Spatial dependence of banded chorus intensity near the magnetic equator
TL;DR: In this article, the spatial dependence of banded chorus intensity near the magnetic equator under conditions of moderate magnetic disturbance (Kp≤ 5) was investigated. And the authors showed that the intensity of the chorus emissions generally increased exponentially with distance, z, away from the Magnetic Equator according to the relation I = Ioeaz.
Abstract: [1] Data from 12 different Cluster orbits containing banded chorus emissions, with observations from 22 different events spread across the four Cluster spacecraft, are used to show the spatial dependence of banded chorus intensity near the magnetic equator under conditions of moderate magnetic disturbance (Kp≤ 5). The intensities for upper band (UB) and lower band (LB) chorus were manually determined from frequency-time spectrograms generated from WBD data. Out of the 22 events, 5 of the spacecraft observations showed a complete absence of chorus emissions within 0.5° of the magnetic equator. The intensity,I, of the chorus emissions generally increased exponentially with distance, z, away from the magnetic equator according to the relation I = Ioeaz, where α is the spatial growth factor. The exponential distribution of α provides a new boundary condition for consideration in current and future chorus generation models.
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TL;DR: In this article, the authors used 11 years of measurements of the STAFF-SA instruments onboard the four Cluster spacecraft to systematically build maps of wave propagation parameters as a function of position.
Abstract: Lower-band whistler-mode emissions can influence the dynamics of the outer Van Allen radiation belts. We use 11 years of measurements of the STAFF-SA instruments onboard the four Cluster spacecraft to systematically build maps of wave propagation parameters as a function of position. We determine probability distributions of wave vector angle weighted by the wave intensity. The results show that wave vector directions of intense waves are close to a Gaussian-shaped peak centered on the local magnetic field line. The width of this peak is between 10 and 20 degrees. The cumulative percentage of oblique waves is below 1015%. This result is especially significant for an important class of whistler-mode emissions of lower-band chorus at higher latitudes, well outside their source region, where a simple ray tracing model fails and another mechanism is necessary to keep the wave vectors close to the field-aligned direction.
80 citations
TL;DR: In this article, a physical model of the wave normal angle distribution along a field line is presented, which provides insight on how the wave norm distribution varies as chorus waves propagate away from the equatorial source region.
Abstract: [1] The propagation and attenuation characteristics of lower band and upper band chorus waves are investigated by ray tracing, and the evaluation of Landau damping based on an empirical suprathermal electron model derived from Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. The rate of Landau damping is found to increase at larger L-shell, for more oblique wave normal angles, and for higher geomagnetic activity. Damping is also larger on the nightside than on the dayside and is more pronounced in the upper band than in the lower band. These features can account for the statistical pattern of chorus waves observed away from the equatorial source region. A physical model of the wave normal angle distribution along a field line is presented, which provides insight on how the wave normal angle distribution varies as chorus waves propagate away from the equatorial source region. Our modeling shows that wave emissions at low latitudes (λ ≲ 30°) come predominately from the equatorial source at the same L-shell and that their wave normal angles increase with increasing latitude due to wave refraction caused by magnetic gradients and curvature. However, at high latitudes (λ ≳ 30°), the wave normal angle distribution along a particular field line is affected by chorus waves that arrive from an equatorial source at lower L because of significant cross-L propagation. As a consequence, the lower band wave normal angle tends to decrease with increasing latitudes, while the upper band wave normal angle can either increase or decrease depending on the equatorial source at lower L. The effect of cross-L propagation might also explain why observed wave normal angle distribution tends to become more field-aligned at high latitudes. Interestingly, the upper band chorus at such high latitudes originates from lower band waves originating near the equator at lower L. A global model of wave normal variation along a field line constructed in this study is not currently available from observations but is nonetheless critically important for evaluating bounce-averaged diffusion coefficients for future radiation belt modeling.
79 citations
TL;DR: There have been more than 2000 refereed papers published using Cluster data but in this paper we will, of necessity, refer to only a small fraction of the published work as discussed by the authors.
Abstract: Plasmas are ubiquitous in nature, surround our local geospace environment, and permeate the universe. Plasma phenomena in space give rise to energetic particles, the aurora, solar flares and coronal mass ejections, as well as many energetic phenomena in interstellar space. Although plasmas can be studied in laboratory settings, it is often difficult, if not impossible, to replicate the conditions (density, temperature, magnetic and electric fields, etc.) of space. Single-point space missions too numerous to list have described many properties of near-Earth and heliospheric plasmas as measured both in situ and remotely (see http://www.nasa.gov/missions/#.U1mcVmeweRY for a list of NASA-related missions). However, a full description of our plasma environment requires three-dimensional spatial measurements. Cluster is the first, and until data begin flowing from the Magnetospheric Multiscale Mission (MMS), the only mission designed to describe the three-dimensional spatial structure of plasma phenomena in geospace. In this paper, we concentrate on some of the many plasma phenomena that have been studied using data from Cluster. To date, there have been more than 2000 refereed papers published using Cluster data but in this paper we will, of necessity, refer to only a small fraction of the published work. We have focused on a few basic plasma phenomena, but, for example, have not dealt with most of the vast body of work describing dynamical phenomena in Earth's magnetosphere, including the dynamics of current sheets in Earth's magnetotail and the morphology of the dayside high latitude cusp. Several review articles and special publications are available that describe aspects of that research in detail and interested readers are referred to them (see for example, Escoubet et al. 2005Multiscale Coupling of Sun-Earth Processes, p. 459, Keith et al. 2005Sur. Geophys.26, 307339, Paschmann et al. 2005Outer Magnetospheric Boundaries: Cluster Results, Space Sciences Series of ISSI. Berlin: Springer, Goldstein et al. 2006Adv. Space Res.38, 2136, Taylor et al. 2010The Cluster Mission: Space Plasma in Three Dimensions, Springer, pp. 309330 and Escoubet et al. 2013Ann. Geophys.31, 10451059).
