Search

In February 2004 the Ulysses spacecraft had its second flyby at Jupiter at 0.8 AU distance from the planet. Twenty-eight dust streams emanating from the jovian system were measured between November 2002 and August 2005 while the spacecraft was within 4 AU of the planet, scanning jovigraphic latitudes from + 75 ∘ to - 25 ∘ . The highest dust fluxes were measured in mid 2004 at the passage of the equatorial plane of the planet when more than 2000 impacts per day were measured. The grain impact direction is correlated with the polarity and strength of the interplanetary magnetic field. At high jovigraphic latitudes, the impact rates show a periodicity of 26 days, closely matching the solar rotation period, while at the jovian equator the streams fluctuate with twice this period. The 14-day subharmonic streams alternate in arrival direction and are correlated with the pointing of the interplanetary magnetic field. Dust fluxes measured above and below the equatorial plane roughly decrease with the inverse square of the distance from the planet while along the equatorial plane dust fluxes are enhanced by up to 2 orders of magnitude.

Observations of ∼ MeV ions obtained in Jupiter's magnetosphere with the COSPIN Anisotropy Telescope experiment during Ulysses' encounter with Jupiter in February 1992 are presented. Ulysses is the fifth spacecraft to have visited Jupiter's magnetosphere, all of which approached Jupiter from the mid-morning sector close to the equatorial plane. Ulysses' outbound pass was, however, through a previously unexplored region of the dusk meridian at moderate southerly latitudes. The jovian magnetosphere has proved very variable, and the particle anisotropy seen during Ulysses' visit is of very different character to that seen on previous visits, even on the inbound pass. A time series of the three-dimensional anisotropy of ∼ MeV protons is presented. The first-order anisotropy transverse to the field can be caused either by a bulk flow of the plasma (i.e. a Compton-Getting anisotropy), or by a spatial gradient in the flux, or by some combination of both. The plasma bulk velocity and the flux gradient scale length based on these alternative interpretations are presented, and the plausibility, or otherwise, of each are commented on.

An analysis is presented of the fluxes and anisotropies of energetic (MeV) ions and electrons measured by instruments of the COSPIN package during the first inbound traversal of the Jovian dayside magnetospheric boundary region by the Ulysses spacecraft in February 1992. The aim of the study is to elucidate the nature of the multiple structures observed and reported previously in magnetic field, plasma and wave data. The combined data suggest the presence of a layer of depleted magnetosheath density which lay outside but adjacent to the magnetopause, and a boundary layer which lay within. The width of each of these structures is inferred to be ∼4 R J , assuming an outward boundary expansion speed of ∼100 km s −1 . The magnetic field rotated slowly (over ∼10 min) towards the magnetospheric orientation just outside the depletion layer, and subsequently (after ∼50 min) turned rapidly through a small angle (∼20°) to point almost due south at the magnetopause. The major changes in energetic particle properties between interplanetary and outer magnetospheric values took place at the latter boundary. Energetic ion fluxes decreased sharply by a factor of ∼2 from interplanetary to magnetospheric values, while energetic electron fluxes increased by about an order of magnitude. Detailed analysis of the ion distributions indicates that the magnetopause was locally closed at the time of the crossing, i.e. a tangential discontinuity, as expected from the small magnetic shear present. Outside the magnetopause the electron fluxes had previously increased from background values by about two orders of magnitude over a 2 h period as the magnetosphere approached, indicating an escaping magnetospheric population in the magnetosheath similar to that at Earth. These electrons generally streamed antiparallel to the field away from the subsolar region, opposite to the observed streaming direction of the interplanetary ions in the magnetosheath. Inside the magnetospheric boundary layer the energetic ions and electrons exhibited weaker variable counterstreaming along the field lines, suggesting an intermittent presence of open flux tubes which were connected through the magnetopause first to the south and then to the north of the spacecraft. It is suggested that these changes were associated with switches in the east-west magnetic field in the magnetosheath, a suggestion which is supported by the observed magnetic field perturbations and field-per-pendicular ion flows within the layer. The fact that the spin-averaged electron flux did not show significant changes despite the field-aligned streaming is suggested to be due to a sufficiently rapid cross-field drift of these particles. In the outer magnetosphere proper the ion and electron distributions finally approach isotropy.

Using data from the Anisotropy Telescopes on the Ulysses spacecraft obtained during the February 1992 encounter with Jupiter, we have derived the bulk velocity of ions in the energy range 0.7–1.3 MeV which were detected in Jupiter's outer magnetosphere (at a jovicentric distance of between ∼80 and ∼110 R J ). The derived ion bulk velocity typically has a component of ∼200 km s −1 in the sense opposite to corotation with Jupiter, and a component directed radially inward towards Jupiter, of comparable magnitude. Velocities consistent with appropriate corotation were only observed after Ulysses had entered Jupiter's middle magnetosphere at jovicentric distances of less than 80 R J . The ion energy spectrum at ∼ 1 MeV is well characterized by a power law, with a spectral index of ∼2.2–3 in the outer magnetosphere, but significantly less (∼2) in the solar wind, magnetospheric boundary layer, and magnetosheath plasma. The estimated proton to alpha ratio at ∼ 5 MeV is ∼2–5 in the outer magnetosphere, ∼ 2 in the magnetosheath and boundary layer, and ∼ 1.5 in the solar wind, immediately prior to the Jupiter encounter.

