E. M. Harnett

University of Washington Seattle, Seattle, WA, United States

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Publications (71)73.41 Total impact

  • Erika M. Harnett, Robert M. Winglee
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    ABSTRACT: The Moon spends 20% of its orbit within the terrestrial magnetosphere. During this time it experiences a dynamic plasma environment, including high-speed streams, flux ropes and a flux of heavy ions from ionospheric outflows. 3D multi-fluid simulations of the Moon within the magnetosphere during a substorm shows that a highly variable plasma flow can develop in the vicinity of the Moon due to the passage of a flux rope. The transit of a flux rope past the Moon potentially leads to a plasma wake that is mis-aligned from the optical wake by nearly 30°30°. This will have implications when determining the range of space weathering and surface charging the lunar surface experiences.
    Advances in Space Research 07/2013; 52(2):243–250. · 1.18 Impact Factor
  • Erika M. Harnett, Michele Cash, Robert M. Winglee
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    ABSTRACT: The Moon spends 20% of its orbit within the terrestrial magnetosphere. During this time it experiences a flux of ions from the Earth, which can be accelerated within the magnetosphere prior to impacting the Moon. Both 3D multi-fluid simulations and particle tracking were used to study the lunar environment while the Moon is inside the magnetosphere and quantify the energy and flux of ionospheric ions at lunar orbits for times ranging from moderate substorms to severe storms. The results from the fluid simulations indicate that during storms, the flux of ionospheric oxygen ions to the Moon can increase by two orders of magnitude and the bulk of those oxygen ions have energies up to 100s of keV. Particle tracking indicates that the peak energy of particles in the tail of the distribution approaches 1 MeV for the most intense storms, as particles from the ring current can be lost downtail to lunar orbits. Periods of increased flux of ionospheric particles intercepted by the Moon over geologic time periods would have played a role in weathering of the lunar surface and delivery of water components to the surface.
    Icarus 05/2013; 224(1):218–227. · 3.16 Impact Factor
  • AGU Fall Meeting, San Francisco; 12/2012
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    ABSTRACT: The Cassini spacecraft has encountered multiple plasmoids in Saturn's magnetotail thought to be produced by tail reconnection. However, single spacecraft measurements make it difficult to determine plasmoid size, where they form, the composition, and the geometry of the plasma sheet when plasmoids are produced. This paper examines these issues using 3D multifluid simulations of the Kronian magnetosphere. Plasmoids may develop in multiple sectors, form at different distances from the planet, and grow to sizes large relative to the system (˜25 RS), with varying widths and lengths. These plasmoids are composed primarily of water group ions and move downtail with speeds of ˜250 km/s (the local Alfvén speed). The plasma sheet is hinged upward both prior to and following plasmoid formation. Plasmoids can be externally triggered by both flips in the orientation of the interplanetary magnetic field (IMF) as well as a pulse in the solar wind dynamic pressure.
    Journal of Geophysical Research 07/2012; 117(A7):7206-. · 3.17 Impact Factor
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    ABSTRACT: The origin of the periodicities in the radio, plasma and magnetic fields of Saturn has long been debated. Given the high degree of alignment of Saturn's dipole with its rotation axis no strong rotational periodicities are expected. However, detailed analysis of the Cassini data demonstrated the existence of such periodicities not only in Saturn's kilometric radio emissions (SKR), but in the plasma and magnetic field signatures throughout the Kronian magnetosphere as well. Several mechanisms have been proposed to explain these periodicities, yet each have difficulties self-consistently explaining the features detected across all the observations. The results presented will show that many of the observed features can be explained by the development of the centrifugal interchange instability. The centrifugal interchange instability leads to the formation of outward moving cold dense plasma fingers interspersed with inward moving hot tenuous magnetospheric plasma. While each individual interchange finger is sub-corotational, the global nature of the instability leads density and magnetic perturbations at any fixed observing point that have the rotational period embedded in the perturbations. In other words, a stationary observer will see a plasma finger (but not necessarily the same finger) sweep past at the rotation period. We show that while the local perturbations can change in intensity from solar wind forcing, the periodicity remains approximately constant, although there are specific solar conditions that yield the strongest periodicity signal. Reconnection processes that generate plasmoids in the tail also generate density and magnetic fluctuations. However these processes occur over a much longer scale and as such are not the direct source of the observed periodicities. The interchange instability not only provides a means of describing both sub-corotational and rotational features but also provides a natural explanation for the difference in the periodicities seen between the northern and southern hemispheres.
