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Solar Wind-Ionosphere Interaction at
Mars/Venus and the Earth:
Does a strong magnetic
field protect a planetary
atmosphere from stellar
winds?
H. Nilsson, M. Yamauchi, and S. Barabash
Swedish Institute of Space Physics, Kiruna
EANA-2012 (P4.28, 2012-10-15)
Various escape processes
process
Jeans escape
Hydrodynamic
blow-off (Fig.3)
Photochemical
heating
Ion pickup & subsequent sputtering
(Fig. 5)
Mechanism
thermal,
neutral
semi-thermal,
neutral & ion
chemical,
neutral
non-thermal +
thermal,
ion
non-thermal,
ion
Energization by E//
& EM wave (Fig.3,
4)
Large-scale
non-thermal,
momentum transfer ion
2
Specie Explanation
H, He Thermal tail exceeds
(& O)
escape velocity
main external cause
Hot exosphere / hot
ionosphere (high EUV)
all
The same as SW and
Polar Wind
Hot exosphere / hot
ionosphere (high EUV)
H, He
(& O)
Release of energy of
excited state molecule
Hot exosphere / hot
ionosphere (high EUV)
H, He
(& O)
Newly ionized neutral
Extended exosphere
inside SW takes cycloid (high EUV) / thin
motion
ionosphere (high SW PD)
all
Local deposit of SW
Active SW PD/IMF/SEP
energy to ionosphere
generates EM field
Bulk plasma interaction Active SW PD/IMF
at the boundary region.
all
Escape of planetary atmosphere takes place mainly in two forms: as neutral atoms (thermal,
photochemical and after charge exchange from ions) and as ions. Since the ion are trapped
by the magnetic field, total loss in the form of ions might be strongly influenced by the
presence of a planetary intrinsic magnetic field.
3
Observation
Using similar ion instrument (IMA
for Mars/Venus and CIS for the
Earth), we derived atmospheric
loss in the form of ions that exceed
the escape velocity at Venus (10
km/s), Earth (11 km/s) and Mars
(5km/s).
note on observation:
Strong spatial dependency (must
consider spacecraft location)
Limited angular coverage of the
ion instrument of IMA (must
integrate large set of data to make
statistical distribution function)
Figure 1: examples of obtained velocity distribution
for Mars. By fitting these to shifted Maxwellian, we
obtain distribution function and flux.
4
distribution
Mars
Venus
Earth
Figure 2: spatial distribution of
total flux (most are escaping)
5
summary of escape
Venus
SW H+
precipitation
SW He++
precipitation
O+ escape at solar
minimum
O+ escape at solar
maximum
1021~22/s
1020~21/s
3 x 1024/s
0.6~5 x 1025/s
(Brace et al. 1987,
McComas et al. 1986)
Earth
1024/s
1023/s
1025/s
1025~26/s
(Lennartsson et al. 2004)
Mars
1021~22/s
1022/s
3 x 1024/s
3 x 1025/s
(Lundin et al. 1989)
We found that for current conditions the loss rate is similar for the
three planets. The atmospheric escape may actually be stronger
from a magnetized planet during increased solar wind activities.
The intrinsic magnetic field does not protect the atmosphere
from the solar wind for Earth’s case.
note: May protect for strong magnetic field (Jupiter), but SW energy
is high enough to make polar cap area large for the Earth.
examples of acceleration
Figure 3: outflowing ions above the Earth’s
ionosphere (thick red lines: polar wind:
fluctuation: wave acceleration)
Figure 4: cold ions are accelerated
as they travel tailward of the Mars.
6
Venus-Mars difference: gravity and size
foreshock beam
(reflected SW)
pickup ion of exospheric
SW reflection?
origin
SW
SW
BS
Pickup ion is too weak to detect
Figure 5: VEX and MEX frontside observations
BS
No foreshock at X>0
but SW reflection occurs
7
Future work: Geological scale?
Ancient condition:
(a) Most likely high EUV/FUV flux
(b) Most likely high SW (solar wind) PD = vsw2
(c) Most likely strong & active Bsw (IMF) due to faster rotation
(d) Most likely frequent & intense SEP (Solar Energetic Particle)
(e) Most likely less planetary dynamo & more crust B-field
(f) Different atmospheric composition
(a)-(d) => must compare storm time observations to a statistical average
- note: Different response times between magnetized and unmagnetized planets
- note: Different atmospheric composition between planets
- note: Different scale height between planets
Example:
Response to geoeffective solar flare (Futaana et al. 2008)
Response to CIR at Earth and Mars (Wei et al. 2012)
Statistical response to CIR, energy > 50 eV (Edberg et al. 2010)
Statistical response to CIR, all ASPERA-3 energies (Nilsson et al. 2011)
Some indications of stronger response at Mars (10 times more outflow)
Statistical CIR response significantly smaller (~2.5 at Mars, 1.9 at Venus)
Smaller or same total outflow (Smaller for solar minimum, same for solar maximum)
At earth cold plasma is protons, what is the case for Mars and Venus?
8
Mars-Venus difference
11.5
11
10.5
10
9
9.5
8
-3
-2
-1
0
1
Mars-Sun axis [RM]
2
9
10