L. H. Lyu

Interplanetary small‐ and intermediate‐sized magnetic flux ropes during 1995–2005

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). (2) Average speeds of the individual small- and intermediate-sized interplanetary magnetic flux ropes varied from 289 to 790 km/s with a mean value of 420 +/- 86 km/s. Most small- and intermediate-sized interplanetary magnetic flux ropes were found to have a propagation speed similar to typical slow speed solar wind speed, and only a few small- and intermediate-sized interplanetary magnetic flux ropes had speeds comparable to the typically high speed solar wind. (3) Average magnetic field strength for small- and intermediate-sized interplanetary magnetic flux ropes is less than the average magnetic field strengths of MCs, while it is larger than that of background solar wind. (4) The distributions of the axial orientations for small- and intermediate-sized interplanetary magnetic flux ropes are also similar to that of MCs. The results show that small- and intermediate-sized interplanetary magnetic flux ropes and MCs have many similar (or relative) characters. So we suggest that both MCs and small- and intermediate-sized interplanetary magnetic flux ropes originate from solar eruptions.

Observations of an intermediate shock in interplanetary space

An interplanetary intermediate shock is identified from the bulk velocity, number density, and temperature of the solar wind protons and the 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 from the Sun. It is shown by a best fit procedure that the measured 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 Alfven-Mach number (MA = Vn*/VA) 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 (Msl = Vn*/Vsl) in the postshock state are less man unity, but the slow-mode Mach number is greater than unity in the preshock state. (3) The projected components 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 postshock states. In the above expression, VA is the Alfven speed based on the magnetic 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 Vsl 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 Walens relation is not satisfied. The identified intermediate, shock has MA = 1.04, θBn = 37°, and β = 0.56, where θBn is the angle between the preshock magnetic field and the shock normal direction and β is the ratio of thermal to magnetic 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 magnetic fields of the pre-and postshock states agree with those of the observed values. The same parameters are used to simulate 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 compared with high-resolution magnetic field data with a time resolution of 1.92-s averages. The shock thickness of about 70 c/ωpi predicted from the hybrid code agrees with the observations. The general behavior of the magnetic field in the shock transition region is also very similar for the simulated and observed results. The macro- and microstructures of the intermediate shock obtained from the MHD and hybrid models resemble the observed structures.

Simulation of Electrostatic Noise Amplification in Gyrotrons

Electrostatic noise generated by the electrostatic cyclotron instability is simulated in slab geometry with dc space charge effects neglected, The noise is found to be characterized by a broad spectrum, low saturation levels, and a strong disturbing effect on the electron energy distribution.

Quasi-parallel collisionless shocks

The magnetic field and plasma data from the ISEE 1, 2, and 3 spacecraft have greatly increased our knowledge of the quasi-parallel collisionless shock in space. Hybrid-code simulations have provided us with valuable insights into the physics of the quasi-parallel shock. Unfortunately, theoretical understanding of the nonlinear physics of the quasi-parallel shock is still in a qualitative stage of development. Generation of large-amplitude whistler waves and hydromagnetic waves observed in the quasi-parallel shock has been discussed either in terms of linear instabilities or qualitative nonlinear arguments. It appears that the ion reflection, ion heating, and leakage of the shock-heated downstream ions at the quasi-parallel shock can all be explained in terms of nonadiabatic scatterings of ions by the large-amplitude whistler-magnetosonic waves with frequencies near the ion gyrofrequency and wavelength near the ion inertial length. The nonadiabatic scattering is defined by the non-conservation of the magnetic moment. Future study of the quasi-parallel shock should focus on developing quantitative theoretical models for the nonlinear physical processes fundamental to the quasi-parallel shock.

Ionospheric signatures of patchy‐intermittent reconnection at dayside magnetopause

Ionospheric signatures predicted by the patchy-intermittent (P-I) dayside reconnection model during southward interplanetary magnetic field are discussed. The P-I reconnection can lead to spiky convection electric fields. The spiky convection electric fields in turn drives enhanced convection channels on closed field lines in the dayside ionosphere. The observed convection speed in the enhanced convection channel ranges from ∼0.5 to ∼2 km/s. The P-I reconnection model also predicts that the dayside auroral forms should move noonward along enhanced convection channels. Poleward shift of dayside auroral forms occurs as the enhanced convection channels shift poleward due to reconfiguration of magnetic field following enhanced dayside reconnection. Multiple brightenings of dayside auroral forms should occur on a timescale of ∼2 to 4 min, equal to twice the Alfven bounce period between the ionosphere and the equatorial plane. Patchy reconnection is intrinsically intermittent due to the rereconnection of elbow-shaped open flux tubes. The resulting reclosed flux tubes should contain a mixture of magnetosheath-magnetospheric plasmas as observed in the low-latitude boundary layer.

