High energy particle dispersion events observed by interball-1 and -2

Time period from October 1996 until January 1998 was checked on high energy resolution DOK2 energetic particle instrument measurements on Interball-1 and Interball-2 for the ion (> 20 keV) dispersive events (EDIS) with the exclusion of Interball-1 orbit parts in the tail. A variety of energy dispersive events, both in ion and electron spectra with different duration is found in the auroral regions, in the outer magnetosphere and near the cusp. While EDIS were observed in all sectors of MLT, the best conditions for their observation were in the afternoon local time. The characteristics of dispersive events observed by DOK2 are consistent with their explanation by the gradient-curvature drift of particles from the injection point(s) in the night local time sector given in Lutsenko at al., 2000a, b.     

The local interstellar medium viewed through pickup ions, recent results and future perspectives

Over the last 10 years the experimental basis for the study of the very local interstellar medium (VLISM) has been substantially broadened by the direct detection of pickup ions and of neutral helium. The strength of these methods lies in the local measurement of the particles. By scanning the gravitational focusing cone of the interstellar wind, a consistent set of interstellar helium parameters, neutral density, temperature and relative velocity, has been derived. However, the accuracy of these parameters is still hampered by uncertainties in some of the crucial ionization rates and in the pickup ion transport. Recent observations have shown that the scattering mean free path of pickup ions is comparable with the large scale variation of the interplanetary magnetic field (IMF) in the inner heliosphere. This requires a substantial modification in the modeling of the ion distribution and more detailed measurements, tasks that can be addressed in the near future.     

Galactic cosmic ray radiation levels in spacecraft on interplanetary missions.

Using the Langley Research Center galactic cosmic ray (GCR) transport computer code (HZETRN) and the computerized anatomical man (CAM) model, crew radiation levels inside manned spacecraft on interplanetary missions are estimated. These radiation-level estimates include particle fluxes, LET (linear energy transfer) spectra, absorbed dose, and dose equivalent within various organs of interest in GCR protection studies. Changes in these radiation levels resulting from the use of various different types of shield materials are presented.     

Solar-proton polar-cap intensity structures as a test of magnetic field models

The entry of energetic solar protons to the polar caps offers an interesting way to test models of the geomagnetic field. In this brief report, we present a comparison between SAMPEX observations of solar-particle intensity structure during a polar cap traversal with numerical trajectory calculations using the IGRF + T96 field model.     

Plasma production by secondary impacts: Implications for velocity measurements by in-situ dust detectors

A number of in-situ cosmic dust detectors derive the dust particle velocities from measurement of the risetimes of the impact plasma signal. Extensive calibration of these instruments has established a reliable empirical relationship but a quantitative explanation has not been available, with the result that confidence in flight data outside the range of the calibration data is hard to assess. Recent measurements taken at the dust accelerator facilities at the University of Kent (UK) and at MPI-K (Germany), supported by a theoretical analysis, have demonstrated that the relationship results from the time-spread of secondary impacts coupled with the mobility of ions in the impact plasma cloud, which is in turn determined by the magnitude and geometry of the applied electric field and on the ion species present. Results of the current investigations are presented, and the implications of measurements based on this principle at high particle velocities, at masses unobtainable in calibration studies, and for other instrument geometries, are considered.     

Evolution of the Martian water cycle.

The current Martian water cycle is extremely asymmetric, with large amounts of vapor subliming off a permanent north polar water ice cap in northern summer, but with no apparent major source of water vapor in the southern hemisphere. Detailed simulations of this process with a three-dimensional circulation model indicate that the summertime interhemispheric exchange (Hadley cell) is very much stronger than transport by eddies in other seasons. As a result, water ice would be distributed globally were it not for the buffering action of regolith soil adsorption which limits the net flux of water vapor off the north polar cap to amounts that are insignificant even on the scale of thousands of years. It has been suggested that the polar layered deposits are the result of exchange on these long time scales, driven by changes in Martian orbital parameters. We therefore are conducting simulations to test the effect of varied orbital parameters on the Martian water cycle. We find that when the perihelion summer pole is charged with a polar water ice cap, large quantities of water are quickly transfered to the aphelion summer pole, setting up an annual cycle that resembles the present one. Thus, the adsorptivity of the Martian regolith may be in the narrow range where it can limit net transport from the aphelion but not the perihelion pole.     

Monte Carlo track structure studies of energy deposition and calculation of initial DSB and RBE.

Estimation of exposure due to environmental and other sources of radiations of high-LET and low-LET is of interest in radiobiology and radiation protection for risk assessment. To account for the differences in effectiveness of different types of radiations various parameters have been used. However, the relative inadequacy of the commonly used parameters, including dose, fluence, linear energy transfer, lineal energy, specific energy and quality factor, has been made manifest by the biological importance of the microscopic track structure and primary modes of interaction. Monte Carlo track structure simulations have been used to calculate the frequency of energy deposition by radiations of high- and low-LET in target sizes similar to DNA and higher order genomic structure. Tracks of monoenergetic heavy ions and electrons were constructed by following the molecular interaction-by-interaction histories of the particles down to 10 eV. Subsequently, geometrical models of these assumed biological targets were randomly exposed to the radiation tracks and the frequency of energy depositions obtained were normalized to unit dose in unit density liquid water (l0(3) kg m-3). From these data and a more sophisticated model of the DNA, absolute yields of both single- and double-strand breaks expressed in number of breaks per dalton per Gray were obtained and compared with the measured yields. The relative biological effectiveness (RBE) for energy depositions in cylindrical targets has been calculated using 100 keV electrons as the reference radiation assuming the electron track-ends contribution is similar to that in 250 kV X-ray or Co60 gamma-ray irradiations.     

Calibration techniques for magnetometers implementing on-board de-spinning algorithms

The Fluxgate Magnetometer experiments on-board the European Space Agency’s four spacecraft Cluster Mission have the capability to store sampled magnetic field vectors in the instrument memory, either as a full resolution ‘event capture’ or as spin-resolution vectors transformed into a non-spinning co-ordinate system (de-spun). The latter capability has ensured a dataset is available which extends the partial orbital coverage achieved during nominal operations in the first years of operation. The on-board de-spin is achieved using a Walsh function with Haar coefficients and allows for up to 27 h additional data per non-coverage interval. A number of commissioning orbits were used to verify the accuracy of data collected by the de-spin mode, whereby individual spacecraft were operated separately in a number of standard normal sampling and de-spin mode combinations. Up to the present time, this data has not been available to the Cluster community. We present results here comparing the performance of the on-board de-spin algorithm versus the normal sampling modes over a number of boundary layer crossings, describe the techniques used for calibration and timeline recovery, and outline the context in which the data may be usable in future studies.     

Results from the ESA SREM monitors and comparison with existing radiation belt models

The Standard Radiation Environment Monitor (SREM) is a simple particle detector developed for wide application on ESA satellites. It measures high-energy protons and electrons of the space environment with a 20° angular resolution and limited spectral information. Of the ten SREMs that have been manufactured, four have so far flown. The first model on STRV-1c functioned well until an early spacecraft failure. The other three are on-board, the ESA spacecraft INTEGRAL, ROSETTA and PROBA-1. Another model is flying on GIOVE-B, launched in April 2008 with three L-2 science missions to follow: both Herschel and Planck in 2008, and GAIA in 2011). The diverse orbits of these spacecraft and the common calibration of the monitors provides a unique dataset covering a wide range of B-L* space, providing a direct comparison of the radiation levels in the belts at different locations, and the effects of geomagnetic shielding. Data from the PROBA/SREM and INTEGRAL/IREM are compared with existing radiation belt models.