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Nakamura-Messenger, K. Righter, and S. Mass of aggregates as a function of the compactness factor. The radius of the constituent monomers is given in the legend. In this section, the charging of aggregates is examined by including both plasma currents and secondary electron emission.
The aggregates are charged using parameters for conditions in the heliosheath, the region between the termination shock and the heliopause. It is shown that the collective charge on an aggregate consisting of nano-sized grains is appreciably enhanced due to the small particle effect on each subunit. Two models for approximating charge on aggregates are proposed and the charge-to-mass ratio of the aggregates is compared to that of the spheres with the same mass. Before estimating the equilibrium surface charge on the aggregates, the time to reach the equilibrium condition needs to be considered, for depending on the plasma parameters and the dynamic processes being considered, the equilibrium condition is not always satisfied for grains of all sizes.
As the grain charges, the relative contribution of the non-dominant current increases to balance the dominant current. Due to the high temperature of the plasma near the heliopause, secondary electron emission is the dominant charging process, determining Q. Since the typical distance between the heliopause and termination shock is 50 AU Schwenn , all the aggregates in the simulation are assumed to reach equilibrium within traveling a distance of 1 AU. Figure 4. Figure 5. Surface charge on aggregates as a function of the number of constituent monomers.
The radius of the constituent monomers is indicated in the legend. Figure 6. The aggregate charge can be predicted based on both the number of the monomers or the compactness factor. However, it is difficult, if not impossible, to determine the number of monomers within an aggregate measured in situ, while the compactness factor can be obtained through remote observation. As such, relating the charge to the compactness factor may serve as a useful tool when investigating the dynamics of interstellar dust grains in the outer heliosphere. Each group can be fit with a straight line of the same slope on a log—log plot with charge related to the number of constituent monomers by.
This indicates that using the surface potential of a sphere of an equivalent mass to calculate the charge on an aggregate leads to charge underestimation. It is evident that the collective contribution of the higher potential achieved by nm-radius grains within the much larger aggregate caused by the small particle effect is significant and needs to be taken into account when estimating the charge on aggregate structures. The surface potential of an aggregate in this case is defined as. Overall, the surface potential of aggregates shows larger differences for a given mass and is generally greater than that of a sphere with the same mass, due to the greater surface area of the aggregate.
Furthermore, in the experiment, the position of the grain in the sheath is determined by using small grains as markers for the sheath edge. Nonetheless, even given these considerations, the observed charge is much higher than expected and points to the effect of the porosity of the grains on the enhanced charging, similar to the results presented here.
Figure 7. Comparison of the surface potential on aggregates data points and spheres having the same mass solid line. The monomer radius within the aggregates is indicated by the legend. Plasma and UV radiation parameters vary greatly over spatial distance and with time. However, the current purpose is to demonstrate the charging of aggregate grains compared to spherical grains, so more emphasis is placed on the characteristics of aggregate charging rather than modeling a specific environment.
Only electrons and singly ionized hydrogen are considered, with other plasma components neglected due to their relatively small contribution Schwenn The small particle effect for photoemission is also neglected due to the large radius of the grains. This is due to the porous structure of the aggregate. By the same token, when the photoemission current is very strong, the porous aggregate has more surface exposed to the UV photons, yielding a greater positive charge.
Figure 8. Evolution of charge on aggregates compared to that of an equivalent sphere. The photoemission current density is 1. Again, we characterize the equilibrium charge on the aggregates due to plasma and photoelectric charging using both the number of monomers and the compactness factor. Using a photoemission current density of 1. Figure 9. Surface charge on aggregates as a function of a the number of monomers and b the compactness factor. The same exponential factor for both monomer sizes serves as strong evidence that aggregate charge is a function of the aggregate structure. The charge on aggregates can be estimated either by the number of the constituents or the structural characteristics fluffiness of the aggregate.
While determining the charge on an aggregate based on the number of constituent monomers seems intuitive, the information is often hard or infeasible to obtain. Structural characteristics, on the other hand, can be obtained through the scattering and absorption interaction between aggregates and light. The power-law relation between the compactness factor and the aggregate charge also provides an indirect but a rather accurate method of determining the morphology of interplanetary dust.
If the composition and size distribution of these grains is known, along with the solar wind conditions, the structure of these grains may be obtained based on the charge estimate models proposed above. A numerical model has been used to calculate the charge on aggregate structures in astrophysical environments, including primary plasma currents, secondary electron emission, and photoemission.
In general, porous aggregates, with their greater surface area, are more highly charged than an equivalent mass sphere, with the sign of the charge being determined by the dominant charging current.
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This is a result of the LOS factor for an aggregate being independent of the monomer size within the aggregate, as long as all of the spherical monomers have the same radius. The relationship between charge and aggregate structural characteristics for polydisperse monomer populations is the subject of current research. Finally, the relationship between the charge on an aggregate and the charge on an equivalent sphere can vary greatly depending on the magnitude of the non-plasma currents. The values used for the photoemission current density in the three cases shown are all within the range expected for solar UV flux at 1AU, which varies greatly depending on solar activity.
Thus, the charging history of aggregates in space can vary greatly over time and is markedly different from the charging history of a spherical grain. Further results exploring these differences will be presented in an upcoming paper. Crossref Google Scholar.
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