The Galileo Plasma wave investigation

The purpose of the Galileo plasma wave investigation is to study plasma waves and radio emissions in the magnetosphere of Jupiter. The plasma wave instrument uses an electric dipole antenna to detect electric fields, and two search coil magnetic antennas to detect magnetic fields. The frequency range covered is 5 Hz to 5.6 MHz for electric fields and 5 Hz to 160 kHz for magnetic fields. Low time-resolution survey spectrums are provided by three on-board spectrum analyzers. In the normal mode of operation the frequency resolution is about 10%, and the time resolution for a complete set of electric and magnetic field measurements is 37.33 s. High time-resolution spectrums are provided by a wideband receiver. The wideband receiver provides waveform measurements over bandwidths of 1, 10, and 80 kHz. These measurements can be either transmitted to the ground in real time, or stored on the spacecraft tape recorder. On the ground the waveforms are Fourier transformed and displayed as frequency-time spectrogams. Compared to previous measurements at Jupiter this instrument has several new capabilities. These new capabilities include (1) both electric and magnetic field measurements to distinguish electrostatic and electromagnetic waves, (2) direction finding measurements to determine source locations, and (3) increased bandwidth for the wideband measurements.Deceased     

Evaluation of various cleaning methods to remove bacillus spores from spacecraft hardware materials

A detailed study was made of the biological cleaning effectiveness, defined in terms of the ability to remove bacterial spores, of a number of methods used to clean hardware surfaces. Aluminum (Al 6061) and titanium (Ti 6Al-4V) were chosen for the study as they were deemed the two materials most likely to be used in spacecraft extraterrestrial sampler construction. Metal coupons (1 cm x 2.5 cm) were precleaned and inoculated with 5.8 x 10(3) cultivable Bacillus subtilis spores, which are commonly found on spacecraft surfaces and in the assembly environments. The inoculated coupons were subsequently cleaned using: (1) 70% isopropyl alcohol wipe; (2) water wipe; (3) multiple-solvent flight-hardware cleaning procedures used at the Jet Propulsion Laboratory (JPL); (4) Johnson Space Center-developed ultrapure water rinse; and (5) a commercial, semi-aqueous, multiple-solvent (SAMS) cleaning process. The biological cleaning effectiveness was measured by agar plate assay, sterility test (growing in liquid media), and epifluorescent microscopy. None of the cleaning protocols tested completely removed viable spores from the surface of the aluminum. In contrast, titanium was capable of being cleaned to sterility by two methods, the JPL standard and the commercial SAMS cleaning process. Further investigation showed that the passivation step employed in the JPL standard method is an effective surface sterilant on both metals but not compatible with aluminum. It is recommended that titanium (Ti 6Al-4V) be considered superior to aluminum (Al 6061) for use in spacecraft sampling hardware, both for its potential to be cleaned to sterilization and for its ability to withstand the most effective cleaning protocols.     

A reappraisal of the Space Shuttle programme

The conventional wisdom holds that the Space Shuttle programme has been a ‘policy failure’ because NASA compromised its original concept in the face of weak political commitment and inadequate funding. However, a detailed reappraisal of the history shows that this reasoning is ambiguous, counterfactual and contrary to experience. Congressional and presidential support for the Shuttle has consistently been generous despite flawed and shifting justifications for the programme advanced by NASA. Among the lessons to be learned are the need for more rigorous congressional oversight and the development of smaller, quicker and independent civil space programmes.     

