Magnetospheric Plasma Analyzer Research Articles

Magnetotail Dipolarizations and Ion Flux Variations During the Main Phase of Magnetic Storms

AbstractNear‐Earth tail dipolarizations are usually associated with fast plasma flows and an increase in energetic particle fluxes. Yet, the role of these dipolarizations in energetic ion flux transport toward the inner magnetosphere and in ring current (including partial ring current) development during the main phase of a geomagnetic storm is not fully understood. We investigated observations from Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes A, D, and E with apogees at X ≈ −13 RE and Los Alamos National Laboratory (LANL) geosynchronous spacecraft in the midnight magnetic local time (MLT) sector (21.5–1.5 h MLT) during the main phases of storms with integrated Dst exceeding 600 nT*hr from the 2010 to 2016 THEMIS tail seasons. We selected 10 storms during which at least one of the THEMIS probes was in the midnight sector during a storm’s main phase and identified 39 dipolarization events with a Bz increase exceeding 10 nT within 500 s during these storms’ main phases. In 21 of the 39 events, an increase in the magnetic field elevation angle Θ estimated from LANL magnetospheric plasma analyzer (MPA) data was detected at geosynchronous orbit (GEO) within ±15‐min‐long time window around dipolarization onset detected by THEMIS. In only 10 of those 21 events, however, did ion fluxes at energies from 50 to 500 keV increase in three or more consecutive energy bins at LANL. Comparisons of ion spectra collected by THEMIS and LANL in these 10 events revealed a similar increase in fluxes at energies E > 20 keV. Our results indicate that a dipolarization that occupies a large portion of the nightside magnetosphere from GEO to ∼12 RE is a necessary but not sufficient condition for energetic ion flux enhancements at GEO. As suggested in previous studies, an increase in the azimuthal electric field (i.e., enhanced earthward magnetic flux transport) is most likely required for an ∼100 keV ion flux increase at GEO.

On-orbit calibration of geostationary electron and proton flux observations for augmentation of an existing empirical radiation model

Challenges faced in the quantitative use of long-term radiation belt data sets include establishing their relative accuracy and correcting for on-orbit degradation. An existing empirical model of energetic plasmas (0.001–40 keV) in geosynchronous orbit has been extended in energy to several hundred keV by incorporating observations from the Geostationary Operational Environmental Satellite (GOES) 13 and 15 magnetospheric electron detector (MAGED) and magnetospheric proton detector (MAGPD). In order to ensure the accuracy of this energy range extension, the following steps were taken: (1) removing noise bursts; (2) intra-calibrating the nine solid state telescopes comprising each MAGED or MAGPD; (3) cross-calibrating GOES 13 and 15; and (4) cross-calibrating magnetospheric plasma analyzer (MPA) and GOES fluxes, the necessary final step in augmenting an MPA-based model with GOES data. The MAGED and MPA electron fluxes were demonstrated to agree well at the energy (40 keV) where they overlap, while the MAGPD proton fluxes exhibited a severe long-term degradation compared to MPA. This problem is related to the well-known long-term degradation of the proton fluxes from the similar medium energy proton and electron detector (MEPED) proton telescopes on the POES and Metop satellites in low-Earth orbit. Results of this on-orbit calibration work are used to reconsider long-standing hypotheses about the cause of degradation in similar proton telescopes.

