GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L01104, doi:10.1029/2011GL049883, 2012
Bursty escape fluxes in plasma sheets of Mars and Venus
E. Dubinin,1 M. Fraenz,1 J. Woch,1 T. L. Zhang,2 J. Wei,1 A. Fedorov,3 S. Barabash,4
and R. Lundin4
Received 4 November 2011; revised 23 November 2011; accepted 24 November 2011; published 11 January 2012.
[1] High resolution measurements of plasma in the plasma
sheets of Mars and Venus performed by almost identical
plasma instruments ASPERA-3 on the Mars Express spacecraft and ASPERA-4 on Venus Express reveal similar features of bursty fluxes of escaping planetary ions. A period
of bursts lasts about 1–2 min. Simultaneous magnetic field
measurements on Venus Express show that these burst-like
features arise due to flapping motions of the plasma sheet.
Their occurrence can be related to large-amplitude waves
propagating on the plasma sheet surface and launched by
reconnection in the magnetic tails. Citation: Dubinin, E.,
M. Fraenz, J. Woch, T. L. Zhang, J. Wei, A. Fedorov, S. Barabash,
and R. Lundin (2012), Bursty escape fluxes in plasma sheets of
Mars and Venus, Geophys. Res. Lett., 39, L01104, doi:10.1029/
2011GL049883.
1. Introduction
[2] Mars and Venus do not have a global magnetic field
and, as a result, the solar wind interacts directly with their
atmospheres and ionospheres inducing magnetospheres by
pile-up of the interplanetary magnetic field around the conductive ionospheric shells. Generally, solar wind interactions with both planets are similar although some differences
may appear due to stronger gravity on Venus and local
crustal fields on Mars (see more about this type of magnetospheres in work by Russell and Vaisberg [1983] and
Luhmann [1986]). The magnetotails of Mars and Venus
consist of two lobes of opposite polarity of the magnetic
field separated by a plasma sheet. The plasma sheet is one of
the main escape channels of planetary plasma that is a subject of major interest for Mars and Venus [Barabash et al.,
2007b, 2007c; Fedorov et al., 2006, 2008, 2011; Dubinin
et al., 1993, 2011]. The magnetic normal and tangential
stresses push the planetary plasma tailward acting like a jet
engine. Since the magnetic tensions on the nightside are
determined by pile-up of the field lines on the dayside the
energy gained by ions in the plasma sheet and fluxes of
escaping ions also vary with solar wind because the pile-up
varies with solar wind dynamic pressure [Dubinin et al.,
2008, 2011]. Short-term variations arising as a result of
dynamic pressure pulses of solar wind/bow shock origin and
impinging on the ionosphere or large-amplitude waves
1
Max Planck Institute for Solar System Research, Katlenburg-Lindau,
Germany.
2
Space Research Institute, OAW, Graz, Austria.
3
CNRS, Institut de Recherche en Astrophysique et Planetologie,
Toulouse, France.
4
Swedish Institute of Space Physics, Kiruna, Sweden.
Copyright 2012 by the American Geophysical Union.
0094-8276/12/2011GL049883
generated within the induced magnetosphere may also affect
fluxes of planetary ions forced to escape Mars and Venus.
[3] Large-amplitude periodic oscillations in electron fluxes
with periods of 1–2 min is a typical feature of the induced
Martian magnetosphere [Winningham et al., 2006]. They
were observed in different regions sampled by Mars Express
(MEX) and probably trace ion wave modes. The upper
ionosphere of Mars is also dominated by large density and
field variations [Gurnett et al., 2010; Halekas et al., 2011].
Ultra low frequency (ULF) waves in the magnetosheath of
Mars were recorded by the magnetometer on Mars Global
Surveyor, although closer to the planet and in the tail their
amplitudes were much smaller [Espley et al., 2004]. Energetic Neutral Atoms (ENAs) which appear due to chargeexchange between shocked solar wind and the Martian
atmosphere on the dayside and, therefore, tracing ion fluxes,
also reveal quasi-periodic (1 min) variations [Futaana
et al., 2006]. Interpretation of the observations on MEX
is strongly constrained by absence of a magnetometer. At
Venus, low-frequency waves were studied extensively
using the magnetometer measurements on Venus Express
(VEX) [see, e.g., Du et al., 2010]. However their possible
association with plasma fluxes was not analyzed yet.
[4] We have the unique situation now that almost identical
plasma instruments explore the plasma environment of two
unmagnetized terrestrial planets, Mars and Venus at the
same time. It will be shown in this paper that apart from
long-term changes caused by solar wind variations a periodic (1–2 min) bullet-like ejection of planetary ions occurs.
These pulses are probably produced by internal processes in
the plasma sheets of induced magnetospheres.
