X-Ray Observation of Mars at Solar Minimum with Suzaku

Abstract

Mars was observed in X-rays during 2008 April 3-5 for 82 ks with the Japanese Suzaku observatory. Mars has been known to emit X-rays via the scattering of solar X-rays and via charge exchange between neutral atoms in the exosphere and solar-wind ions. Past theoretical studies suggest that the exospheric neutral density may vary by a factor of up to 10 over the solar cycle. To investigate a potential change of the exospheric charge exchange emission, Mars was observed with Suzaku at solar minimum. Significant signals were not detected at the position of Mars in the energy band of 0.2-5 keV. The 2 $\sigma$ upper limit of the O II line flux in 0.5-0.65 keV was 4.3 $\times$ 10 $^{-5}$ photons cm $^{-2}$ s $^{-1}$ . Comparing this upper limit to past Chandra and XMM-Newton observations conducted near solar maximum, it was found that the exospheric density at solar minimum does not exceed that near solar maximum by more than 6-70 times.