First observations of the PMSE overshoot effect and its use for investigating the conditions in the summer mesosphere



[1] We report from the first campaign, with the EISCAT radars and heating facility, which looked for a proposed new PMSE overshoot effect resulting when PMSE is being affected by a specific cycling of artificial electron heating. The overshoot was predicted to appear after unaffected PMSE dusty plasma had been acted upon, for a comparatively short time, by the EISCAT Heating facility. We show model results and observational examples of the overshoot effect. For the overshoot to be clearly observable, a long relaxation time after the heater is switched off is necessary to get the dust charges back to those of dust unaffected by the heated electrons or, to bring unaffected PMSE dusty plasma into the radar beams by horizontal wind transport. The overshoot characteristic curve (OCC) contain a substantial amount of information on the conditions during PMSE and must lead to new possibilities in the study of PMSE and the conditions close to the summer mesopause.

1. Introduction

[2] It has recently been discovered that the radar phenomenon PMSE (Polar Mesospheric Summer Echoes, [Cho and Röttger, 1997]), which occur in the summer mesosphere between about 80 and 90 km, can be weakened if it is affected by artificial electron heating. This weakening of PMSE has been observed with the EISCAT VHF radar at 224 MHz [Chilson et al., 2000; Belova et al., 2003], and the EISCAT UHF radar at 933 MHz [La Hoz et al., 2003]. The EISCAT heating facility [Rietveld et al., 1993] used in these experiments was run in equal and short on and off intervals (from 10 to 20 sec) and it was found in many cases, but not all, that the PMSE strength was rapidly weakened when the heater was turned on, and that it also returned rapidly [Belova et al., 2003] to approximately the pre-heater value when the heater was turned off again.

[3] Rapp and Lübken [2000] showed that the electron density gradients within a collection of dust particles, which are a natural consequence if spatial density variations in charged dust are present [Havnes et al., 1984, 1990, 1992; Lie-Svendsen et al., 2003], will be rapidly smoothed if the electron temperature is raised by a considerable factor, such as is expected from artificial heating. When the heater is switched off, the electrons almost immediately cool to the neutral (and ion) temperature, and the electron spatial irregularities are re-established and the PMSE is strengthened again. In the observations by Chilson et al. [2000], Belova et al. [2003] and La Hoz et al. [2003], all run with equal and short on and off intervals, the PMSE returns approximately to the strength it had in the former off phase.

[4] With a time dependent version of the standard dusty plasma (dust-electron-positive ion) model [Havnes et al., 1984, 1990; Verheest, 2000], Havnes [2003] modelled the variations of the PMSE strength during overshoot heating cycles with the heater on for 20 sec and thereafter off for 160 sec. With dust, or aerosols, we here mean any solid particle (icy or mixed with e.g., metals) with sizes from about a nanometer and upwards. The PMSE modelling is based on the PMSE to be caused by scattering from many electron density gradients with characteristic lengths of the order of a half radar wavelength. In our case the electron gradients are within and are controlled by similar small dust density spatial irregularities, which we will call a dust cloud. The dust particles in a cloud are charged by electron and ion collisions and attachment, and the cloud itself will also be charged up and have a general electric potential V. Both the dust charges and dust cloud potential depend on the dust density and therefore on the position in the cloud, they also depend on the electron and ion temperature and their density outside the cloud, and on the ion mass. The potential V has its largest (negative) value in the centre of the dust cloud where the dust density is highest. At the summer mesospheric conditions, with its low temperature, the dust charges are low and predominantly negative with charge numbers normally somewhere in the range 0 to −4. The electric potential of the cloud due to the dust charging is also low, with a maximum value of the order of a few times the plasma temperature measured in eV, or around −.02 Volt. The electron and ion densities nα are Boltzman distributed in the cloud potential

