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The Ganymede laser-altimeter (GALA) is one of 10 instruments on ESA’s Jupiter Icy Moons Explorer (JUICE) mission. The scientific goals cover a wide range from geology, geophysics to geodesy of the icy moons Ganymede, Europa and Callisto. JUICE will explore Jupiter, its magnetosphere and satellites first in orbit around Jupiter before going finally into polar orbit around Ganymede. GALA is developed under responsibility of the DLR Institute of Planetary Research in collaboration with industry and institutes from Germany, Japan, Switzerland and Spain. GALA has two main objectives: (1) providing Ganymede’s topography from global to local scales (2) determination of Ganymede's tidal variations of surface elevations. GALA is a single-beam laseraltimeter: a laser pulse (1064 nm) is emitted by using a Nd:YAG laser firing at 30 Hz (nominal). After about 3 msec (500 km altitude) the reflection of the pulse from the surface of Ganymede is received by a telescope and transferred to the detector (Avalanche Photo Diode). The signal is digitized and transferred to the range finder module, which determines (a) time of flight (b) pulse shape, and (c) energy of the received pulse. Including information on the spacecraft position and attitude the height of the terrain above a reference surface is determined for each shot from time-of-flight measurements. The GALA flight model was delivered to ESA in August 2021. After several tests on instrument level the integration on the JUICE spacecraft started in September 2021 and first tests were performed successfully in October 2021. With the launch scheduled for 2023, GALA will go through several tests, among them an end-to-end test including laser-receiver measurements. Here we present the instrument's current status with respect hardware integration and regarding the verification of its performance.

Full lists of authors: Thomas, N.; Hussmann, H.; Spohn, T.; Lara, L. M.; Christensen, U.; Affolter, M.; Bandy, T.; Beck, T.; Chakraborty, S.; Geissbuehler, U.; Gerber, M.; Ghose, K.; Gouman, J.; HosseiniArani, S.; Kuske, K.; Peteut, A.; Piazza, D.; Rieder, M.; Servonet, A.; Althaus, C. Behnke, T.; Gwinner, K.; Hüttig, C.; Kallenbach, R.; Lichopoj, A.; Lingenauber, K.; Lötzke, H. -G.; Lüdicke, F.; Michaelis, H.; Oberst, J.; Schrödter, R.; Stark, A.; Steinbrügge, G.; del Togno, S.; Wickhusen, K.; Castro, J. M.; Herranz, M.; Rodrigo, J.; Perplies, H.; Weigel, T.; Schulze-Walewski, S.; Blum, S.; Casciello, A.; Rugi-Grond, E.; Coppoolse, W.; Rech, M.; Weidlich, K.; Leikert, T.; Henkelmann, R.; Trefzger, B.; Metz, B.--This is an Open Access article distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/.Open Access funding provided by Universitat Bern., The BepiColombo Laser Altimeter (BELA) is the first European laser altimeter constructed for interplanetary flight. BELA uses a 50 mJ pulsed Nd:YAG laser operating at 10 Hz with a 20 cm aperture receiver to perform the ranging. The instrument also uses a digital approach for range detection and pulse analysis. The ranging accuracy is expected to be better than 2 metres and ∼20 cm in optimum conditions. With the given, only slightly elliptical, orbit, BELA should return a consistent data set for the most if not all of the planet. The instrument is required to function in an extreme environment with the thermal issues being particularly demanding. Novel solutions have been taken to resolve these issues. BELA is described in detail and its predicted performance outlined on the basis of pre-flight testing. © The Author(s) 2021, This work was supported by the Swiss State Secretariat for Education, Research and Innovation (SERI) under the PRODEX scheme (contract number C90191). The opinions expressed and arguments employed herein do not necessarily reflect the official views of the Swiss Government. DLR would like to thank all involved collaborators in the BELA transmitter team. Financial support was provided by the DLR Space Administration on behalf of the Federal Ministry of Economics and Technology of Germany. We acknowledge the financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award SEV-2017-0709, and from the research projects ESP 2014-54062-R, ESP 2016-76076-R and PGC 2018-099425-B-I00. Open Access funding provided by Universitat Bern.

