Continued progress in electrophonic fireball investigations

The mechanism proposed in 1980 by Keay to explain the occasional observation of instantaneous electrophonic sounds from large meteor fireballs continues to gain support. This mechanism accounts for many of the empirical features of the phenomenon, and the detection of ELF electromagnetic waves by direct transduction explains various other geophysical electrophonic phenomena including early sounds from seismic events. The extension of Ceplecha's fireball model to include Revelle's criterion for turbulence leads to realistic estimates for the frequency of occurrence of electrophonic fireballs. It also appears possible that the type of electrophonic sound observed relates to the composition of the fireball.Geophysical electrophonic phenomena may explain many baffling reports from ancient historical writings.     

Altitudinal Distribution Of 20 Persistent Meteor Trains: Estimates Derived From Metro Campaign Archives

Observations of persistent meteor trains are limited because of the extreme rarity of the phenomenon. The altitudinal distribution of persistent trains has previously been investigated via limited instances of simultaneous observation from multiple sites, however, a statistical study of persistent trains has yet to be realized. The meteor train observation (METRO) campaign was established in Japan in 1998 to obtain images of persistent trains. From 1998 to 2002, the METRO campaign, involving Japanese amateur collaborators, captured more than 400 image sequences of persistent trains, resulting in 53 simultaneous multi-site observations. Several Japanese observers were involved in imaging persistent trains prior to the METRO campaign, producing 6 simultaneous observations over the period 1988–1997. In this paper, simultaneous multi-site observations of high spatial and temporal resolution are used to determine, via triangulation, the altitudinal distribution of persistent trains for 20 events. The altitudinal range of 2 Orionid trains was slightly higher than that of 18 Leonid trains. The Leonid train data reveal no clear dependence of upper and lower altitude on the brightness of the associated fireball. The upper altitude of the 18 observed Leonid trains were almost invariant with respect to local time (LT) of observation, however, a possible dependence of lower altitude on LT of observation was also found, indicating a near-constant penetrating path-length in the upper atmosphere for each train. The average upper altitude of persistent Leonid trains was 99.8 km, while the average central altitude was 93.0 km. These trends are probably determined by atmospheric conditions such as the abundance of O and O₃.     

An extended Heitler–Matthews model for the full hadronic cascade in cosmic air showers

The Heitler–Matthews model for hadronic air showers will be extended to all the generations of electromagnetic subshowers in the hadronic cascade. The analysis is outlined in detail for showers initiated by primary protons. For showers initiated by iron primaries the part of the analysis is given for as far as it differs from the analysis for a primary proton. Predictions for shower sizes and the depth of maximum shower size are compared with results of Monte Carlo simulations. The depth of maximum as it follows from the extrapolation of the Heitler–Matthews model restricted to the first generation of electromagnetic subshowers is too small with respect to Monte Carlo predictions. It is shown that the inclusion of all the generations of electromagnetic subshowers leads to smaller predictions for the depth of maximum and to smaller predictions for the elongation rate. The discrepancy between discrete model predictions and Monte Carlo predictions for the depth of maximum can therefore not be explained from the number of generations that is taken into consideration. An alternative explanation will be proposed.     

Radio pulses from ultra-high energy atmospheric showers as the superposition of Askaryan and geomagnetic mechanisms

Radio emission in atmospheric showers is currently interpreted in terms of radiation due to the deviation of the charged particles in the magnetic field of the Earth and to the charge excess (Askaryan effect). Each of these mechanisms has a distinctive polarization. The complex signal patterns can be qualitatively explained as the interference (superposition) of the fields induced by each mechanism. In this work we explicitly and quantitatively test a simple phenomenological model based on this idea. The model is constructed by isolating each of the two components at the simulation level and by making use of approximate symmetries for each of the contributions separately. The results of the model are then checked against full ZHAireS Monte Carlo simulations of the electric field calculated from first principles. We show that the simple model describes radio emission at a few percent level in a wide range of shower-observer geometries and on a shower-by-shower basis. As a consequence, this approach provides a simple method to reduce the computing time needed to accurately predict the electric field of radio pulses emitted from air showers, with many practical applications in experimental situations of interest.