M. Fraenz

Ionospheric storms on Mars: Impact of the corotating interaction region

Measurements made by the ASPERA-3 and MARSIS experiments on Mars Express have shown, for the first time, that space weather effects related to the impact of a dense and high pressure solar wind (corotating interaction region) on Mars cause strong perturbations in the martian induced magnetosphere and ionosphere. The magnetic barrier formed by pile-up of the draped interplanetary magnetic field ceases to be a shield for the incoming solar wind. Large blobs of solar wind plasma penetrate to the magnetosphere and sweep out dense plasma from the ionosphere. The topside martian ionosphere becomes very fragmented consisting of intermittent cold/low energy and energized plasmas. The scavenging effect caused by the intrusions of solar wind plasma clouds enhances significantly (by a factor of ≥10) the losses of volatile material from Mars.

Plasma environment of Mars as observed by simultaneous MEX-ASPERA-3 and MEX-MARSIS observations

[1] Simultaneous in situ measurements carried out by the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instruments on board the Mars Express (MEX) spacecraft for the first time provide us with the local parameters of cold ionospheric and hot solar wind plasma components in the different regions of the Martian magnetosphere and ionosphere. On the dayside, plasma of ionospheric and exospheric origin expands to large altitudes and gets in touch with the solar wind plasma. Formation of the magnetic field barrier which terminates the solar wind flow is governed by solar wind. The magnetic field rises up to the value which is just sufficient to balance the solar wind pressure while the position of the magnetospheric boundary varies insignificantly. Although, within the magnetic barrier, solar wind plasma is depleted, the total electron density increases owing to the enhanced contribution of planetary plasma. In some cases, a load caused by a planetaiy plasma becomes so strong that a pileup of the magnetic field occurs in a manner which forms a discontinuity (the magnetic pileup boundary). Generally, the structure of the magnetospheric boundary on the dayside varies considerably, and this variability is probably controlled by the magnetic field orientation. Inside the magnetospheric boundaiy, the electron density continues to increase and forms the photoelectron boundary which sometimes almost coincides with the magnetospheric boundary. The magnetic field strength also increases in this region, implying that the planetary plasma driven into the bulk motion transports the magnetic field inward. A cold and denser ionospheric plasma at lower altitudes reveals a tailward cometary-like expansion. Large-amplitude oscillations in the number density of the ionospheric plasma are another typical feature. Crossings of plasma sheet at low altitudes in the terminator region are characterized by depletions in the density of the ionospheric component. In some cases, density depletions correlate with large vertical components of the crustal magnetic field. Such anticorrelation in the variations of the densities of the cold ionospheric and hot magnetosheath/plasma sheet plasmas is also rather typical for localized aurora-type events on the nightside.

Capture of solar wind alpha‐particles by the Martian atmosphere

Integration along He++ test-particle trajectories in the self-consistent electromagnetic fields generated by three-dimensional hybrid simulations of the solar wind/Mars interaction is used to evaluate the removal of solar wind α-particles due to charge-exchange processes with neutral species of the Martian exosphere. The total removal rate of solar wind He++ ions, transformed into either singly ionised or neutral helium, is equal to 6.7 × 1023 s−1, which corresponds approximately to 30% of the flux of solar α-particles through the planetary cross-section. The deposition rate of helium neutral atoms, created by double electronic capture on exospheric oxygen, impacting the exobase, and penetrating below where it can be trapped, is about 1.5 × 1023 s−1. That means an important contribution of the solar wind source to the helium balance of the Martian atmosphere. The implantation of the solar helium into the Martian atmosphere shows an asymmetry related to the orientation of the motional electric field of the solar wind, −VSW × BIMF.

Magnetic fields in the Venus ionosphere: Dependence on the IMF direction—Venus express observations

The structure of the magnetized ionosphere of Venus is investigated using the magnetometer and plasma (Analyzer of Space Plasmas and Energetic Atoms 4) data from the Venus Express spacecraft. Observations surveying the low-altitude (h ≤ 250 km) ionosphere were made at solar zenith angles ≥ 75°. The magnetic field permeating the Venus ionosphere at solar minimum conditions increases at low altitudes and reaches a maximum at an altitude of ∼200 km. The orientation of the magnetic field in the peak is almost insensible to the magnetic field direction in the solar wind. For both sector polarities of the IMF, the magnetic field vector has a dominant dawn-dusk component. The topology of the magnetic field also occurs different for different signs of the cross-flow component of the IMF revealing either a sudden straightening with liberation of the magnetic field stresses or a closing into a loop. We discuss different mechanisms of the peak formation including local magnetization, a weak intrinsic planetary field, a dipole field induced by eddy currents, a remnant origin, or giant flux ropes. All of them fail to explain most of the observed features. We suggest that a decoupling of ion and electron motion at low altitudes due to ion-neutral collisions results in currents which produce different field configurations depending on the IMF orientation.

Upper ionosphere of Mars is not axially symmetrical

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.

