Ion Composition Instrument Research Articles

In‐Situ Measurements of Ion Density in the Martian Ionosphere: Underlying Structure and Variability Observed by the MAVEN‐STATIC Instrument

AbstractMeasurement of the dense cold thermal plasma in planetary ionospheres via orbiting spacecraft is challenging because ion energies are small (0–4 eV), densities can vary by four orders of magnitude, composition varies with altitude, spacecraft charging varies in time and must be measured very accurately, and instrumental effects (e.g., detector dead‐time and background) can be significant. The SupraThermal And Thermal Ion Composition instrument team has recently released a new set of data products that contain density moments of the primary ion species at Mars, including those derived at periapsis, subject to the full suite of calibration factors required. This article discusses the challenges associated with deriving these densities and provides examples of the key caveats that users of the data should be aware of. A preliminary statistical study of this new data set focuses on the structure and variability of Mars' ionosphere, demonstrating that solar zenith angle effects, the crustal magnetic fields, and electron precipitation on the nightside, drive the strongest structural features, consistent with photochemical theory and previous studies. Dayside ionospheric density profiles are highly repeatable below altitudes of 200 km, marking the region where photochemistry and collisions dominate. In the upper dayside ionosphere (altitudes >300–400 km) changes in the solar wind dynamic pressure on timescales of Mars Atmosphere and Volatile EvolutioN's orbit (hr) drive the largest (factors of 1–3) variability in ionospheric density. In contrast variability in ionospheric density peaks between 150 and 250 km altitude on the nightside (factors of 1–2), consistent with electron precipitation driving ionization in this region.

Formation of Coronal Mass Ejection and Posteruption Flow of Solar Wind on 2010 August 18 Event

The state of the space environment plays a significant role in the forecasting of geomagnetic storms produced by disturbances of the solar wind (SW). Coronal mass ejections (CMEs) passing through the heliosphere often have a prolonged (up to several days) trail with declining speed, which affects propagation of the subsequent SW streams. We studied the CME and posteruption plasma flows behind the CME rear in the event on 2010 August 18 that was observed in quadrature by several space-based instruments. Observations of the eruption in the corona with EUV telescopes and coronagraphs revealed several discrete outflows followed by a continuous structureless posteruption stream. The interplanetary coronal mass ejection (ICME) associated with this CME was registered by the Plasma and Suprathermal Ion Composition instrument aboard the Solar Terrestrial Relations Observatory between August 20, 16:14 UT and August 21, 13:14 UT, after which the SW disturbance was present over 3 days. Kinematic consideration with the use of the gravitational and drag-based models has shown that the discrete plasma flows can be associated with the ICME, whereas the posteruption outflow arrived in the declining part of the SW transient. We simulated the Fe ion charge distributions of the ICME and post-CME parts of the SW using the plasma temperature and density in the ejection region derived from the differential emission measure analysis. The results demonstrate that in the studied event, the post-ICME trailing region was associated with the posteruption flow from the corona rather than with the ambient SW entrained by the CME.

In Situ Measurements of Thermal Ion Temperature in the Martian Ionosphere.

In situ measurements of ionospheric and thermospheric temperatures are experimentally challenging because orbiting spacecraft typically travel supersonically with respect to the cold gas and plasma. We present O2+ temperatures in Mars' ionosphere derived from data measured by the SupraThermal And Thermal Ion Composition instrument onboard the Mars Atmosphere and Volatile EvolutioN spacecraft. We focus on data obtained during nine special orbit maneuvers known as Deep Dips, during which MAVEN lowered its periapsis altitude from the nominal 150 to 120 km for 1 week in order to sample the ionospheric main peak and approach the homopause. We use two independent techniques to calculate ion temperatures from the measured energy and angular widths of the supersonic ram ion beam. After correcting for background and instrument response, we are able to measure ion temperatures as low as 100 K with associated uncertainties as low as 10%. It is theoretically expected that ion temperatures will converge to the neutral temperature at altitudes below the exobase region (∼180–200 km) due to strong collisional coupling; however, no evidence of the expected thermalization is observed. We have eliminated several possible explanations for the observed temperature difference between ions and neutrals, including Coulomb collisions with electrons, Joule heating, and heating caused by interactions with the spacecraft. The source of the energy maintaining the high ion temperatures remains unclear, suggesting that a fundamental piece of physics is missing from existing models of the Martian ionosphere.

