Abstract Based on the daily SE2 temperature tide at MLT altitudes (70–108 km altitudes) from Li et al. (2017; https://doi.org/10.1002/2016JA023435 ), which is picked up from TIMED/SABER observations at 45°S–45°N latitudes during a solar cycle (2002–2012), we decompose the high resolution (1 day) SE2 tide into a set of orthogonal Hough functions, which are orthogonal, and analyze the characteristics (i.e., annual, interannual, and day‐to‐day variations) of their coefficients. Our results show that the first antisymmetric and second symmetric Hough functions ((2, 3) and (2, 4)) are stronger than other Hough functions. The properties of the climatological variation are that: the (2, 3) function manifests an obvious semiannual variation that the monthly mean values are larger during equinoxes than solstices; the (2, 4) function presents two peak values are, respectively, in June and November with June maxima. Actually, the higher resolution SE2 tide was utilized to reveal the meteorological (day‐to‐day variabilities) of the Hough functions. It is worth to point out that the major two ranks ((2, 3) and (2, 4)) both dominate in high altitudes and all manifest remarkable semiannual variations and weak annual variations that they are larger during solstices than equinoxes with June solstice maxima. The further discussion reveals the contributions from the dominant two ranks of Hough functions absolute amplitudes play more significant role in the day‐to‐day variability of their Hough coefficients, than their phases.
Abstract Utilizing high-resolution data from the Magnetospheric Multiscale mission, we present new observations of lower-hybrid drift waves (LHDWs) in terrestrial magnetotail reconnection with guide field levels of ∼70% and asymmetric plasma density ( N high / N low ∼ 2.5). The LHDWs, driven by lower-hybrid drift instability, were observed in correlation with magnetic field and density gradients at separatrices on both sides of the reconnection current sheet. The properties of the LHDWs at both sides of the separatrices are different: (1) At high-density side separatrices, the LHDWs with wavelength kρ e ∼ 0.41 propagated away from the X-line mainly in the L–M plane; (2) at the low-density side separatrices, the LHDWs with wavelengths kρ e ∼ 0.76 and kρ e ∼ 0.35 propagated mainly along the outflow direction and current sheet normal. It is also found that the perpendicular magnetic field fluctuations were comparable to the parallel component. Wave potential of the LHDWs was 20% ∼ 35% of the electron temperature. The LHDWs contributed to electron demagnetization and energy dissipation. Our study can promote understanding of properties of LHDWs during magnetic reconnection.
Abstract The traditional annular closed-loop E × B drifting Hall thruster is limited by its compact structure. Two-dimensional (2D) distribution of the plasma parameters inside the discharge channel cannot be accurately measured, thus directly hindering further understanding of the discharge process. In this paper, we propose to employ an unclosed E × B drift thruster with a visible (UDTv) ionization zone to conduct research into the effects of the crossed electric field and magnetic field on the plasma distribution inside the discharge channel. Experiments showed that the UDTv presented discharge similar to a Hall thruster, especially the low-frequency oscillation related to the discharge voltage. A 2D symmetrically crescent-shaped distribution of plasma inside the channel with a hollow zone located near the maximum of the magnetic field was clearly captured by optical imaging and an emission spectrometer. Correlation between the location of the maximal magnetic gradient and the 2D ionization zone configuration was verified. A decreased magnetic mirror effect at the location of the maximum magnetic field enhanced the electron–wall interaction, inducing near-wall conduction and secondary electron emission. The electron temperature presented a canyon distribution, resulting in a bimodal configuration of the plasma density. Increased flowrate lowered the ionization inside the channel and transformed the plasma distribution into a unimodal structure because of enhanced electron conduction and the lower electron temperature. Generally, the ability to capture the correlation between 2D plasma distribution and the magnetic field inside the discharge channel was successfully demonstrated, thus proposing new ideas for further research into the internal plasma of Hall thrusters.
Abstract A few to tens of keV electron precipitation that carries substantial energy source down to the upper atmosphere to create aurora is manifested as an important magnetosphere‐ionosphere coupling process. The precipitation is usually caused by scattering processes associated with plasma waves in the magnetosphere. The scattering process is often quantified by wave diffusion rates that indicate how fast an electron is scattered. Global models commonly use diffusion coefficients that are derived from statistical wave models. However, due to the statistical nature, many localized, transient features could be smeared out. In this study, we investigate electron precipitation using event‐specific diffusion coefficients that are obtained based on simultaneous in‐situ measured/inferred, rather than statistical, chorus wave dynamics. We find that the application of the event‐specific diffusion coefficients associated with a more dynamic and intense chorus wave model leads more electrons, particularly at several to tens of keV in the dawn‐to‐noon sector at L > 3, to precipitate than using statistical coefficients. The new simulation roughly captures both the intensity and variability of the precipitating flux as detected by the NOAA/POES satellites. Ionospheric electron density in the lower E region (100–120 km) observed by the mid‐latitude Millstone Hill radar is also much better reproduced, while the case using statistical diffusion coefficients underestimates the ionization rate. This study implies the importance of using event‐specific diffusion rates in simulating the diffuse electron precipitation and understanding the magnetosphere‐ionosphere coupling.
