The dawn–dusk asymmetry of ion density in the dayside magnetosheath and its annual variability measured by THEMIS

by Andrew P. Dimmock
Department of Radio Science & Engineering, Aalto University, Finland


The solar wind is a continuous stream of charged particles, which are ejected from the Sun and propagate through interplanetary space at approximately 400 km/s. The supersonic solar wind flow is decelerated so subsonic speeds upon encountering the terrestrial magnetic field and forms a standing shock wave: the bow shock. The plasma processed by the bow shock has its energy redistributed to other degrees of freedom and in general the “shocked” plasma is slower, hotter, more turbulent, and denser. This shocked plasma is housed in the magnetosheath and extends to the magnetopause; which marks the boundary in which the solar wind dynamic pressure balances the magnetic pressure of the inner magnetosphere. Figure 1 below shows a diagram depicting these regions and boundaries. The region highlighted in orange is the data collection region of the present study.

Figure 1. Figure 1: Diagram of the dayside magnetosheath and significant regions and boundaries. The filled orange sectors demonstrate the data collection region employed in the present study. Please note the following: (1) the figure is not to scale and (2) in physical space, the collection region scales with the magnetosheath width and is therefore larger on the dusk-side.

An appropriate definition of the magnetosheath is a filter/interface coupling the interplanetary (solar wind) and inner magnetosphere (behind the magnetopause) regions. In practise, this coupling is realised by the fact that the global and local magnetosheath properties regulate the mechanisms that transport mass and momentum across the magnetopause. However, the magnetosheath state is particularly difficult to predict and quantify since the spatial distribution of plasma properties are sensitive to upstream conditions and asymmetric about the equatorial x-axis. Nevertheless, quantifying and understanding the magnetosheath state in close proximity to the magnetopause is fundamental to the understanding of plasma transport mechanisms such as the Kelvin Helmholtz Instability. Here we highlight a study that quantifies the dawn-dusk asymmetry of ion density close to the magnetopause. In addition, we study the temporal evolution of the asymmetry over the THEMIS era between 2008 and 2015.


The THEMIS spacecraft consisted of 5 probes [now 3, 2 probes are now the ARTEMIS mission] in approximate equatorial orbits covering the terrestrial magnetosphere. As a result, their extensive coverage and orbital bias prove to be well suited to statistical studies of equatorial magnetosheath asymmetries. Their relative longevity (2008-2015) also allow for investigating the temporal variability of magnetosheath properties, which is what has been utilised here. To account for the substantial magnetosheath boundary motion, each data point was normalised between the estimated (model) boundaries. This enables direct comparison between measurements made under different upstream conditions collected over extended time periods. Based on these data, we created statistical maps by binning the data onto a uniform grid-space (0.5x0.5 Re), and also computed statistical properties of the dataset and all extracted temporal sub-sets. Figure 2 below shows a statistical map and dawn-dusk asymmetry estimate of ion density.

Figure 2. Statistical data of ion number density collected in the dayside magnetosheath. Panel (a) is a statistical map whereas panels (b) and (c) are the dawn–dusk asymmetry and cross sectional cuts, respectively.

Panels (b & c) demonstrate the asymmetry close to the magnetopause and the dawn-dusk profile as a function of the angle from the sub-solar point (Y = 0). The data suggests a well-defined dawn favoured asymmetry, which in general, increases with distance from the sub-solar point. Since the data-coverage differs over time, when measuring the temporal variability, we restricted our data collection to the regions highlighted in Figure 1. To measure the asymmetry, we computed the mean and median averages of ion densities in each dawn-dusk sector bin. This was implemented for 1-year sub-sets that were incremented by 6 months so each window overlapped by 50%. To ensure outliers were not influencing our results, we compared the probability distributions of densities and confirmed this was not the case, nor were mismatched dawn-dusk distributions a factor in the asymmetry estimates. Figure 3 shows the dawn-dusk asymmetry as a function of year.

Figure 3. Dawn–dusk asymmetry of ion density in the dayside magnetosheath as a function of year from 2008 to 2014 (a). The grey and orange bars in panel (a) represent the dawn–dusk asymmetry calculated using mean and median averages, respectively. The blue solid and dashed horizontal lines in the same panel show the mean and median dawn–dusk asymmetry for the entire interval between 2008 through 2014, respectively. The error bar limits are determined based on the maximum variation of the asymmetry based on the standard error about the mean SEM. Panel (b) shows monthly mean averages of solar wind conditions over the same time period as in panel (a).

The maximum asymmetry is close to the solar minimum in 2009 but this decreased during the rising phase from 2011 onwards. The results are in agreement with previous estimates using THEMIS data over a similar period (Walsh et al, 2012) but differed to other studies who observed a peak at solar maximum (Paularena et al 2001) as opposed to minimum. Investigation into solar wind parameters suggested a complex multi-parameter dependence. These results imply that the occurrence of the Kelvin Helmholtz instability and the associated plasma transport could also be solar cycle (or temporally) dependent since the local ion density plays a role in its growth. This may have repercussions on low latitude boundary later structure and also inner magnetospheric regions such as the cold dense plasma sheet. Finally, these results are relevant to the modelling community who aim to accurately recreate magnetosheath conditions, and thus should be aim for reproducibility under comparable initial conditions.


Based on our results, we have the following conclusions:

1. In general, there is a dawn-favoured asymmetry of ion density in close proximity to the magnetopause of approximately 10%.

2. The dawn-favoured asymmetry appeared to increase with distance from local noon.

3. During the THEMIS era, the dawn-favoured asymmetry was at its maximum during the 2009 solar maximum and then decreased thereafter.

4. Although our results are consistent with similar estimates using THEMIS, discrepancies with other studies using different observations (albeit further tail-ward) suggest an inconsistent solar cycle dependency and a complex underlying dependency on a nonlinear combination of solar wind parameters.

5. The temporal variability of magnetosheath dawn-dusk asymmetries implies that the efficiency of plasma transport processes (which are influenced by local plasma conditions) may also be affected.


A. P. Dimmock, T. I. Pulkkinen, Osmane, A., K. Nykyri, The dawn-dusk asymmetry of ion density in the dayside magnetosheath and its annual variability measured by THEMIS, Ann. Geophys., doi:10.5194/angeo-34-511-2016, 2016.

Biographical Note

Andrew Dimmock is a postdoctoral research fellow at the Department of Radio Science & Engineering at Aalto University in Finland. His research interests include solar wind - magnetosheath coupling and especially the role played by magnetosheath properties.

Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu