J. E. Borovsky

Concordia University–Ann Arbor, Ann Arbor, Michigan, United States

Are you J. E. Borovsky?

Claim your profile

Publications (254)470.28 Total impact

  • Fei Xu, Joseph E. Borovsky
    [Show abstract] [Hide abstract]
    ABSTRACT: A 3-parameter algebraic scheme is developed to categorize the solar wind at 1 AU into 4 plasma types: coronal-hole-origin plasma, streamer-belt-origin plasma, sector-reversal-region plasma, and ejecta. The three parameters are the proton specific entropy Sp = Tp/np2/3, the proton Alfvén speed vA, and the proton temperature Tp compared with a velocity-dependent expected temperature. Four measurements are needed to apply the scheme: the proton number density np, the proton temperature Tp, the magnetic-field strength B, and the solar-wind speed vsw. The scheme is tested and is found to be more accurate than existing categorization schemes. The categorization scheme is applied to the 1963–2013 OMNI2 data set spanning 4 solar cycles and to the 1998–2008 ACE data set. The statistical properties of the 4 types of plasma are examined. The sector-reversal-region plasma is found to have statistically low alpha-to-proton density ratios and high Alfvén Mach numbers. The statistical relations between the proton and alpha-particle specific entropies and oxygen and carbon charge-state density ratios Sp, Sα, O7+/O6+, and C6+/C5+ from ACE are examined for the four types of plasma: the patterns observed imply a connection between sector-reversal-region plasma and ejecta and a connection between streamer-belt-origin plasma and coronal-hole-origin plasma. Plasma occurrence rates are examined and solar-cycle patterns are found for ejecta, for coronal-hole-origin plasma, and for sector-reversal-region plasma.
    Journal of Geophysical Research: Space Physics 12/2014; · 3.44 Impact Factor
  • M. H. Denton, J. E. Borovsky
    [Show abstract] [Hide abstract]
    ABSTRACT: The outer plasmasphere is eroded when the strength of the convection electric field increases. Following a return of lower levels of convection, the outer plasmasphere " refills" from the ionosphere. In-situ observations of the cold (~1 eV) ion number density from geosynchronous orbit indicate that within ~48 hours after plasmaspheric erosion events the number density may return to a level of ~100 cm-3, consistent with previously reported values. Current theoretical estimates of refilling rates at geosynchronous orbit are inconsistent with such rapid refilling. In order to shed light on this issue a theoretical investigation is carried out to determine the major factors governing the refilling process. While theoretical estimates of the refilling rate reported here still fall below observed levels by at least a factor of two, the results of this study indicate that the morphology of the neutral atmosphere, (particularly the neutral atmosphere number density), is critical in controlling the rate of refilling at geosynchronous orbit. The strength of vertical E × B drifts, and horizontal neutral winds are found to play only a minor role.
    Journal of Geophysical Research: Space Physics 10/2014; · 3.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Long-lived (weeks) plasmaspheric drainage plumes are explored. The long-lived plumes occur during long-lived high-speed-stream-driven storms. Spacecraft in geosynchronous orbit see the plumes as dense plasmaspheric plasma advecting sunward toward the dayside magnetopause. The older plumes have the same densities and local-time widths as younger plumes, and like younger plumes they are lumpy in density and they reside in a spatial gap in the electron plasma sheet (in sort of a drainage corridor). Magnetospheric-convection simulations indicate that drainage from a filled outer plasmasphere can only supply a plume for 1.5 - 2 days. The question arises for long-lived plumes (and for any plume older than about 2 days): Where is the plasma coming from? Three candidate sources appear promising: (1) substorm disruption of the nightside plasmasphere which may transport plasmaspheric plasma outward onto open drift orbits, (2) radial transport of plasmaspheric plasma in velocity-shear-driven instabilities near the duskside plasmapause, and (3) an anomalously high upflux of cold ionospheric protons from the tongue of ionization in the dayside ionosphere, which may directly supply ionospheric plasma into the plume. In the first case the plume is drainage of plasma from the magnetosphere; in the second case it is not. Where the plasma in long-lived plumes is coming from is a quandary: to fix this dilemma further work, and probably full-scale simulations, are needed.
