Relaxation probe measurement in the Earth's and Titan's atmospheres: Effect of shadow

In this work, we investigate the shadowing effect of the boom and the gondola on electrical conductivity measurement in the stratosphere using a Relaxation Probe (RP). By taking the examples of the terrestrial flight for the Huygens experiment and the Huygens mission to Titan, our numerical simulations demonstrate that this effect should be taken into account, particularly for the resistance of the electrode-ionized medium during both the apogee and descent of a balloon. The fact that a signal seems a perfect exponential does not allow one to accept the hypothesis of a measurement at face value, and a correction factor should be applied. Our numerical simulations show that the resistance and the capacitance are almost constant, independent of the potential of the electrode. This makes the correction of the data an easy task. This analysis was carried out using finite element methods to model a complex electrode-boom-gondola geometry. In our model, the potential around the electrode-boom-gondola system is described by a Laplace equation, and this corresponds well to the stratospheric environment. We shall emphasize the crucial importance of estimating the floating potential of the gondola during a flight from the relaxation data of the potential. From our numerical simulations, it appears that the resistance may be too high by a factor 1.72, depending of course on the length of the boom. Also, the measurement of the positive ions by the RP for the Huygens mission around Titan does not seem too reliable. In this case, we found a heavy shadowing effect of the gondola. We hope that this work will aid in the optimal design of future planetary missions, taking into account the length of boom and, given the constraints of a mission, whether the boom should be an insulator, or a conducting body.