R. M. Skoug

A cyclotron resonance model of VLF chorus emissions detected during electron microburst precipitation

VLF chorus, consisting of narrowband rising frequency emissions, has often been observed in association with microburst electron precipitation. We present the first simultaneous rocket observations of these two phenomena, with emphasis on understanding the source of the VLF emissions. The rocket experiment was launched on May 6, 1993, from Poker Flat, Alaska (L = 5.6). In this work, the observed 1-4 kHz chorus emissions are interpreted in terms of a cyclotron resonance interaction. The frequency range of the risers and the observed electron energy range agree with those required for this interaction. Using a criterion derived from the conservation of energy during an interaction, it is shown that a cold plasma cyclotron resonance interaction can produce the lower-frequency portions of the observed chorus risers, from ≃1000 Hz to ≃2500 Hz, while a warm plasma model is required to produce frequencies > 2500 Hz. The warm plasma model assumes a two-component plasma, with an isotropic cold component and a bi-Maxwellian warm component. The effect of the warm component is to change the wave dispersion relation, allowing the production of the higher-frequency risers. A portion of the anisotropy required to produce the high-frequency emissions can also be provided by a loss cone distribution. The chorus source is estimated from this cyclotron resonance theory to be located near the equatorial plane.

Upstream and magnetosheath energetic ions with energies to ≈2 MeV

We present the first observations of ≃2 MeV ion bursts detected in the upstream region and the magnetosheath by the three-dimensional (3D) plasma and energetic particles instrument on the WIND spacecraft. This instrument measures the full 3D distribution of particles from a few eV to several MeV, and allows characterization of the upstream ions in both pitch angle and energy. The new feature observed is the presence of bursts of ions at energies extending up to ≃2 MeV, both upstream and in the magnetosheath. The observation of MeV ions has strong implications for the ion source and acceleration mechanisms.

Observations of the solar wind, the bow shock and upstream particles with the WIND 3D plasma instrument

Abstract The 3-D Plasma and Energetic Particle Instrument on the WIND spacecraft provides high sensitivity 3-D electron and ion measurements from solar wind plasma up to ∼300 keV energy. We review a few of the many new results, including: 1) the detection of a quiet-time population (the “superhalo”) of electrons extending up to ∼102 keV energy; 2) solar impulsive electron events extending down to ∼0.5 keV energy; 3) the remote sensing of the jump in magnetic field and the electric potential of the Earths bow shock, using angular distributions of backstreaming electrons; and 4) the detailed distribution functions for the electrons producing solar type III radio bursts.

Modeling of upstream energetic particle events observed by WIND

Energetic particle events observed upstream of the bow shock can have two possible sources, shock acceleration of solar wind particles or leakage from the magnetosphere. Three-dimensional global fluid simulations in conjunction with particle tracking in global fields are used to investigate the sources of the energetic particles. Acceleration of particles from the bow shock can account for many of the lower intensity events but the magnetosphere appears to be an important source during the more energetic particle events where 2 MeV particles are observed.

Analysis and modeling of microburst precipitation

Observations from a recent rocket experiment that measured electrons over the energy range of 1–300 keV shows that microburst temporal structures exist from about 20 keV to >120 keV. Simultaneous observations at five different pitch-angles (0°–90°) show the distribution is nearly isotropic during the bursts, while at low activity (quiet times and the valleys between microbursts) the distribution is anisotropic with higher fluxes at larger pitch-angles. The energy spectra were reasonably fit with a Maxwellian, with an e-folding energy Eo≈6–9 keV for α=0°, which increases to ≈ 10 keV at 67°. A modeling of electron spectra using the theory of pitch-angle diffusion can reproduce the observed spectra, suggesting pitch-angle scattering of electrons due to a wave-particle interaction is the primary mechanism responsible for microbursts.

