Geophysical Research Letters 01/2011; 38(2):L02102. · 3.79 Impact Factor
ABSTRACT: Cold ionospheric ions dominate the plasma escape from Mars. The flow
pattern versus altitude, latitude and local time suggests a fairly
symmetric transport of ionospheric plasma from the dayside into the
nightside/tail region of Mars. An interesting aspect of the plasma
escape from Mars is the large abundance of molecular ions. This implies
that the outflow source region extends down to the lower ionosphere
where molecular ions dominate. It also implies that the primary
energization does not contribute much to the molecular dissociation. The
gentle increase of ion velocity leading to escape may explain another
finding, the outflow of ionized molecular hydrogen, H2+. Because the
cold ionospheric ion outflow is dominated by H+, H2+, O+ and O2+, we
have made a stoichiometric analysis of the escape. Adding the total
outflow of hydrogen and oxygen respectively, and taking their ratio
(∑H/∑O), we get ∑H/∑O ≈1.5. Considering measurement
uncertainties and other hydrogen escape processes, such as thermal/Jeans
escape, the escaping H and O atoms most likely originate from water. The
obvious implication is that water, a minor constituent in the Martian
atmosphere, is not only highly unstable and prone to escape Mars, but in
fact dominates the non-thermal mass escape from Mars.
Geophysical Research Letters 01/2008; 35(9):L09203. · 3.79 Impact Factor
ABSTRACT: Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth's also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.
Nature 12/2007; 450(7170):650-3. · 36.28 Impact Factor
ABSTRACT: In Dec. 2006, single sunspot region produced a series of proton solar
flares, up to X9.0 level on 5 Dec 2006 10:35 UT. One unique feature of
this X9.0 flare is that MeV particles originated from this proton flare
were observed at Venus and Mars by Venus Express (VEX) and Mars Express
(MEX), which are respectively located away from Earth by nearly
+160° and -160° as viewed from the Sun. On 5 Dec 2006, the
plasma instruments ASPERA-3 and ASPERA-4 on board MEX and VEX have
detected a large enhancement in their respective background count level,
which is a typical signature of intensive MeV particle flux. The timing
of these enhancements were consistent with the estimated field-aligned
travel time along the Parker spiral from the site of X9.0 flare to
Venus and Mars. The Mars Express data indicate a one-order enhancement
in the heavy ion outflow from the Martian atmosphere during the SEP
period. This is the first observation of the increase of escaping flux
at Mars during a violent solar activity. This suggests that the solar
EUV flux levels also significantly affect the atmospheric loss from
AGU Fall Meeting Abstracts. 11/2007; -1:1087.
ABSTRACT: As a part of the global plasma environment study of Mars and its response to the solar wind, we have analyzed a peculiar case
of the subsolar energetic neutral atom (ENA) jet observed on June 7, 2004 by the Neutral Particle Detector (NPD) on board
the Mars Express satellite. The “subsolar ENA jet” is generated by the interaction between the solar wind and the Martian
exosphere, and is one of the most intense sources of ENA flux observed in the vicinity of Mars. On June 7, 2004 (orbit 485
of Mars Express), the NPD observed a very intense subsolar ENA jet, which then abruptly decreased within ∼10 sec followed
by quasi-periodic (∼1 min) flux variations. Simultaneously, the plasma sensors detected a solar wind structure, which was
most likely an interplanetary shock surface. The abrupt decrease of the ENA flux and the quasi-periodic flux variations can
be understood in the framework of the global response of the Martian plasma obstacle to the interplanetary shock. The generation
region of the subsolar ENA jet was pushed towards the planet by the interplanetary shock; and therefore, Mars Express went
out of the ENA jet region. Associated global vibrations of the Martian plasma obstacle may have been the cause of the quasi-periodic
flux variations of the ENA flux at the spacecraft location.
Space Science Reviews 09/2006; 126(1):315-332. · 3.61 Impact Factor
ABSTRACT: By identifying peaks in the photoelectron spectrum produced by photoionization of CO2 in the Martian atmosphere, we have conducted a pilot study to determine the locations of these photoelectrons in the space
around Mars. The significant result of this study is that these photoelectrons populate a region around Mars bounded externally
by the magnetic pileup boundary, and internally by the lowest altitude of our measurements (∼250 km) on the dayside and by
a cylinder of approximately the planetary radius on the nightside. It is particularly noteworthy that the photoelectrons on
the nightside are observed from the terminator plane tailward to a distance of ∼3 R
M, the Mars Express apoapsis. The presence of the atmospherically generated photoelectrons on the nightside of Mars may be
explained by direct magnetic field line connection between the nightside observation locations and the Martian dayside ionosphere.
Thus the characteristic photoelectron peaks may be used as tracers of magnetic field lines for the study of the magnetic field
configuration and particle transport in the Martian environment.
