Multiple-frequency studies of the high-latitude summer mesosphere : implications for scattering processes

.lmmul Printed

of Almo.rphe,_k and Terresfrial in Great Britain.

Physics,

Vol. 52. No. IO/l I, pp. 907-926,

1990.

0021-9169,‘90$3.00+ .oo Pergamon Press plc

Multiple-frequency studies of the high-latitude summer mesosphere : implications for scattering processes U.-P. HOPPE,* D. C. Fmr-rs,t

I. M. REID,$$ P. CZECHOWSKY,~ C. M. HALL j/ and T. L. HANSEN11

Establishment, N-2007 Kjeller, Norway ; t Geophysical Institute, University of Alaska, Fairbanks, AK 99775-0800, U.S.A.; 1 Max-Planck-Institut fur Aeronomie, D-341 1 Katlenburg-Linda”, F.R.G. ; 11 The Aurora1 Observatory, N-9001 Tromsa, Norway

* Norwegian Defence Research

(Receiaed in ,find

form 22 Maq‘ 1990)

Abstract-The characteristics of polar mesosphere summer echoes (PMSE) are studied at 53.5 and 224 MHz. Observations at 2.78 MHz, simultaneous with the ones at the other two frequencies, were carefully compared for indications of PMSE, but no obvious relation was found. Relationships between relative scattering cross-section, spectral width and vertical velocity are studied for the 224MHz radar, and observations at 53.5 MHz are compared with those at 224 MHz. Results of aspect sensitivity measurements at 53.5 MHz are presented. The implications of these characteristics for several possible scattering mechanisms are discussed. We rule out incoherent scatter and chemically induced fluctuations from the evidence that we have. In view of the extremely low temperatures near the high-latitude mesopause in summer, we discuss several scenarios involving heavy cluster ions and charged aerosol particles.

1. INTRODUCTION

power at the two higher frequencies and examine the influence of vertical motions on scattering cross-sccVery strong VHF radar echoes at frequencies near tion and spectral width in Section 3. This reveals much 50MHz have been known to occur in the summer of the variance of scattering cross-section and spectral polar mesopause region for some time (see, for examwidth to be due to advection. The cross-correlation ple, BALSLEYet uf., 1983 ; CZECHOWSKYand ROOSTER, between scattering cross-section and spectral width, 1985). These so-called polar mesopause summer as well as the aspect sensitivity of the PMSE, also give echoes (PMSE) have also recently been observed at an insight into the possible scattering mechanism. 224 MHz (HOPPE rt al., 1988), and even at 933 MHz Similar observations were also carried out with simul(R~~TTGERet ul., private communication). At these taneous in-situ observations of electron density. higher frequencies, the coherent nature and strength Results of this last experiment are reported elsewhere of the PMSE cannot be explained by incoherent scat(KELLEY et cd.,1990).The implications of our findings ter theory (e.g. FUKUYAMA and KOFMAN, 1980), just for the nature of PMSE are discussed in Section 4. as the strength of the observations near 50 MHz cannot be entirely explained by volume scattering from turbulence (KELLEY and ULWICK, 1988; REID et al.. 2. THE EXPERIMENTS 1988). During the MAC/SINE campaign, three radars The 224MHz radar used was the EISCAT VHF with frequencies of 2.78,53.5 and 224 MHz were opersystem located at Ramfjordmoen in northern Norway ated simultaneously within 130 km of one another in (BARON, 1986). In the period 10 June-17 July 1987, it northern Norway. These observations make it posoperated from 9 to 13 UT (IO-14 LST) regularly on sible to compare characteristics of the PMSE at the three days of each week. The mobile SOUSY VHF three frequencies. We will show that, while we observe 53.5 MHz radar, presently located near Bleik on the PMSE at 53.5 and 224 MHz, we do not at 2.78 MHz. island of Andoya, operated from 15 June to 19 July. The radar systems and their data collection proIt had two operation modes, observing either concedures are described in Section 2. We present the tinuously or for 10 min of every hour. Andoya is cross-correlations between the relative scattered located 130 km southwest of Ramfjordmoen. The 2.78 MHz partial reflection QPresent address: Department of Physics and ematical Physics, Univesity of Adelaide, Australia.

experiment (PRE) (HOLT located at Ramfjord10 June to 19 July, partly

et al., 1980) is, like EISCAT,

Math-

moen. 907

It operated

from

U.-P.

908 Table Instrument

HOPPE et al.

I. The locations of the radars

EISCAT SOUSY PRE

Coordinates

(MHz)

Frequency

69&N, 19.2”E 69.3”N. 16.O”E 69.6’ N, 19.2 ‘E

224 53.5 2.78

during the same periods as EISCAT. locations of the three radars.

Table 1 lists the

2. I. The EISCA T ohsercations Table 2 summarizes the characteristics of the mode used for the EISCAT observations. It is important to note that pulse-to-pulse correlation is used. The autocorrelation function contains information on the received power, the spectral width, and the radial velocity. All received power plotted and discussed here has been corrected for range and normalized to transmitted power and to a calibration noise source periodically connected to the receiver for this purpose. This relative scattered power is therefore independent of possible variations of sky background noise or receiver noise. It is proportional to the scattering cross-section per unit volume. The spectral width is the inverse correlation time of the distribution of scatters in the volume and is a function of (a) the time constants of their formation and decay, as well as (b) their radial velocity distribution. If (a) were dominating, a Lorentzian spectrum would result; if (b) dominated, the spectrum would be Gaussian (for a Maxwellian velocity distribution). The limited frequency resolution of 10.7Hz of the modulation employed does not allow us to discriminate between these contributions. We can, however, estimate the spectral width (at half-maximum) down to a fraction of one hertz from the decay time of the autocorrelation function. The main difference between a Lorcntzian and a Gaussian spectral shape occurs in Table 2. EISCAT

experimental

characteristics

Operating frequency Peak transmitter power Duty cycle Antenna aperture Half-power beamwidth Beam position

