Upper ionosphere of Mars is not axially symmetrical
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The measurements carried out by the ASPERA-3 and MARSIS experiments on board the Mars Express (MEX) spacecraft show that the upper Martian ionosphere (h ≥ 400 km) is strongly azimuthally asymmetrical. There are several factors, e.g., the crustal magnetization on Mars and the orientation of the interplanetary magnetic field (IMF) which can give rise to formation of ionospheric swells and valleys. It is shown that expansion of the ionospheric plasma along the magnetic field lines of crustal origin can produce bulges in the plasma density. The absense of a magnetometer on MEX makes the retrieval of an asymmetry caused by the IMF more difficult. However hybrid simulations give a hint that the ionosphere in the hemisphere (E−) to which the motional electric field is pointed occurs more inflated than the ionosphere in the opposite (E+) hemisphere.
Key wordsMars ionosphere crustal magnetization solar wind
The Martian ionosphere, formed by the photoionization of the major neutral constituents CO2 and O with subsequent molecular reactions creating O2+ as the major ionospheric ion species and O+ becoming comparable at altitudes ≥300 km, was extensively studied by radio occultation measurements (Kliore, 1992; Mendillo et al., 2004; Pätzold et al., 2005), radar remote sounding (Gurnett et al., 2005, 2008), in-situ measurements by retarding potential analyzer (Hanson et al., 1977; Hanson and Mantas, 1988) and ‘plasma wave diagnostics’ (Gurnett et al., 2005, 2008). Since Mars is not screened by a large-scale magnetic field the solar wind has direct access to the ionosphere providing momentum and energy transfer to the upper layers of the ionospheric plasma. While the ionosphere at the heights ≤200 km is in photochemical equilibrium and the height profile and solar zenith dependence rather closely follow the Chapman model (Gurnett et al., 2008; Morgan et al., 2008; Withers, 2009; Lillis et al., 2010), deviations from the model become essential at larger altitudes. At altitude 300–550 km the median electron density ceases to follow Open image in new window dependence and remains almost constant for all solar zenith angles up to SZA ~ 80° (Duru et al., 2008).
It is reasonable to assume that dynamics of the upper ionosphere is strongly influenced by the interaction between solar wind and ionospheric plasma mediated by the IMF draped around the planet. This interaction can introduce asymmetry in the plasma distribution. Here we present data which show that in contrast to the low-altidude ionosphere, which is almost axially symmetrical (e.g. the dawn-dusk asymmetry does not exceed 5–25%, Lillis et al., 2010), the top-side ionosphere is very asymmetrical.
The MEX spacecraft is in a highly eccentric polar orbit around Mars with periapsis and apoapsis of about 275 and 10000 km, respectively. The measurements were made by the ASPERA-3 (Analyzer of Space Plasma and Energetic Atoms) and MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) experiments. ASPERA-3 comprises two plasma sensors: the Ion Mass Analyzer (IMA) and ELectron Spectrometer (ELS) (Barabash et al., 2006). The Ion Mass Analyzer (IMA) determines the composition, energy, and angular distribution of ions in the energy range ≈ 10 eV -30 keV. Mass (m/q) resolution is provided by a combination of an electrostatic analyzer with deflection of ions in a cylindrical magnetic field set up by permanent magnets. In the energy range ≥50 eV, IMA measures fluxes of different (m/q) ion species with a time resolution of ~3 min and a field of view of 90° x 360° (electrostatic steering provides an elevation coverage of ±45°). The measurements of the cold/low-energy component (≤50 eV) are carried out without the elevation coverage, and therefore, the time-resolution of these measurements increases up to 12 s. The ELS sensor measures 2D distributions of the electron fluxes in the energy range 1 eV-20 keV (δE/E = 8%) with a field of view of 4° × 360° and a time resolution of ~4 s.
The MARSIS radar sounder (f ± 0.1 to 5.5 MHz), designed to monitor the ionospheric height profile and the subsurface of the planet, consists of a 40 m tip-to-tip electric dipole antenna, a radio transmitter, a receiver, and a digital signal processing system. For the normal ionospheric sounding mode used by MARSIS the transmitter steps through 160 frequencies (Δf/f ≈ 2%) from 100 kHz to 5.5 MHz. The receiver has a bandwidth of 10.9 kHz centered on the frequency of the transmitted pulse. A complete scan through all 160 frequencies takes 1.26 s, and the basic sweep cycle is repeated once every 7.54 s. In addition to remote radio sounding, the local electron density and the magnetic field strength can also be retrieved from the MARSIS data by measuring the frequencies of local electron plasma oscillations and their harmonics and electron cyclotron waves excited by the radar transmitter in the nearby plasma. These measurements were made in the region around periapsis, at the altitudes ≤1300 km (Gurnett et al., 2005, 2008; Duru et al., 2008).