26 citations
TL;DR: In this paper, the authors used the data of the DEMTER satellite during the magnetic storm on 14 April 2006, and found the first observational evidence of penetration of high-latitude chorus into the plasmasphere.
Abstract: [1] Using the data of the DEMTER satellite during the magnetic storm on 14 April 2006, we study the storm time VLF electromagnetic waves, and find the first observational evidence of penetration of high-latitude chorus into the plasmasphere. During this geomagnetic storm, “banded” emissions of a few hertz to 20 kHz are observed to be intensified and to be organized in the frequency range of 0.1–0.5fce (equatorial electron cyclotron frequency) in high-latitude region of magnetic latitude between ~40°and ~60°. The signatures in the wave power spectra suggest that these emissions are likely lower-band chorus. The observed chorus waves are generally outside the plasmasphere. However, interestingly, these waves are observed inside the plasmasphere at regions with low L value during the main phase and early recover phase, which has never been reported in previous studies of chorus in low-latitude regions.
24 citations
TL;DR: Goldstein et al. as discussed by the authors reviewed some of the plasma physics problems that have been investigated by the Cluster mission, and concentrated on a few investigations that emphasize fairly general plasma physics phenomena, including measurement of the threedimensional properties of turbulence using multi-point measurements of electromagnetic energy, and observations of electron-scale turbulence made feasible by the high time resolution of the electric and magnetic field instrumentation.
Abstract: The talk reviews some of the plasma physics problems that have been investigated by the Cluster mission. The material is based on a paper that contains details of the topics mentioned here (Goldstein, et al., J. Plasma Physics (2015), vol. 81, 325810301, doi:10.1017/S0022377815000185, Multipoint observations of plasma phenomenamade in space by Cluster). The four Cluster spacecraft were launched a month apart into a polar orbit of 4 × 19.6 RE by two Soyuz-Fregat rockets from Baikonur on 16 July and 9 August 2000. The mission was designed so that the orbits of the four spacecraft would form the vertices of a regular tetrahedron in areas of scientific interest so that for the first time one couldmakemeasurements in three dimensionswith the ability to distinguish between spatial and temporal changes. Thsy design, coupledwith the capability of being able to change the spacecraft separation, opened new regions of the Earth’s plasma environment to exploration. The spacecraft separation has been changed more than 25 times. The separation distances have ranged from 4 to 36 ,000 km. In 2005, to study magnetofluid turbulence in the solar wind, the spacecraft separation was adjusted to form a 10,000 km tetrahedron. Because spacecraft 3 (C3) is in a very similar orbit to spacecraft 4 (C4), those two have, on occasion, been moved to within 4 km of each other. The talk concentrated on a few investigations that emphasize fairly general plasma physics phenomena. Cluster’s times in the solar wind have produced new insights into the properties of magnetofluid turbulence down to electron scales. Cluster data have been used together with data from other spacecraft (e.g., ACE and WIND) to determine the correlation scale of interplanetary (and plasmasheet) turbulence and the four-point capability has led to significant progress in understanding plasma turbulence in the solar wind and magnetosphere. Progress has been most dramatic in two areas: (i) measurement of the three-dimensional properties of turbulence using multi-point measurements of electromagnetic energy, and (ii) observations of electron-scale turbulence made feasible by the high time resolution of the electric and magnetic field instrumentation. We now have a description of an anisotropic cascade of magnetofluid turbulence down to electron scales, while at larger scales vorticity has been measured in both the solar wind and the magnetotail. Cluster data have also been used to characterize the intermittency of the turbulence. The Taylor
21 citations
References
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TL;DR: In this paper, the post-midnight chorus was detected in the midnight sector of the magnetosphere in conjunction with magnetospheric substorms and the characteristics of these emissions such as their frequency time structure, emission frequency with respect to the local equatorial electron gyrofrequency, intensity-time variation, and the average intensity were investigated.