224 MHz 2 MW 8% 40x 120m 0.6 x 1.7’ Vertical

Phase code Height resolution Height range

I .05 km 74-l 13 km

Data aquisition No. of lags Lag increment Integration time

Correlation 21 2.222 ms IO s

13 bit 7 ps Barker

function

the wings; from the maximum to half the maximum, the two are identical to within 6%. The GENI I modulation (TURUNEN, 1986) we used has the advantages of allowing a useful height resolution at a high duty cycle and of virtually cancelling noise, clutter and low-frequency hum. This gives us confidence in our spectral width estimates, as there is no unknown noise floor to consider. In the case of very large vertical gradients of scattered power. GENl I has the disadvantage of contaminating neighboring range gates through modulation sidelobes (POLLARI rt a/., 1989). It is possible to numerically correct the data for this effect. but at the very large vertical variations of scattered power cncountered when PMSE occur, this is difficult. We chose therefore to concentrate our present analysis on the nine range gates (centered on 81.6-90.0 km) where the largest values of received power occur. GENl 1 has only small sidelobes, d - 25 dB at k 13 range gates, and the scattered power at these ranges is sufficiently similar so that modulation sidelobes are small compared to the scattered power at the ranges they contaminate. The modulation sidelobes have different magnitudes at the different lags, so that a contamination of the data by them shows up clearly in the autocorrelation function. We are therefore confident that modulation sidelobe effects are small in the results presented here. The modulation sidelobes discussed above must not be confused with antenna sidelobes. The antenna sidelobes within 10” of boresight are suppressed by < - 12 dB. These do not pose a problem for the results reported here. The data in Figs l-3 consist of one estimate each of relative scattered power, spectral width and vertical velocity for every 10s in the time interval 09301135 UT on 12 June, at 9 heights from 8 1 to 90 km. Each represents an average value for the scattering volume of N I km3, weighted with the spatial distribution of scatterers within that volume. The scattered power and the spectral width were estimated from the intercept and the slope, respectively, of a straight line fitted to the absolute autocorrelation function. Assuming a Lorentzian spectrum, the absolute autocorrelation function actually has an A exp (- Bt) form (HOPPE and HANSEN. 1988). Assuming a Gaussian spectrum, it actually has an A . exp ( - Bt’) form. For the long correlation times (narrow spectra) encountered here, however, both cases-or any intermediate one-are approximated well by a straight line out to the longest occurring lag of 46.7ms. If there were any background noise to consider (but see above), it would contaminate the zero lag, and it would lead to a noisy scatter at the

Multiple-frequency

studies of the summer 12 Cogf

Fig. 1. Normalized

-06 ret_.

909

_ 1987 Power

1

power received with the EISCAT 224 MHz radar from 9 height gates on 12 June 1987. Each dot represents a 10-s average.

following lags. The GENI 1 modulation employed has no real zero lag, as a pseudo-zero lag of 112ps is used. Because of the very large scattered power, a scatter of the autocorrelation function at non-zero lags around the fitted line was observed to be negligible. The probable error (lo) for the velocity values was estimated from the goodness-of-fit of a straight line to the phase of the complex autocorrelation function. The calculation of velocity and its probable error is described in more detail by FRITTS et al. (1990). Velocity values with a probable error larger than 1 m/s were discarded and not used. It should be pointed out that this is already a stringent requirement at a time resolution of IO s. In many cases, it was possible to tighten this threshold to 0.3 m/s, and even 0.1 m/s. 2.2. The SOUSY

mesosphere

observntions

The mobile SOUSY VHF radar has been described comprehensively by CZECHOWSKY et al. (1984).

Briefly, the radar operates at a frequency of 53.5 MHz, corresponding to a wavelength of 5.6m. The peak pulse power used for these observations was 100 kW, and the duty cycle 4%, resulting in an average power of 4 kW. Using an 8-bit complementary code, the pulses were phase coded, with elements of 2 or 4,~~slong, corresponding to range resolutions of 300 or 600m, respectively. At a range resolution of 600 m, ranges of 9.6-30.0 and 54.0-l 10.4km were sampled. and at a range resolution of 300 m, 60.0-100.2 km were sampled. Operation at 300m range resolution was continuous, and at 600 m, for 10 min in the hour. The antenna consists of a phased array of 576 fourelement Yagi antennae, covering an effective area of 8880m2, producing a gain of 35.5 dB, and a one-way beam width of 3’. Tapering is applied across the array to reduce sidelobes, and all are more than 2 1 dB below the main lobe. The antenna polar diagrams relevant to the current results are shown in CZECHOWSKY pt al.

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Fig. 2. Spectral

width received with the EISCAT 224MHz radar from 9 height gates on 12 June 1987. Each dot represents a IO-s average. Values are missing where the signal was not sufficient to determine the spectral width.