Advection of Martian Crustal Magnetic Fields by Ionospheric Plasma Flow Observed by the MAVEN Spacecraft

AbstractThe plasma environment of Mars is highly influenced by its crustal magnetic fields. In this work, we investigate whether the crustal magnetic fields can be displaced from their original positions due to advection by ionospheric plasma flow. The ranges of interest are altitudes between km and solar zenith angles between . We examine magnetic field data using measurements from the Magnetometer (MAG) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We conduct statistical analyses ( years of data) in order to investigate if horizontal shifts are observed between the data and the crustal magnetic field model by Langlais et al. (2019), https://doi.org/10.1029/2018je005854. We also show two case studies of individual measurements within a specific region of Mars. The results show statistical and observational evidence that the crustal magnetic fields are often being advected towards the night‐side of the planet. The displacement seems to increase exponentially as a function of altitude, reaching a horizontal shift of 100 km at 650 km altitude. Moreover, we use measurements from the SupraThermal And Thermal Ion Composition (STATIC) instrument in order to compare the dynamic pressure of ionospheric plasma flow to the magnetic pressure of the crustal magnetic fields. There are two possible causes for the displacement: a forcing by external magnetic pressure or an advection of the field to the electron fluid, which follows the ion fluid. In this paper, we argue that by the observed relation between ion fluid and displacement, the latter explanation is more probable.

Properties and sources of the dayside martian magnetosphere

This study is based on measurements of the energy-mass analyzer Suprathermal and Thermal Ion Composition instrument (STATIC, McFadden et al., 2015) and magnetometer (MAG, Connerney et al., 2015) on the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft (Jakosky et al., 2015). 48 passes of MAVEN at SZA < 70° in the northern hemisphere on the day side of Mars (in order to avoid the influence of remnant magnetic field) were selected with condition to have continuous observations of STATIC from ionosphere to magnetosheath (“complete” observation crossings). It was found that in the vast majority of such “complete” crossings there is a layer between the ionosphere and magnetosheath, where two oxygen ion populations are observed: (1) heated ionospheric ions and (2) higher energy oxygen ions. The heated ionospheric ions are located just above the cold ionospheric ions and is identified by the increase of energy and temperature and decrease of number density. The second ion population has a wide energy spread from a few eV to hundreds of eV and has a velocity distribution typical to pick-up ions. The quite similar characteristics of the pick-ions just upstream of the magnetopause and characteristics of the second ions component in the layer between magnetosheath and ionosphere suggests their common origin. Pick-up ions in this layer have energies from a few to hundred electron volts have gyroradii comparable or significantly larger (from ~80 to ~570 km) than the thickness of this layer (~90 km).Pick-up ions from magnetosheath penetrate into upper ionosphere and interact with ionospheric ions and upper atmosphere. The population of pick-up ions provides additional energy source to the ionosphere and the upper atmosphere of Mars.The layer between dayside magnetosheath and ionosphere is not sufficiently studied and has many different names (see Espley, 2018). The physical state and properties of this layer (formed by combination of hydrodynamic and kinetic processes) are very much different from magnetosheath and ionosphere. This is defined as the dayside magnetosphere of Mars (Vaisberg et al., 2018).

The Pitch-angle Distributions of Suprathermal Ions near an Interplanetary Shock

Abstract We present a case study of the pitch-angle distributions (PADs) of suprathermal H+, He2+ at ∼10–40 keV/nuc and He+ at ∼8–20 keV/nuc near a reverse shock of a stream interaction region observed by the Plasma and Suprathermal Ion Composition instrument on board the Solar Terrestrial Relations Observatory Ahead spacecraft on 2008 March 9. We find that in both the downstream and upstream region close to the shock, the shocked particles of all three species appear to have a power-law-like spectrum at these suprathermal energies. The PADs of these three species show very similar behavior: in the downstream region, the phase space density appears to be significantly higher in the direction perpendicular to the interplanetary magnetic field (IMF) than in the parallel direction, along which particles accelerated at the shock front are supposed to escape into the downstream region. In the upstream region, the PADs of all three species show a clear beam in the direction antiparallel to the IMF due to the escaping particles from the shock into the upstream region. In addition, we find that suprathermal He+ shows a signature of bidirectional beams in the upstream region very close to the shock. These results suggest that H+, He2+ at ∼10–40 keV/nuc and He+ at ∼8–20 keV/nuc could be accelerated similarly at interplanetary shocks and that shock drift acceleration likely plays an important role in the in situ acceleration of low-energy suprathermal ions.

