Nuggets of MIST science, summarising recent MIST papers in a bitesize format.
By Arianna Sorba, Department of Physics and Astronomy, University College London, UK.
At Saturn, the planet’s rotation axis and the dipole axis are aligned to within 0.01° [Dougherty et al., 2018], and so the magnetosphere’s magnetic field should be extremely azimuthally symmetric. However the Cassini space mission, which orbited Saturn from 2004-2017, observed mysterious periodic variations in the magnetic field at a period close to the planetary rotation rate. These observations suggested that the outer magnetosphere’s equatorial current sheet was `flapping’ above and below the rotational equator once per planetary rotation, to a first approximation acting like a rotating, tilted disc [Arridge et al., 2011].
However this ‘flapping’ picture does not fully explain the observed magnetic field periodicities. More recently, some studies have suggested the magnetosphere may also display ‘breathing’ behaviour; a periodic large-scale compression and expansion of the system, associated with a thickening and thinning of the current sheet [Ramer et al., 2016, Thomsen et al., 2017]. In Sorba et al. , we investigate these two dynamic behaviours in tandem by combining a geometric model of a tilted and rippled current sheet, with a force-balance model of Saturn’s magnetodisc. We vary the magnetodisc model system size with longitude to simulate the breathing behaviour, and find that models that include this behaviour agree better with the observations than the flapping only models. This can be seen in the figure below, which shows that for an example Cassini orbit, both the amplitude and phase of the magnetic field variations are better characterised by the flapping and breathing model, especially for the meridional component (middle panel).
The underlying cause of this periodic dynamical behaviour is still an area of active research, but is thought to be due to two hemispheric magnetic field perturbations rotating at different rates. The study by Sorba et al.  provides a basis for understanding the complex relationship between these perturbations and the observed current sheet dynamics.
For more information, please see the paper below:
Sorba, A.M., N. Achilleos, P. Guio, C.S. Arridge, N. Sergis, and M.K. Dougherty. (2018), The periodic flapping and breathing of Saturn's magnetodisk during equinox, J. Geophys. Res. Space Physics, 123. https://doi.org/10.1029/2018JA025764
Figure: Radial (a), meridional (b), and azimuthal (c) components of the magnetic field measured by Cassini along Rev 120 Inbound. Magnetometer data shown in black, flapping only model shown in red, and flapping and breathing model shown in blue. Annotation labels underneath the time axis give the cylindrical radial distance of Cassini from the planet centre, and Saturn magnetic local time.
By Beatriz Sánchez-Cano, Department of Physics and Astronomy, University of Leicester, UK.
On the 19th October 2014, an Oort-cloud comet named Comet C/2013 A1 (Siding Spring) passed Mars at an altitude of 140,000 kilometres (only one third of the Earth-moon distance) during a single flyby through the inner solar system. This rare opportunity, where an event of this kind occurs only once every 100,000 years, prompted space agencies to coordinate multiple spacecraft to witness the largest meteor shower in modern history and allow us to observe the interaction of a comet’s coma with a planetary atmosphere. However, the event was somehow masked by the impact of a powerful Coronal Mass Ejection from the Sun that arrived at Mars 44 hours before the comet, creating very large disturbances in the Martian upper atmosphere and complicating the analysis of data.
Sánchez-Cano et al.  present energetic particle datasets from the Mars Atmosphere and Volatile EvolutioN (MAVEN) and the Mars Odyssey missions to demonstrate how the Martian atmosphere reacted to such an unusual external event. Comets are believed to have strongly affected the evolution of planets in the past and this was a near unique opportunity to assess whether cometary energetic particles, in particular O+, constitute a notable energy input into Mars’ atmosphere. The study found several O+ detections while Mars was within the comet’s environment (at less than a million kilometers distance, see period A in the figure below). In addition, the study discusses several other very interesting showers of energetic particles that occurred after the comet’s closest approach, which are also indicated in the figure below. These detections seem to be related to comet dust tail impacts, which were previously unnoticed. This unexpected detections strongly resemble the tail observations that EPONA/Giotto made of comet 26P/Grigg-Skjellerup in 1992. In conclusion, the authors found that the comet produced a large shower of energetic particles into the Martian atmosphere, depositing a similar level of energy to that of a large space weather storm. This suggests that comets had a significant role on the evolution of the terrestrial planet’s atmospheres in the past.
For more detailed information, please go to the paper:
Sánchez – Cano, B., Witasse, O., Lester, M., Rahmati, A., Ambrosi, R., Lillis, R., et al (2018). Energetic Particle Showers over Mars from Comet C/2013 A1 Siding‐Spring. Journal of Geophysical Research: Space Physics, 123.https://doi.org/10.1029/2018JA025454
Figure: MAVEN and Mars Odyssey observations as a function of time of a powerful Coronal Mass Ejection on 17th October 2014, and of comet Siding-Spring flyby on 19th October 2014. It can be seen that from the point of view of energetic particles, the comet deposited a similar amount of energy than a solar storm on Mars’ atmosphere. (a) MAVEN-SEP ion energy spectra (b) Mars Odyssey-HEND energy profile from higher-energy channels. (c) Same as in (b) but for lower-energy channels. Periods A and B indicate the comet O+ detections at Mars. Period C shows similar detections although the particle identity cannot be determined. Finally, periods D and E shows dust tail impacts on the instrument.
