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MSSL Space Plasma Science Nuggets

The Earth’s foreshock: simulations and in-situ satellite data

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Figure 1 from Kempf et al. [2015] showing modelled density variations in the vicinity of the bow shock

The super-magnetosonic solar wind impinging the Earth’s magnetic field creates the bow shock, the giant bow-shaped boundary shielding the Earth’s magnetosphere from the interplanetary environment. At this boundary, the plasma is compressed and heated while slowing down to sub-magnetosonic flow speeds. In fluid theory no information can travel upstream of a shock, but kinetic processes can cause solar wind particles to be reflected back off a shock and propagate upstream along the magnetic field lines. The upstream region magnetically connected to the bow shock, where reflected particles can interact with the solar wind, is called the foreshock. As the foreshock cannot be described by plasma fluid theory, the kinetic plasma simulations are required to understand the large-scale foreshock dynamics. 

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.

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