Variability of Precipitating Ion Fluxes During the September 2017 Event at Mars

AbstractIn this work, we study the influence of the September 2017 solar event on the precipitating heavy ion fluxes toward Mars' atmosphere as seen by Mars Atmosphere and Volatile EvolutioN/Solar Wind Ion Analyzer, an energy and angular ion spectrometer and by Mars Atmosphere and Volatile EvolutioN/Suprathermal and Thermal Ion Composition instrument, an energy, mass, and angular ion spectrometer. After a careful reconstruction of the background induced by the Solar Energetic Particle event in the Mars Atmosphere and Volatile EvolutioN/Solar Wind Ion Analyzer spectrometer, we investigate the precipitating ion flux responses to the space weather events that took place in September 2017. This period is a unique opportunity to analyze the respective role of various possible drivers of heavy ion precipitation into Mars' atmosphere with a wide range of different space weather events occurring during the same month. This study shows an increase in the precipitation flux by more than 1 order of magnitude during the arrival of the September Interplanetary Coronal Mass Ejection compared to the average flux during quiet solar conditions. We also showed that among the possible solar drivers, the solar wind dynamic pressure is the most significant during September 2017.

Multispecies and Multifluid MHD Approaches for the Study of Ionospheric Escape at Mars

AbstractA detailed model‐model comparison between the results provided by a multispecies and a multifluid magnetohydrodynamic (MHD) code for the escape of heavy ions in the Martian‐induced magnetosphere is presented. The results from the simulations are analyzed and compared against a statistical analysis of the outflow of heavy ions obtained by the Mars Atmosphere and Volatile EvolutioN/Suprathermal and Thermal Ion Composition instrument over an extended period of time in order to estimate the influence of magnetic forces in the ion escape. Both MHD models are run with the same chemical reactions and ion species in a steady state mode under idealized solar conditions. Apart from being able to reproduce the asymmetries observed in the ion escape, it is found that the multifluid approach provides results that are closer to those inferred from the ion data. It is also found that the j × B force term is less effective in accelerating the ions in the models when compared with the Mars Atmosphere and Volatile EvolutioN results. Finally, by looking at the contribution of the plume and the ion escape rates at different distances along the tail with the multifluid model, it is also found that the escape of heavy ions has important variabilities along the tail, meaning that the apoapsis of a spacecraft studying atmospheric escape can affect the estimates obtained.

Spectral variation of suprathermal protons associated with stream interaction regions: STEREO A/PLASTIC observations

Context. The observation of power-law spectra of suprathermal particles is typically associated with the occurrence of stream interaction regions (SIRs), indicating that these particles are accelerated close to the observer. However, recent observations have identified the existence of sunwards streaming particles at low suprathermal energies following SIRs. In addition, the observational evidence for turnover spectra in the low suprathermal energies has also been presented, suggesting that these particles might be accelerated at remote shocks and travel back to the Sun along the interplanetary magnetic field lines. Aims. We investigate the spectral evolution and variation of suprathermal protons from SIR to SIR as the observer moves from inside the compression regions of SIRs to the outside undisturbed solar wind regions away from the reverse shocks. Methods. The spectral analysis in the range from solar wind to suprathermal energies was based on proton data, which are obtained by the Plasma and Suprathermal Ion Composition instrument (PLASTIC) on the Solar Terrestrial Relations Observatory mission (STEREO). Results. All spectra in the compressed fast wind regions (F′ regions) of twelve SIRs exhibit power-law suprathermal tails. Six of them show clear turnover spectra at velocities below 2500 km s−1 in the undisturbed fast solar wind regions (F regions) following the compression regions, while the remaining six events exhibit continuous power-law spectra. Overall, the spectra at velocities higher than 2500 km s−1 harden in the F regions, consistent with previous observations.

