Heliospheric Termination Shock Research Articles

Injection Process of Pickup Ion Acceleration at an Oblique Heliospheric Termination Shock

The injection process of pickup ion acceleration at a heliospheric termination shock is investigated. Using two-dimensional fully kinetic particle-in-cell simulation, accelerated pickup ions are self-consistently reproduced by tracking long time evolution of shocks with an unprecedentedly large system size in the shock normal direction. Reflected pickup ions drive upstream large-amplitude waves through resonant instabilities. Convection of the large-amplitude waves causes shock surface reformation and alters the downstream electromagnetic structure. A part of pickup ions are accelerated to tens of upstream flow energy in the timescale of ∼100 times inverse ion gyrofrequency. The initial acceleration occurs through the shock surfing acceleration (SSA) mechanism followed by the shock drift acceleration mechanism. Large electrostatic potential accompanied by the upstream waves enables the SSA to occur.

A Comparison of Particle-in-cell and Hybrid Simulations of the Heliospheric Termination Shock

We compare hybrid (kinetic proton, fluid electron) and particle-in-cell (kinetic proton, kinetic electron) simulations of the solar wind termination shock with parameters similar to those observed by Voyager 2 during its crossing. The steady-state results show excellent agreement between the downstream variations in the density, plasma velocity, and magnetic field. The quasi-perpendicular shock accelerates interstellar pickup ions to a maximum energy limited by the size of the computational domain, with somewhat higher fluxes and maximal energies observed in the particle-in-cell simulation, likely due to differences in the cross-shock electric field arising from electron kinetic-scale effects. The higher fluxes may help address recent discrepancies noted between observations and large-scale hybrid simulations.

The Role of Pickup Ions in the Interaction of the Solar Wind with the Local Interstellar Medium. I. Importance of Kinetic Processes at the Heliospheric Termination Shock

The role of pickup ions (PUIs) in the solar wind interaction with the local interstellar medium is investigated with 3D, multifluid simulations. The flow of the mixture of all charged particles is described by the ideal MHD equations, with the source terms responsible for charge exchange between ions and neutral atoms. The thermodynamically distinct populations of neutrals are governed by individual sets of gas dynamics Euler equations. PUIs are treated as a separate, comoving fluid. Because the anisotropic behavior of PUIs at the heliospheric termination shocks is not described by the standard conservation laws (a.k.a. the Rankine–Hugoniot relations), we derived boundary conditions for them, which are obtained from the dedicated kinetic simulations of collisionless shocks. It is demonstrated that this approach to treating PUIs makes the computation results more consistent with observational data. In particular, the PUI pressure in the inner heliosheath (IHS) becomes higher by ∼40%–50% in the new model, as compared with the solutions where no special boundary conditions are applied. Hotter PUIs eventually lead to charge-exchange-driven cooling of the IHS plasma, which reduces the IHS width by ∼15% (∼8–10 au) in the upwind direction, and even more in the other directions. The density of secondary neutral atoms born in the IHS decreases by ∼30%, while their temperature increases by ∼60%. Simulation results are validated with New Horizons data at distances between 11 and 47 au.

Pickup Ion–Mediated Magnetic Reconnection in the Outer Heliosphere

Pickup ions (PUIs) play a crucial role in the heliosphere, contributing to the mediation of large-scale structures such as the distant solar wind, the heliospheric termination shock, and the heliopause. While magnetic reconnection is thought to be a common process in the heliosphere due to the presence of heliospheric current sheets, it is poorly understood how PUIs might affect the evolution of magnetic reconnection. Although it is reasonable to suppose that PUIs decrease the reconnection rate since the plasma beta becomes much larger than 1 when PUIs are included, we show for the first time that such a supposition is invalid and that PUI-induced turbulence, heat conduction, and viscosity can preferentially boost magnetic reconnection in heliospheric current sheets in the distant solar wind. This suggests that it is critical to include the effect of the turbulence, heat conduction, and viscosity caused by PUIs to understand the dynamics of magnetic reconnection in the outer heliosphere.

