GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 17, 1877, doi:10.1029/2003GL017842, 2003
Solar wind and magnetospheric ion impact on Mercury’s surface
E. Kallio and P. Janhunen
Finnish Meteorological Institute, Geophysical Research, Helsinki, Finland
Received 28 May 2003; revised 2 July 2003; accepted 29 July 2003; published 3 September 2003.
[1] The impact of the solar wind protons on the surface of
Mercury is studied by a global hybrid model. In the threedimensional self-consistent quasi-neutral hybrid (ions are
particles, electrons are a fluid) model ions can hit the surface
of the planet. The response of the impact to several solar wind
conditions is studied by analyzing four upstream cases: pure
northward interplanetary magnetic field (IMF), pure
southward IMF, the Parker spiral IMF, and atypically high
solar wind speed. Three different high impact regions on the
surface can be identified: (1) ‘‘auroral’’ impact on the
equatorward side of the open/closed field line boundary,
(2) ‘‘cusp’’ impact on the dayside in the noon midnight
meridian plane, and (3) ‘‘nose’’ impact around the subsolar
point when the solar wind dynamic pressure is high. The
particle flux also has North-South and dawn-dusk
INDEX TERMS: 2753 Magnetospheric Physics:
asymmetries.
Numerical modeling; 2756 Magnetospheric Physics: Planetary
magnetospheres (5443, 5737, 6030); 2784 Magnetospheric Physics:
Solar wind/magnetosphere interactions; 5443 Planetology: Solid
Surface Planets: Magnetospheres (2756); 6235 Planetology: Solar
System Objects: Mercury. Citation: Kallio, E., and P. Janhunen,
Solar wind and magnetospheric ion impact on Mercury’s surface,
Geophys. Res. Lett., 30(17), 1877, doi:10.1029/2003GL017842,
2003.
Killen et al., 2001; Sarantos et al., 2001; Ip and Kopp,
2002; Kallio and Janhunen, 2003b], the motion of ions in
the Hermean magnetosphere produced from the Hermean
exosphere and emitted the surface [Ip, 1987; Delcourt et al.,
2002, 2003; Killen and Sarantos, 2003], and the motion of
the solar wind protons injected from the tail [Lukyanov et
al., 2001] and during SEP (Solar Energetic Particle) event
[Leblanc et al., 2003]. The role of the impact of the solar
wind protons has received recently a noticeable interest
because the ion sputtering was proposed as a potential
candidate to explain the rapid temporal variations in the
Hermean sodium exosphere observed in Earth-based remote
sensing measurements [Potter et al., 1999].
[4] In this paper the impact of the solar wind protons is
studied for a first time by a self-consistent quasi-neutral
hybrid model. The hybrid model takes into account ions
finite gyroradius effects and it provides a powerful approach
to study Hermean, or Martian, size magnetospheres [Kallio
and Janhunen, 2003a]. The paper is organized as follows.
Macroscopic parameters and the morphology of the magnetic field are illustrated after a brief description of the used
hybrid model. Then the flux of the solar wind protons at
four different upstream parameters is studied and the three
high impact flux regions on the surface are presented.
1. Introduction
2. Model Description
[2] Mercury has a weak intrinsic magnetic field and
tenuous atmosphere that makes the surface of the planet a
target for solar wind particles. The weak intrinsic magnetic
field results to a miniature magnetosphere in which the
subsolar distance of the bow shock and magnetopause have
been estimated to be only 1.8 RM and 1.4 RM (RM =
2440 km), respectively [Ogilvie et al., 1977]. The solar
wind particles near the surface can freely hit the surface of
the planet because the planet has an exosphere without an
atmosphere [Killen and Ip, 1999].
