The objective of the research is to use the low-frequency wave spectrum measured by the Plasma Wave Instrument on the DE-1 spacecraft to include the wave-particle interaction (WPI) in the different polar wind models that are available at Utah State University. A Monte Carlo technique was used to simulate the ion diffusion in the velocity space due to scattering by the waves. This enabled us to study the effect of WPI on the magnitude as well as the composition of the outflow of the ionospheric ions. In particular, in the first year the 1-D semi-kinetic code developed by Barakat and Schunk (1983) was modified in order to include the effect of the WPI

Barakat, Abdallah R.

English

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fSpace Physics Guest Investigator
Semi-Annual Report for
NASA Research Grant]VIAG5 1546
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Principal Investigator
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Abdallah R. Barakat
Utah State University
Center for Atmospheric and Space Sciences
Logan, Utah 84322-4405
Reporting Period
May 1, 1991 to October 31, 1991
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Introduction
The outflow of ionospheric ions at high latitudes is an important component in the
ionosphere-magnetosphere (I-M) coupling. A significant portion of the magnetospherie
ions is from an ionospheric origin. The conventional wisdom is that the wave-particle
interaction (WPI) is negligible outside the auroral regions. Therefor, in all the polar wind
models developed so fat, this effect was neglected. However, a close study of the
measured levels of the waves in the region above the polar cap indicate that the wave-
particle interaction can play an important role in the behavior of the outflowing ions. A self
consistent solution of the plasma and waves is beyond our current computational ability. A
feasible alternative is to use the wave data from Plasma Wave Instrument aboard the DE-1
satellite. This data is introduced to an existing semi-kinetic code [e.g. Barakat and Schunk,
1983] using a Monte Carlo approach,
The objective of the research funded by the grant NAG5-1546 is to use the low-
frequency wave spectrum measured by the Plasma Wave Instrument on the DE- 1 spacecraft
to include the WPI in the different polar wind models that are available at Utah State
University. A Monte Carlo technique will be used to simulate the ion diffusion in the
velocity space due to scattering by the waves. This will enable us to study the effect of WPI
on the magnitude as well as the composition of the out flow of the ionospheric ions. In
particular, in the first year the 1-D semi-kinetic code developed by Barakat and schunk
(1983) will be modified in order to include the effect of the WPI. This code can help us
answer questions such as: (1) Is the WPI enough to reduce the large temperature
anisotropies predicted by the semi-kinetic model? (2) To what extent the ion velocity
distribution and its moments are modified? etc...
A Ph.D. student was chosen to work on this project as part of his dissertation. We
started the training of the student by gradual introduction of the different concepts that are
neededto solve such problem (e.g. Monte Carlo technique, Semi-kinetic polar wind
models). In the next two sections we present the concepts that the student was introduced
to in order to be able to address the problem outlined above. The simulation of particle
diffusion in velocity space is introduced in section 2, the general features of the Semi-
kinetic model is given in section 3, and a survey of the data is presented in section 4. In
section 5, the preliminary results of the code are presented, and the future plans are briefly
discussed in section 6.
2. Diffusion in the Velocity Space
As we mentioned above the student was introduced to the basic concepts of the
project in a gradual manner. In order to get experience in simulating ion diffusion in
velocity space, he developed a Monte Carlo algorithm that simulates the effect of Coulomb
collisions on the ion outflow in the polar wind. The effect of Coulomb collisions is similar
to that of the WPI ( form an algorithmic point of view) in that both of them can be
represented by diffusion in the velocity space which can be simulated by a 'random walk'.
This model included the the effects of polarization electric field, gravitational forces,
and the divergent geomagnetic field lines. It covered the collision dominated region, the
collisionless region and the transition between the them. It was concluded that the transition
region play an important role in determining the characteristics of the ion outflow. In this
region, the gradient of drift velocity reaches a maximum, both the parallel and
perpendicular temperatures maximize, the heat flow changes its sign, and the distribution
function change in a rapid and complicated manner from Maxwellian (at the barosphere) to
kidney shape (at the exosphere). A copy of this paper is enclosed.
3. Polar Wind Collisiordess Model
The base for thiswork isthe semi-kineticmodel for the collisionlesspolar wind