Exploring the icy moons of Jupiter: The Ganymede Laser Altimeter (GALA) for ESA’s JUICE Mission Fabian Lüdicke (1), Hauke Hussmann (1), Kay Lingenauber (1), Reinald Kallenbach (1), Keigo Enya (2), Masanori Kobayashi (3), Nicolas Thomas (4), Luisa Lara (5), Kazuyuki Touhara (2) and the GALA team. (1) DLR Institute of Planetary Research, Berlin, Germany (fabian.luedicke@dlr.de), (2) ISAS/JAXA, (3) Chiba Institute of Technology, Japan, (4) Physikalisches Institut, University of Bern (UBE), (5) CSIC, Instituto de Astrofísica de Andalucía (IAA), Granada, Spain Abstract The Ganymede Laser Altimeter (GALA) is one of ten instruments selected for ESA’s Jupiter Icy Moons Explorer (JUICE) mission. We will present an overview on the scientific goals as well as on the status of the instrument development and performance analysis. Introduction ESA’s JUICE mission will explore Jupiter, its magnetosphere and satellites first in orbit around Jupiter before going finally into polar orbit around Ganymede [1]. GALA is one of ten payloads on-board the spacecraft and is developed under responsibility of the DLR Institute of Planetary Research in collaboration with industry and institutes from Germany, Japan, Switzerland and Spain. Its major objectives are to measure the surface topography and the tidal deformation of the satellite. The JUICE Mission JUICE (Jupiter Icy Moons Explorer) will be the first orbiter around a moon (other than Earth’s moon) in solar system exploration. Its launch is planned for May/June 2022 followed by an interplanetary cruise of 7.6 years. Jupiter orbit insertion will take place by the end of 2029. An orbit maneuver will bring the spacecraft into a 500-km circular orbit in which it will be staying for at least 132 days until end of nominal mission. The latter phase will be the main period for GALA taking data. In addition data will be taken at Europa, Ganymede, and Callisto at closest approaches of flybys in the Jupiter orbiting phase. GALA Science GALA has two main objectives: (1) by range measurements it shall obtain Ganymede’s topography on global, regional and local scales. This will reveal how surface features have formed and how they are connected with the shallow interior ice shell. Global shape measurements will tell us whether the satellite is in hydrostatic state with respect to rotational and tidal forces. (2) Obtaining range measurements distributed in time along the orbital cycle, tidal variations of surface elevations will be measured. The tidal amplitudes are indicative for the presence of a subsurface ocean and would (together with complementary measurements) constrain the ice-I shell thickness [2]. GALA GALA is a single-beam altimeter: a laser pulse (at 1064 nm wavelength) is emitted by using an actively Q-switched Nd:YAG laser firing at 30 Hz in nominal operation. A small fraction of the pulse is guided through fiber optics onto the detector characterizing the outgoing pulse and time of emission. After about 3 msec the Lambertian reflection of the pulse from the surface is received by a aperture telescope and transferred to the detector, an Avalanche Photo Diode. The signal is digitized at a sampling rate of 200 MHz and transferred to the range finder module, which determines (a) the time of flight between the emission and receiving of the pulse (b) the pulse shape, in particular the pulse-width, and (c) the energy of the received pulse. From the time of flight of the wave-package and the spacecraft position and attitude, the distance for each shot can be converted into height above a reference surface in post-processing of the data. (1) (2) (3) Figure 1: (1) GALA Tranceiver, (2) Electronics Unit (ELU), (3) Laser Electronics Unit (LEU) In 2019 the GALA EM (Engineering Model) was delivered to Airbus Defence and Space Toulouse and was successfully integrated into the JUICE EM and tested. The EM is used to test the functionality of GALA; it can be operated only under ambient conditions. Later it will be used as a ground reference model for checks for the Flight Model. For the GALA delivery in 2021 the different GALA flight model subsystems are currently tested and integrated at DLR, and at industries in Japan (Meisei) and Germany (Hensoldt Optronics). Flight hardware of the Power converter Module (PCM) and the Range Finder Module have been received from our partner institutes in Spain (IAA, CSIC) and Switzerland (UBE). The main units of GALA are the TRU, the LEU and the ELU, see Figure 1. The TRU consists of the laser/detector system, including an Avalanche Photo Diode. ELU includes the main CPU, Data Processing Module, Range Finder Module for the analysis of the laser pulses and the Power Converter Module which provides the power. A detailed description of GALA can be found in [3]. All parts were tested in terms of functionality and performance, here in particular the detector. Besides electrical testing a big part is the testing of the on-board software, the Application Software. Mainly it manages the commanding of the above mentioned parts, the memory management, FDIR (Fault Detection Isolation Recovery) and the interface (I/F) to the spacecraft (S/C). For I/F testing to the S/C a SIS (Spacecraft Interface Simulator) is used to check the GALA commanding and the data output. After testing at DLR the above mentioned GALA parts will be assembled to the GALA PFM at Hensoldt Optronics Oberkochen. Here additional tests will take place like thermal vacuum tests, EMC and further functional testing, especially for the laser system, but also for the complete system before it will be delivered for final integration into the JUICE spacecraft in 2021. References [1] Grasset, O. and 17 colleagues (2013), JUpiter ICy moons Explorer (JUICE): an ESA mission to orbit Ganymede and to characterise the Jupiter system, Planet. Space Sci., 78, 1-21. [2] Steinbrügge, G., A. Stark, H. Hussmann, F. Sohl, and J. Oberst (2015), Measuring tidal deformations by laser altimetry. A performance model for the Ganymede Laser Altimeter, Planet. Space Sci., 117, 184-191. [3] Hussmann, H. and 38 colleagues 2019. The Ganymede Laser Altimeter (GALA): key objectives, instrument design, and performance. CEAS Space Journal (Special Issue on Space Lidar and Space Optics) 11, 381 – 390.