We examine the energetic (∼MeV) ion data obtained by the Anisotropy Telescopes instrument of the Ulysses COSPIN package during two northern high-latitude excursions prior to closest approach to Jupiter, when the spacecraft left the region of trapped fluxes on closed magnetic field lines at lower latitudes and entered a region of open field lines which we term the polar cap. During these intervals the ion fluxes dropped by 4–5 orders of magnitude to low but very steady values, and the ion spectrum was consistent with the observation of an essentially unprocessed interplanetary population. Ion anisotropies observed at these distances (within ∼16 R J , of Jupiter) indicate that in the low-latitude, high-flux regions the flows are principally azimuthail and in the sense of corotation, with speeds which are within a factor of ∼2 (in either direction) of rigid corotation. In the higher latitude trapped flux regions the flows rotate to become northward as the polar cap is approached, while in the polar cap itself the flows rotate further to become anti-corotational (and anti-sunward in the morning sector) and northward. These results provide primary evidence of the existence of solar wind-driven flows in the outer Jovian magnetosphere mapping to the high-latitude ionosphere. Investigation of concurrent magnetic data for the signatures of related field-aligned currents reveals only weak signatures with an amplitude of order ∼1 nT. The implication is that the height-integrated Pedersen conductivity of the ionosphere to which the spacecraft was connected was low, of order 0.01 mho or less. We also examine the ion observations during the two northern high-latitude excursions previous to those discussed above. These data indicate that the spacecraft approached but did not penetrate the open flux region during these intervals.

The electron analyzer of the solar wind plasma experiment aboard the Ulysses spacecraft measured the plasma properties of the Jovian magnetosphere and surrounding regions during the February 1992 encounter with Jupiter. In addition to sampling a new region of the Jovian magnetosphere, the Ulysses electron measurements were unique in that they included fine directional sampling over nearly 4π sr in look direction, for electron energies up to 862 eV. In this paper we present an overview of electron bulk parameters and a sampling of distribution shapes for the magnetosphere and adjacent plasma regions. The magnetopause was crossed 10 times, with each crossing characterized by a boundary layer with mixed magnetospheric and magnetosheath-like electron distributions. The spacecraft transited the dayside, prenoon, equatorial magnetosphere inbound, and the near-terminator, dusk magnetosphere at mid-latitudes outbound. The middle and inner magnetosphere contained a distinct plasma sheet, well-ordered in magnetic latitude, while the outer magnetosphere was less cleanly structured. Field-aligned electron anisotropies were observed throughout the magnetosphere. Unique electron spectra were seen in probable open-field line regions at the highest sampled magnetic latitudes in the inner magnetosphere. Azimuthal flow velocities indicated significant departures from rigid corotation, perhaps representing a combination of corotation and tailward flow. Radial plasma flows were slightly outward in the prenoon plasma sheet and strongly outward throughout the dusk terminator region, but were inward at higher latitudes on the dayside. Strong southward flows were observed in the outer magnetosphere on the dusk side.

During its February 1992 inbound journey into Jupiter's magnetosphere, Ulysses made several near-encounters with (and actual traversals of) the Jovian magnetodisk. In at least one such event (investigated here) the anticipated variations in energetic particle fluxes, plasma density, magnetic field intensity, and magnetic field direction at the planetary rotation period were further modulated (strongly and quasi-periodically) at a higher frequency, as if the magnetodisk and magnetic-equatorial surface were “flapping” relative to the position of the spacecraft. This additional modulation occurred around 14: 00–19: 00 U.T. on 7 February (Day 38) 1992, with Ulysses at a nominal (dipolar) magnetic latitude ≈ 11°–22° and at a zenocentric distance r ≈ 18–22Rj. The quasi-period 2π/Ω revealed by autocorrelation analysis of the energetic-proton data (and confirmed by visual examination of plotted data from the various instruments) was about 6.8 ks (≈1.9 h). Oscillations in energetic particle fluxes and plasma density were typically 180° out-of-phase with those in magnetic field intensity. The 6.8 ks period is consistent with oscillation of the magnetic shell of radius r ≈ 21Rj at the second poloidal harmonic, which is the mode that would correspond to a “flapping” of the magnetodisk and (more generally) of the magnetic-equatorial surface.

The literature from 1970 to 1987 on nightglow variability, particularly from the mesosphere and lower thermosphere, is surveyed and classified. Earlier work is referenced via reviews. The work is classified both in terms of its coverage of short period, diurnal and long term variations, and also by the observational methods used.

An account is given of the design, construction, and performance of a six channel spectrometer using tilting interference filters. Spectra of resolution of approximately 2 nm and extending over a range of about 15 nm are obtained once every 2 min at each of six wavelength regions in the visible and near infrared, and are recorded digitally for subsequent analysis. Examples are given of data obtained between 1981 and 1984 showing how the variation of intensities of atomic and molecular emissions and of the temperature deduced from the OH bands can be displayed.