Statistical analysis of the reflection of incident O+ pickup ions at Mars: MAVEN observations

AbstractAnalyzing ~1.3 year data set of O+ ion velocity distribution functions obtained from the Suprathermal and Thermal Ion Composition instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, we statistically investigate reflections of incident O+ pickup ions (&gt;10 keV) occurring below the Martian bow shock. To quantitatively evaluate importance of the reflection of incident O+ pickup ions, we estimate reflection ratios by calculating average inward and outward O+ ion fluxes above the Martian bow shock. Our result shows that an average reflection ratio is 14.1 ± 6.7%. We also investigate dependences of the reflection ratio on the solar wind, solar EUV flux, and southern hemispheric crustal magnetic field on Mars. We find that the reflection ratio strongly depends on the magnitude of the interplanetary magnetic field (IMF), solar wind dynamic pressure, and gyroradius of O+ pickup ions in the solar wind, rather than the solar EUV flux and the crustal magnetic field. We suggest that the reflection ratio is determined by the magnetic field both in the solar wind and below the bow shock that mainly controls gyroradius of incident O+ pickup ions in these regions. We also find that the reflection ratio increases to 38.9 ± 10.2% as these parameters become extreme conditions (i.e., IMF magnitude &gt;6 nT). Since incident O+ pickup ions are known to be a major source of atmospheric sputtering escape from Mars, our result suggests that ion reflections may have a role to reduce the sputtering escape from Mars under the extreme solar wind condition.

Deep nightside photoelectron observations by MAVEN SWEA: Implications for Martian northern hemispheric magnetic topology and nightside ionosphere source

AbstractThe Mars Atmosphere and Volatile EvolutioN (MAVEN) mission samples the Mars ionosphere down to altitudes of ∼150 km over a wide range of local times and solar zenith angles. On 5 January 2015 (Orbit 520) when the spacecraft was in darkness at high northern latitudes (solar zenith angle, SZA &gt;120°; latitude &gt;60°), the Solar Wind Electron Analyzer (SWEA) instrument observed photoelectrons at altitudes below 200 km. Such observations imply the presence of closed crustal magnetic field loops that cross the terminator and extend thousands of kilometers to the deep nightside. This occurs over the weak northern crustal magnetic source regions, where the magnetic field has been thought to be dominated by draped interplanetary magnetic fields (IMF). Such a day‐night magnetic connectivity also provides a source of plasma and energy to the deep nightside. Simulations with the SuperThermal Electron Transport (STET) model show that photoelectron fluxes measured by SWEA precipitating onto the nightside atmosphere provide a source of ionization that can account for the density measured by the Suprathermal and Thermal Ion Composition (STATIC) instrument below 200 km. This finding indicates another channel for Martian energy redistribution to the deep nightside and consequently localized ionosphere patches and potentially aurora.

O+ ion beams reflected below the Martian bow shock: MAVEN observations

AbstractWe investigate a generation mechanism of O+ ion beams observed above the Martian bow shock by analyzing ion velocity distribution functions (VDFs) measured by the Suprathermal and Thermal Ion Composition instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. In the solar wind near Mars, MAVEN often observes energetic O+ ion beams (~10 keV or higher). Accompanied with the O+ ion beam events, we sometimes observe characteristic ion VDFs in the magnetosheath: a partial ring distribution. The partial ring distribution corresponds to pickup ions with a finite initial velocity (i.e., not newborn pickup ions), and its phase space density is much smaller than that of local pickup O+ ions of the magnetosheath. Thus, the partial ring distribution is most likely produced by the reflection of pickup O+ ions precipitating from the upstream solar wind below the bow shock. After being injected into the magnetosheath from the solar wind, the precipitating O+ ions are subject to the significantly enhanced magnetic field in this region and start to gyrate around the guiding center of the plasma frame in the magnetosheath. Consequently, a part of precipitating O+ ions are reflected back to the solar wind, generating O+ beams in the solar wind. The beams direct quasi‐sunward near the subsolar region but have large angle with respect to the sunward direction at high solar zenith angles (&gt;50°). The reflected O+ beams are accelerated by the convection electric field of the solar wind and may escape Mars.

