Adv. Space Res. Vol.12, No. 10, pp.(10)81-(10)84, 1992 0273-1177/92 $15 oo Pnnted m Great Bntam. All nghts reserved Copyright © 1992 COSPAR

OBSERVATIONS OF ATMOSPHERIC DYNAMICS IN THE LOW- AND MIDDLE-MESOSPHERE WITH THE EISCAT INCOHERENT SCATI'ER RADAR DURING SOLAR PROTON EVENTS

M. T. Rietveld,* P. N. Collis** and J. R6ttger**,***

* EISCAT, N-9027 Ramfiordbom, Norway ** EISCAT, S-98128 Ktruna, Sweden

ABSTRACT

Prof'des of meridional neutral wind velocities in the height range 54-81 km with high temporal (minutes) and spatial resolution (1 km) from the EISCAT Incoherent Scatter Radar (ISR) at 933 MHz are presented. The 4_at_a, obtained during three different solar proton events causing polar cap absorption (PCA) in 1989-1990, sometimes show slowly descending structures in the velocity prof'de which may be the signature of waves with vertical wavelengths as low as 10 km and periods from 9-14 hours.

INTRODUCTION

Radar probing of the atmosphere at mesosphere/thermosphere heights with the ISR technique depends on sufficient ionisation being present to return a detectable signal. In the region 50-70 km it is difficult to measure winds with conventional radars because of the very weak and seasonally variable signals received. Mathews /I/ has reviewed the D-region incoherent scatter radar technique and Balsley /2/ the Mesosphere-Stratosphere-Troposphere (MST) radar technique. In the absence of photo-ionisation, high- latitude ISR's can still make observations provided that particle precipitation ionises the atmosphere. During the special conditions of energetic solar proton precipitation causing the PCA events, the ISR technique can be applied at much lower altitudes than usual. Here we discuss observations from the EISCAT 933 MHz radar during recent PeA events when spectral measurements were obtained from altitudes as low as 55 km. Previously the lowest altitude from which mesospheric velocities were determined using EISCAT was about 70 km/3L A summary of EISCAT's contributions to the study of the mesosphere and D-region is given by Collis and R6ttger/4/.

THE EXPERIMENTS

The results come from the EISCAT UP-1-A (Unusual-Programme) experiment on the UHF (933 MHz) radar situated at 69.6 deg north. The EISCAT system is described by Folkestad et al./5/and Baron/6/. UP-1 is a monostatic D-region experiment using an advanced pulse-to-pulse correlation scheme (otherwise known as GEN-11) with 1.05 km range resolution described by Turnnen/7/, run at times of unusual geophysical conditions such as during PCA's. Table 2 of Collis and Rietveld /8/ lists the PCA's and the corresponding EISCAT operations for 1989. The UP-1-A experiment normally uses a vertically pointing antenna to measure from 70 to 113 km in altitude. Because the signals were seen to be strong at the lowest height during the PCA's, the antenna was tilted to lower elevations (typically 45 deg to the north) to get data from heights down to 50 km. This also enabled us to measure the Doppler shifts due to winds in the north-south direction. This scheme was used for PCA's on 20 October, 23 October, 1 December 1989, and 20 March 1990. The duration of these experiments varied from about 2 to 5 hours although often other experiments were run during other parts of the PCA's (see Collis and Rietveld/8/).

*** On leave from Max-Planck-lnstaut f~r Aeronomie, D3411 Katlenburg-Linda~ Germany

(10)81

(10)82 M.T. Raetveld et al.

ANALYSIS

Figure 1 shows a series of spectra as a function of altitude from 1 December 1989. Lorentzian curves were fitted to the spectra and the offset from zero frequency used to derive the line-of-sight velocity and hence the horizontal velocity assuming the vertical component to be small, which is well justified at elevation angles of 45 deg. The Lorentz form has been shown to be a good approximation for the incoherent backscatter spectrum at these heights from both theoretical and experimental work/9/. Aliasing of the spectra resulted from the low elevation and the consequently large Doppler frequencies due to meridional winds. By assuming continuity of velocity with height the Doppler shifts were de- aliased. The de-aliased fitted spectra are shown by broken lines. The spectral widths provide valuable information on ion-neutral collision frequency and other quantities and are analysed in /10/.

75 km I " "1 . . . . . I - " - I - - ¢

I - - - " I " - I / 7

I . . . . . . I -

72 km

/ i d . . . . "1 . . . . v . . . . r . . . . .