Relating Energetic Ion Spectra to Energetic Neutral Atoms

Heliospheric energetic neutral atoms (ENAs) originate from energetic ions that are neutralized by charge exchange with neutral atoms in the heliosheath and very local interstellar medium (VLISM). Since neutral atoms are unaffected by electromagnetic fields, they propagate ballistically with the same speeds as parent particles. Consequently, measurements of ENA distributions allow one to remotely image the energetic ion distributions in the heliosheath and VLISM. The origin of the energetic ions that spawn ENAs is still debated, particularly at energies higher than ∼keV. In this work, we summarize five possible sources of energetic ions in the heliosheath that cover the ENA energy from a few keV to hundreds of keV. Three sources of the energetic ions are related to pickup ions (PUIs): those PUIs transmitted across the heliospheric termination shock (HTS), those reflected once or multiple times at the HTS, i.e., reflected PUIs, and those PUIs multiply reflected and further accelerated by the HTS. Two other kinds of ions that can be considered are ions transmitted from the suprathermal tail of the PUI distribution and other particles accelerated at the HTS. By way of illustration, we use these energetic particle distributions, taking account of their evolution in the heliosheath, to calculate the ENA intensities and to analyze the characteristics of ENA spectra observed at 1 au.

Oblique and rippled heliosphere structures from the Interstellar Boundary Explorer

Past analysis has shown that the heliosphere structure can be deduced from correlations between long-scale solar wind pressure evolution and energetic neutral atom emissions. However, this required spatial and temporal averaging that smoothed out small or dynamic features of the heliosphere. In late 2014, the solar wind dynamic pressure increased by roughly 50% over a period of 6 months, causing a time and directional-dependent rise in around 2–6 keV energetic neutral atom fluxes from the heliosphere observed by the Interstellar Boundary Explorer. Here, we use the 2014 pressure enhancement to provide a simultaneous derivation of the three-dimensional heliospheric termination shock (HTS) and heliopause (HP) distances at high resolution from Interstellar Boundary Explorer measurements. The analysis reveals rippled HTS and HP surfaces that are oblique with respect to the local interstellar medium upwind direction, with significant asymmetries in the heliosphere structure compared to steady-state heliosphere models. We estimate that the heliosphere boundaries contain roughly ten astronomical unit-sized spatial variations, with slightly larger variations on the HTS surface than the HP and a large-scale, southwards-directed obliquity of the surfaces in the meridional plane. Comparisons of the derived HTS and HP distances with Voyager observations indicate substantial differences in the heliosphere boundaries in the northern versus southern hemispheres and their motion over time.

Explanation of Heliospheric Energetic Neutral Atom Fluxes Observed by the Interstellar Boundary Explorer

Interstellar neutral atoms propagating into the heliosphere experience charge exchange with the supersonic solar wind (SW) plasma, generating ions that are picked up by the SW. These pickup ions (PUIs) constitute ∼25% of the proton number density by the time they reach the heliospheric termination shock (HTS). Preferential acceleration of PUIs at the HTS leads to a suprathermal, kappa-like PUI distribution in the heliosheath, which may be further heated in the heliosheath by traveling shocks or pressure waves. In this study, we utilize a dynamic, 3D magnetohydrodynamic model of the heliosphere to show that dynamic heating of PUIs at the HTS and in the inner heliosheath (IHS), as well as a background source of energetic neutral atoms (ENAs) from outside the heliopause, can explain the heliospheric ENA signal observed by the Interstellar Boundary Explorer (IBEX) in the Voyager 2 direction. We show that the PUI heating process at the HTS is characterized by a polytropic index larger than 5/3, likely ranging between γ ∼ 2.3 and 2.7, depending on the time in solar cycle 24 and SW conditions. The ENA fluxes at energies >1.5 keV show large-scale behavior in time with the solar cycle and SW dynamic pressure, whereas ENAs < 1.5 keV primarily exhibit random-like fluctuations associated with SW transients affecting the IHS. We find that ≲20% of the ENAs observed at ∼0.5–6 keV come from other sources, likely from outside the heliopause as secondary ENAs. This study offers the first model replication of the intensity and evolution of IBEX-Hi ENA observations from the outer heliosphere.

