Interplanetary Tangential Discontinuity Research Articles

Sub-ion-gyro scale magnetic field compressions generated by the solar wind interaction with the moon

Short-period magnetic enhancements were detected by the MAP-LMAG magnetometer onboard Kaguya orbiting the moon in the solar wind at an altitude of 100 km. The duration was typically 10 s, which corresponds to 0.5 degrees in latitude along the Kaguya orbit and a scale size of 15 km. The magnitude of the magnetic field was enhanced up to 1.5–3.6 times as large as that of the preceding quiet periods. No such magnetic enhancements were found in the upstream solar wind magnetic field. The short-period magnetic enhancements were categorized into 2 groups. One is the sub-ion-gyro-scale limb compression detected at the terminator region of the moon in a nearly constant solar wind magnetic field. The magnetic field flared away from the moon consistently with the previously known limb compressions. The scale size deduced from the duration was 11 km, 85 times as small as that of previously reported limb compressions. It is significantly smaller than the typical proton gyroradius 50–100 km in the solar wind at 1AU. The other types of magnetic enhancements appeared at crossings of magnetic discontinuities of the solar wind. Some of them were found on the nightside of the moon. A possible explanation is that they were magnetic fields compressed by the solar wind ions reflected at the moon channeled back along the current sheet of an interplanetary tangential discontinuity, similar to the hot flow anomalies observed at the Earth’s bow shock. The reflected ions themselves were not detected on the nightside of the moon, while the magnetic field compressed by the expanding region can penetrate through the moon to be detected as magnetic field enhancements on the nightside of the moon.Graphical

Investigation of the Interaction Between Magnetosheath Reconnection and Magnetopause Reconnection Driven by Oblique Interplanetary Tangential Discontinuity Using Three‐Dimensional Global Hybrid Simulation

AbstractMagnetosheath reconnection due to the interaction of an interplanetary directional discontinuity with the bow shock and Earth's magnetosphere under an initially northward interplanetary magnetic field (IMF) has been investigated in previous simulations (e.g., Guo et al., 2018, https://doi.org/10.1029/2018ja025679). Under an initially southward IMF, the magnetosheath reconnection could interact with reconnection at the magnetopause. In this study, using three‐dimensional (3D) global‐scale hybrid simulations, we present cases with incoming tangential discontinuities (TDs) in an initially southward IMF, which possess various magnetic field rotation angles (ΔΦ) and half‐width (w), to study the effects of pre‐existing magnetopause reconnection on the formation of magnetosheath flux ropes, as well as the subsequent interaction between the magnetopause reconnection and magnetosheath reconnection, with downstreams of both Quasi‐perpendicular (Q‐) and Quasi‐parallel (Q‐||) shock examined. The initial IMF is assumed to be oblique, with a finite Bx and Bz, similar to that in Guo et al. (2018), https://doi.org/10.1029/2018ja025679 but with a southward Bz < 0. Compared with the cases with an initially northward IMF, magnetopause reconnection weakens the compression processes of the TD and leads to less frequent reconnection in the magnetosheath. The existence and the structure of magnetosheath reconnection are found to strongly depend on the parameters w and of the TD. When interacting with the magnetopause reconnection, the magnetosheath flux ropes can re‐reconnect with the geomagnetic dipole field lines, forming new structures of magnetopause flux ropes. The resulting evolution of flux rope configuration is illustrated.

Magnetosheath Reconnection Before Magnetopause Reconnection Driven by Interplanetary Tangential Discontinuity: A Three‐Dimensional Global Hybrid Simulation With Oblique Interplanetary Magnetic Field

