One of the most difficult experimental problems in space physics is making detailed measurements of the properties of the cold thermal plasma (ionized gas) in the ionosphere and plasmasphere. Contemporary models of the thermodynamics and transport properties 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. In order to validate these models and make further progress, the scientific requirement on the experiments is designed to 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. The design and prototype testing of a cold plasma analyzer will provide the capability of measuring the thermal plasma velocity distribution function in much greater detail than is presently possible. In this context, UH researchers will use the term "cold" to mean plasmas with temperature T£10eV with particular emphasis on temperature T£leV.
There are many reasons why these measurements are very difficult to make. First, an in situ spacecraft lacks a true ground. The operating point of plasma measuring instruments on a spacecraft is often determined by reference to telemetry or buss ground. Depending on the types of instruments in operation, the amount of insulation, and other factors, this potential may vary by ~±10kT/e or more with respect to ambient plasma potential. In these circumstances, cold plasma particles may be repelled or accelerated in the plasma sheath around the spacecraft. Techniques exist that will mitigate and correct for these problems. However, these techniques introduce uncertainties that make it difficult to measure higher order moments of the thermal plasma velocity distribution function. Second, cold plasma particles lack sufficient energy to exceed the noise or window thresholds of many detectors based on particle counting techniques commonly used to study the properties of more energetic space plasmas. Third, the density of most cold plasmas is large enough to overwhelm or saturate particle counting techniques with high counting rates. Spaceborne in situ measurement of cold ionospheric thermal plasmas is usually accomplished by means of Langmuir probes, Faraday cups, or rf probes. These instruments have very coarse angular resolution. They are capable only 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.
This project supported the task of developing a preliminary idea of how to develop this experimental problem into a sufficiently detailed instrument design to enable the preparation and submission of a proposal to NASA for prototype construction.
Relationship to Previous Work. The idea was initially suggested by Prof. T. S. Huang, a theorist at the Physics Department of Prairie View A&M University, based on a recent publication.[1] The proposed work will be carried out in conjunction with Dr. Huang, who will carry out a detailed theoretical analysis and simulation of charged particle trajectories in the various alternative configurations that will emerge during the design process.
Prof. Bering's dissertation work was based upon Langmuir probe studies of high moments of the plasma distribution function. He has more than 25 years experience with probe and particle detector studies of thermal plasmas in the ionosphere, plasma distribution functions, and plasma waves.
Scientific Need. Several scientific questions can be addressed by the instrument being proposed for development. These questions include the identification of the component of the plasma that carries the downward Birkeland currents near auroral arcs. Historically, it has been assumed that this current is carried by an upward bulk drift of the entire thermal electron population in the ionospheric plasma. However, evidence has accumulated to indicate that this current may instead be carried by a magnetically aligned beam of suprathermal ionospheric electrons.[2,3] Verification of this conclusion and elucidation of the driving mechanism responsible for producing such beams are important unsolved questions in auroral physics. A second major area of investigation concerns the development of strongly non-Maxwellian plasma distribution functions in the ionosphere whenever the electric field exceeds 70mV/m.[4,5,6,7,8] This model prediction has been supported by evidence inferred from high latitude incoherent scatter radar data, but has never been directly verified.[7] Finally, the growth and propagation of wave modes in the ionospheric plasma depends entirely on the details of the plasma velocity space distribution function. The absence of data on the plasma distribution function at thermal energies has required data analysts and modelers alike to use approximations and assumptions that limit the ability of researchers to predict instabilities and plasma wave spectra.[9]
Technologically, it is important and interesting to monitor the plasma velocity space distribution function in several applications. These include any space-based industrial processes that occur in vacuum in low earth orbit, many industrial plasma processes, and controlled thermonuclear fusion reactors. The proposed instrument will have useful applications in these areas.
Relation to Other NASA Programs. The scientific questions that might be answered through spaceborne use of the proposed instrument are directly relevant to the scientific objectives of programs such as TIMED, UARS, and FAST. Once developed, test flights will be proposed for both a sounding rocket and a SPARTAN package. Further use would depend on what projects or opportunities were announced in the time frame of the early 2000's.
Methodology
Experimental. The proposed instrument exploits the unique kinetic and
thermodynamic properties of a convecting plasma in a 2-dimensional dipole
magnetic field.[1] In a recent paper, Huang and Birmingham have shown that the
guiding center motion of charged particles is considerably simpler if the
magnetic field is that of a 2-D rather than a 3-D 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. In the jargon of high energy particle and accelerator physics, this type of magnet is called a racetrack magnet. A magnetic field of the order of 1T is required to keep the gyroradius of thermal ions small compared to the dimensions of the detector surface. For initial strawman purposes, assume superconducting cables separated by 2 cm, carrying 4 x 10e5A, with the long axis in the y-direction, as shown in Fig. 1. Currents of this magnitude would involve prohibitive power dissipation for spacecraft use if ordinary conductors were used, so this design is based on a superconducting magnet.