Anisotropy of the He+, C+, N+, O+, and Ne+pickup ion velocity distribution functions

Context. Interstellar and inner-source pickup ions (PUIs) are produced by the ionization of neutral atoms that originate either outside or inside the heliosphere. Just after ionization, the singly charged ions are picked up by the magnetized solar wind plasma and develop strong anisotropic toroidal features in their velocity distribution functions (VDF). As the plasma parcel moves outwards with the solar wind, the pickup ion VDF gets more and more affected by resonant wave-particle interactions, changing heliospheric conditions, and plasma drifts, which lead to a gradual isotropization of the pickup ion VDF. Past investigations of the pickup ion torus distribution were limited to He+ pickup ions at 1 astronomical unit (AU).Aims. The aim of this study is to quantify the state of anisotropy of the He+ , C+ , N+ , O+ , and Ne+ pickup ion VDF at 1 AU. Changes between the state of anisotropy between PUIs of different mass-per-charges can be used to estimate the significance of resonant wave-particle interactions for the isotropization of their VDF, and to investigate the numerous simplifications that are generally made for the description of the phase-space transport of PUIs.Methods. Pulse height analysis data by the PLAsma and SupraThermal Ion Composition instrument (PLASTIC) on board the Solar Terrestrial RElations Observatory Ahead (STEREO A) is used to obtain velocity-spectra of He+ , C+ , N+ , O+ , and Ne+ relative to the solar wind, f (w sw ). The w sw -spectra are sorted by two different configurations of the local magnetic field – one in which the torus distribution lies within the instrument’s aperture, φ ⊥ , and one in which the torus distribution lies exclusively outside the instrument’s field of view, φ ∥ . The ratio of the PUI spectra between φ ⊥ and φ ∥ is used to determine the degree of anisotropy of the PUI VDF.Results. The data shows that the formation of a torus distribution at 1 AU is significantly more prominent for O+ (and N+ ) than for He+ (and Ne+ ). This cannot be explained by resonant wave-particle interactions as the sole mechanism for the isotropization of the PUI VDF. The anisotropy of the O+ VDF compared to He+ is highly fluctuating but consistently higher over an observation period of six years and therefore unlikely to be related to either specific heliospheric conditions or solar activity variations. To our surprise, we also found a clear signature of a C+ torus distribution at 1 AU very similar to the one of He+ , although as an inner-source PUI, C+ should have a considerably different spectral and spatial injection pattern than interstellar PUIs.

Long-term variations in the plasma sheet ion composition and substorm occurrence over 23 years

The Geotail satellite has been operating for almost two solar cycles (~23 years) since its launch in July 1992. The satellite carries the energetic particle and ion composition (EPIC) instrument that measures the energetic ion flux (9.4–212 keV/e) and enables the investigation of long-term variations of the ion composition in the plasma sheet for solar cycles 22–24. From the statistical analysis of the EPIC data, we find that (1) the plasma ion mass (M) is approximately 1.1 amu during the solar minimum, whereas it increases to 1.5–2.7 amu during the solar maximum; (2) the increases in M seem to have two components: a raising of the baseline levels (~1.5 amu) and a large transient enhancement (~1.8–2.7 amu); (3) the baseline level change of M correlates well with the Mg II index, which is a good proxy for the solar extreme ultraviolet (EUV) or far ultraviolet (FUV) irradiance; and (4) the large transient enhancement of M is caused by strong magnetic storms. We also study the long-term variations of substorm occurrences in 1992–2015 that are evaluated with the number of Pi2 pulsations detected at the Kakioka observatory. The results suggest no clear correlation between the substorm occurrence and the Mg II index. Instead, when the substorms are classified into externally triggered events and non-triggered events, the number of the non-triggered events and the Mg II index are negatively correlated. We interpret these results that the increase in the solar EUV/FUV radiation enhances the supply of ionospheric ions (He+ and O+ ions) into the plasma sheet to increase M, and the large M may suppress spontaneous plasma instabilities initiating substorms and decrease the number of the non-triggered substorms. The present analysis using the unprecedentedly long-term dataset covering ~23 years provides additional observational evidence that heavy ions work to prevent the occurrence of substorms.

