ALBERT

Description: In this work, Van Allen Probes data are used to derive terrestrial plasmaspheric hiss wave power distributions organized by (1) distance away from the plasmapause and (2) plasmapause distance from Earth. This approach is in contrast to the traditional organization of hiss wave power by L parameter and geomagnetic activity. Plasmapause-sorting reveals previously unreported and highly repeatable features of the hiss wave power distribution, including: a regular spatial distribution of hiss power with respect to the plasmapause, a standoff distance between peak hiss power and the plasmapause, and frequency-dependent spatial localization of hiss. Identification and quantification of these features can provide insight into hiss generation and propagation, and will facilitate blackimproved parameterization of hiss wave power in predictive simulations of inner magnetosphere dynamics.

Description: Using the Van Allen Probes we investigate the enhancement in the large scale duskward convection electric field during the geomagnetic storm (Dst ~ −120 nT) on June 1, 2013 and its role in ring current ion transport and energization, and plasmasphere erosion. During this storm, enhancements of ~1-2 mV/m in the duskward electric field in the co-rotating frame are observed down to L shells as low as ~2.3. A simple model consisting of a dipole magnetic field and constant, azimuthally westward, electric field is used to calculate the earthward and westward drift of 90° pitch angle ions. This model is applied to determine how far earthward ions can drift while remaining on Earth's night side, given the strength and duration of the convection electric field. The calculation based on this simple model indicates that the enhanced duskward electric field is of sufficient intensity and duration to transport ions from a range of initial locations and initial energies characteristic of (though not observed by the Van Allen Probes) the earthward edge of the plasma sheet during active times ( L ~ 6–10 and ~1-20 keV) to the observed location of the 58–267 keV ion population, chosen as representative of the ring current (L ~3.5 – 5.8). According to the model calculation, this transportation should be concurrent with an energization to the range observed, ~58-267 keV. Clear coincidence between the electric field enhancement and both plasmasphere erosion and ring current ion (58–267 keV) pressure enhancements are presented. We show for the first time, nearly simultaneous enhancements in the duskward convection electric field, plasmasphere erosion, and increased pressure of 58–267 keV ring current ions. These 58–267 keV ions have energies that are consistent with what they are expected to pick up by gradient B drifting across the electric field. These observations strongly suggest that we are observing the electric field that energizes the ions and produces the erosion of the plasmasphere.

Description: Abstract Electron velocity distributions in Mars's magnetosheath show a systematic erosion of the energy spectrum with distance downstream from the bow shock. Previous attempts to model this erosion invoked assumptions to promote electron ionization impact collisions with Mars's neutral hydrogen exosphere. We show that the near collision‐free magnetosheath requires a kinetic description; the population of electrons at any location is a convolution of electrons arriving from more distant regions that ultimately map directly to the solar wind. We construct a simple model that captures all the essential physics. The model demonstrates how the erosion of the electron distributions is the result of the trapping, escape, and replacement of electrons that traverse the global bow shock; some are temporarily confined to the expanding cavity formed by the cross‐shock electrostatic potential. The model also has implications for the ability of solar wind electrons to reach altitudes below the pileup boundary.

Description: Poloidal ULF waves are capable of efficiently interacting with energetic particles in the ring current and the radiation belt. Using Van Allen Probes (RBSP) data from October 2012 to July 2014, we investigate the spatial distribution and storm-time occurrence of Pc4 (7-25 mHz) poloidal waves in the inner magnetosphere. Pc4 poloidal waves are sorted into two categories: waves with and without significant magnetic compressional components. Two types of poloidal waves have comparable occurrence rates, both of which are much higher during geomagnetic storms. The non-compressional poloidal waves mostly occur in the late recovery phase associated with an increase of Dst toward 0, suggesting that the decay of the ring current provides their free energy source. The occurrence of dayside compressional Pc4 poloidal waves is found correlated with the variation of the solar wind dynamic pressure, indicating their origin in the solar wind. Both compressional and non-compressional waves preferentially occur on the dayside near noon at L~5-6. In addition, compressional poloidal waves are observed at MLT 18-24 on the nightside. The location of the Pc4 poloidal waves relative to the plasmapause is investigated. The RBSP statistical results may shed light on the in-depth investigations of the generation and propagation of Pc4 poloidal waves.

Description: The twin Van Allen Probe spacecraft, launched in August 2012, carry identical scientific payloads. The Electric and Magnetic Fields Instrument Suite and Integrated Science (EMFISIS) suite includes a plasma wave instrument (Waves) that measures three magnetic and three electric components of plasma waves in the frequency range of 10 Hz to 12 kHz using triaxial search coils and the Electric Fields and Waves (EFW) triaxial electric field sensors. The Waves instrument also measures a single electric field component of waves in the frequency range of 10 to 500 kHz. A primary objective of the higher frequency measurements is the determination of the electron density n e at the spacecraft, primarily inferred from the upper hybrid resonance frequency f uh . Considerable work has gone into developing a process and tools for identifying and digitizing the upper hybrid resonance frequency in order to infer the electron density as an essential parameter for interpreting not only the plasma wave data from the mission, but also as input to various magnetospheric models. Good progress has been made in developing algorithms to identify f uh and create a data set of electron densities. However, it is often difficult to interpret the plasma wave spectra during active times to identify f uh and accurately determine n e . In some cases there is not a clear signature of the upper hybrid band and the low-frequency cutoff of the continuum radiation is used. We describe the expected accuracy of n e and issues in the interpretation of the electrostatic wave spectrum.