    AGU Fall Meeting Abstracts. 12/2011;
  • M. D. Cash, E. M. Harnett, R. Winglee
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    ABSTRACT: Our 3D multifluid code has been used to model the magnetosphere for substorm and storm conditions and does well predicting substorm onset and storm commencement. It does, however, under predict the intensity of currents in the inner magnetosphere. To resolve this issue, the code has been modified to incorporate temperature anisotropies. Previous studies using single-particle tracking to investigate the energization and injection of ring current ions from ionospheric outflow showed asymmetries between H+ and O+ energization mechanisms and injection into the ring current, as well as the importance of the O+ contribution of the storm time ring current energy density during storm main phase. The current work applies single-particle tracking with time varying fields to calculate changes in the number density, energy density, temperature and temperature anisotropies during the development of storms. The bulk velocities and temperatures of plasma sheet ions are examined in three directions and distribution functions are compared between the single-particle tracking results and an anisotropy model. It is observed that in order for particles to contribute to the development of the storm time ring current, a large perpendicular velocity component is necessary; particles with high parallel velocities are lost. Such results not only suggest the importance of including temperature anisotropies, but also point toward the underlying physical processes driving ring current enhancements.
    AGU Fall Meeting Abstracts. 12/2011;
  • R. M. Winglee, E. Harnett
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    ABSTRACT: Multifluid/multiscale simulations are used to examine the influence of ionospheric outflows on two substorms that occurred on August 13, 2001. Both substorms had well defined onsets in Cluster ion spectrometer (CIS) data of ion composition of the plasma sheet. It is shown that the model is able to account for two orders of magnitude variation in the H+ density and one order of magnitude change in the O+ density in the plasma sheet in association with the ionosphere generating peak outflows of 4 × 1025 H+ ions/s and 2 × 1025 O+ ions/s. The model shows that the growth phase is associated with the venting of solar wind plasma in association with the formation of an X-line and the ejection of a plasmoid. This reconnection occurs in a proton-dominated plasma and occurs well before onset. After this venting, a Y-line configuration develops with the ionospheric plasma being the dominant source of the plasma sheet. Lobe reconnection involves heavy enriched O+ lobe field lines produced by enhanced outflows that start at the beginning of the growth phase. This O+ produces enhanced dissipation and localized formation of flux ropes prior to substorm onset. For the isolated substorms considered here, the acceleration of O+ in the plasma sheet keeps the density low in this region but leads to the buildup of both number and density at the inner edge of the plasma sheet. O+ can contribute nearly 50% of the total energy density in this region just prior to onset. This results in over pressurization and dipolarization in conjunction with onset and associated increases in the nightside auroral currents.
    Journal of Geophysical Research 11/2011; 116(A11):11212-. · 3.17 Impact Factor
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    ABSTRACT: Analysis of spectra from the Clementine ultraviolet-visible and near-infrared cameras of small, immature craters and surface soils both on and adjacent to the lunar swirls at Mare Ingenii has yielded the following conclusions about space weathering at a magnetic anomaly. (1) Despite having spectral characteristics of immaturity, the lunar swirls are not freshly exposed surfaces. (2) The swirl surfaces are regions of retarded weathering, while immediately adjacent regions experience accelerated weathering. (3) Weathering in the off-swirl regions darkens and flattens the spectrum with little to no reddening, which suggests that the production of larger (>40 nm) nanophase iron dominates in these locations as a result of charged particle sorting by the magnetic field. Preliminary analysis of two other lunar swirl regions, Reiner Gamma and Mare Marginis, is consistent with our observations at Mare Ingenii. Our results indicate that sputtering/vapor deposition, implanted solar wind hydrogen, and agglutination share responsibility for creating the range in npFe0 particle sizes responsible for the spectral effects of space weathering.
