Acknowledgement. We thank Science for their permission to use an excerpt from;

Stone, E. C., et al. 1989. Energetic charged particles in the magnetosphere of Neptune. Science 246 (4936), 1489-1494. (Excerpt from pp. 1493-1494).

Copyright AAAS, December 15, 1989.

Energetic Charged Particles in the Magnetosphere of Neptune

Small satellite and ring absorption. When energetic charged particles trapped in the magnetosphere strike satellites or ring material in orbit about Neptune, the charged particles may be absorbed completely, or they may lose so much energy that they fall below the energy thresholds of the CRS detectors. Thus, orbiting satellites and ring material produce absorption signatures that are a kind of shadow of that material in the energetic particle flux (10). Because charged particles are directed by the magnetic field, these signatures or shadows are cast along the magnetic field line direction and eventually around the entire drift shell. The analysis of such signatures at Jupiter, Saturn, and Uranus has yielded important and unique information about the structure and dynamics of those magnetospheres.

The close approach of Voyager 2 to Neptune provided an opportunity to observe the signatures of the several newly discovered rings and small satellites, which orbit between 2 and 3 R_N from the planet. We present observations of the energetic electron flux in this region during the time interval from 0425 to 0455 on day 237 (Fig. 8). These data, which provide the highest available time resolution, are from a LET detector that nominally responds to protons but in regions of high electron flux responds primarily to pileup of electrons with energies greater than 20 keV. Each data point is the number of counts in the 6-s accumulation time, so that statistical fluctuations occur with a standard deviation approximately equal to the square root of the value.

A broad decrease in flux from about 0428 to 0442 is shown in Fig. 8, superimposed on the general trend of increasing flux in this region. The decrease can also be seen in the >~1-MeV electron rate at G in Fig. 1a. From about 0444 to 0447 there is another significant decrease, and throughout the region there are flux decreases of varying width and depth that are well above the level of statistical fluctuations. After 0500 the flux drops continuously into the deep minimum associated with absorption by 1989N1.

From 0430 to 0500 the spacecraft L was increasing from ~2 to 4. The satellites and rings absorb charged particles at a rate that is strongly peaked near their minimum L values, where they spend the longest time. The minimum L is approximately the orbital radius of the body, so that Fig. 8 covers the region of maximum expected absorption for all of the inner satellites and rings. However, identification of the features in Fig. 8 with particular absorbers is complicated by their relative proximity and the complex magnetic field geometry. When the magnetic field is not azimuthally symmetric, the satellites will have multiple local minimum L values occurring at different phases in their orbits. For example, in the OTD magnetic field model the satellites (and rings) have two local minimum L values, associated with an approach to the magnetic equator when the dipole offset is either toward or away from the satellite. In the OTD model, we would therefore expect either two distinct absorption features from each satellite, or one broad one in the case that radial diffusion has caused the two to merge. The two minima are displaced inward and outward from the mean orbital radius by ~0.25 R_N. The major flux decrease in Fig. 8 contains distinct local minima and must be the result of absorption by several of the satellites and rings. However, the width of the local features is less than 0.5 R_N, and their spacing is on the order of 0.5 R_N. The interpretation of these features will therefore require further analysis and modeling of the absorption process in the complex magnetic field.

The interpretation of Fig. 8 is simplified considerably if, inside of 3 R_N, the azimuthal asymmetries relative to the dipole axis of the OTD model are small compared to the width of the observed absorption features (<~0.2 R_N). Then the major satellite absorption regions would be well separated, leading to a unique ordering of the absorption features with radial distance. In this case, a tentative identification is possible for the four major absorption regions labeled in Fig. 8. The features M, N and O would likely be due to absorption by 1989N3 and the 53k ring combined, 1989N4, and 1989N2, respectively, whereas the feature P corresponds to the 1989N1 signature labeled as I in Fig. 1a. Given that the features occur at L values equal to the orbital radii of the absorbers, a smooth variation of L with time is obtained and shown by the dashed curve in Fig. 1b for the time period from 0430 to 1510. More detailed comparisons will be required to evaluate the consistency of such an L dependence with the magnetic field observations.

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