69 krn ~

66 km ~

63 km

60 km

-200 0 200 [Hz]

F~g. 1 Altitude stack of spectra obtained during a PCA on 1 December 1989. The dotted line shows the fitted de-aliased Lorentz spectra and the broken line shows the Doppler shifted peak, illustrating the effect of aliaslng. The fitted spectra in the range 66-69 km are shifted to the left, out of the measured frequency window.

Figure 2 shows profiles of meridional velocity measured during two intervals with a separation of three days during a PCA in October. Each profile is an average over two minutes, and the separation between profiles is about 30 minutes, with northward velocities being positive. Figure 3 shows similar prof'des for the other two events, one in December 1989 and the other in March 1990. In most of the events there is a generally northward background wind with structures of much larger velocity on smaller vertical scales of 7-10 km superimposed. In three of the four d_ata sets there is a pronounced peak of northward velocity ranging between 50 and 80 m/s at around 70 km. Examining the longest data set from the 23 October more closely, we see that the peak in velocity at 70 km at 1300 UT decreases slowly m height at a rate of 0.85 kin/hr. If the broader peak near 79 km is the next peak of the same wave (broken sloping lines), then the vertical wavelength is about 10 km which gives a wave period of 11 hours. If the uncertainty in the vertical wavelength from this data is 2 kin, then the period could be in the range 9 -14 hours which is a candidate for the semidiumal tide, or a 10 hour period or possibly even an 8 hour period wave. Waves of 8 hour period have been reported by Balsley et a l . /11/ in VI-IF radar data from Poker Flat, Alaska, and by Rilster et al . /12/in SOUSY VHF radar data from AndCya, not far from Troms¢. 10 km seems, however, a rather short vertical wavelength for the semidiurnal tide although at altitudes between 25 and 60 km measurements indicate the presence of higher order modes with wavelengths of 15 km or less (Forbes and Groves/13/). The amplitude is also rather high compared to measurements at slightly greater altitudes at Andoya (/12/). Moreover, modelling by Forbes /14/ suggests that amplitudes should decrease with decreasing altitude and measurements between 25 and 60 km indicate amplitudes of 3-12 m/s (/13/). Clearly, longer time series are needed to resolve such long period waves. There is also a peak which decreases with height in the 1 December data in Figure 3, shown by the broken line. The rate of decrease is 1.6 km/h and the vertical wavelength is probably greater than 10 km although the measurements do not allow a reliable estimate to be made. This implies a period for the wave of > 6 hours. Again, this may be related to waves reported by other workers ~IlL/12/.

Atmospheric Dynarmes dining Solar Proton Events (10)83

20 October 1989 23 October 1989

[km]

" ° ' ~ i: i :

E L' 1 1 i i

r° E = = 1

i i ' ' i 1 1: 1

t ~ t ~ t

i ] i ~ i

~5 | i i ; I ~'" b -50 0 50 100 150 200 250 0 0

ve~oc~y [m/s] 1002UT ~ 1032UT ~ 110~UT

1134UT --N-- 1202UT ~ 1;~3~UT

[Wn] !10

?0

6O

-50 5O

I • i t

100 150 200 250

~10=='¢,/[m,'s]

! t 1

300 350

I : )00UT "-'t-" I:)gl0UT " 'NI- 1400UT ~ 14S0UT

"-)4"- t 5~0UT " e - - 1530UT --dk-- 1 0 ~ U T

Fig. 2 Northward velocity profiles obtained from spectra like those in Figure 1 for the October 1989 PCA. Each profile is an average over 2 minutes and each profile is offset by 50 m/s to the right of the previous one. The sloping solid lines show how the velocity peaks descend with height.

[km] I!0

75

70

65

60

55

1 December 1989

1

50 100 150 200

vtao¢~¢ [m/s l 0928 UT

~068 UT

250

[kin] 80

zs- %~

7 0 -

J

8 5 -

8 0 - ?

55 - 5 0 0 50

IO04UT "-)N'- ~028UT ~ 1;~4~,UT " ) ( ' - 112~UT ~ $ 4 ~ U T

20 March 1990

1 0 0 1 5 0

- -+ ' - 1~106 UT

"-)<-- 14~5 UT

- - I t ' - 1 = ,16 U T

3 0 0

Fig. 3 Northward velocity profiles, as in Figure 2, for two other PCA events. The December event shows a minimum in velocity (sloping solid line) which also descends with height.

