on the future NASA mission IMAP will be dedicated to exactly this purpose. Another method is direct detection of atoms by instruments like IBEX-Lo. The heliospheric glow is caused by the fluorescence radiation of hydrogen atoms that are illuminated by the solar spectral line Lyman-alpha – the same one that is the source of the resonance radiation pressure effect.

Scientists from CBK PAN Dr. Izabela Kowalska-Leszczyńska, Prof. Maciej Bzowski, Dr. Justyna Sokół and MSc Marzena Kubiak examined how the new radiation pressure model affects the interstellar neutral hydrogen distribution in the heliosphere and the distributions of its derivatives populations of particles. First of all, they compared the densities of hydrogen predicted by the old model and by the new one. It turned out that during the low solar activity there should be more hydrogen according to the new model. The situation is reversed during the high solar activity, when the new model predicts less gas. The largest differences between the two considered models are in a region located opposite to the direction of inflow of the interstellar gas to the heliosphere.

Another aspect considered by the research team was the value of the density of interstellar neutral hydrogen at the termination shock. Beforehand, it was estimated by scientists from CBK PAN in 2008 using the old model of radiation pressure and the flux of pick up ions observed by the Ulysses mission. The new value of hydrogen density calculated using the new model of radiation pressure turned out to be statistically consistent with the old one due to the large measurement uncertainty, which was at the level of 25% of the measured value.

Another important question was the influence of the new radiation pressure model on the hydrogen atoms flux seen by the IBEX-Lo detector. Scientists from USA and Russia discovered that the ratios of fluxes of hydrogen atoms observed by IBEX-Lo in different energy bands are not consistent with the models. They suggested that the reason for this may be an insufficient understanding of radiation pressure acting on hydrogen atoms in the heliosphere. Simulations performed by the scientists from CBK PAN showed that indeed, the expected fluxes of h atoms are very sensitive to details of the radiation pressure model, but even using the newest and the more accurate model does not remove the observed discrepancy.

Therefore scientific team from CBK PAN have challenged the existing paradigm how the radiation pressure actually works. According to the current views, if the flux of photons from the Sun decreases with the square of solar distance, then the force due to the radiation pressure should behave in the same way. Since the gravity force also decreases with the square of solar distance, then the ratio of these two forces acting on hydrogen atom should be constant no matter where it is measured. But is that assumption true, if some of the photons are scattered by hydrogen atoms? The scientists from CBK PAN calculated how many of the original photons emitted by the Sun are scattered on hydrogen atoms depending on the location in the heliosphere. They found that even at relatively small distances (within approximately 10 astronomical units – around Saturn’s orbit) the scattering losses can reach 30% of the Lyman-α photons within the spectral sensitivity band of hydrogen. Thus, the force caused by radiation pressure decreases much faster with the distance than previously thought. The modification of radiation pressure force due to the absorption effect is larger than the differences between the two radiation pressure models. Therefore, developing of a new radiation pressure model in the heliosphere is needed. The new model should include absorption processes. However, this new model depends on the distribution of interstellar neutral hydrogen in the heliosphere – the two phenomena are closely related to each other.

Results of this analysis have been presented by the scientific team lead by dr. Izabela Kowalska-Leszczyńska in a paper published The Astrophysical Journal.

Izabela Kowalska-Leszczyńska, Maciej Bzowski

Last Updated on Thursday, 22 November 2018 15:22
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