Abstract: The ELF emissions were detected in the midnight sector of the magnetosphere in conjunction with magnetospheric substorms. The emissions were observed at local midnight and early morning hours and are accordingly called 'post-midnight chorus.' The characteristics of these emissions such as their frequency time structure, emission frequency with respect to the local equatorial electron gyrofrequency, intensity-time variation, and the average intensity were investigated. The occurrence of the chorus in the nightside magnetosphere was investigated as a function of local time, L shell, magnetic latitude, and substorm activity, and the results of this analysis are presented. Specific features of postmidnight chorus are discussed in the context of possible wave-particle interactions occurring during magnetospheric substorms.
626 citations
TL;DR: In this paper, the authors present CRRES data on the spatial distribution of chorus emissions during active conditions and calculate the pitch angle and energy diffusion rates in three magnetic local time (MLT) sectors and obtain a timescale for acceleration.
Abstract: [1] Electron acceleration inside the Earth's magnetosphere is required to explain increases in the ∼MeV radiation belt electron flux during magnetically disturbed periods. Recent studies show that electron acceleration by whistler mode chorus waves becomes most efficient just outside the plasmapause, near L = 4.5, where peaks in the electron phase space density are observed. We present CRRES data on the spatial distribution of chorus emissions during active conditions. The wave data are used to calculate the pitch angle and energy diffusion rates in three magnetic local time (MLT) sectors and to obtain a timescale for acceleration. We show that chorus emissions in the prenoon sector accelerate electrons most efficiently at latitudes above 15° for equatorial pitch angles between 20° and 60°. As electrons drift around the Earth, they are scattered to large pitch angles and further accelerated by chorus on the nightside in the equatorial region. The timescale to accelerate electrons by whistler mode chorus and increase the flux at 1 MeV by an order of magnitude is approximately 1 day, in agreement with satellite observations during the recovery phase of storms. During wave acceleration the electrons undergo many drift orbits and the resulting pitch angle distributions are energy-dependent. Chorus scattering should produce pitch angle distributions that are either flat-topped or butterfly-shaped. The results provide strong support for the wave acceleration theory.
622 citations
TL;DR: In this paper, the minimum electron energy for cyclotron resonant interaction with various electromagnetic waves was calculated for conditions representative of storm-times, and the possibility of electron stochastic energization to relativisitic energies (≥ 1 MeV) via resonant waveparticle interactions during a magnetic storm was explored.
Abstract: The possibility of electron stochastic energization to relativisitic energies (≥ 1 MeV) via resonant wave-particle interactions during a magnetic storm is explored. The minimum electron energy Emin for cyclotron resonant interaction with various electromagnetic waves is calculated for conditions representative of storm-times. Since Emin > 1 MeV for resonance with L-mode ion cyclotron waves, intense stormtime EMIC waves could contribute to relativistic electron loss, but not acceleration. Inside the plasmapause whistler mode waves, and highly oblique magnetosonic waves near the lower hybrid frequency, can resonate with electrons over the important energy range from ∼ 100 keV to ∼ 1 MeV. In low density regions outside the plasmapause, the whistler, RX, LO and Z modes can resonate with electrons over a similar energy range. These waves have the potential to contribute to the stochastic acceleration of electrons up to relativistic energies during magnetic storms.
574 citations
TL;DR: In this paper, an electron analyser was used to measure the three-dimensional velocity distribution of electrons in the energy range from 0.59 eV to 26.4 keV on the four spacecraft of the Cluster mission.
Abstract: An electron analyser to measure the three-dimensional velocity distribution of electrons in the energy range from 0.59 eV to 26.4 keV on the four spacecraft of the Cluster mission is described. The instrument consists of two sensors with hemispherical electrostatic energy analysers with a position-sensitive microchannel plate detectors placed to view radially on opposite sides of the spacecraft. The intrinsic energy resolutions of the two sensors are 12.7% and 16.5% full width at half maximum. Their angular resolutions are 2.8° and 5.3° respectively in an azimuthal direction and 15° in a polar direction. The two sensors will normally measure in different overlapping energy ranges and will scan the distribution in half a spacecraft rotation or 2 s in the overlapped range. While this is the fastest time resolution for complete distributions, partial distributions can be recorded in as little as 62.5 ms and angular distributions at a fixed energy in 7.8 ms. The dynamic range of the instrument is sufficient to provide accurate measurements of the main known populations from the tail lobe to the plasmasheet and the solar wind. While the basic structure of the instrument is conventional, special attention has been paid in the design to improving the precision of the instrument so that a relative accuracy of the order of 1 % could be attained in flight in order to measure the gradients between the four spacecraft accurately; to decreasing the minimum energy covered by this technique from 10 eV down to 1 eV; and to providing good three dimensional distributions.
564 citations
TL;DR: It is shown, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth, that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz.
Abstract: The Van Allen radiation belts are two regions encircling the Earth in which energetic charged particles are trapped inside the Earth's magnetic field. Their properties vary according to solar activity and they represent a hazard to satellites and humans in space. An important challenge has been to explain how the charged particles within these belts are accelerated to very high energies of several million electron volts. Here we show, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth, that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz. Wave acceleration can increase the electron flux by more than three orders of magnitude over the observed timescale of one to two days, more than sufficient to explain the new radiation belt. Wave acceleration could also be important for Jupiter, Saturn and other astrophysical objects with magnetic fields.
552 citations