Space Science Reviews 09/2006; 126(1):389-402. · 3.61 Impact Factor
ABSTRACT: We have studied the loss of O+ and O+
2 ions at Mars with a numerical model. In our quasi-neutral hybrid model ions (H+, He++, O+, O+
2) are treated as particles while electrons form a massless charge-neutralising fluid. The employed model version does not
include the Martian magnetic field resulting from the crustal magnetic anomalies. In this study we focus the Martian nightside
where the ASPERA instrument on the Phobos-2 spacecraft and recently the ASPERA-3 instruments on the Mars Express spacecraft
have measured the proprieties of escaping atomic and molecular ions, in particular O+ and O+
2 ions. We study the ion velocity distribution and how the escaping planetary ions are distributed in the tail. We also create
similar types of energy-spectrograms from the simulation as were obtained from ASPERA-3 ion measurements. We found that the
properties of the simulated escaping planetary ions have many qualitative and quantitative similarities with the observations
made by ASPERA instruments. The general agreement with the observations suggest that acceleration of the planetary ions by
the convective electric field associated with the flowing plasma is the key acceleration mechanism for the escaping ions observed
Space Science Reviews 09/2006; 126(1):39-62. · 3.61 Impact Factor
ABSTRACT: The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on board the Mars Express spacecraft found that solar wind plasma and accelerated ionospheric ions may be observed all the way down to the Mars Express pericenter of 270 kilometers above the dayside planetary surface. This is very deep in the ionosphere, implying direct exposure of the martian topside atmosphere to solar wind plasma forcing. The low-altitude penetration of solar wind plasma and the energization of ionospheric plasma may be due to solar wind irregularities or perturbations, to magnetic anomalies at Mars, or both.
Science 10/2004; 305(5692):1933-6. · 31.20 Impact Factor
ABSTRACT: The ASPERA-3 (Analyser of Space Plasma and Energetic Atoms) instrument
of Mars Express is designed to study the solar wind-Mars atmosphere
interaction and to characterise the plasma and neutral gas environment
in near-Mars space through energetic neutral atom (ENA) imaging and
local charged-particle measurements. The studies address the fundamental
question: how strongly do the interplanetary plasma and electromagnetic
fields affect the martian atmosphere? This question is directly related
to the problem of martian dehydration. The instrument comprises four
sensors; two ENA sensors, and electron and ion spectrometers. The
Neutral Particle Imager (NPI) measures the integral ENA flux (0.1-60
keV) with no mass and energy resolution but with high angular
resolution. The Neutral Particle Detector (NPD) measures the ENA flux,
resolving energy (0.1-10 keV) and mass (H and O) with a coarse angular
resolution. The electron spectrometer (ELS) is a standard top-hat
electrostatic analyser of a very compact design. These three sensors are
mounted on a scanning platform providing 4π coverage. The instrument
includes an ion mass composiotion sensor, IMA (Ion Mass Analyser).
Mechanically, IMA is a separate unit connected by a cable to the
ASPERA-3 main unit. IMA provides ion measuremets in the energy range
0.01-40 keV/q for the main ion components H+,
He2+, He+, O+, with 20-80 amu/q.
ABSTRACT: Knowledge learnt on the dynamics and topology of the polar cusp from in
situ plasma measurements primarily on board low and medium altitude
satellite is reviewed. The polar cusp, first identified from low
altitude orbiting satellites, is the only region in the topside
terrestrial ionosphere that maintains continuous contact with plasma.
The continuous inflow of plasma through the cusp leads to a direct
transfer of solar wind energy and momentum to the ionosphere and
atmosphere. Thus, the cusp represents a particular 'hot spot' in the
solar terrestrial relationship. New improved magnetic field mapping
models have helped elucidating the connection of the low altitude cusp
to the magnetospheric boundary region.
Geophysical Research Letters, v.35 (2008).
Icarus, v.182, 413-423 (2006).
Icarus, v.182, 343-349 (2006).
Nature, v.450, 650-653 (2007).
Geophysical Research Letters, v.36 (2009).
Geophysical Research Letters, v.33 (2006).
ABSTRACT: Measurements of energetic neutral atoms (ENA) generated in the magnetosheath at Mars are reported. These ENAs are the result of charge exchange collisions between solar wind protons and neutral oxygen and hydrogen in the exosphere of Mars. The peak of the observed ENA flux is . For the case studied here, i.e., the passage of Mars Express through the martian magnetosheath around 20:15 UT on 3 May 2004, the measurements agree with an analytical model of the ENA production at the planet. It is possible to find parameter values in the model such that the observed peak in the ENA count rate during the spacecraft passage through the magnetosheath is reproduced.
Journal of Geophysical Research, v.115 (2010).
Space Science Review, v.126, 239-266 (2006).