Growth rate measurement of ULF waves in the ion foreshock

Abstract

We report the first satellite measurement of the ultralow frequency (ULF) wave growth rate in the upstream region of the Earth's bow shock. We employ the two identical ARTEMIS spacecraft orbiting the Moon to characterize crescent‐shaped reflected ion beams and relatively monochromatic ULF waves. The event presented here features spacecraft separation of ∼2.5 Earth radii (0.9 ± 0.1 wavelengths) in the solar wind flow direction along a nearly radial interplanetary magnetic field. The ULF wave growth rate is estimated and found to fall within dispersion solver predictions during the initial growth time. Observed frequencies and wave numbers are also within the predicted range. Other ULF wave properties such as the phase speed, obliquity, and polarization are consistent with expectations from resonant beam instability theory and prior satellite measurements. These results will inform future missions near bow and interplanetary shocks as well as future nonlinear studies related to turbulence and dissipation in the heliosphere.

Foreshock Properties at Typical and Enhanced Interplanetary Magnetic Field Strengths: Results From Hybrid‐Vlasov Simulations

“…A recent study by Dorfman et al () provides an observational estimate of the growth rate of the ULF waves in the terrestrial foreshock, using data from the ARTEMIS mission around the Moon (Angelopoulos, ). The spacecraft, initially located outside of the foreshock, moved inside the ULF foreshock after a change in the IMF direction.…”

“…The spacecraft, initially located outside of the foreshock, moved inside the ULF foreshock after a change in the IMF direction. From the variation of the amplitude of the waves from one satellite to the other, Dorfman et al () estimated the growth rate to be γ /Ω ci =0.035. The median normalized growth rates in our simulations, γ /Ω ci =0.042 for Run 1 and γ /Ω ci =0.041 for Run 2, are very comparable to the observational estimate, despite the very different solar wind conditions at the time of ARTEMIS measurements.…”

Near‐Earth Solar Wind: Plasma Characteristics From ARTEMIS Measurements

“…Previous investigations of such correlations were mostly performed using observations from four Cluster spacecraft e.g., Knetter et al, 2004;Mistry et al, 2015;Paschmann et al, 2013; that mostly measure the solar wind close to the Earth's bow shock) or from spacecraft separated by tens of Earth radii (see examples in Horbury et al, 2001;Němeček et al, 2013;Phan et al, 2006). Therefore, the two ARTEMIS probes allow for the first-time investigation of the inner structure of small-scale magnetic field/plasma variations (see an example in Dorfman et al, 2017).…”

Different Types of Ion Populations Upstream of the 2013 October 8 Interplanetary Shock

“…This type of combined distributions have recently been used in several ARTEMIS/THEMIS studies of different plasma regions (see e.g. Dorfman et al 2017;Hietala et al 2015Hietala et al , 2017Runov et al 2015). We first removed the bins that were at or below the one-countlevel from the measurements.…”