The Moon is periodically deformed by the tidal forces exerted on it by the Earth and the Sun. The tidal Love number h2 describes the magnitude of the radial component of these deformations at the monthly frequency, which have an amplitude of up to ∼10 cm. Like the potential Love number k2, h2 depends on the density and rheological properties of the materials in the lunar interior and their distribution. We analyze > 3.6 · 109 measurements of the Lunar Orbiter Laser Altimeter (LOLA) obtained during the 27-month circular orbit phase of the Lunar Reconnaissance Orbiter (LRO) at 50 km altitude, when LOLA reached global coverage. We simultaneously invert these observations for the Love number h2 and a global topographic model. The topography is parametrized as an expansion in 2D cubic B-spline basis functions, which are defined on a global equirectangular grid. This parametrization is more computationally efficient than an expansion in spherical harmonics, but still allows for a high smoothness. To deal with data gaps, we constrain the solution by minimizing the second derivative of the topography. We find that the h2 solution depends on the choice of resolution of the equirectangular grid. We determine the accuracy for each investigated resolution (from 6 km to 1 km at the equator) from a Monte Carlo simulation using 100 synthetically generated sets of observations. The topographic signal in the synthetic data follows a power law extrapolated from the real lunar topography. At large scales, the topography is generated using a spherical harmonic expansion, at smaller scales it is generated using Gaussian process regression. Finally, we use the inverse of the root-mean-square h2 obtained from the Monte Carlo simulation as weights for determining a weighted mean of the h2 results for different grid resolutions. The final result of h2 = 0.0386 ± 0.0022 agrees within one standard deviation with a previous result obtained from the same data, but utilizing crossover points of LOLA profiles. This validates the method of simultaneous inversion for tides and topography, especially with regard to future laser altimeter experiments at other planetary bodies, such as Mercury and Ganymede. However, our result also confirms a discrepancy between laser altimeter measurements of h2 and the k2 result of the Gravity Recovery and Interior Laboratory (GRAIL) mission, which needs to be resolved through better modelling of the lunar tidal response.

Context.The Love numberh2describes the radial tidal displacements of Mercury’s surface and allows constraints to be set on the inner core size when combined with the potential Love numberk2. Knowledge of Mercury’s inner core size is fundamental to gaining insights into the planet’s thermal evolution and dynamo working principle. The BepiColombo Laser Altimeter (BELA) is currently cruising to Mercury as part of the BepiColombo mission and once it is in orbit around Mercury, it will acquire precise measurements of the planet’s surface topography, potentially including variability that is due to tidal deformation.Aims.We use synthetic measurements acquired using BELA to assess how accurately Mercury’s tidal Love numberh2can be determined by laser altimetry.Methods.We generated realistic, synthetic BELA measurements, including instrument performance, orbit determination, as well as uncertainties in spacecraft attitude and Mercury’s libration. We then retrieved Mercury’sh2and global topography from the synthetic data through a joint inversion.Results.Our results suggest thath2can be determined with an absolute accuracy of ± 0.012, enabling a determination of Mercury’s inner core size to ± 150 km given the inner core is sufficiently large (>800 km). We also show that the uncertainty ofh2depends strongly on the assumed scaling behavior of the topography at small scales and on the periodic misalignment of the instrument.

The Ganymede Laser Altimeter (GALA) is one of the ten scientific instruments selected for the Jupiter Icy Moons Explorer (JUICE) mission currently implemented under responsibility of the European Space Agency (ESA). JUICE is scheduled for launch in mid 2022; arrival at Jupiter will be by end of 2029 with the nominal science mission—including close flybys at Ganymede, Europa, and Callisto and a Ganymede orbit phase—ending by mid 2033. GALA’s main objective is to obtain topographic data of the icy satellites of Jupiter: Europa, Ganymede, and Callisto. By measuring the diurnal tidal deformation of Ganymede, which crucially depends on the decoupling of the surface ice layer from the deep interior by a liquid water ocean, GALA will obtain evidence for (or against) a subsurface ocean in a 500 km orbit around the satellite and will provide constraints on Ganymede’s ice shell thickness. In combination with other instruments, it will characterize the morphology of surface units on Ganymede, Europa, and Callisto providing not only topography but also surface roughness and albedo (at 1064 nm) measurements. GALA is a single-beam laser altimeter operating with up to 50 Hz (nominal 30 Hz) shot frequency at a wavelength of 1064 nm and pulse lengths of 5.5 ± 2.5 ns using a Nd:YAG laser. The return pulse is detected by an Avalanche Photo Diode (APD) with 100 MHz bandwidth and is digitized at a sampling rate of 200 MHz providing range measurements with a subsample resolution of 0.1 m and surface roughness measurements from pulse-shape analysis on the scale of the footprint size of about 50 m at 500 km altitude. The instrument is developed in collaboration of institutes and industry from Germany, Japan, Switzerland, and Spain. © 2019, CEAS., Financial support was provided under grant 50 QJ 1401 on behalf of the DLR Space Administration by the German Bundesministerium fur Wirtschaft und Energie. This research has been supported by the Spanish Ministerio de Economia y Competitividad under Contract ESP 2016-76076-R.

Based on lessons learnt with the BepiColombo Laser Altimeter (BELA) [1], electronics design improvements have been implemented for the Ganymede Laser Altimeter (GALA), to be launched on ESA's JUICE spacecraft in 2022 [2]. Improvements include multi-point grounding and separation of “aggressor” printed circuit boards (PCBs) for the high-voltage Q-switch electronics and the pulsed high-power current source of the diode-laser pumped Nd:YAG laser from “victim” PCBs for the sensitive Si avalanche photodiode detector (APD) operating at a noise floor of about 300 fA/sqrt(Hz) in a bandwidth of 100 MHz. Most importantly, the detector amplifier has increased phase margin to avoid feedback of undamped filter resonances, and its output is digitized as differential signal very close to the APD.