Longitudinal expansion of field line dipolarization

We examine the substorm expansions that started at 1155 UT 10 August 1994 in the midnight sector focusing on the longitudinal (eastward) expansion of field line dipolarization in the auroral zone. Eastward expansion of the dipolarization region was observed in all of the H, D, and Z components. The dipolarization that started at 1155 UT (0027 MLT) from 260° of geomagnetic longitude (CMO) expanded to 351°(PBQ) in about 48 min. The expansion velocity was 0.03–0.04°/s, or 1.9 km/s at 62°N of geomagnetic latitude. The dipolarization region expanding to the east was accompanied by a bipolar event at the leading edge of the expansion in latitudes equatorward of the westward electrojet (WEJ). In the midnight sector at the onset meridian, the Magnetospheric Plasma Analyzer (MAP) on board geosynchronous satellite L9 measured electrons and ions between 10 eV and 40 keV. We conclude from the satellite observations that this dipolarization was characterized by the evolution of temperature anisotropies, an increase of the electron and ion temperatures, and a rapid change in the symmetry axis of the temperature tensor. The field line dipolarization and its longitudinal expansion were interpreted in terms of the slow MHD mode triggered by the current disruption. We propose a new magnetosphere-ionosphere coupling (MI-coupling) mechanism based on the scenario that transmitted westward electric fields from the magnetosphere in association with expanding dipolarization produced electrostatic potential (negative) in the ionosphere through differences in the mobility of collisional ions and collisionless electrons. The field-aligned currents that emerged from the negative potential region are arranged in a concentric pattern around the negative potential region, upward toward the center and downward on the peripheral.

Storm time equatorial magnetospheric ion temperature derived from TWINS ENA flux

AbstractThe plasma sheet plays an integral role in the transport of energy from the magnetotail to the ring current. We present a comprehensive study of the equatorial magnetospheric ion temperatures derived from Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) energetic neutral atom (ENA) measurements during moderate to intense (Dstpeak < −60 nT) storm times between 2009 and 2015. The results are validated using ion temperature data derived from the Geotail low‐energy particle energy analyzer and the Los Alamos National Laboratory magnetospheric plasma analyzer. The ion temperatures are analyzed as a function of storm time, local time, and L shell. We perform a normalized superposed epoch analysis of 48 geomagnetic storms and examine the spatial and temporal evolution of the plasma as a function of storm phase. This analysis illustrates the spatial and temporal variation of the ions from the plasma sheet into the inner magnetosphere. We find that the ion temperature increases approaching the storm peak. This enhancement has the largest magnetic local time extent near 12 RE distance downtail.

On the origin of low‐energy electrons in the inner magnetosphere: Fluxes and pitch‐angle distributions

AbstractAccurate knowledge of the plasma fluxes in the inner magnetosphere is essential for both scientific and programmatic applications. Knowledge of the low‐energy electrons (approximately tens to hundreds of eV) in the inner magnetosphere is particularly important since these electrons are acted upon by various physical processes, accelerating the electrons to higher energies, and also causing their loss. However, measurements of low‐energy electrons are challenging, and as a result, this population has been somewhat neglected previously. This study concerns observations of low‐energy electrons made by the Helium Oxygen Proton Electron instrument on board the Van Allen Probes satellites and also observations from geosynchronous orbit made by the Magnetospheric Plasma Analyzer on board Los Alamos National Laboratory satellites. The fluxes of electrons from ~30 eV to 1 keV are quantified as a function of pitch‐angle, McIlwain L parameter, and local time for both quiet and active periods. Results indicate two sources for low‐energy electrons in this energy range: the low‐energy tail of the electron plasma sheet and the high‐energy tail of the dayside ionosphere. These populations are identified primarily as a result of their different pitch‐angle distributions. Field‐aligned outflows from the dayside ionosphere are observed at all L shells during quiet and active periods. Our results also demonstrate that the dayside electron field‐aligned fluxes at ~30 eV are particularly strong between L values of 6 and 7, indicating an enhanced source within the polar ionosphere.

An empirical model of electron and ion fluxes derived from observations at geosynchronous orbit

Knowledge of the plasma fluxes at geosynchronous orbit is important to both scientific and operational investigations. We present a new empirical model of the ion flux and the electron flux at geosynchronous orbit (GEO) in the energy range ~1 eV to ~40 keV. The model is based on a total of 82 satellite-years of observations from the Magnetospheric Plasma Analyzer instruments on Los Alamos National Laboratory satellites at GEO. These data are assigned to a fixed grid of 24 local-times and 40 energies, at all possible values of Kp. Bi-linear interpolation is used between grid points to provide the ion flux and the electron flux values at any energy and local-time, and for given values of geomagnetic activity (proxied by the 3-hour Kp index), and also for given values of solar activity (proxied by the daily F10.7 index). Initial comparison of the electron flux from the model with data from a Compact Environmental Anomaly Sensor II (CEASE-II), also located at geosynchronous orbit, indicate a good match during both quiet and disturbed periods. The model is available for distribution as a FORTRAN code that can be modified to suit user-requirements.

