S. J. Monson

Langmuir waves in a fluctuating solar wind

The Langmuir waves which are resonant with typical type III solar radio burst electrons have a frequency so little above the ambient plasma frequency that they should be strongly affected by known density fluctuations. Some consequences of this observation are worked out, and the expected consequences are demonstrated in the observations of the Langmuir waves from two quite different bursts, of November 4, 1997, and January 19, 1998.

Electrostatic Turbulence and Debye-Scale Structures Associated with Electron Thermalization at Collisionless Shocks

We analyze measurements of bipolar, Debye-scale electrostatic structures and turbulence measured in the transition region of the Earths collisionless bow shock. In this region, the solar wind electron population is slowed and heated, and we show that this turbulence correlates well in amplitude with the measured electron temperature change. The observed bipolar structures are highly oblate and longitudinally polarized and may instantaneously carry up to 10% of the plasma energy ψ ≡ e/kbTe ≈ 0.1 before dissipating. The relationship between ψ and the field-aligned scale size Δ∥ of the Gaussian potential suggests that the bipolar structures are BGK trapped particle equilibria or electron hole modes. We suggest a generation scenario and a potential role in dissipation.

Transverse z‐mode waves in the terrestrial electron foreshock

We examine the phase relation between two orthogonal electric field components for several hundred waveform measurements of intense electron plasma waves in the terrestrial electron foreshock. In general, the phase shift at the carrier frequency is not zero or π as would be expected if the waves were purely electrostatic Langmuir waves, but is a function of the angle between the antennas and the interplanetary magnetic field (IMF). When the antennas are field aligned, the phase shift between the components is large; this value recedes smoothly to zero as the antenna is rotated away from the IMF direction. When solar wind density fluctuations are considered, this is consistent with the dispersion of the electromagnetic z-mode and we assert that the electron foreshock is populated by transverse z-mode waves, not purely longitudinal Langmuir waves. This has implications for conversion to freely propagating modes and large-amplitude saturation mechanisms.

On the amplitude of intense Langmuir waves in the terrestrial electron foreshock

Waveforms of large-amplitude Langmuir oscillations were recorded by the Wind spacecraft in the Earths upstream electron foreshock region. We present some statistics of the waveforms and discuss them in the context of various saturation mechanisms. In particular, it is found that the value of Epeak/Erms is not large, as previously suggested, and that the largest-amplitude Langmuir waveforms are generally somewhat sinusoidal and lack structure on small spatial scales. The measured probability distribution of electric field amplitude and dimensionless energy suggest that some stochastic process may play a role in wave generation. The values of dimensionless energy needed to arrest Langmuir wave collapse occur with very small probability and the value of Epeak/Erms for large fields suggests that, statistically, Langmuir wave collapse is not an important process in the terrestrial foreshock.

Evidence for Langmuir wave collapse in the interplanetary plasma

With the Fast Envelope Sampler part of the URAP experiment on Ulysses, we have observed much rapidly varying structure in plasma waves in the solar wind. Here we discuss extremely narrow (1 ms) structures observed together with electrostatic Langmuir waves, as well as some broader Langmuir wave packets

Observations of plasma waves during a traversal of the Moon's wake

The Wind satellite passed through the solar wind wake of the moon at a distance of about 8 lunar radii on 27 Dec. 1994. Several different wave modes were observed, which we describe. Some of these modes are not certainly identifiable with modes of a uniform plasma, and are probably associated with the density discontinuity, and possibly with a velocity shear, at the boundary of the wake. Modes which are identifiable, at least tentatively, include electromagnetic waves at both fp and 2 fp from outside the wake, and a mode at the local plasma frequency of the wake.

Early Wind observations of bow shock and foreshock waves

Some preliminary results of time-resolved waveforms of Langmuir waves in the foreshock region are presented. There is evidence for both three-wave parametric decay and modulational instability. Also strong incoherent waves in the bow shock are shown.

On the beam speed and wavenumber of intense electron plasma waves near the foreshock edge

Using high time resolution particle and wave data from the Wind spacecraft, we examine several crossings of the electron foreshock-solar wind boundary. We show that the most intense electron plasma waves, observed near the foreshock boundary, often occur coincident with a flux of electrons with energies between 1 keV and 27 keV. This corresponds to electron beam speeds of 9vth ≤ vb ≤ 50vth, rather than vb ≈ 5vth, as is inferred from reduced distribution functions obtained by other instruments. Assuming Landau coupling, the resonant index of refraction is then 3 ≤ N0 ≤ 15, which implies that the waves are susceptible to strong scattering by ambient density fluctuations. The most intense electric fields are not well correlated with beam speed, and the distribution of electric field occurrence is broadly aligned with the interplanetary magnetic field direction. We compare the estimated maturity and bandwidth of the instability with those expected of the electrostatic decay instability and nonlinear Landau damping and find that our observations show a narrower bandwidth than expected. We suggest that the observations are consistent with scattering of electrostatic Langmuir waves to small wavenumber. The observed transverse polarization may be explained by the electromagnetic nature of the small-wavenumber z-mode, or as evidence of incident, reflected, and transmitted electrostatic components. The generation of electromagnetic emission at fpe and 2fpe is discussed in the context of the observations.

Phase coupling in Langmuir wave packets: Possible evidence of three-wave interactions in the upstream solar wind

Some theories of the generation of 2ωpe radiation, in the upstream solar wind, invoke the notion of nonlinear wave-wave interactions. We present observations, from the WIND spacecraft, of an event in the upstream solar wind with first and second harmonic emissions. The bicoherence spectrum of this event shows coherence amongst components that suggest three-wave interactions of the type described in (Cairns and Melrose, 1985). We discuss this result and its theoretical context.

A search for Langmuir solitons in the Earth's foreshock

The Earths foreshock region, the region which is magnetically connected to the bow shock, is a region of intense Langmuir waves. Some of these waves have an envelope similar to that expected for one-dimensional Langmuir solitons. A computer search for such waveforms has identified a number sufficient for statistical investigation. It is concluded that there is no evidence for stable waveforms like one-dimensional Langmuir solitons.