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VII The Near-Earth Environment

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Dust characterisation in the near Earth environment N. McBride *

Planetary and Space Sciences Research Institute, The Open University Milton Keynes MK7 6AA, UK

Exposure of impact sensitive surfaces in low Earth orbit (LEO) has led to an improved definition of the near Earth space environment in terms of time-averaged fluxes. In LEO, separation into meteoroids and orbital debris has been possible, and the fluxes to individual faces of a spacecraft can be quite confidently modelled. This paper considers some aspects of dust flux characterisation in the near Earth region, and 'tests' whether the much used Grün et al. dust flux is consistent with recent expansive datasets.

1. INTRODUCTION

In the effort to characterise the interplanetary dust complex in the near Earth region, analyses generally focus on three mains areas: (i) definition of the dust flux at 1 AU (i.e. defining the mass distribution, and the directionality of dust sources), (ii) identification of impactor chemical residues from retrieved space-flown surfaces (i.e. obtaining information on the impactor composition, and the impact processes involved), and (iii) intact capture and retrieval of dust particulates (i.e. obtaining significant amounts of the actual source body). In this paper, I will focus mainly on dust fluxes.

Dust fluxes are generally derived from three main sources: (i) the study of meteors in the atmosphere, (ii) the study of lunar rock sample microcraters, and (iii) data, returned from dust impact experiments in space, and the study of retrieved space-flown surfaces. It is the last area that I will be mainly concentrating on in this paper. I will consider some aspects of interpreting flux data from dust instruments (or via inspection of retrieved surfaces), review some of the more important past missions, and then present a consolidation of much of the work on the topic performed by the author and co-workers.

2. MEASURING DUST FLUXES

In characterising the interplanetary dust flux, an important aim is to determine the flux as a function of particle mass. However, when a dust particle impacts a typical dust instrument, a signal is produced (or some damaged-related feature is produced) which is proportional to both the particle mass M and impact velocity v i.e. proportional to Mavb, where a and b are constants. As the (cumulative) dust flux is generally characterised by a mass distribution, which can be approximated over a given mass region by F(> M)=kM~a (where A; is a constant and a is the cumulative mass distribution index;

'Formerly at The Unit for Space Sciences & Astrophysics, University of Kent at Canterbury, UK.

Figure 1. For dust impacting at a given velocity uj, a detector will have a given mass threshold, giving rise to a flux F\ being detected (left figure). However, for a velocity v2 (where v2 > Ui), the mass threshold is lower, so that the detector is sensitive to particles 'further up' the dust mass distribution, and thus detects a flux F2 (where F2 > Fi).

Figure 1. For dust impacting at a given velocity uj, a detector will have a given mass threshold, giving rise to a flux F\ being detected (left figure). However, for a velocity v2 (where v2 > Ui), the mass threshold is lower, so that the detector is sensitive to particles 'further up' the dust mass distribution, and thus detects a flux F2 (where F2 > Fi).

which is typically of order 1), then it is clear that the number of events giving signals above an instrument detection threshold is dependent on a, b and a. This is demonstrated graphically in Figure 1, where we can visualise a dust flux distribution being 'sampled' by a generic detector. For particles impacting at velocity uj, the detector has a given mass threshold (left diagram), which gives rise to the detection of a flux i^. However, for particles with velocity V2 (where V2 > t>i), the mass threshold is lowered, and thus more (smaller) particles are detected (flux F2). The ratio of the fluxes at the two velocities is given by F-i/Fi — (w2/wi)oi^a- The ratio b/a is often referred to as the factor 7.

For detectors sensitive to impact momentum detection (i.e. oc Mv) then 7=1, whereas for detectors sensitive to impact energy (oc Mv2) then 7 = 2, and for detectors sensitive to impact plasma generation (oc Mv~3'5) then typically 7 =3-4. Thus detectors (particularly plasma detectors) have a strong velocity dependence. In interpreting data to deduce flux distributions, it is often not advisable to attempt to directly invert the data, but to use a model incorporating a flux distribution, a velocity distribution, and the detector threshold relationship, and iterate until the fitting is consistent with the data. However to first order, mean velocities can be applied (allowing direct inversion of the data), although the use of weighted means (weighted by aj) is generally required.

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