Showing papers by "Roger R. Anderson published in 1999"

On relative timing in substorm onset signatures

Abstract: We investigate the timing of popular substorm onset signatures to understand their temporal relationship. Proxies for substorm onsets include auroral breakups, high-latitude magnetic bays, low-latitude Pi2 bursts, dispersionless injections at geostationary orbits, and auroral kilometric radiation. We use the auroral breakup, identified with Polar UVI images, as a common reference time frame to calibrate the others. Results, illustrated by two well-defined auroral substorms, unambiguously indicate that none of the four frequently used substorm onset proxies can provide a consistent timing of substorm onset. This inconsistency in substorm onset timing is attributed as a consequence of temporal and spatial limitations on each observational technique. A delay between the proxy identifiers and the auroral breakup is found to be typical. It is therefore strongly suggested from this study result that a common reference time frame for substorm onset is necessary, and we propose it should be auroral breakups. We argue that there is a need for an intercalibration of magnetospheric substorm phenomenology by using a unified definition of the substorm onset.

“Pancake” electron distributions in the outer radiation belts

Abstract: Electron pitch angle distributions sharply peaked at 90 degrees pitch angle were first recorded in the energy range 50 eV < E < 500 eV by the GEOS1 and GEOS2 spacecraft in 1977/1978, from the plasmapause out to geostationary orbit. At the time they were explained as the remnants of pitch angle diffusion driven solely by electron cyclotron harmonic (ECH) waves. Here we report new observations by the Low Energy Plasma Analyser on board the Combined Release and Radiation Effects Satellite, which measured the complete pitch angle distribution over the energy range 100 eV < E < 30 keV. The pancake distributions are seen to develop from injected distributions that are nearly isotropic in velocity space, on a timescale that is greater than 2 hours. The freshly injected distributions are associated with strong ECH and whistler mode waves suggesting that the pancake distributions are likely to be caused by a combination of both wave types. Outside L = 6.0 the fitting analysis at energies in the range 100 eV < E < 1 keV shows that in the marginally stable state the phase space density contours lie approximately along the characteristic curves for diffusion by whistler mode waves determined independently from the plasma wave data. However, inside L = 6.0, significant departures are observed. Our results suggest that whistler mode waves play a dominant role in the formation of pancake distributions outside L = 6.0, whereas inside L = 6.0 and, in particular, in the vicinity of the plasmapause, the ECH waves also play a significant role. Consequently, both types of waves should be considered in any attempt to explain the diffuse aurora and the variation with L taken into account.

Electrostatic solitary waves carried by diffused electron beams observed by the Geotail spacecraft

Abstract: We study waveforms of electrostatic solitary waves (ESW) and corresponding electron velocity distribution functions observed by the Geotail spacecraft. When we observe a series of ESW in the plasma sheet boundary layer of the Earth's magnetotail, we find enhanced fluxes of high-energy electrons flowing along the ambient magnetic field. We find good correlation between the propagation direction of the ESW and the direction of the enhanced high-energy electron flux. This supports the model proposed by computer simulations that ESW are formed by the electrons modulated through the bump-on-tail instability. The enhanced electron flux is regarded as the diffused electron beam after the saturation of the instability. From comparison between the variation of waveforms and the velocity of the electron beams, the spatial scales and depths of the ESW potentials and their propagation directions are estimated. We found both tailward and earthward ESW in the plasma sheet boundary layer, even in the deep magnetotail.

Generation mechanism of ESW based on GEOTAIL plasma wave observation, plasma observation and particle simulation

Abstract: Data of electrostatic waves and plasma particles in the deep magnetotail (X ∼ -209R E ) respectively obtained by Plasma Wave Instrument and Comprehensive Plasma Instrument onboard the GEOTAIL spacecraft are presented. When the GEOTAIL spacecraft experienced multiple crossings of the plasma sheet boundary layer, broadband electrostatic noise (BEN) and Langmuir wave were observed alternatively. The dynamic frequency spectra of BEN are very bursty in time, and their waveforms are a series of electrostatic solitary waves (ESW). The LW is observed when an enhancement of an electron flux is found on a high-energy tail of a relatively cold velocity distribution function of the major thermal electrons. The ESW, on the other hand, are observed in the presence of a hot thermal electron distribution function, in which electrons responsible for the ESW are embedded. These plasma conditions are in agreement with the ESW generation model based on particle simulations.

