D.T. Moriarty

Laboratory reproduction of arecibo experimental results: HF wave-enhanced Langmuir waves

Laboratory experiments at MIT using the Versatile Toroidal Facility (VTF) have produced “cascading” and “frequency-upshifted” spectra of HF wave-enhanced Langmuir waves resembling the spectra observed in Arecibo experiments. The VTF experiments are well-explained using the source mechanism proposed by Kuo and Lee [1992] to interpret observed Langmuir wave spectra at Arecibo, Puerto Rico. This mechanism is referred to as a nonlinear scattering of parametric decay instability (PDI)-excited Langmuir waves by “pre-existing” lower hybrid waves to preferentially produce anti-Stokes (i.e., frequency-upshifted) Langmuir waves. Recent radar spectral observations of anti-Stokes Langmuir waves at Arecibo with improved range and time resolution [Sulzer and Fejer, 1994] can be reasonably understood in terms of this mechanism.

LABORATORY STUDY OF SOME LIGHTNING-INDUCED EFFECTS IN THE IONOSPHERIC PLASMA

Abstract Laboratory experiments have been conducted at MIT, using the student-built Versatile Toroidal Facility (VTF), to investigate some ionospheric plasma effects produced by lightning-induced whistler waves. Lower hybrid waves, generated by the lightning-induced whistler waves, can cause a chain of extensive plasma effects, such as the acceleration of electrons and ions and the spectral broadening of plasma waves. Two mechanisms by which whistler waves generate lower hybrid waves can be important in the ionosphere. One is the simultaneous excitation of lower hybrid waves and low-frequency mode waves by intense whistler waves. The other is the nonlinear mode conversion of whistler waves into lower hybrid waves in the presence of short-scale field-aligned density striations. The effect of lower hybrid waves on the spectra of Langmuir waves has been investigated. The results of VTF laboratory experiments show that lower hybrid waves can beat with Langmuir waves to produce frequency-upshifted and frequency-downshifted Langmuir waves, broadening the spectra of Langmuir waves. The intensity of these beat waves, however, depends upon the angle of wave propagation with respect to the background magnetic field.

Versatile toroidal facility (VTF) for space plasma research

Summary form only given. The versatile toroidal facility (VTF) is a student-built large toroidal plasma machine having a helical magnetic field, that guides electrons emitted from heated LaB/sub 6/ filaments to flow from the bottom side to the topside of the plasma chamber. These upward flowing electrons serve two purposes: (1) creating a background plasma through electron collisions with neutral gases (e.g., H/sub 2/, Ar, O/sub 2/) and (2) forming a field-aligned electric current. The VTF plasma machine is ideal for the study of ionospheric plasma heating and space plasma processes, in a plasma environment characterized by: /spl omega//sub pe///spl omega//sub ce//spl ges/3 and T/sub e//spl sim/T/sub i//spl sim/5 e/spl nu/ where /spl omega//sub pe/, /spl omega//sub ce/, T/sub e/, and T/sub i/ represent the electron plasma frequency, the electron gyrofrequency, the electron temperature, and the ion temperature in the VTF, respectively. The VTF plasma has sharp density gradient in the radical direction and intense magnetic field-aligned electric currents. These VTF plasma conditions can properly simulate auroral plasma condition for the excitation of low-frequency ionospheric density fluctuations.

RF effects on current-driven plasma instabilities

Summary form only given. The Versatile Toroidal Facility (VTF) is a large laboratory plasma machine of 1 meter major radius used to out investigations of ionospheric plasma turbulence. LaB/sub 6/ hot-cathode electron emitters produce hydrogen plasmas of densities of 5.10/sup 17/ ions per cubic meter. Characteristics of the VTF plasma such as density gradients and field-aligned currents are in good agreement with those of the auroral and upper ionospheric regions. Spectral analysis has been performed on plasmas produced by the electron emitters. Interest has focused on the low frequencies below the lower hybrid resonance where ion acoustic and current-convective modes have been observed. Microwaves injected from a 3000 watt magnetron produce dramatic changes to the low frequency spectrum. First, the parametric decay instability intensifies the ion acoustic modes in the region of plasma heated by the microwaves. Second, the normally dominant current-convective modes are greatly suppressed in the heated region due to the oscillating electric field of the pump wave. When we probe beyond the heated region, these two pump wave effects are no longer observed, presumably because the microwaves are denied access to beyond the heated region due to the high plasma density. A theory which describes the suppression of current-convective modes in the presence of a pump wave will account for subtle differences in geometry between the ionosphere and the VTF machine. Results of recent experiments in the VTF machine and from Tromso, Norway are compared with the theory.