Statistical analysis of the geomagnetic response to different solar wind drivers and the dependence on storm intensity

AbstractGeomagnetic storms start with activity on the Sun that causes propagation of magnetized plasma structures in the solar wind. The type of solar activity is used to classify the plasma structures as being either interplanetary coronal mass ejection (ICME) or corotating interaction region (CIR) driven. The ICME‐driven events are further classified as either magnetic cloud (MC) driven or sheath (SH) driven by the geoeffective structure responsible for the peak of the storm. The geoeffective solar wind flow then interacts with the magnetosphere producing a disturbance in near‐Earth space. It is commonly believed that a SH‐driven event behaves more like a CIR‐driven event than a MC‐driven event; however, in our analysis this is not the case. In this study, geomagnetic storms are investigated statistically with respect to the solar wind driver and the intensity of the events. We use the Hot Electron and Ion Drift Integrator (HEIDI) model to simulate the inner magnetospheric hot ion population during all of the storms classified as intense (Dstmin ≤ −100 nT) within solar cycle 23 (1996–2005). HEIDI is configured four different ways using either the Volland‐Stern or self‐consistent electric field and either event‐based Los Alamos National Laboratory (LANL) magnetospheric plasma analyzer (MPA) data or a reanalyzed lower resolution version of the data that provides spatial resolution. Presenting the simulation results, geomagnetic data, and solar wind data along a normalized epoch timeline shows the average behavior throughout a typical storm of each classification. The error along the epoch timeline for each HEIDI configuration is used to rate the model's performance. We also subgrouped the storms based on the magnitude of the minimum Dst. We found that typically the LANL MPA data provide the best outer boundary condition. Additionally, the self‐consistent electric field better reproduces SH‐ and MC‐driven events throughout most of the storm timeline, but the Volland‐Stern electric field better reproduces CIR‐driven events. Contrary to what we expect, examination of the HEIDI model results and solar wind data shows that SH‐driven events behave more like MC‐driven events than CIR‐driven storms.

Evolution of mass density and O+ concentration at geostationary orbit during storm and quiet events

AbstractWe 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 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 spacecraft or ne from the Radio Plasma Imager instrument on the Imager for Magnetopause‐to‐Aurora Global Exploration 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 corotate with the Earth, sudden changes in density for both ρm and ne often appear between dusk and midnight magnetic local time, 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 plasma sheet‐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.

In situ signatures of residual plasmaspheric plumes: Observations and simulation

AbstractWe compare in situ observations of the Los Alamos National Laboratory (LANL) Magnetospheric Plasma Analyzers with output of a dynamic, plasmapause test particle (PTP) simulation for the moderately disturbed interval 18–20 January 2000. In the model, weakly enhanced convection on 18 January creates a narrow drainage plume (plume A) that wraps completely around the main torus. Moderate convection on 19 January triggers significant plasmaspheric erosion, forming a second plume (B) that coexists with the narrow, wrapped, residual plume A. We fly three virtual LANL satellites through the simulation domain. The observations are globally consistent with the PTP simulation; LANL data contain several intervals of plume plasma in the model's predicted magnetic local time (MLT) sector. The modeled durations of plume sector transits are in good agreement with the LANL data. On a subglobal scale, the MLT widths and timings of the simulated plumes do not precisely agree with observations. However, several observation intervals exhibit good morphological agreement with virtual spacecraft signatures of two distinct, coexisting plumes (A and B). The fine‐scale structure in the PTP model arises from the merging of residual plume A with the newer plume B. Plume merging is one theoretical means of generating fine structure in the plasmasphere: during multiple cycles of erosion and recovery, successive layers of wrapped, residual plumes can merge with newer plumes, creating layers of filamentary density structure. The model‐data comparisons suggest that the plasmaspheric density distribution may preserve some memory of prior epochs of erosion and recovery.