    Journal of Geophysical Research 01/2011; 116. · 3.17 Impact Factor
  • M. D. Cash, R. Winglee, E. M. Harnett
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    ABSTRACT: The issue of the influence of ionospheric plasma on the development of substorms remains controversial. Multi-fluid simulations are used to investigate the sources of plasma during Aug 13, 2001 period of activity that was well observed by Cluster at 18 Re down the tail. It is shown that the ionospheric outflow from the multi-fluid code, which has been documented previously, is able to account for the Cluster observations during the period of several hours of magnetospheric activity, including the changes in the heavy ion density and relative density. The fact that the model is able to account for features both at low altitudes and in the magnetotail suggest that the transport of the plasma within the model is realistic. The influences of the dayside and nightside ion fountains are clearly identified in the model. It is shown that for the smaller substorms that occur during the event, ionospheric H+ plays an important role during substorm onset, while for the more intense substorm, O+ is an important factor. In the latter case relative densities can increase from a few percent to a few tens of percent over an extend region of the magnetosphere. At the inner edge of the plasma sheet, O+ can contribute more than 50% of the local energy density, though its number density in the central plasma sheet remains low. The deposition of this energy at the inner edge of the plasma sheet is an important factor for substorm onset. In addition, while the dayside outflows produce the greatest fluxes to the tail, the nightside outflows are primary involved in substorm onset.
    AGU Fall Meeting Abstracts. 12/2010;
  • A. Kidder, R. Winglee, E. M. Harnett, C. S. Paty
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    ABSTRACT: Cassini has observed a handful of plasmoids in Saturn's magnetotail and will likely observe more when its future orbits move it out of the equatorial plane. While Cassini's orbits are unfavorably positioned to observe the dynamic nature of Saturn's crosstail current sheet, our new multi-scale/multi-fluid model of the Kronian system allows for a global view with high resolution gridding in the magnetotail, to capture small-scale size dynamics associated with plasmoid development. The multi-scale nature of the model enables us to resolve structures whose sizes are similar to or less than the grid length of previous simulations, and how these structures affect the rest of the simulation, as they develop and travel into the lower resolution simulation regions. Measurements from the Cassini spacecraft are one dimensional, making it difficult to determine the location of reconnection sites as well as the speed and composition of plasmoid structures. Results are validated against Cassini observations and show a crosstail current sheet, bent up into a bowl shape, as well as quantitative information regarding current sheet thickness, kinking, flapping, hinging location, and tilt angle.
    AGU Fall Meeting Abstracts. 12/2010;
  • R. Winglee, E. M. Harnett, J. Waldock
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    ABSTRACT: Multi-fluid/Multi-Scale simulations are used to investigate the induced magnetosphere around Io and the mass loading on the Jovian magnetosphere from the induced outflows from Io. The simulations have been improved so that a stable torus plasma within the Jovian magnetosphere is established before inserting Io through the multi-scale aspects of the code. It is shown that because of the strong magnetic field at Io, most of the outflow is launch along the field lines leading to an ion tail that is a few Jovian radii in height. This sheet of plasma then convective drifts in the azimuthal direction. This wall of plasma is subject to small-scale interchange instabilities that lead to the rippling plasma with resultant enhancements in both number and current density. The resultant density profile is compared with published profiles of the Io plasma torus. These enhancements are proposed as a mechanism for the patchy auroral emissions in Jupiter’s ionosphere that are associated with Io’s plasma interaction with the Jovian magnetosphere.
    AGU Fall Meeting Abstracts. 12/2010;
  • E. M. Harnett, M. D. Cash, R. Winglee
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    ABSTRACT: The timing of substorm onset in relation to magnetotail dynamics is still rather controversial, particularly with regard to the link to reconnection in the tail. 3D global multi-scale/multi-fluid simulations are used to study the dynamics of the magnetotail with high (~ 400 km) resolution prior to, during, and after substorm onset. Case studies include examples of both internally and externally triggered events. Simulation results are validated against satellite data and observations of the auroral oval. Particle tracking is used to investigate how small-scale features such as kinks in the current sheet can accelerate ionospheric particles and lead to an enhancement in the ring current.
    AGU Fall Meeting Abstracts. 12/2010;
  • Michele Cash, Robert Winglee, Erika Harnett
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    ABSTRACT: During storms, induced electric and magnetic fields within the magnetosphere lead to the build up of energetic particles within the ring current and radiation belts. Using single-particle tracking with time-dependent global magnetic and electric fields, we explore the mechanisms responsible for the acceleration and injection of plasma sheet ions into the inner magnetosphere. We examine the contribution from various ionospheric source regions to the storm-time ring current. Solar wind boundary conditions are used as inputs for a self-consistent 3D multi-fluid model, which produces time-dependent global electric and magnetic fields that are read into our single-particle code. Ionospheric H^+ and O^+ are injected into the simulation from various ionospheric regions. Results show that the energization and trapping of ionospheric H^+ and O^+ are highly dependent on the location where the outflowing ions are initialized, and small scale structures in the current sheet are correlated with particle convection and energization. The same magnetic features that produce intensification of auroral current lead to the injection of energetic particles into the inner magnetosphere.
