M. Witte

Ionization Processes in the Heliosphere — Rates and Methods of Their Determination

The rates of the most important ionization processes acting in interplanetary space on interstellar H, He, C, O, Ne and Ar atoms are critically reviewed in the paper. Their long-term modulations in the period 1974 – 1994 are reexamined using updated information on relevant cross-sections as well as direct or indirect data on variations of the solar wind/solar EUV fluxes based on IMP 8 measurements and monitoring of the solar 10.7 cm radio emission. It is shown that solar cycle related variations are pronounced (factor of ∼ 3 between maximum and minimum) especially for species such as He, Ne, C for which photoionization is the dominant loss process. Species sensitive primarily to the charge-exchange (as H) show only moderate fluctuations ∼ 20% around average. It is also demonstrated that new techniques that make use of simultaneous observations of neutral He atoms on direct and indirect orbits, or simultaneous measurements of He+ and He++ pickup ions and solar wind particles can be useful tools for narrowing the uncertainties of the He photoionization rate caused by insufficient knowledge of the solar EUV flux and its variations.

The Interstellar Boundary Explorer (IBEX)

The Interstellar Boundary Explorer (IBEX) mission is exploring the frontiers of the heliosphere where energetic neutral atoms (ENAs) are formed from charge exchange between interstellar neutral hydrogen atoms and solar wind ions and pickup ions. The geography of this frontier is dominated by an unexpected nearly complete arc of ENA emission, now known as the IBEX ‘Ribbon’. While there is no consensus agreement on the Ribbon formation mechanism, it seems certain this feature is seen for sightlines that are perpendicular to the interstellar magnetic field as it drapes over the heliosphere. At the lowest energies, IBEX also measures the flow of interstellar H, He, and O atoms through the inner heliosphere. The asymmetric oxygen profile suggests that a secondary flow of oxygen is present, such as would be expected if some fraction of oxygen is lost through charge exchange in the heliosheath regions. The detailed spectra characterized by the ENAs provide time-tagged samples of the energy distributions of the underlying ion distributions, and provide a wealth of information about the outer heliosphere regions, and beyond.

REVISITING ULYSSES OBSERVATIONS OF INTERSTELLAR HELIUM

We report the results of a comprehensive reanalysis of Ulysses observations of interstellar He atoms flowing through the solar system, the goal being to reassess the interstellar He flow vector and to search for evidence of variability in this vector. We find no evidence that the He beam seen by Ulysses changes at all from 1994-2007. The direction of flow changes by no more than ~03 and the speed by no more than ~0.3 km s–1. A global fit to all acceptable He beam maps from 1994-2007 yields the following He flow parameters: V ISM = 26.08 ± 0.21 km s–1, λ = 75.54 ± 019, β = –5.44 ± 024, and T = 7260 ± 270 K; where λ and β are the ecliptic longitude and latitude direction in J2000 coordinates. The flow vector is consistent with the original analysis of the Ulysses team, but our temperature is significantly higher. The higher temperature somewhat mitigates a discrepancy that exists in the He flow parameters measured by Ulysses and the Interstellar Boundary Explorer, but does not resolve it entirely. Using a novel technique to infer photoionization loss rates directly from Ulysses data, we estimate a density of n He = 0.0196 ± 0.0033 cm–3 in the interstellar medium.

Interplanetary Lyman α remote sensing with the Ulysses Interstellar Neutral Gas Experiment

The Ulysses neutral gas instrument obtained celestial sphere maps of interplanetary Lyman α emission from neutral hydrogen in 1991–1996. These maps are unique because the spacecraft was located at a wide range of heliocentric ecliptic latitudes. Eleven of these maps are compared with the predictions of an interstellar wind hydrogen model previously used to study Galileo and Pioneer Venus Lyman α data obtained near the ecliptic plane. The model provides reasonable agreement with the Ulysses maps. The lifetime of the interstellar hydrogen atoms against charge exchange with solar wind protons is shown to be more latitudinally isotropic at solar maximum in 1991 than at solar minimum in 1994 and 1995. This result agrees with white light coronagraph data that generally show a more isotropic coronal brightness pattern at solar maximum. The transition from a more symmetric to a more asymmetric solar wind charge exchange rate also explains differences between published Galileo data from solar maximum and Solar and Heliospheric Observatory (SOHO) Solar Wind Anisotropy Experiment (SWAN) data from solar minimum.

The low energy particle detector SLED (∼30 keV-3.2 MeV) and its performance on the PHOBOS Mission and its moons

Abstract A low energy particle detector system (SLED) is described which was designed to measure the flux densities of electrons and ions in the energy range from ≈30 keV to a few MeV in (a) the varying solar aspect angles and temperatures pertaining during the Cruise Phase of the Phobos Mission and (b) in the low temperature environment (reaching −25° C) pertaining during Mars Encounter. Representative data illustrating the excellent functioning of SLED during both phases of the mission are presented.