2. Observations
2.1. Instrumentation
[5] This paper focuses on results obtained from observations on the MEX and VEX spacecraft, especially those
from the ASPERA-3 and ASPERA-4 instruments [Barabash
et al., 2006, 2007a], respectively. The Mars Express (MEX)
spacecraft is in a highly eccentric polar orbit around Mars
with periapsis and apoapsis altitudes of about 275 and
10000 km, respectively. The orbital period is 6.75 h. Venus
Express (VEX) has a highly elliptical polar orbit with a 24 h
period and pericenter and apocenter of 250–350 km and
66000 km, respectively. The Ion Mass Analyzer (IMA/
ASPERA-3) on MEX measures ions in the 10 eV/q
30 keV/q energy range and 1–44 amu/charge range, including both solar wind and planetary ions. At E/q ≥ 50 eV IMA
measures fluxes of different (m/q) ion species with time
resolution of 192 s and field of view 90° 360°. Scanning
in the elevation direction (45°) is performed using an
electrostatic deflector. The measurements of the low-energy
(E/q ≤ 50 eV) ions are carried out without the elevation
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Figure 1. Crossing of the plasma sheet on Mars by MEX. (left, top to bottom) The electron number density, energy-time
spectrogram of electron fluxes, and energy-time spectrogram of ion fluxes. (right, top to bottom) The variations in the density, the electron fluxes, and the power spectrum of the density variations in a zoom.
steering but with increased time-resolution of 12 s and the
instantaneous field of view is 4.5° 360°. To get a high
time-resolution in the energy range E/q ≥ 50 eV the electrostatic scanning system was switched off on some orbits
enabling to perform the 2D measurements of ion fluxes with
a sampling time of 12 s. The ion sensor on ASPERA-4/VEX
is almost a replica of IMA/ASPERA-3 but the electrostatic
elevation steering operates for the whole energy range.
[6] The ELS sensors on both experiments ASPERA-3,4
measure a 2D distribution (16 sectors) of electron fluxes in
the energy range 5 eV 20 keV with a time resolution of 4 s.
The plasma observations on VEX are supported by magnetometer measurements [Zhang et al., 2006]. In this paper we
use the magnetic field measurements carried out with 4 s
resolution. MEX carried no magnetometer.
[7] A spiky behavior of plasma in the Martian magnetosphere is a common feature. It is clearly observed in the
electron data sampled with high temporal resolution. The
number of orbits when the IMA/ASPERA-3 sensor has
operated in the high resolution mode is very small and not
sufficient for statistical analysis. However whenever twodimensional ion measurements carried out in this mode
detected the bulk plasma flow we clearly observe that ion
measurements replicate the electron data, at least, qualitatively and show a bursty origin of ion fluxes. There are also
only few VEX orbits when the electrostatic deflector of
IMA/ASPERA-4 was turned off making possible to perform
high resolution (12 s) measurements. In the following we
study typical cases of this type of observations in plasma
sheets of Mars and Venus.
2.2. Mars
[8] The energy-time spectrogram of ion fluxes (Figure 1,
bottom left) measured by ASPERA-3 when MEX crossed
the plasma sheet (06:25–06:32 UT) illustrates energization
of oxygen ions by the j B force. Since shear stresses of the
draped field lines are the strongest in the center of plasma
sheet, the energy gained by ions gradually increases, reaches
a maximum and again decreases [Dubinin et al., 1993,
2011]. Ion measurements carried out at E/q ≥ 50 eV with
time resolution of 192 s provide us with a periodic (3 min),
drop-like picture of ion fluxes. This is an effect of low time
resolution and ion flows with Vbulk Vth (here Vbulk and Vth
are the bulk and thermal ion speeds, respectively). However,
the electron measurements (Figure 1, top left and middle
left) made with much higher resolution (4 s) reveal a real
bursty structure of the central plasma sheet. Bursts of electron fluxes are observed with a period of 1–2 min and the
variations in the electron number density reach one order of
magnitude. Figure 1 (right) shows from top to bottom a
zoom of the electron density variations (ne ne), where ne
and ne are the densities measured with a high resolution and
averaged over fast oscillations, respectively, the spectrogram
of electron fluxes and a power spectrum of the density
oscillations evaluated using a sliding Fourier Transform
Technique. A clear maximum at f 10 mHZ is seen.
[9] Similar bursts in ion fluxes are exposed when IMA
operates with a high temporal resolution (12 s). Examples of
bursts of tailward streaming oxygen ions are shown in
Figure 2. Amplitude of their flux variations reach a factor of
10–30. Since in this instrumental mode IMA performs only
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Figure 2. Examples of a fine structure of the plasma sheet on Mars. (top to bottom) Energy-time spectrogram of electron
fluxes, energy-time spectrogram of ion (O+) fluxes, the number density of the electrons and the partial number density of
oxygen ions, and fluxes of oxygen ions.
two-dimensional measurements the evaluated values of the
number density and flux are the partial ones. Supplementary
periodic variations in the electron number density confirm
that the bursts are not related to a possible field-off-view
offset of the IMA sensor, but are a characteristic feature of
plasma sheet at Mars.
