MIST

Magnetosphere, Ionosphere and Solar-Terrestrial

Latest news

MIST recognised in 2018 RAS awards

MIST Council would like to congratulate those who have been recognised for contributions to the field by the Royal Astronomical Society recently, but particularly we would like to congratulate those members of the MIST community who are to be honoured at the next National Astronomy Meeting.

Emma Bunce has won the Chapman Medal for outstanding contributions to the understanding of the magnetospheres of gas giants, Matt Taylor has won the Service Award for his exceptional work in co-ordinating and contributing to ESA's Rosetta mission, and Jim Wild has been awarded the James Dungey lectureship for his excellent and highly relevant work on substorms and reconnection in the magnetotail. We would also like to congratulate Kerri Donaldson Hanna for winning the Winton Award for planetary science.

MIST Council applauds each of the winners, alongside the other academics who will be recognised in Liverpool this spring!

More details are available at the RAS website.

New MIST councillors in 2017

Congratulations to Jasmine Sandhu and Jonny Rae, both at MSSL, who have been elected (and, in Jonny’s case, re-elected) to MIST Council. They join Ian McCrea (Chair - RAL), Sarah Badman (Lancaster), Luke Barnard (Reading) and John Coxon (Southampton), all of whom continue in their posts.

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Rishbeth Prizes 2017

Congratulations to Jade Reidy (University of Southampton) and Mervyn Freeman (British Antarctic Survey) for winning this year's Rishbeth prizes for their presentations at the National Astronomy Meeting at the University of Hull this July.

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Nigel Wade

Nigel Wade
Nigel Wade - University of Leicester

It is with deep sadness that we have to inform the MIST community of the untimely death after a short illness of Nigel Wade who worked in the Radio and Space Plasma Physics (RSPP) group at Leicester for over 30 years.

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New Members of MIST Council

After a hard-fought campaign by the five candidates, the results of the MIST Council elections are now in!

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

By Lloyd Woodham, Mullard Space Science Laboratory, University College London, UK

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.

We use magnetic field and ion moment data from the MFI and SWE instruments on-board the Wind spacecraft to study the nature of solar wind turbulence at ion-kinetic scales. We analyse the spectral properties of magnetic field fluctuations between 0.1 and 5.5 Hz over 2012 using an automated routine, computing high-resolution 92 s power and magnetic helicity spectra. To ensure the spectral features are physical, we make the first in-flight measurement of the MFI ‘noise-floor’ using tail-lobe crossings of the Earth's magnetosphere during early 2004. We utilise Taylor's hypothesis to Doppler-shift into the spacecraft frequency frame, finding that the spectral break observed at these frequencies is best associated with the proton-cyclotron resonance scale, 1/kc, compared to the proton inertial length di and proton gyroscale ρi. This agreement is strongest when we consider periods where βi,perp ~ 1, and is consistent with a spectral break at di for βi,par « 1 and ρi for βi,perp » 1.

Histograms for 2012 of the estimated helicity onset frequency, fb, versus the three characteristic plasma scales, converted into frequencies using Taylor's hypothesis - fL represents fkc, fdi, and fρi, for each column respectively. The data used are for periods where 0.95 ≥ βi,perp ≥ 1.05. The colour-bar represents the column-normalised number of spectra. The black dashed lines represent fb = fL and similarly, the red dashed lines are fb = fL√2 and fb = fL√2, which give the resolution of the wavelet transform about the line fb = fL due to the finite width of the Morlet wavelet in frequency space. We see the best agreement between fb and fkc during these periods.

We also find that the coherent magnetic helicity signature observed at these frequencies is bounded at low frequencies by 1/kc and its absolute value reaches a maximum at ρi. These results hold in both slow and fast wind streams, but with a better correlation in the more Alfvénic fast wind where the helicity signature is strongest. We conclude that these findings are consistent with proton-cyclotron resonance as an important mechanism for dissipation of turbulent energy in the solar wind, occurring at least half the time in our selected interval. However, we do not rule out additional mechanisms.