The Sun is the largest reservoir of matter in the solar system, which formed 4.6 Gyr ago from the protosolar nebula. Data from space missions and theoretical models indicate that the solar wind carries a nearly unfractionated sample of heavy isotopes at energies of about 1 keV/amu from the Sun into interplanetary space. In anticipation of results from the Genesis mission’s solar-wind implanted samples, we revisit solar wind isotopic abundance data from the high-resolution CELIAS/MTOF spectrometer on board SOHO. In particular, we evaluate the isotopic abundance ratios 15N/14N, 17O/16O, and 18O/16O in the solar wind, which are reference values for isotopic fractionation processes during the formation of terrestrial planets as well as for the Galactic chemical evolution. We also give isotopic abundance ratios for He, Ne, Ar, Mg, Si, Ca, and Fe measured in situ in the solar wind.

Laser altimeters on board the MESSENGER and BepiColombo missions will accurately map Mercury's topography including the time-dependent elevation from solar tides, which are predicted to be of order 1 m. The amplitude of solar tides, characterized by the Love number h2, reveals information on the interior structure of Mercury, in particular on the state and the size of the core. Koch et al., [Koch et al., (2008). Planet. Space Sci. 56, 1226–1237] have directly decomposed a global synthetic altimetry record into the time-dependent tidal signal and into a spherical harmonic expansion of the static topography. The precision for h2 depends on the maximum degree l of the expansion which is practically limited because the computation time increases very strongly with l. Here, we show that the computation time to achieve the same precision can be substantially reduced if we describe the static topography in terms of local basis functions defined on a rectangular grid. The subsequent projection of the grid data onto the orthonormal spherical harmonic functions is very rapid. The new method is promising to achieve high precision to test current models of Mercury's interior and additionally delivers a topographic map with approximately 4.7 million grid points.

Interstellar neutral gas enters the inner heliosphere where it is ionized and becomes the pickup ion population of the solar wind. It is often assumed that this population will subsequently cool adiabatically, like an expanding ideal gas due, to the divergent flow of the solar wind. Here, we report the first independent measure of the effective adiabatic cooling index in the inner heliosphere from SOHO CELIAS measurements of singly charged helium taken during times of perpendicular interplanetary magnetic field. We use a simple adiabatic transport model of interstellar pickup helium ions, valid for the upwind region of the inner heliosphere. The time averaged velocity spectrum of helium pickup ions measured by CELIAS/CTOF is fit to this model with a single free parameter which indicates an effective cooling rate with a power-law index of gamma = 1.35 +- 0.2. While this average is consistent with the 'ideal-gas' assumption of gamma = 1.5, the analysis indicates that such an assumption will not apply in general, and that due to observational constraints further measurements are necessary to constrain the cooling process. Implications are discussed for understanding the transport processes in the inner heliosphere and improving this measurement technique.

Solar tidal forces generate elevation changes of Mercury's surface of the order 1 m within one Hermean year, and solar torques on the non-symmetric permanent mass distribution of the planet cause an uneven rotation of Mercury's surface with a libration amplitude of the order of 40 arcsec. Knowledge of the precise reaction of the planet to tidal forcing, expressed by the Love numbers h 2 and k 2 , as well as accurate knowledge of the amplitude of forced libration Φ lib , puts constraints on the internal structure, for example the state and the size of the core. The MESSENGER and BepiColombo missions to Mercury carry laser altimeters, whose primary goal is to accurately map the topography. Here we investigate if the Love number h 2 and the amplitude of forced libration can be determined together with the static topography of the planet from a global altimetry record. We do this by creating synthetic altimeter data for the nominal orbit of BepiColombo over the nominal mission duration of approximately four Mercury years and inverting them for the static and time-dependent parts of the topography. We assume purely Gaussian noise. We find that it is possible to extract both parameters h 2 and Φ lib with an accuracy of approximately 10%, while the static topography coefficients of a spherical harmonic expansion can be determined simultaneously with an accuracy at the centimetre level. Extraction of the static topography to higher harmonic degrees improves the precision of the measurement of h 2 and Φ lib . The simulation results demonstrate that it seems feasible to test current models on Mercury's interior with sufficient precision using BepiColombo Laser Altimeter data.

Enhancements of suprathermal particles observed at 1AU often can be related to Corotating Interaction Regions (CIRs). The compression regions associated with CIRs and their driven shocks which typically form at a few AU distance to the Sun can efficiently accelerate particles. If accelerated at the trailing edge of a CIR these particles can travel sunward along the ambient magnetic field and thus enhanced fluxes can be observed even if the acceleration region has passed over the spacecraft. We have analysed a CIR that has been observed at L1 by ACE/SWICS and SOHO/CELIAS/STOF on days 207 and 208 in 2003. The combination of SWICS and STOF data allowed us to study suprathermal Helium ranging from its onset at solar wind bulk energies up to 330 keV/nuc. Here we present our results for the temporal evolution of the flux, energy spectra and the He+/He++ ratio. In particular we present observational evidence for a turnover of the energy spectra at lower energies after the CIR passage which has been theoretically predicted but never been observed so far.