2.3. Venus
[10] Measurements performed by ASPERA-4 on VEX
show that similar effects are also observed on Venus.
The simultaneous magnetic field measurements essentially
complement these observations and provide us with a new
insight on possible mechanisms of bursty ion fluxes in the
induced magnetotails. Figure 3 depicts an example of the
plasma sheet crossing in the Venusian tail (04:30–04:40 UT).
IMA operated in the low-resolution mode while the electron
measurements show the fine structure of the plasma sheet.
Periodic spikes in the electron fluxes are very similar to the
ones observed at Mars. The period of oscillations is close to
the characteristic value of 2 min typical for the bursts on
Figure 3. Crossing of the plasma sheet on Venus by VEX. (left, top to bottom) The electron number density, energy-time
spectrogram of electron fluxes, energy-time spectrogram of fluxes of oxygen ions, and the total value and the Bx component
of the magnetic field. (right, top to bottom) The variations in the density, electron fluxes, power spectrum of the density variations, and the magnetic field components in a zoom.
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Figure 4. (a) Example of a fine structure of the plasma sheet on Venus. (top to bottom) The value and the Bx component of
the magnetic field, the electron number density, the partial number density of oxygen and hydrogen ions, and the Vx components of the velocity of oxygen and hydrogen ions. (b) Vectors Byz along the VEX orbit crossing the plasma sheet in VSO
coordinates. Vectors of the IMF in the solar wind on the inbound (Bin)and outbound (Bout) legs of the orbit are also shown.
(c) The magnetic field in the principal axes (Min (k), Int (j), Max(i)) shows the field variations for the time interval marked
by the dotted black vertical lines in Figure 4a. Eigenvalues li, lj, lk for both time intervals are (112.3, 11.9, 0.7) and
(58.2, 5.5, 0.8), respectively. Green dotted vertical line marks time when the component Bx changes sign. (d) The inferred
magnetic field configuration is plotted in the axes i, j, k. The directions of the out-of-plane Hall magnetic field components
are shown. The projections of the magnetic field along the VEX trajectory (green vectors) are imposed on the spacecraft
trajectory.
Mars. The magnetometer data confirm that the bursts are
observed in the current sheet separating two magnetospheric
lobes. The transition through the current sheet is not smooth
but characterized by strong oscillations with multiple
crossings. The electron bursts correlate with the dips in the
magnetic field strength which mark crossings of the current
sheet or rapid excursions of the spacecraft to it. It is worth
noting that the magnetic field oscillations in the adjacent
lobes are much weaker indicating that their strong amplification in the plasma sheet has an internal origin. Figure 3
n e, energy(right) shows in a zoom oscillations in ne
time spectrogram of electron fluxes, a power spectrum of
the density oscillations and all components of the magnetic
field. The curve of the oxygen gyrofrequency is imposed. It
is seen that emissions in the spacecraft frame occur at the
frequency close to the oxygen gyrofrequency.
[11] Figure 4a shows an example of a plasma sheet
crossing on Venus when IMA operated in the high-resolution mode. As in the previous case the transition from one
lobe of the magnetic tail to another one occurs with multiple
crossings of the current sheet. Although the attitude of the
IMA-measuring plane was not very favorable (the partial
number densities of outflowing oxygen ions are rather low)
the appearance of ion spikes generally correspond to the
plasma sheet crossings and have their counterparts in the
electron data. The magnetic field variations indicate at
multiple rotations of the magnetic field vector. Figure 4b
shows how the projection of the magnetic field vector onto
the YZ-VSO plane (Byz) varies during the plasma sheet
crossing. The vectors Byz in the inbound and outbound solar
wind are pointed approximately in the +Y direction implying
that the current sheet separating two magnetospheric lobes
is stretched almost along the Z-axis. It is observed that the
By-component several times changes sign indicating strong
perturbations of the current sheet.
3. Discussion
[12] There are several possible mechanisms which can
be responsible for the observed periodic bursts. Largeamplitude coherent pressure pulses generated upstream the
bow shock by ion beams [Mazelle et al., 2004] can impact
the magnetosphere producing periodic pulses in forces
pushing planetary plasma tailward. Pressure pulses can also
arise at the bow shock and are convected to the magnetosheath, which then is decomposed into a sequence of
periodic compressive waves [Winningham et al., 2006].
