Interaction of Titan's ionosphere with Saturn's magnetosphere

Abstract

Titan is the only Moon in the Solar System with a significant permanent atmosphere. Within this nitrogen-methane atmosphere, an ionosphere forms. Titan has no significant magnetic dipole moment, and is usually located inside Saturn's magnetosphere. Atmospheric particles are ionized both by sunlight and by particles from Saturn's magnetosphere, mainly electrons, which reach the top of the atmosphere. So far, the Cassini spacecraft has made over 45 close flybys of Titan, allowing measurements in the ionosphere and the surrounding magnetosphere under different conditions. Here we review how Titan's ionosphere and Saturn's magnetosphere interact, using measurements from Cassini low-energy particle detectors. In particular, we discuss ionization processes and ionospheric photoelectrons, including their effect on ion escape from the ionosphere. We also discuss one of the unexpected discoveries in Titan's ionosphere, the existence of extremely heavy negative ions up to 10 000 amu at 950 km altitude.

Negative ion chemistry in Titan's upper atmosphere

“…The spectrum (Figure 3 in Coates et al (2007a)) shows evidence for negatively charged ions up to ~10 000 amu/q as well as two distinct peaks at 12-20 and 28-45 amu/q. Negative ions have been detected on all Titan encounters when the spacecraft altitude was low enough and pointing conditions were favorable (Coates, 2008;Coates et al, 2007a;Coates et al, 2008).…”

“…Cassini CAPS-ELS observations of Titan's ionosphere have revealed the existence of heavy negative ions at altitudes between ~1200 and 950 km with masses up to ~10 000 amu/q (Coates, 2008;Coates et al, 2007a;Coates et al, 2008;Waite et al, 2007). The ELS sensor is a hemispherical, electrostatic top-hat analyzer, which sweeps through a 63-point energy spectrum ranging from 0.6 to 28 000 eV/q for negatively charged particles.…”

Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols?

“…There is a lot of free energy within Titan's upper atmosphere from bombarding highenergy photons (UV ' IEUV and X-rays) and magnetospheric charged particles. This high-energy input results in high thermal electron temperatures T,,,-1000 K (Wahlund et al, 2005) and hot suprathermal photoelectrons T h -20,000 K ( Hartle et al, 2006a,b;Coates, 2009a). But, the ion-neutral and neutral-neutral collisions that lead to the growth of the heavy ions occur for T-150 K. The high electron temperatures may affect ion charge state and formation of radicals.…”

“…(1998) were made at relatively high electron temperatures -300 K or more and that it is difficult to achieve lower electron temperatures T-150K in the laboratory (Goulay et al, 2004). Titan's upper atmosphere and ionosphere are relatively cold with neutral and ion temperatures -150 K (Waite et al, 2005(Waite et al, , 2007, but the electron temperatures can be quite hot with thermal electron temperatures T,,,-1000K (Wahlund et al, 2005) and hot component photoelectron temperatures Te,i,,000 K (Hartle et al, 2006a,b;Coates, 2009a;Cravens et al, 2009). It is important to note that electron attachment to fullerenes Coo and C70 has an exponential temperature dependence that increases rapidly for Te> 500 K with relatively high rate coefficient /1e-3 x 10-8 em 3 /s (Smith et al, 1993;Spanel and Smith, 1994;Spanal and Smith, 1995;Jaffke et al, 1994) at Tee-1000K (Wahlund et al, 2005), so electron attachment to fullerenes seems allowed.…”

Preface

“…The influence of Titan's space environment on its upper atmosphere and exosphere is also discussed by Johnson (2009) whose calculations suggest that magnetosphere coupling processes provide an important energy source for the atmosphere. Coates (2009) discusses the ion population discovered in the vicinity of Titan, of which the heavy negative ions are particularly noteworthy. This issue is rounded off by Bertucci (2009), who characterizes the magnetic field geometry in the vicinity of Titan's orbit around Saturn.…”