Energetic particle studies at Mars by SLED on Phobos 2

A preliminary overview of particle records obtained by the SLED instrument on Phobos 2, February–March, 1989 during Mars encounter, is presented. Data obtained while in close elliptical orbit around the planet (pericenter < 900 km), in both spin and three axis stabilised mode, display evidence of energy related particle shadowing by the body of Mars. This effect was also observed, under favourable conditions, in certain circular orbits (altitude 6330 km above the planet). Flux enhancements, inside the magnetopause, in the approximate range 30–350 keV, recorded in the same general location at < 900 km above Mars over an 8 day period during three consecutive elliptical orbits, are described. Possible explanations of these enhancements include the presence of quasi-trapped radiation at the planet and the detection of the propagation of accelerated particles along the boundary of the magnetopause from the day to the night side of Mars. Large anisotropic ion flux increases (1–1.5 orders of magnitude) in the approximate range 30–200 keV recorded in front of the bow shock (inbound and outbound) during certain circular orbits, provide evidence that the spacecraft traversed strongly anisotopic jets of energetic particles. These are suggested to have constituted O+ ions. The pickup process would have been sufficient to accelerate such ions to their observed energies in the prevailing solar wind conditions. Alternatively, they might have comprised particles that had leaked from inside the magnetopause, perhaps undergoing shock drift acceleration in the process. Significant flux enhancements were also sometimes identified in the magnetotail (approximate energy range 30–50 keV). These are suggested to represent the signatures of O+ beams, impelled by acceleration processes similar to those associated with terrestrial ion beams.

Energetic particle composition measurements from Phobos 2: Results of the LET experiment

Abstract The Low Energy Telescope (LET) experiment carried on board the Phobos spacecraft measured the flux, spectra and elemental composition of nuclei from hydrogen up to iron, in the energy range ∼ 1–75 MeV nucleon −1 . Isotope separation for helium was also achieved. We present the results of a study of solar energetic particle (SEP) composition using LET data acquired during the period July 1988 to March 1989. The set of particle events selected for study comprises six large solar flare events, two 3 He-rich events and two energetic storm particle increases associated with interplanetary shocks. Three of the six large flare events occurred during a period of unusually high solar activity in March 1989. In two of these events, large Fe/O ratios were measured (0.44 ± 0.05 and 0.95 ± 0.24). The Fe/O ratios determined for the complete set of large flare events show an inverse correlation with the spectral index of oxygen, suggesting that the acceleration mechanism that produces events showing enhanced heavy-ion abundances is different from that responsible for lower Fe/O ratios. Our results in the case of the 3 He-rich events are in agreement with earlier work, showing an enrichment in heavy ions relative to the average SEP composition. The abundances measured for the shock-associated increases are consistent with the acceleration of the ambient population of solar flare particles.

Low energy charged particles in near Martian space from the SLED and LET experiments aboard the Phobos-2 spacecraft

Abstract The charged particle detector SLED on the Phobos-2 spacecraft has recorded, during a number of circular orbits about Mars, significant fluxes of ions with energies up to 200 keV in close spatial association with the Martian bow shock. The observed characteristics of these enhancements suggest that different shock acceleration mechanisms were operative in producing individual events

An Interstellar Neutral Atom Detector (INAD)

Direct detection of interstellar neutrals is a powerful technique for enlarging our knowledge about the media surrounding our solar system. We present in this paper a combination of two telescopes and a pointing device that would enable precise and detailed measurements of the density, velocity, temperature, and composition of the neutral particles that penetrate through the heliospheric bow shock and the heliopause.

EXPLORING THE POSSIBILITY OF O AND Ne CONTAMINATION IN ULYSSES OBSERVATIONS OF INTERSTELLAR HELIUM

We explore the possibility that interstellar O and Ne may be contributing to the particle signal from the GAS instrument on Ulysses, which is generally assumed to be entirely He. Motivating this study is the recognition that an interstellar temperature higher than any previously estimated from Ulysses data could potentially resolve a discrepancy between Ulysses He measurements and those from the Interstellar Boundary Explorer (IBEX). Contamination by O and Ne could lead to Ulysses temperature measurements that are too low. We estimate the degree of O and Ne contamination necessary to increase the inferred Ulysses temperature to 8500 K, which would be consistent with both the Ulysses and IBEX data given the same interstellar flow speed. We find that producing the desired effect requires a heavy element contamination level of ~9% of the total Ulysses/GAS signal. However, this degree of heavy element contribution is about an order of magnitude higher than expected based on our best estimates of detection efficiencies, ISM abundances, and heliospheric survival probabilities, making it unlikely that heavy element contamination is significantly affecting temperatures derived from Ulysses data.