The Geophysics of Mercury: Current Status and Anticipated Insights from the MESSENGER Mission

Current geophysical knowledge of the planet Mercury is based upon observations from ground-based astronomy and flybys of the Mariner 10 spacecraft, along with theoretical and computational studies. Mercury has the highest uncompressed density of the terrestrial planets and by implication has a metallic core with a radius approximately 75% of the planetary radius. Mercury’s spin rate is stably locked at 1.5 times the orbital mean motion. Capture into this state is the natural result of tidal evolution if this is the only dissipative process affecting the spin, but the capture probability is enhanced if Mercury’s core were molten at the time of capture. The discovery of Mercury’s magnetic field by Mariner 10 suggests the possibility that the core is partially molten to the present, a result that is surprising given the planet’s size and a surface crater density indicative of early cessation of significant volcanic activity. A present-day liquid outer core within Mercury would require either a core sulfur content of at least several weight percent or an unusual history of heat loss from the planet’s core and silicate fraction. A crustal remanent contribution to Mercury’s observed magnetic field cannot be ruled out on the basis of current knowledge. Measurements from the MESSENGER orbiter, in combination with continued ground-based observations, hold the promise of setting on a firmer basis our understanding of the structure and evolution of Mercury’s interior and the relationship of that evolution to the planet’s geological history.     

Reception of the NKL (Jim Creek) transmitter onboard GEOS-1

At the beginning of the GEOS lifetime, some attempts have been made for taking advantage of the passes over Alaska. GEOS was then commanded in a fixed mode and the corresponding telemetry data were recorded at the NASA stations. For two passes over Jim Creek (48°2N–121°9W) where a powerful VLF transmitter (f ₀ = 18.6 kHz) is located, GEOS was put in a specific mode in order to study the magnetospheric electromagnetic field in the vicinity of f ₀. The results of one pass (June 11, from 0755 UT) are presented here.During this pass, a strong enhancement of all the e.m. components at f ₀ has been observed for a specific period of time, when GEOS was very near to the exact conjugacy with NKL. The distance, as measured on the ground, over which the signal was above -6 dB from the maximum is of the order of 800 km. During the corresponding period of time (0740–0750 UT), the satellite altitude varied between 8000 and 6000 km. The magnetospheric region where the signal is strong appears to be structured, as if there were many ducts.Preliminary results concerning the polarization characteristics of the signal are presented. In the absence of precise measurements of these characteristics, the comparison between the electric and magnetic components of the received signal is not easy to interpret. An examination of the onboard computed correlograms (in the frequency range from f ₀ -0.6 kHz to f ₀ +3.3 kHz) shows that, for this pass, no VLF emissions were triggered by NKL, at the altitude of the satellite.     

Progrès récents dans l'étude des ondes T.B.F. et E.B.F.

Conclusion Il est intéressant de dégager le bilan actuel des deux dernières années en matière de phénomènes T.B.F. et E.B.F., et d'établir sur ces bases quelques possibilités d'expériences nouvelles.Nous avons décrit les nouveaux modes de propagation dans le domaine T.B.F. (sifflements protoniques, propagation transverse, ondes de plasma ioniques) et montré l'importance des phénomènes de résonance. Ces résultats ont été obtenus grâce à l'emploi des satellites, et en particulier du satellite Alouette-1.Mais les expériences au sol ont été tout aussi profitables (études des sifflements de haute latitude et des ondes hydromagnétiques). Dans ce domaine, l'utilisation d'émetteurs artificiels s'est aussi avérée un moyen d'étude important.Du point de vue théorique, les mécanismes d'interaction entre ondes et particules ont retenu l'attention de nombreux chercheurs, ainsi que l'absorption Landau par les particules thermiques. Ces études, qui touchent à la physique fondamentale des milieux ionisés, n'en sont encore qu'à leurs débuts, mais les conclusions qu'elles doivent permettre d'atteindre sont d'une grande importance du point de vue de la dynamique de la magnétosphère.En ce qui concerne les expériences nouvelles qu'il est possible de suggérer sur la base de ces études, elles peuvent se décomposer en mesures de basses altitudes et mesures de hautes altitudes.Dans les premières, il serait par exemple intéressant de mesurer la. polarisation des ondes, dans la gamme de fréquence 0.1–1 kHz. On peut espérer obtenir ainsi une meilleure mesure de la fréquence de croisement, l'extension vers les basses fréquences permettant de détecter éventuellement les helium-whistlers. Il est important de mesurer également les composantes électriques du champ, mais de nombreux satellites sont déjà prévus pour cette mesure. Enfin, il ne faut pas oublier qu'aux altitudes inférieures à 1000 km, les fréquences de l'ordre du MHz appartiennent encore au domaine T.B.F. ( < _B); la mesure des bruits radioélectriques dans cette gamme de fréquence serait d'un grand intérêt du point de vue de la compréhension des mécanismes d'interaction.Aux altitudes élevées (4R _E), où les particules de basse énergie ne sont pas absorbées par l'atmosphère, des mesures simultanées, T.B.F. et particules, seraient utiles à la compréhension des mêmes mécanismes. Enfin, à ces altitudes où le champ magnétique terrestre est faible, il n'est pas exclu d'envisager une mesure des ondes E.B.F., surtout si l'on dispose d'un satellite orienté magnétiquement et presque stationnaire. Placé à 5 ou 6 rayons terrestres, disposant de sondes magnétiques 0.1–5 Hz orientées perpendiculairement au champ directeur, de sondes d'ionisation et de compteurs de particules de faible énergie (5–2000 keV pour des protons; 0.1–100 keV pour des électrons) il pourrait étudier la faille d'ionisation ainsi que les mécanismes d'interaction qui prennent certainement naissance au voisinage de cette discontinuité.     

