MSSL Space Plasma Science Nuggets

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.

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.

Automated determination of auroral breakup during the substorm expansion phase using all-sky imager data

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An example of the difficulty to visually define an time and location for auroral break-up, but how well an automated algorithm picks out this period of brightening. From Murphy et al. (2014)

MSSL researchers participated in the development of a novel method for quantitatively and routinely identifying auroral breakup following substorm onset using the THEMIS (Time History of Events and Macroscale Interactions during Substorms) all-sky imagers.

The detailed spatial structure of field-aligned currents comprising the substorm current wedge

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. Field-Aligned currents observed by the AMPERE mission and ground perturbations of the Hall current components  of the substorm current wedge during three substorm expansion phases.  The polarisation ellipses point towards the centre of the substorm current wedge, and the integrated FACs from AMPERE show a significantly complex current structure results in a net upward and downward current structure as first identified by McPherron et al. [1973]. From Murphy et al. (2014)

We present a comprehensive two-dimensional view of the field-aligned currents (FACs) during the late growth and expansion phases for three isolated substorms utilizing in situ observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and from ground-based magnetometer and optical instrumentation from the Canadian Array for Realtime Investigations of Magnetic Activity and Time History of Events and Macroscale Interactions during Substorms ground-based arrays.

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