Interplanetary Field Enhancements: The Interaction between Solar Wind and Interplanetary Dusty Plasma Released by Interplanetary Collisions
Interplanetary field enhancements (IFEs) are unique large-scale structures in the solar wind. During IFEs, the magnetic-field strength is significantly enhanced with little perturbation in the solar-wind plasma. Early studies showed that IFEs move at nearly the solar-wind speed and some IFEs detected at 0.72AU by Pioneer Venus Orbiter (PVO) are associated with material co-orbiting with asteroid Oljato. To explain the observed IFE features, we develop and test an IFE formation hypothesis: IFEs result from interactions between the solar wind and clouds of nanoscale charged dust particles released in interplanetary collisions.
This hypothesis predicts that the magnetic field drapes and the solar wind slows down in the upstream. Meanwhile the observed IFE occurrence rate should be comparable with the detectable interplanetary collision rate. Based on this hypothesis, we can use the IFE occurrence to determine the spatial distribution and temporal variation of interplanetary objects which produce IFEs.
To test the hypothesis, we perform a systematic survey of IFEs in the magnetic-field data from many spacecraft. Our datasets cover from 1970s to present and from inner than 0.3AU to outer than 5AU. In total, more than 470 IFEs are identified and their occurrences show clustering features in both space and time. We use multi-spacecraft simultaneous observations to reconstruct the magnetic-field geometry and find that the magnetic field drapes in the upstream region. The results of a superposed epoch study show that the solar wind slows down in the upstream and there is a plasma depletion region near the IFE centers. In addition, the solar-wind slowdown and plasma depletion feature are more significant in larger IFEs. The mass contained in IFEs can be estimated by balancing the solar-wind pressure force exerted on the IFEs against the solar gravity. The solar-wind slowdown resultant from the estimated mass is consistent with the result in superposed epoch study.
The interplanetary collision rate is estimated based on the flux model of Ceplecha  and collision model of Grün [et al., 1985]. A debris distribution model of Fujiwara [et al., 1977] is modified to estimate the mass carried by nanoscale dust particles. The integrated collision rate inside a detectable volume, which is a truncated cone starting from 0.2AU, is used to compare with the observed IFE rate. At 1AU, we find that in the same mass range, the two rates are comparable. Inside 1AU, both rates increase slowly as the heliocentric distance increases.
We reanalyze the PVO observations and confirm the association between IFEs and co-orbiting material of asteroid 2201 Oljato. An analogous study is performed at 1AU and we find that material co-orbiting with asteroid 138175 produces many IFEs there. We then compare the earlier PVO observations with the present Venus Express (VEX) observation and find that the IFE production rate of the material co-orbiting with Oljato has decreased in the past three decades. A comparison between earlier IMP 8 observations and current observations shows a similar decrease in the rate of IFEs associated with asteroid 138175. Such a rate decrease can be explained by the gravitational scattering of co-orbiting material accompanying both asteroids, as they make occasional close passes of the Earth and Venus. Simulations show that due to the gravitational perturbations from the Earth and Venus, gaps can be formed in the otherwise continuous debris trails in periods of decades [Connors et al., 2014a].
The importance of this IFE study is discussed in this thesis. We now have a better understanding of a previous mysterious phenomenon, sufficient to use the IFE occurrence to identify small interplanetary objects. Material of tens of meters across co-orbiting with near-Earth objects is too small to detect by traditional survey methods, but still can cause great property and civilian damage once it enters the Earth's atmosphere. In addition, due to gravitational perturbations, the co-orbiting material can be spread along and across the orbits of their parent bodies, which otherwise might be considered to have been well determined and safe. With our new small-object-identification technique, we can obtain the spatial distribution of the potentially hazardous material and develop a planetary defense stratagem.