(exosphere) such as those developed by barakat and Schunk (1983). Understanding the
code was an essential part of the student's training. He studied the code thoroughly in order
to understand its algorithm. Then he ran the code for a given set of physical parameters (Te
= T[O +} = T[H +] = 3000 K, n[O +] = 200 cm -3, n[H +] = 100 cm'3), and computed the
potential energy d>, the density n, the drift velocity u, the parallel and perpendicular
temperatures (T IIand T.L), the paratlel and perpenicular heat fluxes (qll and q_L). A two
folded objective was achieved by these runs. First, by reproducing the results given by
Barakat and Schunk (1983), which can be used as a pinch mark to calibrate the new code.
Second, the profile of d> will be used as an input to the new (Monte Carlo) code.
4.Preliminary Data Study
As we pointed out earlier,the WPI resultsin a particlediffusionin the velocity
space.During an intervalof time _ theparticleperpendicularvelocityisincremented by
Avj_such that
<(zlv±) 2 >= 2D_[zlt
where D'_k is given by
D, =g"riq___2 )IE(¢°=_)12
± k4m z
and where q and m are the ion charge and mass, respectively, _c is the ion cyclotron
frequency, andr/is the proportion of the spectral density that corresponds to the left-
polarized wave. [ E 21 is the spectral density of the wave.
A dimensional study of the above equations shows that the effectof WPI is
significantifthe normalized diffusioncoefficientis of order of, or greaterthan unity
3
(D.I__I).Where D± [e roB'thisthe normalized diffusioncoefficient,rois the geocentric
distanceof the lower boundary, and vth ision thermal speed.A general literaturesurvey
was carriedout inorderto have a rough estimateof thesignificanceof theWPI on the ion
outflow.Itwas found thatthe normalized diffusioncoefficientinthe velocityspace (D±)
ranges from 0.I to 10. Therefor,we should expect thatthe WPI can have a significant
cffccton theionoutflow alongtheopen gcomagnctic fieldlinesabove thepolarcap.
5. WPI Algorithm
After the student gained the necessary experience as explained above, he started to
develop a Monte Carlo simulation for the polar wind ion outflow under the influence of the
WPI. The range of simulation was taken high enough to neglect the effect of collisions.
The electrostatic polarization field was taken from the results of the semi-kinetic code
explained above. For simplicity, the value of D± was assumed to range from 0.01 to 10,
and to be independent of altitude, latitude (provided that it is still in the polar cap), season,
solar activity, and geomagnetic activity. This simplified approach should give us a rough
idea about the magnitude as well as the nature of the effects of the WPI on the results.
In this first crack at the problem, we concentrated on the behavior of the H + ions.
The main conclusions are:
1. At very weak WPI (D±-_-O), we reproduce the results of Barakat and schunk
(1983).
2. Near the exobase (1.7 Re), the shape of the H+ velocity distribution is not
affected by the WPI.
3. For higher altitudes (>5 Re), the WPI reduces the anisotropy due to
perpendicular heating. For relatively large values of WPI (D_I__'0.1), the heating
due to WPI overcomes the adiabatic cooling, and hence, the ion perpendicular
temperature increases with altitude near the top of the model.
4. The lower order moments (density and drift velocity) of H + ions are not very
sensitive to WPI for moderate values of D± (< 1). For larger values of D±
(>1), the WPI can change these lower order moment in a quantitative, rather
than qualitative, manner .(e.g. see fig. 1).
5. As the WPI becomes stronger, the ion perpendicular temperature increases
especially at high altitudes. For strong enough WPI (D± > 1), T± can become
monotonically increasing(e.g, see fig. 2).
6. The parallel temperature decreases with D± for weak to moderate WPI
(D±< 1).For very strong WPI (D±>>I), the parallel temperature reaches a
minimum and then increase very rapidly with altitude.
6. Future Plans
In the rest of the first year we plan to address the following points:
1. The behavior of the O + ions will be studied.
2. The resulting ion densities will be used to compute improved profiles for the
polarization electric field, which in turn.wiU be used to compute new profiles of
the ions densities. This iteration will be continued until we reach a consistent
solution.
3. The code will be modified to be able to handle profiles of D_l_ that are altitude
dependent. The modified code will use more realistic profiles of D_l_ (from
measurements) to study the effect of altitude variation of D± on the results.
4. Finally if thetime will permit,a larger data-base will be binned with respect to
season and geomagnetic activity to investigate the dependance of the WPI
strength on these two factors. Then the code will be used to study the effect of
season and geomagnetic activity on the ion outflow characteristics.
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Effect of wave-particle interaction on the outflow of ions at high latitudes

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