MSSL Space Plasma Science Nuggets

A physical explanation for the magnetic decrease ahead of dipolarization fronts

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Bursty Bulk Flows (BBFs) are intervals of fast Earthward plasma and magnetic flux transport in the plasma sheet, and are usually considered as the most important carriers of mass and energy towards the near-Earth region. A BBF consists of one or more individual flow bursts (FBs) [Angelopoulos et al., 1992]. Both the plasma velocity and the north-south component of the magnetotail’s magnetic field inside the BBF are significantly larger than in the surrounding region. They carry a stronger magnetic field and current density on their leading edge than in the surrounding magnetotail. The front of the BBF is often associated with a sharp increase in the northward magnetic field component B_z and is thus known as the dipolarization front (DF) [Nakamura et al., 2002; Sergeev et al., 2009]. This is usually a kinetic-scale structure of width of the order of an ion gyro-radius, i.e. ~1000km.

Statistical characterisation of the growth and spatial scales of the substorm onset arc

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During southward IMF reconnection on the dayside leads to a build up of magnetic energy in the tail. As flux is piled into the tail the configuration becomes unstable leading to an explosive release in magnetic energy, termed a substorm. The rearrangement of the magnetic field is accompanied by highly dynamic substorm aurora. 

Influence of solar wind variability on magnetospheric plasma waves

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ULF wave power spectral density as a function of solar wind variability

Solar wind impacts the Earth’s magnetic cavity driving various waves and instabilities inside the magnetosphere. The waves in the range of few mHz (ultra low frequency range, ULF) are particularly important for the dynamics of radiation belts, the populations of energetic particles trapped inside the Earth’s magnetosphere. The physical mechanisms behind driving ULF wave power are not fully understood but they are known be strongly dependent on the upstream solar wind conditions. The time-average solar wind parameters, such as average solar wind speed and density, are typically used to characterise the upstream solar wind conditions. In this work, the alternative approach is taken and the solar wind conditions are characterised by the dynamic variability of solar wind parameters, statistically quantified by their standard deviations. For the statistical study, the nine-year dataset of GOES satellite observations at the geostationary orbit is processed to characterise the magnetospheric ULF wave power, while the variability of solar wind is characterised using solar wind data from the Lagrangian L1 point. It is demonstrated that the magnetospheric wave power in ULF frequency range is the most sensitive to the variability of interplanetary magnetic field vector rather than variabilities of other solar wind parameters (plasma density, solar wind speed and temperature). The work results from collaboration between MSSL, NASA Goddard Space Flight Center and the University of Alberta.

Transpolar arc observation after solar wind entry into the high latitude magnetosphere

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Aurora picture from TIMED/GUVI, and the footpoints of Cluster and DMSP

During periods of northward Interplanetary Magnetic Field (IMF), geomagnetic activity is generally quiet, but solar wind plasma can penetrate and be stored in the magnetosphere. Recently, a new region of solar wind plasma entry into the terrestrial magnetosphere, in the lobes tailward of the cusp was reported and high latitude magnetic reconnection was suggested to be the most probable mechanism of the entry [Shi et al., 2013]. Higher energy ions have been found by Fear et al. [2014] and interpreted as due to magnetotail reconnection during periods of northward IMF. Since these events are rare, the fate of the entered plasma has not been widely studied. It is not known whether those plasmas entry will contribute to aurora. In this study, with very unique conjugate observations of aurora and high latitude in-situ observations, we investigate a possible link between solar wind entry and the formation of transpolar arcs in the polar cap.

The magnetospheric substorm at Mercury

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The evolution of Mercury’s magnetosphere during the substorm.

Magnetospheric substorms are space weather disturbances powered by the rapid release of magnetic energy stored in the lobes of planetary magnetic tails. Despite the comprehensive observations of substorm at Earth, there are rare detail observations of substorm processes at Mercury.

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.

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