    Journal of Geophysical Research: Space Physics 08/2014; · 3.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We investigated mass density ρm and O + concentration ηO + ≡ nO +/ne (where nO + and ne are the O + and electron density, respectively)during two events, one active and one more quiet. We found ρm from observations of Alfvén wave frequencies measured by the Geostationary Operational Environmental Satellites (GOES), and we investigated composition by combining measurements of ρm with measurements of ion density nMPA,i from the Magnetospheric Plasma Analyzer (MPA) instrument on Los Alamos National Laboratory (LANL) spacecraft or ne from the Radio Plasma Imager (RPI) instrument on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. Using a simple assumption for the He + density at solar maximum based on a statistical study, we found ηO + values ranging from near zero to close to unity. For geostationary spacecraft that co-rotate with the earth, sudden changes in density for both ρm and ne often appear between dusk and midnight MLT, especially when Kp is significantly above zero. This probably indicates that the bulk (total) ions have energy below a few keV and that the satellites are crossing from closed or previously closed to open drift paths. During long periods that are geomagnetically quiet, the mass density varies little, but ne gradually refills leading to a gradual change in composition from low density plasma that is relatively cold and heavy (high average ion mass M ≡ ρm/ne) to high density plasma that is relatively cold and light (low M) plasmasphere-like plasma. During active periods we observe a similar daily oscillation in plasma properties from the dayside to the nightside, with cold and light high density plasma (more plasmasphere-like) on the dayside, and hotter and more heavy low density plasma (more plasmasheet-like) on the nightside. The value of ne is very dependent on whether it is measured inside or outside a plasmaspheric plume, while ρm is not. All of our results were found at solar maximum; previous results suggest that there will be much less O + at solar minimum under all conditions.
    Journal of Geophysical Research: Space Physics 08/2014; 119(8). · 3.44 Impact Factor
  • Joseph E. Borovsky
    [Show abstract] [Hide abstract]
    ABSTRACT: Canonical correlation analysis (CCA) will evaluate the degree of correlation between two multivariate data sets and will uncover patterns of correlation between the two data sets. Here CCA is applied to the multivariate solar-wind data set and the multivariate geomagnetic-index data set. CCA creates a new set of solar-wind variables and a new set of Earth variables. The first of the new solar-wind variables can be used as a solar-wind driver function for the magnetosphere; the conjugate Earth variable can be used as an Earth vector to describe global geomagnetic activity in the magnetosphere-ionosphere system. The CCA-generated driver functions are found to be superior in accuracy to other driver functions in the literature. CCA of the combined data sets provides information about: (1) differences in the solar-wind driving of high-latitude geomagnetic indices (AE, AU, AL, and PCI) versus magnetospheric-convection geomagnetic indices (Kp and MBI), (2) the properties of electric-field-based driver functions versus reconnection-based driver functions, and (3) improvements to solar-wind/magnetosphere correlations produced by time averaging the solar-wind clock angle. The CCA process tends to focus on the magnetospheric-convective indices over other indices: this may indicate that there is more predictable variance in the global-convective indices than in the others.
    Journal of Geophysical Research: Space Physics 06/2014; · 3.44 Impact Factor
  • Joseph E. Borovsky, S. Peter Gary
    [Show abstract] [Hide abstract]
    ABSTRACT: This report explores the feasibility of explaining the observed proton heating in the inner heliosphere (1) by tapping the field-aligned relative drift between alpha particles and protons in the solar-wind plasma and (2) by tapping the strahl-electron heat flux from the Sun. The observed reduction of the alpha-proton drift kinetic energy from 0.3 AU to 1 AU and the observed reduction of electron heat flux from 0.3 AU to 1 AU are each about half of the energy needed to account for the observed heating of protons from 0.3 AU to 1 AU. A mechanism is identified to transfer the free energy of the alpha-proton relative drift into proton thermal energy: the alpha-proton magnetosonic instability. A mechanism is identified to transfer kinetic energy from the strahl-electron heat flux into proton thermal energy: weak double layers. At the current state of knowledge, the plausibility of heating the solar-wind protons via the alpha-proton magnetosonic instability is high. The properties of the weak double layers that have been observed in the solar wind are not well known; more data analysis and plasma simulations are needed before the plausibility of heating the solar-wind protons by the double-layer mechanism can be evaluated.
