ALBERT

Description: An overview of the observational results on the plasma environment at Uranus is given, and the implications of these observations for magnetospheric physics at Uranus are discussed. During the Voyager 2 encounter with Uranus, an extended magnetosphere filled with a tenuous plasma was detected. This low-energy plasma was found to consist of protons and electrons, with no significant heavy ion contribution, and with a density in the regions sampled by the spacecraft of at most three electrons per cubic centimeter. The plasma electrons and ions exhibit both a thermal component (with temperatures of tens of eV) and a hot component (with temperatures of a few keV). The thermal ion component is observed both inside and outside an L-shell value near 5, whereas the hot ion and electron component is excluded from the region inside of that L-shell. The source of the thermal component of the plasma is either the planetary ionosphere or the neutral hydrogen corona surrounding Uranus, whereas the hot component is convected in from the magnetotail, with probably an ionospheric source.

Description: The rings and most of the satellites of the outer planets orbit within the radiation belts of their parent bodies. This is an environment with intense fluxes of energetic electrons. As a result, these objects are strong emitters of X-rays. The characteristic X-ray lines from these bodies depend on atomic composition, but they are not sensitive to how the material is arranged in compounds or mixtures. X-ray fluorescence spectral analysis has demonstrated its unique value in the laboratory as a qualitative and quantitative analysis tool. This technique has yet to be fully exploited in a planetary instrument for remote sensing. The characteristic X-ray emissions provide atomic relative abundances. These results are complementary to the molecular composition information obtained from IR, visible, and UV emission spectra. The atomic relative abundances are crucial to understanding the formation and evolution of these bodies. They are also crucial to the proper interpretation of the molecular composition results from the other sensors. The intensities of the characteristic X-ray emissions are sufficiently strong to be measured with an instrument of modest size. Recent developments in X-ray detector technologies and electronic miniaturization have made possible space-flight X-ray imaging and nonimaging spectrometers of high sensitivity and excellent energy resolution that are rugged enough to survive long-duration space missions. Depending on the application, such instruments are capable of resolving elemental abundances of elements from carbon through iron. At the same time, by measuring the bremsstrahlung intensity and energy spectrum, the characteristics of the source electron flux can be determined. We will discuss these concepts, including estimated source strengths, and will describe a small instrument capable of providing this unique channel of information for future planetary missions. We propose to build this instrument using innovative electronics packaging methods to minimize size and weight.

Description: The unusual configuration of the Uranian magnetosphere leads to differences in the relative effects of solar wind induced magnetospheric convection and plasma corotation from those at the other planets. At the present epoch the orientation of the rotation axis of Uranus with respect to the solar wind flow direction leads to a decoupling of the convective and corotational flows, allowing plasma from the tail to move unimpeded through the inner magnetosphere. As Uranus progresses in its orbit around the sun, corotation plays a gradually more important role and the plasma residence times within the magnetosphere increase. When the rotation axis finally becomes perpendicular to the solar wind flow, corotation is dominant.

Description: The locations of features in the Voyager 2 energetic particle data from Neptune are combined with uncertainties in the multipole expansion of the planetary magnetic field to derive new magnetic field models that are consistent both with various interpretations of the particle features and with the magnetic field data. While assumptions as to the origin of the features must be made, they do not provide sufficient constraints to obtain significant new information on any of the unknown multipole coefficients. However, the magnetic L shell positions of the particle features, which are interpreted primarily as absorption signatures of Neptune's satellites, can, in general, be brought into agreement with expected values.

Description: The Voyager 2 cosmic ray system (CRS) measured significant fluxes of energetic greater than or equal to about 1 megaelectron volt (MeV) trapped electrons and protons in the magnetosphere of Neptune. The intensities are maximum near a magnetic L shell of 7, decreasing closer to the planet because of absorption by satellites and rings. In the region of the inner satellites of Neptune, the radiation belts have a complicated structure, which provides some constraints on the magnetic field geometry of the inner magnetosphere. Electron phase-space densities have a positive radial gradient, indicating that they diffuse inward from a source in the outer magnetosphere. Electron spectra from 1 to 5 MeV are generally well represented by power laws with indices near 6, which harden in the region of peak flux to power law indices of 4 to 5. Protons have significantly lower fluxes than electrons throughout the magnetosphere, with large anisotropies due to radial intensity gradients. The radiation belts resemble those of Uranus to the extent allowed by the different locations of the satellites, which limit the flux at each planet.

Description: The magnetosphere of Neptune changes its magnetic configuration continuously as the planet rotates, leading to a strong modulation of the convection electric field. Even though the corotation speed is considerably larger, the modulation causes the small convection speed to have a cumulative effect, much like the acceleration of particles in a cyclotron. A model calculation shows that plasma on one side of the planet convects out of the magnetosphere in a few planetary rotations, while on the other side it convects slowly planetward. The observation of nitrogen ions from a Triton plasma torus may provide a critical test of the model.

Description: The unique orientation of Uranus at the time of the Voyager 2 encounter results in a convection dominated magnetosphere. Plasma and magnetic field data from the tail magnetosheath are presented. Velocity decreases of 5-10 percemt seem to occur with a 17-hour period. At least four repetitions of this decrease are observed, in all cases when flow passes over the dayside polar cap. One possible interpretation of these features is that they are signatures of dayside reconnection. The cause of the velocity decreases would be drag on the reconnected flux tubes which are coupled via Birkeland currents to the ionosphere. The coupling efficiency for power transfer between the solar wind and Uranian magnetosphere implied by these decreases is consistent with previous determinations of this quantity.

Description: The cosmic ray system on Voyager 2 found a trapped radiation environment in Neptune's inner magnetosphere which is controlled primarily by absorption at the rings and satellite surfaces. The intensity of electrons with kinetic energies approximately greater than 1 MeV shows particularly strong and narrow signatures associated with absorption by the satellite 1989N1 at an orbital radius of 4.75 Neptune radii. Closer to the planet are several signatures of the inner satellites and rings. Absorption limits the intensity of the inner radiation belt sufficiently for the maximum intensity to occur outside the orbit of 1989N1 at a magnetic L shell of about 7. Radial profiles of the electron phase space density show that electrons diffuse inward from a source in the outer magnetosphere. Many of the inward-diffusing electrons are absorbed upon reaching a satellite orbital radius, but the finite absorption efficiency allows some of the electrons to pass by unaffected. The locations of the satellite and ring signatures also provide constraints on the nondipolar components of the planetary magnetic field.