    11/2010;
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    ABSTRACT: Our analysis of Mare Ingenii points to the importance of solar wind implanted protons in creating npFe^0 on the lunar surface both within and outside the influence of a magnetic field.
    03/2010;
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    ABSTRACT: We present initial results from the first community-wide effort to compare global plasma interaction model results for Mars. Seven modeling groups participated in this activity, using MHD, multi-fluid, and hybrid assumptions in their simulations. Moderate solar wind and solar EUV conditions were chosen, and the conditions were implemented in the models and run to steady state. Model output was compared in three ways to determine how pressure was partitioned and conserved in each model, the location and asymmetry of plasma boundaries and pathways for planetary ion escape, and the total escape flux of planetary oxygen ions. The two participating MHD models provided similar results, while the five sets of multi-fluid and hybrid results were different in many ways. All hybrid results, however, showed two main channels for oxygen ion escape (a pickup ion ‘plume’ in the hemisphere toward which the solar wind convection electric field is directed, and a channel in the opposite hemisphere of the central magnetotail), while the MHD models showed one (a roughly symmetric channel in the central magnetotail). Most models showed a transition from an upstream region dominated by plasma dynamic pressure to a magnetosheath region dominated by thermal pressure to a low altitude region dominated by magnetic pressure. However, calculated escape rates for a single ion species varied by roughly an order of magnitude for similar input conditions, suggesting that the uncertainties in both the current and integrated escape over martian history as determined by models are large. These uncertainties are in addition to those associated with the evolution of the Sun, the martian dynamo, and the early atmosphere, highlighting the challenges we face in constructing Mars’ past using models.
    Icarus 03/2010; · 3.16 Impact Factor
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    E. M. Harnett, R. M. Winglee, T. Lerud
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    ABSTRACT: Multiscale-multifluid simulations are used to provide high-resolution (˜470 km) simulations of the 26 February 2008 substorm that was well observed by the THEMIS spacecraft to investigate whether the substorm was internally or externally triggered. This substorm occurred during an extended (1 h 45 min) period of weak (-2.5 to -1.5 nT) southward interplanetary magnetic field (IMF). Simulation results show that this substorm was internally triggered well before the IMF start to become more northerly. The southward IMF leads to an enhancement in the cross-polar cap potential, strong thinning of the current sheet, and pseudo-breakup in association with patchy reconnection. The southward IMF also produces an enhancement in the outflow of ionospheric oxygen into the current sheet. Because of the tilt of the magnetic field the strongest outflows originate from the southern polar cap. The arrival of these heavy ions in the tail occurs about 10 min prior to the observed substorm onset and leads to enhanced tail reconnection, including flux rope development and fast earthward flows. The interaction of these fast flows at the inner edge of the plasma sheet is closely associated with auroral onset, indicating that the substorm is internally triggered. Comparison with THEMIS data suggests that reconnection occurred earlier than THEMIS observations. The results show onset is associated with processes at the inner edge of the plasma sheet and is not directly related to the onset of reconnection. Comparison with THEMIS data indicates that processes at scale lengths less than 500 km are occurring.
    Journal of Geophysical Research 01/2010; 115. · 3.17 Impact Factor
  • M. D. Cash, R. M. Winglee, E. M. Harnett
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    ABSTRACT: Single-particle tracking with time-dependent global magnetic and electric fields is used to investigate the generation of the ring current from ionospheric outflows during an internally driven substorm. We show that the energization of the ions is not correlated with the time that the ions leave the ionosphere; instead energization is correlated with the formation of an injection front driven by an earthward moving flux rope at onset. Because of the large gyroradius of the O+ ions, they experience strong dawn-dusk acceleration in the vicinity of the injection front. The acceleration is strongly influenced by small-scale structures including the Hall electric field and the development of kinks across the tail. H+ is mainly energized by betatron acceleration as it is injected into the inner magnetosphere with less average energy than the O+ ions. In this paper we investigate the conditions that lead to the formation of the injection front and small-scale structures (˜1 RE) in the current sheet, such as tail kinking and flux ropes, that are correlated with particle convection and energization at substorm onset. High-resolution capabilities allow us to resolve these small-scale processes within a thin (<1000 km) current sheet, and we show that simulations with coarse grid resolution underestimate the energization of ring current particles. The role of the interplanetary magnetic field (IMF) Bz on dayside particle loss is also examined. It is found that northerly turning IMF at or shortly after onset is important in producing a symmetric ring current, but the degree of turning is not as critical.