In this investigation, using a two-dimensional particle code, we have exploredthe influence of the fast flows in plasma sheet on the triggering of substorms. Wehave found that, the local speedy flows in plasma sheet could trigger the magneticreconnection process and cause the fast release of magnetic energy stored in mag-netotail. However, continuing, stable and homogeneous fast convections in plasmasheet may depress the magnetic reconnection processes in magnetotail. The resultsobtained in this study indicate that the local Bursty Bulk Flows (BBFs) can lead tothe onsets of magnetospheric substorms; nevertheless, when the IMF keeps south-ward for a long time, there may be no substorm onset, which has been called SteadyMagnetospheric Convection processes.
Abstract Magnetic holes (MHs) are transient magnetic structures responsible for energy conversion in space plasma. Using single-spacecraft measurements from Mars Atmosphere and Volatile EvolutioN (MAVEN), the existence of MHs on Mars has been confirmed. However, due to the limitations of single-spacecraft observations, significant uncertainty also arises on the identification of the spatial scale and 3D geometry of MHs. In this study, we report a series of MHs successively detected by Tianwen-1 near the high-latitude magnetopause and by the MAVEN spacecraft near the subsolar magnetopause. The large separation between Tianwen-1 and MAVEN (∼4 R M ) suggests these MHs are macroscale structures extending along the axial direction. Additionally, we observe whistler waves generated by electron perpendicular anisotropy in one of the macroscale MHs. This study is the first joint observation of Martian MHs, shedding light on the research of transient magnetic structures on Mars.
Ion-driven magnetic nozzles (Ti > Te) are designed as intrinsic parts of cutting-edge propulsive technologies such as variable specific impulse magnetoplasma rockets (VASIMRs) and applied-field magnetoplasmadynamic thrusters. Employing a two-dimensional axisymmetric particle-in-cell (PIC) code, in the ion-driven magnetic nozzle, the compositions and distributions of azimuthal currents in different axial regions are investigated under various inlet ion temperatures Ti0 and found to differ dramatically from that in the electron-driven magnetic nozzles. Previously reported to be all paramagnetic and vanishing under a high magnetic field, the azimuthal currents resulting from the E × B drift are shown to turn diamagnetic and sustain a considerable magnitude when Ti0 is considered. The previously reported profile of diamagnetic drift current is altered by the introduction of inlet ion temperature, and the paramagnetic part is significantly suppressed. Moreover, a wide range of paramagnetic currents appear downstream due to the inward detachment of ions, which can also be reduced by increasing inlet ion temperature. Albeit considered in this paper, the azimuthal currents resulting from grad-B and curvature drift are still negligible in all cases of interest. The magnitude of diamagnetic azimuthal currents increases with amplifying Ti0, indicating a clear physical image of energy transformation from ion thermal energy to the directed kinetic energy through electromagnetic processes in the magnetic nozzle. Additionally, the magnetic inductive strength also has noticeable impacts on the azimuthal currents, the current magnitude tends to decrease as the magnetic field increases, and over-increment of it may result in larger divergence angles and lower nozzle efficiency.
Electric propulsion (EP) has become one of the most promising options for the motion control of small satellites. The physics behind the ejected plasma plumes of EP thrusters has attracted significant interest due to their interactions with the critical components of satellites. Axisymmetric plume assumptions are widely used in simulations and plume diagnostics. However, we show here that the plasma plume of a parallel-plate pulsed plasma thruster (PPT) is asymmetrically distributed along the centerline of the electrodes, contrary to the inherent axisymmetric assumption. To study this asymmetric plume structure in depth, a triple Langmuir probe was used to obtain the electron density of a two-dimensional plume area over the operating period of a PPT. The electron density results show that the plasma forms an 'I' shape plume at 2 μs after the initial main discharge. However, over time, the plume appears to cant significantly towards the cathode. The physical mechanism behind the asymmetric plume structure is studied through inter-electrode magnetic probe measurements and plasma trajectory analyses. The successive magnetic profiles indicate that the plasma is accelerated by a non-symmetrical electromagnetic force between the electrodes, which results in the plasma exhausting out with an entirely asymmetric distribution shifted upwards towards the cathode. This was also verified using varying sets of discharge voltage experiments. This work also indicates that care must be taken in the selection of the measurement points in PPT plume diagnostics. The measurement points should be chosen above the centerline of the PPT exit as the plasma parameters along the centerline may not be the most energetic part as previously believed. Furthermore, the asymmetric acceleration component of the electromagnetic force between the electrodes can enlighten us in the design of electromagnetic field configurations for future discharge channel optimization.
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