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. 

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