MSSL Space Plasma Science Highlights

The role of localised compressional Ultra-Low Frequency waves in energetic electron precipitation

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Global-scale electromagnetic wave activity known as Ultra-Low Frequency (ULF) waves have been historically discussed as playing an indirect role in the acceleration and loss of radiation belt electrons. This is primarily due to the fact that ULF waves cannot easily interact with the gyration of electrons causing acceleration, or their bounce motion causing them to be lost. However, certain assumptions on the global-scale nature of these ULF wave fields are made to arrive at this conclusion. In this paper, we explore the validity of the assumptions that go into our current thinking, and provide a thought experiment on how a localised, large-amplitude electromagnetic wave field could interact with relativistic particles and play a direct role in radiation belt losses as a result. We conclude that localized ULF wave fields may provide an additional and, importantly, complementary means to more established processes that are known to precipitate electrons from the radiation belts during geomagnetic storms.

Variations of high-latitude geomagnetic pulsation frequencies: A comparison of time-of-flight estimates and IMAGE magnetometer observations

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The terrestrial magnetosphere is subjected to an abundance of perturbations, both externally from the solar wind and internal plasma dynamics, which result in oscillations of the magnetic field lines at their given natural frequencies. These resonant oscillations of the field lines are a fundamental mechanism for the transfer of energy and momentum within the magnetosphere. Therefore, it is valuable to understand how these oscillation frequencies vary spatially throughout the magnetosphere.

How 'Coronal' Are Solar Wind Electrons?

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We aim to understand the link between the Sun's atmosphere, the corona, and the constant stream of plasma which escapes it, the solar wind. To do so we test how similar energetic electron populations (the isotropic 'halo' and the beamed 'strahl') in the solar wind are to their expected earlier state in the corona. Models for the formation of these electron populations in the corona suggest that their energy content should depend on the local temperature, for which we can use solar wind oxygen ionisation state measurements as a proxy. Comparing electron halo temperature and strahl energy content to these ionisation states, we find only a very weak link which varies with the type of solar wind stream and the 11-year solar cycle. We find minor evidence to suggest that this is due to solar wind processing during its outward flow.

The structure of PSBL during an storm-time intense reconnection

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The transition region lying between the plasma sheet and the lobe, is called the plasma sheet boundary layer (PSBL).  This layer is formed by magnetic reconnection operating farther down the magnetotail, which drives the accelerated electron and ion beams along the magnetic field towards the Earth. The information that electrons and ions carry in PSBL, is essential to understanding the temporal and spatial variation at the reconnection site. We aim to utilise these information including energy dispersion and pitch angle of particles in the highest cadence possible to analyse the dynamics of the reconnection site.

A magnetospheric plasma mass density model for varying geomagnetic activity

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A figure showing the spatial distribution of electron density, average ion mass, and mass density for quiet (bottom) and active (top) conditions.

The terrestrial magnetosphere, a region around the Earth where the motion of positively and negative charged particles (plasma) is largely controlled by the geomagnetic field, is a highly variable and structured environment. The variations in the density and composition of the plasma is an important factor in shaping how the global magnetic field responds to perturbations and how energy propagates throughout the system. A key phenomenon associated with the variability is the geomagnetic storm. In this study, observations of the plasma are used to construct a model describing how the number density, composition, and mass density of the magnetospheric plasma changes in response to storm conditions.

Electromagnetic waves and their respective roles in driving substorm onset

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Substorm onset is marked in the ionosphere by the rapid and poleward expansion of the aurora around local midnight and corresponds to a huge amount of energy release in the stretched tail of Earth’s magnetic environment. With the auroral display, a repeatable signature of substorm onset is the launching of electromagnetic waves across all frequencies, that at lower frequencies are invoked to carry the required electrical currents, and at higher frequencies are implicated in auroral particle acceleration.

How does rapid magnetospheric rotation drive magnetic reconnection?

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Magnetic reconnection is an essential process in driving energy conversion and mass transportation for nebular flares, solar flares, and planetary magnetospheric energization. Plasma heating and energization during reconnection are often identified at magnetopause and nightside magnetotail of the Earth and other planets. However, both the solar wind and fast rotation may drive a reconnection at Saturn and Jupiter, the two giant and fast rotating planets in our solar system.

The unexpected behaviour of the solar wind

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A schematic of the WIND spacecraft which has been located at the L1 Lagrangian point since 2004

The solar wind is a plasma flow that emanates from the Sun. It is very tenuous (only about five particles per cubic centimetre) and very hot (multiple hundred thousands degrees). Therefore, collisions among plasma particles are very rare. We call such a plasma "collisionless". Collisionless plasmas behave very differently from collisional fluids like the air and usually require a more complicated theoretical framework for their treatment.

Strahl Electron Beams in the Solar Wind

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Plasma in the solar atmosphere is too hot to be confined to the Sun. It streams outwards into the solar system, pulling the Sun’s magnetic field with it, to form the solar wind and interplanetary magnetic field (IMF). Beams of high speed electrons are observed to travel outwards away from the Sun along the IMF lines, we call these ‘strahl’. We do not know the exact nature of the solar origins of the strahl nor do we have a complete picture of their in-transit interactions. We do know that in the absence of particle or wave interactions, these beams should narrow as distance from the Sun increases. Hence, in this study we examined strahl beam width over the largest radial distance range to date, to observe how strahl electron beams changed with distance from the Sun. To achieve this we used plasma particle and magnetic field data from the Cassini-Huygens mission on its journey to Saturn. We found that strahl beam width increased with distance from the Sun, to just beyond the orbit of Jupiter, and was likely too broad and low density to be observed on the approach to Saturn. It was concluded that some form of wave-particle interaction is required to produce scattering of the strahl beam. 

Remote sensing the substorm instability

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Auroral beads (inset) which form along an auroral arc in the minutes prior to an auroral substorm. A schematic showing the beads which are mapped from the ionosphere along magnetic field lines into the magnetotail where we can understand their source.

The particles which generate the auroral are funnelled into the polar atmospheres along magnetic field lines, having originated deep within the Earth’s magnetosphere.  At the start of auroral substorms, quiet auroral arcs rapidly brighten and expand poleward, leading to a bright and dynamic auroral display. The processes in the magnetosphere which result in this auroral signature are not well understood, but a recent statistical study of the onset arc has provided new insights. 

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