S. J. Schwartz

Comprehensive study of the magnetospheric response to a hot flow anomaly

We present a comprehensive observational study of the magnetospheric response to an interplanetary magnetic field (IMF) tangential discontinuity, which first struck the postnoon bow shock and magnetopause and then swept past the prenoon bow shock and magnetopause on July 24, 1996. Although unaccompanied by any significant plasma variation, the discontinuity interacted with the bow shock to form a hot flow anomaly (HFA), which was observed by Interball-1 just upstream from the prenoon bow shock. Pressures within and Earthward of the HFA were depressed by an order of magnitude, which allowed the magnetopause to briefly (∼7 min) move outward some 5 RE beyond its nominal position and engulf Interball-1. A timing study employing nearby Interball-1 and Magion-4 observations demonstrates that this motion corresponded to an antisunward and northward moving wave on the magnetopause. The same wave then engulfed Geotail, which was nominally located downstream in the outer dawn magnetosheath. Despite its large amplitude, the wave produced only minor effects in GOES-8 geosynchronous observations near local dawn. Polar Ultraviolet Imager (UVI) observed a sudden brightening of the afternoon aurora, followed by an even more intense transient brightening of the morning aurora. Consistent with this asymmetry, the discontinuity produced only weak near-simultaneous perturbations in high-latitude postnoon ground magnetometers but a transient convection vortex in the prenoon Greenland ground magnetograms. The results of this study indicate that the solar wind interaction with the bow shock is far more dynamic than previously imagined and far more significant to the solar wind-magnetosphere interaction.

Role of the magnetosheath flow in determining the motion of open flux tubes

We have constructed a model which examines the motion of reconnected magnetic flux tubes over the surface of the magnetopause. For a given interplanetary magnetic field (INT) we first determine the draping and strength of the magnetosheath magnetic field, flow velocity, and density over the entire surface of a paraboloid magnetopause. For a given magnetopause location we apply a test for steady state reconnection occurring between the magnetosheath field and a modeled magnetospheric field at that point. We trace the subsequent motion of these tubes along the surface of the magnetopause and into the magnetotail. Results are shown for a range of cases. The model has applications in testing various hypotheses about the location of reconnection events and, for example, IMF B-Y effects. In particular, it highlights the dominance of the magnetosheath flow model in determining the motion of the tubes and the necessity of sub-Alfvenic magnetosheath flows for the occurrence of steady state reconnection poleward of the cusp during periods of northward IMF. Our model may also be used to identify likely reconnection sites on the dayside and near-Earth nightside magnetopause and to identify possible locations for steady state reconnection.

Quasi‐parallel shocks: A patchwork of three‐dimensional structures

Collisionless shocks at quasi-parallel geometries, i.e., for which the average magnetic field direction upstream of the shock is close to the shock normal, reveal temporally varying quantities, a variety of boundary crossing and kinetic signatures, and magnetic structures, often convecting, of finite extent. These results can be put together by a framework in which the shock can be viewed as an extended region containing three-dimensional Short Large Amplitude Magnetic Structures (SLAMS) which represent individual semi-cycles of the ambient upstream low frequency waves associated with diffuse ions in the ULF foreshock. As SLAMS convect with the flow they grow to large amplitudes and entrain inter-SLAMS regions to form an inhomogeneous downstream state. Their finite transverse extent is probably related to, and interacts with, ion beams, to produce a patchy transition zone which accounts for the variety of spacecraft signatures observed.

Conditions for the formation of hot flow anomalies at Earth's bow shock

Hot flow anomalies (HFAs) result from the interaction of an interplanetary current sheet with Earths bow shock and were discovered over a decade and a half ago. The deflected flow and hot interior of an HFA are consequences of ions reflected at the bow shock being channeled along the current sheet. Previous studies have shown that this requires a solar wind motional electric field pointing toward the current sheet on at least one side and that the current sheet must be a tangential discontinuity. Recent reports of a rapid displacement of the magnetopause by 5 Re as the result of an HFA have led us to explore the interplanetary conditions surrounding all reported HFAs. The kinetic aspects of HFA formation suggest that current sheets should pass relatively slowly along the bow shock; that is, their normals should have large cone angles. This hypothesis is confirmed. Individual multispacecraft case studies confirm that the underlying current sheets are tangential discontinuities, but most HFAs have relatively small jumps in field magnitude from before to after and thus would fail traditional identification tests as definite tangential discontinuities. The combination of our results suggests that HFAs should occur at a rate of several per day, and thus they may play a significant role in the solar-terrestrial dynamics.

Hot flow anomalies near the Earth's bow shock

Abstract The discovery in 1984/85 of short intervals (a few minutes) near to the Earths bow shock of very hot plasma flowing roughly perpendicular to the nominal solar wind direction sparked some intrigue as to the underlying cause(es) and local process(es). Typical characteristics of these events, now known as Hot Flow Anomalies (HFAs), include one or both edges of compressed/mildly shocked solar wind, a low and turbulent magnetic field in the extremely hot interior, a flow direction which is organized by local time, and a gross change in interplanetary magnetic field orientation from before to after the events. These latter two properties suggest that HFAs arise from the interaction of an interplanetary current sheet with the bow shock, as subsequently investigated theoretically. A crucial role is played by the dynamics of reflected protons in that the orientation of the magnetic field can lead to either a deficit of particles near the current sheet or a focussing along the current sheet into the upstream region.

