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MSSL Space Plasma Science Highlights

How 'Coronal' Are Solar Wind Electrons?

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Macneil2017

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.

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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Rae-2017

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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Yao2017_saturn

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

Publication date:

Graham2017

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. 

Measuring the currents that power the aurora

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Artist's impression of the Swarm spacecraft. Copyright: ESA/AOES Medialab

The interaction between charged particles flowing from the Sun with the magnetic field of the Earth drives enormous electrical currents in space. At any time, currents of around 1 million Amps can be flowing into and out of the upper atmosphere of the Earth, driven by the Sun-Earth connection. These currents close by flowing through the ionosphere, a charged layer of our atmosphere that extends upwards from around 80 km altitude, and thus are an important part of the connection between the atmosphere and space.

Effects of ULF waves on the Earth’s radiation belts

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This image was created using data from the Relativistic Electron-Proton Telescopes on NASA's twin Van Allen Probes. It shows the emergence of a new third transient radiation belt. The new belt is seen as the middle orange and red arc of the three seen on each side of the Earth. Image Credit: APL, NASA

Relativistic particles with energies of up to few Megaelectron Volts are trapped by the Earth’s main magnetic field in the regions known as Van Allen radiation belts. The intense radiation environment imposes danger for satellite operations and needs to be forecasted and modelled using numerical simulations and data assimilation. Electromagnetic ultra low frequency (ULF) oscillations in the range of 150-600 s periods, produced by the interaction between solar wind and the Earth’s magnetosphere, play a substantial role in the acceleration, transport and loss of radiation belt particles. Properties of ULF waves need to analysed to improve the modelling of radiation belts.

Substructures within a Dipolarization Front Revealed by High-temporal Resolution Cluster Observations

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

A Dipolarization Front (DF) is usually considered as the leading edge boundary of a reconnection outflow in the magnetotail, and is characterised by a dramatic magnetic field enhancement, typically on Bz component in GSM coordinates. This Bz ramp usually lasts for a few seconds, which is comparable to the spin period of a Cluster or a THEMIS spacecraft.

What effect do substorms have on the radiation belts?

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Artists impression of particles in Earth's Van Allen belts. Courtesy NASA SVS

The Van Allen radiation belts are a torus of high-energy charged particles trapped on magnetic field lines at the Earth. Consisting mainly of near-relativistic electrons, these belts stretch out from a few thousand kilometres altitude to around geosynchronous orbit and pose a very real hazard to satellites flying through or inhabiting this space. One of the mysteries of the radiation belts is how they get there - most of the plasma in the magnetosphere or coming off the Sun is at much lower energies. One theory is that dynamic events in the magnetosphere known as substorms, that also result in bright auroral displays, might energise particles in the magnetosphere or provide a mechanism by which particles might be accelerated to these exceptionally high energies.

ULF Waves above the Nightside Auroral Oval during Substorm Onset

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(eft panels) false colour images, and (right) 3 second difference images from the FSMI and GILL ASIs for the three consecutive auroral bead onsets from (a) ~0503 UT, (b) ~0510 UT, and (c) ~0524 UT.

The first indication of substorm onset is a sudden brightening of one of the quiet arcs lying in the midnight sector of the oval, and an explosive auroral displays covering the entire night sky follows.  In space, this corresponds to a detonation that releases a huge amount of energy stored in the stretched night-time magnetic fields and charged particles. This chapter reviews historical ground-based observations of electromagnetic waves and their role in detonating the substorm, and highlights new research linking these electromagnetic waves explicitly to substorm onset itself. The chapter focuses on the properties of ultra-low frequency (ULF) electromagnetic waves that are seen in two-dimensional images of the aurora and discusses a wider range of physical processes that could be responsible for the azimuthally structured auroral forms along the substorm onset arc immediately before it explosively brightens.  

Student Sounding Rockets to train the next generation of space scientists

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Launch of the first CaNoRock. Image courtesy: Kolbjorn Blix Dahle

The Canada-Norway Student Sounding Rocket (CaNoRock) program is a multi-national, multi-university collaboration to train undergraduate students in space science or engineering, and to recruit them into space-related graduate studies or industry.

A New Technique for Determining Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE)

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An auroral substorm observed by the IMAGE FUV-WIC instrument. Courtesy: H. U. Frey/IMAGE/NASA

Substorms are a fundamental mode of variability of the solar wind-magnetosphere-ionosphere system. Previous studies have shown that they can process over 1000 TJ of captured solar wind energy and, in so doing, divert magnetospheric currents through the ionoshpere. This diversion of currents results in a distinct signature in ground-based magnetometer measurements at auroral latitudes. In a new paper, Forsyth et al [2015] have developed a technique for identifying all parts of a substorm from this ground-magnetometer data.

Lightning as a Space Weather Hazard

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Average UK thunder days (RTH;top) and lightning rates (RL; second). RL is shown in the remaining panels split by A to T or T to A current sheet crossings on 80 and 10 day intervals.

UK lightning rates previously have been shown to be influenced by large compressed regions of solar wind known as corotating interaction regions (CIRs). CIRs are often co-located with the heliospheric current sheet (HCS) at 1AU. A catalogue of all HCS crossings from 2000 to 2007 is computed using the change in the magnetic field direction. The average lightning rates (RL; from the UK MetOffice’s radio network) and average thunder days (RTH; from audio records) were then computed for 40 days either side of the HCS crossing. These results are shown in the top two rows of the figure. 13.5-and 27-day peaks in thunderstorm activity is observed corresponding to the regularity of HCS crossings of the Earth as they rotate around with the Sun.

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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