Magnetic field lines lie entirely in the transverse x, z plane and field intensity B drops off with cylindrical radius r = (x2 + z2)1/2 as r-2. 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 = + ~40cm. An electric field of ~3000V/m will be applied in the -y-direction. A position sensitive detector (see below) will be located at z = +6 cm in the x-y plane, as shown in Figs. 2 and 3.
In the detection region, the magnetic field intensity will be such that the gyroradius will be £ lcm for 10eV 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 de and mirror colatitude angle qm are equal (taking the acute value of each). The drift velocity is given by the three usual contributions, the E x B, the magnetic gradient and the magnetic curvature components. For the proposed configuration, the E x B 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 coordinate on the position sensitive detector and initial pitch angle of the particle. As they drift inwards, the kinetic energy of the particles will increase as z-2, where z is the midplane crossing distance of the magnetic field line threading the guiding center.
In the absence of space charge, the guiding center trajectory of a particle within the detector will be given by
where subscript "0" means the initial value. Thus, 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. Depending on ambient density and available telemetry, it should be possible to obtain statistically significant images in 100 ms.
The fact that an appreciable amount of thermal plasma will be flowing into the interior of the instrument means that the applied electric field will be opposed by a polarization field owing to the separation of charges within the previously neutral plasma. The nominal applied field within the instrument, 3000V/m, is greater than the largest polarization field of ~1000V/m that could be produced by an ambient 10l2m-3 plasma, assuming a 1 cm wide entrance aperture, allowing for abiabatic compression within the instrument. The effect of polarization will be to reduce the convection velocity within the instrument and to change the y dependence of the instrument response. Modeling the effect of polarization is one of the major remaining design tasks listed in our NASA proposal.
UH researchers have already conducted an extensive review of possible position sensitive detectors for use in this configuration. The relatively low energy of the incoming particles (~1-50eV) and the relatively high currents (which may reach 10nA 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 (Fig. 4) situated on the detector plane immediately behind the detector surfaces. Preliminary discussions with the staff of Texas Components, Inc. of Houston, Texasm indicates 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 detector array will output a set of 128 analog voltages proportional to the currents being collected by the patch probes. These voltages will be sampled and digitized and read out to the telemetry bus of whatever vehicle the instrument is being flown on. A block diagram of the instrument electronics is shown in Fig. 4.
The cooling scheme envisioned for the proposed prototype would utilize externally supplied cryogenics. This approach is adequate for sounding rocket and short orbital missions. The experience of NASA with spaceborne infrared observatories has shown that with suitable insulation schemes, it is possible to maintain liquid helium temperatures on orbit for intervals of at least a year.
The conceptual sketch is most of the design of this instrument concept that exists at the present time. The development of this sketch to this level of detail was supported by ISSO. Concepts that need to be developed in substance include details of the magnet, many of the 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.
A proposal entitled "Development of a Thermal Plasma Moment Analyzer Based on a Superconducting Line (2-D) Dipole Electromagnet" was submitted to NASA on August 19, 1996, in response to the 1996 Solar Connection SR&T NRA. The preparation and submission of this proposal was the major objective of this ISSO project. Support is being sought for prototype construction and laboratory testing.
References
1T. S. Huang and T. J. Birmingham. "Kinetic and Thermodynamic
Properties of a Convecting Plasma in a Two-Dimensional Dipole Field," J.
Geophys. Res. 99 (1994): 17295-308.
2H. L. Collin, R. D. Sharp, and E. G. Shelley. "The Occurrence and
Characteristics of Electron Beams Over the Polar Regions," J. Geophys. Res.
87 (1982): 7504.
3D. M. Klumpar and W. J. Heikkila. "Electrons in the Ionospheric Source
Cone: Evidence for Runaway Electrons as Carriers of Downward Birkeland
Currents," Geophys. Res. Lett. 9 (1982): 873-76.
4R. W. Schunk and J. C. G. Walker. "Transport Processes in the E Region
of the Ionosphere," J. Geophys. Res. 76 (1971): 6159.
5R. W. Schunk. "Transport Equations for Aeronomy," Planet. Space
Sci. 2S (1975): 437.
6R. W. Schunk, W. J. Raitt, and P. M. Banks. "Effect of Electric Fields
on the Daytime High-Latitude E and F Regions," J. Geophys. Res. 80 (1975):
3121.
7R. W. Schunk, L. Zhu, and J. J. Sojka. "Ionospheric Response to
Traveling Convection Twin Vortices," Geophys. Res. Lett. 21 (1994): 1759-62.
8J.-P. St.-Maurice and R. W. Schunk. "Ion Velocity Distributions in the
High Latitude Ionosphere," Rev. Geophys. 17 (1979): 99-134.
9E. A. Bering, J. E. Maggs, and H. R. Anderson. "The Plasma Wave
Environment of an Auroral Arc, 3. VLF Hiss," J. Geophys. Res. 92 (1987):
7581-605.