The process by which the solar wind assimilates newly ionized atoms is important for understanding the presence of planetary or interstellar helium in the solar wind, the dynamics of the Active Magnetospheric Particle Tracer Explorers (AMPTE) lithium releases in front of the earth's bow shock, and the formation of cometary tails. In this paper we examine how newborn ions can be coupled to the solar wind in the direction parallel to the magnetic field by means of electromagnetic instabilities driven by the distribution of newborn ions. The linear properties of three instabilities are analyzed and compared with numerical solutions of the linear dispersion equation, while their nonlinear behavior is followed by means of computer simulation to obtain the characteristic time for the pickup process. With a primary emphasis on the AMPTE lithium releases, various degrees of realism are introduced into the calculations to model the upstream conditions and the intersection of the lithium with the bow shock. It is shown that a time-dependent shock model is needed to correctly reproduce the amount of lithium which is transmitted through the shock and that the resulting lithium ion distribution is still likely to be subject to the same type of instabilities in the magnetosheath. Application of these results to comets, in particular the artificial comet expected to be generated by the AMPTE barium release in the magnetosheath, is also briefly discussed. INTRODUCTIONSince its discovery, the solar wind has been the subject of intense and diverse research; the investigation of its chemical composition, its hydrodynamic and kinetic behavior, and its interaction with other objects of the solar system have all been actively pursued. One important unresolved problem is how the solar wind, in •he absence of collisions, assimilates an ensemble of newly created ions, which may be heavier than the solar wind protons. One example where such a process occurs involves helium atoms of interstellar or planetary origin. It is well known that interstellar gas of our galaxy consists of neutral helium atoms which can enter the heliosphere, penetrate to a heliocentric distance of less than 1 AU, and then become ionized because of solar UV radiation. Immediately after ionization the initial velocities of such ions, which are equal to those of the parent atoms and thus are directed on the average toward the sun, can be fairly large in relation to the solar wind velocity. However, when such helium ions were observed [Wolf et al,, 1966; Bame et al., 1968], they had been assimilated by the solar wind; i.e., they were traveling at the same bulk speed as the solar wind. Holzer and Axford [1971], Fe!dman et al. 1'1972a, b], and Wu et al. [1973] were among the first who were interested in the interaction of these ions with the solar wind by means of a collisionless process, The same physical problem is relevant to a number of other astrophysica! situations as well, such as the study of planetary ion exospheres under the influence of the solar wind [Hartle,...

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In this paper, aerodynamic actuation characteristics of radio-frequency (RF) discharge plasma are studied and a method is proposed for shock wave control based on RF discharge. Under the static condition, a RF diffuse glow discharge can be observed; under the supersonic inflow, the plasma is blown downstream but remains continuous and stable. Time-resolved schlieren is used for flow field visualization. It is found that RF discharge not only leads to continuous energy deposition on the electrode surface but also induces a compression wave. Under the supersonic inflow condition, a weak oblique shock wave is induced by discharge. Experimental results of the shock wave control indicate that the applied actuation can disperse the bottom structure of the ramp-induced oblique shock wave, which is also observed in the extracted shock wave structure after image processing. More importantly, this control effect can be maintained steadily due to the continuous high-frequency (MHz) discharge. Finally, correlations for schlieren images and numerical simulations are employed to further explore the flow control mechanism. It is observed that the vortex in the boundary layer increases after the application of actuation, meaning that the boundary layer in the downstream of the actuation position is thickened. This is equivalent to covering a layer of low-density smooth wall around the compression corner and on the ramp surface, thereby weakening the compressibility at the compression corner. Our results demonstrate the ability of RF plasma aerodynamic actuation to control the supersonic airflow.

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Particle image velocimetry measurement on shock wave/boundary layer interaction in a Mach 2.0 supersonic wind tunnel is performed to quantitatively reveal the plasma flow control effect in this paper. The typical flow structure is produced by a 24-degree compression ramp model and the streamwise plasma actuator array with five pulsed spark discharge plasma actuators is adopted as the control device. In the midspan plane, the results show that although the separation region exhibits an obvious extension, the foot of the separation wave moves upstream and the shock wave angle decreases from 41.6° to 22.3°, proving the decline in shock intensity. The shock wave drag is estimated to be reduced by 45%. According to the phase-averaged velocity field, the reason that the high-frequency actuation plays a key role in achieving the continuous control effect is revealed through the temporal evolution of the separation region area. Also, another interesting phenomenon that the flow deflects when passing through the actuation region is found, which may induce the upwash and downwash motions of the boundary layer and further reduce the flow separation on both sides of the actuation region. At last, a preliminary conceptual model is proposed to reveal the probable flow control mechanism.