R. B. Torbert

University of New Hampshire, Дарем, New Hampshire, United States

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Publications (233)372.36 Total impact

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    ABSTRACT: Based on particle-in-cell simulations of collisionless magnetic reconnection, the spatiotemporal evolution of electron velocity distributions in the electron diffusion region (EDR) is reported to illustrate how electrons are accelerated and heated. Approximately when the reconnection rate maximizes, electron distributions in the vicinity of the X-line exhibit triangular structures with discrete striations and a temperature (Te) twice that of the inflow region. Te increases as the meandering EDR populations mix with inflowing electrons. As the distance from the X-line increases within the electron outflow jet, the discrete populations swirl into arcs and gyrotropize by the end of the jet with Te about three times that of the X-line. Two dominant processes increase Te and produce the spatially and temporally evolving EDR distributions: (1) electric field acceleration preferential to electrons which meander in the EDR for longer times, and (2) cyclotron turning by the magnetic field normal to the reconnection layer.
    03/2015; 42(8). DOI:10.1002/2015GL063601
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    ABSTRACT: We examine several Cluster and Polar crossings of the magnetopause at high latitudes poleward of the cusp under strongly northward interplanetary magnetic field (IMF) in the years 2001-2008. In this effort, we only study crossings under IMFs whose clock angle is less than 45 degrees for a continuous interval of over 2 hours. As shown by several numerical simulations, theoretical analyses, and a few in-situ observations during recent years, asymmetries in plasma density, magnetic field and flow give rise to major features which are significantly different from those observed in a collisionless diffusion region/reconnection layer when conditions are symmetric. Our study of this specific region, i.e. poleward of the cusp, contains examples of a wide dynamic range of values of these physical parameters. Specifically, we focus on counter-streaming flows and the effects of density asymmetries, ranging up to 2 orders of magnitude and a wide spectrum of guide field values on the structure of the diffusion region.
    AGU Fall Meeting, San Francisco, CA; 12/2014
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    ABSTRACT: We present Polar observations of a reconnection layer during an inbound pass at high northern latitudes. The interplanetary field of 20 nT pointed strongly northward continuously for 13 hours. Reverse polar cap convection observed repeatedly by the DMSP F13 satellite provided direct evidence of continued reconnection. Polar observed sunward and southward jets. The event was hallmarked by a density asymmetry 140 and moderate guide field. Disturbances in fields and plasma were much more intense on the magnetosphere (MSP) side of the current sheet (CS). A density cavity was observed at both separatrices. Isolated EN peaks occurred at the density cavity regions. The intense electric field fluctuations (≤60 mV/m) were mainly in the component normal to the CS, EN. The guide field pointed opposite to the Hall field, leading to an overall weakening of the out-of-plane magnetic field. A magnetic island was observed in the outflow jet. The field reversal at the CS occurred before the outflow jet, which we argue to be due to the large density asymmetry. The stagnation line was strongly shifted towards the MSP side of the CS. We compare observations with simulations which emphasize the density asymmetry [Tanaka et al.,2008] and which also include a guide field [Pritchett and Mozer, 2009] and we find good agreement. Remaining discrepancies may be explained by a density asymmetry much larger than in simulations. This is to our knowledge the first study of a high latitude reconnection layer with (1) an extreme density asymmetry, (2) steady and continuously strong interplanetary Bz.
    Journal of Geophysical Research Atmospheres 09/2014; 119(9):7343. DOI:10.1002/2014JA019879 · 3.44 Impact Factor
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    ABSTRACT: Results from two-dimensional particle-in-cell simulations of collisonless magnetic reconnection with zero guide field discussed in this paper reveal that around the time when the reconnection rate peaks, electron velocity distributions become highly structured in magnetic islands and open exhausts. Rings, arcs, and counter-streaming beams are generic and lasting components of the exhaust electron distributions. The temporal dependence of electron distributions provides a perspective to explain an outstanding discrepancy concerning the degree of electron anisotropy in reconnection exhausts, and enables inference of the reconnection phase based on observed anisotropic electron distributions. Some of the structures predicted by our simulations are confirmed by measurements from the Cluster spacecraft during its encounter with reconnection exhausts in the magnetotail.