[3] Ion impact at Mercury has been a subject of several
analyses motivated by Mariner 10 (M10) magnetic field and
electron measurements made in 1974 – 1975. Unfortunately,
there are no direct ion measurements available from M10
flybys. The Hermean magnetosphere is commonly regarded
as a scaled down version of the Earth’s magnetosphere. The
knowledge of the Hermean ion environment soon after M10
flybys relied therefore strongly on the measurements made
in the Earth’s magnetosphere [Ogilvie et al., 1977; Slavin
and Holzer, 1979; Goldstein et al., 1981]. Quantitative
models about the solar wind-Mercury-interaction studies
have focused to analyzing the role of the interplanetary
magnetic field [Luhmann et al., 1998; Kabin et al., 2000;
[5] The quasi-neutral hybrid model is described in detail
elsewhere [Kallio and Janhunen, 2003a] and here only the
properties that are most important for the present study are
briefly listed. In the model the ions are treated as particles
and they are accelerated by the Lorentz force. The model
thus includes automatically finite ion gyroradius effects.
The velocity distribution of the ions is fully three-dimensional. Ions can hit the surface of the planet and if they do
so they are taken away from the simulation. Electrons form
a charge neutralizing (quasi-neutrality is assumed) massless
fluid. In this paper H+ ion impact in four runs is shown. The
upstream parameters of the analyzed stationary cases are
shown in Table 1. It should be noted that while the used
solar wind density of 76 cm3 represents a typical solar
wind density value at Mercury at the pericenter the used
total IMF is smaller than its typical pericenter value of about
46 nT [Slavin and Holzer, 1981]. Also, the used velocity
value of 860 km/s does not represent any specific solar wind
event and it was chosen only to represent an artificial high
solar wind speed case. The coordinate system is the following: The solar wind flows in the -X direction, USW =
(jUSWj, 0, 0), the Hermean intrinsic magnetic field is
modeled by a magnetic dipole moment that points the -Z
direction, and the Y coordinate completes the right hand
coordinate system. The value of the magnetic dipole
moment was chosen to give [300, 0, 0] nT magnetic field
Copyright 2003 by the American Geophysical Union.
0094-8276/03/2003GL017842$05.00
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KALLIO AND JANHUNEN: IMPACT ON MERCURY’S SURFACE
Table 1. The Total Particle Fluxes of the Impacting Solar Wind
Protons in the Four Analyzed Cases
Name of
the run
IMF
[nT]
USW
[kms1]
qtot
[1025s1]
cabsa
[%]
qNb
[%]
qDb
[%]
North IMF
South IMF
Parker IMF
High USW
[0, 0, 10]
[0, 0, – 10]
[32,10, 0]
[0, 0, 10]
430
430
430
860
3.9
3.4
2.7
30
6
6
4
25
53
44
36
48
53
43
59
33
In all cases the density of the solar wind, nSW, was 76 cm3 and the sonic
Mach number 7.7. The values of qtot, qN, and qD show the total flux of
impacting H+ ions, how many percent of the total H+ flux goes to the
Northern hemisphere(z > 0) and to the dawn hemisphere (y < 0),
respectively.
a
cabs qtot/(nSW USW p RM2 ), RM = 2440 km.
b
qN qtot(z > 0)/qtot; qD qtot (y < 0)/qtot.
on the surface of the planet in the magnetic equator. The
used grid size is 305 km (=0.125 RM).
3. Solar Wind Protons Near Mercury:
An Overview
[6] Figure 1 shows the density of the solar wind protons
and the magnetic field lines in the analyzed four cases. The
black solid lines give the position and shape for the bow
shock and the magnetopause in the North IMF case (see
Kallio and Janhunen, 2003b, for the details of the used
analytical functions for these two boundaries). Figure 1
illustrates several characteristic features about the solar
wind-Mercury-interaction. First, the smallness of the Hermean magnetosphere compared with the Earth’s magneto-
sphere which implies that the high proton density region is
close to the planet. Second, how the IMF Bx component in
the Parker spiral angle case results in a North-South
asymmetry, the other hemisphere being magnetically
connected to the solar wind. Third, how the bow shock
and the magnetopause can be located very close to the
surface of the planet at high solar wind dynamic pressure.
4. Impacting Solar Wind Protons
[7] Figure 2 gives a three-dimensional view of the
particle flux of the solar wind protons at the surface and
the open/closed field lines in the pure north IMF and in the
high Usw cases. Figure 2 shows how the particle flux is low
near the magnetic poles on the nightside and that the highest
particle fluxes can be found on the dayside in the noon
midnight meridian plane between the magnetic poles and
the magnetic equation. Note also how the magnetic poles
are surrounded by a high particle flux region. These flux
pattern is analyzed in more detail later. Finally, Figure 2b
depicts how the compression of the Hermean magnetosphere in a high USW case increases the impact of the solar
wind protons, especially around the subsolar point.