(1988).Since CZECHOWSKY~~~. (1984) described the radar, the number of beams has been increased from four to six. In the configuration applied here, all six were utilized. Using electronically controlled phase shifters, the antenna beam was directed towards the six beam positions sequentially. The beam positions are noted in Table 3. Radar returns were coherently integrated for 0.107 s, and for each beam configuration 64 such complex samples were obtained in each of the I35 range gates. Data were thus obtained for 6.8 s in each beam direction. These 6.X-s data sequences were Fourier transformed off-line to obtain the corresponding Doppler spectra Spectra were required to have S/N ratios of 3 dB or more before they were accepted, and they were checked for aliasing. The latter occurred when radial velocities exceeded 13.1 m s- ‘. Aliasing was removed by applying a special numerical tech-

nique, which takes into account the spatial and temporal variation of the signals at each range gate and beam position. 2.3. The PRE observations The partial reflection experiment (PRE) at Ramfjordmoen and its abilities have been extensively described on several occasions (e.g. HOLT et al., 1980; RASTOGIet al., 1982 ; BREKKEet al., 1985 ; THRANE et al., 1987). Basically, it is a pulsed HF radar with a narrow-bean1 antenna, an altitude resolution of 3 km and a time resolution down to 20 ms. The principal characteristics are summarized in Table 4. During the MAC/SINE campaign, PRE was operated for 64 h on 25 days in the period IO--l 3 UT. There were simultaneous observations with EISCAT for 14 h distributed on 7 days. Echoes in the 80-90 km height range are a common phenomenon with PRE.

911

Fig. 3. Vertical velocity observed with the EISCAT 224 MHz radar from 9 height gates on 12 June 1987. Each dot represents a IO-s average. Values are missing where the signal was not sufticient to determine the vertical velocity with a precision better than I m;s.

On the seven days of observations simultaneous with EISCAT, such echoes were constantly present on five days and absent on two. We have performed a detailed comparison between the echoes observed with EISCAT and those observed with PRE, but found no relationship, neither in time nor in altitude. The distribution of PRE echoes throughout the 64h of summer observations shows a general increase of occurrence from 75 to 95 km with no significant peaks. SCHLEGEL etal.(I 978)did find a peak in the 80-85 km region, analysing a large amount of non-summer data. They did not discriminate between different times of the day. Although a detailed investigation based on a much larger dataset than ours may reveal a peak similar to

that of SCHLEGEL et al. (19781, we conclude that any relation between PMSE and PRE echoes is weak or non-existent.

3.ECHO

CHARACTERISTICS

AND STRUCTURE

We present in this section examples and a discussion of the echo characteristics observed with the SOUSY and EISCAT VHF radars during the MAC/SINE campaign. Observations with these systems include the backscattered power, an estimate of the spectral width, and an estimate of the radial velocity for each beam direction. EISCAT was directed vertically. Parameters that could be obtained utilizing the six beams of the SOUSY radar are discussed in the preliminary

U.-P. HOPPE et ai.

912

Table 3. Mobile SOUSY radar experimental characteristics Operating frequency Peak transmitter power Duty cycle Antenna aperture (effective) Two-way 3 dB beamwidth Beam positions

S3.5 MHz 0.2 MW 4% X880 m’ 1.7‘ 4’ off-zenith towards northeast, northwest and southeast, 5.6 off-zenith north- and westward, vertical

Phase code Height resolution Height range

X-bit 2 ps complementary at 300 m (g-bit 4 us at 600 m) 300 or 600 m 9.6 -30.0 km and 54.0~110.4 km at 600 m 6O&IOO.2 km at 300 m

Data aqtlisition Coherent integration time

6.8-s Doppler spectra 0.107 s

Table 4. PRE experimental characteristics Operating frequency Peak transmitter power Pulse length Pulse repetition time Antenna

2.78 MHz 30 kW 20 /is 20 ms I6 crossed

dipoles in a 300 x 300 m

array Antenna gain Halfpower be~tnwidth Beam position Height resolution Polarization

17dB 17 Vertical 3 km o- or between

x-mode, the two

or

alternatiq

work of REID ct ul. (I 988), CZEC’FIOWSKYet a/. (1988) ROOSTERand REID (1990). Examples of the derived scattered power and vertical velocity data obtained with the EISCAT VHF radar are shown in Figs 1 and 3 for a 2-h interval (0930-I 135 UT) on 12 June 1987. These data reveal velocity fluctuations as large as 2: 10 m/s with observed periods of -3min-I h. Relative scattered power varied by almost four decades and exhibited fluctuations that also occur on temporal scales of r 3 min-1 h. Because of the highly variable power profile at each IO-s interval, and the possible correlation of the power fluctuations with the vertical velocity field. we also present the power profiles averaged for 30 min in Fig. 4. These reveal significant variability of scattered power even at much longer time scales, and yield a mean power profile consistent with those observed by several VHF systems near the high-latitude summer mcsopause (BALSLEYet al., 1983; CZE~HOWSKY et ul., 1988; HOPPE ef ui., 1988). The spectral width for this period likewise exhibits considerable variability, but discussion of these data will be deferred until later in this section. and

We have chosen 24 June 1987 for comparing the power scattered at two radar frequencies. Figure 5 shows the backscattered power as a function of height and time for the SOUSY radar. Figure 6 shows the corresponding data from EISCAT on the same time and height scales. It is obvious that the PMSE occur at the same heights, and that they have similar variability with time and height. The strongest echo on both datasets is generally l-2 km thick and centered around 84 km. This layer widens to 4-5 km every I .5-2 h. The echo remains wide for 30-60min. it expands mostly upwards, but there are short bursts of enhanced power stretching downward to about 82 km. These downward expansions tend to occur in the interval between upward expansions (see also the discussion in 3.2.). The radars have not been calibrated to allow a direct comparison of absolute scattering cross-sections with the present data. We can, however, make a rough and very qualitative c~)nlpar~son of the scattering cross-sections within and just outside the PMSE. The ratio of maximum power to power scattered in the background atmosphere is -2OdB for SOUSY and rl~ 15 dB for EISCAT (that is to say, the scattering cross-section increases by ~3.2 for a wavelength increase of -4.2). No direct correlation between the datasets is apparent. That may not be surprising, though. The scattering volumes are = 130 km apart. The wind at the height of the echo had a velocity of -27 m/s and a direction of 236” (towards southwest) at this time (MATSON et al., 1990). The azimuth from EISCAT to SOUSY is 257 . A parcel of air that was above EISCAT at a given time would thus not approach SOUSY closer than = 50 km. and this would be the case after w 8 1 min.

