Geophysical Research Letters

Bursty escape fluxes in plasma sheets of Mars and Venus

Authors


Abstract

[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.

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 byRussell 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 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 charge-exchange 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. AtE/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 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 rangeE/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 two-dimensional 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 withVbulkVth (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 (neinline imagee), where ne and inline imagee 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.

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.

[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 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.

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.

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 3depicts 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 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 (right) shows in a zoom oscillations in neinline imagee, energy-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.

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 theBx 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.

[11] Figure 4ashows 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 theBy-component several times changes sign indicating strong perturbations of the current sheet.

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 λi, λj, λk 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.

3. Discussion

[12] There are several possible mechanisms which can be responsible for the observed periodic bursts. Large-amplitude 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 dataSergeev 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 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,Vdownstream) 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 componentBj probably appears due to the contribution from the Hall currents [Halekas et al., 2009].

Figure 5.

Examples of distributions of H+ and O+ ion fluxes in the XY VSO plane measured on the orbit on October 4, 2008.

[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.

Acknowledgments

[15] 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.