MSSL Space Plasma Science Nuggets

Near-Earth Cosmic Ray Decreases Associated with Remote Coronal Mass Ejections

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An ENLIL model run of a remote CME associated with an unusual Forbush Decrease that was observed on 30th May 2012

Galactic cosmic rays (GCRs) are highly energetic, charged particles that originate from outside of the heliosphere. The flux of GCRs reaching us varies in response to the magnetic field in which the particles propagate. In time-scales of hours, GCR flux can be suppressed by coronal mass ejections (CMEs) due to the increased magnetic field strength and from scattering by turbulence within the magnetic field. The GCR flux incident on Earth is inferred by measuring neutrons at the surface which are generated when GCRs interact with atmospheric particles. Therefore, when a CME passes over Earth, neutron monitors give a sudden decrease of a number of percent which then recovers slowly as the CME passes out into the outer heliosphere. This change in the neutron monitor data is known as a “Forbush Decrease”.

Solar Ejecta through the Heliosphere

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The solar flare that occurred on 7th June 2011 was not unusually bright, nor was it unexpected. It was classified as a medium-sized event and its effects were barely felt back here on Earth.

Origin of polar auroras revealed

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Auroras are the most visible manifestation of solar wind driven magnetosphere-ionosphere coupling, but many aspects of these spectacular displays are still poorly understood. A paper by Fear et al. published in Science in December 2014 has answered a long standing question about what produces the unusual ‘theta aurora’. Theta aurora are so named because when seen from above it looks like the Greek letter theta – an oval with a line crossing through the centre. The unusual aspect is the ‘line through the centre’ due to aurorae occurring closer to the poles than the normal aurora, which are found about 65–70° degrees north or south of the equator in an area called the ‘auroral oval’ that is reasonably well understood by scientists.

Increases in plasma sheet temperature with solar wind driving during substorm growth phases

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Plot of magnetotail properties against solar wind driving: (a) Magnetic pressure in the lobes; (b) total pressure in the plasma sheet (magnetic pressure + H+ + O+); (c) plasma sheet ion temperature; and (d) plasma sheet ion density. The overlaid boxes show the median (blue line), upper and lower quartiles (large box) and upper and lower deciles (small box) of the ordinate data split into deciles of the solar wind driving from the entire data set. The grey lines show the fits to our semiempirical model. The solid lines show fits of these models to the whole data set, and the dashed lines show fits to the shown median values. From Forsyth et al., GRL, 2014

Through its interaction with the solar wind, Earth's magnetosphere can store 1015 J of magnetic energy in its magnetotail. This energy is explosively released during magnetospheric substorms; events during which the stored magnetic energy is directed into the ionosphere to cause the aurora, heats in the plasma in the magnetotail and is ejected back into the solar wind behind the magnetosphere.

Inner magnetospheric onset preceding reconnection and tail dynamics during substorms: Can substorms initiate in two different regions?

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Figure 1.  Auroral observations during a  substorm. (a) and (b) North-south slice through the aurora from two auroral cameras close-by in white-light, and (c) and (d) in red-line and blue-line auroral emission, respectively.   (e)-(h) shows east-west slices through the auroral cameras, showing the formation and evolution of wave-like auroral beads at the start of this substorm.

The explosive release of energy within a substorm marks the beginning of one of the most dynamic and vibrant auroral displays seen in the night-time skies.  Stored magnetic energy is quickly converted to plasma kinetic energy, resulting in dramatic changes both in the large-scale magnetic topology of the Earth’s night-side magnetic field, and in energetic particles being accelerated towards Earth.

Waves in the ionosphere detected by ground GPS receiver network

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Ionospheric waves observed by EISCAT radar in Tromso, Norway

Ground networks of GPS receivers can be used to characterise ionospheric perturbations. As the dual frequency navigational GPS signal propagates through the ionosphere, due to dispersive properties of the ionised media it carries information about the total ionospheric electron content (TEC). With careful analysis, ionospheric perturbations due to various natural drivers can be detected. Ground networks of GPS receivers in Japan have been used to detect small ionospheric effects from propagating extra long ocean waves, those causing catastrophic tsunamis as they reach the shore. In Scandinavia and Canada, the effects from auroral activity and from magnetospheric plasma waves have been observed in GPS TEC measurements. Such effects can be of crucial importance for the precise GPS positioning but can be also utilised to monitor the Earth’s magnetosphere and in particular the radiation belts.

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.

Particle Distributions in the Magnetotail

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Artists impression of the plasma regions of the magnetosphere. (c) UCL 2011

For the first time, Walsh et al. have examined, in detail, the particle distributions in the magnetotail to determine the average pitch angle distributions. 

Calculating currents from four spacecraft

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Artist's impression of the Cluster quartet. (c) ESA

Ampere's law tells us that the curl of a magnetic field is proportional to current density. In order to measure the curl of a magnetic field in space, one needs to know approximate the variation of the magnetic field between four non-coplanar points. Such measurements are achieved by the Cluster quartet.

Discovery of the 'Travelling Magnetopause Erosion Region'

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A 3D cut showing regions of the magnetosphere. FTE formation occurs on or near the subsolar magnetopause (yellow circle).

Recent work by Owen et al. has shed new light on the structure of the magnetopause following bursts of reconnection through the discovery of 'Travelling Magnetopause Erosion Regions'.

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