E. Belova

First artificially induced modulation of PMSE using the EISCAT Heating Facility

A coordinated experiment involving ionospheric heating and VHF observations of polar mesosphere summer echoes (PMSE) has recently been conducted at the EISCAT facility near Tromso, Norway. We have demonstrated for the first time that ionospheric heating can influence VHF radar returns associated with PMSE. Artificially elevating the electron temperatures within the PMSE layer has been shown to reduce the echo power. Based on this and other results from the experiment, it is suggested that the observed reduction in PMSE power is related to an enhancement of the electron diffusivity through the heating.

The effect of electron bite-outs on artificial electron heating and the PMSE overshoot

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 90km, where noctilucent clouds (NLC) and the radar phenomenon PMSE (Polar Mesospheric Summer Echoes) are observed. With an electron density profile without bite-outs, 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 bite-out 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 EISCAT 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 density 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 (933MHz) 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.

Electron temperature dependence of PMSE power: experimental and modelling results

Abstract Joint PMSE - ionospheric heating experiments using the EISCAT VHF radar and the Heating facility were conducted at the EISCAT site near Tromso during the period July 9–14, 1999. Height profiles of backscattered power for four different combinations of heating parameters and ionospheric conditions are presented and analysed with the help of an ionospheric heating model. The decrease of PMSE power during heating has a height dependence and varies from one case to another. This variability of the heating effect on PMSE power may be related to height-dependence and case-to-case variation of the electron temperature enhancements due to the ionospheric heating as well as to differing undisturbed levels of PMSE power. The possible relation of electron recombination, diffusion, and aerosol charging to the observed reduction of PMSE power during heating is discussed. An increase of electron diffusivity due to the temperature enhancement seems to be the most likely factor affecting PMSE during the experiments.

Reply to comment by M. Rapp and F.‐J. Lübken on “The response time of PMSE to ionospheric heating”

[1] The comment by Rapp and Lubken [2003b] (hereinafter referred to as R&L) on our paper [Belova et al., 2003] (hereinafter referred to as BCKR) contains two major points. The first pertains to an extension to their own work [Rapp and Lubken, 2000] reducing the relevant time constant. The second relates to the correctness of using the electron Debye length in formula (1) of BCKR for plasma where the electron temperature Te is much more than the ion temperature Ti. We reply first to the second point, which is the most important one, and then consider the first.