Mesospheric temperature response to variations in geomagnetic activity

Planet. Space Sci, 1973, Vol. 21, pp. 1447 to 1453. Pergamon Press. Printed in Northern Ireland

MESOSPHERIC

TEMPERATURE RESPONSE TO VARIATIONS IN G E O M A G N E T I C ACTIVITY

S. RAMAKRISHNA and R. SESHAMANI Department of Aeronautical Engineering, Indian Institute of Science, Bangalore-12, India (Received in final form 19 December 1972)

Abstract--Temperature data collected over several years from rocket grenade and other experiments at Point Barrow (Alaska), Fort Churchill (Canada) and Wallops Island (Virginia) have been analysed to determine the effect of geomagnetic activity on the neutral temperature in the mesosphere and to study the latitudinal variation of this effect. An analysis carried out has revealed almost certainly significant correlations between the temperature and the geomagnetic indicies K~ and A~ at Fort Churchill and marginally significant correlations at Barrow and Wallops. This has also been substantiated by a linear regression analysis. The results indicate two types of interdependence between mesospheric temperature and geomagnetic field variations. The first type is the direct heating effect, during a geomagnetic disturbance, which has been observed in the present analysis with a time lag of 3-15 hr at the high latitudes and 36 hr at the middle latitudes. The magnitude of this heating effect has been found to decrease at the lower altitudes. The second type of interrelation which has been observed is temperature perturbations preceding geomagnetic field variations, both presumably caused by a disturbance in atmospheric circulation at these levels. INTRODUCTION Several studies of the effect of geomagnetic activity on neutral temperatures have been carried out so far by Jacchia et al., (1) Devries et al. (~) and Roemer, (z) using density data obtained from satellite drag analysis. Hicks and Justus (4) have studied the response of winds in the lower thermosphere to variations in geomagnetic activity from a series of rocket borne chemical release experiments at Eglin, Florida (30°N) while Shefov (5) has analysed the effect of geomagnetic activity on the intensity and rotational temperature of the hydroxyl emission recorded by observatories spanning a wide range of latitudes (0°-70°N). In this paper we have attempted to study the correlation between temperatures in the mesosphere and geomagnetic activity as characterised by the 3-hourly quasi-logarithmic index K~ and the daily index A~. The temperature data have been taken from several NASA reports. (6,7~ These data have been collected from a series of rocket grenade experiments conducted at Point Barrow, Alaska (71°N 157°W), Fort Churchill, Canada (59°N 94°W) and Wallops Island, Virginia (38°N 75°W), during the period 1956-1969. Some of the soundings during the earlier years of this period were carried out using falling sphere and pitot tube probes. The geomagnetic indicies were obtained from Euler's (s) tabulations of indices of solar and geomagnetic activity for the years 1955 through 1969. METHOD OF ANALYSIS The correlation co-efficient C(TL, Xr, ~, L) between the temperature (T) and the geomagnetic index (X -----K~, A~), over different altitude regions (L) and varying time lags (~-) relative to the launch time were calculated (4,9) using the expression 2I

~_, (r~,~ C ( T L , Xr, ~, L) =

T~)(x,,,~ - ~ )

n=l

(T~,L -- TL)~ n=~E(X~,~ -- X~)~t 1447

i448

s. RAMAKRISHNA and R. SESHAMANI

where X,,T is the value of X at a time "r with respect to the launch time for the nth sou~din!z and N is the total number of soundings at each station. T,~.L is the corresponding average temperature in a specific layer L..VT and Tr~ are the averages of X,,~ and 7)~.~: respec i~'l?. for all the soundings, For the K~ index, the correlation coefficients were computed for the period 1-1 ....... i:5 i~rH + 42 hr where H is the hour of launch. Similarly, for the A~ index, the period D 10 d a y s - D -+- 9 days, where D is the day of launch, was considered. The significance ot the computed correlation coefficients was obtained by applying the Student's t-test. ~°~ In this analysis, a significance level of 10 per cent was considered marginally significant and a level of 1 per cent as almost certainly significant. The variation of the extent of correlation with ~- was obtained from the C - ~r plots. Correlation peaks occurring in the region of negative ~- values signify temperature changes lagging behind the geomagnetic disturbance while those in the region of positive ~" values indicate a temperature change preceding the geomagnetic field variations. It has to be noted that this analysis is incapable of detecting features with a finer time resolution than 3 hr for the K~ analysis and 1 day for the A~ analysis, since these indices are available only at 3-hourly and daily intervals, respectively. The parameter ( A T / A X ) L , , was computed from a linear regression analysis to obtain a quantitative estimate of the effect of variations in the geomagnetic indices X on the temperatures T - - a s a function of ~-, in different altitude regions. In the above analysis, individual errors in the measured temperatures were not considered as it was found that the overall error, over the large layer thicknesses considered, was quite small (I°K-2°K). The absence of accurate seasonal models at the high latitudes has precluded the elimination of the seasonal component in the temperature data. However, a study of the effect of seasonal variations using available data and models constructed from these has shown that the conclusions drawn in this paper will not be significantly modified by this alternate analysis. RESULTS AND DISCUSSION The characteristics of the principal correlation coefficients at each of the three stations in the altitude region 61-90 km and the number of soundings used in each case are given in Table 1. The correlation coefficients are also plotted as a function of ~- in Figs. 1 and 2 for all the three stations, over the altitude region 61-90 km. TABLE 1. CORRELATIONOF T ~

