MSSL Space Plasma Science Highlights

Seasonal and Temporal Variations of Field‐Aligned Currents and Ground Magnetic Deflections During Substorms

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Figure 3 from Forsyth et al. [2018]. Superposed epoch analysis results with respect to substorm onset of the substorm FACs (SU‐MLT and SD‐MLT) from AMPERE, calculated by removing the median current in the hour before onset. The top and bottom rows show the upward substorm FAC (SU) and downward substorm FAC (SD), respectively, in each MLT sector. As per the above, the results are subdivided into seasons of 90 days centered on the solstices and equinoxes.

Earth is surrounded by electrical currents flowing in space. These currents, which can be 10,000 times greater than domestic electrical supplies, can flow along the Earth's magnetic field and into the upper atmosphere and are linked to aurora. The size of this current depends on atmospheric conditions, with the upper atmosphere being a better conductor when it is sunlit, and the interaction between particles flowing from the Sun and the Earth's magnetic field. During space weather events known as substorms, which happen several times per day on average, the aurora brightens massively and the currents flowing into the upper atmosphere increase. Using data from the Iridium communications satellites, measured these currents during substorms.

The Role of Proton Cyclotron Resonance as a Dissipation Mechanism in Solar Wind Turbulence

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The solar wind contains turbulent fluctuations that are part of a continual cascade of energy from large scales down to smaller scales. At ion-kinetic scales, some of this energy is dissipated, resulting in a steepening in the spectrum of magnetic field fluctuations and heating of the ion velocity distributions, however, the specific mechanisms are still poorly understood. Understanding these mechanisms in the collisionless solar wind plasma is a major outstanding problem in the field of heliophysics research.

NHDS: Calculating the properties of plasma waves

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NHDS: The New Hampshire Dispersion relation Solver

Waves are a very important mechanism to transfer energy in a plasma and to heat the particles efficiently. These processes occur in astrophysical, space, and laboratory plasmas. Like all electric charges, the plasma particles react on electric and magnetic fields, while they also change the electric and magnetic fields themselves. This makes plasmas a lot more complicated than normal (neutral) gases. As a consequence of this behaviour, there are many different types of waves that can exist in a plasma, while a gas like the air we are breathing only carries one type of wave (the sound wave). 

Understanding Waves and Instabilities in Collisionless Plasmas with ALPS

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ALPS Logo.

The solar wind is a plasma in which collisions are very rare. Many plasmas in the universe are in this so-called "collisionless" state. This applies, for example, to a common type of accretion disks around black holes in the centres of galaxies, the very dilute medium between galaxies, magnetospheres around planets and comets, as well as pulsar winds in supernova remnants. In all collisionless plasmas, the behaviour of plasma waves, which are the fundamental building blocks of many important plasma processes, is more complicated to understand than in a collision-dominated plasma. Therefore, we have to rely on computer models to calculate the properties of plasma waves. With their help, it is also possible to calculate whether the plasma is in a stable or unstable state, a question of great importance for understanding the plasma behaviour.

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. 

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.

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