It is shown that the interplanetary magnetic field has different characteristics on different scales, and it is noted that a given physical theory may not be applicable or relevant on all scales. Four scales are defined in terms of time intervals on which the data may be viewed. Many discontinuities in the magnetic-field direction are seen on the mesoscale (≈ 4 days, ≈ 1 AU). The characteristics of such directional discontinuities which were observed by Pioneer 6 during the period December 16, 1965-January 4, 1966 are presented, with special emphasis on their distribution in time. Previously, it was suggested that such discontinuities are simply boundaries of spaghetti-like filaments extending from the sun to the earth. Here it is shown that on the mesoscale unique filaments with sharp boundaries containing well-ordered magnetic fields are not always seen although discontinuities are always present at ≈ 1 AU. Thus, the interplanetary medium appears to be discontinuous rather than filamentary. The filamentary model implies that discontinuities originate at the sun and are convected with the solar wind. The discontinuous model allows the additional possibility that the discontinuities form in the interplanetary medium far from the sun.

Directional discontinuities in the interplanetary magnetic field.

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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JA01989

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

[1] The global BATS-R-US MHD code is used to simulate the interaction of a moderately strong interplanetary shock with the Earth's magnetosphere. The model predicts the propagation of a transmitted fast shock through the magnetosheath and magnetosphere and the reflection of this shock from the inner numerical boundary. The reflected fast shock propagates sunward through the dayside magnetosphere and magnetosheath. The passage of the transmitted shock causes the bow shock and magnetopause to move inward, while the passage of the reflected fast shock causes these boundaries to move outward, consistent with previously reported in situ observations. A supplementary study employing a one-dimensional MHD model addresses the interaction of the forward fast shock with the plasmapause. This study demonstrates that most of the energy associated with the fast shock energy penetrates into the plasmasphere. Consequently, the transmitted fast wave must reach the ionosphere, produce the well known sudden impulse signatures, and then be reflected due to the shielding effects of ionospheric currents. The predictions of the numerical simulations are consistent with observations of the dayside geosynchronous magnetic field.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007JA012627

MHD simulation for the interaction of an interplanetary shock with the Earth's magnetosphere

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

One-dimensional resistive MHD and hybrid simulations are carried out to study the manner by which variations of the interplanetary magnetic field (IMF) direction generate dynamic pressure pulses in the magnetosheath. The reaction of the magnetosheath to the temporal IMF variation is modeled as the interaction between the bow shock (BS) and an interplanetary rotational discontinuity (RD), an Alfven wave pulse (AW), or an Alfven wave train. The resistive MHD simulation indicates that the arrival of an RD produces two time-dependent intermediate shocks (TDISs) and two slow shocks downstream of the bow shock, which propagate through the magnetosheath toward the Earth's magnetopause. An enhancement of plasma density is present throughout the TDISs and slow shocks. A plasma dynamic pressure pulse is formed in this region. In the hybrid simulation, the two TDISs are replaced by rotational discontinuities. For a bow shock with a shock normal angle θBn > 45°, the pulse in the dynamic pressure ρV2 causes the total pressure (P + B2/2μ0 + ρV2) in the magnetosheath to increases by about 0–100% of the background value. The strength of the pressure pulse increases with the field rotation angle across the incident rotational discontinuity, while it decreases with the Mach number or upstream plasma beta of the bow shock. The pressure pulse propagates toward the magnetopause with nearly a constant amplitude. On the other hand, the BS/AW interaction leads to the generation of Alfven waves downstream of the bow shock, and large-amplitude dynamic pressure pulses are generated in the downstream Alfven wave. Pressure pulses impinging on the magnetopause may produce magnetic impulse events (MIEs) observed in the high-latitude ionosphere.

Generation of dynamic pressure pulses downstream of the bow shock by variations in the interplanetary magnetic field orientation

The magnetohydrodynamic (MHD) aspects of the interaction between the Earth's bow shock and interplanetary shocks or rotational discontinuities are systematically studied in this paper by MHD simulations. The interplanetary shocks under consideration include forward fast shocks, reverse fast shocks, forward slow shocks, and reverse slow shocks. As an incident forward shock transmits through the bow shock, a fast shock, a slow expansion wave, a slow shock, and a contact discontinuity are generated downstream of the bow shock. At the same time, the bow shock is modified and moves earthward. If the incident shock is a reverse shock, the generated fast shock becomes a fast expansion wave, and the bow shock moves away from the Earth. The generated fast shock or fast expansion wave carries most of the total pressure variation imposed by the incident shock. The contact discontinuity also carries a significant part of the pressure variation. The slow expansion wave and the slow shock are both generated with a small variation in plasma density and pressure but a large variation in magnetic field. When the solar wind Alfven Mach number is small, the density variations associated with the slow shock or slow expansion wave can be significant. Through the interaction between an incident rotational discontinuity and the bow shock, a plateau in the plasma density and thermal pressure is formed. The magnetic pressure is depressed accordingly and anticorrelated with the thermal pressure. If the incident rotational discontinuity propagates toward the Earth (Sun) in the solar wind frame, the leading (trailing) edge of the plateau consists of an intermediate shock or a time-dependent intermediate shock and a slow shock, while the trailing (leading) edge is mainly a slow shock. The generated structure with enhanced plasma density and thermal pressure and a depressed magnetic pressure agrees very well with the observed slow-mode structure in the magnetosheath. Strong dynamic pressure variations associated with this structure may impinge on the magnetopause as a strong pressure pulse.

M. Yan

Papers

Interaction of interplanetary shocks and rotational discontinuities with the Earth's bow shock