[8] A more detailed view to the high particle flux regions
is shown in Figure 3. In all four analyzed cases, high particle
flux bands can be found at latitudes about 20– 60 deg. in
both the Northern and Southern latitudes. These bands are
near, but slightly equatorward, from the open/closed field
line boundary. The proton impact at these two high flux
bands is referred to as the auroral impact in this paper in
analogy with the Earth’s auroral ovals around the Earth’s
Figure 1. The density of the solar wind protons (cm3) and the magnetic field lines in the (a) pure northward IMF,
(b) pure southward IMF, (c) in the Parker spiral angle IMF, and (d) in the high solar wind speed (Usw = 860 km s1) cases.
The magnetic field lines are shown by red lines. The solid black lines show the position of the bow shock and the
magnetopause in the pure northward IMF case.
KALLIO AND JANHUNEN: IMPACT ON MERCURY’S SURFACE
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also expected to increase the gyroradius of the ions. For a
reference, an H+ ion moving at speed 430 km s1 and 860 km
s1 perpendicular to a 10 nT magnetic field has a gyroradius
of 0.18 RM, and 0.37 RM, respectively.
[9] The total particle fluxes in the analyzed cases are
summarized in Table 1. When USW was 430 km s1, the total
impact rate of the solar wind protons is 5% of the
impact rate that would take place without the shield of the
magnetosphere. It should be noted that the total H+ flux
seems to be slightly larger in the north IMF than in the south
IMF cases, that is, in the ‘‘closed’’ magnetosphere than in
the ‘‘open’’ magnetosphere. An explanation for that could
be that in the ‘‘closed’’ (the north IMF case) magnetosphere
the closed field lines occupy a larger fraction in the
magnetosphere and, consequently, the H+ ions can easily
be trapped into the closed field lines and finally hit the
surface. Table 1 also shows that when USW increases from
430 km s1 to 860 km s1 the absorbtion increases from
5% to 25%. The orientation of the IMF contributes also
to the distribution of the solar wind protons negative IMF
Bx resulting in higher total particle flux in the Southern
hemisphere than in the Northern hemisphere.
5. Discussion
[10] In this paper the impact of the solar wind protons at
Mercury was studied for the first time with a quasi-neutral
hybrid model. Although a detailed quantitative comparison
of the presented results and the previous studies is beyond the
scope of this paper, it is worthwhile to note the following
qualitative similarities.
[11] The impact of the solar wind protons was found to be
intense in the auroral region (Figure 3), much in agreement
with the former analysis based on an analogy with the Earth’s
magnetosphere [Ogilvie et al., 1977]. When the formation of
the auroral impact formed in the simulation were studies by a
test particle simulation (figure not shown) those ions which
Figure 2. Three-dimensional view of the flux of the
impacting solar wind protons at the surface of Mercury and
the magnetic field lines in the (a) pure northward IMF, and (b)
in the high Usw cases. The open (closed) magnetic field lines
are marked by red(blue) lines. The colors at the surface of the
planet show the particle flux of impacting H+ ions (see
Figure 3 for the color scale used). The colors at the surface of
the planet show the particle flux of impacting H+ ions (see
Figure 3 for the color scale used). Note the intense ‘‘auroral’’
impact regions around the magnetic poles, the ‘‘cusp’’ impact
near the noon-midnight meridian plane in a), and the ‘‘nose’’
impact in b).