This paper reports on EMC issues of the laser transmitter, the photodiode receiver, and the communication port of the BepiColombo Laser Altimeter BELA during integration on ESA's Mercury Planetary Orbiter MPO. Parasitic currents originating in the active Q-switch electronics of BELA's diode-laser pumped Nd: YAG laser and from the high-power laser diode driver electronics interfered with the sensitive Si avalanche photodiode sensor operating at a noise floor of about 0.3 pA/root Hz in a bandwidth of 20 MHz, and with BELA's Spacewire port, corrupting about 3% of telemetry data to the MPO memory. The problems had been eliminated by implementing multi-point grounding.

We use recent Voyager 1 measurements of the ion fluxes beyond the termination shock and the SOHO HSTOF/CELIAS measured fluxes of the energetic neutral hydrogen and helium at 1 AU to estimate the neutral hydrogen column density NH in the heliosheath. Our best estimate is NH = 1.13 × 1014 cm-2. The result corresponds to an average thickness of the heliosheath, in the forward part of the heliosphere (within 155° from the nose direction and 17° from the ecliptic), of L ~ 75 AU for an average hydrogen density of nH = 0.1 cm-3 in the heliosheath.

Context. Energetic particle enhancements that are associated with corotating interaction regions (CIRs) are typically believed to arise from the sunward propagation of particles that are accelerated by CIR-driven shocks beyond 1 AU. It is expected that these sunward-travelling particles will lose energy and scatter, resulting in a turnover of the energy spectra below ~0.5 MeV/nuc. However, the turnover has not been observed so far, suggesting that the CIR-associated low-energy suprathermal ions are accelerated locally close to the observer. Aims. We investigate the variability of suprathermal particle spectra from CIR to CIR as well as their evolution and variation as the observer moves away from the rear shock or wave. Methods. Helium data in the suprathermal energy range from the Solar and Heliospheric Observatory/Charge, Element, and Isotope Analysis System/Suprathermal Time-of-Flight (SOHO/CELIAS/STOF) were used for the spectral analysis and were combined with data from the Advanced Composition Explorer/ Solar Wind Ion Composition Spectrometer (ACE/SWICS) in the solar wind energies. Results. We investigated sixteen events: nine clean CIR events, three CIR events with possible contamination from upstream ion events or solar energetic particles (SEPs), and four events that occurred during CIR periods that were dominated by SEPs. Six of the nine clean CIR events showed possible signs of a turnover between ~10−40 keV/nuc in the fast solar wind that trails the compression regions. Three of them even showed this behaviour inside the compressed fast wind. The turnover part of the spectra became flatter and shifted from lower to higher energies with increasing connection distance to the reverse shock. The remaining three clean events showed continuous power-law spectra in both the compressed fast wind and fast wind regions, that is, the same behaviour as reported from previous observations. The spectra of the seven remaining events are more variable, that is, they show power law, turnover, and a superposition of these two shapes.

Using observations of the Charge Time-of-Flight(CTOF) charge and mass spectrometer of the Charge, Element and Isotope Analysis System (CELIAS), and of CELIAS/proton monitor on board the Solar and Heliospheric Observatory (SOHO), we present an overview of speeds and kinetic temperatures of minor ions and protons in the solar wind near solar minimum, covering the Carrington Rotations 1908 to 1912. In the case of a collision-dominated solar wind the speed of minor ions is expected to be lower or equal to the speed of the protons, and all species are expected to have equal temperatures. On the other hand, minor ions can be accelerated and heated by wave-particle interaction. In this case, equal thermal speeds of all species are expected. CTOF data allow the determination of the kinetic parameters of various ions with high accuracy and with high time resolution. The mean O6+ speed of the observed period is 390 km s−1. The speeds of Si7+ and Fe9+ correlate well with O6+, the linear correlation coefficient being 0.96 or higher. Our results also indicate that silicon and iron tend to lag behind oxygen with a speed difference of ∼20 km s−1 at 500 km s−1. At the same time, the kinetic temperature of the ions under investigation exhibit the well-known mass proportionality, which is attributed to wave-particle interactions. During the period of low solar activity in consideration, many cases are observed where the kinetic temperature is extraordinarily low (104 K for O6+).

Solar wind abundance ratios of magnesium isotopes measured with the high resolution Mass Time-of-Flight spectrometer (MTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO) are presented. MTOF, as part of CELIAS, is, because of its high time and mass resolution, an excellent tool for isotope abundance measurements in the solar wind. From the data analysis we have found that the isotopic composition of magnesium in the solar wind agrees with the terrestrial composition within the experimental uncertainty. We have obtained isotopic ratios of 24Mg/25Mg = 7.7 ± 0.4 and 24Mg/26Mg = 7.0 ± 0.5. These values are consistent with the terrestrial values of 24Mg/25Mg = 7.90 ± 0.01 and 24Mg/26Mg = 7.17 ± 0.03. Furthermore, these investigations also show that with the given uncertainties the abundance ratios do not vary significantly within a solar wind velocity range from 375 km/s to 530 km/s.

The High-Energy Suprathermal Time-of-Flight sensor (HSTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) on the Solar and Heliospheric Observatory (SOHO) near the Lagrangian point L1 is capable of identifying energetic hydrogen atoms (EHAs) between 55 and 80 keV. Between 1996 February 13 and 1997 August 31, near solar minimum, there were 285 "quiet" days when the interplanetary charged-particle flux was low. During these quiet times, HSTOF scanned the apex of the heliosphere once and the antiapex twice. The flux level and time profile, and hence the arrival direction, of the EHAs accumulated during these quiet times are best interpreted as fluxes of EHAs coming from the heliosheath.

