ALBERT

Description: 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.

Description: A number of relativistic electron loss processes exist in the inner magnetosphere, and the extent to which MeV electron precipitation into Earth's atmosphere plays a role in radiation belt dynamics is a topic of much debate. In this work, we investigate the contribution of electron precipitation to radiation belt losses, looking at what times and locations precipitation is important. Through high-cadence low-altitude measurements from the SAMPEX satellite, we examine the distributions of millisecond (microburst) as well as longer duration (band-type) precipitation and the relative contributions of these two precipitation types to radiation belt dynamics during high speed stream (HSS) driven storms. Different local time and radial distributions between microbursts and precipitation bands suggest different scattering mechanisms as the causes of the two precipitation types. In a superposed epoch study of 42 HSS-driven storms, enhanced main and recovery phase losses to the atmosphere are observed. Microburst occurrence rates peak in the recovery phase of the storms, while their magnitudes remain fairly constant over storm phase. Precipitation bands show an increase in both occurrence and magnitude at storm onset, particularly at the inner edge of the outer radiation belt. The observations, enabled by the high time resolution and large geometric factor and field of view of the SAMPEX/HILT instrument, reveal when and where microburst and band-type precipitation are contributing to radiation belt dynamics during HSS-driven storms.

Description: The distribution of mass density along the field lines affects the ratios of toroidal (azimuthally oscillating) Alfvén frequencies, and given the ratios of these frequencies we can get information about that distribution. Here we assume the commonlyused power law form for the field line distribution, ρ m  =  ρ m , eq ( LR E / R ) α , where ρ m , eq is the value of the mass density ρ m at the magnetic equator, L is the L shell, R E is the Earth's radius, R is the geocentric distance to a point on the field line, and α is the power law coefficient. Positive values of α indicate that ρ m increases away fromthe magnetic equator, zero value indicates that ρ m is constant along the magnetic field line, and negative α indicates that there is a local peak in ρ m at the magnetic equator. Using 12 years of observations of toroidal Alfvén frequencies by the Geostationary Operational Environmental Satellites (GOES), we study the typical dependence of inferred values of α on the magnetic local time (MLT), the phase of the solar cycle as specified bythe F10.7 extreme ultraviolet solar flux, and geomagnetic activity as specified by the auroral electrojet (AE) index. Over the mostly dayside range of the observations, we find that α decreases with respect to increasing MLT and F10.7, but increases with respect to increasing AE. We develop a formula that depends on all three parameters, α 3Dmodel  = 2.2 + 1.3 ⋅  cos (MLT ⋅ 15 ∘ ) + 0.0026 ⋅ AE ⋅  cos ((MLT − 0.8) ⋅ 15 ∘ ) + 2.1 ⋅ 10 − 5  ⋅ AE ⋅ F10.7 − 0.010 ⋅ F10.7, that models the binned values of α within a standard deviation of 0.3. While we do not yet have a complete theoretical understanding of why α should depend on these parameters in such a way, we do make some observations and speculations about the causes. At least part of the dependence is related to that of ρ m , eq ; higher α , corresponding to steeper variation with respect to MLAT, occurs when ρ m , eq is lower.

Description: We simulate whistler mode waves using a hybrid code. There are four species in the simulations, hot electrons initialized with a bi-Maxwellian distribution with temperature in the direction perpendicular to background magnetic field greater than that in the parallel direction, warm isotropic electrons, cold inertialess fluid electrons and protons as an immobile background. The density of the hot population is a small fraction of the total plasma density. Comparison between the dispersion relation of our model and other dispersion relations shows that our model is more accurate for lower frequency whistlers than for higher frequency whistlers. Simulations in 2-D Cartesian coordinates agree very well with those using a full dynamics code. In the 1-D simulations along the dipole magnetic field, the predicted frequency and wave number are observed. Rising tones are observed in the 1/14 scale simulations that have larger than realistic magnetic field spatial inhomogeneity. However, in the full scale 1-D simulation in a dipole field, the waves are more broadband and do not exhibit rising tones. In the 2-D simulations in a meridional plane, the waves are generated with propagation approximately parallel to the background magnetic field. However, the wave fronts become oblique as they propagate to higher latitudes. Simulations with different plasma density profiles across L -shell are performed to study the effect of the background density on whistler propagation.