On the Energization of Pickup Ions Downstream of the Heliospheric Termination Shock by Comparing 0.52–55 keV Observed Energetic Neutral Atom Spectra to Ones Inferred from Proton Hybrid Simulations

We present an unprecedented comparison of ∼0.52–55 keV energetic neutral atom (ENA) heliosheath measurements, remotely sensed by the Interstellar Boundary Explorer (IBEX) mission and the Ion and Neutral Camera (INCA) on the Cassini mission, with modeled ENAs inferred from interstellar pickup protons that have been accelerated at the termination shock, using hybrid simulations, to assess the pickup ion energetics within the heliosheath. This is the first study to use hybrid simulations that are able to accurately model the acceleration of ions to tens of keV energies, which is essential in order to model ENA fluxes in the heliosheath, covering the full energy range observed by IBEX and CASSINI/INCA. The observed ENA intensities are an average value over the time period from 2009 to the end of 2012, along the Voyager 2 (V2) trajectory. The hybrid simulations upstream of the termination shock, where V2 crossed, are constrained by observations. We report an energy-dependent discrepancy between observed and simulated ENA fluxes, with the observed ENA fluxes being persistently higher than the simulated ones. Our analysis reveals that the termination shock may not accelerate pickup ions to sufficient energies to account for the observed ENA fluxes. We, thus, suggest that the further acceleration of these pickup ions is most likely occurring within the heliosheath, via additional physical processes like turbulence or magnetic reconnection. However, the redistribution of energy inside the heliosheath remains an open question.

Density Turbulence and the Angular Broadening of Outer Heliospheric Radio Sources at High Latitudes and in the Ecliptic Plane

Abstract Density irregularities are responsible for the scattering of radio waves in the solar wind and astrophysical plasmas. These irregularities significantly affect the inferred physical properties of radio sources, such as size, direction, and intensity. We present here a theory of angular broadening due to the scattering of radio waves by density irregularities that improves the existing formalism used to investigate radio wave scattering in the outer heliosphere and the very local interstellar medium. The model includes an inner scale and both latitudinal and radial dependencies for the density fluctuation spectra and propagation paths for the radiation both near and out of the ecliptic plane. Based on the pickup-ion-mediated solar wind model (PUI model) of Zank et al., we estimate the turbulence and solar wind quantities for the high-latitude fast solar wind. The predictions include the density variance, inner/dissipation scale, velocity correlation length, mean magnetic field, and proton temperature. The density turbulence amplitude is estimated in two ways. First, a simple scaling technique is used to extend the theoretical predictions of the PUI model for the high-latitude wind beyond the heliospheric termination shock. Second, the solar wind and turbulence quantities are calculated near the ecliptic plane using plasma and magnetometer data from the Voyager 2 spacecraft over the period 1977–2018. Based on the turbulence models and observations, we calculate the scattering angle of the radio sources in the high-latitude and near-ecliptic wind. Finally, we compare the numerical results with the analytic predictions from Cairns and Armstrong et al. and the observed source sizes.

Energetic pickup proton population downstream of the termination shock as revealed by IBEX-Hi data

ABSTRACT Pickup protons originate as a result of the ionization of hydrogen atoms in the supersonic solar wind, forming the suprathermal component of protons in the heliosphere. While they are being picked by the heliospheric magnetic field and convected into the heliosheath, the pickup protons may suffer stochastic acceleration from the solar wind turbulence in the region from the Sun up to the heliospheric termination shock, where they can also experience shock-drift acceleration or reflection from the cross-shock potential. These processes create a high-energy tail in the pickup ion energy distribution. The properties of this energetic pickup proton population are still not well defined, in spite of the fact that they are vital for models that simulate energetic neutral atom fluxes. We consider two scenarios for the pickup proton velocity distribution downstream of the heliospheric termination shock (a filled shell with an energetic power-law tail, and bi-Maxwellian). Based on a numerical kinetic model and observations of the energetic neutral atom fluxes from the inner heliosheath by the IBEX-Hi instrument, we characterize the pickup proton distribution and provide estimations of the properties of the energetic pickup proton population downstream of the termination shock.

Interaction between Multiple Current Sheets and a Shock Wave: 2D Hybrid Kinetic Simulations

