Determining the wavelength of Langmuir wave packets at the Earth's bow shock

TL;DR

This study measures the wavelength of Langmuir wave packets near Earth's bow shock, revealing larger-than-expected wavelengths likely caused by strong plasma inhomogeneities in the solar wind.

Contribution

First measurement of Langmuir wave packet wavelengths upstream of Earth's bow shock using satellite data, highlighting the impact of plasma inhomogeneities.

Findings

Wavelengths are 2-3 times larger than standard theory predicts.

Wavelengths are consistent with strong plasma inhomogeneities.

Results challenge traditional weak beam instability models.

Abstract

The propagation of Langmuir waves in plasmas is known to be sensitive to density fluctuations. Such fluctuations may lead to the coexistence of wave pairs that have almost opposite wave-numbers in the vicinity of their reflection points. Using high frequency electric field measurements from the WIND satellite, we determine for the first time the wavelength of intense Langmuir wave packets that are generated upstream of the Earth's electron foreshock by energetic electron beams. Surprisingly, the wavelength is found to be 2 to 3 times larger than the value expected from standard theory. These values are consistent with the presence of strong inhomogeneities in the solar wind plasma rather than with the effect of weak beam instabilities.