engleski

Electrophonic Sounds and VLF Radiation from Meteors

ntroduction Meteors - "falling stars", are quite common: on a clear, dark night one can see 5-10 per hour, more than 100 on the nights when meteor showers are at their maximum. Meteors are pieces of comets and asteroids, with masses between 1 mg and 10 kg. They hit our atmosphere with speeds between 20 and 70 km/s, glow and burn-out at heights of about 100 km. The so called bolids (meteors brigter than about full Moon) are very rare: few per year can be seen. Strange sounds and even smells are occasionally related with them. The problem is that sound would need several minutes to reach the observer, so reports of sounds heard simultaneously with a bolide are often disregarded with a common explanation of beeing "physological effects!". This problem is very old: old chinese records from 817 mention a meteor that produced a sound "simmilar to birds in flight". Sir Edmund Halley in the beginning of 18th century regarded them to be psychological effects, which stopped any serious research for the next two centuries. Modern research started again only about 20 years ago. Today it is known that these sound are of electrical origin, thus the name "electrophonic" sounds. What is not clear yet is the actual physics of their production, but there are indications that meteor-produced VLF (very low frequency, ca 300Hz-30 kHz) electromagnetic waves may be responsible. Research in Croatia Research of natural VLF has a great educational potential because valuable data can be gathered even with an amateur equipment. The first activities in Croatia started in 1992 at the Višnjan Observatory, when a group of secondary school pupils and students under my supervision tried to detect a meteor-produced VLF radiation. These activities were continued in the following years, with the group working more and more independently. Also, contacts with the NASA-supported INSPIRE project (Interactive NASA Space Physics Ionosphere Radio Experiments targeted to high school students) were made, and we started to use receivers they designed for natural VLF reception. Finally, in 1998 the group planned, organized and carried out an expedition to Mongolia to watch for VLF activity during the Leonid meteor storm of that year. It is during this expedition that first positive instrumetal recording of an electrophonic sound was obtained, and later published in the prestigious Journal of Geophysical Research. The group, whose members in the meantime finished their studies, continued to work on this project, although its members are now spread all over the world doing graduate and postdoctoral research in unrelated fields. In 2000 they started out the Global Electrophonic Field Survey (GEFS) devoted to gathering, standardising and analysing reports of meteor produced electrophonic sounds. Thus far, about 100 reports are gathered and analysed, and resuls are published in the WGN, the Journal of the International Meteor Organization. Also, a new generation of pupils under the leadership of members of the first group has stared their research on the meteor phenomena at the Višnjan Observatory.

VLF radiation

nije evidentirano