The proton monitor, a small subsensor in the Charge, Element, and Isotope Analysis System/Mass Time-of-Flight (CELIAS/MTOF) experiment on the SOHO spacecraft, was designed to assist in the interpretation of measurements from the high mass resolution main MTOF sensor. In this paper we demonstrate that the proton monitor data may be used to generate reasonably accurate values of the solar wind proton bulk speed, density, thermal speed, and north/south flow direction. Correlation coefficients based on comparison with the solar wind measurements from the SWE instrument on the Wind spacecraft range from 0.87 to 0.99. On the basis of the initial 12 months of observations, we find that the proton momentum flux is almost invariant with respect to the bulk speed, confirming a previously published result. We present observations of two interplanetary shock events, and of an unusual solar wind density depletion. This large density depletion, and the correspondingly large drop in the solar wind ram pressure, may have been the cause of a nearly simultaneous large increase in the flux of relativistic magnetospheric electrons observed at geosynchronous altitudes by the GOES 9 spacecraft. Extending our data set with a 10-year time span from the OMNIWeb data set, we find an average frequency of about one large density depletion per year. The origin of these events is unclear; of the 10 events identified, 3 appear to be corotating and at least 2 are probably CME related. The rapidly available, comprehensive data coverage from SOHO allows the production of near-real time solar wind parameters that are now accessible on the World Wide Web.

The CELIAS particle experiment on SOHO contains the Charge Time Of Flight (CTOF) mass spectrometer which measures the ionic and elemental composition of minor ions in the solar wind. In this paper we present iron freeze-in temperatures derived with a time resolution of 5 min. They indicate that some of the filamentary structures of the inner corona observed in Hα survive in the interplanetary medium as far as 1 AU.

We present first results taken from the high-resolution mass time-of-flight spectrometer (MTOF) of the charge, element, and isotope analysis system (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO) spacecraft launched in December 1995, concerning the abundance ratios of neon isotopes in the solar wind. We obtain the isotopic ratios 20Ne/22Ne = (13.8 ± 0.7) and 20Ne/21Ne = (440 ± 110), which agree with the values obtained from the Apollo foil solar wind experiments and which have been derived from measurements of solar particles implanted in lunar and meteoritic samples.

The CELIAS experiment on SOHO is designed to measure the mass, ionic charge and energy of the low and high speed solar wind, of suprathermal ions, and of low energy flare particles. Through analysis of the elemental and isotopic abundances, the ionic charge state, and the velocity distributions of ions originating in the solar atmosphere, the investigation focuses on the plasma processes on various temporal and spatial scales in the solar chromosphere, transition zone, and corona. CELIAS includes 3 mass- and charge-discriminating sensors based on the time-of-flight technique: CTOF for the elemental, charge and velocity distribution of the solar wind, MTOF for the elemental and isotopic composition of the solar wind, and STOF for the mass, charge and energy distribution of suprathermal ions. The instrument will provide detailed in situ diagnostics of the solar wind and of accelerated particles, which will complement the optical and spectroscopic investigations of the solar atmosphere on SOHO. CELIAS also contains a Solar Extreme Ultraviolet Monitor, SEM, which continously measures the EUV flux in a wide band of 17 – 70 nm, and a narrow band around the 30.4 nm He II line.

Supra-thermal neutral atoms such as H and He have been detected with the particle instrument CELIAS/HSTOF onboard SOHO since 1996. While the observed energetic neutral hydrogen (ENH) fluxes are well distinct from background ion fluxes, the helium (ENHe) fluxes have to be corrected for the local He ion and alpha particle fluxes in the same energy region. The latter ion background originates from well known sources such as solar particle events or co-rotating interaction regions and the ion particle flux and spectrum is a function of time. The resulting H/He ratio in the supra-thermal energy region is lower than expected from the anomalous cosmic ray abundances. Heavier neutral particles are not identified in the CELIAS/HSTOF dataset. We will discuss the neutral energetic flux measurements in view of the H/He ratio and potential source regions from the heliosheath region and beyond.

Charge exchange properties of H atoms passing through thin carbon foils at incident energies from 0.5 to 120 keV/u are discussed in the context of charge transfer models. A model is presented in which the charge state equilibrium in the solid is explained by the overlap of the atomic and the solid‐state electron wave functions in k space. Outside the solid, near the surface, charge exchange occurs by tunneling of electrons between the carbon surface and the exiting projectile.

The BepiColombo Laser Altimeter (BELA) is one of 11 instruments aboard ESA's Mercury Planetary Orbiter (MPO) scheduled for launch in 2014. BELA will record the surface profile of the planet while orbiting around it at a distance of 400km to 1500km 1 . The altimetry data constitute an important prerequisite for a number of remote sensing and observation techniques residing on the same orbiter. The BELA instrument comprises a laser transmitter and a receiver part, the design of the former is being presented and discussed in this paper. The laser transmitter encompasses a pair of diode-pumped, actively Q-switched Nd:YAG rod oscillators which have been miniaturized, light-weighted and dimensioned for high electrical to optical efficiency. The key performance parameters of the laser will be presented. Laser design trades which are relevant for a space mission to Mercury and the BELA instrument in particular are discussed. An overview is given to the laser qualification programme which includes performance and environmental tests. Test results are presented which have been recorded during the qualification test campaign currently in progress at Carl Zeiss Optronics.

