Kanako Seki

MAVEN observations of the response of Mars to an interplanetary coronal mass ejection

Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

Statistical properties and possible supply mechanisms of tailward cold O+ beams in the lobe/mantle regions

We investigate statistical properties of cold O+ beams (COBs) streaming tailward at the velocity nearly equal to the major H+ component, which were observed by Geotail/low-energy particle (LEP) instrument in the tail lobe/mantle regions at geocentric distance between 8 and 210 RE (Earth radii) during the solar-minimum period (October 1993 to March 1995). The average O+ density is ∼1.3×10−3 cm−3, which corresponds to ∼1.2 % of the proton component. Properties of the flow velocity show that it is not the weakening of the magnetospheric convection but the large parallel velocity which enables the O+ ions to remain still in the distant lobe/mantle regions. The occurrence frequency of COBs suggests that O+ ions tend to exist in the mantle-like region rather than tenuous “pure lobe” and that their existence has a clear correlation with the geomagnetic activity. On the basis of the EMF By dependence of the double-peaked COB distribution along dawn-dusk direction, it is shown that COBs exist mostly on loaded quadrants in the north–south and dawn-dusk asymmetry of sheath plasma entry caused by the IMF By effect on the dayside reconnection process. The concentration on the loaded quadrants can be seen even in geomagnetic storms. It suggests that frequent COB occurrence at active times is mainly due to the southward orientation of IMF rather than the increase of dynamic pressure itself during the geomagnetic storms. That is, the statistics show that COBs are abundant at geomagnetically active times on loaded quadrants resulting from the dayside reconnection process, where the ions of solar wind origin bear the major component, and their field-aligned velocity is larger than usual. These COBs should originate in the dayside magnetosphere and/or the polar cap regions. From the COB energy of several keV, which is rather higher than that of cusp/cleft ion outflows, the necessity of extra energization(s) to elevate parallel velocity is suggested. Clear IMF By dependence, on one hand, provides other possibilities of the COBs source such as the energetic UFI beams and the equatorially trapped ions. The requirements for each candidate so as to be a main contributor to COBs are also discussed.

The spatial distribution of planetary ion fluxes near Mars observed by MAVEN

We present the results of an initial effort to statistically map the fluxes of planetary ions on a closed surface around Mars. Choosing a spherical shell ~1000 km above the planet, we map both outgoing and incoming ion fluxes (with energies >25 eV) over a 4 month period. The results show net escape of planetary ions behind Mars and strong fluxes of escaping ions from the northern hemisphere with respect to the solar wind convection electric field. Planetary ions also travel toward the planet, and return fluxes are particularly strong in the southern electric field hemisphere. We obtain a lower bound estimate for planetary ion escape of ~3 × 1024 s−1, accounting for the ~10% of ions that return toward the planet and assuming that the ~70% of the surface covered so far is representative of the regions not yet visited by Mars Atmosphere and Volatile EvolutioN (MAVEN).

Cold ions in the hot plasma sheet of Earth's magnetotail

Most visible matter in the Universe exists as plasma. How this plasma is heated, and especially how the initial non-equilibrium plasma distributions relax to thermal equilibrium (as predicted by Maxwell–Boltzman statistics), is a fundamental question in studies of astrophysical and laboratory plasmas. Astrophysical plasmas are often so tenuous that binary collisions can be ignored, and it is not clear how thermal equilibrium develops for these ‘collisionless’ plasmas. One example of a collisionless plasma is the Earths plasma sheet, where thermalized hot plasma with ion temperatures of about 5 × 107 K has been observed. Here we report direct observations of a plasma distribution function during a solar eclipse, revealing cold ions in the Earths plasma sheet in coexistence with thermalized hot ions. This cold component cannot be detected by plasma sensors on satellites that are positively charged in sunlight, but our observations in the Earths shadow show that the density of the cold ions is comparable to that of hot ions. This high density is difficult to explain within existing theories, as it requires a mechanism that permits half of the source plasma to remain cold upon entry into the hot turbulent plasma sheet.

Coexistence of Earth-origin O+ and solar wind-origin H+/He++ in the distant magnetotail

In the lobe/mantle region at ∼159 RE away from the Earth during a geomagnetically disturbed period, we have found the coexistence of three ion species, H+, He++, and O+, streaming tailward with nearly the same flow velocity ∼200–500 km/s. Both H+ and O+ ions are detected almost continuously from near plasma sheet to near magnetopause region. From a positive correlation between the proton density and their velocity component parallel to the magnetic field VH+║, we conclude that most of protons have come from the solar wind. The existence of He++ further supports this conclusion, which implies the importance of solar wind contribution to the magnetotail. The existence of O+, on the other hand, suggests that the ions of ionospheric origin have mixed with those of solar wind origin. The lack of positive correlation between O+ density and VO+║ is consistent with the idea that O+; ions have some source mechanism different from that of protons. Simultaneously, curious velocity differences are also observed: VO+¶ appears to be often faster than VH+║; by ΔV¶ = 20–30 km/s. This observation may provide a key for further discussion.

