Ion Demagnetization Research Articles

Observations of Energy Conversion Caused by Magnetic Reconnection at a Dipolarization Front

AbstractDipolarization fronts (DFs) are widely believed to host energy conversion processes. However, which mechanism is responsible for the energy conversion is still obscure. Using data from the Magnetospheric Multiscale mission, a current sheet is observed at a DF. This current sheet is caused by interchange instability bending the edge of the DF. Inside the current sheet, Hall electromagnetic field, super Alfvénic electron jets, demagnetization of ions and electrons, strong energy conversion, and steady ion flow and temperature are observed, indicating an electron‐only reconnection at the DF. The duskward plasma flow, which may be deflected by the DF, compresses the bent edges of the DF. As a result, the width of the current sheet between two adjacent bent edges of the DF reduces, and then reconnection begins. Our observations give direct evidence that magnetic reconnection results in energy conversion at a DF.

MMS Observations of Multiscale Hall Physics in the Magnetotail

AbstractWe present Magnetospheric Multiscale mission (MMS) observations of Hall physics in the magnetotail, which compared to dayside Hall physics is a relatively unexplored topic. The plasma consists of electrons, moderately cold ions (T∼1.5 keV) and hot ions (T∼20 keV). MMS can differentiate between the cold ion demagnetization region and hot ion demagnetization regions, which suggests that MMS was observing multiscale Hall physics. The observed Hall electric field is compared with a generalized Ohm's law, accounting for multiple ion populations. The cold ion population, despite its relatively high initial temperature, has a significant impact on the Hall electric field. These results show that multiscale Hall physics is relevant over a much larger temperature range than previously observed and is relevant for the whole magnetosphere as well as for other astrophysical plasma.

Three‐scale structure of diffusion region in the presence of cold ions

AbstractKinetic simulations and spacecraft observations typically display the two‐scale structure of collisionless diffusion region (DR), with electron and ion demagnetization scales governing the spatial extent of the DR. Recent in situ observations of the nightside magnetosphere, as well as investigation of magnetic reconnection events at the Earth's magnetopause, discovered the presence of a population of cold (tens of eV) ions of ionospheric origin. We present two‐dimensional particle‐in‐cell simulations of collisionless magnetic reconnection in multicomponent plasma with ions consisting of hot and cold populations. We show that a new cold ion diffusion region scale is introduced in between that of hot ions and electrons. Demagnetization scale of cold ion population is several times (∼4–8) larger than the initial cold ion gyroradius. Cold ions are accelerated and thermalized during magnetic reconnection and form ion beams moving with velocities close to the Alfvén velocity.

Cold ion demagnetization near the X‐line of magnetic reconnection

AbstractAlthough the effects of magnetic reconnection in magnetospheres can be observed at planetary scales, reconnection is initiated at electron scales in a plasma. Surrounding the electron diffusion region, there is an Ion‐Decoupling Region (IDR) of the size of the ion length scales (inertial length and gyroradius). Reconnection at the Earth's magnetopause often includes cold magnetospheric (few tens of eV), hot magnetospheric (10 keV), and magnetosheath (1 keV) ions, with different gyroradius length scales. We report observations of a subregion inside the IDR of the size of the cold ion population gyroradius (∼15 km) where the cold ions are demagnetized and accelerated parallel to the Hall electric field. Outside the subregion, cold ions follow the E × B motion together with electrons, while hot ions are demagnetized. We observe a sharp cold ion density gradient separating the two regions, which we identify as the cold and hot IDRs.

Electron scale structures and magnetic reconnection signatures in the turbulent magnetosheath

AbstractCollisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The Magnetospheric Multiscale (MMS) mission provides the first serious opportunity to verify whether small ion‐electron‐scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi‐parallel bow shock geometry. We observe multiple small‐scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures. Within these structures, we see signatures of ion demagnetization, electron jets, electron heating, and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales.

Ion demagnetization in the magnetopause current layer observed by MMS

AbstractWe report ion velocity distribution functions (VDFs) observed by Magnetospheric Multiscale Mission (MMS) and present evidence for demagnetized ion Speiser motion during magnetopause reconnection. The demagnetization is observed in the vicinity of the X line, as well as near the current sheet midplane about tens of ion skin depths (di) away from the X line. Close to the X line before the outflow is built up, the VDFs are elongated, and the elongated part of VDFs rotates from the out‐of‐plane current direction toward the outflow directions downstream from the X line. Farther downstream, demagnetized ions exhibit a characteristic half‐ring structure in the VDFs, as a result of the mixture of ions that have experienced different amounts of cyclotron turning around the magnetic field normal to the current sheet. Signatures of acceleration by electric fields are more pronounced in the VDFs near the X line than downstream.