Implementation of Rogowski coil and Taylor discharge for ionospheric plasma chamber experiments in the Versatile Toroidal Facility (VTF)

Summary form only given. The VTF is a large torus used to to simulate ionospheric plasmas. Hydrogen plasmas are formed in the chamber at a base pressure of the order of 10/sup -7/ torr. Currently plasmas can be generated by electron emission from four LaB/sub 6/ cathodes, spaced around the bottom of the chamber, by electron cyclotron resonance heating (ECRH) with microwaves injected radially into the chamber from a 3 kilowatt magnetron, and by a Taylor discharge apparatus. Electron beams travel in a helical path till they reach a collector plate at the top of the chamber, and produce plasma currents from 500 to 1500 amps. Eighteen large toroidal field coils, spaced equally around the chamber, produce a toroidal magnetic field of 0.8 Tesla. A 0.01 Tesla uniform vertical field is added to the toroidal field to yield a helical magnetic field that guides the electron beams along their path. Recently, a Rogowski coil has been calibrated to quantitatively measure current in the plasma. A 4.2 meter coil of approximately 5500 turns encircles the torus vertically. As a changing current goes through the coil, a changing magnetic field is produced perpendicular to each of the coils turns. Each turn produces a small voltage, and the sum of all the voltages from all the turns in the coil is proportional to the change in plasma current. The voltage signal is integrated and the result is the plasma current circulating toroidally inside the chamber. Good results measuring electron beam plasma current have been obtained and are reported.

Langmuir wave turbulence generated by electromagnetic waves in the laboratory and the ionosphere

Summary form only given, as follows. We will present some recent results of our laboratory experiments at MIT, using a large plasma device known as the versatile toroidal facility (VTF). These experiments are aimed at cross-checking our ionospheric plasma heating experiments at Arecibo, Puerto Rico using an HF heating facility (heater). The plasma phenomenon under investigation is the spectral characteristic of Langmuir wave turbulence produced by ordinary (o-mode) electromagnetic pump waves. The Langmuir waves excited by o-mode heater waves at Arecibo have both a frequency-upshifted spectrum and a frequency-downshifted (viz., cascading) spectrum. While the cascading spectrum can be well explained in terms of the parametric decay instability (PDI), we have interpreted the frequency-upshifted Langmuir waves to be anti-Stokes Langmuir waves produced by a nonlinear scattering process as follows. Lower hybrid waves created presumably by lightning-induced whistler waves can scatter nonlinearly the PDI-excited mother Langmuir waves, yielding obliquely propagating Langmuir waves with frequencies as the summation of the mother Langmuir wave frequencies and the lower hybrid wave frequencies. This suggested process has been confirmed in our laboratory experiments, that can reproduce the characteristic spectra of Langmuir wave turbulence observed in our Arecibo experiments.

Current-driven plasma waves in the Versatile Toroidal Facility (VTF)

Summary form only given, as follows. We have been conducting laboratory experiments to investigate plasma turbulence that can affect the propagation of electromagnetic waves. This work is aimed at simulating the ionospheric plasma turbulence and cross-checking our radar experiments at Arecibo, Puerto Rico. The large toroidal plasma device, called the Versatile Toroidal Facility (VTF), can produce a radially inhomogeneous plasma imposed in a helically shaped magnetic field. VTF plasma has a sharp density gradient and an intense magnetic field-aligned current, simulating well the plasma environment in the auroral ionosphere. A broad spectrum of plasma waves can be excited in VTF by the injected microwaves and electron beams. In this paper, we discuss the excitation of low-frequency plasma waves driven by electric currents, including ion acoustic waves and current-convective modes. A good agreement has been found between the experimental results and the theories developed for the VTF plasmas. However, our experimental results are quite different from those obtained in the rocket/space shuttle experiments. The differences in the laboratory experiments and space experiments arise from two facts. One is that the plasma inhomogeneity does not play a significant role in the space experiments. The other is that the electron beam injected in the space experiments does not produce a drifting Maxwellian plasma as seen in the VTF plasmas. The contrast between the VTF experiments and the active space experiments show that VTF can adequately simulate the ionospheric plasma turbulence and complement the rocket/space shuttle experiments.

