Showing papers by "Richard B. Horne published in 2012"

Global model of lower band and upper band chorus from multiple satellite observations

Abstract: Gyroresonant wave particle interactions with whistler mode chorus play a fundamental role in the dynamics of the Earth’s radiation belts and inner magnetosphere, affecting both the acceleration and loss of radiation belt electrons. Knowledge of the variability of chorus wave power as a function of both spatial location and geomagnetic activity, required for the computation of pitch angle and energy diffusion rates, is thus a critical input for global radiation belt models. Here we present a global model of lower band (0.1fce < f < 0.5fce) and upper band (0.5fce < f < fce) chorus, where fce is the local electron gyrofrequency, using data from five satellites, extending the coverage and improving the statistics of existing models. From the plasmapause out to L* = 10 the chorus emissions are found to be largely substorm dependent with the largest intensities being seen during active conditions. Equatorial lower band chorus is strongest during active conditions with peak intensities of the order 2000 pT2 in the region 4 < L* < 9 between 2300 and 1200 MLT. Equatorial upper band chorus is both weaker and less extensive with peak intensities of the order a few hundred pT2 during active conditions between 2300 and 1100 MLT from L* = 3 to L* = 7. Moving away from the equator midlatitude chorus is strongest in the lower band during active conditions with peak intensities of the order 2000 pT2 in the region 4 < L* < 9 but is restricted to the dayside between 0700 and 1400 MLT.

Modeling the properties of plasmaspheric hiss: 1. Dependence on chorus wave emission

Abstract: [1] There is increasing evidence that plasmaspheric hiss is formed by the evolution of a portion of chorus waves that are excited in the plasmatrough and propagate into the plasmasphere. Comparison between the statistical spatial distributions of these two emissions in the morning sector during active times from THEMIS over ∼3 years shows that the two emissions have comparable peak intensities but are distinct in their spatial distributions. We present a modeling study of the hiss spectrum, based on ray tracing, by taking the observed chorus source region as an input in the magnetosphere, which contains cold and suprathermal electrons. Our modeling results show that we are able to reproduce the main features of typical hiss, including the frequency spectrum, wave normal angle and spatial distribution. However, the simulated hiss intensity is weaker (∼15 dB less) than the observed intensity, which suggests some modest internal amplification inside the plasmasphere. The responses of hiss to variations in the spatial distribution, wave normal angle distribution and frequency distribution of the source chorus are examined. We find that the majority of hiss formation is due to a small portion of chorus emission originating within ∼3 RE from the plasmapause, with wave normal directions pointing toward the Earth at an angle of 30°–60°, and over a frequency range of 0.1–0.3 fce. If the chorus power is made to increase closer to the plasmapause, the hiss intensity and the peak frequency also increases, which roughly mimics active geomagnetic conditions. Variations of the chorus source distribution do not significantly affect the wave normal angle distribution and frequency distribution of hiss, but does impact the absolute intensity of the resulting hiss.

Efficient diffuse auroral electron scattering by electrostatic electron cyclotron harmonic waves in the outer magnetosphere: A detailed case study

Abstract: [1] This paper is a companion to a paper by Liang et al. (2011) which reports a causal connection between the intensification of electrostatic ECH waves and the postmidnight diffuse auroral activity in the absence of whistler mode chorus waves at L = 11.5 on the basis of simultaneous observations from THEMIS spacecraft and NORSTAR optical instruments during 8–9 UT on February 5, 2009. In this paper, we use the THEMIS particle and wave measurements together with the magnetically conjugate auroral observations for this event to illustrate an example where electrostatic electron cyclotron harmonic (ECH) waves are the main contributor to the diffuse auroral precipitation. We use the wave and particle data to perform a comprehensive theoretical and numerical analysis of ECH wave driven resonant scattering rates. We find that the observed ECH wave activity can cause intense pitch angle scattering of plasma sheet electrons between 100 eV and 5 keV at a rate of >10−4 s−1 for equatorial pitch angles αeq < 30°. The scattering approaches the strong diffusion limit in the realistic ambient magnetic field to produce efficient precipitation loss of <∼5 keV electrons on a timescale of a few hours or less. Using the electron differential energy flux inside the loss cone estimated based upon the energy-dependent efficiency of ECH wave scattering for an 8-s interval with high resolution wave data available, the auroral electron transport model developed by Lummerzheim (1987) produced an intensity of ∼2.3 kR for the green-line diffuse aurora. Separately, Maxwellian fitting to the electron differential flux spectrum produced a green-line auroral intensity of ∼2.6 kR. This is in good agreement with the ∼2.4 kR green-line auroral intensity observed simultaneously at the magnetic foot point (as inferred using the event-adaptive model of Kubyshkina et al. (2009, 2011)) of the location where the in situ observations were obtained. Our results support the scenario that enhanced ECH emissions in the central plasma sheet (CPS) can be an important or even dominant driver of diffuse auroral precipitation in the outer magnetosphere. This paper is an important compliment to recent work that has shown lower band and upper band chorus to be mainly responsible for the occurrences of diffuse aurora in the inner magnetosphere.

Modeling the properties of plasmaspheric hiss: 2. Dependence on the plasma density distribution

Abstract: [1] Wave propagation plays an important role in linking the chorus source in the plasma trough region and the hiss emission in the plasmasphere. We investigate the dependence of plasmaspheric hiss on the background plasma density, which essentially controls the evolution of chorus after its generation at MLT = 10. The variation of plasma density under study includes variation of the location and width of the plasmapause, and the variation of the density distribution in the trough. We demonstrate that the plasmapause shape does not affect the peak hiss intensity significantly, but it does regulate the hiss intensity near the field-aligned direction. As the plasmasphere contracts or the plasmapause width increases, the field-aligned component of plasmaspheric hiss become relatively more intense, as does the intensity of hiss in the inner region, although the peak hiss intensity remains about the same. The trough density distribution, on the other hand, directly influences the accessibility of chorus rays into plasmasphere, and therefore plays an important role in the peak hiss intensity. As the density gradient in the plasma trough increases toward the plasmasphere, the resulting peak in the hiss intensity can increase by up to 10 dB.

Gyroresonant interactions between the radiation belt electrons and whistler mode chorus waves in the radiation environments of Earth, Jupiter, and Saturn: A comparative study

Abstract: In the current study we perform a comparative analysis of the gyroresonant interactions of whistler mode waves with radiation belt electrons in the magnetospheres of Earth, Jupiter, and Saturn. Our primary goal is to evaluate the effect of resonant wave-particle interactions with chorus waves and determine whether chorus waves can produce net acceleration or net loss of radiation belt electrons on the outer planets. The ratio of plasma frequency to gyrofrequency is a key parameter that determines the efficiency of the pitch angle and energy resonant scattering. We present a comparison of statistical maps of the ratio of plasma frequency to gyrofrequency for Jupiter, Saturn and Earth in terms of radial distance and latitude. Preliminary maps of the plasma frequency to gyrofrequency ratio and 2D simulations of pitch angle and energy diffusion using the Versatile Electron Radiation Belt (VERB) indicate that the Kronian plasma environment is not likely to support as efficient gyroresonant interactions with whistler mode chorus waves as in the Terrestrial or Jovian environments. Inefficiency of the local acceleration by whistler mode waves in the Kronian environment raises important questions about the origin of the relativistic electrons in the Saturn's radiation belts. Two-dimensional diffusive simulations of local acceleration and loss to the atmosphere using the VERB code confirm previous suggestions that the acceleration of electrons may be very efficient in the outer radiation belt of Jupiter. However, sensitivity simulations also show that the result of the competition between acceleration and loss in the Jupiter's magnetosphere strongly depends on the currently unknown latitudinal distribution of chorus waves that will be provided by the upcoming Juno mission. If waves extend to high latitudes, it is likely that the loss rates due to whistler mode waves will exceed energization rates.

Chorus, ECH, and Z mode emissions observed at Jupiter and Saturn and possible electron acceleration

Abstract: [1] In this paper we compare and contrast chorus, electron cyclotron harmonics (ECH), and Z mode emissions observed at Jupiter and Saturn and relate them to recent work on electron acceleration at Earth Intense chorus emissions are observed near the magnetic equator, the likely source region, but the strongest intensities are on either side of the magnetic equator Chorus intensities at Jupiter are generally about an order of magnitude larger than at Saturn, and the bandwidth of chorus at Jupiter can reach 7 or 8 kHz (∼06 fc), while at Saturn it is typically <2 kHz (∼06 fc, also) No higher-latitude information is available at Jupiter; however, high inclination orbits at Saturn by Cassini reveal strong chorus intensities at latitudes extending to over 30° At Jupiter, initial studies reveal the chorus intensities are sufficient to accelerate electrons by a stochastic process; however, the high density levels near the source region of chorus at Saturn indicate a less efficient process except for local regions such as within plasma injection regions The role of Z mode in electron acceleration and the role of ECH waves in pitch angle scattering at both Jupiter and Saturn require further study

Forecasting the Radiation Belts in Europe

Abstract: Weather forecasting has expanded to space weather. As of 1 March 2012, satellite operators and the general public will be able to obtain forecasts of the Earth’s radiation belts, thanks to the SPACECAST project. The opening of the first European system to forecast Earth’s radiation belts, part of the SPACECAST project, is funded by the European Union Framework 7 Programme and provides a forecast of high-energy electrons up to 3 hours in advance (updated every hour), as well as a risk index for the satellite operations and service industry.