J. Bortnik

Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft

[1] Whistler mode chorus waves are receiving increased scientific attention due to their important roles in both acceleration and loss processes of radiation belt electrons. A new global survey of whistler-mode chorus waves is performed using magnetic field filter bank data from the THEMIS spacecraft with 5 probes in near-equatorial orbits. Our results confirm earlier analyses of the strong dependence of wave amplitudes on geomagnetic activity, confinement of nightside emissions to low magnetic latitudes, and extension of dayside emissions to high latitudes. An important new finding is the strong occurrence rate of chorus on the dayside at L > 7, where moderate dayside chorus is present >10% of the time and can persist even during periods of low geomagnetic activity. Citation: Li, W., R. M. Thorne, V. Angelopoulos, J. Bortnik, C. M. Cully, B. Ni, O. LeContel, A. Roux, U. Auster, and W. Magnes (2009), Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft, Geophys. Res. Lett., 36, L09104, doi:10.1029/2009GL037595.

The unexpected origin of plasmaspheric hiss from discrete chorus emissions.

Plasmaspheric hiss is a type of electromagnetic wave found ubiquitously in the dense plasma region that encircles the Earth, known as the plasmasphere. This important wave is known to remove the high-energy electrons that are trapped along the Earth’s magnetic field lines, and therefore helps to reduce the radiation hazards to satellites and humans in space. Numerous theories to explain the origin of hiss have been proposed over the past four decades, but none have been able to account fully for its observed properties. Here we show that a different wave type called chorus, previously thought to be unrelated to hiss, can propagate into the plasmasphere from tens of thousands of kilometres away, and evolve into hiss. Our new model naturally accounts for the observed frequency band of hiss, its incoherent nature, its day–night asymmetry in intensity, its association with solar activity and its spatial distribution. The connection between chorus and hiss is very interesting because chorus is instrumental in the formation of high-energy electrons outside the plasmasphere, whereas hiss depletes these electrons at lower equatorial altitudes.

An Observation Linking the Origin of Plasmaspheric Hiss to Discrete Chorus Emissions

Chorus Hissing Plasmaspheric hiss, a type of unstructured broadband, low-frequency radio emission, has long been known to exist in Earths plasmasphere, but its origin has been uncertain. The source of hiss could be a different type of radio wave, called chorus, which originates outside the plasmasphere during geomagnetic storms. Both types of radio wave influence the behavior of energetic electrons in the near-Earth space environment, with implications for spacecraft and astronaut safety, but a correlation between the two has been difficult to establish experimentally. Recently, two of the five satellites of the THEMIS constellation were fortuitously able to record 4 minutes of electromagnetic wave data at high resolution during geomagnetically active conditions, detecting both chorus and hiss. An analysis of the data by Bortnik et al. (p. 775; see the Perspective by Santolik and Chum) revealed that the two sets of waves were well correlated, with hiss lagging behind chorus as expected, implying that one indeed evolved into the other. The radio waves that remove energetic electrons from Earth’s radiation belts originate outside the plasmasphere. A long-standing problem in the field of space physics has been the origin of plasmaspheric hiss, a naturally occurring electromagnetic wave in the high-density plasmasphere (roughly within 20,000 kilometers of Earth) that is known to remove the high-energy Van Allen Belt electrons that pose a threat to satellites and astronauts. A recent theory tied the origin of plasmaspheric hiss to a seemingly different wave in the outer magnetosphere, but this theory was difficult to test because of a challenging set of observational requirements. Here we report on the experimental verification of the theory, made with a five-satellite NASA mission. This confirmation will allow modeling of plasmaspheric hiss and its effects on the high-energy radiation environment.