equation image

Here n0 is the plasma density outside the dust, the plasma particle charge and temperature is qα and Tα respectively while kB is the Boltzman constant. The model also includes equations for a time dependent dust charging with plasma currents to the dust, and charge equilibrium. It is assumed that the plasma adjustment to the changes in electron temperature is short compared to the heating time. As examples of model results we show in Figure 1 two cases computed for a plasma density n0= 4 × 109 m−3 and an increase in the electron temperature from 150°K without the heater, to 390°K when the heater is on. We have used two different dust sizes as given in the figure. The dust density is nd = 109 m−3 for the case with particles of radius r = 10 nm and nd = 4 × 107 m−3 for the 50 nm large particles. These are values close to what can be observed in the PMSE region [von Cossart et al., 1999], and the values have been chosen so that the product ndr2 is the same for the two cases. The charging is by plasma attachment only. The relative PMSE backscatter is taken to be proportional to the electron density gradient in what we take as an average dust cloud. This will be a structure where at the center the dust density is nd, and at a distance of typically λ/2 from the center, the density for the cases in Figure 1 is reduced to 0.8 nd. The variation of the relative electron gradient (the varying gradient divided by the initial gradient) is not much dependent on the initial electron gradient but is mainly a function of the electron density, the dust size and density and the increase in electron temperature. For a calculation of the absolute value of the radar scattering, the absolute values of the electron gradients will be of importance. We have not considered such calculations. The different phases of the overshoot characteristic curves (OCC) in Figure 1 are indicated by the numbers (0, 1, 2 and 3). Initially the PMSE plasma is undisturbed with dust charges and plasma densities in equilibrium at the temperatures Te = Te = 150°K until point (0) when the heater is switched on, resulting in a rapid electron temperature increase [e.g., Kero et al., 2000; Belova et al., 2001]. The main effect of a temperature increase can be seen from Equation (1) which shows that the electron density profile will be flattened and its gradient therefore reduced. This results in the PMSE power drop from (0) to (1). If the electron temperature increase is large, the exponential term in Equation (1) will be ∼1 and the electron density will be approximately constant through the cloud at point (1). The magnitude of this drop is therefore mainly determined by the increase in the electron temperature. During the recovery, or heating phase (1–2) the dust will be charged more negatively by the electrons which, with their increased energy and increased density within the cloud, can charge the dust to a negative charge considerably above the charges at and above (0). This increases the potential V to a more negative value, and Equation (1) shows that this will to some degree restore the electron depletion at the dust cloud centre and cause some recovery of the PMSE. When the heater is switched off at (2), and the electron temperature falls back to the value of Te= Ti, the increased dust charges and increased potential V at (2) compared to its value at (0) and (1) will lead to a stronger electron depletion in the cloud centre at (3) than before the heater was switched on. This leads to a larger electron gradient also and consequently a PMSE overshoot where the PMSE strength is larger at (3) than at (0). After (3) the PMSE dusty plasma relaxes back to its undisturbed state, in a process where the dust is gradually losing negative charge until it reaches the lower equilibrium charges for Te = Ti ∼ 150°K.

Figure 1.

Model calculations of PMSE variations for a heater cycling with the heater on for 20 sec (starting at t = 20 sec) and thereafter off for 160 sec. The full cycle period is not shown. The calculations are for dust radius of 10 and 50 nm, the electron temperature increase from 150°K when the heater is off to 390°K when the heater is on. The electron density is 4 × 109 m−3. The dust densities are nd(r = 10 nm) = 109 m−3 and nd(r = 50 nm) = 4 × 107 m−3. The ion mass is 50 amu.

[5] The reason for the long off phase in the model calculations and in the new observations, was to give the dusty plasma sufficient time to relax back to its undisturbed conditions, or to allow horizontal wind transport to bring undisturbed dusty plasma into the radar beam. Model calculations show that with equal and short heater on and off intervals, the dust particles continue to be charged up by subsequent heater on phases, to a saturation value where de-charging during an off phase balanced the charging during an on phase, or alternatively to a maximum charging set by the time for horizontal winds to transport new PMSE dusty plasma into the radar beam. For the former high duty-cycle heater cycling, the difference in electron densities in subsequent off phases will be small and give the appearance that the PMSE returns to its undisturbed value as soon as the heater is switched off. In fact, the electron density gradients in the saturated off intervals will be considerably stronger than for identical PMSE conditions, which are not affected by the heater. The strength of the PMSE during saturated off phases will therefore in general be stronger than that of an identical undisturbed PMSE. The difference in PMSE strength during saturated on and off phases will therefore be larger than the difference between the strength of an undisturbed PMSE and its reduced intensity as the electrons are heated. One of the advantages of the new cycling is that if the heater is allowed to operate on undisturbed PMSE, the difference in PMSE power between the phase just before the heater is turned on and the phase just after it is turned off is maximized. The relaxation time, and other details of the overshoot will lead to additional information on the PMSE conditions.

[6] An observational campaign looking for and finding the overshoot effect was performed on the days 27 June, and 1 and 2 July 2003. The data are still being analysed but we here show some examples of the overshoot, which was repeatedly seen during the three hours we observed on the 2nd of July 2003.

