Edgar Bering, Ph.D., Professor, Physics, UH
One of the difficult problems in space physics is making detailed measurements of the
thermal plasma in the ionosphere and plasmasphere. Contemporary models of the
thermodynamics of the ionosphere and electrodynamics of the aurora have developed a
considerable degree of detail in their treatment of the velocity space distribution
function of the thermal plasma. Further progress will require that experiments specify the
thermal plasma velocity distribution function in sufficient detail to calculate the lowest
order twenty velocity moments, including electric current density, heat flow vector, and
the off-diagonal terms of the pressure tensor.
Figure 1. A block diagram
of the y-z plane of the proposed instrument. The magnet coil is an
elongated "O" or racetrack with its long axis in the y-direction lying
in the plane of the paper within the box labeled "Superconducting Magnet."
Magnetic field lines lie entirely in the transverse x-z plane and field
intensity B drops off as the inverse square of cylindrical radius. Plasma will be
admitted to the region of this field through a narrow slit lying in the x-y
plane with its long axis parallel to B located at z = ~380mm. An
electric field of ~100-300 V/m will be applied in the -y-direction. The thick
dark line located at z = +70 is an edge view of the 2-D senstive Langmuir probe
array.
The project required preparatory work necessary for writing proposals to research
agencies requesting support for the construction and prototype testing of a unique new
plasma analyzer. The analyzer we designed and proposed will be capable of measuring the
thermal plasma velocity distribution function with much greater detail than is currently
available. Space-borne in situ measurement of cold ionospheric thermal plasmas is usually
accomplished by means of Langmuir probes, Faraday cups, or rf probes. These instruments
have poor angular resolution. They are only capable of measuring density, temperature,
temperature and/or pressure anisotropy, and bulk flow velocity. This level of capability
is inadequate to meet the needs of contemporary models or to solve a number of vexing
observational controversies.
Several questions can be addressed by the instrument that we began to design this year,
inclusive of the identification of the component of the plasma that carries downward
Birkeland currents near auroral arcs. Science has assumed that this current is carried by
an upward bulk drift of the thermal electron population. However, evidence indicates that
this current may, instead, be carried by a magnetically aligned beam of suprathermal
ionospheric electrons. Verification of this conclusion and elucidation of the mechanism
responsible for producing such beams are important questions in auroral physics.
Figure 2. A top-down view
of the x-z plane of the proposed instrument. The cryostat,
superconducting cables, and the main pole pieces of the magnetostatic shielding are
outlined by the circles and arcs. The thick dark line is the orthogonal edge on view of
the probe array.
A second area of investigation concerns the development of strongly non-Maxwellian
plasma distribution functions in the ionosphere whenever the electric field exceeds 70
mV/m. This model prediction has been supported by evidence inferred from high latitude
incoherent scatter radar data, but has never been directly verified. Finally, the growth
and propagation of wave modes in the ionospheric plasma depend entirely on the details of
the velocity space distribution function. The absence of data on the distribution function
at thermal energies has required data analysts and modelers alike to use assumptions that
limit our ability to predict instabilities and plasma wave spectra.
The proposed instrument exploits the unique kinetic and thermodynamic properties of a
convecting plasma in a two-dimensional dipole magnetic field. In a recent paper, Huang and
Birmingham (1994) showed that the guiding center motion of charged particles is
considerably simpler if the magnetic field is that of a 2D rather than a 3D dipole. The
former is also known as a line dipole field. It is produced by pair of closely spaced
wires carrying oppositely directed currents equal in magnitude and infinitely extended in
what we define to be the y-direction. This type of magnet is called a racetrack magnet.
One of the major tasks undertaken this year was initiation of the magnet design process.
In the detection region, the magnetic field intensity will be ~1 T, such that the
gyroradius will be <1 cm for 10 eV protons and much less for electrons. Thus, the
guiding center approximation can be used to treat particle motion within the detector. In
the guiding center approximation, for the line dipole, equatorial pitch angle and mirror
colatitude angle are equal (taking the acute value of each). The drift velocity is given
by the sum of the ExB, magnetic gradient and magnetic curvature drifts. The ExB drift will
convect plasma from the entrance slit to the detector surface. The guiding centers will
drift in such a way that their equatorial pitch angles and, therefore, mirror
"latitudes" remain constant. The x coordinate of position on the
detector surface is proportional to the tangent of the "latitude" angle. Thus,
there will be a unique and well understood correspondence between x coordinates
on the position sensitive detector and initial pitch angle of the particle. Since the
gradient and curvature drift speeds depend on particle energy, the y value of detector
intercept is a linear function of initial particle energy. In other words, the image that
appears on the detector is an image in energy-pitch angle coordinates of the thermal
plasma distribution function, a quantity no other instrument has ever been able to
measure. In fact, two images would be obtained, one of ions and the other of electrons.
The first task accomplished with ISSO support was an extensive review of possible
position sensitive detectors for use in this configuration. The relatively low energy of
the incoming particles (~150 eV) and the relatively high currents (which may reach 10 nA
in the central pixels) make most of the detectors used in high energy physics
inappropriate for this application. The detectors that appear most likely to work are
small planar Langmuir probes operating at fixed potential with linear current to voltage
converter preamplifiers situated on the detector plane immediately behind the detector
surfaces.
Preliminary discussions with the staff of Texas Components, Inc. of Houston, Texas
indicate that it should be possible to assemble hybrid preamps in a small enough package
to fit an 8 x 16 array into a 20 cm x 40 cm area.
This design has some concepts that need to be fleshed out including: details of the
magnet, some details of the detection scheme, a workable design for external magnetic
shielding, details of the electrostatics of the entrance aperture, and the method to be
used for maintaining the entrance aperture guard ring at plasma potential.
Under the auspices of this ISSO project, researchers submitted a TATP proposal in July
requesting support for design work and for prototype construction and preliminary testing.
We will construct the prototype to "sounding rocket" specifications so that it
can be further tested via either a dedicated sounding rocket or a SPARTAN flight.
Prototype construction will also provide good estimates of power, weight and volume
requirements, parameters which will be needed for development of an actual spaceflight
proposal. We anticipate submitting proposals to NASA based on this work as well as
proposals to the Innovative Research program in January, 1996 and the Space Physics
Division SR&T NRA in the summer of 1996.
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Edwin Carrasquillo-Molina, Ph.D., Associate Professor, Chemistry, UH
In previous work, we had developed a new approach in our laboratory for spectroscopic
study at high levels of vibrational excitation. The new approach, now referred to as
collision assisted spectroscopy (CAS), makes use of collisions to populate levels
inaccessible to direct laser excitation. Earlier research had implemented this approach to
characterize the vibrational structure of HCN between 4,000 cm-1-2,000
cm-1, essential for the study of exothermic reactions which produce the
vibrationally energized species. Research during this funding period extended the
spectroscopic characterization of HCN and led to highly sensitive laser induced
fluorescence detection techniques for this molecule to well above its HCN
<-> HNC isomerization barrier (Evib ª 16,600 cm-1). Research
determined that selective implementation of one photon laser induced fluorescence of HCN
via its A state for detection or two photon resonant enhanced photodissociation leading to
CN fragment B --> X emission permits small numbers of
hydrogen cyanide molecules vibrationally excited to energies between 3,500-18,000 cm-1
to be state-selectively detected. The efficient detection of vibrationally hot HCN
over the latter energy range makes possible a detailed probe of the CN + H2
reaction.
Contents
ISSO -- Institute for Space Systems Operations
1994-1995 Annual Report
