Geoscience Reference
In-Depth Information
magnetospheric plasma, but also is a powerful tool to obtain information
about 3-dimensional (3D) exospheric and plasma properties.
9
,
10
In fact,
even if this information needs a deconvolution process to be unfolded from
2-dimensional (2D) ENA images, ENA imaging has the advantage to give a
global
and
instantaneous
information from a remote vantage point. More-
over, since ENAs retain approximately all the energy of the colliding ions,
their energy distribution gives information about energy distribution of the
magnetospheric plasma.
The result of ion-sputtering is, in most cases, a neutral with a few eV
energy. However, a fraction of the sputtered neutrals may have enough
energy to travel on straight lines and escape from planetary gravity: in this
paper, we will refer to those neutrals as ENAs too. Detection and mass-
analysis of this ENA signal will give information about: effectiveness of
ion-sputtering as surface loss process and exospheric source; relative surface
composition; map of ion-impact on Mercury's surface.
11
The purpose of the present study is the estimation of the neutral flux
generated by CE and ion-sputtering and the discussion of the feasibility
of the neutral-particle imaging of the plasma circulation and the surface.
Here we use a single-particle model to reconstruct the spatial distribution
and the energy spectrum of H
+
ions. We will show that this plasma is able
to generate a noticeable amount of neutral particles, both via CE with the
neutral exosphere and via ion-sputtering processes on its surface. Moreover,
we will show that the neutral flux coming from both those processes is
extremely sensible to the external conditions, thus leading to a “neutral
imaging” investigation technique for Mercury's magnetosphere and surface,
through directional neutral atoms detection.
2. Proton Circulation Model
The magnetic and the electric field models developed in this study orig-
inate, respectively, from Tsyganenko T96
12
and Volland
13
models, which
refer to the Earth. They have been modified considering the different intrin-
sic magnetic field of Mercury and the different properties of the solar wind at
the orbit of Mercury (0.31-0.47 AU).
11
,
14
Here we have assumed an unper-
turbed solar wind density of 52 cm
−
3
, a solar wind velocity of 430 km/s and
a dynamic pressure (nmv
2
)of16nPa.
11
The magnetic field model accepts the
z
and the
y
components of
IMF as independent inputs. Even if the magnetic reconnection at the
dayside magnetopause is essentially driven by the IMF
B
z
component,
11
