MIST

by Alexander Bader, Lancaster University, UK.

Saturn's highly dynamic auroras are generated by electrons precipitating along the magnetic field lines into the planet's polar ionospheres due to currents along the magnetic field lines. Therefore, the aurora provide information about the location and strength of these field-aligned currents. Two types of large-scale current systems have been observed in magnetic field measurements: one a quasi-static system associated with flow shears between plasma rotating at different speeds in the outer magnetosphere. The other significant type are field-aligned current systems rotating according to the planetary period oscillation (PPO) systems. Both the northern and the southern hemisphere are associated with one such system each, superimposed on the quasistatic system and causing roughly 10.7-hour periodic oscillations throughout the Kronian magnetosphere.

Upward and downward field-aligned currents in the northern ionosphere were found to be modulated by rotating patterns imposed by both the northern and southern PPO systems, the latter modulation being facilitated through interhemispheric current closure. The auroral intensity is hence also expected to be modulated accordingly, such that the northern aurora is brightest at roughly Ψ_N/S = 90°, where the currents have also been shown to maximize. Due to the two PPO systems rotating at slightly different angular velocities, this results in a double modulation.

In this study we analyzed the statistical behavior of Saturn's ultraviolet auroral emissions over the full Cassini mission using all suitable Cassini-UVIS images acquired between 2007 and 2017. This study shows for the first time that both hemispheres' auroral intensities are modulated by both the PPO system associated with the same hemisphere (primary system, Fig. 1a) and the opposite hemisphere (secondary system, Fig. 1b), relatively. The modulation by the primary system is found to be more intense than the one caused by the secondary system. This confirms that both PPO systems' field-aligned currents traverse the entire magnetosphere and close at least partly in the hemisphere opposite to where the generating perturbation is located.

For more information, see our paper below:

Bader, A., Badman, S. V., Kinrade, J., Cowley, S. W. H., Provan, G., & Pryor, W. R. (2018). Statistical planetary period oscillation signatures in Saturn's UV auroral intensity. Journal of Geophysical Research: Space Physics, 123, 8459–8472. https://doi.org/10.1029/2018JA025855

Figure 1: Average northern UV auroral intensity maxima per local time (4/3 h bin size) and PPO phase Ψ_N/S (20° bin size), shown in a logarithmic color scale. (a) Northern hemisphere auroral intensity ordered by the northern PPO system and (b) northern hemisphere auroral intensity ordered by the southern PPO system. Two Ψ phase cycles are plotted for clarity, the expected locations of maximum upward current are indicated by dashed white lines. On the top and to the side of each 2D histogram the averages of the mean intensity maxima over the Ψ_N/S and LT dimensions are shown in black, respectively. Separate histograms showing the PPO intensity modulation in the dawn-noon (blue) and dusk-midnight (red) regions are calculated from the parts of the histogram marked with colored boxes and shown to the right side (note the logarithmic intensity scale). The histogram over the full LT range (black) has been fitted with a simple sine (gray). Its maxima are marked with vertical dash-dotted lines, its peak-to-peak (pk-pk) amplitude and the Ψ_N angle with the highest intensity are given in the top right corner of each panel.

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