Ionospheric photoelectrons: comparing Venus, Earth, Mars and Titan
Coates, Andrew J. and Tsang, S.M.E. and Wellbrock, Anne and Frahm, R.A. and Winningham, J.D. and Barabash, S. and Lundin, R. and Young, D.T. and Crary, F.J. (2011) Ionospheric photoelectrons: comparing Venus, Earth, Mars and Titan. Planetary and Space Science 59 (10), pp. 1019-1027. ISSN 0032-0633.
The sunlit portion of planetary ionospheres is sustained by photoionization. This was first confirmed using measurements and modelling at Earth, but recently the Mars Express, Venus Express and Cassini-Huygens missions have revealed the importance of this process at Mars, Venus and Titan, respectively. The primary neutral atmospheric constituents involved (O and CO2 in the case of Venus and Mars, O and N2 in the case of Earth and N2 in the case of Titan) are ionized at each object by EUV solar photons. This process produces photoelectrons with particular spectral characteristics. The electron spectrometers on Venus Express and Mars Express (part of ASPERA-3 and 4, respectively) were designed with excellent energy resolution (ΔE/E=8%) specifically in order to examine the photoelectron spectrum. In addition, the Cassini CAPS electron spectrometer at Saturn also has adequate resolution (ΔE/E=16.7%) to study this population at Titan. At Earth, photoelectrons are well established by in situ measurements, and are even seen in the magnetosphere at up to 7RE. At Mars, photoelectrons are seen in situ in the ionosphere, but also in the tail at distances out to the Mars Express apoapsis (not, vert, similar3RM). At both Venus and Titan, photoelectrons are seen in situ in the ionosphere and in the tail (at up to 1.45RV and 6.8RT, respectively). Here, we compare photoelectron measurements at Earth, Venus, Mars and Titan, and in particular show examples of their observation at remote locations from their production point in the dayside ionosphere. This process is found to be common between magnetized and unmagnetized objects. We discuss the role of photoelectrons as tracers of the magnetic connection to the dayside ionosphere, and their possible role in enhancing ion escape.
|Keyword(s) / Subject(s):||Earth, Venus, Mars, Titan, ionosphere, photoelectrons|
|School:||Birkbeck Schools and Departments > School of Science > Earth and Planetary Sciences > UCL/Birkbeck Centre for Planetary Sciences|
|Date Deposited:||25 Jul 2011 11:11|
|Last Modified:||17 Apr 2013 12:21|
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