Energetic neutral atoms (ENAs) are produced wherever energetic ions interact with a neutral atmosphere. Solar wind ENAs have been observed at every planet in the Solar System where ENA instrumentation has been available - for Earth (Collier et al. 2001), Mars (Futaana et al. 2006) and Venus (Galli et al. 2008). In case of ENAs, the ions have a much higher velocity thant the thermal velocities of the exospheric neutrals. During the charge exchange processes shown in Fig. 1, an electron is transferred from the neutral to the ion, resulting in a neutral atom and an ionized neutral.

Because of the large relative velocities of the ions and the exospheric neutrals, the momenta of the individual atoms are preserved to a good approximation. Thus, the produced ENAs will have the same velocity distribution as the source population of ions. This mechanism is well known in the Solar System, it was recently proposed by Holmström et al. (2008) as a possible origin of the atomic hydrogen corona revealed by Hubble Space Telescope observations of HD 209458b, which is a Jupiter-type gas giant with a mass of 0.65M_Jup and a size of 1.32R_Jup that orbits at 0.045 AU (Knutson et al. 2007) around its host star HD 209458, which is a solar-like G-type star with an age about 4 Gyr. HD 209458b is an exoplanet found to transit the disk of its parent star. Observations have revealed a large population of high-velocity atomic hydrogen around the "hot Jupiter" HD 209458b during transit.

Holmström et al. (2008) and Eckenbäck et. al (2008) described the stellar plasma flow and how well the obtained ENA production fit the HST observations (See Presentation: Hot_Jupiter_ENA_seminar07.pdf for further details). A slice of the simulations by Eckenbäck et al. (2008) is shown in Fig.3, displaying stellar wind protons and the hydrogen atoms.

To Fig.3: The flow of stellar wind protons and the exospheric hydrogen cloud seen from above the direction of the negative z-axis (perpendicular to the orbital plane). Each point corresponds to neutral hydrogen (red), or a proton (blue) meta particle in the slice -10⁷ ≤ z ≤ 10⁷m (10⁷m = 0.13 R_Jup). The scale on the axes are 10⁹m (0.14 R_Jup). The circle without particles corresponds to the inner boundary of the simulation, and the large area without protons corresponds to the assumed obstacle to the stellar wind, on which the protons are reflected (Eckenbäck et al. 2008).

One can see that the hydrogen cloud reaches (with significant density) outside the obstacle boundary (magnetospause) and can produce ENAs by charge exchange with stellar wind protons.

Magnetic moment estimations:

Since ENA production depends on the planetary obsacle to the stellar wind, ENA observations can yield information on a magnetosphere if an extrasolar planet. The size of the magnetosphere of an extrasolar planet cannot be measured directly, but it has implications on many processes. It determines the intensity of planetary radio emission (Desch and Kaiser 1984; Zarka et al. 1997; Farrell et al. 1999; Grießmeier et al. 2007) as well as the protection of the planetary atmosphere against atmospheric loss by the solar wind and by CMEs (Grießmeier et al. 2004; Khodachenko et al. 2008). By varying the obstacle standoff distance by Eckenbäck et al. (2008) and shown in Table 1 the model and observations generate the best fit when the obstacle distance (magnetosphere) is decreased down to about 4 R_Jup.