Cometary X-rays - Solar wind charge exchange in cometary atmospheres

Thesis Summary:

‘Cometary Charge Exchange Emission’

The interaction of the solar wind with the planets, moons and the interstellar medium is of key importance for understanding the evolution of our solar system. The interaction with Earth’s atmosphere is best known for the northern light. In case of Mars, the interaction with the solar wind might have lead to the erosion of its atmosphere. Solar wind-atmosphere interactions can be studied particularly well in cometary atmospheres, because in that case the solar wind flow is not attenuated by a planetary magnetic field and interacts directly with its atmosphere, the coma. The size of the cometary atmosphere (in the order of 10⁴-10⁵ km) allows remote tracking of the ions as they penetrate into the comet’s atmosphere, offering a unique window on the cometary atmosphere, the solar wind and the interaction of these two plasmas. 

When solar wind ions fly through an atmosphere they are neutralized via charge exchange reactions with the neutral gaseous species. These reactions depend strongly on target species and collision velocity. The resulting X-ray and Far-UV emission can therefore be regarded as a fingerprint of the underlying reaction, with many diagnostic qualities.

To explore the diagnostics of this emission, I performed experimental studies of charge exchange reactions typical for cometary and planetary atmospheres by means of a technique called ‘Photon Emission Spectroscopy’. Here, ions fly through a neutral gas jet. The velocity of the ions can be controlled via ion optics. The light emitted after electron capture is observed via a Far Ultraviolet spectrometer and this allows for the measurement of state-to-state charge transfer cross sections and the resulting emission cross sections. Among the typical experiments performed were collisions between solar wind ions (He²⁺, O^6-7+, N⁷⁺, etc) and target gasses relevant for cometary- and planetary atmospheres, such as H₂O, CO₂, CO and CH₄, all at velocities typical for the solar wind (200-1500 km/s). These experiments were the first comprehensive study that was fully designed for its astronomical application. It showed that for velocities typical for the solar wind multiple electron capture, a process that thus far had not been accounted for, becomes the most important reaction channel in some comet-wind interactions.

Based upon the charge exchange cross sections measured in the lab, I have developed an astrophysical model that calculates cometary Far UV spectra. This model was used to analyze existing observations by the Extreme Ultraviolet Explorer of the helium emission lines of comets Hale-Bopp and Hyakutake. By combining the model with solar wind data from the instruments on board ACE and Ulysses, we were able to analyze the observations of these two comets in terms of solar wind and coma characteristics. In particular the case of Hale-Bopp was of great interest, as our results indicate severe post–bow shock cooling of the solar wind in this extraordinary large comet. As such, our studies were the first remote, quantitative observations ever of local plasma conditions like temperature and density in the interaction zone, which were thus far only accessible by in situ exploration.

The model was then further expanded to calculate the much more complex cometary X-ray emission, involving charge exchange of solar wind C, N, and O ions with cometary H, O, and H₂O species. Modern X-ray observatories, such as Chandra and XMM provide the observer with spatial, temporal and spectral data. Our charge exchange model was used to analyze several cometary observations, most notably the Chandra observations of comet Tempel 1 when it was hit with the 375 kg impactor of the Deep Impact mission. The nearly 30 day time span of our observations sampled several severe changes in the solar wind and outbursts in the comet's activity, which we could clearly identify in the observed X-ray light curve, emission morphology and spectra.

Last but not least, the model was also used to develop observational strategies and it served as the basis for successful proposals to observe comet Schwassmann-Wachmann 3 (SW3) with XMM, Chandra and SWIFT.  SW3 is a unique comet, because both of its orbit and because of its state. In 1995, SW3 suddenly broke into three pieces and during its extremely close encounter (<0.07 AU) in 2006, some of these cores fragmented even further. The comet’s extremely close encounter in May 2006 provided an unprecedented spatial resolution of up to 300 km in the areas around the nucleus. As charge exchange emission is excellent for tracing thin gas, this allowed for an unprecedented study of the interaction of the solar wind with the neutral coma, the macroscopic structure of the magneto-hydrodynamic flow, and the microscopic physical processes. Even more, these observations allow for a direct comparison with independent, simultaneous in situ measurements of solar wind conditions measured by near-Earth satellites like SOHO and ACE. This study provides a test bed for our insight in such interactions not only for the solar-wind-coma case but also in the wider context of physical processes in wind-environment collisions.

Thus, my thesis studies have focussed on all aspects relevant for X-ray emission from comets: experimental studies of state-to-state charge exchange cross sections, observations of X-ray emission from comets using all X-ray satellites (Chandra, XMM, and Swift), and theoretical modelling of the interaction of solar wind ions with cometary gasses and the resulting X-ray emission spectrum. Together, this has greatly improved our understanding of the interaction of the solar wind with solar system objects and in more general, of physical processes in wind-environment collisions. The thorough understanding of cometary charge exchange emission has opened the door to the direct observation of more complex solar wind interactions such as those with Mars and Venus.