Multiple-li-equency 12.06.

studies of the summer 4987

30

09:30

mcnute

-

14

913

mesosphere :30

Ul

overages

I

8 CogC Tel_.

Power

I

Fig. 4. Same data as Fig. 1. but averaged for the periods OY30~-1000, 1000 1030. 1030 1100 and 1100 I I30 UT. The horizontal scale is correct [or the t’irst profile ; subsequent profiles were shifted by two orders of magnitude

The data presented in Figs I and 3 reveal an apparent correlation between the fluctuating vertical velocities and the scattered power for several data intervals and observed wave periods. Examples of this relationship are the data exhibiting high-frequency (T e 2.5 min) oscillations at upper heights between 0930 and 1000 UT and the lower-frequency (T rr 20min) fluctuations at intermediate heights between about 1030 and I 130 UT. In order to examine these relationships more closely, we show the vertical velocity, backscattered power, and spectral width estimates together for the interval 1025S1135 at heights of 83.65S 87.85 km in Fig. 7. Clearly seen here is a tendency for the 2 20 min fluctuation in the vertical velocity to lead the minimum of the = 20 min oscillation of power by about l/4 cycle. suggesting a quadrature relationship between these two quantities. To exatnine the possible cause of this relationship,

each.

we assume that the relative backscattered power or, equivalently, scattering cross-section, can be characterized as being due to a quantity with specific but unquantified sources and sinks (density of scatterers D, which should include their number and intensity). These sources and sinks can be thought of as processes such as turbulence arising from M’ave instability (FRITTS and RASTOGI, 1985) or enhanced scattering cross-section due to heavy ion clusters (KELLEY et t/l., 1987) that act to increase or decrease the radar crosssection at a given frequency. We may then write a continuity equation for D of the form dD dt

= P+L.

where P and L represent the processes contributing to the production and loss of scatterer density. We now expand the time derivative of D and write iD it + b’-on

= P+L.

(2)

U.-P.

914

thPYE L’/ d

94

SOUSY 24.06.1987 log(Power)

93

[dB]

” 91 90

189 E A 88 ‘87 N 86 85 84 19.4 -

25.0

13.9 9.3 -

19.4 13.9

1.3 -

9.3 4.6

4.6 -

--, d !_A r--~

I

83

82

81 8

0.1-

BELOW -0.1 0.4 - -0.1 0.4 0 1.3 1

Fig. 5. Height -time intensity

10

11

t

plot of the blackscattered power vertical direction.

13

[t5;T]

measured

14

with the SOUSY

15

radar

in the

94 EISCAT

93

24.06.1987

log(Power)

[dB]

g2 91~

N 86 85 84 m

ABOVE

m

277

83

22:9 - 277

82 1

13.5 19.3 15.8 - 22 19 15.8 9 3

a i-

11.4 - 13 5

1-j

i 1

7.5 9.1 - 11.4 9 I

.I

5.8 -

j BELOW

81

8

9

10

11 t

[ET1

13

7.5 58

Fig. 6. As Fig. 5, but measured

with the EISCAT

224 MHz radar.

14

15

915

Multiple-frequency studies of the summer mesosphere 12 -06 v

(al

t

lo:30

_ 1987 Cm/s

1

i 1:s.o

1 i Tcme

CUT1

Fig. 7(a).

Assuming that the velocity may be written as the sum of a horizontal mean plus a perturbation, $ = 6 + tj’, and that the mean velocity is only in the x-direction, we obtain

Thus, D may change locally in response to advcction of a vertical gradient of D by the fluctuating vertical velocity or of a horizontal gradient of D by the mean and fluctuating horizontal wind. Now assuming that velocity and D oscillate as exp (i~$) with 4 = kx + mz - kct (k and m are a horizontal and vertical wavenumber, respectively, and c is the phase speed), and that the primary source of advective changes is the vertical velocity (this assumption is reasonable provided that the dominant gradient of D is vertical), we see from equation (3) that

positive fluctuations of D and thus scattering power, will lead positive velocities by 90” for 2Djaz > 0 and lag positive velocities by 90, for a negative vertical gradient of D. Noting that the mean gradient of scattered power is positive at lower levels for the data interval presented in Fig. 7, we see from the first two panels of this figure that the N 20 min fluctuations do have this approximate phase relationship. This is most apparent at the lowest heights in Fig. 7, because of the strong (positive) vertical gradient of scattered power resulting from very weak signals at lower heights. A second example of the relationships between vertical velocity, scattered power and spectral width from data collected during the observation period on 24 June 1987 is shown in Fig. 8 from 1000 to 1130UT at heights of 82.6-86.8 km (see also Fig. 6). In this example, the vertical gradient of scattered power or scattering cross-section changes sign with height,

916

U.-P. HOWE et ul 12 -06. cogc

(b)

10

1987

reC.

