ALBERT

Description: 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.

Description: The historical (1972–2013) gaps of the solar wind parameters are filled-in by smooth modes of co-variability using the continuous geomagnetic indices with Singular Spectrum Analysis (SSA). A systematic study using experiments with synthetic gaps has been performed to determine optimal SSA parameters for reconstruction of gaps over full solar cycle, and to assess the reconstruction skill. We assessed the accuracy of the SSA gap-filling for solar wind reconstruction in T96 and TS05 empirical magnetic field models by using GOES measurements at geostationary orbit, and compared it to results based on parameters from existing Qin and Denton interpolation. The SSA gap-filling method does improve accuracy of empirical magnetic field models, especially when gaps are large and in particular for the TS05 magnetic field model.

Description: We simulate electromagnetic ion cyclotron (EMIC) wave growth and evolution within three regions, the plasmasphere (or plasmaspheric plume), the plasmapause, and the low density plasmatrough outside the plasmapause. First we use a ring current simulation with a plasmasphere model to model the particle populations that give rise to the instability for conditions observed on 9 June, 2001. Then, using two different models for the cold ion composition, we do a full scale hybrid code simulation in dipole coordinates of the EMIC waves on a meridional plane at MLT = 18 and at 1900 UT within a range of L shell from L  = 4.9 to 6.7. EMIC waves were observed during June 9, 2001 by Geostationary Operational Environmental Satellite (GOES) spacecraft. While an exact comparison between observed and simulated spectra is not possible here, we do find significant similarities between the two, at least at one location within the region of largest wave growth. We find that the plasmapause is not a preferred region for EMIC wave growth, though waves can grow in that region. The density gradient within the plasmapause does, however, affect the orientation of wave fronts and wave vector both within the plasmapause and in adjacent regions. There is a preference for EMIC waves to be driven in the He+ band (frequencies between the O+ and He+ gyrofrequencies) within the plasmasphere, although they can also grow in the plasmatrough. If present, H+ band waves are more likely to grow in the plasmatrough. This fact, plus L dependence of the frequency and possible time evolution toward lower frequency waves can be explained by a simple model. Large O+ concentration limits the frequency range of or even totally quenches EMIC waves. This is more likely to occur in the plasmatrough at solar maximum. Such large O+ concentration significantly affects the H+ cutoff frequency, and hence the width in frequency of the stop band above the He+ gyrofrequency. EMIC wave surfaces predicted by cold plasma theory are altered by the finite temperature of the ring current H+.

Description: 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.

Description: . We study the variation of plasma mass density in the outer magnetosphere over a solar cycle using mass density estimated from the frequency of fundamental toroidal standing Alfvén waves observed by the Geotail spacecraft. We identify wave events using ion bulk velocity data covering 1995–2006 and use events in the 0400–0800 magnetic local time sector for statistical analysis. We find that the F 10.7 index is a dominant controlling factor of the mass density. For the equatorial mass density that is normalized to the value at L  = 11, we obtain an empirical formula = − 0.136 + 1.78 × 10 − 3 F 10.7 , where the units of and F 10.7 are amu cm − 3 and 10 − 22 W m − 2 Hz − 1 (solar flux units, sfu), respectively. This formula indicates that changes by a factor of 1.8, if F 10.7 changes from 70 sfu (solar minimum) to 210 sfu (solar maximum). A formula derived in a similar manner using GOES magnetometer data ( Takahashi et al ., 2010) indicates that, for the same range of F 10.7 , the mass density at L  ∼ 7 varies by a factor of 3.5. We attribute the smaller factor at L  = 11 to the lower O + /H + number density ratio at higher L , the stronger F 10.7 dependence of the O + outflow rate than the H + outflow rate, and entry of solar wind H + ions to theouter magnetosphere.

Description: We analyze data acquired by the Van Allen Probes on 8 November 2012, during a period of extended low geomagnetic activity, to gain new insight into plasmaspheric ultra-low-frequency (ULF) waves. The waves exhibited strong spectral power in the 5–40 mHzband and included multiharmonic toroidal waves visible up to the 11th harmonic, unprecedented in the plasmasphere. During this wave activity, the interplanetary magnetic field cone angle was small, suggesting that the waves were driven by broadband compressional ULF waves originating in the foreshock region. This source mechanism is supported by the tailward propagation of the compressional magnetic field perturbations at a phase velocity of a few hundred kilometers per second that is determined bythe cross phase analysis of data from the two spacecraft. We also find that the coherence and phase delay of the azimuthal components of the magnetic field from the two spacecraft strongly depend on the radial separation of the spacecraft and attribute this feature to field line resonance effects. Finally, using the observed toroidal wave frequencies, we estimate the plasma mass density for L  = 2.6–5.8. By comparing the mass density with the electron number density that is estimated from the spectrum of plasma waves, we infer that the plasma was dominated by H + ions and was distributed uniformly along the magnetic field lines. The electron density is higher than the prediction of saturated plasmasphere models, and this “super saturated” plasmasphere and the uniform ion distribution are consistent with the low geomagnetic activity that prevailed.

Description: We investigated mass density ρ m and O + concentration η O +  ≡  n O + / n e (where n O + and n e are the O + and electron density, respectively)during two events, one active and one more quiet. We found ρ m from observations of Alfvén wave frequencies measured by the Geostationary Operational Environmental Satellites (GOES), and we investigated composition by combining measurements of ρ m with measurements of ion density n MPA,i from the Magnetospheric Plasma Analyzer (MPA) instrument on Los Alamos National Laboratory (LANL) spacecraft or n e 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 η O + values ranging from near zero to close to unity. For geostationary spacecraft that co-rotate with the earth, sudden changes in density for both ρ m and n e 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 n e gradually refills leading to a gradual change in composition from low density plasma that is relatively cold and heavy (high average ion mass M  ≡  ρ m / n e ) 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 n e is very dependent on whether it is measured inside or outside a plasmaspheric plume, while ρ m 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.

Description: 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.

Description: The Naval Research Laboratory SAMI3 (Sami3 is Also a Model of the Ionosphere) code is used to model observed plasmasphere dynamics for 2001 February 1–5, a period of quiet time refilling. The SAMI3 model is driven at high latitudes by the magnetospheric potential calculated by the Weimer05 empirical model, using the observed solar wind. At mid-to-low latitudes, the self-consistent dynamo potential is included, driven by specified winds. During this quiet period we find that the shape of the plasmasphere, at any given time, varies significantly with the wind model even as a similar degree of model-data agreement is recovered for each of the three wind models used. Diurnal oscillations in the model electron density, which are strong when plotted at fixed magnetic local time, are consistent with the degree of variation seen in the measured densities. In all three cases, SAMI3 compares favorably to electron density measured in situ by the IMAGE spacecraft. Results with no winds or with specific wind effects excluded show that wind-driven E × B drifts shape the plasmasphere, relative to a round plasmasphere with no winds, and reduce the refilling rate, relative to the higher refilling rate found without winds.

Description: The ULF magnetospheric indices S gr , S geo , T gr , and T geo 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 S grd , S geod , T grd , and T geod . 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 S grd and T grd are more predictable than the geosynchronous indices S geod and T geod . In the analysis a difference index ΔS mag  ≈ S grd - 0.693 S geod is derived: the time integral of ΔS mag 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 S grd and S geod 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 (r corr  = 0.926) between the two canonical variables are obtained.