Simulation of the Martian diffuse auroral emissions and photochemistry induced by solar energetic particles

概要

The response of the Martian atmosphere to changing conditions of energetic particles and
radiation coming from the Sun, i.e., the space weather on Mars, is one of the biggest interests of
Martian exploration in order to understand the potential habitability of Mars. The notable feature
of Mars’ environment in terms of space weather is the absence of a global intrinsic magnetic field
(e.g., Leblanc et al., 2002). Earth’s magnetosphere plays a significant role in preventing solar
wind from precipitating into Earth’s atmosphere, while the absence of an intrinsic magnetic field
on Mars leads to a direct interaction between the solar wind and the Martian upper atmosphere.
The crustal magnetic fields localized in the southern hemisphere act as local shielding against
incoming charged particles (e.g., Leblanc et al., 2002; Lillis et al., 2011). Another feature of Mars
in terms of space weather is the thin Martian atmosphere. The surface pressure of the Martian
atmosphere is 6 mbar, more than 100 times thinner than Earth’s atmosphere, allowing energetic
particles to penetrate deeper into the atmosphere. Owing to the absence of a global intrinsic
magnetic field and thin atmosphere on Mars, solar energetic particles (SEPs), high energy charged
particles emitted from the Sun associated with solar flares and coronal mass ejections, can easily
precipitate deep into the Martian atmosphere. When SEPs precipitate into planetary atmospheres,
they have various effects on the atmosphere. SEPs that precipitate into planetary atmospheres
cause ionization, dissociation, and excitation of atmospheric molecules, leading to auroral
emissions (e.g., Sandford, 1961; Schneider et al., 2015) and changes in atmospheric composition
(e.g., Crutzen et al., 1975; Rusch et al., 1981; Solomon et al., 1981). The impacts of SEPs on
Earth’s atmosphere have been intensively studied for the past decades, e.g., the depletion of the
ozone density has been observed in the polar mesosphere and stratosphere during a large SEP
event occurring in October-November 2003, which is known as the Halloween event (e.g.,
Seppälä et al., 2004; Jackman et al., 2005; Randall et al., 2005). As for Mars, our understanding
of the impacts of SEPs on the Martian atmosphere is gradually progressing thanks to the recent
discovery of a new type of the Martian aurora, diffuse aurora, detected by the Imaging Ultraviolet
Spectrograph (IUVS) instrument onboard the Mars Atmosphere and Volatile Evolution (MAVEN)
spacecraft (Schneider et al., 2015). ...