Abstract Particle acceleration behind a shock wave due to interactions between magnetic islands in the heliosphere has attracted attention in recent years. The downstream acceleration may yield a continuous increase of particle flux downstream of the shock wave. Although it is not obvious how the downstream magnetic islands are produced, it has been suggested that current sheets are involved in the generation of magnetic islands due to their interaction with a shock wave. We perform 2D hybrid kinetic simulations to investigate the interaction between multiple current sheets and a shock wave. In the simulation, current sheets are compressed by the shock wave and a tearing instability develops at the compressed current sheets downstream of the shock. As the result of this instability, the electromagnetic fields become turbulent and magnetic islands form well downstream of the shock wave. We find a “post-cursor” region in which the downstream flow speed normal to the shock wave in the downstream rest frame is decelerated to ∼ 1V A immediately behind the shock wave, where V A is the upstream Alfvén speed. The flow speed then gradually decelerates to 0 accompanied by the development of the tearing instability. We also observe an efficient production of energetic particles above 100 E 0 during the development of the instability some distance downstream of the shock wave, where E 0 = m p V A 2 and m p is the proton mass. This feature corresponds to Voyager observations showing that the anomalous cosmic-ray intensity increase begins some distance downstream of the heliospheric termination shock.

Turbulent Acceleration of Interstellar Pickup Ions at the Heliospheric Termination Shock Forms the Global ENA Spectrum

Abstract The heliospheric energetic neutral atom spectrum observed by the Interstellar Boundary Explorer (IBEX) reveals that the heliosheath proton distribution is consistent with a power law. The origin of the spectrum is likely from interstellar pickup ions (PUIs) accelerated at the heliospheric termination shock (HTS). We present an explanation of the proton spectrum origin using a test particle simulation of PUIs accelerated at the HTS. PUIs experience preferential heating by the motional electric field in the shock foot, but do not develop a power-law tail without the presence of turbulence at wavenumbers (k) close to the PUI gyroradius scale (R g). Voyager 2 observations of the magnetic field downstream of the HTS indicate a moderate amount of turbulence at kR g ≅ 1, δ B / B 0 2 ≅ 0.01 , which we find to be sufficient for producing a downstream suprathermal PUI tail but not at intensities observed by IBEX. Within the shock ramp, however, Voyager observed the turbulence power at much smaller scales to be nearly 100 times stronger, suggesting the possibility of strong turbulence at the PUI gyroradius scale. We show that a proton distribution can develop a power law downstream of the HTS consistent with IBEX observations if δ B / B 0 2 ≳ 0.1 at kR g ≅ 1 in the shock foot. Shock drift acceleration of PUIs by the motional electric field is aided by interactions with turbulence upstream of the shock overshoot. Steepening of the IBEX proton spectrum in directions farther from the heliospheric nose suggests the HTS compression ratio and/or turbulence power weakens near the heliotail.

Boundary Conditions at the Heliospheric Termination Shock with Pickup Ions

Abstract In a collisionless shock the directed flow energy is converted mainly in thermal energy of the plasma species. At the termination shock (TS) a substantial portion of energy goes into heating of pickup ions (PUIs), while heating of the solar wind protons (SW) is weaker than it would be without PUI. Heating of both species is nonadiabatic. Downstream pressure of the mixture is determined by conservation laws for the whole mixture. SW heating is sensitive to the details of the shock front, while heating of PUI is not. The profile is an analytical approximation of the observed TS. Here the downstream temperature and pressure of PUIs are obtained for the first time using test particle analysis in a model shock profile for various magnetic compression ratios and shock angles. The profile is an analytical approximation of the observed TS. The results of the analysis are used in the pressure balance equation and the corresponding SW heating is estimated. The analysis is supported by full particle simulations, except for the SW heating that was not studied using test particle data due to its apparent dependence on fine structure of the shock front.

Strength of the Termination Shock Inferred from the Globally Distributed Energetic Neutral Atom Flux from IBEX

Abstract In this study, we estimate the heliospheric termination shock (HTS) compression ratio at multiple directions in the sky from a quantitative comparison of the observed and simulated inner heliosheath (IHS) energetic neutral atom (ENA) fluxes. We use a 3D steady-state simulation of the heliosphere to simulate the ENA fluxes by postprocessing the MHD plasma using a multi-Maxwellian distribution for protons in the IHS. The simulated ENA fluxes are compared with time exposure–averaged IBEX-Hi data for the first 3 yr of the mission. The quantitative comparison is performed by calculating the fractional difference in the spectral slope between the observed and simulated ENA fluxes for a range of compression ratios, where the simulated ENA spectrum is varied as a function of downstream pickup ion temperature as a function of compression ratio. The estimated compression ratio in a particular direction is determined by the minimum value of the fractional difference in spectral slope. Our study shows that the compression ratio estimated by this method is in close agreement with the large-scale compression ratio observed by Voyager 2 in its travel direction. Also, the compression ratio in other directions near the ecliptic plane is similar to the compression ratio at the Voyager 2 direction. The weakest shock compression is found to be on the port side of the heliosphere at direction (27°, 15°). This is the first study to estimate the HTS compression ratio at multiple directions in the sky from IBEX data.