Woodham et al., 2018, The Role of Proton Cyclotron Resonance as a Dissipation Mechanism in Solar Wind Turbulence: A Statistical Study at Ion-kinetic Scales, ApJ, 856, 49, DOI: 10.3847/1538-4357/aab03d

 

The Broadband Excitation of 3-D Alfvén Resonances (FLRs) in a MHD Waveguide

By Tom Elsden, Department of Mathematics and Statistics, University of St. Andrews, St. Andrews, UK

Field line resonance (FLR) has been the theoretical mechanism used to explain a myriad of ground and spaced based observations of ultra low frequency (ULF) waves in Earth’s magnetosphere. FLR is a plasma physics process whereby energy from a global oscillation (fast mode) can be transferred to local oscillations along magnetic field lines (Alfvén mode), where the fast mode frequency matches the local Alfvén frequency. This process was first studied analytically where the plasma was only inhomogeneous in the radial direction (mathematically 1D) [Southwood, 1974, Chen and Hasegawa, 1974] and has since been extended both analytically and numerically to more complicated systems [e.g. Lee and Lysak, 1989, Chen and Cowley, 1989, Wright and Thompson, 1994, Russell and Wright, 2010].

A feature of FLRs in complicated geometries, such as a dipole, is that the poloidal (radial) and toroidal (azimuthal) Alfvén frequencies are different [e.g. Radoski, 1967]. This infers that the location where the FLR will occur is dependent on the polarisation of the Alfvén wave. This property has recently been explored theoretically in 3D [Wright and Elsden, 2016] and forms the basis of this current work. The magnetosphere is asymmetric and therefore requires an understanding of FLR in 3D. We look at wave coupling in an excessively asymmetric waveguide in order to study the physics clearly.

The figure below taken from Elsden and Wright [2018], displays cuts in the equatorial plane from a 3D MHD waveguide simulation using a 2D dipole magnetic field geometry. In each panel, the x-axis is the radial direction (α) and the y-axis the azimuthal direction (β), and the density varies with azimuth. The left panel shows the energy density (dimensionless units) integrated along a field line, showing an accumulation of energy along curved resonance paths, where the FLR polarisation is between poloidal and toroidal. The middle and right panels show the square root of the kinetic energy in the equatorial plane, revealing ridges which develop by phase mixing in 3D. We find that with a broadband driver it is the natural fast waveguide modes which drive FLRs. Such modes are fairly insensitive to the form of the driver, and hence the resonances are seen at the same locations for many different driving stimuli. This means that the resonances are a property of the medium, and can hence be used as a seismological tool to infer properties of the equilibrium. Finally, the key point is that traditionally FLRs are regarded as having a strictly toroidal polarisation. However, here we have shown in 3D that they can have other polarisations.

Elsden, T. and A. N. Wright (2018), The Broadband Excitation of 3D Alfvén Resonances in a MHD Waveguide, J. Geophys. Res. Space Physics, 123, doi:10.1002/2017JA025018

Figure: Left: Energy density integrated along a field line. Black dashed line represents a theoretical prediction of the main FLR location. Middle: Square root of the the kinetic energy in the equatorial plane. Right: Same as middle but annotated for use in other plots in the paper.



AuroraWatch UK: An Automated Aurora Alert System

By Nathan A. Case, Department of Physics, Lancaster University, Lancaster, UK

The aurora borealis, though most often visible from more northerly latitudes, can occasionally be seen from the UK too. To help the public in their endeavour to see the northern lights from the UK, Lancaster University’s AuroraWatch UK issues alerts of when the aurora might be visible.

As the currents driving the aurora intensify, they produce disturbances to the local magnetic field. Since its inception in September 2000, AuroraWatch UK has been using its own suite of magnetometers to record these disturbances and issue real-time alerts about where in the UK an aurora might be seen.

We have now combined and standardised these alerts, using the latest alert algorithm to produce a 17-year dataset of UK aurora alerts. This dataset, along with the real-time data, is freely available for the community and the general public to use. We find that the alerts match well with the wider Kp index and the solar cycle.

