The Lorentz force on a charged dust particle near the ecliptic plane is mostly either upward or downward depending on the polarity of the magnetic field. In the magnetic reference frame, the meteoroid moves inward at about the same speed because its orbital speed is comparatively small. For an observer or a meteoroid in interplanetary space, the magnetic field sweeps outward at the speed of the solar wind (400 to 600 km/s). The polarity of the magnetic field can be positive or negative depending on the polarity at the base of the field line in the solar corona, which varies spatially and temporally. Due to the rotation of the Sun (at a period of 25.7 days), magnetic field lines are drawn in a spiral, like water from a lawn sprinkler. The outward-streaming (away from the Sun) solar wind carries a magnetic field away from the Sun. UV radiation releases photoelectrons, electrons, and ions from the solar wind plasma, and they are collected the impact of energetic particle radiation releases secondary electrons. Charging processes of meteoroids in interplanetary space. In the dense plasma of the inner Saturnian magnetosphere, dust particles at –2 V potential have been found.įIGURE 13. These measurements indicate a dust potential of +5 V. Electric charges on dust particles in interplanetary space have been measured by the Cassini Cosmic Dust Analyzer.
The timescale for charging is seconds to hours depending on the size of the particle small particles charge slower. The final charging state is reached when all currents to and from the meteoroid cancel. Only at times of very high solar wind densities does the electron flux to the particle dominate and the particle gets charged negatively. Because of the predominance of the photoelectric effect in interplanetary space, meteoroids are mostly charged positively at a potential of a few volts. Whether electrons or ions can reach or leave the grain depends on their energy and on the polarity and electrical potential of the grain. Energetic ions and electrons then cause the emission of secondary electrons. Electrons and ions are collected from the ambient solar wind plasma. Irradiation by solar ultraviolet (UV) light frees photoelectrons, which leave the grain. Particular attention will be given to the nature of the radio reflections from meteors and to illustrating the capabilities, strengths, and weaknesses of such radars for studying the dynamics of the meteor region of the atmosphere.Īny meteoroid in interplanetary space will be electrically charged, and several competing charging processes determine the actual charge of a meteoroid ( Fig. Many types of radar can detect meteors, but here we will concentrate on purpose-built meteor radars used to investigate the dynamics and structure of the mesosphere and lower thermosphere.
Meteor radars have thus played an important role in elucidating many aspects of the dynamics of this part of the atmosphere, particularly with regard to the mean winds, tides, and planetary waves that dominate the flow at these heights. These regions are notoriously difficult to investigate experimentally because they are too high for in situ measurements other than those made by rockets. This is the upper part of the middle atmosphere and spans the upper mesosphere, the mesopause, and the lower thermosphere. Most radio meteor echoes are detected at heights between ∼70 and 110 km. Meteor trails are carried by the winds at the height where they form, and so HF- and VHF-radar Doppler measurements of the drift velocities of radio meteors allow determination of these winds – essentially using the meteors as tracers of atmospheric motion. Measuring radio waves reflected from the ionized trails that meteors create has proved a powerful tool in investigations of meteors and the region of the atmosphere in which they occur. Meteors, colloquially ‘shooting stars’, are among the most beautiful and striking phenomena of the naked-eye night sky. Mitchell, in Encyclopedia of Atmospheric Sciences, 2003 Introduction
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