Data from the Mass and Charge Time-of-Flight spectrometers MTOF and CTOF on board SOHO have been accumulated for time periods in which coronal hole and non-coronal hole type plasma has been detected by using different methods in order to determine the abundance ratios of magnesium isotopes in these two different source regions of the solar wind. As indicators for the two different solar wind type plasma we have used several plasma parameters such as, the freeze-in temperatures, the Fe/O and the He/H ratio, which are known as reasonable indicators. For this study we have used the refractory element Mg which is an authentic whitness for matter in the solar nebula because temperatures of the Sun never have been high enough to change their isotopic composition by nuclear burning. In addition Mg has three isotopes with abundance ratios greater than about 10% and therefore they are easy to observe.

A continuous wave ultraviolet source with extreme frequency stability is described. A ring dye laser with internal electro-optic phase modulator reaches sub-Hertz linewidths relative to a passive reference cavity with the help of an FM stabilization technique. A barium-β-borate frequency doubler crystal inside a second injection-locked ring dye laser generates output powers of 30 mW near 243 nm.

A single-frequency Ti:sapphire laser has been stabilized to a reference cavity with the simple side-of-fringe locking technique and a piezo-controlled mirror as compensating element. A frequency stability of 1 kHz rms with respect to the locking etalon is obtained. This Ti:sapphire laser, optimized for 972.5 nm, will be used for two-photon excitation of the 2S–4S transition in atomic hydrogen.

The observations of magnetic field fluctuations and suprathermal ion spectra upstream of interplanetary shocks driven by coronal mass ejections (CMEs) give information on two time scales: (1) the eddy turn‐over time scale i.e. the non‐linear cascading time scale of solar wind turbulence, and (2) the growth time scale for waves amplified by energetic protons accelerated at the shock. These two time scales scale differently with wave number. From an analysis of data from the ACE magnetometer and the suprathermal particle sensor SOHO/STOF we derive the solar‐wind eddy turn‐over time.

The observations on magnetic field fluctuations and suprathermal ion spectra near the shocks driven by the coronal mass ejections during the time periods of the Bastille Day 2000 event and the Halloween 2003 events are summarized.

Since the launch of the Solar and Heliospheric Observatory (SOHO) on Dec. 2, 1995, the Suprathermal Time‐Of‐Flight (STOF) energetic particle sensor of the Charge, Element and Isotope Analysis System (CELIAS) has observed gradual and impulsive solar particle events from solar activity cycle minimum to maximum. The instrument CELIAS/STOF has the capability of detecting energetic ions and determine the mass, energy and charge of each particle. We report on the measurements of the detected energetic particle events from 1996 to 2001, i.e. the velocity dispersion of energetic He ions in the energy/charge range between 30 and 800 keV/nuc. For energetic particle events we analyse the velocity dispersion of the suprathermal He and O ions (78 and 43 events, resp.) and estimate the onset time of the solar event and set limits on the propagation length along the connecting magnetic field line.

[1] The Solar and Heliospheric Observatory/Charge, Element, and Isotope Analysis System/(Highly) Suprathermal Time of Flight ((H)STOF) mass spectrometer measures the elemental composition and the charge state distribution of ions with suprathermal energies in the range 10–4000 keV amu−1 from above the solar wind particle distribution up to low-energy flare particle energies. We analyze the time period around the large coronal mass ejection event on 2–3 May 1998. Using data from (H)STOF, we study the charge state distribution of He, the CNO group, and Fe. In addition, we investigate the energy dependence of the mean ionic charge state of Fe in the energy-per-nucleon range 12–100 keV amu−1. Furthermore, we also report energy spectra of H+, He++, and He+ and the variations in the elemental abundance ratios He/H, He/CNO, and Fe/CNO. These observations greatly extend the energy range in which particles associated with this time period have been measured. Because of their elevated energies, suprathermal particles are the prime seed population for further acceleration in gradual events. We identify interstellar pickup ions as an important nonsolar seed population. The observed low mean ionic charge states of Fe and the small Fe/CNO ratio are typical for large gradual events. The energy dependence of the Fe charge states may be a result of the presence of different iron populations or of the thermal history of the accelerated material. The temporal variations of Fe/CNO and He/CNO indicate a stronger confinement of low-rigidity particles at the acceleration site.

The state of the very early Universe, when it was sufficiently small to presumably appeal to our laureate, Prof. Theodor W. Hansch, is yet unexplored. The forthcoming joint ESA-NASA mission LISA (Laser Interferometer Space Antenna) may probe this epoch. This mission will apply lasers with a frequency stability comparable to that developed in one of Theodor Hansch’s laboratories during the late 1980s. Relativistic length changes on the order of an atom’s size due to the oscillating curvature of space in primordial gravitational waves may be measured over a distance of five million kilometers in space.

This report emphasizes new observational aspects of CIR ions revealed by advanced instruments launched on the Ulysses, WIND, SOHO, and ACE spacecraft, and by the unique vantage point of Ulysses which carried out the first survey of Corotating Interaction Region (CIR) properties over a very wide range of heliolatitudes. With this more complete observational picture established, this review is the basis to consider the status of theoretical models on origin, injection, and acceleration of CIR particles reported by Scholer, Mann et al. (1999) in this volume.

