Mechanism Of Charge Production Research Articles

Characteristics of Sprite-Producing Positive Cloud-to-Ground Lightning during the 19 July 2000 STEPS Mesoscale Convective Systems

During the summer of 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) program deployed a three-dimensional Lightning Mapping Array (LMA) near Goodland, Kansas. Video confirmation of sprites triggered by lightning within storms traversing the LMA domain were coordinated with extremely low frequency (ELF) transient measurements in Rhode Island and North Carolina. Two techniques of estimating changes in vertical charge moment (Mq) yielded averages of ;800 and ;950 C km for 13 sprite-parent positive polarity cloud-to-ground strokes (1CGs). Analyses of the LMA’s very high frequency (VHF) lightning emissions within the two mesoscale convective systems (MCSs) show that 1CGs did not produce sprites until the centroid of the maximum density of lightning radiation emissions dropped from the upper part of the storm (7‐11.5 km AGL) to much lower altitudes (2‐5 km AGL). The average height of charge removal (Zq) from 15 sprite-parent 1CGs during the late mature phase of one MCS was 4.1 km AGL. Thus, the total charges lowered by spriteparent 1CGs were on the order of 200 C. The regional 08C isotherm was located at about 4.0 km AGL. This suggests a possible linkage between sprite-parent CGs and melting-layer/brightband charge production mechanisms in MCS stratiform precipitation regions. These cases are supportive of the conceptual MCS spriteproduction models previously proposed by two of the authors (Lyons and Williams).

Photoconductivity of α-Methylnaphthalene Alone and with an Incorporated Solute

Exposure of α-methylnaphthalene to strongly absorbed uv light and externally applied voltage produced photocurrent in the range of 10−9 to 10−7 A/cm2; their magnitude depended on the polarity of the illuminated electrode. The illumination through the positive electrode gave a photocurrent 15 times greater than when the negative electrode was illuminated. The mobilities of the positive and negative carriers were determined to be 1.2×10−4 and 5×10−5 cm2/V·sec, respectively. The former value is of the same order of magnitude as the theoretical value calculated for a positively charged α-methylnaphthalene ion. The temperature dependence of the mobility of the positive carrier indicated that the carrier may be either an ion or a hole with a long hopping time. Current—voltage measurements show that saturation is not reached at the highest field strength employed, 40 000 V/cm, and most likely would not be reached below fields of 106 V/cm. On this basis a theoretical efficiency of 1% for charge production was estimated. A non-Ohmic behavior of the photocurrent was observed, which suggested the presence of either a potential well at the electrode or the influence of space charge effects. The incorporation of a solute, 9,10-diphenylanthracene, reduced the current to 25% of that for the solvent alone, indicating extensive energy transfer to the solute and not simply to differences in mobility. The mechanism of charge production was interpreted either as an exciton interacting with the electrode to produce holes or as ionization of the solvent near the electrode with differences in recombination rates of the charges with the electrode.