Dual spacecraft observations of lobe magnetic field perturbations before, during and after plasmoid release

Abstract: This study examines a data set returned by IMP 8 and Geotail on January 29, 1995 during a substorm which resulted in the ejection of a plasmoid. The two spacecraft (s/c) were situated in the north lobe of the tail and both observed a traveling compression region (TCR). We show that in this instance dual s/c measurements can be used to model all three dimensions of the underlying plasmoid and to estimate its rate of expansion. For this event plasmoid dimensions of ΔX ∼18, ΔY ∼30, and ΔZ ∼10 Re are determined from the IMP 8 and Geotail observations. Furthermore, a factor of ∼2 increase in the amplitude of the TCR occurred in the 1.5 min it took to move from IMP 8 to Geotail. Modeled using conservation of magnetic flux, this increase in lobe compression implies that the underlying plasmoid was expanding at a rate of ∼140 km/s. Finally, a reconfiguration of the lobe magnetic field followed plasmoid ejection which moved magnetic flux tubes into the wake behind the plasmoid where they would become available to feed the reconnection region.

CRRES satellite observations associated with low‐latitude Pi2 pulsations

Abstract: We studied the electric and magnetic field variations and their relation to the ambient plasma density observed from the Combined Release and Radiation Effects Satellite (CRRES) during two Pi2 events observed on the ground at the low-latitude (L = 1.2) station Kakioka. At the time of the Pi2s, CRRES was moving outward at L ∼ 5, magnetic local time (MLT) of ∼2200, and magnetic latitude of ∼4°. The satellite was in an apparently detached dense plasmaspheric plasma for the first Pi2 but in a low-density region for the second Pi2. The first Pi2 had a period of 120 s at Kakioka (MLT ∼ 2000), and the satellite observed a nearly identical oscillation in the (approximately) azimuthal component of the electric field and in the radial and compressional components of the magnetic field. At CRRES the compressional magnetic field and the azimuthal electric field oscillated nearly in quadrature in the decaying phase of the pulsation, which suggests a radially trapped fast mode wave. However, comparison of this observation with numerical models of the plasmaspheric cavity mode is difficult because the plasma density did not have a simple plasmapause structure. The second Pi2 observed at Kakioka exhibited a period of 180 s and was similar to the azimuthal electric field oscillation seen at CRRES but quite different from the compressional magnetic perturbation at the satellite. For this event it is not clear whether the electric field oscillation represents a fast mode wave that was directly related to the ground Pi2 or a local shear Alfven wave that was not directly related to the Pi2. If the first interpretation is the case, the midlatitude Pi2 is not necessarily confined within the plasmasphere by fast mode trapping or cavity resonance. These examples indicate that electric field oscillations measured by equatorial satellites are a sensitive and possibly better indicator of the onset of substorms and that if the plasma density measurements are also available, the electric field data can be useful for identifying the wave mode of Pi2 pulsations.

Simultaneous DMSP and CRRES observation of broadband electrons during a storm-time substorm on March 25, 1991

Abstract: Broadband electrons observed by the DMSP satellites during storm-time substorms are characterized by an unusually intense, flux of precipitating electrons over the broadband energy range from 30 eV to 30 keV near the equatorward edge of the auroral oval (Shiokawa et al., 1996; 1997). During the broadband electron event observed by the DMSP-F9 satellite at 21 MLT on March 25, 1991, the CRRES spacecraft was at a geocentric distance of 6.3 R E and in the same local time as that of DMSP. The CRRES satellite observed 1) a large enhancement of field-aligned electron flux, 2) highly turbulent electric fields up to 10 mV/m, 3) intense west-ward excursion (∼100 nT) of magnetic field (suggesting intense field-aligned current generation) in a highly tail-like field configuration, 4) large O + energy density, and 5) intense low-frequency (below 300 Hz) electrostatic waves. We discuss possible production mechanisms of broadband electrons on the basis of these observations.