MSSL Space Plasma Science Nuggets

High-time-resolution observations of an FTE using Cluster

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Schematic showing the layers of an FTE. From Varsani et al. (2014)

We have presented the Cluster observations of a crater FTE on 12 February 2007, when the quartet was located in the low-latitude boundary layer, and widely separated on the magnetopause plane. The passage of the structure was sequentially observed by Cluster 2, 3, 4 and 1 respectively, analysed in detail. But what are flux transfer events, and why are they important within the magnetosphere?

New and improved analytic expressions for ULF wave radiation belt radial diffusion coefficients

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Azimuthal electric field PSD values derived from ground-based magnetometer measurements of the D-component magnetic field PSD at L = 7.94, 6.51, 5.40, 4.26, 4.21, 2.98, and 2.55. The dashed lines represent constant fits to these PSD values. From Ozeke et al. (2014)

Ozeke et al. [2014] presented 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.

Detailed azimuthal structure of the substorm current wedge

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Currents measured by the Cluster spacecraft as they passed over a statistical auroral oval. The currents inside the statistical oval are associated with the substorm current wedge.

During the most dynamic auroral displays, known as substorm, electrical current is diverted from the magnetosphere through the ionosphere. The passage of this current through the upper atmosphere causes the gas to glow giving us the aurora. Since the 1970s it has been thought that this diverted current forms a "substorm current wedge" with upwards current on the duskward side and downward current on the dawnward side.

Automated determination of auroral breakup during the substorm expansion phase using all-sky imager data

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An example of the difficulty to visually define an time and location for auroral break-up, but how well an automated algorithm picks out this period of brightening. From Murphy et al. (2014)

MSSL researchers participated in the development of a novel method for quantitatively and routinely identifying auroral breakup following substorm onset using the THEMIS (Time History of Events and Macroscale Interactions during Substorms) all-sky imagers.

The detailed spatial structure of field-aligned currents comprising the substorm current wedge

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. Field-Aligned currents observed by the AMPERE mission and ground perturbations of the Hall current components  of the substorm current wedge during three substorm expansion phases.  The polarisation ellipses point towards the centre of the substorm current wedge, and the integrated FACs from AMPERE show a significantly complex current structure results in a net upward and downward current structure as first identified by McPherron et al. [1973]. From Murphy et al. (2014)

We present a comprehensive two-dimensional view of the field-aligned currents (FACs) during the late growth and expansion phases for three isolated substorms utilizing in situ observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and from ground-based magnetometer and optical instrumentation from the Canadian Array for Realtime Investigations of Magnetic Activity and Time History of Events and Macroscale Interactions during Substorms ground-based arrays.

Discovery of the action of a geophysical synchrotron in the Earth’s Van Allen radiation belts

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ULF waves in the Van Allen belts. Figure from Mann et al., (2013)

Although the Earth's Van Allen radiation belts were discovered over 50 years ago, the dominant processes responsible for relativistic electron acceleration, transport and loss remain poorly understood. Here we show evidence for the action of coherent acceleration due to resonance with ultra-low frequency waves on a planetary scale.

The influence of magnetospheric convection and magnetopause motion on Radiation Belt electrons

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ULF wave model outputs for a 1 mHz ULF wave source located along the afternoon sector magnetopause (peaked at 1500 h MLT), showing radial electric right (left), azimuthal electric field (centre) and northward magnetic field perturbation (left). Plots were taken at five wave periods after the beginning of the source wave. Figure 1 from Degeling et al. (2013)

Understanding the acceleration, transport and loss of relativistic electrons in Earth’s magnetosphere is a high-priority international science objective.  Observations indicate that there are a vast number of effects to be considered in this region ranging from large-scale global effects to effects on the electron gyroscale and from the interaction of electrons with electromagnetic wave processes, to global changes in the Earth’s magnetosphere.  

Poleward Boundary Intensifications and Bursty Bulk Flows do not coherently drive the substorm current wedge

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Figure from Rae et al. (2013)

Rae et al. [2013] published a comment on a recent Nishimura et al. [2012] paper that hypothesized that individual flow bursts created the field-aligned currents (FACs) that form the substorm current wedge (SCW).  In their comment, Rae et al. [2013] systematically broke down the underpinning assumptions of the Nishimura paper.

Structure and variability of the auroral acceleration region

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The Aurora Australis seen from the International Space Station (ISS-029). Courtesy: NASA

Bright auroral arc appear when charged particles from the magnetosphere are accelerated into the upper atmosphere. Collisions between charged particles and neutrals excite the electrons in the neutral particles which then de-excite by emitting auroral light. Particles, in particular electrons, are accelerated out of the magnetosphere and into the atmosphere by magnetic-field-aligned electric potential drops in a region known as the auroral acceleration region (AAR). In a recent paper, Forsyth et al. [2012] investigated the temporal variability and spatial structure in one such region.

What is the source of magnetotail flux-ropes?

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Artist impression of the Earth's bow shock. (c) UCL

Travelling compression regions (TCRs) are perturbations in the magnetotail lobe magnetic field caused by structures moving Earthward or tailward within the plasma sheet. Previous works have suggested that these structures are created by either time-dependant reconnection occurring at a single X-line, forming a flux-bulge-type structure, or space-variant reconnection at multiple X-lines, forming flux-rope-type structures. By analysing a TCR and its source structure using the Cluster spacecraft, Beyene et al. (2011) have endeavoured to determine which of these mechanisms creates TCRs.

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