It is observed that the Martian ionosphere at altitudes ≥400 km is strongly azimuthally asymmetrical. Such an asymmetry is closely related to an asymmetry in the flow of the shocked solar wind around Mars. A flow asymmetry can appear due to either variations in the solar wind dynamic pressure or inherent features of solar wind/Mars interaction. For the reason that in many cases the observed asymmetry can not be explained by solar wind variations we consider other factors which might influence the upper ionosphere. The induced magnetosphere of Mars is produced by draping of the IMF. Asymmetry in the pile-up observed by Mars Global Surveyor (MGS) (Vennerstrom et al., 2003) can lead to a more effective screening of the ionosphere in the hemisphere in which the motional electric field is pointed outward the planet (the E+ hemisphere). On the other hand, the forces due to the normal and tangential magnetic field tensions driving the planetary plasma into motion are also stronger in the E+ hemisphere. It will lead to scavenging of the ionospheric plasma, also extraction of the ionospheric ions by the — Vsw × B electric field, and, as the effect, to the ionospheric depletion. Therefore it is not clear what processes will prevail for producing an asymmetry.
The latter asymmetry can appear due to the Parker orientation of the IMF in the solar wind. For the nominal IMF orientation (+Y −X or −Y +X) the induced magnetosphere is stronger protected from the magnetosheath plasma at the dusk side where the draped magnetic field is almost tangential to the magnetospheric boundary while at the dawnside, where the magnetic field is more radial, solar wind more easy gain access to the magnetosphere (Dubinin et al., 2008c). Figure 5(b) shows the orbital segments on which the ionosphere was expanded (red curves) or contracted (blue curves) in the YZmso plane for the 12 selected cases. It is worth noting that the MEX orbit is not suitable to study a dawn-dusk asymmetry because the spacecraft surveyed mainly the low-altitude dusk ionosphere. It is well seen in Fig. 10(a) which shows the map of the fluxes of the low-energy oxygen ions in the XYmso plane. The absense of ion fluxes at the dawn side is due to the fact that MEX sampled this region at much higher altitudes.
Asymmetry of plasma flow can also appear due to crustal magnetic field. Crider et al. (2002) have found that the magnetospheric boundary moves upward with increasing southern latitude. Fraenz et al. (2006) and Dubinin et al. (2008a) have shown that the boundary above strong crustal sources can shift upward by ~400 km as compared to the boundary in the northern hemisphere. Due to the local origin of crustal magnetic fields on Mars, the surface of the magnetospheric cavity occurs corrugated (Brain et al., 2006; Dubinin et al., 2008c) and the ionospheric plasma lifting up along the crustal field lines produces swells in the density distribution. Figure 5(c) shows the trajectories of MEX orbits at which MARSIS examined the upper ionosphere at almost the same solar zenith angles on the inbound and outbound crossings. Blue and red curves correspond to the orbital legs at which the ionosphere was contracted or expanded, respectively. Trajectories are given in the variables: altitude-model radial component of the crustal magnetic field at 400 km. It is seen that although for these selected cases we try to minimize a possible role of crustal sources, the ionospheric expansion usually occurs on the orbital segments with higher values of the crustal field, even if the value of the local field is small to balance the solar wind dynamic pressure.
The importance of this factor is also illustrated in Fig. 9(b, c) which compares the maps of the fluxes of low-energy ionospheric oxygen ions as a function of solar zenith angle and the MEX altitude obtained from the IMA-ASPERA-3 measurements performed in the northern and southern hemispheres in the interval of geographic longitudes in the range of 90°–240°. A clear asymmetry between the northern hemisphere where the pure induced magnetosphere is formed and the southern hemisphere with strongest local magnetization where crustal magnetic field essentially contribute to the solar wind/Mars interaction is observed.
A distinct asymmetry between the altitude profiles of the Martian ionosphere on the inbound and outbound parts of the MEX orbits inferred from the in-situ measurements of the plasma density by ASPERA-3 and MARSIS is observed. It is shown that such an asymmetry is accompanied by the asymmetry in the solar wind flow around Mars. The asymmetry can appear not only due to variations in solar wind dynamic pressure but also due to the inherent features of the solar wind/Mars interaction. In particular, the ionosphere over the regions with strong crustal magnetization occurs more inflated due to a lift of plasma along the crustal magnetic field lines. A possible asymmetry caused by the motional electric field can be disguised by errors in the determination of its direction. However the 3D hybrid simulations reveal such an asymmetry—the ionosphere in the E− hemisphere is more swelled as compared to the E+ hemisphere. A possible dawn-dusk asymmetry due to the Parker IMF could not be studied because of inappropriate dawn-dusk ionosphere sampling by the MEX spacecraft.
E. D., M. F. and J. W. wish to acknowledge the DLR and DFG for supporting this work by grants FKZ 50 QM 0801, O539/17-1 and DFG-grant SPP 1488 W0910/3-1, respectively.
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