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Register a new userIntense cross-tail field-aligned currents in the plasma sheet at lunar distances
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Field-aligned currents in the Earths magnetotail are traditionally associated with transient plasma flows and strong plasma pressure gradients in the near-Earth side. In this paper we demonstrate a new field-aligned current system present at the lunar orbit tail. Using magnetotail current sheet observations by two ARTEMIS probes at $sim60 R_E$, we analyze statistically the current sheet structure and current density distribution closest to the neutral sheet. For about half of our 130 current sheet crossings, the equatorial magnetic field component across-the tail (along the main, cross-tail current) contributes significantly to the vertical pressure balance. This magnetic field component peaks at the equator, near the cross-tail current maximum. For those cases, a significant part of the tail current, having an intensity in the range 1-10nA/m$^2$, flows along the magnetic field lines (it is both field-aligned and cross-tail). We suggest that this current system develops in order to compensate the thermal pressure by particles that on its own is insufficient to fend off the lobe magnetic pressure.
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The spatial distributions of different ion species are useful indicators for plasma sheet dynamics. In this statistical study based on 7 years of Cluster observations, we establish the spatial distributions of oxygen ions and protons at energies from 274 to 955 keV, depending on geomagnetic and solar wind (SW) conditions. Compared with protons, the distribution of energetic oxygen has stronger dawn-dusk asymmetry in response to changes in the geomagnetic activity. When the interplanetary magnetic field (IMF) is directed southward, the oxygen ions show significant acceleration in the tail plasma sheet. Changes in the SW dynamic pressure ($mathit{P}_{dyn}$) affect the oxygen and proton intensities in the same way. The energetic protons show significant intensity increases at the near-Earth duskside during disturbed geomagnetic conditions, enhanced SW $mathit{P}_{dyn}$, and southward IMF, implying there location of effective inductive acceleration mechanisms and a strong duskward drift due to the increase of the magnetic field gradient in the near-Earth tail. Higher losses of energetic ions are observed in the dayside plasma sheet under disturbed geomagnetic conditions and enhanced SW $mathit{P}_{dyn}$. These observations are in agreement with theoretical models.
In an experiment on a magnetic dipole interacting with a laser-produced plasma the generation of an intense field aligned current (FAC) system was observed for the first time in a laboratory. The detailed measurements of the total value and local current density, of the magnetic field at the poles and in the equatorial magnetopause, and particular features of electron motion in the current channels revealed its similarity to the Region-1 current system in the Earth magnetosphere. Such currents were found to exist only if they can close via conductive cover of the dipole. Comparison of conductive and dielectric cases revealed specific magnetic features produced by FACand their connection with electric potential generated in the equatorial part of the magnetopause. To interpret the data we consider a model of electric potential generation in the boundary layer which agrees with experiment and with measurements of the Earth transpolar potential in the absence of an interplanetary magnetic field as well. The results could be of importance for the investigation of Mercury as a magnetic disturbance due to FAC could be especially large because of the small size of the Hermean magnetosphere.
During its two-year mission at comet 67P, Rosetta nearly continuously monitored the inner coma plasma environment for gas production rates varying over three orders of magnitude, at distances to the nucleus from a few to a few hundred km. To achieve the best possible measurements, cross-calibration of the plasma instruments is needed. We construct with two different physical models to cross-calibrate the electron density as measured by the Mutual Impedance Probe (MIP) to the ion current and spacecraft potential as measured by the Rosetta Langmuir Probe (LAP), the latter validated with the Ion Composition Analyser (ICA). We retrieve a continuous plasma density dataset for the entire cometary mission with a much improved dynamical range compared to any plasma instrument alone and, at times, improve the temporal resolution from 0.24-0.74~Hz to 57.8~Hz. The new density dataset is consistent with the existing MIP density dataset and covers long time periods where densities were too low to be measured by MIP. The physical model also yields, at 3~hour time resolution, ion flow speeds as well as a proxy for the solar EUV flux from the photoemission from the Langmuir Probes. We report on two independent mission-wide estimates of the ion flow speed which are consistent with the bulk H$_2$O$^+$ ion velocities as measured by ICA. We find the ion flow to consistently be much faster than the neutral gas over the entire mission, lending further evidence that the ions are collisionally decoupled from the neutrals in the coma. RPC measurements of ion speeds are therefore not consistent with the assumptions made in previously published plasma density models of the comet ionosphere at the start and end of the mission. Also, the measured EUV flux is perfectly consistent with independently derived values previously published from Johansson et al. (2017) and lends support for the conclusions drawn therein.
Knowledge of Marss ionosphere has been significantly advanced in recent years by observations from Mars Express (MEX) and lately MAVEN. A topic of particular interest are the interactions between the planets ionospheric plasma and its highly structured crustal magnetic fields, and how these lead to the redistribution of plasma and affect the propagation of radio waves in the system. In this paper, we elucidate a possible relationship between two anomalous radar signatures previously reported in observations from the MARSIS instrument on MEX. Relatively uncommon observations of localized, extreme increases in the ionospheric peak density in regions of radial (cusp-like) magnetic fields and spread-echo radar signatures are shown to be coincident with ducting of the same radar pulses at higher altitudes on the same field lines. We suggest that these two observations are both caused by a high electric field (perpendicular to $mathbf{B}$) having distinctly different effects in two altitude regimes. At lower altitudes, where ions are demagnetized and electrons magnetized, and recombination dominantes, a high electric field causes irregularities, plasma turbulence, electron heating, slower recombination and ultimately enhanced plasma densities. However, at higher altitudes, where both ions and electrons are magnetized and atomic oxygen ions cannot recombine directly, the high electric field instead causes frictional heating, a faster production of molecular ions by charge exchange, and so a density decrease. The latter enables ducting of radar pulses on closed field lines, in an analogous fashion to inter-hemispheric ducting in the Earths ionosphere.
Imposing an external magnetic field in short-pulse intense laser-plasma interaction is of broad scientific interest in related plasma research areas. We propose a simple method using a virtual current layer by introducing an extra current density term to simulate the external magnetic field, and demonstrate it with three-dimensional particle-in-cell simulations. The field distribution and its evolution in sub-picosecond time scale are obtained. The magnetization process takes a much longer time than that of laser-plasma interaction due to plasma diamagnetism arising from collective response. The long-time evolution of magnetic diffusion and diamagnetic current can be predicted based on a simplified analytic model in combination with simulations.
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