Nuggets of MIST science, summarising recent papers from the UK MIST community in a bitesize format.
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By Hayley J. Allison, British Antarctic Survey / University of Cambridge, UK.
Electrons trapped on the terrestrial magnetic field form the Earth’s electron radiation belts. The dynamics of these structures can be examined using numerical models such as the BAS Radiation Belt Model. Recent work has highlighted the link between increases in the low energy seed population (tens to hundreds of keV electrons) and high-energy relativistic electron flux enhancements in the radiation belts. However, data on the seed population is limited to a few satellite missions.
Low earth orbit satellites, such as the Polar Operational Environmental Satellites (POES), rapidly sample the radiation belt region and provide a wealth of observations of the electron environment. Here we present a method to utilise this dataset to develop event-specific low energy boundary conditions for the British Antarctic Survey 3-D Radiation Belt Model. Such a method can supply realistic low energy boundary conditions for periods outside the Van Allen Probes mission, with a broad magnetic local time coverage.
Using the low energy POES observations presents two main challenges. Firstly, the electron populations measured by the POES satellites are of low equatorial pitch angle. Secondly, the SEM-2 detector supplies integral electron flux, i.e. including all electrons from a lower energy limit up to a threshold. We used activity dependent equatorial pitch angle distributions, derived from Van Allen Probes observations, to map the POES observations to higher pitch angles and explore two methods for obtaining the flux at various electron energies (differential flux) from the integral flux measurements.
The resulting equatorial electron differential flux values were validated against MagEIS observations and showed an average agreement within a factor of 4 for L* > 3.7 when the assumption that electron flux decreased with increasing energy held (white areas in figure). Variations in the MagEIS flux tend to be reproduced in the converted POES dataset. Periods when the electron flux did not fall with energy (shaded grey) were primarily during quiet times when a lack of chorus wave activity meant that these low energy electrons were not accelerated to >900 keV energies.
For more information, please see the paper below:
Allison, H. J., Horne, R. B., Glauert, S. A., & Del Zanna, G. (2018). Determination of the equatorial electron differential flux from observations at low Earth orbit. Journal of Geophysical Research: Space Physics, 123. https://doi.org/10.1029/2018JA025786
Figure: Comparison of the Van Allen Probes Magnetic Electron Ion Spectrometer electron flux (black lines) at five L* values, for energies following a line of constant μ = 100 MeV/G and the electron flux determined from the POES observations using the AE-9 distributions for the integral flux to differential conversion (red line) and using the iterative approach (blue line). Grey regions show periods when the assumptions that the electron flux falls with increasing energy were violated.
By Mathew J. Owens, University of Reading, UK.
In order to forecast space weather, it is necessary to accurately model the solar wind, the continually expanding solar atmosphere which fills the solar system. At present, telescopic observations of the Sun's surface are used to provide the starting conditions for computer simulations of the solar wind, which then propagate conditions all the way from the Sun to Earth. But spacecraft also make direct measurements of the solar wind, which provide useful additional information that is not presently used. In this study we use a simple solar wind model to develop a method to routinely "assimilate" spacecraft observations into the model and thus improve space‐weather forecasts. This data assimilation (DA) approach closely follows that of terrestrial weather prediction, where DA has led to increasingly accurate forecasts. We use artificial and real spacecraft observations to test the new solar wind DA method and show that the error in predicting the near‐Earth solar wind can be reduced by around a fifth using available observations.
For more information, please see the paper below:
Lang, M.S., and M.J Owens. (2018), A variational approach to Data Assimilation in the SolarWind, Space Weather, 16. https://doi.org/10.1029/2018SW001857
Figure: Model near-Earth solar wind speed before (blue) and after (green) assimilation of STEREO in situ observations. The DA enables the model to capture a previously missed fast stream, corrects a false alarm and improves the timing of a third stream
By Martin Archer, School of Physics and Astronomy, Queen Mary University of London, UK.
Earth’s magnetic shield is rife with a symphony of ultra-low frequency analogues to sound waves. These waves transfer energy from outside this shield to regions inside it and therefore play a key role in space weather - how space poses a risk to our everyday lives by affecting power grids, GPS, passenger airlines, mobile telephones etc.
While these waves are too low pitch for us to hear them, Archer et al. [2018] show that we can make our satellite recordings of them audible by dramatically speeding up their playback. These audio versions of the data can be used by school students to contribute to research, by having them explore the data through the act of listening and performing analysis using audio software.
An example of this is presented where school students from Eltham Hill School in London identified “whistling” sounds whose pitch decreased over the course of several days. This event started when a coronal mass ejection, or solar storm, arrived at Earth causing a big disturbance to the space environment. It turned out that the whistling sounds were vibrations of Earth’s magnetic field lines, a bit like the vibrations of a guitar string which form a well-defined note. While the solar storm stripped away much of the material present in Earth’s space environment, as it started to recover following the storm, this started to refill again. It was this refilling that caused the pitch of the sounds to drop slowly over time.
Previously events like these had barely been discussed and therefore were thought to be rare. However, many similar events were discovered in the audio which also followed similar disturbances, revealing that these types of waves are much more common than previously thought.
Video: https://www.youtube.com/watch?v=X6vbST9iMOU
For more information, please see the paper below:
Archer, M.O., M.D. Hartinger, R. Redmon, V. Angelopoulos, and B. Walsh. (2018), First results from sonification and exploratory citizen science of magnetospheric ULF waves: Long‐lasting decreasing‐frequency poloidal field line resonances following geomagnetic storms, Space Weather, 16, https://doi.org/10.1029/2018SW001988
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. [2018], 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. [2018] 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. [2018] 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.