    08/2014; 41(15). DOI:10.1002/2014GL060608
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    ABSTRACT: We report the wave observations, associated plasma measurements, and linear theory testing of electromagnetic ion cyclotron (EMIC) wave events observed by the Van Allen Probes on 28 April 2013. The wave events are detected in their generation regions as three individual events in two consecutive orbits of Van Allen Probe-A, while the other spacecraft, B, does not detect any significant EMIC wave activity during this period. Three overlapping H+ populations are observed around the plasmapause when the waves are excited. The difference between the observational EMIC wave growth parameter (Σh) and the theoretical EMIC instability parameter (Sh) is significantly raised, on average, to 0.10 ± 0.01, 0.15 ± 0.02, and 0.07 ± 0.02 during the three wave events, respectively. On Van Allen Probe-B, this difference never exceeds 0. Compared to linear theory (Σh > Sh), the waves are only excited for elevated thresholds.
    06/2014; 41(12). DOI:10.1002/2014GL060621
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    ABSTRACT: We present an example of a boundary layer tailward of the dawn terminator which is entirely populated by rolled-up flow vortices. Observations were made by Wind on October 24, 2001 as the spacecraft moved across the region at X ~ −13 RE. Interplanetary conditions were steady with a near-radial IMF. Approximately 15 vortices were observed over the 1.5 hr duration of Wind's crossing, each lasting ~5 min. The rolling-up is inferred from the presence of a hot tenuous plasma being accelerated to speeds higher than in the adjoining magnetosheath, a circumstance which has been shown to be a reliable signature of this in single-spacecraft observations [Takagi et al., 2006]. A blob of cold dense plasma was entrained in each vortex, at whose leading edge abrupt polarity changes of field and velocity components at current sheets were regularly observed. In the frame of the average boundary layer velocity, the dense blobs were moving predominantly sunward and their scale size along X was ~ 7.4 RE. Inquiring into the generation mechanism of the vortices, we analyze the stability of the boundary layer to sheared flows using compressible magnetohydrodynamic Kelvin–Helmholtz theory with continuous profiles for the physical quantities. We input parameters from (i) the exact theory of magnetosheath flow under aligned solar wind field and flow vectors [Spreiter and Rizzi, 1974] near the terminator, and (ii) the Wind data. It is shown that the configuration isindeed KH unstable. This is the first reported example of KH-unstable waves at the magnetopause under a radial IMF.
    Journal of Geophysical Research: Space Physics 06/2014; 119(6):4572. DOI:10.1002/2013JA019578 · 3.44 Impact Factor
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    ABSTRACT: We present in situ observations of Pc1 pearl pulsations using the Van Allen Probes. These waves are often observed using ground-based magnetometers, but are rarely observed by orbiting satellites. With the Van Allen Probes, we have seen at least 14 different pearl pulsation events during the first year of operations. These new in situ measurements allow us to identify the wave classification based on local magnetic field conditions. Additionally, by using two spacecraft, we are able to observe temporal changes in the region of observation. The waves appear to be generated at an overall central frequency, as often observed on the ground, and change polarization from left- to right-handedness as they propagate into a region where they are resonant with the crossover frequency (where R- and L-mode waves have the same phase velocity). By combining both in situ and ground-based data, we have found that the region satisfying electromagnetic ion cyclotron wave generation conditions is azimuthally large while radially narrow. The observation of a similar modulation period on the ground as in the magnetosphere contradicts the bouncing wave packet mechanism of generation.
