[1] We present a comprehensive survey of 125 small-and intermediate-sized interplanetary magnetic flux ropes during solar cycle 23 (1995-2005) using Wind in situ observations near 1 AU. As a result, we found the following: (1) The annual number of small-and intermediate-sized interplanetary magnetic flux ropes is not very sensitive to the solar cycle, but its trend is very similar to that of magnetic clouds (MCs).

[1] To study MHD shocks in space, it is important to find the shock frame of reference from the observed plasma and magnetic field parameters. These shock parameters have to satisfy the Rankine-Hugoniot relations. In this study we present a novel procedure for shock fitting of the one-fluid anisotropic Rankine-Hugoniot relations and of the time difference between two spacecraft observations in the case of small He 2+ slippage. Here, a Monte-Carlo calculation and a minimization technique are used. The observed variables including the upstream and downstream magnetic fields, plasma densities, plasma betas, plasma anisotropies, W (the difference between the downstream and upstream velocities, W V 2 À V 1 ), and Dt (the time difference between two spacecraft observations) are used in our procedure where V is defined as the center of mass velocity of plasmas. A loss function based on a difference between the calculated and the observed values is defined, and the best fit solution is found by searching for the minimum loss function value. For shocks that cannot be fitted well, we introduce two new parameters in the modified RH relations, one in the normal momentum flux and the other in the energy flux equations. These two parameters are interpreted as the equivalent ''normal momentum'' and ''heat'' fluxes needed in the RH relations. They provide two degrees of freedom in the system, and their amounts can be estimated from our procedure. Several synthetic shocks are given to verify our procedure. We also apply this procedure to two interplanetary shocks observed by both the WIND and Geotail spacecraft. The results demonstrate that our method works for both the synthetic and the real shocks. We have shown that our method can provide accurate shock normal estimations for perpendicular and parallel shocks as well. Given that our model is based on the RH relations that do not include the effect of alpha particle (He 2+ ) slippage, it can only be applied to the cases with an ignorable slippage pressure tensor. We have investigated the pressure tensor due to alpha particle slippage using the WIND spacecraft data. It is found that in general the slippage pressure is small in comparison with the thermal pressure of the system and can be ignored. Thus our model can be applied to most interplanetary shocks observed near the ecliptic plane. However, when the slippage pressure is large, the magnetic coplanarity theorem is not valid any more. A more general model that involves slippage pressure tensor is a major and important development that is beyond the scope of the present study.

Ion reflection, ion leakage, ion heating and shock‐front reformation are studied based on the simulation results of a supercritical quasi‐parallel shock. The backstreaming ions upstream of the simulated shock are predominantly leakage ions. The leakage of downstream ions is regulated by the large‐amplitude waves in the shock transition region. These waves not only can act as a filter to reduce the leakage ion number density, but also can energize the backstreaming ions to result in the suprathermal upstream ions. The average temperature of the leakage ions is about 100 times that of the upstream incoming ions. The average leakage ion density is about 2% ∼ 3% of the upstream ion density. Ion reflections occur intermittently due to coherent ion scattering by the large‐amplitude whistler waves near the shock front. Most of the coherently reflected ions are subsequently scattered back toward the downstream to contribute to the ion heating. Only a small fraction of the reflected ions can escape upstream with an average number density about 0.1% ∼ 0.2% of the upstream ion density. The shock‐front appears to reform as characterized by the decay and reappearance of the large‐amplitude whistler waves near the shock ramp. We wish to propose a possible explanation for the shock‐front reformation process. Insufficient dissipation in the supercritical shock can lead to a highly‐steepened shock ramp, which in turn can emit large‐amplitude whistler waves to result in the ion reflection event. Since ion reflections can provide additional dissipation, to counter balance the nonlinear steepening and thereby reduce the wave amplitude and turn off the ion reflection event. The cycle of under‐dissipation followed by over‐dissipation is proposed to be a possible cause for the shock‐front reformation process.

[1] Kelvin-Helmholtz (K-H) instability at a magnetohydrodynamic (MHD) tangential discontinuity (TD) is studied by means of two-dimensional MHD simulation. The TD is of finite thickness with both magnetic shear and velocity shear across the TD. Our simulation results indicate that the nonlinear evolution of MHD surface waves at the TD depends on the fast-mode Mach numbers of the plasma flows on two sides of the TD in the surface wave rest frame. When the fast-mode Mach numbers on both sides of the TD are less than 1, the K-H instability can grow into vortices or kink-type surface waves, depending on the orientation of the ambient magnetic field and the plasma beta. When the fast-mode Mach number on either side of the TD is greater than 1, nonlinear fast-mode plane waves are developed from the ridges of the surface waves and extended distance from the TD. A theoretical model based on the fast magnetosonic Mach cone formation is proposed to explain the formation of these nonlinear plane waves. The Mach angle of the fast magnetosonic Mach cone as a function of Mach number, orientation of ambient magnetic field, and plasma beta is derived. The flaring angles of these nonlinear plane waves measured from our simulation results are in good agreement with the Mach angles predicted by the theoretical model. Applications of our results to the Earth's magnetopause and to the solar wind are also discussed.

An interplanetary intermediate shock is identified from the bulk velocity, number density, and temperature of the solar wind protons and tl•e three components of the interplanetary magnetic field observed by Voyager 1 on May 1 (day 122), 1980, when the spacecraft was at a distance of about 9 AU froln the Sun. It is shown by a best fit procedure that the ineasured plasma and magnetic field on both sides of the discontinuity satisfy the Rankine-Hugoniot relations for a magnetohydrodynamic (MHD) intermediate shock.This shock satisfies the following conditions. (1) The normal Alfvtn-Mach nmnber (M^= Vn*/V ^ ) is greater than unity in the preshock state and less than unity in the postshock state. (2) Both the fast-mode Mach number (Mf= Vn*/Vf) in the preshock state and the slow-mode Mach number (Ms• = Vn*/Vs• ) in the postshock state are less than unity, but the slowqnode Mach nmnber is greater than unity in the preshock state. (3) The projected co•nponents of the magnetic fields in the shock front for the pre-and postshock states have opposite signs. (4) The magnitudes of the magnetic fields decrease from the preshock to the ptstshock states. In the above expression, V^ is the Alfvtn speed based on the inagnetic field component normal to the shock front, Vn* is the component of the bulk velocity normal to the shock front and measured in the shock frame of reference, and Vf and Vs• are the speeds of the fast-and slow-mode magnetosonic waves in the direction of the shock normal, respectively. The discontinuity event in our discussion cannot be a rotational discontinuity because the Walen's relation is not satisfied. The identified intermediate, shock has M A =1.04, 0nn =37 ø, and [• =0.56. where 0BniS the angle between the preshock magnetic field and the shock normal direction and [• is the ratio of thermal to inagnetic energy densities. Using these parameters, a numerical solution of the MHD equations for the shock is obtained. The simulated profiles of the bulk velocity, number density, temperature, and xnagnetic fields of the pre-and postshock states agree with those of the observed values. The same parameters are used to siinulate an intermediate shock using a hybrid numerical code in which full ion dynamics is retained while electron inertial force is neglected. The results of this simulation are coinpared with high-resolution magnetic field data with a time resolution of 1.92-s averages. The shock thickness of about 70 c/(Opi predicted from the hybrid code agrees with the observations. The general behavior of the magnetic field in the shock transition region is also very sinfilar for the simulated and observed results. The macro-and nficrostructures of the intermediate shock obtained from the MHD and hybrid models reseinble the observed structures. 17,443 17,444 CIqAO ET AL.' OBSERVATIONS OF AN INTERMEDIATE SHOCK