  Mars Exospheric Neutral Composition Analyzer (MENCA) of Mars Orbiter Mission
(MOM) measures the \emph{in-situ} neutral upper atmospheric constituents of
Mars. Martian lower atmosphere predominated by the presence of $CO_2$ which
photo-dissociates into atomic oxygen ($O$) in higher altitudes much near the
exobase. Atomic $O$ plays a significant role in invoking the stronger presence
of $O_2^+$ in the Martian ionosphere. Primary photo-dissociative species
$CO_{2}$, crossover their neutral abundance with atomic $O$ in the
collisionless heterogenous atmosphere with varying local solar conditions.
Initial measurements from Neutral Gas and Ion Mass Spectrometer (NGIMS)
instrument on Mars Atmosphere and Volatile Evolution (MAVEN) estimated these
crossover/transition altitudes wavering between $\approx$225 km to 240 km
during solar maximum conditions with peak solar illuminations. MENCA sampled
the neutral atmospheric species, below the exobase up to the periareion of
$\approx$160 km, under low solar activity conditions during June 2018.
Observations of partial pressures of $CO_2$ and $O$ in subsequent orbits
reveals that solar inputs are crucial in quantifying these crossing points,
where $[O]/[CO_2]$ remain unity, alongside the influences from temperature. The
multi-spacecraft measurements of the direct influences of solar wind charged
particle fluxes and velocity on the daily variation of neutral
thermospheric/exospheric compositions were observed on the local evening hours
of Mars and presented. It marks the first-ever direct \emph{in-situ}
observation of interaction between the energetic solar particle radiations on
Martian exospheric compositions, potentially contributing to the steady escape
and differing population of atomic $[O]$ in the exosphere.
