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Register a new userA charging model for the Rosetta spacecraft
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Added by
Fredrik Leffe Johansson
Publication date
2020
fields
Physics
and research's language is
English
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Context. The electrostatic potential of a spacecraft, VS, is important for the capabilities of in situ plasma measurements. Rosetta has been found to be negatively charged during most of the comet mission and even more so in denser plasmas. Aims. Our goal is to investigate how the negative VS correlates with electron density and temperature and to understand the physics of the observed correlation. Methods. We applied full mission comparative statistics of VS, electron temperature, and electron density to establish VS dependence on cold and warm plasma density and electron temperature. We also used Spacecraft-Plasma Interaction System (SPIS) simulations and an analytical vacuum model to investigate if positively biased elements covering a fraction of the solar array surface can explain the observed correlations. Results. Here, the VS was found to depend more on electron density, particularly with regard to the cold part of the electrons, and less on electron temperature than was expected for the high flux of thermal (cometary) ionospheric electrons. This behaviour was reproduced by an analytical model which is consistent with numerical simulations. Conclusions. Rosetta is negatively driven mainly by positively biased elements on the borders of the front side of the solar panels as these can efficiently collect cold plasma electrons. Biased elements distributed elsewhere on the front side of the panels are less efficient at collecting electrons apart from locally produced electrons (photoelectrons). To avoid significant charging, future spacecraft may minimise the area of exposed bias conductors or use a positive ground power system.
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A three-dimensional (3-D), self-consistent code is employed to solve for the static potential structure surrounding a spacecraft in a high photoelectron environment. The numerical solutions show that, under certain conditions, a spacecraft can take on a negative potential in spite of strong photoelectron currents. The negative potential is due to an electrostatic barrier near the surface of the spacecraft that can reflect a large fraction of the photoelectron flux back to the spacecraft. This electrostatic barrier forms if (1) the photoelectron density at the surface of the spacecraft greatly exceeds the ambient plasma density, (2) the spacecraft size is significantly larger than local Debye length of the photoelectrons, and (3) the thermal electron energy is much larger than the characteristic energy of the escaping photoelectrons. All of these conditions are present near the Sun. The numerical solutions also show that the spacecrafts negative potential can be amplified by an ion wake. The negative potential of the ion wake prevents secondary electrons from escaping the part of spacecraft in contact with the wake. These findings may be important for future spacecraft missions that go nearer to the Sun, such as Solar Orbiter and Solar Probe Plus.
The relationship between magnetic reconnection and plasma turbulence is investigated using multipoint in-situ measurements from the Cluster spacecraft within a high-speed reconnection jet in the terrestrial magnetotail. We show explicitly that work done by electromagnetic fields on the particles, $mathbf{J}cdotmathbf{E}$, has a non-Gaussian distribution and is concentrated in regions of high electric current density. Hence, magnetic energy is converted to kinetic energy in an intermittent manner. Furthermore, we find the higher-order statistics of magnetic field fluctuations generated by reconnection are characterized by multifractal scaling on magnetofluid scales and non-Gaussian global scale invariance on kinetic scales. These observations suggest $mathbf{J}cdotmathbf{E}$ within the reconnection jet has an analogue in fluid-like turbulence theory in that it proceeds via coherent structures generated by an intermittent cascade. This supports the hypothesis that turbulent dissipation is highly nonuniform, and thus these results could have far reaching implications for space and astrophysical plasmas.
The response of the Earths magnetosphere to changing solar wind conditions are studied with a 3D Magnetohydrodynamic (MHD) model. One full year (155 Cluster orbits) of the Earths magnetosphere is simulated using Grand Unified Magnetosphere Ionosphere Coupling simulation (GUMICS-4) magnetohydrodynamic code. Real solar wind measurements are given to the code as input to create the longest lasting global magnetohydrodynamics simulation to date. The applicability of the results of the simulation depends critically on the input parameters used in the model. Therefore, the validity and the variance of the OMNIWeb data is first investigated thoroughly using Cluster measurement close to the bow shock. The OMNIWeb and the Cluster data were found to correlate very well before the bow shock. The solar wind magnetic field and plasma parameters are not changed significantly from the $L_1$ Lagrange point to the foreshock, therefore the OMNIWeb data is appropriate input to the GUMICS-4. The Cluster SC3 footprints are determined by magnetic field mapping from the simulation results and the Tsyganenko (T96) model in order to compare two methods. The determined footprints are in rather good agreement with the T96. However, it was found that the footprints agree better in the northern hemisphere than the southern one during quiet conditions. If the By is not zero, the agreement of the GUMICS-4 and T96 footprint is worse in longitude in the southern hemisphere. Overall, the study implies that a 3D MHD model can increase our insight of the response of the magnetosphere to solar wind conditions.
Context. The Rosetta spacecraft made continuous measurements of the coma of comet 67P/ Churyumov-Gerasimenko (67P) for more than two years. The plasma in the coma appeared very dynamic, and many factors control its variability. Aims. We wish to identify the effects of solar flares on the comet plasma and also their effect on the measurements by the Langmuir Probe Instrument (LAP). Methods. To identify the effects of flares, we proceeded from an existing flare catalog of Earth-directed solar flares, from which a new list was created that only included Rosetta-directed flares. We also used measurements of flares at Mars when at similar longitudes as Rosetta. The flare irradiance spectral model (FISM v.1) and its Mars equivalent (FISM-M) produce an extreme-ultraviolet (EUV) irradiance (10-120 nm) of the flares at 1 min resolution. LAP data and density measurements obtained with the Mutual Impedence Probe (MIP) from the time of arrival of the flares at Rosetta were examined to determine the flare effects. Results. From the vantage point of Earth, 1504 flares directed toward Rosetta occurred during the mission. In only 24 of these, that is, 1.6%, was the increase in EUV irradiance large enough to cause an observable effect in LAP data. Twenty-four Mars-directed flares were also observed in Rosetta data. The effect of the flares was to increase the photoelectron current by typically 1-5 nA. We find little evidence that the solar flares increase the plasma density, at least not above the background variability. Conclusions. Solar flares have a small effect on the photoelectron current of the LAP instrument, and they are not significant in comparison to other factors that control the plasma density in the coma. The photoelectron current can only be used for flare detection during periods of calm plasma conditions.
The TianQin space Gravitational Waves (GW) observatory will contain 3 geocentric and circularly orbiting spacecraft with an orbital radius of 10^5 km, to detect the GW in the milli-hertz frequency band. Each spacecraft pair will establish a 1.7*10^5 km-long laser interferometer immersed in the solar wind and the magnetospheric plasmas to measure the phase deviations induced by the GW. GW detection requires a high-precision measurement of the laser phase. The cumulative effects of the long distance and the periodic oscillations of the plasma density may induce an additional phase noise. This paper aims to model the plasma induced phase deviation of the inter-spacecraft laser signals, using a realistic orbit simulator and the Space Weather Modeling Framework (SWMF) model. Preliminary results show that the plasma density oscillation can induce the phase deviations close to 2*10^-6 rad/Hz^1/2 or 0.3pm/Hz^1/2 in the milli-hertz frequency band and it is within the error budget assigned to the displacement noise of the interferometry. The amplitude spectrum density of phases along three arms become more separated when the orbital plane is parallel to the Sun-Earth line or during a magnetic storm. Finally, the dependence of the phase deviations on the orbital radius is examined.
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