Pergamon Adv. Spcr Res. Vol. 18, No. 3, pp. (?)79-(3)82, 1996

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STUDIES OF THE E-REGION ION-NEUTRAL COLLISION FREQUENCY USING THE EISCAT INCOHERENT SCATTER RADAR

T. Nygren

Department of Physics, University of Oulu, FIN-90570 Oulu, Finland

ABSTRACT

The ion-neutral collision frequency in the aurora1 E region is one of the parameters which has been investigated using the EISCAT incoherent scatter radar during its history of more than ten years. In this paper results from EISCAT studies of collision frequency are reviewed and the importance of continuation of such work is emphasised.

INTRODUCTION

In the E region altitudes the shape of the ion line in the incoherent scatter spectrum depends on electron density, ion and electron temperatures and ion-neutral collision frequency. Hence the inco- herent scatter method provides a possibility to determine the collision frequency and atmospheric neutral density from the shape of the scattering spectrum. In practice, however, there are serious dif- ficulties due to the fact that increasing the collision frequency and increasing the electron tempera- ture in fixed ion temperature have a fairly similar effect on the spectral shape. Therefore some addi- tional information or assumption has usually been necessary in collision frequency measurements. In the lowest E region one can assume equal ion and electron temperatures but at greater heights, and especially at aurora1 latitudes, this assumption is not necessarily valid. The difficulty can be avoided to some extent by choosing data from quiet geomagnetic conditions but even then some uncertainty remains about the validity of the results.

In spite of the inherent problems of the method, collision frequencies have been determined from incoherent scatter measurements ever since the late 1960’s. After its installation in northern Scandi- navia in the beginning of the 1980’s, the EISCAT UHF radar has been used in collision frequency studies at aurora1 latitudes. Although this work has not been very extensive, it has affected the devel- opment of atmospheric models and its continuation should be encouraged in the future.

EISCAT OBSERVATIONS OF COLLISION FREQUENCY

The first EISCAT studies of ion-neutral collision frequency date from 1985. Kofman and Lath- uillere /l/introduced a new EISCAT experiment and showed, among other parameters, profiles of collision frequency from two days. The data were integrated from the duration of the whole experi- ment selecting periods of low precipitation and Joule heating so that the assumption of equal ion and electron temperatures could be applied. By fitting the results to exponential profiles, scale heights of about 6 km were obtained for the collision frequency.

Fla et nl. /2/calculated collision frequency profiles with a time resolution of 10 min. The data were from selected periods in mid-winter and late spring. The analysis was carried out with two different assumptions; one is the equality of the two temperatures and the other is static equilibrium of the neutral atmosphere. The second alternative implies the determination of collision frequency at a single reference height only, and therefore both temperatures could be determined in the analysis. It

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was observed that the collision frequency profiles given by the latter method were smooth and near- ly identical, whereas the collision frequencies given by the former one were more scattered. An event was also observed where the collision frequency increased by a factor of 1 S-2 in about 1.5 hours.

Huuskonen et al. /3/ showed a few winter-time profiles measured using an experiment with a range resolution of 600 m. This resolution is clearly higher than that in all previous works (2-3 km). The temporal resolution was 20 min. Equal ion and electron temperatures were assumed in the analysis. A fairly good agreement with the CIRA-72 model was found at 100 km altitude but the scale height of the model was clearly greater than that observed.

An effort of using a larger data set was made by Kofman et al. /4/ who showed results from six nights in January, February, March, June and December. The temperatures were assumed to be equal, the data were post-integrated over 5 min in time, and average profiles from each night were calculated. No clear annual variation in the scale height of the collision frequency was observed.

The most extensive collision frequency study by EISCAT was done by Kirkwood /5/. Data with a complete diurnal coverage from selected days in all the other months except March and July were collected for the analysis. Although gaps were created in the results during periods of low data qual- ity, both diurnal and seasonal variation could be studied. The collision frequency was determined at altitudes below 105 km and the equality of the two temperatures was assumed. Comparisons with CIRA-72 and MSIS-83 were made and considerable discrepancies were observed. The results of this paper have later been used in developing the MSIS model /6/.

Huuskonen /7/ calculated collision frequency profiles from several days in February, July and Au- gust. Although the data base of this study is rather large, the distribution of the observations did not allow the determination of annual variations. This is because the work is based on special experi- ments used only during special campaigns. In addition to a high range resolution (600 m and 1.2 km), a specific feature in this work is a careful planning of analysis strategies using advanced error analysis. An agreement between the MSIS-86 model and the observations was found in July but in February and August the observed collision frequencies were greater than the model values.

The collision frequency analysis in all papers cited above was based on the shape of the ion line of the incoherent scatter spectrum, and the resulting profile is limited to a height range of about 90-l 10 km. Attempts have been made to avoid the extra assumptions needed in the collision fre- quency analysis by taking advantage of the other features of the spectrum. Bjorna /g/used simulta- neous observations of the ion and plasma lines and, with the additional information offered by the plasma line, determined both the electron density, the two temperatures and the collision frequency. The resulting collision frequency profile agrees well with the MSIS-83 model in June. When a con- ventional analysis was made using the ion line, the results were generally lower and more scattered.