    03/2014; 41(6). DOI:10.1002/2013GL059187
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    ABSTRACT: We present a comprehensive statistical analysis of small solar wind transients (STs) in 2007–2009. Extending work on STs by Kilpua et al. (2009) to a 3 year period, we arrive at the following identification criteria: (i) a duration < 12 h, (ii) a low proton temperature and/or a low proton beta, and (iii) enhanced field strength relative to the 3 year average. In addition, it must have at least one of the following: (a) decreased magnetic field variability, (b) large, coherent rotation of the field vector, (c) low Alfvén Mach number, and (d) Te/Tp higher than the 3 year average. These criteria include magnetic flux ropes. We searched for STs using Wind and STEREO data. We exclude Alfvénic fluctuations. Case studies illustrate features of these configurations. In total, we find 126 examples, ~81% of which lie in the slow solar wind (≤ 450 km/s). Many start or end with sharp field and flow gradients/discontinuities. Year 2009 had the largest number of STs. The average ST duration is ~4.3 h, 75% < 6 h. Comparing with interplanetary coronal mass ejections (ICMEs) in the same solar minimum, we find the major difference to be that Tp in STs is not significantly less than the expected Tp. Thus, whereas a low Tp is generally considered a very reliable signature of ICMEs, it is not a robust signature of STs. Finally, since plasma β ~ 1, force-free modeling of STs having a magnetic flux rope geometry may be inappropriate.
    Journal of Geophysical Research Atmospheres 02/2014; 119(2):689-708. DOI:10.1002/2013JA019115 · 3.44 Impact Factor
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    ABSTRACT: [1] We apply a semi-analytic magnetohydrodynamic approach to describe effects in the nightside magnetosheath related to accelerated magnetosheath flows caused by the draping of interplanetary magnetic field (IMF). Assuming a northward IMF direction, we show the development of slow mode fronts in the far tail (tailward of ∼ -60 RE). We find that accelerated flows north and south of the equator start to converge towards lower latitudes. The ensuing plasma compression gives rise to slow mode waves in the equatorial region which, further down the tail, evolve into slow mode shocks. These fronts propagating along the magnetic field lines are characterized by sharp increases of plasma density, pressure and temperature and a decrease in the magnetic field strength. The magnetic pressure exhibits an anti-correlation with the plasma pressure, but the total pressure is fairly constant across the fronts. The field–aligned plasma velocity component anti-correlates with the plasma density, while the perpendicular velocity component does not have sharp variations at the fronts. For northward IMF, these fronts appear near the equatorial region and then propagate to higher latitudes. This effect is not very sensitive to the particular shape of the magnetopause. Lowering the upstream Alfvén Mach number increases the strength of the slow mode waves, which also develop closer to Earth. We predict that this effect can be observed by space probes skimming the far tail.
    Journal of Geophysical Research: Space Physics 02/2014; 119(2). DOI:10.1002/2013JA019514 · 3.44 Impact Factor
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    ABSTRACT: We compare the magnetic field data obtained from the flux-gate magnetometer (FGM) and the magnetic field data deduced from the gyration time of electrons measured by the electron drift instrument (EDI) onboard Cluster to determine the spin-axis offset of the FGM measurements. Data are used from orbits with their apogees in the magnetotail, when the magnetic field magnitude was between about 20 and 500 nT. Offset determination with the EDI–FGM comparison method is of particular interest for these orbits, because no data from solar wind are available in such orbits to apply the usual calibration methods using the Alfvén waves. In this paper, we examine the effects of the different measurement conditions, such as direction of the magnetic field relative to the spin plane and field magnitude in determining the FGM spin-axis offset, and also take into account the time-of-flight offset of the EDI measurements. It is shown that the method works best when the magnetic field magnitude is less than about 128 nT and when the magnetic field is aligned near the spin-axis direction. A remaining spin-axis offset of about 0.4 � 0.6 nT was observed for Cluster 1 between July and October 2003. Using multipoint multi-instrument measurements by Cluster we further demonstrate the importance of the accurate determination of the spin-axis offset when estimating the magnetic field gradient.