    Journal of Geophysical Research: Space Physics 06/2014; · 3.44 Impact Factor
  • Joseph E. Borovsky, Michael H. Denton
    [Show abstract] [Hide abstract]
    ABSTRACT: The ULF magnetospheric indices Sgr, Sgeo, Tgr, and Tgeo are examined and correlated with solar-wind variables, geomagnetic indices, and the multispacecraft-averaged relativistic-electron flux F in the magnetosphere. The ULF indices are detrended by subtracting off sine waves with 24-hr periods to form Sgrd, Sgeod, Tgrd, and Tgeod. The detrending improves correlations. Autocorrelation-function analysis indicates that there are still strong 24-hr-period non-sinusoidal signals in the indices which should be removed in future. Indications are that the ground-based indices Sgrd and Tgrd are more predictable than the geosynchronous indices Sgeod and Tgeod. In the analysis a difference index ΔSmag ≈ Sgrd - 0.693 Sgeod is derived: the time integral of ΔSmag has the highest ULF-index correlation with the relativistic-electron flux F. In systems-science fashion, canonical correlation analysis (CCA) is used to correlate the relativistic-electron flux simultaneously with the time integrals of (a) the solar-wind velocity, (b) the solar-wind number density, (c) the level of geomagnetic activity, the (d) ULF indices, and (e) the type of solar-wind plasma (coronal-hole versus streamer belt): the time integrals of the solar-wind density and the type of plasma have the highest correlations with F. To create a solar-wind-Earth system of variables, the two indices Sgrd and Sgeod are combined with seven geomagnetic indices; from this CCA produces a canonical Earth variable that is matched with a canonical solar-wind variable. Very high correlations (rcorr = 0.926) between the two canonical variables are obtained.
    Journal of Geophysical Research: Space Physics 05/2014; 119(6). · 3.44 Impact Factor
  • Joseph E. Borovsky, John T. Steinberg
    [Show abstract] [Hide abstract]
    ABSTRACT: [1] Using measurements from the WIND spacecraft at 1 AU, the heating of protons in the solar wind at locations of intense velocity shear is examined. 4321 sites of intense shear in fast coronal-hole-origin plasma are analyzed. The proton temperature, the proton specific entropy, and the proton number density at the locations of the shears are compared with the same quantities in the plasmas adjacent to the shears. A very slight but statistically significant enhancement of the proton temperature is seen at the sites of the shears, but it is accompanied by a larger enhancement of the proton number density at the sites of the shears. Consequently there is no enhancement of the proton specific entropy at the shear sites, indicating no production of entropy; hence, no evidence for plasma heating is found at the sites of the velocity shears. Since the shearing velocities have appreciable Mach numbers, the authors suggest that there can be a slight adiabatic compression of the plasma at the shear zones.
    Journal of Geophysical Research: Space Physics 03/2014; 119(3). · 3.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A summary is presented of experimental optical observations at 4278 Å from close to a powerful (~150 kW) VLF transmitter (call-sign JXN) with a transmission frequency of 16.4 kHz. Approximately 2.5 seconds after transmitter turn-on, a sudden increase in optical emissions at 4278 Å was detected using a dedicated camera/CCD monitoring system recording at a frequency of 10 Hz. The optical signal is interpreted as a burst of electron precipitation lasting ~0.5 seconds, due to gyro-resonant wave-particle interactions between the transmitted wave and the magnetospheric electron population. The precipitation was centered on the zenith and had no detectable spatial structure. The timing of this sequence of events is in line with theoretical predictions and previous indirect observations of precipitation. This first direct measurement of VLF-induced precipitation at 4278 Å reveals the spatial and temporal extent of the resulting optical signal close to the transmitter.
    Geophysical Research Letters. 03/2014; 41(7).