Description: Electron measurements on board six spacecraft in geosynchronous orbit are superposed-epoch analyzed for 42 high-speed-stream-driven storms. Using pitch angle–resolved fluxes in the range 30 keV to 1.7 MeV, the evolution of the outer electron radiation belt and the suprathermal tail of the electron plasma sheet are studied. The outer electron radiation belt exhibits perpendicular-dominated anisotropies on the dayside and parallel-dominated anisotropies on the nightside consistent with shell splitting in a distorted magnetosphere. The magnitudes of the radiation-belt anisotropies are weak prior to storm onset and become very large during the storms. The magnitudes of the anisotropies lessen with time as the storm ages and the radiation belt heats, probably owing to a weakening of the magnetic field distortion as the storm ages. When a calm before the storm occurs, the dayside radiation belt approaches isotropy, probably owing to pitch angle scattering in the outer plasmasphere that fills during the calm. If no calm before the storm occurs, the dayside radiation belt is strongly perpendicular dominated. The local-time pattern of anisotropy in storms is very different for the suprathermal tail of the electron plasma sheet, which tends to be perpendicular on the nightside and isotropic elsewhere. The magnitudes of the anisotropies of the suprathermal tail are a factor of ∼10 weaker than the anisotropies of the outer electron radiation belt.

Description: While the average ion mass M (normalized to amu) of bulk plasma at geosynchronous orbit has been calculated at solar maximum (during the era of the Combined Release and Radiation Effects Satellite (CRRES)), the solar cycle dependence of bulk ion composition at geosynchronous orbit is not known. Here, we use measurements of mass density ρm from Alfvén wave frequencies measured by the Geostationary Operational Environmental Satellites and ion density measurements by the Magnetospheric Particle Analyzer (MPA) on Los Alamos National Laboratory (LANL) spacecraft to establish the solar cycle dependence of bulk ion composition. We show that there is a strong correlation between the yearly median value of ρm, ρm,yr−med, and the yearly average of the solar EUV flux F10.7, F10.7,yr−av; log10(ρm,yr−med) $\simeq$ 0.5089 + 0.003607F10.7,yr−av (for ρm values adjusted to a magnetic latitude MLAT of 8°). We calibrate the measurements of the MPA instrument on one spacecraft to those from another by using yearly median density values. Then, using close conjunctions of LANL spacecraft with CRRES (for which we have inferred values of ρm and ne), we calibrate the ideal theoretical value of MPA ion density nMPA−th (the value that MPA would measure if it measured all the ions) to the observed values directly measured by the instrument, nMPA−obs. We find that nMPA−th is approximately 1.47 times the value of nMPA−obs measured by the LANL 1994 spacecraft. Using the yearly median values of ρm as a function of F10.7, the yearly median values of nMPA−th from the MPA instruments, and a model for the concentration of He+, we are able to calculate the solar cycle dependence of the average ion mass M and the O+ concentration ηO+ ≡ nO+/ne. We find that M is typically ∼3.8 at solar maximum and near unity at solar minimum. Typical values of ηO+ vary by 2 orders of magnitude over the solar cycle, from about 0.2 at solar maximum to ∼2 × 10−3 at solar minimum. Furthermore, our results also demonstrate that the typical concentration of He+ must also be very low at solar minimum. Since the median yearly values of density are low, characteristic of the plasma trough, our results are most applicable to that region. Considering, however, that the plasmasphere and plume typically have a low concentration of O+, the concentration of O+ at geosynchronous orbit at solar minimum is likely to be low for all conditions (with the possible exception of very low densities for which the high-energy component might dominate).

Description: Electron precipitation from the Earth's inner magnetosphere transmits solar variability to the Earth's upper atmosphere and may affect surface level climate. Here we conduct a superposed epoch analysis of energetic electrons observed by the NOAA POES spacecraft during 42 high-speed solar wind stream (HSS) driven geomagnetic storms to determine the temporal evolution and global distribution of the precipitating flux. The flux of trapped and precipitating E 〉 30 keV electrons increases immediately following storm onset and remains elevated during the passage of the HSS. In contrast, the trapped and precipitating relativistic electrons (E 〉 1 MeV) drop out following storm onset and subsequently increase during the recovery phase to levels which eventually exceed the prestorm levels. There is no evidence for enhanced precipitation of relativistic electrons during the MeV flux drop out, suggesting that flux drop outs during the main phase of HSS-driven storms are not due to precipitation to the atmosphere. On average, the flux of precipitating E 〉 30 keV electrons is enhanced by a factor of ∼10 during the passage of the high-speed stream at all geographic longitudes. In contrast, the precipitating relativistic electron count rate is observed to peak in the region poleward of the South Atlantic Anomaly. During the passage of the high-speed stream, the flux of precipitating E 〉 30 keV electrons peaks in the region from 2100 to 1200 magnetic local time at low L (4 〈 L 〈 7) and in the prenoon sector at high L (7 〈 L 〈 9), suggesting that chorus waves are responsible for the precipitation of E 〉 30 keV electrons in both regions.