:30

Power

I

4 1

4 I :30 Tcme

CUTI

Fig. 7(h).

being positive at the lower heights and negative above. Consistent with this, the relative phase of the velocity and power fluctuations for a period of z 20 min also changes with height. At 83.65 km we note that power variations lead the vertical velocity variations by = 90”. At 85.75 km, between N 1030 and 1110 UT, on the other hand, the reverse phase relationship is apparent, due to the negative vertical gradient of scattered power present during that interval. This example provides strong evidence that advective changes in scattering cross-section due to the vertical velocity are major cause of cross-section variability for PMSE in cases where background gradients are large. By analogy, we should also expect to see similar relationships in the SOUSY data, but no detailed study was undertaken. Despite the good agreement between observed fluctuations and the phase relationship anticipated for advective changes from equation (3), however, we note that there arc also many fluctuations that do not

appear to correlate well with the vertical velocity field. This suggests either horizontal advection effects which cannot be anticipated with our available data or other processes that lead to increases or decreases in scattering cross-section. These will be discussed in some detail in the remainder of this paper. 3.3. Cros.~-correlation

of scattered

pmver und spec.trd

width

We found in the previous section a significant relationship between vertical velocity fluctuations and changes in scattering cross-section brought about by vertical advection ; but at least as strong a correlation exists between the scattering cross-section and the spectral width estimates obtained from this dataset. This correlation is negative and present at both small and large time scales. It also has some important implications for possible scattering mechanisms and may provide useful insights into the occurrence of VHF and UHF echoes from this region of the atmo-

Multiple-frequency

studies of the summer i987

i2.06. CogC

LcnewLdth/Hz

I I

to

:30

917

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i ,

1 I

tt:30 Tcme

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Fig. 7. (a) Vertical velocity observed with the EISCAT 224MHz radar from 5 height gates on 12 June 1987. (b) Normalized power from the same height gates. (c) Spectral width. ~agni~cation of (a) Fig. 3, (b) Fig. 1, and (c) Fig. 2.

Finally, it should be noted that the relationship between vertical velocity and scattering crosssection discussed above could be applied equally well to the relationship between vertical velocity and spectral width, as both scattering cross-section and spectral width appear to be characteristics of the medium that are advected by the vertical and horizontal motions occurring in the middle atmosphere. Two examples of the spectra1 width data obtained during the MAC/SINE campaign are shown in the third panels of Figs 7 and 8 ; and while the spectral width fluctuations are relatively not as large as the ~uctuations of scattered power, the two parameters appear to be negatively correlated to a remarkable degree. This correlation is particularly clear for periods of N 10-30 min in Figs 7 and 8. In addition, the power and spectral width correlation does not appear, at first glance, to depend on dynamical influences. Estimating the spectra1 width in the spectral domain could lead to an apparent inverse relationship between sphere.

scattered power and spectral width, if background noise were present to a significant degree. Because of the data collection and analysis methods outlined in Section 2.1, we consider the relationship we observed to be real. As noted in Section 2, the spectral width depends on both radial velocity Auctuations and the time scales representative of scattering processes at these heights. The strong dependence of scattering cross-section (or spectral width) on the vertical velocity suggests, however, that the source and sink time scales may be sufficiently large such that the spectral width can be used as a crude upper limit on the velocity variance within the radar volume. If this is the case, we may assume that a large spectral width corresponds to a high level of turbulence and that a small spectral width implies relatively less turbulence. Then the negative correlation between scattered power and spectral width implies that the scattering cross-section is a maximum when turbulence is a minimum and vice

U.-P.

