MIST
Magnetosphere, Ionosphere and Solar-Terrestrial
Nuggets of MIST science, summarising recent papers from the UK MIST community in a bitesize format.
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Plasma Heating From Dipolarizations in Saturn's Magnetotail
By Andrew Smith, Department of Physics and Astronomy, University of Southampton, UK.
Magnetic reconnection in a planet's magnetotail allows the stretched field to snap back towards the planet, carrying with it a bundle of plasma. This is known as a dipolarization front, which often manifest in spacecraft data as rapid rotations of the magnetic field accompanied by a change in the local plasma character. Dipolarization fronts have been observed at Earth, Mercury, Jupiter and Saturn and are thought to be linked to bright auroral displays.
We performed a large automated survey of Cassini data, identifying 28 intervals when the spacecraft was in the path of dipolarization fronts sweeping towards Saturn. The changes in plasma properties were investigated, along with the supra-thermal composition. A large dawn-dusk asymmetry was present in the observations, with 79% of the events located post-midnight. Figure 1 shows the change in plasma characteristics from that preceding the front (a) to within the dipolarizing material (b). All of the identified events showed an increase in the electron temperature and a coupled reduction in the electron density. Figures 1c and (d) show the relative change in temperature and density respectively. Overall, the temperature was found to increase by factors between 4 and 12, while the density dropped by factors of 3-10. The variable plasma properties are thought to be linked to a variable reconnection location, particularly post-midnight.
Figure 1: Panels (a) and (b) show the electron density plotted aainst the electron temperature for before (a) and after (b) the dipolarization front. These panels are plotted on the same axes scale for direct comparison. The gray lines indicate how the events move in density-temperature space. Panels (c) and (d) show the electron temperature and density (respectively) before the front plotted against the electron temperature and density after the passage of the front. The points and error bars provided are the mean and standard error of the mean respectively. The diagonal black dashed line shows the location of $y = x$: where the points would lie if there was no change following the passage of the front. The red dashed lines indicate least squares linear fits to the data; the details of the fit parameters are provided on the panels. The color bar for all four panels indicates the radial distance at which the spacecraft encountered the event.
For more information, see the paper below:
Smith, A. W., Jackman, C. M., Thomsen, M. F., Sergis, N., Mitchell, D. G., & Roussos, E. (2018). Dipolarization fronts with associated energized electrons in Saturn's magnetotail. Journal of Geophysical Research: Space Physics, 123, 2714–2735. https://doi.org/10.1002/2017JA024904
The Association of High‐Latitude Dayside Aurora With NBZ Field‐Aligned Currents
By Jennifer Carter, Department of Physics and Astronomy, University of Leicester, UK
Under northward interplanetary magnetic field conditions, when the IMF Bz > 0 nT, non-filamentary auroral emissions may be seen within the dayside polar cap and separate from the main auroral oval. These emissions are associated with lobe reconnection occurring at the high-latitude magnetopause on open field lines. Two mechanisms have been proposed to explain these emissions. The first involves the precipitation of magnetosheath plasma at the footprint of the high-latitude reconnection site, resulting in a “cusp spot”. This cusp spot has been shown to move in response to the east-west (BY) orientation of the solar wind. The second mechanism associates the auroral emissions known as High-Latitude Detached Arcs (HiLDAs) with upward field-aligned currents inside the polar cap. Under northward IMF, twin-cell field-aligned currents (NBZ system) can be found inside of the main region 1-region 2 field aligned current system. Under the influence of positive IMF BY, the upward NBZ cell expands across the noon sector in the Northern Hemisphere, whereas under negative BY, the downward cell will enlarge. The reverse scenario occurs in the Southern Hemisphere for either BYdirection.
Previous observations of HiLDAs have been limited to the Northern Hemisphere for a small data set, and previous authors have linked this phenomenon to season, as the HiLDAs have only been detected during the summer. We used concurrent auroral observations from Defense Meteorological Satellite Program Special Sensor Ultraviolet Spectrographic Imager (SSUSI) experiment, and FAC distributions constructed from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), from the Iridium telecommunication satellite constellation, to perform a large statistical study of HiLDAs under varying IMF for both hemispheres. We observe a patch of auroral emission that is co-located with the upward NBZ FAC in the dayside polar cap in both the Northern and Southern Hemispheres under northward IMF conditions.
We observe the HiLDA emission to move in response to changes in the IMF BYcomponent (e.g. Figure 1), whereby the HiLDAs are seen to move into the polar cap under positive BY, or be pushed up against, and therefore indiscernible from, the main auroral oval under negative BY(Northern Hemisphere case). We also support the hypothesis that these emissions are only detectable in the summer hemisphere, indicating a dependence on ionospheric conductivity via photoionisation in the predominantly sunlit hemisphere.
For more information, see the paper below:
Figure 1: Northern Hemisphere summer auroral emissions in the Lyman-Birge-Hopfield long band with overlaid field-aligned current contours, for the Northern (N, row a) and Southern (S, row b) Hemispheres. Clock angles are given in the left-hand column. Interplanetary magnetic field magnitudes are between 5 and 10 nT. Field-aligned current contours are overlaid for upward (red) and downward (turquoise) currents, at absolute magnitudes of 0.1 (solid line), 0.3 (dashed line), and 0.5 (dotted line) μA/m2.
Solar Wind Dependence of Magnetospheric Ultra-Low Frequency Plasma Waves
By Sarah Bentley, Department of Meteorology, University of Reading, UK
Ultra-low frequency plasma waves (ULF, 1-15 mHz) are implicated in the energisation and transport of radiation belt electrons. Therefore a description of magnetospheric ULF wave power in terms of driving parameters is highly desirable for radiation belt forecasting; in particular, we want to describe power in terms of solar wind properties, as the solar wind is the dominant driver behind these waves.
However, identifying solar wind driving parameters is severely hampered by the nature of the solar wind. All solar wind parameters are highly interrelated due to their common solar sources and the interactions within the solar wind between the Sun and Earth, resulting in the effect that all solar wind properties correlate so strongly with speed vswthat investigating their relationship to magnetospheric properties is difficult.
To circumvent analysis techniques that require properties such as a linear interdependence between these parameters, we use a series of simple yet systematic two-parameter plots (e.g. Figure 1) to identify which parameters are causally correlated to ULF wave power, rather than just correlated via a relationship with speed vsw. We find that speed, the southward component of the interplanetary magnetic field and summed power in proton number density perturbations (vsw, Bz < 0 and δNp) are the three dominant parameters driving power in magnetospheric ultra-low frequency waves. These parameters can be used in future modelling but are also of interest because there is clearly a threshold at Bz = 0, and because ULF wave power depends more on perturbations δNp than the number density Np itself.
For more information, see the paper below or an informal blog post here.
Bentley, S. N., Watt, C. E. J., Owens, M. J., & Rae, I. J. (2018). ULF wave activity in the magnetosphere: Resolving solar wind interdependencies to identify driving mechanisms. Journal of Geophysical Research: Space Physics, 123. https://doi.org/10.1002/2017JA024740
Figure 1: A two-parameter plot taken from Bentley et al., 2018. We bin the ULF power observed at one station (roughly corresponding to geostationary orbit) at one frequency (2.5mHz) and observe whether it increases with increases in solar wind speed vswand/or the component Bz of the interplanetary magnetic field, using fifteen years of data. Cut-throughs at constant speed and Bz are shown in (b) and (c). ULF power increases with speed and with more strongly negative Bz for Bz<0, but only with speed for Bz>0.
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.
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.
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.