AbstractTerrestrial dayside dynamics associated with a southward turning, oblique interplanetary magnetic field (IMF) carried by an interplanetary tangential discontinuity (TD) is investigated by performing a three‐dimensional global‐scale hybrid simulation systematically for cases in which the incoming solar wind TD possesses various magnetic field rotation angles ΔΦ = 90° to 180° and half widths w = 2di0 to w = 30di0, where di0 is the ion inertial length in the solar wind. Overall, the TD is compressed while being transmitted into the magnetosheath, with different compression processes downstream of the quasi‐parallel (Q‐||) and quasi‐perpendicular (Q‐⊥) shocks. It is found that magnetosheath reconnection may take place downstream of both the Q‐|| and Q‐⊥ shocks due to interaction of the directional TD with the bow shock and magnetopause, but the existence of magnetosheath reconnection depends on w and ΔΦ. Magnetosheath flux ropes are formed through three‐dimensional patchy reconnection in the thinned current sheet, with a longer rope lengths under a larger ΔΦ. There exists a dawn‐dusk asymmetry in the spatial extent of the flux ropes, which becomes more significant as ΔΦ decreases. When ΔΦ decreases to 90°, no reconnection flux ropes are found. Magnetopause reconnection is initiated when the magnetic fluxes with a southward turning IMF (on the sunward side) reach the magnetopause, and the magnetopause flux ropes can be mixed with the magnetosheath ones. Our simulation demonstrates that the effects of a southward turning of the IMF may not be a simple field direction change that leads to reconnection only at the magnetopause.

Three‐dimensional hybrid simulation of magnetosheath reconnection under northward and southward interplanetary magnetic field

A three‐dimensional (3‐D) global hybrid simulation is carried out for the generation and structure of magnetic reconnection in the magnetosheath because of the interaction of an interplanetary tangential discontinuity (TD) with the bow shock and magnetosphere. Runs are performed for solar wind TDs possessing different polarization senses of magnetic field (north‐to‐south or south‐to‐north from the leading to trailing side of the incident TD) and initial half‐widths. Two‐step compression processes are shown in the transmitted TD, including a “shock compression,” as the TD passes through the shock followed by a subsequent “convective compression” while the TD is moving in the magnetosheath toward the magnetopause. In cases with a relatively thin solar wind TD, 3‐D patchy reconnection is initiated in the transmitted TD, forming magnetosheath flux ropes. Differences between these flux ropes and those due to magnetopause reconnection are discussed. Multiple components of ion particles are present in the velocity distribution in the magnetosheath merging, accompanied by ion heating. For cases with a relatively wide initial TD, a dominant single X line appears in the subsolar magnetosheath after the transmitted TD is narrowed through the two‐step compression process. Specifically, in the cases with a south‐to‐north field rotation across an incident thin TD, the magnetosheath flux ropes could re‐reconnect with the closed geomagnetic field lines to generate a closed field line region with mixed magnetosheath and magnetospheric plasmas, which may contribute to the transport of solar wind plasma into the magnetospheric boundary layer.

Global hybrid simulation of magnetic reconnection in the magnetosheath

A three-dimensional (3-D) global hybrid simulation is carried out for the generation and structure of magnetic reconnection in the magnetosheath due to interaction of an interplanetary Tangential Discontinuity (TD) with the bow shock and magnetosphere. Runs are performed for solar wind TDs possessing different initial half-widths. As the TD propagates through the bow shock toward the magnetopause, it is greatly narrowed by a two-step compression processes, a "shock compression'' followed by a subsequent ``convective compression''. In cases with a relatively thin solar wind TD, 3-D patchy reconnection is initiated in the transmitted TD, forming magnetosheath flux ropes. Multiple components of ion particles are present in the velocity distribution in the magnetosheath merging, accompanied by ion heating. For cases with a relatively wide initial TD, a dominant single X-line appears in the subsolar magnetosheath after the transmitted TD is narrowed. A shock analysis is performed for the detailed structure of magnetic reconnection in the magnetosheath. Rotational Discontinuity (RD)/Time-Dependent Intermediate Shock (TDIS) are found to dominate the reconnection layer, which and some weak slow shocks are responsible for the ion heating and acceleration.