  • [Show abstract] [Hide abstract]
    ABSTRACT: five spacecraft in geosynchronous orbit, plasmaspheric drainage plumes are located in the dayside magnetosphere and the measured pitch angle anisotropies of radiation belt electrons are compared duskward and dawnward of the plumes. Two hundred twenty-six plume crossings are analyzed. It is found that the radiation belt anisotropy is systematically greater dawnward of plumes (before the electrons cross the plumes) than it is duskward of plumes (after the electrons have crossed the plumes). This change in anisotropy is attributed to pitch angle scattering of the radiation belt electrons during their passage through the plumes. A test database in the absence of plumes finds no equivalent change in the radiation belt anisotropy. The amount of pitch angle scattering by the plume is quantified, scattering times are estimated, and effective pitch angle diffusion coefficients within the plume are estimated. The pitch angle diffusion coefficients obtained from the scattering measurements are of the same magnitude as expected values for electromagnetic ion cyclotron (EMIC) waves at high electron energies (1.5 MeV); however, expected EMIC diffusion coefficients do not extend to pitch angles of 90° and would have difficulties explaining the observed isotropization of electrons. The pitch angle diffusion coefficients obtained from the scattering measurements are of the same magnitude as expected values for whistler mode hiss at lower electron energies (150 keV). Outward radial transport of the radiation belt caused by the pitch angle scattering in the plume is discussed.
    Journal of Geophysical Research: Space Physics 02/2014; 119(3). · 3.44 Impact Factor
  • Joseph E. Borovsky, Joachim Birn
    [Show abstract] [Hide abstract]
    ABSTRACT: [1] Working toward a physical understanding of how solar-wind/magnetosphere coupling works, four arguments are presented indicating that the solar-wind electric field v sw × B sw does not control the rate of reconnection between the solar wind and the magnetosphere. Those four arguments are (1) that the derived rate of dayside reconnection is not equal to solar-wind electric field, (2) that electric-field driver functions can be improved by a simple modification that disallows their interpretation as the solar-wind electric field, (3) that the electric field in the magnetosheath is not equal to the electric field in the solar wind, and (4) that the magnetosphere can mass load and reduce the dayside reconnection rate without regard for the solar-wind electric field. The data is more consistent with a coupling function based on local control of the reconnection rate than the Axford conjecture that reconnection is controlled by boundary conditions irrespective of local parameters. Physical arguments that the solar-wind electric field controls dayside reconnection are absent; it is speculated that it is a coincidence that the electric field does so well at correlations with geomagnetic indices.
    Journal of Geophysical Research: Space Physics 02/2014; · 3.44 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe a new electrostatic particle-in-cell (PIC) code in curvilinear geometry called curvilinear PIC (CPIC). The code models the microscopic (kinetic) evolution of a plasma with the PIC method, coupled with an adaptive computational grid that can conform to arbitrarily shaped domains. CPIC is particularly suited for multiscale problems associated with the interaction of complex objects with plasmas. A map is introduced between the physical space and the logical space, where the grid is uniform and Cartesian. In CPIC, most of the operations are performed in logical space. CPIC was designed following criteria of versatility, robustness, and performance. Its main features are the use of structured meshes, a scalable field solver based on the black box multigrid algorithm and a hybrid mover, where particles' position is in logical space while the velocity is in physical space. Test examples involving the interaction of a plasma with material boundaries are presented.
    IEEE Transactions on Plasma Science 12/2013; 41(12):3577-3587. · 0.95 Impact Factor
  • Joseph E. Borovsky
    [Show abstract] [Hide abstract]
    ABSTRACT: [1] Driver functions for the Earth's magnetosphere-ionosphere system are derived from physical principles. Two processes act simultaneously: a reconnection-coupled MHD generator 𝒢 and a viscous interaction. 𝒢 accounts for the dayside reconnection rate, the length of the reconnection X line, and current saturation limits for the solar wind generator. Two viscous drivers are derived: Bohm viscosity ℬ and the freestream-turbulence effect ℱ. A problematic proxy effect is uncovered wherein the viscous driver functions also describe the strength of reconnection. Two magnetospheric-driver functions written in terms of upstream solar wind parameters are constructed: 𝒢 + ℬ and 𝒢 + ℱ. The driver functions are tested against seven geomagnetic indices. The reaction of the geomagnetic indices to 𝒢 + ℬ and 𝒢 + ℱ is nonlinear: Nonlinear versions of the driver functions are supplied. Applying the driver functions at multiple time steps yields correlation coefficients of ~85% with the AE and Kp indices; it is argued that multiple time stepping removes high-frequency uncorrelated signal from the drivers. Autocorrelation-function analysis shows strong 1 day and 1 year periodicities in the AE index, which are not in the solar wind driver functions; correspondingly, high-pass and low-pass filtering finds uncorrelated signal at 1 day and 1 year timescales. Residuals (unpredicted variance) between the geomagnetic indices and the driver functions are analyzed: The residuals are anticorrelated with the solar wind velocity, the solar F10.7 radio flux, and the solar wind current saturation parameter. Removing diurnal, semiannual, and annual trends from the indices improves their correlation with the solar wind driver functions. Simplified versions of the driver functions are constructed: The simplified drivers perform approximately as well as the full drivers.