2. Observations of the PMSE Overshoot Effect

[7] The heater cycling used was 20 sec on and 160 sec off. The heater transmitted in the O-mode with a frequency 5.423 MHz and 600 MW Effective Radiated Power [Rietveld et al., 1993]. PMSE was present practically all the time at 224 MHz but PMSE was not observed with the UHF radar at 933 MHz. The overshoot effect was seen most of the time a PMSE was present. In Figure 2 we show a sequence of overshoots during a 12 min period starting at around 8.7 hour (UT), when the PMSE layer was comparatively stable in strength and in height. The lower part of the figure shows the raw data, corrected for transmitter power, with a height resolution of 300 m and integration period of 5 sec. The scale gives the relative power of the PMSE backscatter in linear but arbitrary units. The background noise is at approximately 2500 on the scale being used. In the upper figure we show the same data but now smoothed by the MATLAB “contour” program. In the middle panel we show the sum of the three highest values of the PMSE at each 5 sec sample. This can be taken as a measure of the total intensity of the PMSE as a function of time. The heater 20 sec on phase is between the vertical red lines. We see in each cycle the clear and strong overshoot effect in the first sample after the heater is switched off. This is apparent also in the last overshoot where the PMSE is weak. Another, and somewhat surprising effect in these overshoots are the rapid and strong recovery of the PMSE during the heater on phase (1–2). As shown in Figure 1 this is probably a signature of that the PMSE dust is dominated by comparatively large PMSE dust particles. In the cases shown we see that the PMSE strength has recovered and become considerably stronger than before the heater was switched on already during the heater on phase. Normally, as can be seen from Figure 3, we observe some, but much smaller, recovery during the heater on phase. Figure 3 shows the OCC of all 20 heater cycles (each of 3 min duration) in the hour from 0800 to 0900 UT. The intensities have been normalized to 1 at the 3rd sample before the heater was turned on. We clearly see the reduced PMSE intensity as the heater is switched on, the overshoot as it is switched off and the following decay back towards the undisturbed PMSE level. The two cases which does not decay are presumably when the edge of a large PMSE structure enters the beam early in a cycle due to mesospheric winds, and the overall PMSE strength builds up as more of this structure fills the radar beam cross section during the cycle.

Figure 2.

This shows a string of 4 overshoots in a 12 min interval on 2 July 2003. In the bottom figure we show the raw data, corrected for radar transmitter power, while the upper figure are the same data but now smoothed. The relative intensity scale is the same for both figures, and the background noise on this linear scale is at approximately 2500. The middle figure shows the sum of the three highest intensities at each time sample and is a measure of the total PMSE intensity as a function of time. The heater on phase is between the vertical red lines.

Figure 3.

The PMSE intensity of each of the 3 min heating cycles during one hour from 0800 to 0900 UT on July 2, 2003. The PMSE intensities have been normalized to 1 in the first sample. We show only 100 sec of each cycle.

3. Discussion

[8] We have shown examples where the predicted overshoot effect is present in radar PMSE observations, when the artificial electron heater is acting on undisturbed PMSE dusty plasma and where this plasma, after the heater is switched off, is given enough time to relax back to its undisturbed value. So far the overshoot effect has been observed at a frequency of 224 MHz only, but since it has been demonstrated that the heater can weaken saturated PMSE also at 933 MHz [La Hoz et al., 2003], we have little doubt that the overshoot will be observed at this and other frequencies as well.

[9] The observations are influenced by natural fluctuations in the PMSE power, most likely because PMSE horizontal structural variations are carried into and across the radar beam by horizontal mesospheric winds. This will introduce some uncertainty in the results from the different overshoot characteristic curves. The present observations are also hampered by a comparatively long radar PMSE power integration time of 5 sec. This makes it difficult to identify the exact shape of the characteristic curve around point (1), and the minimum value of the PMSE, and also its shape around point (3). Future observations must reduce the radar integration time to 1 sec or lower to resolve the details of the OCC.

[10] Observations of the details of the OCC should lead to information on the dust and plasma conditions in the PMSE. If our model is correct, the reduction in PMSE intensity from (0) to (1) is controlled by the electron temperature increase and will give information on this. The recovery due to the additional charging during the phase (1–2) depends mainly on the amount and size of the dust particles which are present. As we see from Figure 1, a PMSE with small dust particles will recover slowly since their charging time is long, while a PMSE with large dust recover much more rapidly. A small amount of recovery is therefore most likely a signature of small dust while a larger recovery indicates that large dust dominate the PMSE dusty plasma. The overshoot will also depend on the amount of additional charging during phase (1–2): If there is little additional charging the overshoot will be small since the electric dust cloud potential does not change much, but for larger dust and a larger recovery the overshoot will also become larger.

[11] The time scale of the relaxation process from (3) and onwards, is mainly dependent on how fast dust is being de-charged. If the relaxation is faster than what can reasonably be produced by ion attachment on to dust, it will be an indication that photo-detachment [Weingarten and Draine, 2001] is important. The relaxation times of the observations in Figure 3, with halftimes from 15 sec and upwards, can be reproduced for dust of up to 50 nm in size and with electron densities of the order of 4 × 109 m−3. It appears that for the cases in Figure 3 the photo-detachment does not play a decisive role in the charging of PMSE dust.

[12] In vertically extended PMSE layers we will be able to extract information on how conditions vary with heights by measuring the overshoot in each height interval. The potential of the overshoot effect is therefore considerable and we expect that its discovery and exploitation will lead to a new situation in the investigation of conditions near the mesopause during PMSE events.


[13] The EISCAT Scientific Association is funded by SA (Finland), CNRRS (France), MPG (Germany), NIPR (Japan), RCN (Norway), NFR (Sweden), and PPARC (UK). This research was conducted under grants from the Research Council of Norway. The authors would like to thank the two referees for their comments which helped improve the paper.