By Liz Tindale, CFSA, Department of Physics, University of Warwick, UK.
Time series of solar wind variables, such as the interplanetary magnetic field strength, are characteristically “bursty”: they take irregularly spaced excursions to values far higher than their average [Consolini et al., 1996; Hnat et al., 2002]. These bursts can be associated with a range of physical structures, from coronal mass ejections [Nieves-Chinchilla et al., 2018] and corotating interaction regions [Tsurutani et al., 2006] on large scales, down to small-scale transient structures [Viall et al., 2010] and turbulent fluctuations [Pagel and Balogh, 2002]. Over the course of the 11-year solar cycle, changing coronal activity causes the prevalence of these structures in the solar wind to vary [Behannon et al., 1989; Luhmann et al., 2002]. As energetic bursts in the solar wind are often the drivers of increased space weather activity [Gonzales et al., 1994], it is important to understand their characteristics and likelihood, as well as their variation over the solar cycle and between cycles with different peak activity levels.
Tindale et al.  use data from NASA’s Wind satellite to study bursts in the time series of solar wind magnetic energy density, Poynting flux, proton density and proton temperature during 1-year intervals around the minima and maxima of solar cycles 23 and 24. For each variable, the duration of a burst and its integrated size are related via a power law; the scaling exponent of this power law is unique to each parameter, but importantly is invariant over the two solar cycles. However, the statistical distributions of burst sizes and durations do change over the solar cycle, with an increased likelihood of encountering a large burst at solar maximum. This indicates that while the likelihood of observing a burst of a given size varies with solar activity, its characteristic duration will remain the same. This result holds at all phases of the solar cycle and across a wide range of event sizes, thus providing a constraint on the possible sizes and durations of bursts that can exist in the solar wind.
For more information, please see the paper below:
Tindale, E., S.C. Chapman, N.R. Moloney, and N.W. Watkins (2018), The dependence of solar wind burst size on burst duration and its invariance across solar cycles 23 and 24, J. Geophys. Res. Space Physics, 123, doi:10.1029/2018JA025740.
Figure: Scatter plots of burst size, S, against burst duration, τD, for bursts in the time series of solar wind magnetic energy density, B2, extracted from one-year time series spanning i) the minimum of solar cycle 23, ii) the cycle 23 maximum, iii) the minimum of cycle 24, and iv) the cycle 24 maximum. The colours denote bursts extracted over increasingly high thresholds: the 75th, 85th and 95th percentiles of each B2 time series. The solid black line shows the regression of log10(S) onto log10(τD) for bursts over the 85th percentile threshold; the gradient of the regression for bursts over each threshold, alongside the 95% confidence interval, is denoted by α.
By Julia E. Stawarz, Department of Physics, Imperial College London, UK.
In Stawarz et al. , we examine large- and small-scale properties of three ion-scale flux ropes in Earth’s magnetotail. Evidence of variability in the flux rope orientations is found and an electron-scale vortex is discovered inside one of the flux ropes.
Magnetic reconnection, which releases stored magnetic energy and converts it into particle motion, is a key driver of dynamics in Earth’s magnetosphere. However, it is still not fully understood how particles are accelerated and energy is partitioned both within the reconnection diffusion region, where particles decouple from the magnetic field, and within reconnection outflows. Helical magnetic fields known as flux ropes are one type of structure generated by reconnection and often observed within reconnection outflows [Borg et al., 2012; Eastwood & Kiehas, 2015; Sharma et al., 2008], which are both theoretically [Drake et al., 2006; Dahlin et al., 2017] and observationally [Chen et al., 2008] linked with particle energization. Previous observations have shown flux ropes can have substructure and intense electric fields [e.g., Eastwood et al., 2007], but the nature of these electric fields have not been previously determined. Recent high-time-resolution, mutispacecraft measurements with electron-scale separations from NASA’s Magnetospheric Multiscale (MMS) mission finally allow us to examine the detailed substructure of flux ropes.
The three closely spaced flux ropes examined in Stawarz et al.  are observed near a reconnection diffusion region and have different orientations, indicating significant spatiotemporal variability and highlighting the three-dimensional nature of the overall reconnection event. One of the most intense electric fields in the event is found within one of the flux ropes and is linked with an electron vortex (Fig. 1). The intense electric field is perpendicular to the magnetic field and the vortex consists of electrons that are frozen-in and ions that are decoupled from the fields. The resulting difference in motion between the ions and electrons drifting in the electromagnetic fields drives a current perpendicular to the magnetic field that produces a small-scale magnetic enhancement. The presence of such vortices may contribute to accelerating particles, either through inferred parallel electric fields at the ends of the structure or the excitation of waves, and points to the necessity of better understanding the substructure of flux ropes in order to characterize particle energization in magnetic reconnection.