An Ab Initio Approach to Antiproton Modulation in the Inner Heliosphere

Abstract Recent advances in the detection of cosmic-ray (CR) antiproton intensities at Earth have the potential to provide valuable new insights in the search for dark matter. As such, a fuller understanding of the modulation of these particles due to the influence of the Sun is of vital importance. Valuable insights can be gained through the study of galactic CR protons, as the transport parameters for these particles are theoretically expected to be the same as those for antiprotons, barring drift effects. As such, the present study develops a data-driven, 3D time-dependent ab initio model for the modulation of galactic CR protons in the region of the heliosphere dominated by the supersonic solar wind, which yields results in good agreement with spacecraft observations over several solar cycles when an observationally motivated expression for the differential intensity spectrum of these particles at the heliospheric termination shock is employed. This model is then applied to the study of solar-cycle-dependent antiproton modulation using two current estimates for the local interstellar differential intensities of these particles. This approach yields estimates of antiproton intensities at the heliospheric termination shock that are considerably lower than the proposed interstellar spectra, with the implication that a significant amount of antiproton modulation is expected to occur in the heliosheath.

Evidence of magnetic flux ropes downstream of the heliospheric termination shock

Voyager 1 and 2 observed enhanced energetic particle fluxes downstream of the heliospheric termination shock. In this paper, we provide observational evidence of reconnection processes downstream of the shock by applying a wavelet analysis technique to three magnetohydrodynamics (MHD) invariants from the magnetic fleld and plasma fluctuations in the post-HTS region measured by Voyager 2. Our results suggest the existence of possible magnetic islands/flux ropes structures within ∼ 1 AU behind the HTS. The location and scales of these structures are characterized by wavelet spectrograms of the normalized reduced magnetic helicity, normalized cross helicity, and normalized residual energy. Transport theory suggests that these structures may contribute to the acceleration of energetic particles through magnetic reconnection processes. We use a kinetic transport theory to model the energetic proton flux in the region downstream of the HTS. Our results suggest that stochastic acceleration due to magnetic reconnection can explain the ACR proton flux enhancement at a short distance beyond the HTS.

Energetic Neutral Atom Flux from the Inner Heliosheath and Its Connection to Termination Shock Properties

Abstract We present statistical comparisons between energetic neutral atom (ENA) fluxes obtained using a global simulation of the heliosphere and data collected by the Interstellar Boundary Explorer (IBEX) spacecraft. The simulation of the inner heliosheath (IHS) ENA flux is based on a 3D steady-state heliosphere, while the data are from the IBEX-Hi instrument over the time period 2009–2015. The statistical comparison is performed by calculating the chi-square value between the simulated ENA fluxes and the data for each line of sight in the sky. A comparison with exposure-averaged data for solar minimum and solar maximum conditions is also performed to see the effect of solar wind (SW) properties on the IHS ENA fluxes. The model matches well with the data in the flanks and parts of the nose of the heliosphere, whereas the match is poor in the downwind tail, ribbon, and polar regions. We interpret these results to mean that (i) heliosheath plasma in the polar region consists of advected fast (or slow) SW during the solar minimum (or maximum) condition, and (ii) heliospheric termination shock parameters are likely different over the poles. A poor match at around 30° north and south of the downwind direction is likely due to the existence of a mixture of plasma that comes from fast and slow SW. While our results are consistent with a single heliotail, the shape of the heliosphere continues to be an area of active research, and more data and further modeling are needed to determine its true structure.

ACR Proton Acceleration Associated with Reconnection Processes beyond the Heliospheric Termination Shock

Abstract One of the curious observations from the Voyagers is that the intensity of anomalous cosmic rays (ACRs) did not peak at the heliospheric termination shock (HTS) but instead a short distance (within ∼1 au) downstream of the HTS. One possible explanation is that the interaction of the wavy heliospheric current sheet with the HTS enhances magnetic reconnection and generates numerous small-scale magnetic flux ropes in the heliosheath immediately downstream of the HTS. Charged particles are accelerated in this region due to Fermi acceleration and the reconnection electric field. In this work, we provide observational evidence of the presence of magnetic flux ropes in the heliosheath region just downstream of the HTS using a wavelet analysis of the reduced magnetic helicity and Grad–Shafranov reconstruction techniques. The Zank et al. kinetic transport theory for particles propagating through the magnetic islands region is employed to fit the observed energetic proton intensities in the post-HTS region. Our modeling results agree reasonably well with the observations, which suggests that stochastic acceleration via reconnection processes can explain the ACR proton peak beyond the HTS.