91x

HOPPE ct al. 24 -06. 1987 W [In/S1

I F-T-..-_

7..

~~~~~~~~~~~~_~___~~__~ .:‘-

I

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(a)

v

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Fig. S(a).

versa, supporting the view advanced by several authors (e.g. HOCKING, 1987, and references therein) that radar backscatter at VHF and UHF at mesopause heights may be due more to a specular than to a turbulent process. One might ask, then, what could be the nature of this scattering process, and if it proves not to be specular, why do echoes occur at radar wavelengths that are, by most accounts, well within the viscous subrange? 3.4. Aspect sensiticitJ Preliminary calculations of the aspect sensitivity of the PMSE at 53.5 MHz have been presented by REID et ul. (1988) and CZECH~WSKY rt al. (1988). The data which will be discussed here were obtained on 26 June 1987 from 0915 to 1242 UT, and are representative of the whole MAC/SINE observation period. This time interval was selected because the PMSE layer was relatively stable in height and did not exhibit large variations in intensity. Figure 9 shows the echo power

as a function of height and time measured in the vertical beam. The time resolution was 6.8 s for each height profile, and the beam direction was changed every 10 s, so each of the six beam directions was sampled about once per minute. In the present example, the mean height profile for each of the beams was averaged for 207 min and these were used to calculate the aspect sensitivity. The method described by HOCKING et 01. (I 986) and applied by REID et al. (1988) and CZECHOWSKY et al. (1988) to MAC/SINE data to investigate the aspect sensitivity was used here to calculate O,, the half-width of the angular polar diagram of the backscatter, and O,, the effective beam angle for each of the beams. The effective beam angles were found to be 3.8 and 5.3’. These may be compared to the nominal beam angles of 4 and 5.6”. Figure 10 shows the backscattered power relative to that received in the vertical beam plotted at the two effective off-zenith angles. The mean value of 0, was 6.6”, which is in

Multiple-frequency

studies of the summer 24.06 cogc

..

(b)

:

:

mesosphere

919

_ i987

reC.

Power

I

._

10. Fig. 8(h).

good agreement with the results of REID et cd. (I 988) and CZECHOWSKY et al. (I 988), and is representative of the PMSE observed from 8 I to 91 km. Such a value would be consistent with a quasi-specular reflection mechanism at that radar frequency. The height profiles of the mean power measured in the vertical beam P, . of the ratio of the power measured in the oblique beam to that measured in the vertical beam P,,,,,/P$, and of O,s are presented in Fig. I 1.The aspect sensitivity shows a tendency to decrease with increasing height, and almost all minima of 0, arc related to peaks in the power profile. At 224MHz, the few off-zenith observations that we have are inconclusive with respect to aspect sensitivity of the PMSE. 4. DISCuSSlON

The most striking features large scattering cross-sections,

of PMSE are the very the fact that they occur

in discrete layers of < I km to a few km, and that their spectra1 width is O.lLlOHz (corresponding to 0.076.7 m/s) and maybe even smaller. We will now discuss several possible scattering mechanisms with respect to the characteristics described in Section 3. The common denominator of the processes considered is scattering of radiowaves from spatial or temporal fluctuations in the refractive index of the medium. 4.1. Incoherent

scatter

Incoherent scatter results from thermal fluctuations in the electron distribution, which produce fluctuations in the refractive index. Since these fluctuations are spatial, they give rise to scattering of radiowaves if the ionization is sufficiently large. Since the fluctuations are also temporal, the coherence time of the scattered signal is finite and the spectral width becomes non-zero. D~UCHERTY and FAKLEY (1963) have formulated this case, taking account of the effect of ion-neutral collisions, which are important in the

920 24.06.

Log(

()c

1987

L cnewcdth/Hr

I

&-----

40

Fig. 8. As Fig. 7, but fcr 24 June 1987.

II-region, and WALDTEUFEL (1965) has taken a hydrodynamic approach. FUUJYAMA and KOFMAN(1980) have utilized the hydrodynamic treatment of TANENBAUM (1968) to account for the effects not only of electrons and positive ions, but also of negative ions. KOFMANPI crl. (1984) have expressed the resulting terms for (relative) total scattered power and (relative) spectral width at half-maximum in terms of straightforward atmospheric parameters. The incoherent scatter spectrum from the I®ion is shown to have approximately Lorentzian shape. It is possible to compute the absolute spectral width at halfmaximum from known parameters, C,N,( 1+2i) Power = ~_---(2(1 tJJ+x*)(l +x2)

(4)

(KUFMANet al., 1984), where

(6) (GANGULYand COCO,1987). C, is the system constant of the radar, N,. is the electron density, d the number density ratio of negative ions to electrons, , t( = 4&,/l,, L, = 4 eoksT/(N,e2) the Debye length, L, the radar wavelength, k, Boltzmann’s constant, E,, the ~~ittivity of vacuum, e the elementary charge, T the tern~~tu~, which is assumed to be equal for neutrais, electrons and ions, and mj the mean ion mass. The ion-neutral collision frequency for momentum transfer, v,,~,is given as a function of ion and neutral mass and neutral density by HILL and BOWHILL (1977). In summer 1987, the mean temperature at 86 km and 69” N was measured to be T = 133 K and the average density p = 9.13 -iO.-6 kg * m - ’ (L~.YJBKEN et

SOUSY

VHF

RADAR

921

studies of the summer mesosphere

Multiple-frequency

Andenes

26 Jun 1987 >48

39-46 m 29-38 z L. s 19-28 2 9-18 O-8 0730

0800

0830

0900

0930

1000

1030

t I LT Fig.5).Heighl-time

intensity plot of the backscattered power measured (LT = UT+2 h).

al., 1990). For these values, and assuming CI= 0, and i = 0, and a mean ion mass of 31 a.m.u. (average of NO+ and O:), the spectral width is 28Hz for the EISCAT VHF radar. A value of 1 greater than zero ANQENES

SOUSY VHF RADAR 26 JUNE 1988

0715-1042 LT

0

x\

x

in the vertical

direction

would only increase this value. Under the extreme assumption that the dominant ion was H+(H,O),, with an n value of 20 (see Section 4.3), the mean ion mass would be 361 a.m.u. and the spectral width 22 Hz. The surprisingly weak dependence of the spectral width on mass is due to the fact that vi,, is almost IO-times smaller for the IO-times larger ion mass. For mass, the mi >> m,, the average neutral molecular product mi- v,, in the denominator of equation (6) becomes independent of nri, and Wconverges towards

-10

%

aty>

an

number density in m ‘. For the EISCAT VHF in summer at 86 km, W converges towards 21 Hz. We observed spectral widths which where N,, is the neutral

pS =exp-sin* e

-20

P”

e, = 6.570 ~

-

0

i sin* 8,

I

I

I

2

4

6

I

I

6 10 8ldeg

) I

I

1’2 14

16

Fig. IO. Backscattered power relative to the power received from the vertical direction. The crosses denote the effective off-zenith angle 0, of the beam.