    Journal of Geophysical Research: Space Physics 11/2013; 118(11). · 3.44 Impact Factor
  • Joseph E. Borovsky, Michael H. Denton
    [Show abstract] [Hide abstract]
    ABSTRACT: [1] A corotating interaction region (CIR) is formed when fast coronal hole origin solar wind overtakes slow solar wind and forms a region of compressed plasma and magnetic field. The slow wind upstream of the coronal hole fast wind can be either of helmet streamer origin or pseudostreamer origin. For a collection of 125 CIR-driven geomagnetic storms, the slow wind ahead of each CIR is examined; for those storm not containing ejecta, each CIR is categorized as a helmet streamer CIR (74 of the 125 storms) or a pseudostreamer CIR (11 of the 125 storms). Separate superposed epoch studies are performed on the two groups to discern the differences between storms driven by pseudostreamer CIRs and those driven by helmet streamer CIRs. A major difference is that pseudostreamer CIR storms tend not to have a calm before the storm, so the outer plasmasphere does not refill before storm onset, and the outer electron radiation belt does not exhibit a pre-storm decay. The superdense plasma sheet is weaker for pseudostreamer CIR storms, and the dropout of the electron radiation belt is weaker. Pseudostreamer CIR storms and helmet streamer CIR storms tend to be of the same strength as measured by the magnitude of Kp, MBI (midnight boundary index), or Dst.
    Journal of Geophysical Research: Space Physics 09/2013; 118(9). · 3.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Estimates are calculated for the storm time reduction of solar wind/magnetosphere coupling by the mass density ρm of the magnetospheric plasma. Based on the application of the Cassak‐Shay reconnection‐rate formula at the dayside magnetopause, a numerical factor M is developed to quantify the effect of ρm on the dayside reconnection rate. It is argued that the mass loading of dayside reconnection by ρm also makes reconnection more susceptible to shutoff by magnetosheath velocity shear: a formula is developed to estimate the shortening of the dayside reconnection X‐line by ρm. Surveys of plasmaspheric drainage plumes at geosynchronous orbit during high‐speed‐stream‐driven storms and coronal mass ejection (CME)‐driven storms are presented: in the surveys the CME‐driven storms are separated into sheath‐driven portions and magnetic‐cloud‐driven portions. The storm time mass density of the warm plasma cloak (ionospheric outflows into the electron plasma sheet) is obtained from Alfven‐wave analysis at geosynchronous orbit. A methodology is developed to extrapolate geosynchronous‐orbit plasma measurements to the dayside magnetopause. For each of the three plasmas, estimates of the fractional reduction of the total dayside reconnection rate vary, with typical values of tens of percent; i.e., solar wind/magnetosphere coupling is reduced by tens of percent during storms by oxygen in the ion plasma sheet, by the plasmaspheric drainage plume, and by the plasma cloak. Dependence of the reduction on the F 10.7 solar radio flux is anticipated. Via these ionospheric‐origin plasmas, the magnetosphere can exert some control over solar wind/magnetosphere coupling. Pathways to gain a fuller understanding of the physics of the solar wind‐driven magnetosphere‐ionosphere system are discussed.
