The idea of using a high-voltage electron beam with substantial current to actively probe magnetic field line connectivity in space has been discussed since the 1970s. However, its experimental realization onboard a magnetospheric spacecraft has never been accomplished because the tenuous magnetospheric plasma cannot provide the return current necessary to keep spacecraft charging under control. In this work, we perform Particle-In-Cell simulations to investigate the conditions under which a high-voltage electron beam can be emitted from a spacecraft and explore solutions that can mitigate spacecraft charging. The electron beam cannot simply be compensated for by an ion beam of equal current, because the Child-Langmuir space charge limit is violated under conditions of interest. On the other hand, releasing a high-density neutral contactor plasma prior and during beam emission is critical in aiding beam emission. We show that after an initial transient controlled by the size of the contactor cloud where the spacecraft potential rises, the spacecraft potential can settle into conditions that allow for electron beam emission. A physical explanation of this result in terms of ion emission into spherical geometry from the surface of the plasma cloud is presented, together with scaling laws of the peak spacecraft potential varying the ion mass and beam current. These results suggest that a strategy where the contactor plasma and the electron beam operate simultaneously might offer a pathway to perform beam experiments in the magnetosphere.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014JA020608

Future beam experiments in the magnetosphere with plasma contactors: How do we get the charge off the spacecraft?

Experiments where a high-voltage electron beam emitted by a spacecraft in the low-density magnetosphere is used to probe the magnetospheric configuration could greatly enhance our understanding of the near-Earth environment. Their challenge, however, resides in the fact that the background magnetospheric plasma cannot provide a return current that balances the electron beam current without charging the spacecraft to such high potential that in practice prevents beam emission. In order to overcome this problem, a possible solution is based on the emission of a high-density contactor plasma by the spacecraft prior to and after the beam. We perform particle-in-cell simulations to investigate the conditions under which a high-voltage electron beam can be emitted from a magnetospheric spacecraft, comparing two possible routes that rely on the high-density contactor plasma. The first is an “electron collection” route, where the contactor has lower current than the electron beam and is used with the goal of connecting to the background plasma and collecting magnetospheric electrons over a much larger area than that allowed by the spacecraft alone. The second is an “ion emission” route, where the contactor has higher current than the electron beam. Ion emission is then enabled over the large quasi-spherical area of the contactor cloud, thus overcoming the space charge limits typical of ion beam emission. Our results indicate that the ion emission route offers a pathway for performing beam experiments in the low-density magnetosphere, while the electron collection route is not viable because the contactor fails to draw a large neutralizing current from the background.

Future beam experiments in the magnetosphere with plasma contactors: The electron collection and ion emission routes

The APEX mother-daughter project (Active Plasma EXperiments) was launched into an elliptical polar orbit (440 –-- 3080 km) in December 1991. It consisted of the main Russian Interkosmos--25 (IK--25) satellite and the Czech MAGION--3 subsatellite, both with international scientific payloads. The mission used intensive modulated electron beam emissions and xenon plasma or neutral releases from the main satellite for studies of dynamic processes in the magnetosphere and upper ionosphere. Its main scientific objectives were to simulate an artificial aurora and to study optical and radio emissions from the aurora region, and to investigate the dynamics and relaxation of modulated electron and plasma jets, artificially injected into the ionospheric plasma. The experiments studied the Critical Ionization Velocity phenomenon and a diamagnetic cavity formation during the xenon releases, local and distant effects of the electron beam injection, spacecraft charging and potential balance, and plasma-wave interactions during the artificial emissions. Attempts were performed to utilize the modulated electron beam as an active transmitting antenna in the space. The theory of ballistic wave propagation across plasma barrier was tested in a joint active experiment with the Dushanbe ionospheric heater facility. In the paper, we give a short overview of the IK--25/MAGION--3 scientific instrumentation and methodology of experiments with artificial beam injections and we provide a review of the main APEX active experiments results, many of which have been published only in the Russian language so far. From a historical 25-years-long perspective, we try to put the results of the APEX experiments into the context of other active experiments in the space plasma.

/pdf/overview-of-apex-project-results-3aba6j2y3r.pdf

Overview of APEX Project Results

A spacecraft-charging mitigation scheme necessary for the operation of a high-power electron beam in the low-density magnetosphere is analyzed. The scheme is based on a plasma contactor, that is, a high-density charge-neutral plasma emitted prior to and during beam emission and its ability to emit high ion currents without strong space-charge limitations. A simple theoretical model for the transient of the spacecraft potential and contactor expansion during beam emission is presented. The model focuses on the contactor ion dynamics and is valid in the limit when the ion contactor current is equal to the beam current. The model is found in very good agreement with particle-in-cell simulations over a large parametric study that varies the initial expansion time of the contactor, the contactor current, and the ion mass. The model highlights the physics of the spacecraft-charging mitigation scheme, indicating that the most important part of the dynamics is the evolution of the outermost ion front, which is pushed away by the charge accumulated in the system by the beam. The model can be also used to estimate the long-time evolution of the spacecraft potential. For a short contactor expansion (0.3- or 0.6-ms helium plasma or 0.8-ms argon plasma, both with 1-mA current) it yields a peak spacecraft potential of the order of 1–3 kV. This implies that a 1-mA relativistic electron beam would be easily emitted by the spacecraft.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2017JA024926

Spacecraft-Charging Mitigation of a High-Power Electron Beam Emitted by a Magnetospheric Spacecraft: Simple Theoretical Model for the Transient of the Spacecraft Potential

. The APEX project (Active Plasma Experiment) has been launched into a polar orbit in December 1991 and consists of two satellites (IK-25 and MAGION-3), with a distance between them from 200 km to 10 000 km. The mission used intensive electron beam emission, complemented by a low-energy Xenon plasma generator during the electron beam injection, for the study of dynamic processes in the magnetosphere and upper ionosphere.  The paper deals with short, intensive bursts of field-aligned electrons observed during the APEX mission on board the MAGION-3 satellite. These events are located pre-dominantly at the middle geomagnetic latitudes in the day-side magnetosphere. The time-energy structure of these electron bursts is similar to the inverted-V one, but the pitch-angle width is less than 10°. Electrons with an energy up to 700 keV are often observed during the events. We analyze the observed events, discuss the possible mechanisms of the particle spreading, and the role of the main satellite’s activity as a possible source of these events. Key words. Ionosphere (particle acceleration; particle precipitation) – Space plasma physics (active perturbation experiments)

pdf/actively-produced-high-energy-electron-bursts-within-the-1rbi3ih8bs.pdf

Actively produced high-energy electron bursts within the magnetosphere: the APEX project

Short time high energy electron fluxes in day-side magnetosphere

The APEX project (Active Plasma Experiment) has been launched into a polar orbit in December 1991 and consists of two satellites (IK-25 and MAGION-3), with a distance from ≍ 200 km to 10 000 km between them. The paper deals with short intensive bursts of field-aligned electrons observed during the APEX mission on board the MAGION-3 satellite. These events are predominantly located at the middle geomagnetic latitudes in the day-side magnetosphere. The time–energy structure of these electron bursts is similar to the inverted-V one but the pitch–angle width is less than 10°. Electrons with an energy up to 700 keV are often observed during the events. We analyse the observed events, discuss the possible mechanisms of the particle spreading and the role of the activity of the main satellite as a possible source of these events.

Propagation of electron bursts in the low-latitude magnetosphere

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Actively produced high-energy electron bursts within the magnetosphere: the APEX project

Short time high energy electron fluxes in day-side magnetosphere

Propagation of electron bursts in the low-latitude magnetosphere