Indeed the MEX observations partly support such type of
scenario showing the existence of large-amplitude waves
penetrating inside the induced magnetosphere and modulating ouflowing fluxes of planetary ions [Dubinin et al.,
2011]. However, it seems unlikely that pressure pulses are
able to produce very strong variations of ion fluxes as it is
observed, for example, in Figure 2. Moreover the simultaneous magnetic field and plasma measurements on VEX
clearly show that ion bursts correlate with the crossings of
the tail current sheet. Therefore it seems more reasonable to
assume that a bursty behavior of plasma fluxes appears, at
least partly, because of plasma sheet flapping caused either
by the sausage-type or kink-like perturbations. It is worth
noting that flapping motions are also typical for the Earth’
plasma sheet although the origin of such waves has not been
established yet. Analyzing the Geotail data Sergeev et al.
[2006] have found a close relationship between the occurrence of flapping motions and the occurrence of bursty bulk
plasma flows. It was suggested that both phenomena might
be triggered by the same mechanism - the magnetic field
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Figure 5. Examples of distributions of H+ and O+ ion fluxes in the XY VSO plane measured on the orbit on October 4,
2008.
reconnection. The localized reconnection can launch the
transverse motions of the current sheet propagating outward
of the reconnection site. Reconnection features in the Mars
magnetotail were recently identified by the characteristic
Hall magnetic field signatures arising due to the different
dynamics of ions and electrons [Eastwood et al., 2008;
Halekas et al., 2009]. It is worth noting that the near Mars/
Venus tail (1
3RM,V downstream) is assumed to be a
favorable region for reconnection during solar minimum
when the IMF penetrates deep inside the ionosphere. The
field lines then are mass-loaded by the ionospheric plasma
and become strongly stretched while being ejected from the
nightside ionosphere [Zhang et al., 2010, Halekas et al.,
2009].
[13] Reconnection features are also observed in the VEX
measurements. As a result of reconnection between the
draping magnetic fields in the magnetotail the magnetic
island with ‘O’-type neutral point arises. Such a field configuration suggests a disruption of the plasma jets accelerated by the magnetic tensions and the appearance of sunward
flows planetward from the reconnection site. Figure 4a
(bottom) depicts the X-component of the bulk speed of the
planetary ions (H + and O+). It is observed that at time
interval 08:01–08:05:30 UT the component Vx(H +) changes
sign from tailward to planetward. At the same time the
oxygen component becomes almost stagnant. Then with a
change of sign of the Bx-component the proton flow again
becomes tailward and its speed sharply increases. Accelerated oxygen ions appear to be observed later, at 08:09 UT
and also gain a strong acceleration in the antisunward
direction. Such a picture of ion fluxes is consistent with the
successive entry of the spacecraft into the magnetic field
island and then into the outflow region. Figure 5 shows
examples of the distribution of ion fluxes in the XY plane.
The region corresponding to the island is characterized either
by the planetward fluxes or by the counterstreaming ion jets.
Tailward jets observed later correspond to the region tailward from the X-point. The magnetic field data for time
intervals 08:01–08:03:40 and 08:04–08:06:40 UT plotted in
the principal axes (i, j, k) frame found from the Minimum
Variance Analysis generally suggest the crossing of the tail
configuration arising after reconnection and are shown in
Figure 4d. The out-of-plane component Bj probably appears
due to the contribution from the Hall currents [Halekas
et al., 2009].
[14] It is worth noting that bursty escape fluxes in plasma
sheets of Mars and Venus are probably difficult to explain
only by flapping effects. The amplitude of the flapping
motions is about of the sheet thickness [Sergeev et al., 2006]
while the width of the ion outflow region is larger than the
current sheet thickness (see, e.g., Figures 1 and 2). Therefore
an almost total disappearance of ion fluxes in the time
intervals between the bursts - as often observed - indicates
the existence of another, additional mechanism which is able
to break off the operation of a jet-engine in the plasma sheet.
A transient/pulsed reconnection in the induced tails can be
such a mechanism which sporadically releases the magnetic
field tensions in the tail. A question about the 1–2 min
periodicity still remains open and suggests a possible link
between periodic large-amplitude pressure pulses and the
instability of the plasma sheet and/or reconnection. The
bursty origin of ion fluxes in the tails of Mars and Venus
have implications for evaluation of nonthermal escape from
these planets.
[15] Acknowledgments. E.D., M.F., J.W. and J.W. wish to acknowledge the DLR and DFG for supporting this work by grants FKZ 50 QM
0801, MO539/17-1, SPP 1488 W0910/3-1, respectively.
[16] The Editor wishes to thank Jasper Halekas and an anonymous
reviewer for their assistance evaluating this paper.
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S. Barabash and R. Lundin, Swedish Institute of Space Physics, Kiruna
SE-901 87, Sweden. ([email protected]; [email protected])
E. Dubinin, M. Fraenz, J. Wei, and J. Woch, Max Planck Institute for
Solar System Research, Katlenburg-Lindau, D-37191, Germany. (dubinin@
mps.mpg.de)
A. Fedorov, CNRS, Institut de Recherche en Astrophysique et
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