Terrestrial nitrogen and noble gases in lunar soils

The nitrogen in lunar soils is correlated to the surface and therefore clearly implanted from outside. The straightforward interpretation is that the nitrogen is implanted by the solar wind, but this explanation has difficulties accounting for both the abundance of nitrogen and a variation of the order of 30 per cent in the 15N/14N ratio. Here we propose that most of the nitrogen and some of the other volatile elements in lunar soils may actually have come from the Earths atmosphere rather than the solar wind. We infer that this hypothesis is quantitatively reasonable if the escape of atmospheric gases, and implantation into lunar soil grains, occurred at a time when the Earth had essentially no geomagnetic field. Thus, evidence preserved in lunar soils might be useful in constraining when the geomagnetic field first appeared. This hypothesis could be tested by examination of lunar farside soils, which should lack the terrestrial component.

Early MAVEN Deep Dip campaign reveals thermosphere and ionosphere variability

The Mars Atmosphere and Volatile Evolution (MAVEN) mission, during the second of its Deep Dip campaigns, made comprehensive measurements of martian thermosphere and ionosphere composition, structure, and variability at altitudes down to ~130 kilometers in the subsolar region. This altitude range contains the diffusively separated upper atmosphere just above the well-mixed atmosphere, the layer of peak extreme ultraviolet heating and primary reservoir for atmospheric escape. In situ measurements of the upper atmosphere reveal previously unmeasured populations of neutral and charged particles, the homopause altitude at approximately 130 kilometers, and an unexpected level of variability both on an orbit-to-orbit basis and within individual orbits. These observations help constrain volatile escape processes controlled by thermosphere and ionosphere structure and variability.

Low-energy charged particle measurement by MAP-PACE onboard SELENE

MAP-PACE (MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment) is one of the scientific instruments onboard the SELENE (SELenological and ENgineering Explorer) satellite. PACE consists of four sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measure the distribution function of low-energy electrons below 15 keV, while IMA and IEA measure the distribution function of low energy ions below 28 keV/q. Each sensor has a hemispherical field of view. Since SELENE is a three-axis stabilized spacecraft, a pair of electron sensors (ESA-S1 and S2) and a pair of ion sensors (IMA and IEA) are necessary for obtaining a three-dimensional distribution function of electrons and ions. The scientific objectives of PACE are (1) to measure the ions sputtered from the lunar surface and the lunar atmosphere, (2) to measure the magnetic anomaly on the lunar surface using two ESAs and a magnetometer onboard SELENE simultaneously as an electron reflectometer, (3) to resolve the Moon-solar wind interaction, (4) to resolve the Moon-Earth’s magnetosphere interaction, and (5) to observe the Earth’s magnetotail.

The Energization and Radiation in Geospace (ERG) Project

The Energization and Radiation in Geospace (ERG) project for solar cycle 24 will explore how relativistic electrons in the radiation belts are generated during space storms. This geospace exploration project consists of three research teams: the ERG satellite observation team, the ground-based network observation team, and the integrated data analysis/simulation team. Satellite observation will provide in situ measurements of features such as the plasma distribution function, electric and magnetic fields, and plasma waves, whereas remote sensing by ground-based observations using, for example, HF radars, magnetometers, optical instruments, and radio wave receivers will provide the global state of the geospace. Various kinds of data will be integrated and compared with numerical simulations for quantitative understanding. Such a synergetic approach is essential for comprehensive understanding of relativistic electron generation/loss processes through crossenergy and cross-regional coupling in which different plasma populations and regions are dynamically coupled with each other. In addition, the ERG satellite will utilize a new and innovative measurement technique for wave-particle interactions that can directly measure the energy exchange process between particles and plasma waves. In this paper, we briefly review some of the profound problems regarding relativistic electron accelerations and losses that will be solved by the ERG project, and we provide an overview of the project.

Cold flowing O+ beams in the lobe/mantle at Geotail: Does FAST observe the source?

The Geotail spacecraft observed high-energy (∼3–10 keV) cold O+ beams (COBs) streaming tail ward together with protons entering from the magnetosheath in the northern dusk lobe/mantle when the IMF (interplanetary magnetic field) By and Bz are steadily negative. He+ beams were also observed intermittently. During the same period the FAST satellite passed across the dayside northern polar regions from dawn to dusk at low altitudes (1200–3400 km) and observed O+ precipitation on both closed and open field lines. There are regions where the magnetosheath and dayside plasma sheet/ring current components coexist and are precipitating together. In the open field line regions the precipitating O+ seem continuous with the precipitation in the closed regions, while the H+ and He++ precipitations are denser and typically have lower energy than O+. The phase space density (PSD) of the precipitating ions is highly isotropic except for the loss cone in the upward direction. Utilizing Liouvilles theorem, we have compared the PSD of locally mirroring O+ at FAST with the PSD of COBs observed at Geotail. This comparison shows that the PSD in the high-energy precipitation region on closed field lines is comparable to that of the COBs. In regions where the magnetosheath and dayside magnetosphere ions coexist, the O+ PSD is typically smaller than that of the COBs, but at low latitudes it sometimes reaches values comparable to that of the COBs. These results suggest that the high-energy O+ ions in the dayside magnetosphere are a promising candidate for the source of COBs in the lobe/mantle. The ion dynamics on reconnected flux tubes needs to be examined further to clarify the possible energization mechanisms and their effect on the O+ ions.