were at least 2-200 times narrower than this lower limit, and conclude that incoherent scatter theory cannot account for these observations. Even given that the temperature can be lower and the neutral density larger during short time intervals than the average values by L~~BKENet al. (1990), changes by more than lo-20% seem unrealistic. Only if m,, was very large could the narrow spectral widths we observed be explained with this formalism (see Section 4.3).

SOUSY

VHF RADAR

26 JUNE

0715 -1042 LT

ANOENES

x-x-*

j-D = I+0

-

ZD= 5.6’

88

I

-12 -10 -8 -6 -L -2 p, IdB

I

I

,

2 L

6

8 1012

I

I

8,ideg

Fig. 11. (a) Mean power profile PC measured in the vertical direction for a period 01’207 min. (b) Ratio of powers (P,,hI:lP,) measured in directions 4 and 5.6 off-zenith to that in the vertical. (c) Half-width 0, of the angular polar diagram of the backscatler.

The so-called system constant C,, in equation (4) is determined from a comparison with an independent measurement of N,.. The system constant varies from one radar to another. Using this calibration for EISCAT-VHF, we arrive at N,, = IO”-10” m-3 in a typical PMSE layer, and N, 2: lo”-lO’O m 3 in its immediate vicinity. It seems difiicult to visualize eIectron density changes by factors of IO’ over 1-2 km in height and a few minutes in time. Also, the magnitude of I%!,, as such seems difficult to generate at this height. We do not actually believe the N, cited above for the layer. Aspect sensitivity such as observed at 53.5 MHz is not expected for this scattering process.

KOCKARTS and WIS~MB~RG (I 98 1) and W~SE~BERG and K~CKAKTS (1983) have proposed chemically induced fluctuations of the electron density as an additional process leading to radiowave scattering in the D-region. These fluctuations are due to the formation and destruction of negative ions by the chemical reactions that maintain the equilibrium density. The scattering cross-section for this process is

where r,. is the classical electron radius, and L,. and L. the effective loss rates for electrons and negative ions. WISEMBERG and KOCKARTS (1983) quote these as about 2. IO- ‘s- ’ for clcctrons and 20s ’ for negative ions at 85 km. These values do not give cross-sections and spectral widths consistent with our observations. But the loss rates given by WISEMHEKGand K~CXARTS (1983) are model values, and they can instead be determined from our observations of scattering cross-section and spectral width. With a typical scattering cross-section corresponding to incoherent scatter from = IO” electrons - m ’ and a typical spectral widthof
923

Multiple-frequency studies of the summer mesosphere would not expect to encounter these terminal ions in summer conditions at these latitudes. The negative ion chemistry is discussed in sufficient detail for our purposes by REID (1987). It is conceivable, of course, that other types of ions exist, but we know of no experimental evidence for this. 4.3. Heut~~~ cluster ionslchuryed

purticles

WISEMRERG and K~CKARTS (1980) explicitly state that their model does not include hydration effects on negative ions, but that these could be incorporated. Since the PMSE are only observed near the summer mesopause, the low temperatures enco~lntered here at that time are likely to be important, and this could be due to the generation of heavy cluster ions and charged aerosol particles. It is conceivable that these are not as unstable against photo-dissociation as ordinary negative ions. The lifetime of a negatively charged particle near the high-latitude mesopause in summer may thus be as long as 10s or even longer, ~ollsistent with our observation of spectral width given in the previous section. Our observation of the advection of scatterers (see Section 3.2) suggests that the lifetime of the clusters or particles may be as long as some minutes. in which time they may transform to neutrals or positive ions and back several times. The notion of electrons becoming attached to large clusters or small aerosol particles is also consistent with the observations of KELLEY and ULWICK (1988), which show substantial, localized depletion of electron densities when PMSE occur. If this reasoning is correct, polar mesosphcric clouds should be present when PMSE are observed, and there is evidence for this (JENSEN cf N/.. 1988). While the possibly long lifetimes of the negative cluster ions or charged aerosol particles can explain the observed narrow spectral widths, we observe too small a scattering cross-section to be explained by the chemical fluctuation/heavy negative ion mechanism. The chemical fluctuation model assumes a uniform average ion and electron distribution. If the heavy cluster ions were present in only a fraction of the scattering volume, for example in thin layers as precursors or remnants of noctiiucent clouds/polar mcsospheric clouds, the scattered power could be as we obscrvc it. The organization of heavy cluster ions/small acrosol particles into thin layers could, on the other hand, lead to enhanced echoes because of the electron density gradients produced by such electron absorbing layers. This could lead to partial reflections or Fresnel scatter. Turbulence acting on such layers would not lead to enhancement of the echoes, since the half-

Table 5. Saturation mixing ratio of water at 70 N in summer PIi+ 2 (km)

90 88 86 84 82 80 78 76 74 72 70

Pll~O.~& P4 2.0.10 L( 6.6. lo-’ 2.8 * IO-’ 2.8.10.’ 5.4. lo-o 9.0. IO--i 1.6. IO-’ 2.2. lo-’ 1.4. 10. ’ 4.0, lo- ’ 3.9

Pr0*@a) I.3. lo-’ ’ 2.1 ’ lo- ’ 3.5. IO- ’ 5.7. lo- ’ 9.0.10 ’ 1.4 2.1 3.0 4.2 5.9 8.1

_.

Pr0,. ..~_~

1.5*10-1 3.1.lo-”

8.0. 1o-8 4.9. IO_’ 6.0. IO-& 6.4. lo- ( 7.6. lo-” 7.3*10-’ 3.3. IO 2 6.8. IO_’ 4.8. fo-’

wavelength-at least of EISCAT-is well within the viscous subrange where turbulence is strongly damped. Instead, turbulence would tend to break up the heavy ion layer, mixing it with warmer air, thus destabilizing the ion clusters. In this scenario the spectral width is inversely proportional to the scattering cross-section, in agreement with our observations. A probable candidate for the large cluster ions is H+ (H20),, (e.g. ARNOLD, 1980; PRASAD and TAUBENHEIM, 1987). The temperatures and densities (L~~BKENer al., 1990) allow us to compare total atmospheric pressure with the saturation vapour pressure of Hz0 over flat ice (JANSCO rt nl.,1970) as a function of height (Table 5, Fig. 12). We must remember that the vapour pressures of JAKSCO et af. (1970) may not be valid at the low pressures near the mcsopause and for small particles with a large surface-to-volume ratio. Assuming the mixing ratio of water near the mesopause was 5 p.p.m. = 5. lo-” (ARNOLD, 1980), then the atmosphere would be supersaturated by a factor of = 160 at 88 km {Fig. 12). Any ice particles formed near 88 km would eventually sediment downwards, becoming larger as long as the atmosphere was supersaturated with respect to water, and sublimating when they reached a height where it was sufficiently warm. For the 5p.p.m. water volume mixing ratio that we are assuming, this height would be near 82 km. the lower limit of the PMSE. This sedimentation process would also concentrate the little water that there may be in the coldest region into the region where the particles sublimate. Turbulence would tend to redistribute the water content evenly. This sedimentation vs turbulent mixing process may explain the variability we observe in the PMSE, and it can also explain the asymmetrical height distribution of PMSE reported by HOPP~ et al. (1988). The ratio of scattering cross-sections in PMSE

