26.11. 2015

Data Integration

Data Integration
In the space physics community, progress is constantly being made both with new, more detailed spacecraft observations and statistical investigations of spacecraft data, and in parallel, advances within computer simulations continuously give more detailed insights into the physical processes and the time evolution of various phenomena, which enhance our understanding of the  important physics. Although both of these disciplines play an important part in the scientific progress, comparative studies between observations and simulation are a key component in order to make sense of both the data and the simulations. Such work, in particular studies which concern kinetic effects, are highly relevant for the work conducted within the SHOCK project. Here below is a selection of four recent papers which together highlight the relevance of simulation-observation comparisons, and the scientific gains that may come from such studies.
Electron vortex magnetic holes: A nonlinear coherent plasma structure
C. T. Haynes, D. Burgess, E. Camporeale, and T. Sundberg, Physics of Plasmas,  22, 012309, 2015.
Haynes et al. (2015) report the details of a novel type of electron-scale plasma structure that can evolve directly out of a turbulent background field. Apart from their much smaller scale size, these structures are very similar in their structure to the magnetic holes found in for example the magnetosheath and the solar wind, with a cylindrically symmetric depression in the magnetic field. Although the larger magnetic holes are typically generated by the mirror mode instability, the smaller scale of these structures points to an electron-driven formation. In a closer examination of the plasma data, Haynes et al. (2015) discover that the structures can be explained by a trapping of high-pitch angle electrons in petal like orbits around the center of the depression. Each of these electrons contribute to an azimuthal current which enhances the depression and thereby provides a positive feedback to the magnetic hole. For this reason, these electron vortex magnetic holes (EVMH) can grow self-consistently out of the background turbulence, and they are observed to be stable over at least hundreds of electron gyro periods. These simulations show striking similarities with the sub-proton-scale magnetic holes that have have previously been observed in the Earth's plasma sheet (Ge et al., 2011; Sun et al., 2013), and they can thus provide a theoretical basis for their formation.
Properties and origin of subproton-scale magnetic holes in the terrestrial plasma sheet
T. Sundberg, D. Burgess, and C. T. Haynes, J. Geophys. Res. Space Physics, 120, doi:10.1002/2014JA020856.
Following up on the previous paper by Haynes et al. (2015), this paper further test the EVHM formation theory for the electron scale magnetic depressions observed in the terrestrial plasma sheet, making maximum use of the multi-spacecraft aspects of the cluster observations. Their data set shows that a majority of these events observed during the tail season of 2003, when the cluster spacecraft separation was short, can be described as cylindrical structures approximately 200-300 km wide, and a field-parallel length on the order of 1000 km or more. These structures are found stable on time scale of at minimum 5-10 s, and they are also typically associated with an increase in the perpendicular electron energy flux, with a peak energy of ~3 keV, a value which matches well that predicted from the simulations. All in all, these magnetic depressions show very good agreement with the features seen in the Haynes et al. (2015) simulations. This is shown in Figure 1, which displays the simulated magnetic field on the left (Haynes et al., 2015, Fig 2), and that recreated from the observations on the right (Sundberg et al., 2015, Fig 13). However, not all electron-scale depressions adhere to this picture. There is a sub-class of events which are better described as sheet-like structures, where there is little to no variation in between the four spacecraft. These structures may be better explained by alternative formation mechanisms such as electron-scale soliton waves, however, they are in minority to the circularly symmetric events.
Figure 1: (A) Simulated magnetic field signature of electron-vortex magnetic holes (Haynes et al., 2015), and (B) Reconstructed magnetic field signature of a magnetic hole observed by Cluster (Sundberg et al., 2015)
Energetic ions in dipolarization events.
J. Birn, A. Runov, and M. Hesse, J. Geophys. Res. Space Physics, 120, doi: 10.1002/2015JA021372
This study presents the results of a simulation-observation comparison of the ion distribution associated with dipolarization fronts in the terrestrial magnetotail. The study uses the electromagnetic fields of a 3-D magnetohydrodynamic simulation for test-particle tracing, and the results are compared with a dipolarization event observed by two of the THEMIS spacecraft on 27 Feb 2009, a time period which has previously been analyzed by Runov et al. (2009). This analysis helps clarify the connection between the different ion populations and their source regions. Many similarities exist between the observed and simulated ion distributions, even though there are several features in the magnetic field that is not captured by the simulation, particularly at the smaller scales. Both the observations and the simulations show a rapid drop in energy flux in the lower energies at the arrival of the dipolarization front, together with an increase in the high energy (83.5 keV) ions. These high energy ions are primarily found within the dipolarization bubble, drifting towards the dusk side; their motion is strongly affected by non-adiabatic processes and gradient drifts. Ahead of the dipolarization front, there is a population of medium energy (21 keV) ions, forming an earthward propagating precursor front. These are a typical signature of dipolarizations, and they have been previously been attributed to plasma sheet ions that have been reflected off the dipolarization front. This is confirmed by the present simulation, however, the ions also commonly show multiple encounters with the dipolarization front, and they can thereby gain a larger amount of energy than a singly reflected ion.
ULF foreshock under radial IMF: THEMIS observations and global kinetic simulation Vlasiator results compared
M. Palmroth, M. Archer, R. Vainio, H. Hietala, Y. Pfau-Kempf, S. Hoilijoki, O. Hannuksela, U. Ganse, A. Sandroos, S. von Alfthan, and J. P. Eastwood, J. Geophys. Res. Space Physics, 120, doi:10.1002/2015JA021526.
The authors of this study use global kinetic simulations to investigate the mechanism of oblique propagation of ultralow frequency (ULF) waves in the terrestrial foreshock under radial interplanetary magnetic field (IMF). Simulations are performed using Vlasiator, a 2-D hybrid-Vlasov code which evolves the full ion distribution f(r,v,t) in order to model ion kinetic effects, but treats the electrons as a charge-neutralising fluid. Although this simulation method is computationally expensive, it reduces the numerical noise present in hybrid-PIC methods and provides a better treatment of low density regions of the ion velocity space. The simulation results are compared to observational data from two THEMIS spacecraft in the foreshock on 16 July 2008, during which the IMF was almost antiparallel to the simulated case. The authors demonstrate that the Vlasiator simulations quantitatively reproduce many of the observed properties of the foreshock region sampled by THEMIS, including ULF wave periods, obliquity, polarisation and ion velocity distributions, and therefore justify that the simulations can be used to draw further conclusions about the broader foreshock region and mechanisms for oblique ULF propagation. An example of the simulated foreshock wave distribution is shown in Figure 2. It has been suggested that oblique ULF propagation arises from refraction due to the spatial variability of the suprathermal ions, originating from the ExB drift component (Hada et al. 1987). However, for radial IMF the ExB drift component is small. Using the Vlasiator simulations, the authors demonstrate that additional refraction may be caused by changes in wave propagation at so-called “spine” structures in the foreshock. These spines are shown to be associated with two processes: the presence of transient preferential ion reflection sites near the bow shock leading to regions of higher density beams, and the enhanced density and velocity of back streaming ions at the edges of the foreshock.

Figure 2: Alfvenic wave activity upstream of the bow shock, as modelled by the Vlasiator code. The Bz component is shown in color, and By contours mark the wave fronts. From Palmroth et al. (2015). 
[Written by Torbjorn Sundberg and Peter Gingell]
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