    01/2014; 3(1):1-11. DOI:10.5194/gi-3-1-2014
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    Space Science Reviews 01/2014; DOI:10.1007/s11214-014-0057-3 · 5.87 Impact Factor
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    ABSTRACT: We compare the magnetic field data obtained from the Flux-Gate Magnetometer (FGM) and the magnetic field data deduced from the gyration time of electrons measured by the Electron Drift Instrument (EDI) onboard Cluster to determine the spin axis offset of the FGM measurements. Data are used from orbits with their apogees in the magnetotail, when the magnetic field magnitude was between about 20 nT and 500 nT. Offset determination with the EDI-FGM comparison method is of particular interest for these orbits, because no data from solar wind are available in such orbits to apply the usual calibration methods using the Alfvén waves. In this paper, we examine the effects of the different measurement conditions, such as direction of the magnetic field relative to the spin plane and field magnitude in determining the FGM spin-axis offset, and also take into account the time-of-flight offset of the EDI measurements. It is shown that the method works best when the magnetic field magnitude is less than about 128 nT and when the magnetic field is aligned near the spin-axis direction. A remaining spin-axis offset of about 0.4 ~ 0.6 nT was observed between July and October 2003. Using multi-point multi-instrument measurements by Cluster we further demonstrate the importance of the accurate determination of the spin-axis offset when estimating the magnetic field gradient.
    10/2013; 3:459-487. DOI:10.5194/gid-3-459-2013
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    ABSTRACT: Acceleration of magnetosheath plasma resulting from the draping of the interplanetary magnetic field (IMF) around the magnetosphere can give rise to flow speeds that exceed that of the solar wind (VSW) by up to ~60%. Three case event studies out of 34 identified events are described. We then present a statistical study of draping-related accelerations in the magnetosheath. Further, we compare the results with the recent theory of Erkaev et al. (2011, 2012). We present a methodology to help distinguish draping-related accelerations from those caused by magnetic reconnection. To rule out magnetopause reconnection at low latitudes, we focus mainly on the positive Bz phase during the passage of interplanetary coronal mass ejections (ICMEs), as tabulated in Richardson and Cane (2010) for 1997-2009, and adding other events from 2010. To avoid effects of high-latitude reconnection poleward of the cusp, we also consider spacecraft observations made at low magnetic latitudes. We study the effect of upstream Alfvén Mach number (MA) and magnetic local time (MLT) on the speed ratio V/VSW. The comparison with theory is good. Namely, (i) flow speed ratios above unity occur behind the dawn-dusk terminator, (ii) those below unity occur on the dayside magnetosheath, and (iii) there is a good general agreement in the dependence of the V ratio on MA.
    Annales Geophysicae 10/2013; 31(10):1779-1789. DOI:10.5194/angeo-31-1779-2013 · 1.68 Impact Factor
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    ABSTRACT: The interaction of interplanetary coronal mass ejections (ICMEs) and magnetic clouds (MCs) with the Earth's magnetosphere exhibits various interesting features principally due to interplanetary parameters which change slowly and reach extreme values of long duration. These, in turn, allow us to explore the geomagnetic response to continued and extreme driving of the magnetosphere. In this paper we shall discuss elements of the following: (i) anomalous features of the flow in the terrestrial magnetosheath during ICME/MC passage and (ii) large geomagnetic disturbances when total or partial mergers of ICMEs/MCs pass Earth. In (i) we emphasize two roles played by the upstream Alfvén Mach number in solar wind–magnetosphere interactions: (i) It gives rise to wide plasma depletion layers. (ii) It enhances the magnetosheath flow speed on draped magnetic field lines. (By plasma depletion layer we mean a magnetosheath region adjacent to the magnetopause where magnetic forces dominate over hydrodynamic forces.) In (ii) we stress that the ICME mergers elicit geoeffects over and above those of the individual members. In addition, features of the non-linear behavior of the magnetosphere manifest themselves.