HOPPE et 01

U.-P.

924 SATURATION AT

MIXING 70

OEGR.

RATIO N

IN

OF

WATER

SUMMER

Log(

Mcxmg

Fig. 12. The saturation water mixing ratio calculated from temperatures and pressures Andaya

during

the MAC/SINE

campaign.

between SOUSY and EISCAT is =3.2 (see Section 3.1). As the ratio of wavenumbers k is -0.24, scattering cross-section appears to vary as = k -” ‘. If turbulence were the sole cause for PMSE, it should vary as key ‘, if the wavenumbers of both radars are in the viscous subrange at those heights, which is probable. If the reasoning of KELLEY et al. (1987) is correct, the scattering cross-section should vary as km ‘. They propose that, in the presence of heavy positive ions, a viscous-convective subrange is inserted between the inertial and the viscous subranges. With the necessary caution, as we have only made a crude, and not absolute, comparison of scattering cross-sections, we can conclude that our observations seem to be consistent with this power law.

5. CONCLUSIONS We have shown that the PMSE observed at 53.5 and 224 MHL are very similar in nature. Echoes received

Rotco

measured PMSE occur where this ratio is less than 10~ ‘.

1

over

during the same time periods from the same general height region at 2.78 MHz do not correlate well with the PMSE observed at VHF, neither in height nor in time. A crude comparison of the scattering cross-sections in PMSE at 53.5 and 224 MHz suggests that it follows a rr k _ ’ power law, consistent with the insertion of a viscous-convective subrange between the inertial and viscous subranges. This would be the case if the heavy positive ions formed at the extremely low temperatures near the polar summer mesopause modified the mobility of the electron gas as proposed by KELLEY et ul. (1987). On the other hand, the inverse correlation between scattering cross-section and spectral width that we have fcund seems to rule out turbulence as a primary cause for PMSE, as does the apparent absence of correlation between PMSE and PRE echoes. The vertical advection of the scattering medium that we have observed suggests that the lifetime of

Multiple-frequency

studies of the summer

the scatterers or of the atmospheric state supporting PMSE is at least on the order of Smin. On the other hand, the high temporal variability of PMSE may suggest that this lifetime cannot be much longer than about 5 min. or, alternativelyl that the PMSE are very patchy as they) drift across the radar beam. We have reviewed some of the scattering processes that might he applicable for PMSE. Incoherent scatter must be ruled out, mainly because this process does not explain the narrow spectral widths that we have observed, but also because of the very localized and extrcmcly high electron densities that would have to be invoked. Neither can chemically induced fluctuations involving ordinary negative ions account for the high cross-sections and narrow spectral widths observed. The extremely coid environment encountered near the high-latitude mesopause in summer makes possible the formation of heavy cluster ions and even aerosol pat-tic&. The existence of one or both of these may give rise to PMSE by one of three possible processes : (a) The mechanism proposed by KELLEY ct al. (1987) mentioned above. (b) Chemically induced fluctuations involving charged vs neutral aerosol particles instead of negative ions vs molecules. Charged aerosol particles are likely to have the low effective loss rates needed to explain the narrow spectral width of PMSE. These particics must be organized into wisps filling the scattering volume only partially ; otherwise the scattering cross-section should be subs~~ntialiy larger than observed. (c) Thin layers of aerosol particles absorb the free electrons. creating strong electron density gradients

mesosphere

925

as observed by KELLEY and ULWI~K (1988). These gradients of electron density lead to quasi-specular reflections consistent with the aspect sensitivity found at 53.5 MHz. This mechanism is also consistcnt with the observed inverse correlation between radar cross-section and spectral width. The absence of PMSE in the PRE data indicates that the structures must either be small against the halfwavelength of this radar (54m), or distributed across its first Fresnel zone in such a way that the reflections cancel at the receiver. Our data arc not sufficient to decide which of these three mechanisms may be responsible for PMSE. Indeed. a combination of any two or even ail three may be the cause. To a certain extent, they are different formulations of the same physical process. At present the evidence suggests that they become more probable from (a) to (c), as they also become conceptually less complicated. ilckvro~llrrlyemrnrs-We are grateful for helpful assistance in the EISCAT observations by T. Trondsen and the EISCAT staff. The EISCAT Scientific Asso~iatioil is supported by Norges A~menvit~nskapelige Forskningsrid of Norway, the Max-Planck Cesellschaft of the F.R.G., the Science and Engineering Research Council of the U.K., Naturvetenskapliga Forskningsradet of Sweden. Suomcn Akatemia of Finland, and Centre National de la Recherche Scientihque of France. The partial support from NATO Scientific Affairs Division under Collaborative Research Grant No. 0426!87 for U.-P. Hoppe and D. C. Fritts is .gratefullv acknowledged. Additional support for D. C. Fritts was provided by the Air Force O&e of Scientific Research (AFSCI under contract F496~~-~7-~-0024 and bv the SDIO; fST and managed by the Oftice of Naval Research under contract N00014-86-K-0661. C. Hall was funded by Norges Almenvitenskapelige Forskningsrdd.

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1988 1985

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U.-P. Hopp~ et (II. BKEKKE A. and HANSEN T.

1980

The PRE (partial Ramfjordmoen,

reflection experiment) Tromss. Proceedings

facility

at

of the Vth ESA-PAC Symposium on European Rocket and Balloon Programmes and Related Research, 14-18 April 1980, Bournemouth, pp. 387-392, ESA SP- 152. ESA,

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1988 1988 1970 1988 1987 1988 1990

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J. atmos. terr. Phys. 51, 937.

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HANSENT. L., ZAHN U. VON.MEYER W., CZE~HOWSKYP., SCHMIDTG., WIDDELH.-U and NEUMANN A. TIIKI’NFN T. WALDTEUPEL P. WISEMBERGJ. and KOCKARTS G. WISEMUEK~;J. and K~CKARTS G.