    Journal of Geophysical Research Atmospheres 09/2013; 118(9):5695-5719. · 3.44 Impact Factor
  • Joseph E. Borovsky
    [Show abstract] [Hide abstract]
    ABSTRACT: [1] In a 2008 publication a first principles calculation of the dayside reconnection rate expressed in terms of upstream–solar wind parameters led to a rudimentary solar wind coupling function R1 for the Earth's magnetosphere. Four improvements to that derivation are added in the present paper, resulting in a more correct solar wind control function describing the rate at which solar wind magnetic field lines connect into the Earth's magnetosphere. The first is the inclusion of the effect of β-dependent plasma compressibility on the reconnection rate. The second is a corrected calculation of the inflow of magnetic field lines into the reconnection X-line. The third is a more accurate estimate of the orientation of the reconnection X-line for asymmetric reconnection. The fourth correction accounts for the compression ratio of the Earth's bow shock for arbitrary orientation of the solar wind magnetic field. Two solar wind control functions result: one function, R2CS, is based on the Cassak-Shay equation and another function, R2CSB, is based on the Cassak-Shay-Birn equation. The control functions are tested using solar wind measurements and geomagnetic indices from 1980 to 2012, and some improved correlation coefficients over the rudimentary function R1 are found. Simplified approximations to the new control functions are supplied: one is R2CS-approx = 1.68 × 10−2 sin2(θ/2)no1/2vo2MA−0.3044exp(−[MA/3.18]1/2), where the subscript “o” denotes the solar wind upstream of the bow shock and MA is the solar wind Alfvén Mach number.
    Journal of Geophysical Research: Space Physics 05/2013; 118(5). · 3.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The methodology based on the Error Reduction Ratio (ERR) determines the causal relationship between the input and output for a wide class of nonlinear systems. In the present study, ERR is used to identify the most important solar wind parameters, which control the fluxes of energetic electrons at geosynchronous orbit. The results show that for lower energies, the fluxes are indeed controlled by the solar wind velocity, as was assumed before. For the lowest energy range studied here (24.1 keV), the solar wind velocity of the current day is the most important control parameter for the current day's electron flux. As the energy increases, the solar wind velocity of the previous day becomes the most important factor. For the higher energy electrons (around 1 MeV), the solar wind velocity registered 2 days in the past is the most important controlling parameter. Such a dependence can, perhaps, be explained by either local acceleration processes due to the interaction with plasma waves or by radial diffusion if lower energy electrons possess higher mobility. However, in the case of even higher energies (2.0 MeV), the solar wind density replaces the velocity as the key control parameter. Such a dependence could be a result of solar wind density influence on the dynamics of various waves and pulsations that affect acceleration and loss of relativistic electrons. The study also shows that statistically the variations of daily high energy electron fluxes show little dependence on the daily averaged Bz, daily time duration of the southward IMF, and daily integral ∫Bsdt (where Bs is the southward component of IMF).
    Journal of Geophysical Research Atmospheres 04/2013; 118(4):1500-1513. · 3.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: works have emphasized the significant influence of the solar wind Alfvén Mach number (MA) on magnetospheric dynamics. Here we report statistical, observational results that pertain to changes in the magnetosheath flow distribution and magnetopause shape as a function of solar wind MA and interplanetary magnetic field (IMF) clock angle orientation. We use all Cluster 1 data in the magnetosheath during the period 2001-2010, using an appropriate spatial superposition procedure, to produce magnetosheath flow distributions as a function of location in the magnetosheath relative to the IMF and other parameters. The results demonstrate that enhanced flows in the magnetosheath are expected at locations quasi-perpendicular to the IMF direction in the plane perpendicular to the Sun-Earth line; in other words, for the special case of a northward IMF, enhanced flows are observed on the dawn and dusk flanks of the magnetosphere, while much lower flows are observed above the poles. The largest flows are adjacent to the magnetopause. Using appropriate magnetopause crossing lists (for both high and low MA), we also investigate the changes in magnetopause shape as a function of solar wind MA and IMF orientation. Comparing observed magnetopause crossings with predicted positions from an axisymmetric semi-empirical model, we statistically show that the magnetopause is generally circular during high MA, while is it elongated (albeit with moderate statistical significance) along the direction of the IMF during low MA. These findings are consistent with enhanced magnetic forces that prevail in the magnetosheath during low MA. The component of the magnetic forces parallel to the magnetopause produces the enhanced flows along and adjacent to the magnetopause, while the component normal to the magnetopause exerts an asymmetric pressure on the magnetopause that deforms it into an elongated shape.