Statistical properties of the surface‐charging environment at geosynchronous orbit

AbstractBased on 13 years of data from the Magnetospheric Plasma Analyzers on six Los Alamos National Laboratory (LANL) geosynchronous satellites, the statistical behavior of environmental conditions that cause strong surface charging on the satellites is examined. Analysis of the Magnetospheric Plasma Analyzers data reveals the electron energy range (~5–10 keV) and threshold flux (1.4×103 cm–2 s–1 sr–1 eV–1 at 8 keV) that are most closely associated with satellite surface charging. We also find that the average ambient electron temperature in the plasma sheet correlates with the observed magnitude of the surface potential on the LANL satellites. Analysis of the statistical occurrence rates of (1) the observed surface potential on the LANL satellites, (2) 8 keV electron fluxes above the threshold, and (3) elevated values of the average electron temperature all reveal that an enhanced surface charging probability exists (a) during higher values of Kp, (b) in the local time range from premidnight through dawn, (c) during equinox seasons, and (d) during the declining phase of the solar cycle. With the exception of the solar cycle dependence, these are the same statistical dependences found by Choi et al. (2011) in the satellite anomaly database they examined. The local time dependence of those anomalies is a particularly strong diagnostic of surface charging as a probable cause of a significant number of them.

A comparison of magnetic field measurements and a plasma‐based proxy to infer EMIC wave distributions at geosynchronous orbit

Wave‐particle interactions are fundamental to the dynamics of the outer radiation belt. Electromagnetic ion cyclotron (EMIC) waves can resonate with energetic electrons, causing pitch angle diffusion and scattering of the electrons into Earth's atmosphere. These waves act locally; thus, accurately measuring their spatial and temporal distributions is critical to understanding their contribution to radiation belt electron losses. Using Los Alamos National Laboratory Magnetospheric Plasma Analyzer data from geosynchronous orbit, we examine a plasma‐based proxy for enhanced EMIC wave growth during a set of 52 relativistic electron flux dropout events. This proxy is compared to in situ wave measurements from the GOES satellites, also at geosynchronous orbit, for single‐wave events as well as a superposed epoch statistical analysis. The proxy is extended to calculate an amplitude for the inferred waves, to enable a more quantitative comparison to the in situ GOES EMIC measurements. Signatures of EMIC waves are present in both the proxy and the direct wave observations at similar local times as well as epoch times. The waves are most prevalent in the afternoon sector, with enhanced occurrences beginning half a day before the onset of the dropouts and peaking in the day following. We see agreement in occurrence between the proxy and waves both statistically and in individual instances. This study demonstrates the powerful applications of plasma data to infer wave distributions in space.