Global hybrid simulation of hot flow anomalies near the bow shock and in the magnetosheath

A two-dimensional global hybrid simulation is carried out to examine the generation of hot flow anomalies (HFAs) by interaction of the Earth's bow shock (BS), magnetosheath, and magnetosphere with an interplanetary tangential discontinuity (TD). By including the dayside magnetosphere in the simulation domain, this new study focuses on the formation and internal structure of strong HFAs, both near the bow shock and in the magnetosheath, and their effects on the dayside magnetopause. The simulation is performed for directional TDs, which carry a magnetic field orientation change and propagate to the BS from various directions. In the cases in which the magnetic field orientations across the TD result in a normal component of the upstream motional electric field pointing toward the TD, a strong HFA can be generated by coherent, gyrating reflected ion beams near the BS. In the center of the upstream part of the HFA, a low plasma density and a low magnetic field strength are present. The plasma is heated to a nearly isotropic temperature that is much greater than that in the downstream magnetosheath. The bulk flow speed is greatly reduced, and a significant sunward flow deflection is present in the HFA. The HFA bulges into the solar wind due to an enhancement of the total pressure in the hot cavity. As a result, the local BS and the magnetosheath may expand into the upstream solar wind by several Earth radii. A significant enhancement in the density and magnetic field strength flanks the bulge. The structure of the magnetosheath HFA, which encompasses the transmitted tangential discontinuity, is similar to that for the upstream HFA. Nevertheless, relative to the magnetosheath total pressure perpendicular to the HFA boundaries, a low pressure region is present in the magnetosheath HFA due to pressure balance with the upstream HFA. The magnetopause can expand sunward by several Earth radii as the HFA passes by. In the simulation, the size of the diamagnetic cavity increases with the rotation angle Δ Φ of the magnetic field through the TD, which corresponds to the magnitude of the normal electric field. The maximum strength of the HFA occurs when the angle of the propagation direction of the TD relative to the Sun–Earth line, γ, is nearly 80°. In addition, spatial structures with alternate temperature increases and decreases develop at the quasi-parallel shock. These structures are elongated along field lines, both upstream and downstream of the bow shock. Those with temperature increases and density, magnetic field, and the flow speed decreases appear like weak HFAs, but without strong flow deflections.

Generation of traveling convection vortices and field‐aligned currents in the magnetosphere by response to an interplanetary tangential discontinuity

Three‐dimensional global magnetohydrodynamic simulations are conducted to study the response of the magnetosheath and the magnetosphere to an interplanetary tangential discontinuity (TD), across which the plasma density increases. A fast mode wave and a transmitted TD are generated in the magnetosheath by the interaction between the interplanetary TD and the bow shock. These waves further propagate and disturb the dayside magnetosphere. The fast mode compressional wave propagates downstream out of the dayside region in about 1 minute. The pressure enhancement associated with the transmitted TD later compresses the magnetopause significantly for several minutes. The generation of traveling convection vortices, shear Alfvén waves, and the associated field‐aligned currents (FACs) in the dayside magnetosphere by these pressure pulses is identified in the simulation.

Generation of anomalous flows near the bow shock by its interaction with interplanetary discontinuities

Two‐dimensional hybrid simulations using a curvilinear coordinate system are carried out to study the interaction of the Earth's bow shock (BS) with an interplanetary directional discontinuity. In particular, plasma flow patterns are examined. In the interaction of the bow shock with an interplanetary tangential discontinuity (TD), a bulge of magnetic field and plasma may be present near the intersection between the fronts of the BS and the TD. The bulge expands to the upstream and is embedded in the solar wind. High magnetic field and ion density are present in the boundary regions of the bulge, and the temperature in the bulge is significantly higher than that in the ambient solar wind. A core of low density and sometimes low field is present inside the bulge. The flow speed changes from supersonic in the solar wind to subsonic throughout the bulge. A strong sunward deflection in flow velocity is present both in the bulge and in the magnetosheath behind the bulge. The presence of such hot anomalous flows depends on the direction and symmetry condition of the upstream motional electric field. The formation of the bulge and the associated anomalous flows is found to be mainly due to (1) the reflected ions which are focused to TD under the proper electric field conditions, (2) the unbalanced pressure associated with the geometry change and reformation of the BS which causes the expansion of downstream and the sunward motion of ions, or (3) the occurrence of magnetic reconnection in the current sheet. In the interaction of the BS with an interplanetary rotational discontinuity (RD), the flow may be deflected sunward by the magnetic tension force associated with the resulting rotational discontinuities and slow shocks in the magnetosheath. It is suggested that the observed anomalous flow events upstream of the bow shock may be due to the BS/TD interaction, and both BS/RD and BS/TD interactions may generate strong sunward flow deflections in the magnetosheath.