Ion dynamics in high-mach-number quasi-parallel shocks

Ion heating in high-Mach-number (MA ≥ 3) quasi-parallel shocks is accomplished by ion reflections at the shock ramp and nonadiabatic pitch angle scattering through nonlinear waves in the shock transition region. Two types of ion reflection events are observed in high-Mach-number shock simulations: (1) the reflecting-reentering event and (2) the backstreaming-gyrating event. The behavior of ion reflection depends on the orientation of the ramp field relative to the upstream field. A gyroreflection model is proposed to provide a physical explanation for the two types of ion reflection events observed in the high-Mach-number shocks. The rotation of the ramp field with time leads to alternative occurrence of the two types of ion reflection events, which in turn leads to shock front reformation in the high-Mach-number quasi-parallel shock. The time scale for the shock front reformation is estimated to be about 1 to 2 upstream ion gyroperiods, which is about 10 ion gyroperiods for ions experiencing overshot magnetic field in the shock ramp.

Are all leading shocks driven by magnetic clouds

Magnetic clouds (MCs) are commonly observed in association with shocks at 1 AU, and many authors have claimed that the leading shocks are driven by MCs, without any direct evidence. In this work we surveyed the relations between MCs and their associated shocks. Using the interplanetary plasma and magnetic field data measured by Wind, we have identified 97 MCs near Earth during 1995 to 2007. Sixty-two (64%) of the MCs were associated with leading shocks. As Lepping et al. have pointed out, if a leading shock is driven by an MC, the axis of the driver should be approximately perpendicular to the shock normal. We calculated the angle theta between the axis of the MC and its leading shock normal and found that the angle theta for 21 of the 62 MCs deviates from 90 degrees by more than 25 degrees. Seventeen of the 21 MCs also have the following signatures: (1) The MC leading edge moves more slowly than the preceding shock; and (2) the time interval between the preceding shock and the front boundary of the MC is always long, with an average period of 15.4 h. The speed profiles of the other four events revealed that the leading shock was driven not directly by the MC but by the immediately subsequent flow, and their sheath durations are shorter than the time interval between the preceding shocks and the MC front boundaries. Therefore, at least 21 (34%) of 62 leading shocks were not directly driven by MCs.

Two‐spacecraft observations of an interplanetary slow shock

Slow-mode shocks in space have been identified via one spacecraft observation by many authors since 1970. However, Feng et al. suggested that the analysis using single-spacecraft observation based only on the Rankine-Hugoniot relations could misinterpret a tangential discontinuity as a slow shock. In this paper, we identify a slow-mode shock using a model fitting based on both the Rankine-Hugoniot relations and intraspacecraft timing. As a result, we found a slow-shock solution that, including the local parameters and propagation time, agrees well with the observations. The slow shock may be associated with an asymmetric reconnection exhaust at a heliospheric current sheet.

From Rankine‐Hugoniot relation fitting procedure: Tangential discontinuity or intermediate/slow shock?

To identify an observed intermediate/slow shock, it is important to fit the measured magnetic fields and plasma on both sides using Rankine-Hugoniot (R-H) relations. It is not reliable to determine an intermediate/slow shock only by the shock properties and fitting procedure based on one spacecraft observation, though previous reported intermediate/slow shocks are confirmed in such a way. We investigated two shock-like discontinuities, which satisfy the R-H relations well. One meets the criterions of slow shocks and was reported as a slow shock, and another has all the characters of intermediate shock based on one spacecraft observation. However, both discontinuities also meet the requirements of tangential discontinuities and were confirmed as tangential discontinuities on large-scale perspective by using multi-spacecraft observations. We suggest that intermediate/slow shocks should be identified as carefully as possible and had better be determined by multi-spacecraft.

Observations of an interplanetary switch-on shock driven by a magnetic cloud

A possible interplanetary switch-on shock event prior to a trailing magnetic cloud was observed on August 1, 2002 at 1 AU. We fit the data with the Rankine-Hugoniot (R-H) relations based on both oblique and switch-on shock models. It is found that both models are consistent with the observed data, and the best fit solutions of the two models are close to one another. For the oblique shock model, the best fit upstream shock normal angle, theta(BN1) (= cos(-1) (B(t1)/B(t1))), is as small as 5.55 degrees. The shock has the following characteristics: (1) plasma density, plasma temperature, and the magnetic field strength all increase across the shock, (2) protons are thermalized very efficiently across the shock, but it is not the case for electrons, (3) the fast-mode Mach number is greater than unity in the preshock region and less than unity in the postshock region, and (4) from the oblique shock model we find that the normal Alfven Mach number is very close to unity in the postshock region, while from the switch-on shock model we obtain a solution of unity normal Alfven Mach number. Our results clearly demonstrate the MHD character of a fast shock propagating along the ambient magnetic field. Citation: Feng, H. Q., C. C. Lin, J. K. Chao, D. J. Wu, L. H. Lyu, and L. C. Lee (2009), Observations of an interplanetary switch- on shock driven by a magnetic cloud, Geophys. Res. Lett., 36, L07106, doi: 10.1029/2009GL037354.