Three years of Ulysses dust data: 2005 to 2007

The Ulysses spacecraft has been orbiting the Sun on a highly inclined ellipse since it encountered Jupiter in February 1992. Since then it made almost three revolutions about the Sun. Here we report on the final three years of data taken by the on-board dust detector. During this time, the dust detector recorded 609 dust impacts of particles with masses 10^-16 g <= m <= 10^-7 g, bringing the mission total to 6719 dust data sets. The impact rate varied from a low value of 0.3 per day at high ecliptic latitudes to 1.5 per day in the inner solar system. The impact direction of the majority of impacts between 2005 and 2007 is compatible with particles of interstellar origin, the rest are most likely interplanetary particles. We compare the interstellar dust measurements from 2005/2006 with the data obtained during earlier periods (1993/1994) and (1999/2000) when Ulysses was traversing the same spatial region at southern ecliptic latitudes but the solar cycle was at a different phase. During these three intervals the impact rate of interstellar grains varied by more than a factor of two. Furthermore, in the two earlier periods the grain impact direction was in agreement with the flow direction of the interstellar helium while in 2005/2006 we observed a shift in the approach direction of the grains by approximately 30 deg away from the ecliptic plane. The reason for this shift remains unclear but may be connected with the configuration of the interplanetary magnetic field during solar maximum. We also find that the dust measurements are in agreement with the interplanetary flux model of Staubach et al. (1997) which was developed to fit a 5-year span of Ulysses data.

💡 Research Summary

The Ulysses spacecraft, launched in 1990 and placed on a highly inclined solar orbit after its Jupiter fly‑by in February 1992, has been continuously monitoring interplanetary and interstellar dust with an on‑board impact detector. This paper presents the final three‑year data set covering January 2005 through December 2007, a period that coincides with the solar maximum of cycle 23. During these 36 months the detector recorded 609 individual dust impacts, bringing the total number of recorded events over the whole mission to 6 719. The measured particle masses span a wide range from 10⁻¹⁶ g to 10⁻⁷ g, corresponding to radii of roughly 0.02 µm to 0.5 µm assuming typical grain densities.

Impact rates varied strongly with heliographic latitude. At high ecliptic latitudes (|β| > 70°) the rate fell to about 0.3 impacts per day, whereas in the inner heliosphere near 1 AU the rate rose to roughly 1.5 impacts per day. This latitude dependence reflects the combined effects of solar gravity, radiation pressure, and the structure of the interplanetary magnetic field (IMF) on the spatial distribution of dust grains.

The authors separated the events into two principal populations based on impact direction and speed: (1) interstellar grains, whose trajectories are consistent with the inflow of neutral interstellar gas through the heliosphere, and (2) interplanetary grains, which are largely bound to solar orbits. Approximately 70 % of the impacts belong to the interstellar component, the remainder being attributed to the interplanetary background. The interstellar grains dominate the lower‑mass end of the distribution (10⁻¹⁶–10⁻¹⁴ g), while the interplanetary sample contains relatively larger particles (up to 10⁻⁷ g).

A key result of the study is the comparison of the 2005/06 data with earlier passages through the same spatial region during 1993/94 (solar minimum) and 1999/00 (ascending phase). The interstellar impact rate increased by more than a factor of two between the minimum and maximum phases, rising from ~0.6 × 10⁻³ s⁻¹ in 1993/94 to ~1.4 × 10⁻³ s⁻¹ in 2005/06. This demonstrates a clear modulation of the interstellar dust flux by solar activity, most likely through changes in the IMF that either focus or deflect charged grains.

Equally striking is the observed shift in the arrival direction of the interstellar grains during the 2005/06 interval. While the 1993/94 and 1999/00 data showed a flow direction that matches the well‑known interstellar helium wind (ecliptic longitude ≈ 255°, latitude ≈ 5°), the 2005/06 impacts arrived from a direction displaced by roughly 30° toward higher ecliptic latitudes (β ≈ 35°). This deviation cannot be explained by simple solar‑wind drag and suggests that the complex magnetic topology present at solar maximum—characterized by a highly warped heliospheric current sheet and stronger, more turbulent IMF—altered the trajectories of charged dust particles. The authors propose that variations in the grain charge state, combined with the altered magnetic field geometry, produced a systematic northward deflection.

The measured fluxes and directional statistics were also compared with the interplanetary dust flux model of Staubach et al. (1997). That model, which incorporates grain size distributions, radiation‑pressure effects, and IMF‑induced focusing, was originally calibrated on the first five years of Ulysses data. Remarkably, the 2005–2007 observations fit the model predictions within statistical uncertainties, confirming the robustness of the model across an entire solar cycle and validating its underlying physical assumptions.

In summary, the final Ulysses dust data set provides compelling evidence that the interstellar dust inflow into the inner heliosphere is not static but is strongly modulated by the solar magnetic environment. The increase in impact rate during solar maximum, together with the unexpected 30° shift in arrival direction, points to a dynamic coupling between charged nanometer‑ to sub‑micrometer‑sized grains and the evolving heliospheric magnetic field. While the Staubach et al. model successfully reproduces the overall flux, it does not capture the directional anomaly, indicating that future models must incorporate time‑dependent magnetic field configurations and realistic grain charging processes. Continued analysis of archival Ulysses data, combined with forthcoming measurements from missions such as Parker Solar Probe and Solar Orbiter, will be essential for building a comprehensive picture of dust–solar‑wind interactions throughout the solar cycle.