    Journal of Atmospheric and Solar-Terrestrial Physics 07/2013; 99:14–26. DOI:10.1016/j.jastp.2012.11.014 · 1.75 Impact Factor
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    ABSTRACT: Using Cluster data from the Electron Drift (EDI) and the Electric Field and Wave (EFW) instruments, we revise our empirically-based, inner-magnetospheric electric field (UNH-IMEF) model at 2<L<10. We pick more EFW data during large activities when wake effects are expected to be small. The model is organized by either the interplanetary electric field (IEF) merging with the magnetosphere or the K-p index. IEF and K-p ranges for which we get potential patterns are, respectively: IEF<0.282 mV/m, 0.282<IEF<0.575 mV/m, 0.575<IEF<0.872 mV/m, 0.898<IEF<1.308 mV/m, 1.308<IEF<1.834 mV/m, 1.834<IEF<2.662 mV/m, and IEF>2.662 mV/m; K-p<1, 1K(p)<2, 2K(p)<3, 3K(p)<4, 4K(p)<5, and K(p)4(+). Patterns consist of one set of data and processing for smaller activities, and another for higher activities. As activity increases, the skewed potential contour related to the partial ring current appears on the nightside. With the revised analysis, we find that the skewed potential contours get clearer and potential contours get denser on the nightside and morningside. Since the fluctuating components are not negligible, standard deviations from the modeled values are included in the model. In this study, we perform validation of the derived model more extensively. We find experimentally that the skewed contours are located close to the last closed equipotential, consistent with previous theories. This gives physical context to our model and serves as one validation effort. As another validation effort, the derived results are compared with other models/measurements. From these comparisons, we conclude that our model has some clear advantages over the others.
    Journal of Geophysical Research: Space Physics 07/2013; 118(7). DOI:10.1002/jgra.50373 · 3.44 Impact Factor
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    ABSTRACT: We describe a ~4 hour-long solar wind transient observed by the Wind spacecraft, in which is embedded a pressure-balanced structure. Minimum variance analysis on high resolution (~11 Hz) magnetic field data shows it to be planar to an excellent approximation (ratio of intermediate-to-minimum eigenvalues = 83). The structure starts with a very sharp discontinuity whose orientation coincides within four degrees with that of the structure itself. We find that this discontinuity has a bifurcated magnetic field and plasma flow structure. There is also a velocity depression coextensive with it. Applying a tangential stress balance test (Walén relation) to the discontinuity, we find good agreement of predictions with observations. We show directly the presence of two Alfvén waves propagating in opposite directions. The observation is consistent with the presence of a reconnection region in a hitherto unexplored configuration within a small solar wind transient.
    06/2013; DOI:10.1063/1.4811013
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    ABSTRACT: We use Cluster data from fluxgate magnetometer (FGM) and electron drift instrument (EDI) to determine the magnetic field gradients in the near-Earth magnetotail. Here we use the magnetic field data from FGM measurements as well as the gyro-time data of electrons determined from the time of flight measurements of EDI. The results are compared with the values estimated from empirical magnetic field models for different magnetospheric conditions. We also estimated the spin axis offset of FGM based on comparison between EDI and FGM data and discuss the possible effect in determining the current sheet characteristics.
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    ABSTRACT: During the solar activity minimum 2007-2009 very few large transients (interplanetary coronal mass ejections, ICMEs) were observed. However, an interesting feature was the frequent occurrence of small transients (few hours' duration), as shown, for example over a two-month interval by Kilpua et al. (2009). In this work we present a comprehensive statistical analysis of small transients over the entire three-year period. Identification crieria are : (i) duration between 0.5 and 12 hours; (ii) low proton temperature; (iii) low proton beta; (iv) enhanced magnetic field strength; (v) diminished magnetic field variability; (vi) low Alfven Mach number; and (vii) higher-than-average over the three years of the electron-to-proton temperature ratio. In selecting events, we require small transients to satisfy criteria (i)-(iii) and, in addition, they should satisfy at least two of the other four signatures. We compare their properties with those of the solar wind during the same three-year period, and are thus able to isolate a number of features characterizing these small transients during this solar activity minimum period. We search for small transients using observations acquired by the Wind spacecraft. After removing those which are likely to be Alfvenic strctures, we find 131 examples, about 81 percent of which lie in the slow solar wind (< 450 km/s). We present six case studies to illustrate various interesting aspects of these configurations. We then give statistical results on the whole assembly. The average duration is about 4.3 hours, while 99 events (76 pervent) are shorter than 6 hours. The maximum magnetic field is about twice that of the average solar wind value, the proton beta is about four times smaller than ambient, and the Alfven Mach number is about one half of the average value.
