ALBERT

Description: For decades, monochromatic large-scale ultra low frequency (ULF) waves with a period of about 30 seconds 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-second ULF waves, and compare their properties to previous literature and multipoint 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 backstreaming 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 backstreaming ions.