Low-Energy Electron Exposure of Space Materials

M ATERIALS that reside on the exterior of a spacecraft, and are therefore unshielded, are exposed to the entire space environment. In addition to receiving a substantial dose of ionizing radiation from exposure to solar ultraviolet radiation, external spacecraft materials or surfaces can also be subjected to extremely large fluxes of ionizing radiation from the charged-particle environment. In particular, the electron and proton fluxes impart an extremely large dose of ionizing radiation into the surface layers of exposed materials, which generally produces both optical and mechanical degradation through a variety of mechanisms. These effects have been studied for years on many materials, particularly thermal control coatings [1–19]. Any effective simulation of the space environment for the purpose of material evaluation requires accurate modeling of these charged-particle environments. This requires environment models that describe the electron and proton fluxes as a function of particle energy for all particle energies (within reason). The models AE8 [20] and AP8 [21] have been the standards for describing the trapped-particle environments for electrons and protons, respectively, for all orbits. Unfortunately, corresponding and comprehensive engineeringmodels for the plasma environments do not currently exist for all orbits. Generally speaking, the models AE8 andAP8 are referred to as trapped-particle environment models because their intent is to describe the particle fluxes that are essentially trapped in the Van Allen radiation belts. Historically, this has been defined as particle fluxes with energies greater than 40 keV for electrons and 100 keV for protons. Particle populations with energies lower than these limits are referred to as plasma particles, which are not confined to the idealized radiation belts. The AE8 and AP8 models neglect the lower energy ranges of both electron and proton populations for largely historical reasons: Theflight data upon which they are based spanned only limited energy ranges, which resulted in the development of models that started at relatively high energy levels relative to the plasma environments. Particle populations below these energy levels (40 keV for electrons and 100 keV for protons) were either ignored or approximated through crude extrapolation of the model’s flux-energy curves. Although the Advanced Technology Satellite 6 (ATS-6) model [22] has often served to describe plasma electron and proton environments, primarily at geosynchronous Earth orbit (GEO), the model is based on only 45 days of flight data. An excellent review of the temporal and energy coverage of numerous satellite data and space environment models, including AE8, AP8, and ATS-6, is provided in [23]. Use of the plasma-population-deficient models AE8 and AP8 can easily lead to a two-order-of-magnitude underprediction of the energy deposited into the shallow depths of an external spacecraft material. It is important to note that the fluxes of particle populations increase exponentially with decreasing energy; consequently, there are orders-of-magnitude-more particles below the cutoff range of these trapped-particle models. Since these low-energy particles deposit energy and stop in very shallow depths of materials, their large combined fluence results in a tremendous deposition of energy in the surface of materials. Without consideration of the proper plasma environment, the energy deposition in exterior spacecraft materials is incorrectly predicted by energy-deposition codes. Recently,more comprehensive and accuratemodels of the plasmaparticle environments at GEO have been published. These models use a large data set of orbital measurements as their basis and provide themost accurate description of the lower-energy particle population currently available. The Los Alamos National Laboratory Magnetospheric PlasmaAnalyzer (LANL-MPA)model [24] is based onmore than 16 years of orbital measurements and characterizes GEO electron and proton fluxes from about 45 keV down to approximately Received 29 September 2010; revision received 28 April 2011; accepted for publication 23 May 2011. Copyright © 2011 by The Aerospace Corporation. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0022-4650/11 and $10.00 in correspondence with the CCC. Research Scientist, Materials Science Department, Space Materials Laboratory, 2310 East El Segundo Boulevard, M2-248. Associate of the Technical Staff, Materials Science Department, Space Materials Laboratory, 2310 East El Segundo Boulevard, M2-248. Associate Member of the Technical Staff, Materials Science Department, Space Materials Laboratory, 2310 East El Segundo Boulevard, M2-248. §Distinguished Scientist, Space Materials Laboratory, 2310 East El Segundo Boulevard, M2-248. JOURNAL OF SPACECRAFT AND ROCKETS Vol. 48, No. 6, November–December 2011

Magnetosonic wave instability analysis for proton ring distributions observed by the LANL magnetospheric plasma analyzer

Energetic proton (1–45 keV) distributions measured by the magnetospheric plasma analyzer detector on Los Alamos National Laboratory geosynchronous orbiting satellites are analyzed to study the characteristics of proton ring development during the 2001 April storm. Distinct proton rings are formed over a broad spatial region from noon to premidnight, associated with individual or multiple substorm injections on the nightside. On the basis of observed proton distributions, the convective growth rate of magnetosonic waves is calculated. We find that these proton ring distributions can provide a source of free energy for exciting magnetosonic waves, with convective growth rate mostly in the range 10−5–10−6 m−1, over a broad range of frequency from a few times ΩH+ up to ~35 ΩH+ and over a broad magnetic local time range from 1000 to 2200.

Probing the relationship between electromagnetic ion cyclotron waves and plasmaspheric plumes near geosynchronous orbit

Plasmaspheric plumes created during disturbed geomagnetic conditions have been suggested as a major cause of increased occurrences of electromagnetic ion cyclotron (EMIC) waves at these times. We have catalogued occurrences of strong Pc1 EMIC waves from 1996 through 2003 at three automated geophysical observatories operated by the British Antarctic Survey at auroral zone latitudes in Antarctica (L = 6.28, 7.68, and 8.07) and have compared them to the occurrence of plasmaspheric plumes in space, using simultaneous data from the Magnetospheric Plasma Analyzer on the Los Alamos National Laboratory 1990–095 spacecraft, in geosynchronous orbit at the same magnetic longitude. A superposed epoch analysis of these data was conducted for several categories of disturbed geomagnetic conditions, including magnetic storms, high‐speed streams, and storm sudden commencements. We found only a weak correspondence between the occurrence of strong Pc1 waves observed on the ground and either plasmaspheric plumes or intervals of extended plasmasphere at geosynchronous orbit before, during, or after the onset of any of these categories. Strong Pc1 activity peaked near or slightly after local noon during all storm phases, consistent with equatorial observations by the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer satellite at these L shells. The highest Pc1 occurrence probability was at or 1–2 days before storm onset and during the late recovery phase. Occurrence was lowest during the early recovery phase, consistent with the decrease in solar wind pressure often seen at this time. The peak at onset is consistent with earlier observations of waves in the outer magnetosphere stimulated by sudden impulses and magnetospheric compressions.

Exploring sources of magnetospheric plasma using multispecies MHD

A persistent, unresolved problem in terrestrial magnetospheric physics is determining the dominant source and associated entry mechanism for plasma in the Earth's magnetosphere. This study uses the multispecies MHD code, Block Adaptive Tree Solar Wind Roe‐Type Upwind Scheme (BATS‐R‐US), to investigate this issue. Two proton species, ionospheric origin and solar wind origin, are defined in the system and the evolution of each population is followed under different idealized solar wind conditions. It is found that during southward oriented interplanetary magnetic field (IMF), the dominant source is ionospheric plasma entering deep down tail through reconnecting field lines. During northward IMF, the dominant source is solar wind plasma entering through the flanks of the magnetosphere. This two‐mode behavior is tested through data‐model comparisons of real world simulations. Comparisons of model results against Los Alamos National Laboratory Magnetospheric Plasma Analyzer density, pressure, and inferred oxygen content support the conclusions of the idealized results.

Ion observations from geosynchronous orbit as a proxy for ion cyclotron wave growth during storm times

There is still much to be understood about the processes contributing to relativistic electron enhancements and losses in the radiation belts. Wave particle interactions with both whistler and electromagnetic ion cyclotron (EMIC) waves may precipitate or accelerate these electrons. This study examines the relation between EMIC waves and resulting relativistic electron flux levels after geomagnetic storms. A proxy for enhanced EMIC waves is developed using Los Alamos National Laboratory Magnetospheric Plasma Analyzer plasma data from geosynchronous orbit in conjunction with linear theory. In a statistical study using superposed epoch analysis, it is found that for storms resulting in net relativistic electron losses, there is a greater occurrence of enhanced EMIC waves. This is consistent with the hypothesis that EMIC waves are a primary mechanism for the scattering of relativistic electrons and thus cause losses of such particles from the magnetosphere.

Representation of the measured geosynchronous plasma environment in spacecraft charging calculations

Historically, the characterization of the magnetospheric environment has limited our ability to determine spacecraft surface charging levels. One difficulty lies in the common practice of fitting the plasma data to a Maxwellian or Double Maxwellian distribution function, which may not represent the data well for use in spacecraft charging simulations. We use electron and ion flux spectra measured by the Los Alamos National Laboratory (LANL) Magnetospheric Plasma Analyzer (MPA) during eclipse in September 2001 to examine how the use of different spectral representations of the charged particle environment in computations of spacecraft potentials during magnetospheric substorms affects the accuracy of the results. We examine charging and noncharging flux spectra and the relationships between the density and temperature moments. We then calculate the spacecraft potential (zero net current) using both the measured fluxes and several different fits to these fluxes. The potential computed using the measured fluxes and secondary and backscattered fluxes computed for graphite carbon, with a constant fraction of 81% of secondary electrons escaping, is within a factor of three of the measured potential for 87% of the data. Potentials calculated using a Kappa function fit to the electron flux and a Maxwellian function fit to the ion flux agree with measured potentials nearly as well. Alternative spectral representations give less accurate estimates. The use of all the components of the net flux, along with spacecraft specific average material properties, gives a better estimate of the spacecraft potential than the measured flux from a single high‐energy channel.

