Abstract.We have considered the effect that a local reduction in the electron density (an electron bite-out), caused by electron absorption on to dust particles, can have on the artificial electron heating in the height region between 80 to 90 km, where noctilucent clouds (NLC) and the radar phenomenon PMSE (Polar Mesospheric Summer Echoes) are observed. With an electron density profile without biteouts, the heated electron temperature T e,hot will generally decrease smoothly with height in the PMSE region or there may be no significant heating effect present. Within a biteout T e,hot will decrease less rapidly and can even increase slightly with height if the bite-out is strong. We have looked at recent observations of PMSE which are affected by artificial electron heating, with a heater cycling producing the new overshoot effect. According to the theory for the PMSE overshoot the fractional increase in electron temperature T e,hot /T i , where T i is the unaffected ion temperature = neutral temperature, can be found from the reduction in PMSE intensity as the heater is switched on. We have looked at results from four days of observations with the EIS-CAT VHF radar (224 MHz), together with the EISCAT heating facility. We find support for the PMSE overshoot and heating model from a sequence of observations during one of the days where the heater transmitter power is varied from cycle to cycle and where the calculated T e,hot /T i is found to vary in proportion to the transmitter power. We also looked for signatures of electron bite-outs by examining the variation of T e,hot /T i with height for the three other days. We find that the height variation of T e,hot /T i is very different on the three days. On one of the days we see typically that this ratio can increase with height, showing the presence of a bite-out, while on the next day the heating factor mainly decreases with height, indicating that the fractional amount of dust is low, so that the electron density is hardly affected by it. On the third day there is little heating effect on the PMSE layer. This is probably due to a sufficiently high electron densityCorrespondence to: O. Havnes (oha003@asp.uit.no) in the atmosphere below the PMSE layer, so that the transmitted heater power is absorbed in these lower layers. On this day the D-region, as given by the UHF (933 MHz) observations, extends deeper down in the atmosphere than on the other two days, indicating that the degree of ionization in and below the PMSE layers is higher as well.

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[1] In the present paper, recent radar and heating experiments on the polar mesospheric winter echoes (PMWE) are analyzed with the radar overshoot model. The PMWE dust particles that influence the radar backscatter most likely have sizes around 3 nm. For dust to influence the electrons in the PMWE layers, it must be charged; therefore, we have discussed the charging of nanometer-sized particles and found that the photodetachment effect, where photons of energy less than the work function of the dust material can remove excess electrons, probably is dominant at sunlit conditions. For moderate and low electron densities, very few of the dust smaller than $3 nm will be charged. We suggest that the normal requirement that disturbed magnetospheric conditions with ionizing precipitation must be present to create observable PMWE is needed mainly to create sufficiently high electron densities to overcome the photodetachment effect and charge the PMWE dust particles. We have also suggested other possible effects of the photodetachment on the occurrence rate of the PMWE. We attribute the lack of PMWE-like radar scattering layers in the lower mesosphere during the summer not only to a lower level of turbulence than in winter but also to that dust particles are removed from these layers due to the upward wind draught in the summer mesospheric circulation system. It is likely that this last effect will completely shut off the PMWE-like radar layers in the lower parts of the mesosphere.

[1] We present an analysis of relatively strong Polar Mesospheric Winter Echoes (PMWE) under artificial electron heating that changes the PMWE intensity. A major purpose is to find reliable estimates of the relaxation time of the heater modified PMWE to their undisturbed state during the heater switch-off phase; the implications regarding charge/ discharge mechanisms; and to exploit the diagnostic potential of artificial electron heating. The relaxation time is between 60 to 70 s for the regions with strong PMWE layers and substantial electron heating. This short relaxation time, related to the variation of charges on the nanometer dust which most likely is present in PMWE, rules out ion attachment as the mechanism to bring the dust charges to their equilibrium state. Neutral winds, sweeping the heated electrons out of the radar beam, are unlikely to be the cause of the observed relaxation, since this requires winds of around 100 m s À1 . The most probable cause is photo detachment by which negatively charged dust can lose excess electrons by photon absorption with energies less than the dust material's work function. By comparing the observed heating with heating model profiles, the electron density at 65 km height must have been of the order of 3 Â 10 9 m À3 . This agrees with PMWE occurring mainly during disturbed conditions with high electron densities. Our results also indicate that in the strongest PMWE layers, electron bite-outs exist consistent with the role of charged dust particles in the mechanism of PMWE and implying larger dust densities.

Abstract. We report measurements of dust currents obtained with a small probe and a larger probe during the flight of the ECOMA-4 rocket through the summer polar mesosphere. The payload included two small dust probes behind a larger dust probe located centrally at the front. For certain phases of the payload rotation, the current registered by one of the small dust probes was up to 2 times the current measured with the larger probe, even though the effective collection area of the larger probe was 4 times that of the small one. We analyze the phase dependence of the currents and their difference with a model based on the assumption that the small probe was hit by charged dust fragments produced in collisions of mesospheric dust with the payload body. Our results confirm earlier findings that secondary charge production in the collision of a noctilucent cloud/Polar Summer Mesospheric Echo (NLC/PMSE) dust particle with the payload body must be several orders of magnitude larger than might be expected from laboratory studies of collisions of pure ice particles with a variety of clean surfaces. An important consequence is that for some payload configurations, one should not assume that the current measured with a detector used to study mesospheric dust is simply proportional to the number density of ambient dust particles. The higher secondary charge production may be due to the NLC/PMSE particles containing multiple meteoric smoke particles.