Examining the use of the NeQuick bottomside and topside parameterizations at high latitudes

An examination of the high latitude performance of the bottomside and topside F-layer parameterizations of the NeQuick electron density model is presented using measurements from high latitude ionosonde and Incoherent Scatter Radar (ISR) facilities.For the bottomside, we present a comparison between modeled and measured B2Bot thickness parameter. In this comparison, it is seen that the use of the NeQuick parameterization at high latitudes results in significantly underestimated bottomside thicknesses, regularly exceeding 50%. We show that these errors can be attributed to two main issues in the NeQuick parameterization:(1) through the relationship relating foF2 and M3000F2 to the maximum derivative of F2 electron density, which is used to derive the bottomside thickness, and (2) through a fundamental inability of a constant thickness parameter, semi-Epstein shape function to fit the curvature of the high latitude F-region electron density profile.For the topside, a comparison is undertaken between the NeQuick topside thickness parameterization, using measured and CCIR-modeled ionospheric parameters, and that derived from fitting the NeQuick topside function to Incoherent Scatter Radar-measured topside electron density profiles. Through this comparison, we show that using CCIR-derived foF2 and M3000F2, used in both the NeQuick and IRI, results in significantly underestimated topside thickness during summer periods, overestimated thickness during winter periods, and an overall tendency to underestimate diurnal, seasonal, and solar cycle variability. These issues see no improvement through the use of measured foF2 and M(3000)F2 values. Such measured parameters result in a tendency for the parametrization to produce a declining trend in topside thickness with increasing solar activity, to produce damped seasonal variations, and to produce significantly overestimated topside thickness during winter periods.     

GOCE star tracker attitude quaternion calibration and combination

We analyse the inter-boresight angles (IBA) measured by the star trackers on board the GOCE satellite and find that they exhibit small offsets of 7–9″ with respect to the ones calculated from the rotation of the star tracker reference frames to the satellite reference frame. Further, we find small variations in the offsets with a peak-to-peak amplitude of up to 8″, which correlate with variations of the star trackers’ temperatures. Motivated by these findings, we present a method for combining the attitude quaternions measured by two or more star trackers that includes an estimation of relative attitude offsets between star trackers as a linear function of temperature. The method was used to correct and combine the star tracker attitude quaternions within the reprocessing of GOCE data performed in 2018. We demonstrate that the IBA calculated from the corrected star tracker attitude quaternions show no significant offsets with respect to the reference frame information. Finally, we show that neglecting the star tracker attitude offsets in the processing would result in perturbations in the gravity gradients that are visible at frequencies below 2?mHz and have a magnitude of up to 90?mE. The presented method avoids such perturbations to a large extent.