For more information, see our paper below:
Stawarz, J. E., J. P. Eastwood, K. J. Genestreti, R. Nakamura, R. E. Ergun, D. Burgess, J. L. Burch, S. A. Fuselier, D. J. Gershman, B. L. Giles, O. Le Contel, P.-A. Lindqvist, C. T. Russell, & R. B. Torbert (2018), Intense electric fields and electron-scale substructure within magnetotail flux ropes as revealed by the Magnetospheric Multiscale mission, Geophys. Res. Lett., 45. https://doi.org/10.1029/2018GL079095
Figure 1: Overview of the electron vortex. (a) Electron-scale perturbation to the magnetic field with a 1s running average removed as observed by the four MMS spacecraft. (b,c) Components of the electric field perpendicular to the magnetic field as observed by the four MMS spacecraft. (d,e) Components of the current perpendicular to the magnetic field based on the curl of the magnetic field (black), moments of the ion and electron distribution functions (blue), and assuming the current is driven by electrons drifting in the electric and magnetic fields (red). (f) Diagram of the electron vortex encountered inside of one of the flux ropes. The observed profiles of the electric field and current are consistent with the indicated trajectories through the structure.
By Jade Reidy, Department of Physics and Astronomy, University of Southampton, UK.
The formation mechanism of polar cap arcs is still an open question. Since they were first discovered (over a century ago), there have been conflicting reports of polar cap arcs forming on open field lines [e.g., Hardy et al., 1982; Carlson and Cowley, 2005] and on closed field lines [e.g., Frank et al., 1982; Fear et al., 2014]. It is possible that there are more than one type of formation mechanism [e.g., Newell et al., 2009; Reidy et al., 2017].
Reidy et al.  investigates the interhemispheric nature of polar cap arcs using low-altitude ultraviolet imaging, combined with particle data, to determine whether they occur on open or closed field lines. Figure 1 shows an example of an image from SSUSI (Special Sensor Ultra-Violet Spectrographic Imager) (left) with the corresponding SSJ/4 particle spectrograms (right). The SSUSI instruments, on board DMSP (Defence Meteorological Satellite Program) spacecraft, are UV imagers that scan across the polar regions, building up images over 20 minutes. The SSJ/4 particle spectrometer is also on board DMSP spacecraft and provides measurements of the particle precipitation directly above the spacecraft.
In Fig. 1 the SSUSI image has been projected on to a magnetic local time grid with noon at the top and dawn to the right. The black and grey dashed lines on the particle spectrograms and corresponding black and grey vertical lines on the DMSP footprint (black line on the SSUSI image) give an estimated position of the poleward edge of the auroral for the electrons and ions respectively (see Reidy et al.  for details). Multiple sun-aligned arcs can be seen poleward of this edge, hence assumed to be occurring within the polar cap. The arcs seen on the dawnside of the SSUSI image are associated with ion and electron precipitation (indicated by red bars on both the DMSP track and the particle spectrograms), similar arcs were also seen in the opposite hemisphere. These arcs are consistent with formation on closed field lines [Fear et al., 2014; Carter et al., 2017]. The arc seen on the duskside of the polar cap is associated with electron-only precipitation (indicated by yellow bars). This kind of particle signature is consistent with accelerated polar rain and is hence consistent formation on open field lines [Newell et al., 2009; Reidy et al., 2017].
Reidy et al.  investigated 21 events in December 2015 using SSUSI images and corresponding SSJ/4 data. Nine of these events contained arcs consistent with a closed field line mechanism, i.e. arcs associated with ion and electron precipitation present in both hemispheres (similar to the arcs on the dawnside of Fig. 1). Six of these events contained arcs that were associated with electron-only precipitation, consistent with an open field line mechanism (e.g. the duskside of Fig. 1). Examples of events containing arcs that were not, at first sight, consistent with either an open or a closed field line formation mechanism are also explored. This study shows the complex nature of polar cap arcs and highlights the needs for future study as there is still much to understand about their formation mechanism.
Please see the paper below for more information:
Reidy, J., R.C Fear, D. Whiter, B.S. Lanchester, A.J. Kavanagh, S.E. Milan, J.A. Carter, L.J. Paxton, and Y. Zhang. (2018), Inter‐hemispheric survey of polar cap aurora, J. Geophys. Res. Space Physics, 123. https://doi.org/10.1029/2017JA025153
Figure 1. An image from the SSUSI instrument on board DMSP spacecraft F17 is shown on the left. The time at the top of the image indicates the time when the spacecraft crossed 70 degrees magnetic latitude as it passed from dawn to dusk (i.e. left to right). The corresponding data from the SSJ/4 particle spectrometer is shown on the right with the electron spectrogram in the top panel and the ion spectrogram at the bottom. Precipitation associated with polar cap arcs is indicated on the DMSP track on the SSUSI image (indicated by a black line) and the particle spectrograms in red for ion and electron signatures and orange for electron-only signatures.