Radiation belt electron acceleration by chorus waves during the 17 March 2013 storm

Local acceleration driven by whistler-mode chorus waves is fundamentally important for accelerating seed electron populations to highly relativistic energies in the outer radiation belt. In this study, we quantitatively evaluate chorus-driven electron acceleration during the 17 March 2013 storm, when the Van Allen Probes observed very rapid electron acceleration up to several MeV within ~12 hours. A clear radial peak in electron phase space density (PSD) observed near L* ~4 indicates that an internal local acceleration process was operating. We construct the global distribution of chorus wave intensity from the low-altitude electron measurements made by multiple Polar Orbiting Environmental Satellites (POES) satellites over a broad region, which is ultimately used to simulate the radiation belt electron dynamics driven by chorus waves. Our simulation results show remarkable agreement in magnitude, timing, energy dependence, and pitch angle distribution with the observed electron PSD near its peak location. However, radial diffusion and other loss processes may be required to explain the differences between the observation and simulation at other locations away from the PSD peak. Our simulation results, together with previous studies, suggest that local acceleration by chorus waves is a robust and ubiquitous process and plays a critical role in accelerating injected seed electrons with convective energies (~100 keV) to highly relativistic energies (several MeV).

Modeling the propagation characteristics of chorus using CRRES suprathermal electron fluxes

In the present paper, phase space density functions of the form f(v) = A N /v n are fitted to statistical distributions of suprathermal electron fluxes (E = 0.213–16.5 keV) from the CRRES satellite, parameterized by L-shell, Magnetic Local Time (MLT), and geomagnetic activity. The fitted distributions are used in conjunction with ray tracing to calculate the Landau damping rates of an ensemble of rays representing whistler-mode chorus waves. The modeled propagation characteristics are compared with observations of chorus wave power from the CRRES satellite, as a function of L-shell, MLT, and magnetic latitude, in various frequency bands, and under various geomagnetic conditions. It is shown that the model results are remarkably consistent with many aspects of the observed wave distributions, including frequency, L-shell, MLT, and latitudinal dependence. In addition, the MLT distribution of wave power becomes characteristically asymmetric during active geomagnetic conditions, with small propagation lengths on the nightside which increase with MLT and maximize on the dayside. This asymmetry is shown to be directly related to the dynamics of the Landau resonant suprathermal electrons which drift around the Earth whilst undergoing scattering and loss due to a variety of plasma waves. Consequently, the suprathermal electrons play an important role in radiation belt dynamics, by controlling the distribution of chorus, which in turn contributes to the acceleration and loss of relativistic electrons in the recovery phase of storms.

Statistical properties of plasmaspheric hiss derived from Van Allen Probes data and their effects on radiation belt electron dynamics

Author(s): Li, W; Ma, Q; Thorne, RM; Bortnik, J; Kletzing, CA; Kurth, WS; Hospodarsky, GB; Nishimura, Y | Abstract: ©2015. American Geophysical Union. Plasmaspheric hiss is known to play an important role in controlling the overall structure and dynamics of radiation belt electrons inside the plasmasphere. Using newly available Van Allen Probes wave data, which provide excellent coverage in the entire inner magnetosphere, we evaluate the global distribution of the hiss wave frequency spectrum and wave intensity for different levels of substorm activity. Our statistical results show that observed hiss peak frequencies are generally lower than the commonly adopted value (~550Hz), which was in frequent use, and that the hiss wave power frequently extends below 100Hz, particularly at larger L shells ( g ~3) on the dayside during enhanced levels of substorm activity. We also compare electron pitch angle scattering rates caused by hiss using the new statistical frequency spectrum and the previously adopted Gaussian spectrum and find that the differences are up to a factor of ~5 and are dependent on energy and L shell. Moreover, the new statistical hiss wave frequency spectrum including wave power below 100Hz leads to increased pitch angle scattering rates by a factor of ~1.5 for electrons above ~100keV at L~5, although their effect is negligible at L≤3. Consequently, we suggest that the new realistic hiss wave frequency spectrum should be incorporated into future modeling of radiation belt electron dynamics.