A new international geostationary electron model: IGE‐2006, from 1 keV to 5.2 MeV

Département Environnement Spatial, Office National d'Etudes et de Recherches Aérospatiales (ONERA) has been developing a model for the geostationary electron environment since 2003. Until now, this model was called Particle ONERA‐LANL Environment (POLE), and it is valid from 30 keV up to 5.2 MeV. POLE is based on the full complement of Los Alamos National Laboratory geostationary satellites, covers the period 1976–2005, and takes into account the solar cycle variation. Over the period 1976 to present, four different detectors were flown: charged particle analyzer (CPA), synchronous orbit particle analyzer (SOPA), energetic spectra for particles (ESP), and magnetospheric plasma analyzer (MPA). Only the first three were used to develop the POLE model. Here we extend the energy coverage of the model to low energies using MPA measurements. We further include the data from the Japanese geostationary spacecraft, Data Relay Test Satellite (DRTS). These data are now combined into an extended geostationary electron model which we call IGE‐2006.

Superposed epoch analysis of high‐speed‐stream effects at geosynchronous orbit: Hot plasma, cold plasma, and the solar wind

Superposed epoch analyses of magnetospheric plasma analyzer (MPA) data from Los Alamos National Laboratory (LANL) satellites are performed to reveal the density, temperature and flow velocity behavior of the hot ion plasma (0.1–45 keV), the hot electron plasma (0.03–45 keV) and the cold ion plasma (1–100 eV) at geosynchronous orbit following the arrival of high speed solar wind streams at the dayside magnetopause. The analyses reveal three striking features. (1) The arrival of a high density solar wind plasma at the leading edge of a high speed solar wind stream induces a sharp enhancement in magnetospheric convection which leads to the delivery of a hot, dense “plug” of fresh plasma sheet ions and electrons to the inner magnetosphere. On average, this superdense plasma is observed at geosynchronous orbit for ∼20 h following convection onset. There follows an extended period when exceptionally hot plasma sheet ions and electrons of more usual density are continually convected to the inner magnetosphere ‐ a environment that persists at geosynchronous orbit while the high speed stream prevails. (2) Flow velocities and convection speeds of eroded cold plasma moving toward the dayside magnetopause are calculated from MPA statistical analyses. Average convection speeds of 8–12 km s−1 are typical in plume material moving sunwards following the arrival of high speed solar wind streams at the magnetopause. (3) The density of plume material convecting to the dayside magnetopause during high speed streams which follow very calm periods (Kp ∼ 0) is around double that during high speed streams following periods when higher levels of convection persist (Kp ∼ 2).

Superposed epoch analysis of a whistler instability criterion at geosynchronous orbit during geomagnetic storms

Enhanced whistler mode waves produced by anisotropic hot plasma-sheet electrons outside the storm-time plasmapause have been suggested as one mechanism for accelerating relativistic outer-belt electrons in the aftermath of geomagnetic storms. Using measurements from the Los Alamos Magnetospheric Plasma Analyzers in geosynchronous orbit, we perform a superposed-epoch study of the storm-time behavior of the inferred plasma-sheet whistler growth parameter. Separate analyses are done for storms that result in strong relativistic electron enhancements and those that do not. The inferred whistler instability is strongest in the midnight-to-dawn sector, where freshly injected plasma-sheet electrons drift into and through the inner magnetosphere. During the main phase of both sets of storms, there is a marked drop in the whistler growth parameter, especially in the prime midnight-to-dawn sector. In the early recovery phase, this parameter is elevated and then returns to more typical values over the next few days. The elevation of the whistler growth parameter persists longer for the electron-enhanced storms than for those that do not produce such enhancements. These results suggest that whistler wave generation is greater during storms yielding enhanced levels of relativistic electrons.