. i 

Semi-annual Report for NASA grant NAGW-998, titled 

"Wave Generation and Particle Transport in the Plasma Sheet 

and Boundary Layer' 

The one and two ion beam instability was considered as a possible 

explanation for the observations of broadband electrostatic noise in the 

plasma sheet region of the geomagnetic tail (Dusenbery and Lyons, JGR, 90, 

2915, 1985; Dusenbery, JGR, 91, 12,005, 1986). When only hot streaming plasma 

sheet boundary layer ions were present, no broadband waves were excited. 

Cold, streaming ionospheric Ions can generate electrostatic broadband waves 

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propagating in the slow beam-acoustic mode, but the growth rates of the waves 

were significantly enhanced when warm boundary layer ions were present. (Both 

b the slow and fast beam-acoustic modes can be excited, depending on the 

relative ion drift.) This model predicted that the wave intensity of the 

broadband noise should peak in the plasma sheet boundary layer (PSBL). 

Observations of less intense electrostatic waves in the lobes and plasma sheet 

were likely a result of the absence of warm ion beams or large ion 

temperatures, respectively, which resulted in smaller growth rates. 

The model dependence of the ion beam instability has also been studied 

(Dusenbery and Lyons, Chapman Conference Proceedings, 1986). For cold and 

warm ions streaming in the same direction, we found wave growth peaked for 

wave normal angles e = 0" and wave frequencies -0.1 x the electron plasma 

frequency. However, for anti-parallel streaming cold and warm ions, wave 

growth peaks near 8 = 90" and wave frequencies were an order of magnitude 

smaller. Including counterstreaming warm ions in addition to a cold ion 

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stream resulted in wave growth that was a superposition of that for the above 

two cases. 

Convective growth of slow and fast beam acoustic waves in the PSBL was 

investigated (Dusenbery, JGR, - 92, 2560, 1987). It has been shown previously 

that a cold ion population must be present in order to excite beam-acoustic 

waves in the PSBL. However, growth rates were significantly enhanced when 

warm PSBL ions were present. Net wave growth along a ray path is determined 

by convective growth. This quantity was calculated for particle distribution 

models consistent with the PSBL where the intensity of broadband turbulence is 

observed to peak. Total number density dependence on beam acoustic convective 

growth was evaluated and it was found that even for low density condition 

-O.Ol~rn'~, a measurable level of broadband turbulence was expected. 

tive drift effects between cold and warm ion populations were also considered. In particular, it was found that slow mode convective growth can be 

enhanced when slowly streaming cold ions are present compared to fast ion 

streams. 

RelaBesides understanding the characteristics of this turbulence such as 

frequency range excited, growth rates, and convective growth, it is important 

to determine the interactions such waves have on the PSBL particle distributions present. For normal PSBL conditions, it is unlikely that ion beamacoustic waves will affect warm PSBL electrons much. However, significant 

diffusion of warm PSBL ions in velocity space may occur. Interactions between 

beam-acoustic waves and boundary layer ions may offer a possible explanation 

for heating the boundary layer ions to form the hot ion component of the 

central plasma sheet as has been inferred to occur from observations. 
1 

I 

The wave-particle interactions and resulting diffusion of PSBL ions with 

beam-acoustic waves are a hybrid between the case where there is no magnetic 

field and where the magnetic field dominates. 

The Aerospace Corporation), we refer to such diffusion as unmagnetized 

diffusion. It essentially results from a Landau resonance but affects oblique 

as well as parallel degrees of freedom. 

In a study with Larry Lyons (at 

Besides studying the diffusion times, we also plan to investigate the 

temporal evolution of a warm boundary layer ion distribution. We feel that 

understanding the dynamics of the PSBL is an important study and therefore it 

is important to understand what effects are caused by the interactions which 

can occur between broadband turbulence and PSBL ions. 

Work is continuing on the properties of the beam acoustic mode under both 

plasma sheet and auroral conditions. In addition to the linear and 

quasi-linear properties of the instability, the nonlinear evolution and 

saturation of the excited wave fields are presently being investigated by 

simulation experiments (Dusenbery and Nishikawa, JGR, submitted, 1987).