Competing source and loss mechanisms due to wave-particle interactions in Earth’s outer radiation belt during the 30 September to 3 October 2012 geomagnetic storm

Drastic variations of Earths outer radiation belt electrons ultimately result from various competing source, loss, and transport processes, to which wave-particle interactions are critically important. Using 15 spacecraft including NASAs Van Allen Probes, THEMIS, and SAMPEX missions and NOAAs GOES and POES constellations, we investigated the evolution of the outer belt during the strong geomagnetic storm of 30 September to 3 October 2012. This storms main phase dropout exhibited enhanced losses to the atmosphere at L*  1 MeV electrons and energetic protons, SAMPEX >1 MeV electrons, and ground observations of band-limited Pc1-2 wave activity, we show that this sudden loss was consistent with pitch angle scattering by electromagnetic ion cyclotron waves in the dusk magnetic local time sector at 3  300 nT, and energetic electron injections and whistler-mode chorus waves were observed throughout the inner magnetosphere for >12 h. After this period, Bz turned northward, and injections, chorus activity, and enhancements in PSD ceased. Overall, the outer belt was depleted by this storm. From the unprecedented level of observations available, we show direct evidence of the competitive nature of different wave-particle interactions controlling relativistic electron fluxes in the outer radiation belt.

Interaction of Emic Waves With Thermal Plasma and Radiation Belt Particles

Electromagnetic ion cyclotron (EMIC) waves are excited during the enhanced convective injection of plasmasheet ions into the inner magnetosphere. Waves grow rapidly near the magnetic equatorial plane reaching amplitudes up to 10 nT. Such intense waves induce scattering of cyclotron resonant ions at a rate comparable to the strong diffusion limit, causing rapid ion precipitation into the atmosphere in localized regions where the waves are present. The waves also resonate with relativistic electrons at energies typically above 0.5 MeV Such scattering, which could provide a major loss process for relativistic outer zone electrons during the main phase of a magnetic storm, is confined to high-density regions just inside the plasmapause or within dayside drainage plumes. As EMIC waves propagate to higher latitude, their wave normal angle becomes highly oblique. This allows Landau resonant interaction with thermal electrons, which can heat the electron population in the outer plasmasphere to several eV, contributing to the heat flux that drives Stable Auroral Red (SAR) arcs. During the propagation to higher latitude, EMIC waves can also experience cyclotron resonant damping by heavy thermal ions, leading to ion conic distributions, which are observed near the equator.

Resonant scattering of energetic electrons by unusual low-frequency hiss

We quantify the resonant scattering effects of the unusual low-frequency dawnside plasmaspheric hiss observed on 30 September 2012 by the Van Allen Probes. In contrast to normal (~100–2000 Hz) hiss emissions, this unusual hiss event contained most of its wave power at ~20–200 Hz. Compared to the scattering by normal hiss, the unusual hiss scattering speeds up the loss of ~50–200 keV electrons and produces more pronounced pancake distributions of ~50–100 keV electrons. It is demonstrated that such unusual low-frequency hiss, even with a duration of a couple of hours, plays a particularly important role in the decay and loss process of energetic electrons, resulting in shorter electron lifetimes for ~50–400 keV electrons than normal hiss, and should be carefully incorporated into global modeling of radiation belt electron dynamics during periods of intense injections.

Three-dimensional ray tracing of VLF waves in a magnetospheric environment containing a plasmaspheric plume

A three dimensional ray tracing of whistler-mode chorus is performed in a realistic magnetosphere using the HOTRAY code. A variety of important propagation characteristics are revealed associated with azimuthal density gradients and a plasmaspheric plume. Specifically, whistler mode chorus originating from a broad region on the dayside can propagate into the plasmasphere. After entry into the plasmasphere, waves can propagate eastward in MLT and merge to form hiss. This explains how chorus generated on the dayside can contribute to plasmaspheric hiss in the dusk sector. A subset of waves entering the plasmasphere can even propagate globally onto the nightside. Citation: Chen, L., J. Bortnik, R. M. Thorne, R. B. Horne, and V. K. Jordanova (2009), Three-dimensional ray tracing of VLF waves in a magnetospheric environment containing a plasmaspheric plume, Geophys. Res. Lett., 36, L22101, doi:10.1029/2009GL040451.