M.C. Lee

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.

Generation of ELF and VLF waves by HF Heater‐modulated polar electrojet via a thermal instability process

Generation of ELF and VLF waves via a thermal instability process in a HF-modulated polar electrojet has been investigated. It is shown that a positive feedback through the electron-neutral collisional heating process can cause the transient response of the plasma to the modulated HF heater to grow exponentially. The threshold fields of the instability under normal electrojet conditions are found to be about 2.25 V/m and 1.13 V/m for the operation of the o-mode and x-mode heaters with a 50% duty cycle, respectively. For a heater wave field of, e.g., 1.5 V/m, the instability can be excited by the x-mode heater within a few tens of millisecond. The predicted dependence of the ELF/VLF radiation amplitude on the percentage of the duty cycle of the HF modulation is identical to that of the recent observational results of Barr and Stubbe. 28 refs., 2 figs.

Stimulated thermal instability for ELF and VLF wave generation in the polar electrojet

Generation of ELF and VLF waves in the HF heating wave modulated polar electrojet is studied. Through the Ohmic heating by the amplitude-modulated HF heating wave, the conductivity and thus the current of the electrojet is modulated to set up the ionospheric antenna current. However, it is shown that a stimulated thermal instability is also excited by the amplitude-modulated HF heating wave. This instability introduces an electron temperature modulation more effectively than that by the passive Ohmic heating process and is expected to improve considerably the intrinsic efficiency of ELF and VLF wave generation by the amplitude-modulated HF heating wave. Moreover, the generation efficiency and signal quality also depend on the HF wave modulation scheme. Thus, four amplitude-modulation schemes are examined and compared.

Numerical comparison of two schemes for the generation of ELF and VLF waves in the HF heater‐modulated polar electrojet

Generation of ELF and VLF waves in the HF heater modulated polar electrojet is numerically studied. Illuminated by an amplitude modulated HF heater, the electron temperature of the electrojet is modulated accordingly. This, in turn, causes the modulation of the conductivity and thus the current of the electrojet. Emissions are then produced at the modulation frequency and its harmonics. The present work extends the previous one on a thermal instability to its nonlinear saturation regime. Two heater modulation schemes are considered. One modulates the heater by a rectangular periodic pulse. The other one uses two overlapping heater waves (beat wave scheme) having a frequency difference equal to the desired modulation frequency. It is essentially equivalent to a sinusoidal amplitude modulation. The nonlinear evolutions of the generated ELF and VLF waves are determined numerically. Their spectra are also evaluated. The results show that the signal quality of the second (beat wave) scheme is better. The field intensity of the emission at the fundamental modulation frequency is found to increase with the modulation frequency, consistent with the Tromso observations.

Explosive spread F caused by lightning-induced electromagnetic effects

Abstract The lightning-produced electromagnetic effects may produce significant modifications in the ionospheric plasmas. An outstanding phenomenon investigated in this paper is the so-called “explosive spread F”, whose close link with lightning has been identified ( woodman R. F. and Kudeki E. , 1984, Geophys. Res. Lett . 11 , 1165). The parametric instability excited by the lightning-induced whistler waves is proposed as a potential source mechanism causing the explosive spread F. Some observed striking features of this phenomenon can be reasonably explained by the proposed mechanism.

Interhemispheric propagation of VLF transmissions in the presence of ionospheric HF heating

Recent experiments carried out at the Arecibo Observatory in Puerto Rico and the magnetically conjugate location in Argentina suggest a connection between the prevalence and intensity of the interhemispheric propagation of VLF transmissions and HF heating of the ionosphere. During several nights of the study the O-mode CW heater appeared to enhance the coupling of VLF transmissions into the affected region of plasma. These signals presumably then entered plasmaspheric ducts more efficiently and led to conjugate measurements of relatively high amplitude. No such effects were seen with pulsed or X-mode heating. Short time resolution measurements of fading transmissions were marked by transit delays and amplitudes that remained nearly constant as the interhemispheric VLF signals diminished, consistent with the model of a plasmaspheric duct complemented by favorable plasma structures extending well into the F region but producing the majority of the total transit delay at plasmaspheric altitudes. It is also suggested that weak long-delay receptions often observed in these data are most probably prolongitudinal whistlers that are not ducted but which represent a potential contaminant in ducted VLF experiments of all types.

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.

Parametric excitation of lower hybrid waves by Z-mode waves near electron cyclotron harmonics at Tromso

Z-mode wave converted from O-mode heater wave is considered as the pump wave in Tromso HF heating experiments for the parametric excitation of lower hybrid decay mode together with electron Bernstein sidebands in the region above the O-mode reflection layer. This is the process, suggested by Mishin et al. [1997], which generates superthermal electrons by the excited lower hybrid waves. These superthermal electrons produce plasma waves in the region above the O-mode reflection layer, with frequencies much greater than the heater wave frequency, as observed by Isham et al. [1990, 1996]. Our detailed analyses of this process are carried out for two experimental cases wherein the pump wave frequencies are near the third and fifth harmonic electron cyclotron frequencies, respectively. The results show that the minimum threshold field of the instability in either case is less than 0.2 V m−1. However, because the wavelength of the instability is also short, the collisionless dampings restrict the instability occurrence to narrow wavelength and frequency ranges. For 1 V m−1 pump field intensity, the instability can be excited in less than 1 ms. Since the excited lower hybrid waves are strongly Landau damped, superthermal electrons are efficiently generated by them.

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.