We report on measurements of the He charge states as observed in recurring energetic particle events during the recent solar minimum in 1996. The time-of-flight mass spectrometer CELIAS/STOF on board the SOHO satellite has the capability of detecting energetic ions between 35 and 630 keV/q and determine the mass, energy and charge of each particle. Typically we observe a He+/He2+ ratio of 0.16 to 0.31 in the range of 25 to 150 keV/nuc. We discuss the implications of these observations on the existing models of particle acceleration in corotating interaction regions (CIRs), i.e. the location of the acceleration process of corotating particle events as observed near 1 AU.

The space qualified design of a miniaturized laser for pulsed operation at a wavelength of 1064 nm and at repetition rates up to 10 Hz is presented. This laser consists of a pair of diode laser pumped actively q switched Nd:YAG rod oscillators hermetically sealed and encapsulated in an environment of dry synthetic air. The system delivers at least 300 million laser pulses with 50 mJ energy and 5 ns pulse width (FWHM). It will be launched in 2017 aboard European Space Agency's Mercury Planetary Orbiter as part of the BepiColombo Laser Altimeter which after a 6 years cruise will start recording topographic data from orbital altitudes between 400 and 1500 km above Mercury's surface. (C) 2013 Optical Society of America

A small, low-power suite of fields and particles and imaging experiments is required for fulfilling the critical science objectives for a near-sun flyby mission. We discuss how an integrated instrument suite using novel sensors and advanced detector/microelectronics/packaging techniques can be implemented for such a payload. Critical tradeoffs between science requirements, measurement strategies and these resource limits are discussed, and critical enabling components are identified. The instrument site consists of 6 major investigations, some with multiple sensors, power conditioners for both high and low voltages and a common DPU. The concept design is essentially a dress-rehearsal of how a payload could realistically make the measurements needed to answer the critical science questions while operating within a real-world physics, engineering and technology context.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Planar quadrupole ion traps have been analyzed through numerical and analytic solutions of Laplace's equation. These involve either one or more conducting rings or their analogs, a hole in one or more conducting sheets. The leading terms in the potential are harmonic, corresponding to the Paul trap, but with coefficients that reduce their efficiency and for some traps, the anharmonic terms can be suppressed to eighth-order. Stable ion trapping is predicted for all electrode configurations possessing radial and axial symmetry. A three-hole microtrap with an inner hole radius of 80 \ensuremath{\mu}m trapped from one to many (dense clouds) laser-cooled ${\mathrm{Ba}}^{+}$ ions where the two-ion distance is compressed to 1 \ensuremath{\mu}m, allowing new experiments in quantum optics. Also, arrays of traps for optical clocks are contemplated using photolithographic fabrication.

We have observed the 1S‐2S transition of atomic hydrogen with a resolution of 5 parts in 1011 by means of Doppler‐free two photon spectroscopy in an atomic beam. The significantly asymmetric line shape is strongly affected by the relativistic Doppler effect. The new high resolution can be used for improved measurements of the Rydberg constant, the 1S Lamb shift, and the isotope shift of hydrogen and deuterium.

We have stabilized the frequency of a commercial ring dye laser operating in the blue near 486 nm with an internal electro-optic modulator, using optical heterodyne detection of phase-modulated light reflected from a reference cavity. With a servo bandwidth of 10 MHz, the short-term stability has been reduced below 1 kHz. Frequency doubling of this laser will provide highly monochromatic 243 nm radiation for high resolution spectroscopy of the two-photon hydrogen 1S–2S transition.

We describe a versatile tunable continuous wave ultraviolet source for spectroscopy. An angle-tuned β-barium-borate frequency doubler is placed inside a doubly resonant build-up cavity. By compensating for double refraction, this scheme provides efficient second harmonic generation in a pure TEM00 mode.

Sidebands are observed at 72 GHz off an optical carrier at 633 nm. The laser beam is phase modulated in a LiNbO3 crystal which is exposed to a mm‐wave field inside a Fabry–Perot resonator. Under optimum phase‐matching conditions we obtain a modulation index of 5% derived from 200 mW microwave power. Phase matching is obtained by guiding the light beam on a zig‐zag path under internal total reflections. For this unconventional type of phase matching, the optical wave fronts travel at twice the speed of the modulating wave along the resonator axis.

Energetic hydrogen and helium atoms have been identified and their fluxes monitored by the High‐Energy Suprathermal Time‐of‐Flight sensor (HSTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) on the Solar and Heliospheric Observatory (SOHO) near the Lagrangian point L1 since 1996. The potential sources of energetic neutral atoms (ENAs) in the heliosphere are co‐rotating interaction regions (CIRs), solar energetic particle events, pre‐accelerated pickup ions as well as low‐energy (up to few hundred keV) anomalous cosmic ray (ACR) ions in the outer heliosphere, close to and beyond the solar wind termination shock. The ENAs, neutralized via charge transfer reactions, can penetrate into the inner solar system, unaffected by the interplanetary magnetic field. The observed ENA fluxes in the apex direction of the heliosphere set limits on potential theories of the dominant sources of the energetic neutral atoms and we discuss implications of these ENA observations on modeling of the heliosheath.