Electron beam-plasma interaction experiments with the Versatile Toroidal Facility (VTF)

Summary form only given, as follows. Our laboratory investigation of electron beam-plasma interactions is motivated by the recent space shuttle experiments. Interesting but puzzling phenomena were observed in the shuttle experiments such as the bulk heating of background ionospheric plasmas by the injected electron beams and the excitation of plasma waves in the frequency range of ELF waves. Our plasma machine, the Versatile Toroidal Facility (VTF) can generate a large magnetized plasma with the electron plasma frequency greater than the electron gyrofrequency by a factor of 3-5 similar to the plasma condition in the ionosphere. Short pulses of electron beams are injected into the VTF plasmas in order to simulate the beam injection from spacecrafts in the ionosphere. A Langmuir probe installed at a bottom port of VTF monitors the spatial variation of electron beams emitted from LaB6 filaments. An energy analyzer has been used to determine the particle energy distribution in the VTF plasmas. Several mechanisms will be tested as potential causes of the bulk heating of background plasmas by the injected electron beams as seen in the space shuttle experiments. It is speculated that the observed ELF emissions result from the excitation of purely growing modes detected by the space shuttle-borne detectors. Results of our laboratory experiments will be reported to corroborate this speculation.

Laboratory study of anti-Stokes Langmuir waves in a magnetized plasma

Summary form only given, as follows. Anti-Stokes Langmuir waves were observed in our ionospheric plasma heating experiments at Arecibo, Puerto Rico. They are referred to as frequency upshifted Langmuir waves excited by injected high power HF waves. The frequency shifts are found to be inversely proportional to the HF pump wave frequencies, and of the magnitudes of the lower hybrid wave frequencies in the ionosphere. Kuo and Lee (1992) suggest that these anti-Stokes Langmuir waves are caused by the nonlinear scattering of HF wave-produced Langmuir waves off lightning induced lower hybrid waves. Laboratory experiments with a large toroidal plasma machine known as the Versatile Toroidal Facility (VTF) have been conducted to investigate this nonlinear scattering process. In VTF, a background plasma is created by a Taylor discharge device which has a peak plasma frequency greater than the electron gyro-frequency by a factor of 3-5. This factor has been chosen in order to simulate the ionospheric plasma environment. Overdense heating of the VTF plasma by injected microwaves can generate Langmuir waves and ion acoustic waves via the decay instability. These Langmuir waves may then be scattered by lower hybrid turbulence existing in the background VTF plasmas to produce the anti-Stokes modes. Preliminary results of our laboratory experiments are presented and compared with results of field experiments and theory.

Wave-plasma interaction experiments at Arecibo using vertically and obliquely injected HF waves

Summary form only given. In recent ionospheric heating experiments at Arecibo, Puerto Rico, the spectral characteristics of HF enhanced Langmuir waves, among other phenomena, were investigated. Obliquely propagating HF waves were transmitted from a tilted heater in experiments during August 5-11, 1992, while vertically propagating HF waves were employed in experiments during September 17-22, 1992. The cascading spectra of PDI (parametric decay instabilities) produced Langmuir waves were observed in the vertical heating experiments, as expected. Radar measurements of the HF enhanced plasma lines in the oblique heating experiments also showed the excitation of PDI. Theoretical analyses suggest that the VLF waves produced by lightning or injected from a ground-based transmitter near Arecibo can be guided by the HF-induced ionospheric ducts to reach the radiation belts at L = 1.47 and effectively interact with energetic electrons through pitch angle scattering.