K. Goetz

Bipolar electrostatic structures in the shock transition region: Evidence of electron phase space holes

We present observations of intense, bipolar, electrostatic structures in the transition region of the terrestrial bow shock from the Wind spacecraft. The electric field signatures are on the order of a tenth of a millisecond in duration and greater than 100 mV/m in amplitude. The measured electric field is generally larger on the smaller dipole antenna, indicating a small spatial size. We compare the potential on the two dipole antennas with a model of antenna response to a Gaussian potential profile. This result agrees with a spatial scale determined by convection and gives a characteristic scale size of 2–7 λd. We interpret the observations as small scale convecting unipolar potential structures, consistent with simulations of electron phase space holes and discuss the results in the context of electron thermalization at strong collisionless shocks.

Dust Detection by the Wave Instrument on STEREO: Nanoparticles Picked up by the Solar Wind?

The STEREO wave instrument (S/WAVES) has detected a very large number of intense voltage pulses. We suggest that these events are produced by impact ionisation of nanoparticles striking the spacecraft at a velocity of the order of magnitude of the solar wind speed. Nanoparticles, which are half-way between micron-sized dust and atomic ions, have such a large charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. Since the voltage produced by dust impacts increases very fast with speed, such nanoparticles produce signals as high as do much larger grains of smaller speeds. The flux of 10-nm radius grains inferred in this way is compatible with the interplanetary dust flux model. The present results may represent the first detection of fast nanoparticles in interplanetary space near Earth orbit.

Low-frequency whistler waves and shocklets observed at quasi-perpendicular interplanetary shocks

[1] We present observations of low-frequency waves (0.25 Hz < f < 10 Hz) at five quasi-perpendicular interplanetary (IP) shocks observed by the Wind spacecraft. Four of the five IP shocks had oblique precursor whistler waves propagating at angles with respect to the magnetic field of 20–50 and large propagation angles with respect to the shock normal; thus they do not appear to be phase standing. One event, the strongest in our study and likely supercritical, had low-frequency waves consistent with steepened magnetosonic waves called shocklets. The shocklets are seen in association with diffuse ion distributions. Both the shocklets and precursor whistlers are often seen simultaneously with anisotropic electron distributions unstable to the whistler heat flux instability. The IP shock with upstream shocklets showed much stronger electron heating across the shock ramp than the four events without upstream shocklets. These results may offer new insights into collisionless shock dissipation and wave-particle interactions in the solar wind.

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.

The properties of large amplitude whistler mode waves in the magnetosphere: Propagation and relationship with geomagnetic activity

using waveform capture data from the Wind spacecraft. Weobserved 247 whistler mode waves with at least one electricfield component (105/247 had≥80 mV/m peak!to!peakamplitudes) and 66 whistler mode waves with at least onesearch coil magnetic field component (38/66 had≥0.8 nTpeak!to!peak amplitudes). Wave vectors determined fromevents with three magnetic field components indicate that30/46 propagate within 20° of the ambient magnetic field,though some are more oblique (up to ∼50°). No relationshipwas observed between wave normal angle and GSM lati-tude. 162/247 of the large amplitude whistler mode waveswere observed during magnetically active periods (AE >200 nT). 217 out of 247 total whistler mode waves exam-ined were observed inside the radiation belts. We presenta waveform capture with the largest whistler wave magneticfield amplitude (^8nTpeak!to!peak) ever reported in theradiation belts. The estimated Poynting flux magnitude asso-ciated with this wave is ^300 mW/m

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.

Observation of relativistic electron microbursts in conjunction with intense radiation belt whistler-mode waves

We present multi-satellite observations indicating a strong correlation between large amplitude radiation belt whistler-mode waves and relativistic electron precipitation. On separate occasions during the Wind petal orbits and STEREO phasing orbits, Wind and STEREO recorded intense whistler-mode waves in the outer nightside equatorial radiation belt with peak-to-peak amplitudes exceeding 300 mV/m. During these intervals of intense wave activity, SAMPEX recorded relativistic electron microbursts in near magnetic conjunction with Wind and STEREO. The microburst precipitation exhibits a bursty temporal structure similar to that of the observed large amplitude wave packets, suggesting a connection between the two phenomena. Simulation studies corroborate this idea, showing that nonlinear wave--particle interactions may result in rapid energization and scattering on timescales comparable to those of the impulsive relativistic electron precipitation.

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