David K. Milling

Effect of EMIC waves on relativistic and ultrarelativistic electron populations: Ground-based and Van Allen Probes observations

We study the effect of electromagnetic ion cyclotron (EMIC) waves on the loss and pitch angle scattering of relativistic and ultrarelativistic electrons during the recovery phase of a moderate geomagnetic storm on 11 October 2012. The EMIC wave activity was observed in situ on the Van Allen Probes and conjugately on the ground across the Canadian Array for Real-time Investigations of Magnetic Activity throughout an extended 18 h interval. However, neither enhanced precipitation of >0.7 MeV electrons nor reductions in Van Allen Probe 90° pitch angle ultrarelativistic electron flux were observed. Computed radiation belt electron pitch angle diffusion rates demonstrate that rapid pitch angle diffusion is confined to low pitch angles and cannot reach 90°. For the first time, from both observational and modeling perspectives, we show evidence of EMIC waves triggering ultrarelativistic (~2–8 MeV) electron loss but which is confined to pitch angles below around 45° and not affecting the core distribution.

Monitoring spatial and temporal variations in the dayside plasmasphere using geomagnetic field line resonances

It is well known that the resonant frequency of geomagnetic field lines is determined by the magnetic field and plasma density. We used cross-phase and related methods to determine the field line resonance frequency across 2.4≤<L≤4.5 in the Northern Hemisphere at 78°–106° magnetic longitude and centered on L=2.8 in the Southern Hemisphere at 226° magnetic longitude, for several days in October and November 1990. The temporal and spatial variation in plasma mass density was thus determined and compared with VLF whistler measurements of electron densities at similar times and locations. The plasma mass loading was estimated and found to be low, corresponding to 5–10% He+ on the days examined. The plasma mass density is described by a law of the form (R/Req)−p, where p is in the range 3–6 but shows considerable temporal variation, for example, in response to changes in magnetic activity. Other features that were observed include diurnal trends such as the sunrise enhancement in plasma density at low latitudes, latitude-dependent substorm refilling effects, shelves in the plasma density versus L profile, and a longitudinal asymmetry in plasma density. We can also monitor motion of the plasmapause across the station array. Properties of the resonance were examined, including the resonance size, Q, and damping. Finally, we note the appearance of fine structure in power spectra at these latitudes, suggesting that magnetospheric waveguide or cavity modes may be important in selecting wave frequencies.

Analytic expressions for ULF wave radiation belt radial diffusion coefficients

We present analytic expressions for ULF wave-derived radiation belt radial diffusion coefficients, as a function of L and Kp, which can easily be incorporated into global radiation belt transport models. The diffusion coefficients are derived from statistical representations of ULF wave power, electric field power mapped from ground magnetometer data, and compressional magnetic field power from in situ measurements. We show that the overall electric and magnetic diffusion coefficients are to a good approximation both independent of energy. We present example 1-D radial diffusion results from simulations driven by CRRES-observed time-dependent energy spectra at the outer boundary, under the action of radial diffusion driven by the new ULF wave radial diffusion coefficients and with empirical chorus wave loss terms (as a function of energy, Kp and L). There is excellent agreement between the differential flux produced by the 1-D, Kp-driven, radial diffusion model and CRRES observations of differential electron flux at 0.976 MeV—even though the model does not include the effects of local internal acceleration sources. Our results highlight not only the importance of correct specification of radial diffusion coefficients for developing accurate models but also show significant promise for belt specification based on relatively simple models driven by solar wind parameters such as solar wind speed or geomagnetic indices such as Kp. Key Points Analytic expressions for the radial diffusion coefficients are presented The coefficients do not dependent on energy or wave m value The electric field diffusion coefficient dominates over the magnetic

Wavelet-based ULF wave diagnosis of substorm expansion phase onset

[1] Using a discrete wavelet transform with a Meyer wavelet basis, we present a new quantitative algorithm for determining the onset time of Pi1 and Pi2 ULF waves in the nightside ionosphere with � 20- to 40-s resolution at substorm expansion phase onset. We validate the algorithm by comparing both the ULF wave onset time and location to the optical onset determined by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE)–Far Ultraviolet Imager (FUV) instrument. In each of the six events analyzed, five substorm onsets and one pseudobreakup, the ULF onset is observed prior to the global optical onset observed by IMAGE at a station closely conjugate to the optical onset. The observed ULF onset times expand both latitudinally and longitudinally away from an epicenter of ULF wave power in the ionosphere. We further discuss the utility of the algorithm for diagnosing pseudobreakups and the relationship of the ULF onset epicenter to the meridians of elements of the substorm current wedge. The importance of the technique for establishing the causal sequence of events at substorm onset, especially in support of the multisatellite Time History of Events and Macroscale Interactions During Substorms (THEMIS) mission, is also described.

Pi2 pulsations observed with the Polar satellite and ground stations: Coupling of trapped and propagating fast mode waves to a midlatitude field line resonance

