SUMMARY AND CONCLUSION

Transcription

1 168 CHAPTER 5 SUMMARY AND CONCLUSION A summary of work described in this sis, its main achievements, and a discussion of investigations that can be carried out as an extension, are given in this chapter. 5.1 SUMMARY: The sis describes development of Gerdien condenser payloads for balloon-borne and rocket-borne measurements of atmospheric electrical polar conductivities, ion densities and mobilities. For balloon-borne measurements, a force-aspirated Gerdien condenser has also been designed, fabricated and successfully used in two flights, for first time in India. Four balloon flights and five rocket flights have been carried out with se payloads. All of m, except one rocket flight, have given good data, some of m for first time in India. The rocket measurements have shown minimum in ion density around 62 km. These are some of few flights to have observed this minimum. The positive and negative ion densities match up to an altitude of about 70 km, beyond which negative ion densities are seen to decrease. However, as

2 169 opposed to accepted model profiles, measured negative ion density profile shows a broad maximum around 80 to 85 km. Only one or two previous in-situ measurements conducted from elsewhere have observed such a behaviour. The measured positive and negative ion mobilities agree with those from or measurements. The balloon-borne measurements have given some of first results from Indian zone. One flight has succeeded in obtaining a continuous profile of conductivity from ground to float altitude of around 35 km. The observed ion densities agree with model values, except in case of one flight. The observed reduced mobilities show that ion species remain more or less same in region of measurement. The mobility values are in good agreement with or observations. Measurements during float, covering a period from night to day, do not show any change in any of parameters with sun rise. 5.2 ACHIEVEMENTS OF THE WORK: One of important achievements of work has been development and establishment in India of Gerdien condenser technique for measurement of polar conductivities, ion densities and mobilities in region from surface to about 90 km. The instrument has been almost standardised for balloon-borne and rocket-borne measurments. For balloon-borne measurements, feasibility

3 170 of converting a commercially available lobe type flow meter into a pump, and using it for forced aspiration of Gerdien condenser has been successfully demonstrated. One of balloon-borne measurements has given a continuous profile of polar conductivities from surface to about 35 km, where effect of surface radioactivity and radioactive gases on profile is clearly seen. This is probably first balloon measurement, at least from equatorial region, that has given a continuous profile from surface, and observed effect of radioactivity. The balloon measurements reported here are also first simultaneous measurements from India of polar conductivities, ion densities and mobilities of both positive and negative species in troposphere and stratosphere. The rocket-borne measurements reported here are first simultaneous measurements from India of positive and negative ion densitites and mobilities in mesosphere. The observations of minimum in ion densities, reported here, are some of few such observations. The large negative ion densities around 80 to 85 km seen in last two flights reported here have been observed only very rarely. Only one or two in-situ measurements and a few ground based soundings have observed this earlier.

4 AN OVERVIEW OF THE RESULTS Atmospheric electric polar conductivities have been obtained in region from ground to 35 km, and from 55 to 70 km. Although measurements in two altitude regions have been made from two different stations, namely Thumba at dip equator, and Hyderabad, India, a low latitude station, y can be combined to give altitude profile over a typical low latitude station. The two data sets can be combined because in this region dominated by galactic cosmic ray ionisation, difference between se two stations will not be large. Similarly, ion density and mobility data from two stations also can be combined to give typical profiles for this region. The profile for positive polar conductivity is shown in Figure 5.1, where values from fourth balloon flight (IMAP C05) alone has been shown for lower region. The smooth curve is from empirical equation discussed in Chapter 4. The data for two regions are seen to merge well, showing mutual consistency of measurements. Figure 5.2 shows positive ion density profile for whole region. Here, profile for lower region has been derived from conductivity values obtained in fourth balloon flight. This has been preferred over direct measurements made in earlier flights because fourth flight has shown a greatly improved performance compared to

5 ~ IMAP C st rocket flight... * * * * 2nd rocket flight I I I I I 3rd rocket flight, 5th rocket flight 04-~~~~~~~~~~~~~~~~~~~ 10 -I ~ Positive conductivity (Ohm -1 m - ) FIGURE 5.1 Positive conductivity from rocket and balloon experiments.

6 d. d..do.do.do Rocket First flight Rocket )( )( )( )()( Rocket Second flight Third flight Rocket Fifth flight Balloon IMAP-COS Electron density - - Model ion density 80 w Cl ~ I- ~ 40 4: 20 \ \ \ 'b0~ o~\ ~ ~O~ O~--~~~~~--~~~~~~~~~-, ION/ELECTRON DENSITY. m FIGURE 5.2. Positive ion density from rocket and balloon experiments

7 ors, and data are seen to be of better quality and consistency. The figure also shows a model ~rofile derived 174 from cosmic ray ionisation rates taken from Deshpande et al. (1985) and a parametrised recombination coefficient taken from Datta et al. (1985). The measured profile agrees well with this model profile. In upper region of atmosphere, especially above about 60 to 65 km, ionisation is mainly due to solar ultraviolet radiation, and ion chemical processes are more complex than in lower regions. A number of models have been developed for explaining observations during different normal and disturbed conditions (for instance, Nicolet and Aikin, 1960; Ferguson, 1971; Mitra and Rowe, 1972; Reid, 1977; Chakraborty and Chakraborty, 1978; Deshpande et al., 1985; etc.). In region above about 70 km, negative ions are displaced by electrons. A large number of electron density measurements have been made from Thumba, which can refore be compared with present measurements. Figure 5.2 shows a typical normal days profile for noon time taken from Subbaraya et al. (1983). This is seen to agree well with ion densities reported in this sis. The ion density profiles in this region, shown in figure, have been corrected for variation in mass flow efficiency as reported by Conley (1974). The correction is not exact, since it was made using curve given by Conley (1974) for his sensor for a rocket velocity of Mach 3. However,

8 175 agreement between se values and electron density profile shows that correction has not been far mark. off The measured ion mobilities are plotted in Figure 5.3. The straight line shown is a least square fit for entire data. The fact that straight line fits well with entire data indicates that ion masses are more or less uniform over this region. 5.4 FUTURE PERSPECTIVES Several problems remain to be tackled with regard to area of present investigation. One of gaps in data is between regions covered by balloon measurements and rocket measurements, namely altitude region between about 35 km to about 55 km. While it is difficult to make reliable in-situ measurements in this region, since only access is by using a parachute, lack of data close to region where transition in main ionising source takes place also creates difficulties in obtaining a complete picture. One of problems to be sorted out is regarding effect of aerosols on polar conductivities. While first two balloon flights using self-aspirated Gerdien condensers reported here showed some fluctuations close to tropopause, which were assumed to be due to aerosols, later flights using force-aspirated condensers failed to do so.

9 Q El IMAP C 1- negative,computed. III st rocket fli9ht, positive * * * * * 2nd rocket flight, positive I I I I I 3rd rocket flight, positive 5th rocket flight. positive O~~-rrn~,-.-,,~mo-'-'TTrrrr'-'-TT,"nr-.~ 10 -of FIGURE 5.3. Ion mobility from rocket and balloon experiments

10 Similar fluctuations have been observed by some or 177 experimenters also (for instance, Kondo et al., 1982 a,b). On or hand, Rosen and Hofmann (1885) report that aerosols have hardly any effect on conductivity. A third problem that needs to be looked into is negative ion maximum seen around 80 to 85 km. This contradicts accepted model predictions, and some efforts have been made to interpret m in terms of chemistry. But more observations, and perhaps more model computations are also needed