magnetic poles. The highest fluxes on these auroral ovals can
be found on LT 12:00 (longitude 0 deg.) resembling the
particle impact at the Earth’s magnetic cusps, or cusp entry
layers. Figure 3c, instead, represents a third kind of high
impact region, namely a high particle flux region near the
subsolar point. This ion impact region, referred to as the nose
impact region, occurs when the solar wind speed and,
consequently, its dynamic pressure is high and it does not
have a clear analogy with the ion impact at the Earth. It is
also noteworthy that increasing the solar wind velocity is
Figure 3. The particle flux(cm2s1) of impacting H+ ions
and the open/closed magnetic field line region in the four
analyzed cases: The (a) pure northward IMF, (b) pure
southward IMF, (c) Parker spiral IMF, and (d) high Usw
case. Longitudes 0, 90, 180, and 270 deg. correspond the
local times (LT) of 00:00, 09:00, 12:00, and 18:00,
respectively. The subsolar point is at the center of the
figure and the positive(negative) latitudes corresponds to
the northern(southern) hemisphere. The dashed lines show
the open/closed field line boundaries.
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KALLIO AND JANHUNEN: IMPACT ON MERCURY’S SURFACE
penetrated into the closed field lines were found to move
along the magnetic field line and finally hit the surface of the
planet. Such an ion loss is consistent with the fact that the ions
at Mercury have a large loss cone angle [Ogilvie et al., 1977;
Delcourt et al., 2003] due to which particle can easily hit the
surface. The motion of ions in the Hermean magnetosphere is
different from the motion in the terrestrial magnetosphere
also because the gradient drift plays a larger role at Mercury
that at the Earth [Ogilvie et al., 1977; Delcourt et al., 2002]
Furthermore, increasing solar wind dynamic pressure was
found to push the magnetopause toward the planet (Figure 1),
in agreement with an MHD model runs [Kabin et al., 2000].
In such a case an intense H+ ions impact was found also near
the subsolar point.
[12] Furthermore, the orientation of the IMF was found to
affect the morphology of the Hermean magnetosphere and
to the H+ ion impact. The magnetic cusps were found to be
sited closer to the magnetic equator in IMF Bz < 0 case than
in IMF Bz > 0 case (compare Figures 3a and 3b). Consequently, the south IMF case resulted a larger open field line
region on the surface than the north IMF case (compare
Figures 3a and 3b) which is qualitatively in agreement with
an MHD model [Ip and Kopp, 2002]. The North-South
asymmetry in the Hermean magnetic field caused by the
IMF Bx component [Sarantos et al., 2001; Kallio and
Janhunen, 2003b] resulted in a North-South asymmetry
also in the impact of the solar wind protons: The total H+
ion particle flux is higher on the hemisphere that is
magnetically connected to the solar wind (Figure 1c) than
in the opposite hemisphere. The impacting H+ emit neutral
atoms and ions (see, for example, Killen and Ip, 1999) and,
therefore, the non-uniform H+ impact shown in this paper
forms a non-uniform neutral source for the Hermean exosphere and a non-uniform ion source for its magnetosphere.
[13] It should finally be worth noting that the present
study leaves room for more comprehensible studies in the
future. In addition to the numerical issues (see discussion in
Kallio and Janhunen, 2003a, for details) there are many
physical issues to be studied such as how the Hermean
exospheric ions affect the impact, the role of the resistivity
(which in the presented runs is zero both above and within
the planet), the magnetospheric dynamics associated with
time dependent upstream parameters, and the role of ions’
gyroradius. The model should also be made more selfconsistent by taking into account the emission of neutrals
and ions resulted from the H+ impact.
6. Summary
[14] In this paper the particle flux of the solar wind protons
was calculated self-consistently with a hybrid model. The
impact was found to be high near the rings around the
magnetic poles, in analogy with the ion impact at the Earth’s
auroral ovals. The impact within these rings was most intense
in the dayside near the noon-midnight plane resembling ion
impact at the Earth’s cusps. A substantially different impact
that has no analogy with Earth-like case was found when the
solar wind dynamic pressure is high. In such a case a high
particle flux was found around the subsolar point as well.
Hemispherically asymmetric particle impact controlled by
the orientation of the interplanetary magnetic field was also
found. Overall, the study illustrates that the solar wind ions
are impacting non-uniformly on the surface and that the site
where the high particle impact is high depends on the
upstream parameters.
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E. Kallio and P. Janhunen, Finnish Meteorological Institute, Geophysical
Research, Vuorikatu 15A, FIN-00101, Helsinki, Finland. (Esa.Kallio@
fmi.fi; [email protected])