JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. A10, PAGES 17,879-17,890, OCTOBER 1, 1991 The OEDIPUS Experiment' Analysis of the Current/Voltage Data R. GODARD,• H. G. JAMES, 2 J. G. LAFRAMBOISE, 3 B. MACINTOSH,• A. G. MCNAMARA, 4 S. WATANABE, 4 AND B. A. WHALEN 4 The OEDIPUS A rocket was successfully launchedfrom the Andoya Rocket Range on January 30, 1989, toward a moderate auroral arc. In this experiment, the forward and the aft payloadsconstituted a tether. The forward and the aft payloadswere connectedthrough a thin isolated wire and the two subpayloadswere used as a plasma diagnostictool. Here we examine the results obtained in the current/voltagemode, when the two subpayloads act as a large double probe. In this mode, a voltage sweep from -8 V to +8 V was applied between the two payloads. The measured current/voltage characteristic has been fitted with a theoretical model of the current collected to extract the plasma parameters. The results indicate (1) a temporary contaminationof the surface of the payload by the argon release system, (2) a moderate and reasonableelectron temperature and (3) good correspon- dence with the results of the voltmeter for the measurement of the natural and induced electric fields. 1. INTRODUCTION Tethers in space are a promisingtool for the study of the ionosphere and also for technological applications. Previ- ously, they have been used for active experiments. One of the objectives of the Tethered Payload Experiment (TPE) was the measurement of the vehicle potential [Williamson et al., 1982]. In the Cooperative High-Altitude Rocket Gun Experiment 2 (CHARGE 2) [Sasaki et al., 1988; Neubert et al., 1990], a high-voltagedifferencebetween the forward and the aft payloads was applied through an interconnecting conducting tether to study the response of the space plasma to large potential differencesand the beam-plasmainterac- tion. In the Space Power Experiment Aboard Rockets (SPEAR 1) experiment [Katz et al., 1989], two 10-cm-radius spheres were biasedas high as 46,000 V positivewith respect to the rocket body, even though the SPEAR 1 experiment was not a tether. Such experiments are a natural extension of laboratory plasma physics. Other classes of space experiments involving large-dc spacecraftpotentials in the ionosphere are (1) RF pulses generated by a topside sounder which charged,for example, the ISIS 2 spacecraft to a few hundred volts negative [James, 1987]; (2) large space structures such as the space station in which substantial¾ x B potentials are induced; and (3) the high-level spacecraft charging in the medium- altitude auroral environmentreported by Gussenhoven et al. [1985]. The OEDIPUS A experiment for Observations of Electric Field Distributionsin the Auroral Plasmaa Unique Strategy, was designed by the National Research Council (NRC) of Canada and NASA. Its objectives were to provide observa- tions of the charged particles and electric fields (ac and dc) and to identify the energization mechanisms of auroral particles [James and Whalen, 1991]. Unique to this project was the use of a tether, as a very long (1 km) doubleprobe 1Department of Mathematics, Royal Military College of Canada, Kingston, Ontario. 2Communications Research Centre, Ottawa. 3physics Department, YorkUniversity, Toronto. 4Herzberg Institute of Astrophysics, National Research Council, Ottawa. Copyright 1991 by the American Geophysical Union. Paper number 91JA01089. 0148-0227/91/91JA-01089505.00 to measure Ell, the electric field parallel to themagnetic field B, the plasma, and the wave environment at two points along B. Thus, the concept of the OEDIPUS A experiment was totally different from the CHARGE 2 experiment, although in both experiments the forward and the aft payloads were electrically connected by a thin isolated wire and used as a doubleprobe. The forward and the aft payloads were usedin a number of different modes. In one, we measured the natural potential difference due to parallel electric fields and the V x B effect. In another mode, the so-called low- impedancemode or the current/voltagemode that we shall call the I/V mode, the two payloads were used as a long double probe. In the shorted mode, the two payloads were connectedby a low-impedance ammeter, which was used to study density fluctuations. Several previous experiments, such a TPE and CHARGE 2, have also employed the same operating modes. Other modes were used to study sheath waves [K. Bal- main, personalcommunication, 1991]. The modesof interest to us are illustrated in Figure 1. This paper concerns the analysisof the data from the I/V mode where the fore and the aft payloads act as a large double probe. A voltage sweep from -8 V to +8 V was applied between the two payloads as illustrated in Figure 2. We consider this device as a passive device since no powerful perturbating devices were active at this time. The purpose of the OEDIPUS experiment was to study the natural plasma and, more exactly, the plasma physicswhich is responsible for the parallel electric field. This is why the voltage sweepwas relatively low with respect to other tether experiments. The I/V mode has provided us with a unique opportunity to study the plasma/spacecraftinteraction for relatively large bodiesat potentialsdifferentfrom the floating potential, to estimate the natural plasma parameters, and to examine the performance of the mathematical model used for the collected current [Godard and Laframboise, 1983]. The theory of the double probe is more complex than the theory of a singleelectrostaticprobe. Becausethe systemof the double probe is electrically neutral, if one probe collects a positive current, the other one collects the same negative current. The estimationof the plasmaparameterssuchas the electron temperature is very sensitive to the theoretical model used for the ion current while the electron current is still described by Boltzmann's law. 17,879