Upper Ionosphere of Mars During Solar Quiet and Disturbed Conditions - Planetary Exploration and Science: Recent Results and Advances - page 129

time of arrival at Earth. A second CME-related shock was predicted to reach Mars
on 17 May 2005 at 16:00 UT (Fig. 7.11g ). The arrival of the resultant shock was
predicted to reach Mars between 16 and 17 May 2005 (Fig. 7.11d-g ). The direction
oftheCMEwasawayfromMarson18May(Fig. 7.11h ). The overall effect of
CME arrival at Mars lasted for about 2 days. As a result TEC increased suddenly
(see Fig. 7.8c ) by factors of 2-3 between 16 and 17 May 2005.

7.3.3 Effect of SEPs on the Upper Ionosphere

Solar energetic particle (SEP) events are a part of major disturbances in the
heliosphere (Schnjver and Siscoe 2010 ). These events are mostly composed of
protons with about 10 % He C and <1 % heavier elements. There are two types
of SEP events: impulsive and gradual (Cane et al. 1986 ). Impulsive events are
relatively of short duration (<1 day) with a high proton content. Gradual events
are of longer duration (days), have higher fluxes, display a wider spread in longitude
and are associated with fast CMEs. Mckenna-Lawlor et al. ( 2012 ) reported three
major factors in connection with effect of SEP radiations at the Martian surface:
(1) shadowing by the planet, which cuts off 50 % of SEP primary particle flux,
(2) atmospheric attenuation, which shields out SEP primaries characterised by
relatively low energies and (3) backscattering particles, mostly neutrons, due to the
interaction of high energy particles with soil material. The Mars Energetic Radiation
Environment models (MEREM) were developed by ESA and NASA to study
the SEP radiation close to the Martian environment. The output of these models
gives (1) particle influence, (2) effective dose and (3) ambient dose equivalent with
the Martian atmosphere and in the planetary orbit about Mars (Mckenna-Lawlor
et al. 2012 ).
It is known that the energetic particle densities in the solar wind are significantly
enhanced during SEP events. The observations made by MARSIS have demon-
strated that SEP events modify the ionosphere of Mars (Morgan et al. 2006 ). It
needs to be mentioned that this instrument did not detect its usual reflections from
the surface of Mars during an SEP event, indicating that radio waves which usually
pass smoothly through the ionosphere were fully absorbed (Withers 2011 ). Sheel
et al. ( 2012 ) investigated the effect of SEP event of 29 September 1989 on the
ionosphere of Mars. This event was observed by IMP and GOES satellites during
the disturbed condition of the sun (Lovell et al. 1998 ). Figure 7.12 shows electron
density profiles predicted for the 29 September 1989 SEP event, with and without
photoionisation process. It can be noted that the SEP event caused electron density to
exceed 2 10 5 cm 3 at 120-140 km, a value much larger than typically observed
by MGS in the dayside ionosphere of Mars (cf. Haider et al. 2011 ). Sheel et al.
had carried out two model calculations - photochemical and generalised models.
Solid lines represent the calculation of photochemical equilibrium model. The
black dashed line shows the electron density profile obtained from a generalised
model. The generalised model neglects the energy deposition at high energies.