Quasi‐thermal noise measurements on STEREO: Kinetic temperature deduction using electron shot noise model

AbstractQuasi‐thermal noise (QTN) spectroscopy is an accurate technique for in situ measurements of electron density and temperature in space plasmas. A QTN spectrum is determined by plasma and antenna properties. On STEREO/WAVES, since the antennas are relatively short and thick, the QTN spectrum is dominated by electron shot noise, especially at low frequencies, which reduces the accuracy of the method. Here we use the STEREO low‐frequency receiver, proton density measured by Plasma and Suprathermal Ion Composition instrument, and a QTN and shot noise models to provide electron temperature data from both STEREO A and B spacecraft. This derivation is important since no reliable measurements of electron temperature exist on board these spacecraft. We compare the results of our analysis with the electron temperature provided by the Wind spacecraft during the period when Wind and STEREO B were close to each other. The comparison shows that our technique is reliable when results are integrated on a time scale of the order of 50 to 60 min.

Low‐frequency waves in the Martian magnetosphere and their response to upstream solar wind driving conditions

AbstractWe characterize low‐frequency plasma waves in the Martian magnetosphere and in the upstream region by using transport ratios. To compute the transport ratios, we use Mars Atmosphere and Volatile EvolutioN mission's (MAVEN) solar wind ion analyzer and suprathermal and thermal ion composition instrument measurements of the ion moments and the magnetometer measurements of the magnetic field. We find that the Alfvén waves are the most dominant wave mode in the upstream region and the magnetosheath. Fast waves are found frequently near the bow shock and the magnetic pileup boundary. Mirror and slow waves, on the other hand, occur much less frequently. We also find that the Alfvén and fast wave occurrences vary dominantly near the bow shock in response to the solar wind dynamic pressure.

THERMALIZATION OF HEAVY IONS IN THE SOLAR WIND

Observations of velocity distribution functions from the Advanced Composition Explorer/Solar Wind Ion Composition Spectrometer heavy ion composition instrument are used to calculate ratios of kinetic temperature and Coulomb collisional interactions of an unprecedented 50 ion species in the solar wind. These ions cover a mass per charge range of 1–5.5 amu/e and were collected in the time range of 1998–2011. We report the first calculation of the Coulomb thermalization rate between each of the heavy ion (A > 4 amu) species present in the solar wind along with protons (H+) and alpha particles (He2+). From these rates, we find that protons are the dominant source of Coulomb collisional thermalization for heavy ions in the solar wind and use this fact to calculate a collisional age for those heavy ion populations. The heavy ion thermal properties are well organized by this collisional age, but we find that the temperature of all heavy ions does not simply approach that of protons as Coulomb collisions become more important. We show that He2+ and C6+ follow a monotonic decay toward equal temperatures with protons with increasing collisional age, but O6+ shows a noted deviation from this monotonic decay. Furthermore, we show that the deviation from monotonic decay for O6+ occurs in solar wind of all origins, as determined by its Fe/O ratio. The observed differences in heavy ion temperature behavior point toward a local heating mechanism that favors ions depending on their charge and mass.

Inflow direction of interstellar neutrals deduced from pickup ion measurements at 1 AU

Observations of interstellar pickup ions inside the heliosphere provide an indirect method to access information on the surrounding interstellar medium. The so‐called pickup ion focusing cone and pickup ion crescent, which show an imprint of the related longitudinal distribution of interstellar neutrals in form of two overabundances on the down‐ and upwind side of the sun, are both believed to be aligned along the inflow vector of the interstellar medium. By finding their longitudinal positions, we can give an accurate value for the inflow direction λISM of interstellar matter. For that we performed an epoch analysis of interstellar pickup ions measured by the PLAsma and SupraThermal Ion Composition instrument (PLASTIC) on the Solar TErrestrial RElations Observatory mission (STEREO) and were able to reveal in situ the longitudinal distribution of interstellar He+, O+, and Ne+ pickup ions in the ecliptic plane at 1 AU. The previously accepted values for the inflow direction of interstellar matter in ecliptic longitude, as obtained with Ulysses/GAS (λ = 75.4° ± 0.5°), Prognoz 6 (λ = 74.5° ± 1°), and ACE/SWICS (λ = 74.43° ± 0.33°), are currently debated, especially in view of recent results from the Interstellar Boundary Explorer (IBEX) mission that show an inflow direction of interstellar neutral helium of λ = 79° + 3.0°(−3.5°). Four years of data collected with PLASTIC aboard STEREO A provided statistics sufficient not only to obtain values for the inflow direction of interstellar helium (λCone = 77.4° ± 1.9° and λCrescent = 80.4° ± 5.4°, deduced from an analysis of the He+ focusing cone and crescent, respectively) but also to derive values for the inflow direction of interstellar neon (λCone = 77.4° ± 5.0° and λCrescent = 79.7° ± 2.6°) and oxygen (λCrescent = 78.9° ± 3.1°). Although our values for He+, O+, and Ne+ are consistent with results from ACE, Ulysses, and Prognoz 6, considering the statistical and systematic uncertainties (except λNe,Crescent), they are systematically larger than the previously accepted values of 74.99 ± 0.55° and show a better agreement with the values from IBEX.