    Journal of Geophysical Research Atmospheres 03/2013; 118(3):1089-1100. · 3.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: An unsolved problem in plasma turbulence is how energy is dissipated at small scales. Particle collisions are too infrequent in hot plasmas to provide the necessary dissipation. Simulations either treat the fluid scales and impose an ad hoc form of dissipation (e.g., resistivity) or consider dissipation arising from resonant damping of small amplitude disturbances where damping rates are found to be comparable to that predicted from linear theory. Here, we report kinetic simulations that span the macroscopic fluid scales down to the motion of electrons. We find that turbulent cascade leads to generation of coherent structures in the form of current sheets that steepen to electron scales, triggering strong localized heating of the plasma. The dominant heating mechanism is due to parallel electric fields associated with the current sheets, leading to anisotropic electron and ion distributions which can be measured with NASA's upcoming Magnetospheric Multiscale mission. The motion of coherent structures also generates waves that are emitted into the ambient plasma in form of highly oblique compressional and shear Alfven modes. In 3D, modes propagating at other angles can also be generated. This indicates that intermittent plasma turbulence will in general consist of both coherent structures and waves. However, the current sheet heating is found to be locally several orders of magnitude more efficient than wave damping and is sufficient to explain the observed heating rates in the solar wind.
    Physics of Plasmas 01/2013; 20(1). · 2.25 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The structure of Mercury's internal magnetic field has been determined from analysis of orbital Magnetometer measurements by the MESSENGER spacecraft. We identified the magnetic equator on 531 low-altitude and 120 high-altitude equator crossings from the zero in the radial cylindrical magnetic field component, Bρ. The low-altitude crossings are offset 479 ± 6 km northward, indicating an offset of the planetary dipole. The tilt of the magnetic pole relative to the planetary spin axis is less than 0.8°. The high-altitude crossings yield a northward offset of the magnetic equator of 486 ± 74 km. A field with only nonzero dipole and octupole coefficients also matches the low-altitude observations but cannot yield off-equatorial Bρ = 0 at radial distances greater than 3520 km. We compared offset dipole and other descriptions of the field with vector field observations below 600 km for 13 longitudinally distributed, magnetically quiet orbits. An offset dipole with southward directed moment of 190 nT-RM3 yields root-mean-square (RMS) residuals below 14 nT, whereas a field with only dipole and octupole terms tuned to match the polar field and the low-altitude magnetic equator crossings yields RMS residuals up to 68 nT. Attributing the residuals from the offset-dipole field to axial degree 3 and 4 contributions we estimate that the Gauss coefficient magnitudes for the additional terms are less than 4% and 7%, respectively, relative to the dipole. The axial alignment and prominent quadrupole are consistent with a non-convecting layer above a deep dynamo in Mercury's fluid outer core.
    Journal of Geophysical Research Atmospheres 12/2012; · 3.44 Impact Factor

Publication Stats

4k Citations
470.28 Total Impact Points

Institutions

  • 2014
    • Concordia University–Ann Arbor
      Ann Arbor, Michigan, United States
  • 2013–2014
    • The Space Science Institute
      Boulder, Colorado, United States
  • 2009–2014
    • Lancaster University
      • Department of Physics
      Lancaster, England, United Kingdom
  • 2012
    • University of Michigan
      • Department of Atmospheric, Oceanic and Space Sciences
      Ann Arbor, Michigan, United States
  • 1029–2012
    • Los Alamos National Laboratory
      • • Plasma Physics Group
      • • Space Science and Applications Group
      Los Alamos, California, United States
  • 2011
    • University of New Hampshire
      • Space Science Center
      Durham, New Hampshire, United States
  • 2010
    • Finnish Meteorological Institute
      Helsinki, Southern Finland Province, Finland
  • 1983–1989
    • University of Iowa
      • Department of Physics and Astronomy
      Iowa City, IA, United States