Case, N. A., Marple, S. R., Honary, F., Wild, J. A., Billett, D. D., & Grocott, A. 2017. AuroraWatch UK: An automated aurora alert system. Earth and Space Science, 4, 746–754. https://doi.org/10.1002/2017EA000328

(left) A pie chart illustrating the number of hours spent at each AuroraWatch UK activity level, as a percentage of the total number of hours. (right) A histogram of the percentage of hours spent at an elevated alert level (i.e., yellow or above) per year. Also plotted are (solid line) the percentage of time per year where Kp ≥ 4 and (dashed line) the mean daily sunspot number per year (as a proxy for solar activity). The sunspot number is divided by 10 for scale.

Nugget: Are steady magnetospheric convection events prolonged substorms?

By Maria-Theresia Walach, Department of Physics and Astronomy, University of Leicester, Leicester, UK

The large scale convection of magnetic flux within the Earth’s magnetosphere due to reconnection, also known as the Dungey cycle [Dungey, 1961; 1963], is partially driven by the solar wind. During southward IMF reconnection at the subsolar magnetopause opens flux, which is then added to the magnetotail. Depending on the strength of solar wind-driving, the magnetospheric response can be delayed, episodic or prolonged, also known as “magnetospheric modes” [e.g. Pulkkinen et al., 2007].

Walach and Milan [2015] produced a statistical analysis of the event progression of steady magnetic convection events (intervals where the dayside reconnection is balanced by nightside reconnection [e.g. DeJong et al., 2008]), substorms (dominant dayside reconnection is followed by a delayed interval of dominant nightside reconnection [e.g. Baker et al., 1996]), and sawtooth events (signatures appearing to be quasi-periodic and quasi-global substorms [e.g. Henderson, 2004]). Superposed epoch analyses show that 58% of the studied steady magnetospheric convection events are part of prolonged substorms, where dayside reconnection is at first dominant. Then nightside reconnection is initiated as part of a substorm, but as the solar wind-driving continues the Earth’s magnetosphere then progresses into a state of steady magnetospheric convection, after which the substorm recovery continues.

Walach, M.-T., S. E. Milan (2015), J. Geophys. Res. Space Physics, 120, doi:10.1002/2014JA020631.

walach nugget

Superposed epoch analysis of substorms (red), sawtooth events (orange), steady magnetospheric convection events with preceding substorms (blue) and steady magnetospheric convection events without preceding substorms (green). The onset of the steady magnetospheric convection events with preceding substorms has been shifted to match the preceding substorm onset. The time of the event duration for the steady magnetospheric convection events in superposed epoch analyses in the right column has been normalised.

Nugget: Statistical characterisation of the growth and spatial scales of the substorm onset arc

By Nadine Kalmoni, UCL Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey, UK

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.

The relatively high temporal and spatial resolution of the THEMIS mission All Sky Imagers have allowed recent observations of small scale azimuthal structures, auroral beads, which form in the minutes leading to auroral onset [e.g. Rae et al., 2009]. Conjugate observations in the Northern and Southern hemisphere suggest that the beads have a common magnetotail driver and are the ionospheric signature of a magnetospheric instability [Motoba et al., 2012].

Kalmoni et al. [2015] statistically analyse the growth and spatial scales of clear signatures of auroral beads observed in the minutes leading to substorm onset. The statistical observations are compared with the Shear-Flow Ballooning Instability (SFBI) [Voronkov et al., 1997] and the Cross-Field Current Instability [Lui, 2004 and references therein] which have both been proposed to play a role in substorm onset. Our observations conclude that the SFBI initiated in the near-Earth plasma sheet is the most likely explanation.

Kalmoni, N. M. E., I. J. Rae, C. E. J. Watt, K. R. Murphy, C. Forsyth, and C. J. Owen (2015), J. Geophys. Res. Space Physics, 120, doi:10.1002/2015JA021470.

kalmoni nugget

Normalised growth rate as a function of spatial scale in comparison to (a) the Cross-Field Current Instability for inner-edge and mid-tail plasma sheet parameters and (b) the Shear-Flow Ballooning Instability for varying shear-flow widths.