    Journal of Geophysical Research 01/2010; 115. · 3.17 Impact Factor
  • E. M. Harnett
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    ABSTRACT: The Moon spends 25% of its orbit within the terrestrial magnetosphere. Particle tracking is used to investigate access points of 35 MeV and 760 MeV particles into the magnetosphere for both quiet and disturbed magnetospheric conditions. The results indicate that solar energetic particle (SEP) flux at the Moon can be reduced for storm conditions when the magnitude of the magnetic field in the sheath is enhanced, as particles in the 35 MeV range have limited access to the magnetosphere for storm conditions. Plasmoids are also effective at reducing SEP flux from the tailward direction. The results also indicate that the flux of SEPs from the dawnside of the magnetosphere can be focused into the current sheet, leading to a potential enhancement in SEP flux at the Moon. Particles traveling up the tail for both quiet and storm conditions tended to experience the greatest deflection away from the central tail.
    Journal of Geophysical Research 01/2010; 115. · 3.17 Impact Factor
  • M. D. Cash, R. M. Winglee, E. M. Harnett
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    ABSTRACT: Using single-particle tracking with time-dependent global magnetic and electric fields from multifluid simulations, we model the 10 March 1998 storm and investigate storm time acceleration, injection, and trapping mechanisms associated with the formation of the ring current. We examine the contribution from various ionospheric source regions to the storm time ring current and the effect interplanetary magnetic field Bz has on producing an asymmetric and symmetric ring current; we provide the first maps for the relative importance of ionospheric outflow (H+ and O+) regions as a function of all magnetic local times (MLTs) and latitudes between 60° and 80°. During the early part of the storm, high-latitude outflow regions between 00 and 06 MLT are the most efficient sectors at contributing particle density to the ring current, whereas during the main phase of the storm, there is more even contribution from all MLTs. The sectors that contribute the majority of the energy are consistently the high-latitude regions between 03 and 09 MLT. An increase in the contribution of O+ to the current density is observed from the predawn high-latitude region during each of two decreases in Dst examined for the 10 March 1998 storm, supporting the central role oxygen plays in storm development. Asymmetries are observed between H+ and O+ contributions. The dominant ionospheric species contributing to the ring current energy density is shown to vary during the course of the storm with a significant increase in ionospheric O+ contribution to the ring current associated with large decreases in Dst.
    Journal of Geophysical Research 01/2010; 115. · 3.17 Impact Factor
  • A. Kidder, R. Winglee, E. M. Harnett
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    ABSTRACT: Cassini has observed multiple plasmoids in Saturn's magnetotail that are thought to be produced by substorm-like events. However, the single spacecraft measurements make it difficult to determine the location of a reconnection site as well as the speed and composition of plasmoid-like structures. Measurements by Cassini have determined that solar wind pressure modulations have a stronger effect on reconnection and the development of plasmoids than the orientation of the interplanetary magnetic field. In this paper we use a three-dimensional multi-fluid model to investigate whether substorm-like events are more likely to be influenced by an increase in solar wind pressure that is a result of increased solar wind density or by an increase in solar wind velocity. We demonstrate that initial conditions are very important with large scale plasmoids being associated with IMF turnings anti-parallel to the Kronian magnetosphere. Smaller scale and more frequent flux ropes are generated in the wake of the plasma and/or by increases in the solar wind dynamic pressure. The global nature of this model will enable the determination of the distance, Saturn Local Time, speeds and recurrence rate of any plasmoids. We also examine the three-dimensional movement and geometry of the plasma sheet - in particular its location relative to any plasmoids observed to form in the model and its dayside and nightside thicknesses.
    AGU Fall Meeting Abstracts. 12/2009;

Publication Stats

211 Citations
73.41 Total Impact Points

Institutions

  • 2002–2013
    • University of Washington Seattle
      • Department of Earth and Space Sciences
      Seattle, WA, United States
  • 2010
    • Seattle University
      Seattle, Washington, United States
  • 2005
    • University of Colorado
      Denver, Colorado, United States