| 1. |
- Artemyev, A. V., et al.
(författare)
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Field-Aligned Currents Originating From the Magnetic Reconnection Region : Conjugate MMS-ARTEMIS Observations
- 2018
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 45:12, s. 5836-5844
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Tidskriftsartikel (refereegranskat)abstract
- Near-Earth magnetic reconnection reconfigures the magnetotail and produces strong plasma flows that transport plasma sheet particles and electromagnetic energy to the inner magnetosphere. An essential element of such a reconfiguration is strong, transient field-aligned currents. These currents, believed to be generated within the plasma sheet and closed at the ionosphere, are responsible for magnetosphere-ionosphere coupling during substorms. We use conjugate measurements from Magnetospheric Multiscale (MMS) at the plasma sheet boundary (around x approximate to- 10R(E)) and Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) at the equator (around x approximate to- 60R(E)) to explore the potential generation region of these currents. We find a clear correlation between the field-aligned current intensity measured by MMS and the tailward plasma sheet flows measured by ARTEMIS. To better understand the origin of this correlation, we compare spacecraft observations with results from 3-D particle-in-cell simulations of magnetotail reconnection. The comparison reveals that field-aligned currents and plasma flows start, wax, and wane due to the development of a reconnection region between MMS (near-Earth) and ARTEMIS (at lunar distance). A weak correlation between the field-aligned current intensity at MMS and earthward flow magnitudes at ARTEMIS suggests that distant magnetotail reconnection does not significantly contribute to the generation of the observed near-Earth currents. Our findings support the idea that the dominant role of the near-Earth magnetotail reconnection in the field-aligned current generation is likely responsible for their transient nature, whereas more steady distant tail reconnection would support long-term field-aligned current system. Plain Language Summary Field-aligned currents connect the Earth magnetotail and ionosphere, proving energy and information transport from the region where main energy release process, magnetic reconnection, occurs to the region where the collisional energy dissipation takes place. Therefore, investigation and modeling of the field-aligned current generation is important problem of the magnetosphere plasma physics. However, field-aligned current investigation requires simultaneous observations of reconnection signatures in the magnetotail and at high latitudes. Simultaneous and conjugate operation of two multispacecraft missions, Magnetospheric Multiscale and Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun, for the first time provide an opportunity for such investigation. Combining spacecraft observations with results from 3-D particle-in-cell simulations of magnetotail reconnection, we demonstrate that field-aligned currents and plasma flows start, wax, and wane due to the development of a reconnection region between near-Earth (Magnetospheric Multiscale location) and lunar distant tail (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun location). Our findings support the idea that the dominant role of the near-Earth magnetotail reconnection in the field-aligned current generation is likely responsible for their transient nature, whereas more steady distant tail reconnection would support long-term field-aligned current system.
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| 2. |
- Roberts, O. W., et al.
(författare)
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Estimation of the Electron Density From Spacecraft Potential During High-Frequency Electric Field Fluctuations
- 2020
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Ingår i: Journal of Geophysical Research - Space Physics. - : AMER GEOPHYSICAL UNION. - 2169-9380 .- 2169-9402. ; 125:9
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Tidskriftsartikel (refereegranskat)abstract
- Spacecraft potential has often been used to infer electron density with much higher time resolution than is typically possible with plasma instruments. However, recently, two studies by Torkar et al. (2017, https://doi.org/10.1002/2017JA024724) and Graham, Vaivads, Khotyaintsev, Eriksson, et al. (2018, https://doi.org/10.1029/2018JA025874) have shown that external electric fields can also have an effect on the spacecraft potential by enhancing photoelectron escape from the surface. Consequently, should the electron density derived from the spacecraft potential be used during an event with a large electric field, the estimation would be contaminated and the user would see the effects of the electric field rather than density perturbations. The goal of this paper is to propose a method to remove the electric field effects to allow the density derived from spacecraft potential to be used even during large-amplitude wave events such as Langmuir waves or upper hybrid waves. Plain Language Summary Spacecraft in a plasma become charged due to a number of processes. Often the two most important processes in determining the charge are due to the ambient plasma and the photoelectron emission from the surface of a sunlit spacecraft. The potential itself is a function of the electron density, and consequently, the potential data can be used to infer the electron density if the photoelectron emission can be modeled. However, in the presence of large electric fields, the photoelectron emission can change with the electric field. This means that rather than see fluctuations of density in the spacecraft potential, the effect of the electric field is seen. Here a method is presented to remove the electric field effect on the spacecraft potential such that the density can be estimated even when there are strong electric fields present.
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| 3. |
- Torkar, K., et al.
(författare)
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Improved Determination of Plasma Density Based on Spacecraft Potential of the Magnetospheric Multiscale Mission Under Active Potential Control
- 2019
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Ingår i: IEEE Transactions on Plasma Science. - : Institute of Electrical and Electronics Engineers (IEEE). - 0093-3813 .- 1939-9375. ; 47:8, s. 3636-3647
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Tidskriftsartikel (refereegranskat)abstract
- Data from the Magnetospheric Multiscale (MMS) mission, in particular, the spacecraft potential measured with and without the ion beams of the active spacecraft potential control (ASPOC) instruments, plasma electron moments, and the electric field, have been employed for an improved determination of plasma density based on spacecraft potential. The known technique to derive plasma density from spacecraft potential sees the spacecraft behaving as a plasma probe which adopts a potential at which the ambient plasma current and one of photoelectrons produced at the surface and leaving into space are in equilibrium. Thus, the potential is a function of the plasma current, and plasma density can be determined using measurements or assumptions on plasma temperature. This method is especially useful during periods when the plasma instruments are not in operation or when spacecraft potential data have significantly higher time resolution than particle detectors. However, the applicable current-voltage characteristic of the spacecraft has to be known with high accuracy, particularly when the potential is actively controlled and shows only minor residual variations. This paper demonstrates recent refinements of the density determination coming from: 1) the reduction of artifacts in the potential data due to the geometry of the spinning spacecraft and due to effects of the ambient electric field on the potential measurements and 2) a calibration of the plasma current to the spacecraft surfaces which is only possible by comparison with the variable currents from the ion beams of ASPOC. The results are discussed, and plasma densities determined by this method are shown in comparison with measurements by the Fast Plasma Instrument (FPI) for some intervals of the MMS mission.
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