The mcrease with time of the minimum height of the available velocity data on 23 October (fig. 2) is caused by the decreasing electron density at sunset resulting in signals too weak to derive a Doppler shift. The missing points in the lower parts of the prof'fles for 20 October and 1 December were caused by high antenna elevations and not by a lack of solar illumination. This shows that even under PCA conditions one may not be able to get a long uninterrupted time series of velocity data, depending on the solar zenith angle for the time of year.

SUMMARY AND DISCUSSION

We have shown that velocities with accuracies of about 1 m/s, with a time resolution of less than a few minutes and range resolution of 1.05 km are measurable down to 54 km during PCA events with the

(10)84 M. T. Raetveld et al.

EISCAT UHF radar by tilting the antenna to low elevations. Slowly varying wavelike structures with vertical wavelengths of about 10 km and northward velocities reaching 80 m/s were seen. Whether these structures are related to the PCA event or are already present in the neutral atmosphere is difficult to say. They may be related to 8- or 10-hour waves or semidiurnal tides. Longer time series of both meridional and zonal velocity measurements are necessary to compare the measured winds with models of txdes. Recent longer runs of VI-IF-~dar versions of the same experiment are being analysed to this end. The better sensitivity of the EISCAT VHF radar should allow similar measurements during conditions with weaker electron densities, thereby improving the coverage in time.

ACKNOWLEDGEMENTS

The EISCAT Scientific Association is supported by the Centre National de la Recherche Scientifique of France, Suomen Akatemia of Finland, Max-Planck-GeseUschaft of West Germany, Norges Almenvitenskaplige Forskningsrfid of Norway, Naturvetenskapliga Forskningsrfidet of Sweden and the Science and Engineering Council of the United Kingdom.

REFERENCES

1. J. D. Mathews, The incoherent scatter radar as a tool for studying the ionospheric D region, J. Atmos. Terr. Phys., 46, 975 (1984)

2. B. B. Balsley, The MST technique - a brief review, J. Atmos. Terr. Phys., 43,495 (1981)

3. J. ROttger, and W. Meyer, Tidal wind observations with incoherent scatter radar and metec~logical rockets during MAP~WINE, J. Atmos. Terr. Phys., 49,689 (1987)

4. P. N. Colhs, and J. ROttger, Mesospheric studies using the EISCAT UHF and VI-IF radars: a review of principles and experimental results, J. Atmos. Terr. Phys., in press (1990)

5. K. Folkestad, T. Hagfors, and S. Westerlund, EISCAT: an updated description of technical characteristics and operational capabilities, Radio Sci., 18, 867 (1983)

6. M. Baron, The EISCAT facility, J. Atmos Terr. Phys., 46, 469 (1984)

7. T. Turunen, GEN-system - A new experimental philosophy for EISCAT radars, J. Atmos. Terr. Phys., 48,777 (1986)

8. P. N. Colhs, and M. T. Rietveld, Mesospheric observations with the EISCAT radar during polar cap absorption events: 1. Electron densities and negative ions, Annales Geophysicae, in press (1990)

9. J. D. Mathews, Incoherent scatter radar probing of the 60-100-kin atmosphere and ionosphere, Proc. I E E.E. Trans. Geosc. andRemote Sensing, GE-24,765 (1986)

10. M. T. Rletveld, and P. N. Collis, Mesospheric observations with the EISCAT radar during polar cap absorption events: 2. Spectral Measurements, Annales Geophysicae, submitted (1990)

11. B. B. Balsley, W. L. Ecklund, and D. C. Fritts, VHF Echoes from the High-Latitude Mesosphere and Lower Thermosphere: Observations and Interpretations, Y. Atmos. Sci., 40, 2451 (1983)

12. R. Ruster, P. Czechowsky, and G. Schmidt, VI-IF ~daX observations of tides at polar latitudes in the summer mesosphere, J. Atmos. Terr. Phys., 12, 1041 (1988)

13. J. M. Forbes, and G. V. Groves, Atmospheric Tides below 80 km, MAP Handbook, 16, 157 (1985)

14. J. M. Forbes, Atmospheric Tides 2. The Solar and Lunar Semidiurnal Components, Y. Geophys. Res, 87, 5241 (1982)