Description: During an interval when the interplanetary magnetic field was large and primarily duskward and southward, a stable region of auroral emission was observed on 17 August 2001 by IMAGE at ∼16 magnetic local time, poleward of the main aurora, for 1 h, from before the onset of a large substorm through the recovery phase. In a region where ions showed the energy dispersion expected for the cusp, strong field-aligned currents and Poynting flux were observed by Polar (at 1.8 RE in the Southern Hemisphere) as it transited field lines mapping to the auroral spot in the Northern Hemisphere. The data are consistent with the hypothesis that the long-lasting electron auroral spot maps to the magnetopause region where reconnection was occurring. Under the assumption of conjugacy between the Northern and Southern hemispheres on these field lines, the Polar data suggest that the electrons on these field lines were accelerated by Alfvén waves and/or a quasi-static electric field, primarily at altitudes below a few RE since the in situ Poynting flux (mapped to 100 km) is comparable to the energy flux of the emission while the mapped in situ electron energy flux is much smaller. This event provides the first example of an emission due to electrons accelerated at low altitudes at the foot point of a region of quasi-steady dayside reconnection. Cluster data in the magnetotail indicate that the Poynting flux from the reconnection region during this substorm is large enough to account for the observed nightside aurora.

Description: Abstract We present a statistical analysis with 100% duty cycle and non‐time‐averaged amplitudes of the prevalence and distribution of high‐amplitude 〉 50 pT whistler‐mode waves in the outer radiation belt using 5 years of Van Allen Probes data. Whistler‐mode waves with high magnetic field amplitudes are most common above L=4.5 and between MLT of 0‐14 where they are present approximately 1‐6% of the time. During high geomagnetic activity, high‐amplitude whistler‐mode wave occurrence rises above 25% in some regions. The day‐side population are more common during quiet or moderate geomagnetic activity and occur primarily 〉 5 degrees from the magnetic equator, while the night‐to‐dawn population are enhanced during active times and are primarily within 5 degrees of the magnetic equator. These results are different from the distribution of electric field peaks discussed in our previous paper covering the same time period and spatial range. Our previous study found large‐amplitude electric field peaks were common down to L=3.5 and were largely absent from afternoon and near‐noon. The different distribution of large electric and magnetic field amplitudes implies that the low‐L component of whistler‐mode waves observed previously are primarily highly oblique, while the dayside and high‐L populations are primarily field‐aligned. These results have important implications for modeling radiation belt particle interactions with chorus, as large‐amplitude waves interact non‐linearly with electrons, resulting in rapid energization, de‐energization, or pitch‐angle scattering. This also may provide clues regarding the mechanisms which can cause significant whistler‐mode wave growth up to more than 100x the average wave amplitude.

Description: [1]  On 11 th October 2012, during the recovery phase of a moderate geomagnetic storm, an extended interval (〉 18 hours) of continuous EMIC waves was observed by CARISMA and STEP induction coil magnetometers in North America. At around 14:15 UT, both Van Allen Probes B and A (65 degrees magnetic longitude apart) in conjunction with the ground array observed very narrow (ΔL ~ 0.1-0.4) left-hand polarized EMIC emission confined to regions of mass density gradients at the outer edge of the plasmasphere at L ~ 4. EMIC waves were seen with complex polarization patterns on the ground, in good agreement with model results from Woodroffe and Lysak [2012] and consistent with Earth's rotation sweeping magnetometer stations across multiple polarization reversals in the fields in the Earth-ionosphere duct. The narrow L-widths explain the relative rarity of space-based EMIC occurrence, ground-based measurements providing better estimates of global EMIC wave occurrence for input into radiation belt dynamical models.

Description: [1]  We present Cluster spacecraft observations of large-amplitude ( δ | B |/| B | ~ 1) fast mode magnetosonic waves in the heliospheric current sheet (HCS). The crossings of the HCS and the associated heliospheric plasma sheet (HPS) are encountered by Cluster in the near Earth solar wind and by ACE upstream of Earth. Multiple current layers are detected in correspondence with small-scale discontinuities in the regime of open magnetic field lines within the HCS. Fast magnetosonic waves are observed at one current layer, accompanying the phase-steeped edge of a large amplitude transverse Alfvén wave. The observed fast mode waves are in the frequency range 0.01 Hz-0.2 Hz, characterized by a strong correlation between variations of plasma density and magnetic field strength. Analysis of ratio δE / δB indicate that the fast mode wave packet consists of an anti-sunward propagating component and a larger sunward propagating components in the rest frame of the solar wind. The fast mode waves are not observed by ACE in the upstream solar wind. The generation of the fast mode waves may relate to the development of the phase-steeped Alfvén wave and have profound effects on the evolution of the solar wind plasma.

Description: We present a Cluster spacecraft investigation of plasma sheet-confined waves and the associated Poynting flux in strong ion jets near a reconnection region in the geomagnetic tail at ∼−17 Re. Using phase lag analysis, we determine from multispacecraft measurements the phase front, wavelength, and phase velocity of the waves with spacecraft frame frequencies from 0.03 Hz to 1 Hz. The wave phase velocities were found to be directed nearly perpendicular to the local magnetic field laying in the plasma sheet plane. The strong evidence that the waves were driven by ion velocity shears has been provided. The waves are confined by the plasma sheet and their amplitude diminishes in the tail lobes. The δE/δB ratios are mostly on the order of the Alfven speed. The observed wave characteristics are different from plasma sheet flapping observed at the same time. The field-aligned Poynting flux was found to be directed away from the reconnection region. The values of earthward Poynting flux (100 ergs/cm2s or 0.1 W/m2) and longitudinal scales (10–100 km) mapped to the auroral ionosphere suggest that the observed waves may provide energy needed to accelerate auroral particles and may drive observed motions in auroral curls.