Nonstationarity and reformation of high-Mach-number quasiperpendicular shocks : Cluster observations

[1] A set of experimental data is presented for a high-Mach-number (M f = 5) quasiperpendicular (q Bn = 81°) bow shock layer crossed by Cluster spacecraft on 24 January 2001 at 07:05 – 07:09 UT. The measurements of magnetic field, spectra of electric field fluctuations, and ion distributions reveal that the shock is highly nonstationary. In particular, the magnetic field profiles measured aboard different spacecraft differ considerably from each other. The mean frequency of downshifted waves observed upstream of the shock ramp oscillates with a characteristic time comparable with the proton gyroperiod. In addition, the reflection of ions from the shock is bursty and a characteristic time for this process is also comparable with the ion gyroperiod. All of these features in conjunction are the first convincing experimental evidence in favor of the shock front reformation. Citation: Lobzin, V. V., V. V. Krasnoselskikh, J.-M. Bosqued, J.-L. Pincon, S. J. Schwartz, M. Dunlop (2007), Nonstationarity and reformation of high-Mach-number quasiperpendicular shocks: Cluster observations, Geophys. Res. Lett., 34, L05107,

Hybrid simulations of protons strongly accelerated by a parallel collisionless shock

We present initial results from one-dimensional hybrid simulations which directly address the problems of using such methods to simulate the acceleration of ions to high energy by parallel shocks. As particles are accelerated from the thermal population, they are repeatedly “split,” thereby ensuring statistically valid energy spectra covering a wide dynamic range. In order to model the complex foreshock, as expected if the simulation domain were large enough, and as seen in observations, we introduce a source of upstream turbulence. This turbulence produces an enhanced high energy tail in the upstream particle distribution extending to over a hundred times the plasma flow energy, and a prominent shoulder downstream.

Determination of dispersion relations in quasi-stationary plasma turbulence using dual satellite data

The joint frequency-wavenumber spectrum is one of the basic quantities for analyzing plasma turbulence. It is shown how the full spectrum can be recovered from wavefields measured by two or more satellites via spectral methods based on wavelet transforms. Compared to standard cross-correlation techniques, different branches in the dispersion relation can be resolved and quasi-stationary wavefields can be accessed. Using this new approach, low frequency magnetic field data from the AMPTE-UKS and AMPTE-IRM spacecraft are investigated and the impact of nonlinear processes on wave propagation at the Earths foreshock is revealed.

Electron anisotropy constraint in the magnetosheath: Cluster observations

[1] The whistler anisotropy instability is driven by the condition T⊥ e /T∥ e > 1, where the subscript e denotes electrons and the other subscripts denote directions relative to the background magnetic field B o . Instability growth leads to enhanced field fluctuations which scatter the electrons; theory and simulations show that this scattering imposes an upper bound on the electron anisotropy in the form T⊥ e /T∥ e -1 = S e /β α e∥ e with fitting parameters 0.1? S e ? 1 and 0.5? α e < 0.7 over 0.10 ≤ β∥ e < 1.0 where β∥ e = 8πn e T∥ e /B 2 o . Here measurements from the PEACE instrument on the Cluster 1 spacecraft show that electron anisotropies in two crossings of the dayside terrestrial magnetosheath are constrained statistically by this equation with S e ≃ 0.2 and α e ≃ 0.6. This is the first reported observation of this constraint in a space plasma. Citation: Gary, S. P., B. Lavraud, M. F. Thomsen, B. Lefebvre, and S. J. Schwartz (2005), Electron anisotropy constraint in the magnetosheath: Cluster observations.

Electron temperature effects in the linear proton mirror instability

It is shown that when the electron temperature Te is of the same order of the proton temperature parallel to the background magnetic field, the growth rate of the proton mirror mode in the long-wavelength limit is reduced by the presence of a longitudinal electric field. The field is due to the electron pressure gradient which builds up when Te ≠ 0, because the electrons are dragged by nonresonant protons which are mirror accelerated from regions of high into regions of low parallel magnetic field flux. In return, the longitudinal electric field causes the density of nonresonant protons with a perpendicular velocity smaller than a strongly Te-dependent critical velocity υ⊥,crit to increase (decrease) at maxima (minima) of the parallel magnetic field flux. These nonresonant protons thus behave differently from the “circulating” protons described by Southwood and Kivelson (1993) in the cold electron limit. Although the instability threshold is only weakly affected by changes in Te, quantities like the growth rate, the compressibility, the polarization, and the angle between wave vector and magnetic field for the most unstable mode, as well as the structure of the perturbed proton distribution itself, are strongly modified by variations in the electron temperature. The predictions of the model are shown to agree well with numerical solutions of the full Vlasov dispersion relation, indicating that most long-wavelength aspects of the proton mirror instability are included in the model.