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Ann. Geophys., 25, 623-637, 2007
www.ann-geophys.net/25/623/2007/
© European Geosciences Union 2007
On the secondary charging effects and structure of mesospheric dust particles impacting on rocket probes
O. Havnes and L. I. Næsheim
Institute of Physics, University of Tromsø, Norway
Abstract. The dust probe DUSTY, first launched during the summer of 1994
(flights ECT–02 and ECT–07) from Andøya Rocket Range, northern Norway,
was the first probe to unambiguously detect heavy charged mesospheric
aerosols, from hereon referred to as dust. In ECT–02
the probe detected negatively charged dust particles in the height
interval of 83 to 88.5 km.
In this flight, the lower grid in the detector (Grid 2) measures both
positive and negative currents in various regions, and we find that the
relationship between the current measurements of Grid 2 and the bottom
plate can only be explained by influence from secondary charge
production on Grid 2.
In ECT–07, which had a large coning,
positive currents reaching the top grid of the probe were interpreted as due
to the impact of positively charged dust particles. We have now reanalyzed
the data from ECT–07 and arrived at the conclusion that the measured
positive currents to this grid must have been mainly due to secondary
charging effects from the impacting dust particles. The grid consists of
a set of parallel wires crossed with an identical set of wires on top of
it, and we find that if the observed currents were created from the direct
impact of charged dust particles, then they should be very weakly modulated at
four times the rocket spin rate ωR. Observations show, however, that the
observed currents are strongly modulated at 2ωR. We cannot
reproduce the
observed large modulations of the impact currents in the dust layer if the
currents are due only to the transfer of the charges on the impacted dust
particles.
Based on the results of recent ice cluster impact secondary charging
experiments by
Tomsic (2003),
which found that a small fraction of
the ice clusters, when impacting with nearly grazing incidence,
carried away one negative charge −1e, we have
arrived at the conclusion that similar, but significantly more
effective, charging effects must be
predominantly responsible for the positive currents measured by the top
grid in ECT–07 and their large rotational modulation at 2ωR.
Since the secondary effect is dependent on
the size of the impacting dust, this opens up for the possibility of
mapping the relative dust sizes throughout a dust layer by comparing
the observed direct and secondary currents.
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