Observations of interstellar neon in the helium focusing cone

Extended pickup ion distributions in the heliosphere have been found to originate either from interstellar neutral gas or from the so‐called inner source. Because of the motion of the Sun through the interstellar medium, interstellar neutral gas flows through the inner heliosphere and for some of the species is focused on the downwind side by the gravitation of the Sun. This gravitational focusing cone, which is absent for inner source ions, has been identified for He with UV backscattering and pickup ion observations and is used to diagnose the interstellar parameters. Using the large geometric factor of the Plasma and Suprathermal Ion Composition instrument on the Solar Terrestrial Relations Observatory mission, we have studied heavy pickup ions in the mass‐per‐charge range 12–20 during 2007 and 2008. We have clearly identified C+, N+, O+, the H2O+ group, and Ne+ among the heavy pickup ions. Out of these, Ne+ shows clear enhancements during two consecutive focusing cone passages, as evidenced by concurrent He+ enhancements during these periods, whereas all the other species are evenly distributed. This is the first observation of the interstellar focusing cone for Ne, whose survival probability as neutrals is only slightly lower than that of He and substantially higher than for other elements.

Bi-layer structure of counterstreaming energetic electron fluxes: a diagnostic tool of the acceleration mechanism in the Earth's magnetotail

Abstract. For the first time we identify a bi-layer structure of energetic electron fluxes in the Earth's magnetotail and establish (using datasets mainly obtained by the Geotail Energetic Particles and Ion Composition (EPIC/ICS) instrument) that it actually provides strong evidence for a purely spatial structure. Each bi-layer event is composed of two distinct layers with counterstreaming energetic electron fluxes, parallel and antiparallel to the local ambient magnetic field lines; in particular, the tailward directed fluxes always occur in a region adjacent to the lobes. Adopting the X-line as a standard reconnection model, we determine the occurrence of bi-layer events relatively to the neutral point, in the substorm frame; four (out of the shown seven) events are observed earthward and three tailward, a result implying that four events probably occurred with the substorm's local recovery phase. We discuss the bi-layer events in terms of the X-line model; they add more constraints for any candidate electron acceleration mechanism. It should be stressed that until this time, none proposed electron acceleration mechanism has discussed or predicted these layered structures with all their properties. Then we discuss the bi-layer events in terms of the much promising "akis model", as introduced by Sarafopoulos (2008). The akis magnetic field topology is embedded in a thinned plasma sheet and is potentially causing charge separation. We assume that as the Rc curvature radius of the magnetic field line tends to become equal to the ion gyroradius rg, then the ions become non-adiabatic. At the limit Rc=rg the demagnetization process is also under way and the frozen-in magnetic field condition is violated by strong wave turbulence; hence, the ion particles in this geometry are stochastically scattered. In addition, ion diffusion probably takes place across the magnetic field, since an intense pressure gradient is directed earthward; hence, ions are ejected tailward of akis. This way, in front of akis an "ion capsule region" is formed with net positive charge. In between them a distinct region with an electric field E⊥ orthogonal to the magnetic field is emerged; E⊥ in front of akis is directed earthward. The field-aligned and highly anisotropic energetic electron populations have probably resulted via spatially separated antiparallel and field-aligned electric fields being the very heart of the acceleration source. We assume that the ultimate cause for the field-aligned electric fields are the net positive capsule charge and the net negative charge trapped at the tip of akis; both charges will be eventually neutralized through field aligned currents, but they remain unshielded for sufficient time to produce the observed events.