Energy spectral characteristics of auroral electron microburst precipitation

We consider data from a rocket experiment designed to study auroral microburst precipitation. Our rocket instruments had high energy spectral resolution which allowed us to characterize the electron energy spectra. Data from the field aligned electron detector show that the majority of the spectra fit an exponential form in the 45–70 keV energy range. Approximately 15% of the spectra fit a power law form for high energies (70–150 keV and 45% fit an additional exponential form for low energies (20–45 keV. The multi-component spectra appeared during both microburst and non-burst times. The e-folding energies for the 45–70 keV energy range increased as the flux increased. Some of these features can be explained by a cyclotron wave-particle interaction mechanism. We also observed a weak peak in the energy spectrum at about 45 keV, suggesting the existence of field-aligned potential drops associated with microburst production.

Modeling of microburst electron precipitation using pitch angle diffusion theory

Microburst electron precipitation is characterized by short bursty (duration ∼0.2–0.3 s) quasiperiodic precipitation of electrons in the dayside auroral zone. Pitch angle diffusion of electrons due to the interaction with whistler waves has been suggested previously as a possible mechanism for scattering electrons into the loss cone to cause microburst precipitation. In this paper we investigate the viability of the above mechanism through modeling. We assume the scattering to occur near the equatorial plane and solve the pitch angle diffusion equation numerically to find the time-dependent form of the electron distribution function F, which results from a given time-dependent diffusion coefficient D. Different aspects of our results (burst size, pitch angle dependence, risetime, burst width) are compared with observations from a recent rocket experiment on microburst launched from Poker Flat, Alaska. The comparison shows very good agreement, further supporting the idea that the pitch angle diffusion process is the driving mechanism for microbursts.

Ion beams observed in the near Earth plasma sheet region on May 10, 1996

This Letter reports observations of ion distributions made by the Wind 3D plasma experiment on May 10, 1996 during the 0400 UT substorm. The observations come from the 3D ion analyzer with a geometrical factor two orders of magnitude larger than most of the similar instruments flown through the near-earth geomagnetic tail. This has permitted observations of detailed ion beam characteristics during the passage of the spacecraft across the plasma sheet boundary into the lobe where the density is very low. The plasma initially consists of cold and warm components, with an additional hot component observed as the spacecraft approaches the plasma sheet-lobe interface. The beams originate from the warm component, which is the most dynamic. The Wind detector was even able to detect a weak beam inside the lobe where the density is ≲0.01 cm−3. Some of these observations are new and are not completely explained by current theories.

IMF induced changes to the nightside magnetotail: A comparison between WIND/Geotail/IMP 8 observations and modeling

A 3-D global fluid simulation is used to investigate the changing magnetic field topology of the magnetotail as observed by Geotail and IMP 8. The event studied is of particular interest as the solar wind density and speed as observed by WIND were approximately constant so that the influence of the interplanetary magnetic field (IMF) can be isolated. Loading of the tail fields during southward IMF is seen at high latitudes with IMP 8 moving from the sheath into the magnetosphere while at low latitudes Geotail moves from the plasma sheet into the lobes. The reverse is true for northward turnings. The tail cross-section is shown to be elliptical during southward IMF with an eccentricity of about 0.2 and that this eccentricity is slowly eroded over a period of about an hour during northward IMF.

WIND observations of energetic ions far upstream of the Earth's bow-shock

During the first year of operation, the WIND spacecraft followed a complicated orbit which took it from the Earth to the upstream libration point and back again. During this time, a considerable number of upstream particle events were observed all the way out to the libration point. These events are typically of short duration (a few tens of minutes) and up until now have only been seen in the energetic protons (at energies of a few tens of keV, but extending up to several hundreds of keV). We present here new observations from the Three-dimensional (3D) plasma and energetic particle experiment on the WIND spacecraft of these upstream events, with particular emphasis on the uniqueness of the observations from this instrument: energy spectra measured over the range from a few keV to several hundreds of keV, and complete three-dimensional angular distributions covering the same range of energies. We present here for the first time a complete spectrum of these ions extending from a few eV to a few MeV. This spectrum, with a turnover at one or two keV, shows that the bulk of the energy density of the upstream ions is at around 1 keV. These are most likely the particles responsible for the low-frequency waves which are usually seen accompanying upstream events.