Effects of solar irradiance on the upper ionosphere and oxygen ion escape at Mars. MAVEN observations

We present multi-instrument observations of the effects of solar irradiance on the upper Martian ionosphere and escape fluxes based on the MAVEN data from November 2014 to February 2016. It is shown that fluxes of oxygen ions with E > 30 eV both inside and outside of the Martian magnetosphere are nonsensitive to EUV variations. In contrast, the fluxes of ions with lower energies extracted from the upper ionosphere increase with solar irradiance. Such an enhancement is nonlinear with the EUV variations and exhibits a growth by almost one order of magnitude when the EUV (0.1-50 nm) radiation increases to ≥0.1 W/m2 implying an enhancement of total ion losses of the low-energy component to ∼1.8·1025s−1. The flow of cold ions in the near Mars tail occurs very asymmetrical shifting in the direction opposite to the direction of the the solar wind motional electric field. Fluxes of the low-energy (E ≤ 30 eV) ion component are also nonsensitive to the variations in solar wind dynamic pressure.

Space weather effects on the bow shock, the magnetic barrier, and the ion composition boundary at Venus

We present a statistical study on the interaction between interplanetary coronal mass ejections (ICMEs) and the induced magnetosphere of Venus when the peak magnetic field of the magnetic barrier was anomalously large (>65 nT). Based on the entire available Venus Express data set from April 2006 to October 2014, we selected 42 events and analyzed the solar wind parameters, the position of the bow shock, the size and plasma properties of the magnetic barrier, and the position of the ion composition boundary (ICB). It was found that the investigated ICMEs can be characterized with interplanetary shocks and unusually large tangential magnetic fields with respect to the Venus-Sun line. In most of the cases the position of the bow shock was not affected by the ICME. In a few cases the interaction between magnetic clouds and the induced magnetosphere of Venus was observed. During these events the small magnetosonic Mach numbers inside magnetic clouds caused the bow shock to appear at anomalously large distances from the planet. The positions of the upper and lower boundaries of the magnetic barrier were not affected by the ICMEs. The position of the ICB on the nightside was found closer to the planet during ICME passages which is attributed to the increased solar wind dynamic pressure.

Discrepancy between ionopause and photoelectron boundary determined from Mars Express measurements

The Martian ionosphere directly interacts with the solar wind due to lack of a significant intrinsic magnetic field, and an interface is formed in between. The interface is usually recognized by two kinds of indicators: the ionopause identified from ionospheric density profiles and the photoelectron boundary (PEB) determined from the electron energy spectrum at higher energies. However, the difference between them remains unclear. We have determined the locations of crossings of the ionopause and PEB from Mars Express observations during 2005–2013 and found that the average position of the PEB appears to be ~200 km higher than that of the ionopause, which corresponds to 103 cm–3 in the electron density profile. The discrepancy can be explained by cross-field transport of photoelectrons.

Magnetic fields in the Mars ionosphere of a noncrustal origin: Magnetization features

The magnetic field observations by the Mars Global Surveyor (MGS) performed on the premapping orbits in the years 1997 and 1998 in the low-altitude ionosphere of Mars show the existence of a strong “external” magnetization not related to the “internal” crustal magnetization. A significant increase of the magnetic field strength is observed in the collisional northern ionosphere at altitudes of ∼ 200 km and at 60°–90°solar zenith angles sampled by MGS. The magnetization features and the magnetic field topology vary significantly with the sector structure of the interplanetary magnetic field (IMF). For ByIMF>0 the magnetic flux tubes transported to altitudes of ∼200 km are suddenly straightened releasing their tangential stresses. For ByIMF<0 a rotation of the magnetic field vector by almost 180°occurs. Such an asymmetry in the ionospheric response on Mars is similar to the asymmetry observed on Venus at the periods of low-solar activity indicating its universal origin for magnetized ionospheres. It is suggested that the electric currents generated in the collisional ionosphere where the ions become demagnetized, while the electrons remain magnetized produce the observed features.

Solar zenith angle-dependent asymmetries in Venusian bow shock location revealed by Venus Express

It has been long known that the Venusian bow shock (BS) location is asymmetric from the observations of the long-lived Pioneer Venus Orbiter mission. The Venus Express (VEX) mission crossed BS near perpendicularly not only in the terminator region but also in the near-subsolar and tail regions. Taking the advantage of VEX orbit geometry, we examined a large data set of BS crossings observed during the long-lasting solar minimum between solar cycles 23 and 24 and found that the Venusian BS asymmetries exhibit dependence of solar zenith angle. In the terminator and tail regions, both the magnetic pole-equator and north-south asymmetries are observed in Venusian BS location, which is similar to the Pioneer Venus Orbiter (PVO) observation near terminator. However, in the near-subsolar region, only the magnetic north-south is observed; i.e., the BS shape is indented inward over magnetic south pole and bulged outward over magnetic north pole. The absence of the magnetic pole-equator asymmetry in the near-subsolar region suggests that the magnetic pole-equator asymmetry is mainly caused by the asymmetric wave propagation rather than the ion pickup process. The evident magnetic north-south asymmetry in solar minimum, which is not observed by PVO, suggests that even during the low long-lasting solar minimum, the ion pickup process is very important in Venusian space environment.