R.J. Riddolls

Generation of large sheet-like ionospheric plasma irregularities at Arecibo

Large-scale ionospheric plasma irregularities, generated by O-mode heater waves at Arecibo, are shown for the first time to have “sheet-like” structures. The irregularities are aligned with the magnetic meridional plane and have scale sizes ranging from a few hundred meters to a few kilometers. This interpretation is based on detailed considerations of sequential measurements of radar backscatter power, the controlling magnetic field geometry, and the deduced E × B plasma drift. The alignment of O-mode-generated irregularities with the magnetic meridional plane, and their disappearance during X-mode heating intervals are consistent with predictions of the thermal filamentation instability model.

Augmentation of natural ionospheric plasma turbulence by HF heater waves

HF heating offers a powerful technique for controlled studies of ionospheric plasma turbulence. Heater waves generate large plasma density depletions and sheet-like ionospheric irregularities in the F region, which can give rise to spread F backscatter. Electric fields associated with the induced irregularities can seed plasma instabilities, driven by such environmental causes as density gradients and ambient electric fields, to enhance spread F signatures. Significant reductions in the height-integrated radar backscatter power, measured during the HF heating, indicate the depletion of magnetic flux tubes. Density gradients at the edges of the depletions provide new sources of free energy to augment ionospheric plasma turbulence, enhancing the spread F processes. Furthermore, depleted magnetic flux tubes create and/or alter ionospheric ducts thus affecting radio wave propagation.

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.

Ionospheric plasma bubble generated by Arecibo heater

During recent experiments ionospheric plasma bubbles were excited by the upgraded HF heater at Arecibo. These plasma bubbles were observed by radar in the midnight sector with the entire flux tube in darkness. A simple model is outlined to explain the dynamics of density depletions generated during O-mode wave heating of the F layer. We suggest that thermal expansion of plasma away from the heated volume leads to enhanced recombination along the flux tube. In the absence of photoionization sources, density depletions develop along the excited flux tube. The discontinuity of gravity-driven currents at the walls of the depleted region requires development of polarization electric fields. Eastward polarization electric fields of ∼2.5 mV/m within the flux tube caused an observed plasma bubble to drift vertically at a speed of 70 m/s.

Space plasma disturbances caused by NAU-launched whistler waves

Radio signals from Naval (NAU) transmitter in Puerto Rico can interact effectively with naturally occurring or HF heater wave-induced large-scale ionospheric irregularities, allowing them to propagate as whistler-modes in the ionosphere and to the inner radiation belts. NAU-generated whistler-modes have intensities sufficient to parametrically excite lower hybrid waves and ten-meter and meter-scale ionospheric irregularities over Arecibo. Subsequent heating of electrons and ions by the lower hybrid waves yield a sequence of ionospheric plasma effects such as airglow, short-scale density depletion and plasma line enhancements in a range of altitudes which far exceed that caused by the HF heater. Furthermore, they can interact with trapped energetic electrons in inner radiation belts at L=1.35 and trigger precipitation of electrons into the lower ionosphere. We suggest that disturbances in the ionosphere above NAU caused by whistler-mode signals can significantly affect heater-induced perturbations and partially explain unique results obtained at other heater sites.

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.