A completely different approach based on the Doppler shift of the ion line was made by Nygren ef al. /9/. They used an EISCAT experiment with a vertical beam and measured the electric field and the profile of the vertical ion velocity. Choosing periods of strong electric field, the effect of neutral wind on ion motion could be neglected and the collision frequency could be solved from the verti- cal component of the ion momentum equation. In this way the collision frequency could be deter- mined up to 130 km and, between 100 and 110 km where the conventional method was also appli- cable, the agreement between the two methods was good. The idea was later developed to include the neutral wind /lo/. In this modified version of the method two beam directions (vertical and tilted to the east) were used. Then it was possible to eliminate the (horizontal) neutral wind components and solve the collision frequency as a function of the electric tleld and beam-aligned ion velocities.

A set of collision frequency results collected from several of the above papers is presented in Fig. 1. All these profiles are from 13-15 February and a comparison with the MSIS-86 model from the same period reveals that the observed collision frequencies are, especially at the bottom part of the profile, generally higher than the model values. Close to 110 km the results are more scattered than

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Fig. 1. Selected collision frequency profiles observed by EISCAT.

lower down. This may be caused by an increase of the statistical error with height or temperature ra- tios departing from the assumed value of unity.

An exception in Fig. 1 is the profile by Fla et al. /2J which is in agreement with the MSIS model. The difference between this profile and the others is that FlZi et al., assuming a static equilibrium of the atmosphere, was able to solve both the collision frequency and the two temperatures, whereas in all the other profiles the temperature ratio is fixed to unity.

As a second example, results taken from Nygren et al. /lo/ are shown in Fig. 2. These collision fre- quencies were calculated from two separate time intervals of about 1 h. The bottom parts of the pro- tiles were obtained by the conventional method and the upper parts, reaching the altitude of 130 km, by using ion velocity profiles from two directions of the radar beam. In agreement with the re-

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COLLISION FREQUENCY / kHz

Fig. 2. Collision frequencies calculated using the conventional method (triangles) and ion velocities in two directions of the radar beam (dots) /lo/.

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sults in August by Huuskonen &‘/,higher collision frequencies than those predicted by the MSIS-86 model were observed. Although the profiles are rather scattered at some heights, their average would be rather smooth. When studying the individual velocity profiles, it becomes evident that the most violent kinks in the collision frequency profiles are due to similar kinks in the velocities, probably indicating the presence of wave activity in the background neutral atmosphere.

DISCUSSION

The early EISCAT papers on ion-neutral collision frequency often had a role of testing new ideas and methods or the capabilities of the new radar. Although most of these studies were based on too small data sets to have any wider use, this work has indeed affected the development of the MSIS model. It seems obvious that these investigations should be continued and encouraged.

The data collected during the past years give a possibility to studies of long-term variations of colli- sion frequency and neutral atmospheric density. The modern radar programmes based on new modulation principles as well as the new analysis software allowing a wide choice of analysis strate- gies are expected to lead to improved reliability and accuracy. In order to have the best possible re- sults, even the old data sets should be reanalysed using the modern methods. The application and development of the methods reaching altitudes above 110 km would also be useful.

The new ESR radar to be installed on Svalbard in the nearest future will extend the latitudinal range of the measurements. Then the EISCAT system will be able to provide observations from two sites separated by 8” in latitude. This opens new possibilities for EISCAT in the development of atmo- spheric models in the future.

REFERENCES

1. W. Kofman and C. Lathuillere, EISCAT multiple pulse technique and its contribution to aurora1 ionosphere and thermosphere description. J. Geophys. Res., 90, 3520-3524 (1985).

2. T. I%, S. Kirkwood and K. Schlegel, Collision frequency measurements in the high-latitude E re- gion with EISCAT. Radio Sci., 20, 785-793 (1985).

3. A. Huuskonen, T. Nygren, L. Jalonen, T. Turunen and J. Silen, High resolution EISCAT observa- tions of the ion-neutral collision frequency in the lower E region. J. Armos. Terr. Phys., 48, 827-836 (1986).

4. W. Kofman, C. Lathuillere and B. Pibaret, Neutral atmosphere studies in the altitude range 90-110 km using EISCAT. 1. Afmos. Terr. Phys., 48, 837-847 (1986).

5. S. Kirkwood, Seasonal and tidal variations of neutral temperatures and densities in the high lati- tude lower thermosphere measured by EISCAT. 1. Atmos. Terr. Phys., 48, 817-826 (1986).

6. A.E. Hedin, Extensions of the MSIS thermosphere model into the middle and lower atmosphere. J. Geophys. Res., 96, 1159-1172 (1991).

7. A. Huuskonen, High resolution observations of the collision frequency and temperatures with the EISCAT UHF radar. Planet. space Sci., 37, 211-221 (1989).

8. N. Bjorna, Derivation of ion-neutral collision frequencies from a combined ion line /plasma line incoherent scatter experiment. 1. Geophys. Res., 94, 3799-3804 (1989).

9. T. Nygren, L. Jalonen and A. Huuskonen, A new method of measuring the ion-neutral collision frequency using incoherent scatter radar. Planet. space Sci., 35, 337-343 (1987).

10. T. Nygren, B.S. Lanchester, L. Jalonen and A. Huuskonen, ,A method for determining ion- neutral collision frequency using radar measurements of ion velocity in two directions. Planet. Space Sci., 37, 493-502 (1989).