Magnetospheric response to solar wind dynamic pressure variations: Interaction of interplanetary tangential discontinuities with the bow shock

Some magnetic impulse events observed in the polar region are related to vortices associated with plasma convection in the ionosphere. Recent analyses of satellite and ground data suggest that the interaction of solar wind dynamic pressure pulses and the magnetosphere may lead to the formation of velocity vortices in the magnetopause boundary layer region. This can in turn lead to the presence of vortices in the polar ionosphere. However, before reaching the Earth's magnetopause, these interplanetary pressure pulses must interact with and pass through the bow shock. A variation of the solar wind dynamic pressure (ΔρV²) may be associated with shocks, magnetic holes, or tangential discontinuities (TDs) in the interplanetary medium. We study the interaction of interplanetary TDs with the Earth's bow shock (BS) using both theoretical analysis and MHD computer simulations. It is found that as a result of the collision between a TD and the BS, the jump in the solar wind dynamic pressure associated with the TD is significantly modified, the bow shock moves, and a new fast shock or fast rarefaction wave, which propagates in the downstream direction, is excited. Our theoretical analysis shows that the change in the plasma density across the interplanetary TD plays the most important role in the collision process. In the case with an enhanced dynamic pressure behind the interplanetary TD, the bow shock is intensified in strength and moves in the earthward direction. The dynamic pressure jump associated with the transmitted TD is generally reduced from the value before the interaction. A fast compressional shock is excited ahead of the transmitted TD and propagates toward the Earth's magnetosphere. For the case in which the dynamic pressure is reduced behind the interplanetary TD, the pressure jump across the transmitted TD is substantially weakened, the bow shock moves in the sunward direction, and a rarefaction wave which propagates downstream is excited. We also simulate and discuss the interaction of a pair of tangential discontinuities, which may correspond to a magnetic hole, with the BS.

The Magnetic Field Structure of the Cometary Plasma Environment

AbstractThe plasma surrounding a comet has the interplanetary magnetic field frozen in. The geometric interpretation of this property is considered. The frozen-in character of the magnetic field leads to the draping of magnetic field lines around the inner coma, where, by exclusion of the inner purely cometary ionosphere, a magnetic cavity is formed inside a region of magnetic field pile-up. The consequences of these physical processes can nicely be diagnosed and tested by interplanetary tangential discontinuities serving as tracers of the magnetoplasma flow. The topology of the magnetic field around the cavity and the shape of the ionopause, as well as the formation of the magnetic tail, are discussed. Particularly in the outer regions, the magnetic field is disturbed by strong magnetic turbulence. This turbulence plays a role in accelerating cometary and also solar wind ions to high energies.

Nonlinear oblique interaction of interplanetary tangential discontinuities with magnetogasdynamic shocks

The nonlinear oblique interaction between a plane tangential discontinuity (TD) and a plane fast magnetogasdynamic shock is treated generally and then applied to the interplanetary TD-bow shock interaction. A number of similarity relations are found for the general problem. For the frequent directional TD's the interaction is strongest if the directional change is 90° and one of the magnetic field vectors is parallel to the shock. Bow shock motions of up to several tens of kilometers per second are induced by these TD's. Tangential discontinuities with density jumps held responsible for geomagnetic sudden impulses lead to a much stronger interaction with the bow shock with nonnegligible strength of the newly generated wave. The oblique interaction requires weaker TD's to produce observed bow shock velocities. It is also shown that complex nonlinear wave patterns very often develop, which have not been predicted in the past in this context.