Simultaneous measurements from the Polar satellite and several ground stations of two substorm-related Pi2 pulsation events (separated by ∼6 min) provide evidence for radially trapped and propagating fast mode waves and a coupled field line resonance (FLR). The Pi2 pulsations were observed at five ground stations located between 2130 and 2330 magnetic local time (MLT) ranging from L=1.83 to 3.75, which showed nearly identical waveforms in the H component with a frequency of ∼20 mHz. Five additional ground stations located between L=4.48 and ∼15 (on similar meridians) recorded weaker less-correlated signals. The pulsations were also detected both simultaneously and with a time delay of ∼38 s at two low-latitude stations (L=1.17 and 1.23) on the dayside at ∼1030 and ∼0612 MLT, respectively, indicating the global extend of the pulsations. The nightside ground data showed an amplitude maximum and a phase reversal in the H component between L=3.4 and 3.75. During the oscillations the Polar satellite moved (on the same meridian as the ground stations in the nightside) from 14° to 10° magnetic latitude and from L=4.1 to 3.7. Electric and magnetic field measurements also showed two Pi2 pulsation events (∼20 mHz) in both the compressional (Bz and Ey) and transverse (By and Ex) mode components with waveforms almost identical to the ground signals. Whereas the first Pi2 had a standing wave structure in the compressional mode, the second Pi2 was a propagating wave. Both Pi2s had standing wave signatures in the transverse mode. The amplitude of the compressional magnetic field component (Bz) was ∼40% of that of the azimuthal component (By). Although the two Pi2 events showed equal amplitudes in the H component of ground data, Polar recorded much larger oscillations in the azimuthal magnetic field component (By) of the first Pi2 event; the fast mode amplitude (Bz) was nearly unchanged for both Pi2s. This suggests that Polar was at or near a localized FLR excited by the oscillations of the fast mode waves. During the in situ FLR observation, Polars footpoint was closest to the ground stations which recorded the amplitude maximum and the phase reversal in the H component.

Monitoring the plasmapause using geomagnetic field line resonances

This paper discusses the use of ground magnetometer data to derive plasma mass density profiles of the dayside plasmapause region with spatial and temporal resolution in the range 0.15-0.4 R-E and 20-60 min. This is achieved using cross-phase techniques to identify field line resonance signatures that are not apparent in power spectra. Under quiet conditions, mass density profiles do not show a distinct plasmapause and closely resemble electron density profiles for similar conditions. Under more active conditions the plasmapause can be clearly identified, and its width can be resolved in about 20% of the cases. Spatial integration effects smooth the mass density profiles near the plasmapause boundaries, while comparison of the mass and electron densities allows estimates of the heavy ion mass loading. Temporal variations in the plasmapause position and plasmaspheric density depletions are readily resolved. Sudden changes in solar wind conditions cause a redistribution of plasma within similar to20 min, probably in response to penetration of the magnetospheric electric field into the plasmasphere. Field line resonances occur daily and provide a useful tool for investigating the plasmapause region, especially in conjunction with VLF whistler and in situ particle and imaging experiments. Furthermore, the extensive existing suites of magnetometer data permit retrospective studies of focus intervals.

Electron precipitation from EMIC waves: a case study from 31 May 2013

On 31 May 2013 several rising-tone electromagnetic ion-cyclotron (EMIC) waves with intervals of pulsations of diminishing periods (IPDP) were observed in the magnetic local time afternoon and evening sectors during the onset of a moderate/large geomagnetic storm. The waves were sequentially observed in Finland, Antarctica, and western Canada. Co-incident electron precipitation by a network of ground-based Antarctic Arctic Radiation-belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK) and riometer instruments, as well as the Polar-orbiting Operational Environmental Satellite (POES) electron telescopes, was also observed. At the same time POES detected 30-80 keV proton precipitation drifting westwards at locations that were consistent with the ground-based observations, indicating substorm injection. Through detailed modelling of the combination of ground and satellite observations the characteristics of the EMIC-induced electron precipitation were identified as: latitudinal width of 2-3° or ΔL=1 Re, longitudinal width ~50° or 3 hours MLT, lower cut off energy 280 keV, typical flux 1×104 el. cm-2 sr-1 s-1 >300 keV. The lower cutoff energy of the most clearly defined EMIC rising tone in this study confirms the identification of a class of EMIC-induced precipitation events with unexpectedly low energy cutoffs of <400 keV.

Modeling cross L shell impacts of magnetopause shadowing and ULF wave radial diffusion in the Van Allen belts

We present simulations of the outer electron radiation belt using a new ULF wave-driven radial diffusion model, including empirical representations of loss due to chorus and plasmaspheric hiss. With an outer boundary condition constrained by in situ electron flux observations, we focus on the impacts of magnetopause shadowing and outward radial diffusion in the heart of the radiation belt. Third invariant conserving solutions are combined to simulate the L shell and time dependence of the differential flux at a fixed energy. Results for the geomagnetically quiet year of 2008 demonstrate not only remarkable cross L shell impacts from magnetopause shadowing but also excellent agreement with the in situ observations even though no internal acceleration source is included in the model. Our model demonstrates powerful utility for capturing the cross-L impacts of magnetopause shadowing with significant prospects for improved space weather forecasting. The potential role of the plasmasphere in creating a third belt is also discussed.

Spatial localization and ducting of EMIC waves: Van Allen Probes and ground‐based observations

On 11 October 2012, during the recovery phase of a moderate geomagnetic storm, an extended interval (> 18 h) of continuous electromagnetic ion cyclotron (EMIC) waves was observed by Canadian Array for Real-time Investigations of Magnetic Activity and Solar-Terrestrial Environment Program induction coil magnetometers in North America. At around 14:15 UT, both Van Allen Probes B and A (65° 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 Earths 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.

Reply to comment by K. Liou and Y.-L. Zhang on “Wavelet-based ULF wave diagnosis of substorm expansion phase onset”

Citation: Murphy, K. R., I. J. Rae, I. R. Mann, A. P. Walsh, D. K. Milling, C. E. J. Watt, L. Ozeke, H. U. Frey, V. Angelopoulos, andC. T. Russell (2009), Reply to comment by K. Liou and Y.-L. Zhang on ‘‘Wavelet-based ULF wave diagnosis of substorm expansionphase onset,’’ J. Geophys. Res., 114, A10207, doi:10.1029/2009JA014351.