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    ABSTRACT: Magnetic islands are a common feature in both the onset and nonlinear evolution of magnetic reconnection. In collisionless regimes, the onset typically occurs within ion-scale current layers leading to the formation of magnetic islands when multiple X lines are involved. The nonlinear evolution of reconnection often gives rise to extended electron current layers (ECL) which are also unstable to formation of magnetic islands. Here, we show that the excess negative charge and strong out-of-plane electron velocity in the ECL are passed on to the islands generated therein, and that the corresponding observable distinguishing the islands generated in the ECL is the strongly enhanced in-plane electric fields near the island core. The islands formed in ion-scale current layers do not have these properties of the ECL-generated islands. The above result provides a way to assess the occurrence and importance of extended ECLs that are unstable to island formation in space and laboratory plasmas.
    Physics of Plasmas 11/2012; 19(11). DOI:10.1063/1.4767645 · 2.25 Impact Factor
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    ABSTRACT: The Kelvin-Helmoltz instability (KH) with formation of vortices appears in a wide variety of terrestrial, interplanetary, and astrophysical contexts. We study a series of iterated rolled-up coherent plasma structures (15) that flow in the equatorial Earth's boundary layer (BL), observed on October 24, 2001. The data were recorded during a 1.5 hour-long Wind crossing of the BL at the dawn magnetospheric flank, tailward of the terminator (X≈-13 RE). The interplanetary magnetic field (IMF) was radially directed, almost antiparallel to the magnetosheath (MS) flow. This configuration is expected to be adverse to the KH instability because of the collinearity of field and flow, and the high compressibility of the MS. We analyze the BL stability with compressible MHD theory using continuous profiles for the physical quantities. Upstream, at near Earth sites, we input parameters derived from an exact MHD solution for collinear flows. Further downtail at Wind position we input measured parameters. The BL is found KH unstable in spite of unfavorable features of the external flow. On the experimental side, the passage of vortices is inferred from the presence of low density - hot plasma being accelerated to speeds higher than that of the contiguous MS. It is further supported by the peculiar correlation of relative motions (in the bulk velocity frame): cold-dense plasma drifts sunward, while hot-tenuous plasma moves tailward. This event differs from many other studies that reported BL vortices under strongly northward IMF orientations. This is a case of KH vortices observed under an almost radial IMF, with implicit significance for the more common Parker's spiral fields, and the problem of plasma entry in the magnetosphere.
    Journal of Physics Conference Series 06/2012; 370(1):2003-. DOI:10.1088/1742-6596/370/1/012003

Publication Stats

4k Citations
372.36 Total Impact Points

Institutions

  • 2–2014
    • University of New Hampshire
      • • Department of Physics
      • • Space Science Center
      Дарем, New Hampshire, United States
  • 1999
    • Durham University
      Durham, England, United Kingdom
  • 1993
    • Cornell University
      • Department of Electrical and Computer Engineering
      Ithaca, New York, United States
  • 1988–1990
    • University of Alabama in Huntsville
      Huntsville, Alabama, United States
  • 1989
    • University of Texas at Arlington
      Arlington, Texas, United States
  • 1986
    • University of California, San Diego
      San Diego, California, United States
  • 1977–1984
    • University of California, Berkeley
      • Space Sciences Laboratory
      Berkeley, California, United States
  • 1981
    • Johns Hopkins University
      • Applied Physics Laboratory
      Baltimore, Maryland, United States