Solar magnetic fields and the corona

Results from a seminar on the role of solar magnetic fields in determining the structure of the corona and interplanetary medium are presented. Some topics considered are: (1) polar regions of the sun which have identical magnetic configurations but display a vastly different appearance in the corona, (2) the influence of latitude on coronal and interplanetary microstructure, and (3) the role of the corona in contributing to the solar wind. It is proposed that an out-of-the-ecliptic solar probe mission would provide needed information on the above topics, and the study of solar activity in general

Coronal magnetic fields and the solar wind

Current information is presented on coronal magnetic fields as they bear on problems of the solar wind. Both steady state fields and coronal transient events are considered. A brief critique is given of the methods of calculating coronal magnetic fields including the potential (current free) models, exact solutions for the solar wind and field interaction, and source surface models. These solutions are compared with the meager quantitative observations which are available at this time. Qualitative comparisons between the shapes of calculated magnetic field lines and the forms visible in the solar corona at several recent eclipses are displayed. These suggest that: (1) coronal streamers develop above extended magnetic arcades which connect unipolar regions of opposite polarity; and (2) loops, arches, and rays in the corona correspond to preferentially filled magnetic tubes in the approximately potential field

Results Based Risk Management Education

Risk and Uncertainty, Teaching/Communication/Extension/Profession,

Measurement of the fundamental modulation response of a semiconductor laser to millimeter wave frequencies by active-layer photomixing

The room-temperature modulation response of a GaAs/GaAlAs semiconductor laser (relaxation resonance frequency, vR=6.5 GHz) is measured to 37 GHz using the active-layer photomixing technique. The measured response function agrees with the theoretical ideal, and there is no indication of device parasitic effects. An ultrahigh-finesse Fabry–Perot interferometer is used to detect the optical modulation, which appears as sidebands in the laser field spectrum. With a moderately faster laser diode (i.e., vR~10 GHz), the modulation response should be measurable beyond 100 GHz

Low-temperature measurement of the fundamental frequency response of a semiconductor laser by active-layer photomixing

We use the active-layer photomixing technique to directly modulate the output of a GaAs semiconductor laser operating at temperatures as low as 4.2 K. The technique produces modulation with nearly perfect immunity to device parasitic effects, revealing the laser diode's intrinsic modulation response. At 4.2 K the parasitic corner frequency is estimated to be 410 MHz, yet the response appears ideal out to 15 GHz. We measure the dynamical parameters governing the response function, the relaxation resonance frequency, and the damping rate, and discuss their low-temperature behavior

Amplitude-phase decorrelation: a method for reducing intensity noise in semiconductor lasers

It has been shown that the method of amplitude-phase decorrelation can reduce the fundamental intensity noise floor of semiconductor laser light over a wide bandwidth by the ratio 1/(1+α^2), where α is the linewidth enhancement factor. The method uses a dispersive element to convert phase noise into intensity noise. This technique was recently demonstrated by reducing intensity noise from a DFB (distributed feedback) laser as much as 7 dB below its intrinsic level. In the present work, the authors extend these results by characterizing the frequency dependence of the noise reduction. Optimum reduction is achieved in the flat region of the spectrum and diminishes at higher frequencies approaching the relaxation resonance. The correlation properties of the fluctuations are also investigated

Intensity noise reduction in semiconductor lasers by amplitude-phase decorrelation

Detuned operation of a laser results in coupling of field amplitude and phase fluctuations. In a semiconductor laser, this coupling is known to be very large. We demonstrate that it can be used to significantly reduce intensity noise below its intrinsic limit

Parasitic-free modulation of semiconductor lasers

Active-layer photomixing is a technique for modulating semiconductor lasers with nearly perfect immunity to device parasitics. Measurements of the intrinsic modulation response of a laser diode using this technique at temperatures as low as 4.2 K are discussed. From these measurements, the temperature dependence of important dynamical parameters is determined. In addition, this provides a stringent test of the active-layer photomixing technique since parasitic response is degraded, while the intrinsic response is improved for low-temperature operation. At 4.2 K, the ideal intrinsic response is measured for frequencies as high as 15 GHz despite an estimated parasitic corner frequency of 410 MHz

Equivalent circuit model for active-layer photomixing: Parasitic-free modulation of semiconductor lasers

Direct modulation of a laser diode by active-layer photomixing is studied in terms of an equivalent circuit model. The model shows that this modulation technique achieves nearly perfect immunity to package, chip, and junction-related parasitic effects so that the measured modulation response reflects the intrinsic carrier-photon dynamics. The nonlinear gain effect is included in the model