A possible explanation for the enhancement of energetic particles downstream of the heliospheric termination shock

Voyager 2 observations of the energetic particle “time-intensity” profiles from ~1.8 to ~40 MeV show that the flux peaks downstream of the heliospheric termination shock (HTS), which is inconsistent with the predictions of classical diffusive shock acceleration (DSA). Previous studies suggest that shocks are effective in generating downstream magnetic flux ropes, islands or plasmoids. These dynamically interacting small-scale structures can accelerate charged particles statistically through reconnection-related processes. We present a preliminary study of the magnetic field and plasma properties together with the energetic particle data during the V2 crossing of the HTS. We apply a local stochastic acceleration model associated with solar wind magnetic island dynamics to explain the unusual behavior of energetic particles observed in the vicinity of the HTS. An analytic solution for the particle velocity distribution function derived from the Zank et al. statistical transport theory is used to fit the observed particle flux amplification downstream of the HTS. The results show that stochastic acceleration by interacting magnetic islands can successfully predict the observed (i) peaking of particle intensities behind the HTS, instead of at the shock front; (ii) increasing of the particle flux amplification factor with increasing particle energy; and (iii) increase in distance between the particle intensity peak and the HTS with increasing particle energy; This study illustrates the possibility of local acceleration in the inner heliosheath due to the dynamical interaction of magnetic islands.

Preface

G P ZankEditor, Proceedings of the 18th Annual International Astrophysics Conference,Center for Space Plasma and Aeronomic Research (CSPAR) and Department of SpacePhysics, University of Alabama in Huntsville, Huntsville AL 35805, USAE-mail: garyp.zank@gmail.comThe 18th Annual International Astrophysics Conference was held at the Sheraton Pasadena, Pasadena, California, USA from February 18 to February 22, 2019. The theme of the meeting was "The Physics of Energetic Particles: Universal Processes from the Solar Corona to the Very Local Interstellar Medium and the Physics they Enable.” Energetic particles are ubiquitous throughout the solar wind, being integral to the solar corona, accelerated by interplanetary shock waves, present in the quiet solar wind, possibly accelerated by magnetic flux ropes or magnetic islands in the vicinity of the heliospheric current sheet, at planetary bow shocks, present in the distant heliosphere and the heliospheric termination shock, in the inner heliosheath, and now observed by Voyager 1 in the very local interstellar medium. Although numerous mechanisms have been identified that accelerate particles, no consensus exists as to which is primary, and indeed several acceleration processes can operate simultaneously in close proximity. The acceleration of particles to high energies represents a fundamental dissipative process for the plasma and can therefore modify the underlying plasma physical processes in important ways. The meeting explored observations, theories, and looked forward to anticipated new missions that will shine a light on this theme, the Parker Solar Probe and IMAP. The 18th AIAC explored these topics via a combination of invited 25 minute talks and a few 40 minute plenary talks.The 18th AIAC also celebrated Prof. J.R. (Randy) Jokipii’s 80th birthday. Randy has been a centralfigure in the physics of energetic particles and transport, including particle acceleration, particularly at shock waves, and the modulation of cosmic rays in the heliosphere. His seminal early paper, “Cosmicray propagation. I. Charged particles in a random magnetic field” defined the field for decades after its publication in 1966. This meeting was an opportunity to celebrate Randy’s accomplishments with some selected plenary presentations.Finally, there are several people to thank for their kindness and considerable help with the 18th AIAC. Adele Corona suffered a horrific car accident on the second day of the meeting. She has thankfully largely recovered. However, we are profoundly grateful to Prof.’s Dave McComas and Nathan Schwadron for selflessly stepping forward to taking over the running and management of the meeting after Adele was rushed to hospital. Their kindness and organizational skills ensured that the meeting continued with virtually no interruption. I should like to thank them both and of course Adele Corona for another outstanding meeting, and her help in providing the logistical support for this volume of papers.