ULF foreshock under radial IMF: THEMIS observations and global kinetic simulation Vlasiator results compared

Abstract

For decades, monochromatic large‐scale ultralow frequency (ULF) waves with a period of about 30 s have been observed upstream of the quasi‐parallel bow shock. These waves typically propagate obliquely with respect to the interplanetary magnetic field (IMF), while the growth rate for the instability causing the waves is maximized parallel to the magnetic field. It has been suggested that the mechanism for the oblique propagation concerns wave refraction due to the spatial variability of the suprathermal ions, originating from the E × B drift component. We investigate the ULF foreshock under a quasi‐radial IMF with Vlasiator, which is a newly developed global hybrid‐Vlasov simulation solving the Vlasov equation for protons, while electrons are treated as a charge‐neutralizing fluid. We observe the generation of the 30 s ULF waves and compare their properties to previous literature and multipoint Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft observations. We find that Vlasiator reproduces the foreshock ULF waves in all reported observational aspects. We conclude that the variability of the density and velocity of the reflected back streaming ions determines the large‐scale structure of the foreshock, which affects the wave frequency, wavelength, and oblique propagation. We conclude that the wave refraction may also be at work for radial IMF conditions, which has earlier been thought of as an exception to the refraction mechanism due to the small E × B drift component. We suggest that additional refraction may be caused by the large‐scale spatial variability of the density and velocity of the back streaming ions.

Cavitons and spontaneous hot flow anomalies in a hybrid-Vlasov global magnetospheric simulation

“…Due to the IMF orientation, the foreshock develops in front of the southern part of the bow shock, as evidenced by the fluctuations of B y in this region. The oscillations of B y from positive to negative values and the coherent wave fronts extending perpendicular to the IMF direction show that the foreshock is permeated by so-called 30 s ULF waves , with properties similar to those analysed by Palmroth et al (2015) in another Vlasiator run. Wave activity is also visible throughout the magnetosheath, with stronger perturbations downstream of the quasi-parallel portion of the bow shock.…”

“…Vlasiator (von Alfthan et al, 2014;Palmroth et al, 2015;Pfau-Kempf, 2016) is a unique hybrid-Vlasov code capable of performing global simulations of the Earth's magnetosphere and the surrounding space environment. It models kinetic proton-scale physics by simulating the proton distribution function through a Cartesian 3-D velocity grid and cellaveraged values, instead of relying on particle-in-cell methods and statistical sampling.…”

“…Many works in the past have focused on studying the bow shock and the kinetic processes taking place in it, such as Cluster mission results in Issues 1-4, Vol. 118 from Space Science Reviews (available at: https://link.springer.com/journal/11214/118/1/ page/1, last access: 1 August 2018), Omidi et al (2005), Blanco-Cano et al (2006), Burgess and Scholer (2013), and Palmroth et al (2015). However, there are still many open questions about how the solar wind is processed at the bow shock and in the regions upstream and downstream of it.…”

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

“…Our results imply that the role they play is time dependent and this needs to be taken into consideration in the analysis of long duration data sets of both magnetosheath and magnetospheric measurements. Furthermore, the present results are of importance to numerical simulations (Alfthan et al, 2014;Karimabadi et al, 2014;Palmroth et al, 2015) attempting to accurately reproduce the magnetosheath behaviour in response to upstream conditions. Open questions remain on the detailed solar wind driver of the ion density asymmetry, and further work is required to use the state-ofthe-art numerical models to resolve the causal relationships.…”

“…Thus, although the ion density dawn-dusk asymmetry does vary with the 11-year solar cycle, the behaviour of the plasma properties during each solar cycle have to be taken into account. With the increased sophistication and resolution of numerical models and simulations (Alfthan et al, 2014;Karimabadi et al, 2014;Palmroth et al, 2015), in conjunction with the long-term monitoring of solar wind conditions, we should be well equipped to resolve the drivers of this asymmetry.…”