In situ evidence of breaking the ion frozen-in condition via the non-gyrotropic pressure effect in magnetic reconnection

Abstract. For magnetic reconnection to proceed, the frozen-in condition for both ion fluid and electron fluid in a localized diffusion region must be violated by inertial effects, thermal pressure effects, or inter-species collisions. It has been unclear which underlying effects unfreeze ion fluid in the diffusion region. By analyzing in situ THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft measurements at the dayside magnetopause, we present clear evidence that the off-diagonal components of the ion pressure tensor is mainly responsible for breaking the ion frozen-in condition in reconnection. The off-diagonal pressure tensor, which corresponds to a non-gyrotropic pressure effect in this event, is a fluid manifestation of ion demagnetization in the diffusion region. From the perspective of the ion momentum equation, the reported non-gyrotropic ion pressure tensor is a fundamental aspect in specifying the reconnection electric field that controls how quickly reconnection proceeds.

Observations of kinetic scale field line resonances

AbstractWe identify electromagnetic field variations from the Van Allen Probes which have the properties of Doppler shifted kinetic scale Alfvénic field line resonances. These variations are observed during injections of energetic plasmas into the inner magnetosphere. These waves have scale sizes perpendicular to the magnetic field which are determined to be of the order of an ion gyro‐radius (ρi) and less. Cross‐spectral analysis of the electric and magnetic fields reveals phase transitions at frequencies correlated with enhancements and depressions in the ratio of the electric and magnetic fields. Modeling shows that these observations are consistent with the excitation of field‐line resonances over a broad range of wave numbers perpendicular to the magnetic field (k⊥) extending to k⊥ρi ≫ 1. The amplitude of these waves is such that E/Bo ≳ Ωi/k⊥ (E, Bo, and Ωi are the wave amplitude, background field strength, and ion gyro‐frequency, respectively) leading to ion demagnetization and acceleration for multiple transitions through the wave potential.

Effects of magnetized ions on the lower-hybrid-drift instability

The present paper investigates the effects of magnetized ions on the lower-hybrid-drift instability (LHDI). Since LHDI is operative over a wide frequency range covering multiple harmonics of ion-cyclotron frequency, it is expected that the instability must essentially correspond to a multiple-harmonic drift-ion-cyclotron instability if the assumption of ion demagnetization is not imposed, and that LHDI is only an approximation which results when the individual harmonic structure is smoothed over. However, such a speculation has not been explored in detail to this date. The present paper addresses just such a problem. It is shown that indeed, LHDI is a result of the coupling of the drift mode and multiple-harmonic ion-cyclotron modes. It is found, however, that the unstable mode forms a smooth unbroken curve in frequency-wave-number dispersion space, intersected at regular frequency intervals by each ion-cyclotron harmonic. Thus, it is concluded that treating the ions as “unmagnetized” at the outset is largely justifiable. However, it is also found that the unmagnetized treatment leads to lower growth rate for LHDI.

2.5D Particle and MHD simulations of mini‐magnetospheres at the Moon

2.5D Particle simulations of the solar wind interaction with the magnetized regions on the surface of the Moon confirm the earlier 2D MHD result that mini‐magnetospheres can form around the magnetic anomalies. A dipole buried 100 km below the surface with a field strength equal to 50 nT at the surface and 10 nT at 100 km above the surface held the solar wind off of the surface and caused a bow shock and a magnetopause to form. However, the boundary separating the bow shock and magnetopause becomes ambiguous as the two structures merge due to the small‐scale size of the mini‐magnetospheres. Acceleration of solar wind particles occurs at the shock. Inside the magnetopause, the Lunar electrons remained highly magnetized and exhibited fluid‐like behavior. The Lunar ions, on the other hand, become demagnetized. Outside of the magnetopause, the solar wind ions and electrons exhibited fluid‐like behavior. Small‐scale and nonideal MHD effects can be included into fluid simulations by adding Hall and pressure gradient terms in Ohm's Law, creating a magnetoplasma dynamics (MPD) model. The small‐scale effects allow for field‐aligned currents and electric fields that look qualitatively similar to those in the particle simulations, but they do not appear to change the overall shape of the mini‐magnetosphere. The extra components of the electric field indicate the presence of charge separation at the shock surface, due to the momentum difference between ions and electrons, and the near the Lunar surface, due to nonideal MHD behavior inside the mini‐magnetosphere. The 2.5D MPD model can replicate the ion demagnetization seen in the 2.5D particle simulations.

Possible Role of ion Demagnetization in the Plasma Sheet in Auroral arc and Substorm Generation

Ion demagnetization in the plasma sheet causes the formation of field-aligned current that can trigger a magnetosphere-ionosphere coupling feedback instability, which may play an important role in substorm and auroral arc generation. Since field-aligned currents close ionospheric currents, their magnitude is controlled by ionospheric conductivity. The cause of instability is the impact of increasing upward field-aligned currents on ionospheric conductivity, which in turn stimulates an increase in the field-aligned currents. When the magnitude of these currents becomes sufficiently large for the acceleration of precipitating electrons, a feedback mechanism becomes possible. Upward field-aligned currents increase the ionospheric conductivity that stimulates an explosion-like increase in field-aligned currents. It is believed that this instability may be related to substorm generation. Demagnetization of hot ions in the plasma sheet leads to the motion of magnetospheric electrons through a spatial gradient of ion population. Field-aligned currents, because of their effect on particle acceleration and the magnitude of ionospheric conductivity, can also lead to another type of instability associated with the breaking of the earthward convection flow into convection streams. The growth rate of this instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to the formation of auroral arcs with widths of the order of 10 km. This instability is able to explain many features of auroral arcs, including their conjugacy in opposite hemispheres. However, it cannot explain very narrow (less than 1 km) arcs.

Super Dual Auroral Radar Network observations of velocity‐divergent structures in the F region ionosphere

This paper describes Super Dual Auroral Radar Network observations of an unusual mesoscale (scale size L ≈ 300 km) structure in the auroral convection pattern. This structure is characterized by an expansion motion of the plasma in a plane perpendicular to the magnetic field and then by an anomalously high velocity divergence. This study describes those observations and eliminates possible artifacts in the data analysis. Because the radar data strongly suggest that the structure is located in the F region, the observations are thus in opposition with the well‐known divergence‐free motion hypothesis. They are interpreted in terms of an ion demagnetization process, in which the collisionless ions become locally collisional in the F region.

A possible role of ion demagnetization in substorm generation

A new model for substorm generation based on a stimulated ion diffusion across the magnetic field lines in the magnetotail plasma sheet is studied. The essence of the model is as follows: A partial ion demagnetization in the plasma sheet produces outward ion diffusion, which leads to charge separation and to the formation of field‐aligned currents removing these charges. The magnitude of field‐aligned currents is controlled by the, ionospheric conductivity. The effect of field‐aligned currents on the magnitude of the ionospheric conductivity can lead to the instability, consisting of an increase in these currents, a sharp increase in the ion diffusion, the dipolarization of the magnetotail magnetic field, and the substorm onset. The model includes the following issues: (1) Before the substorm onset the magnetotail plasma sheet approaches the Earth and magnetic field lines are stretched from the Earth. (2) Partially demagnetized (because of their large Larmor radius) ions in the magnetotail plasma sheet move from the Earth. This produces a radially directed electric field which restricts the ion escape and produces a system of field‐aligned currents, with upward current at the boundary of ion demagnetization and downward current poleward of this boundary. (3) The magnitude of field‐aligned currents is controlled by the ionospheric conductivity. For quiet conditions the ionospheric conductivity is small and field‐aligned currents are weak. An increase in the ion diffusion leads to an increase of field‐aligned currents. This stimulates the growth of the ionospheric conductivity, which leads to a new increase in the ion diffusion and to the substorm onset.

Theory of magnetized, coupled, rf-driven plasma sheaths in one dimension

A set of equations for magnetized plasma sheaths in rf capacitance discharges is developed within the adiabatic electron, fluid ion framework. It differs from previous approaches in that (a) ion demagnetization, i.e., the detachment of the ion flow from the magnetic lines caused by electrostatic gradients, is introduced self-consistently, (b) the ion injection velocity from the presheath is reevaluated to ensure the ion-electron flux balance, and (c) two symmetrically opposed coupled sheaths are driven by a sinusoidal driving voltage instead of a sinusoidal current. It is found that the sheath potential and thickness increase considerably with increasing magnetic inclination \ensuremath{\theta} relative to the electric field compared to the unmagnetized results; the latter are recovered at the parallel magnetic field limit. Also, the ion injection velocity along the magnetic lines is subsonic and depends on the magnetic inclination. Finally, a sinusoidal ac voltage drives an anharmonic sheath current, while a sinusoidal current drives anharmonic voltage.

Ion heating in oblique low‐Mach number shocks

The contribution of the directly transmitted ions to ion heating in the low‐Mach number oblique shock is determined analytically. We derive approximate expressions for the pressure tensor inside the ramp of a low‐Mach number low‐β shock. In the thin shock limit the ion state equation p ∝ n³ is recovered within the ramp, which corresponds to the complete demagnetization of ions and effective one‐dimensional behavior. We derive the estimate for the maximum downstream temperature of the heated ion distribution, which depends only on the magnetic compression ratio and cross‐shock potential, and may be verified observationally.