Kp AND A~ FOR THE ALTITUDE REGION 61--90

Total no. of soundings

No. of soundings used

Parameter

Point Barrow 71°N Fort Churchill 59°N Wallops Island 38°N

46 83 112

33 42 43

K~ Kp K,

71°N Fort Churchill 59°N

46 83

33 42

A.

Wallops Island 38°N

112

43

A~

Station

Point Barrow

A~

Time of correlation peak H -HH -H -H+ D+ D -D -D --

3 15 6 36 24 7 8 5 9

km

COrrelation Regression coefficient coefficient 0-34 0.53 0"27 0.27 0.30 0.49 0'46 0-45 0.32

3-74 9.69 2-37 2.58 2.88 0.41 0.83 0.97 0-63

RESPONSE

TO VARIATIONS

IN GEOMAGNETIC

ACTIVITY

1449

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•Time relot-ive~'launch, hi"

PLOT OF THE CORRELATION COEFFICIENT C , BETWEEN THE AVERAGE TEMPERATURE T IN THE 6 1 - 9 0 km REGION AND K ~ , v s THE TIME LAG ~" IN HOURS.

t

o4 Borow

Churchill 06

-

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04

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FIG. 2.

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Time relofive t0 launch, days \--"•'~.,,/•

PLOT OF THE CORRELATION COEFFICIENT C , BETWEEN THE AVERAGE TEMPERATURE THE 61-90 km REGION AND ,4v, VS THE TIME LAG 7" IN DAYS.

T

IN

1450

s. RAMAKRISHNA and R. SESHAMANI

('a) Correlation with K v

It can be seen from Fig. 1 that there is a strong correlation between K~, and the average temperature in the 61-90 km region at Fort Churchill. This peak at H ....... 15 hr indicates that temperature changes lag 15 hr behind K~ changes. A peak is also observed, through of lesser significance, at Point Barrow ( H - 3). There are two peaks observed at Wallops Island, one at H -- 36 and another at H--- 6, but these have lower significance leveis. The high positive correlation at Churchill between K~ and the temperature with a time lag of 15 hr suggests a direct heating mechanism at altitudes extending down t~ the 61-90 km region. Such direct heating effects have already been observed during periods of magnetic disturbance by JacchiaJ ~) Devries ~2~ and Roemer ~3) at altitudes of 140 kin. From a theoretical study of the effects of magnetic storms on the neutral composition of the upper atmosphere, Mayr and Volland ~ have concluded that the predominant heat source in the direct heating effect observed during geomagnetic disturbances is Joule dissipation in the auroral electrojet. In a recent study, Devries c~2) has reported the results of the LOGACS experiment, which studied the direct heating effect in the 140-.300 km region by using satellite borne low-g accelerometers. A time lag of about 1.5 hr was observed between the magnetic disturbance and associated atmospheric perturbation at high latitudes, the lag increasing to about 6 hr at low and middle latitudes. It was also found that this heating generated waves which spread out radially from the auroral electrojet region. The wave amplitudes were found to increase with altitude but to decrease as they penetrated downwards into the regions of higher density. The value of 15 hr for the lag at Churchill and 36 hr at Wallops could be attributed to the time required for these disturbances to be felt in the 61-90 km region. The amplitude of the heating effect (Air' = 9.7K~,) at Churchill also appears to be reasonable, taking into account the relation (AT == 28K~) (~) at the higher levels and the fact that this is expected to decrease as we go downwards into the more dense region. The time lag values should only be taken as approximate, in view of the limited time resolution of this analysis. It should be noted that data are not available during all magnetically disturbed periods, but only at specific times. Let us consider a situation where a magnetic disturbance, as observed by an enhanced value of K~, has occurred at a particular time. If this is to result in an enhanced temperature with a time lag r~, the correlation value obtained would very much depend on whether the temperature measurement was made at a time corresponding to this time lag or at some other time. Measurements made at other times would result in apparent correlations appearing at such time lags, though with smaller values. It is this factor which is causing a broadening of the correlation peak and also the occurrence of secondary peaks at other time lag values. However, using a sufficiently large number of firings and obtaining a statistically significant result removes this problem to some extent. It is observed that, of all peaks, there is one peak at each station, which remains prominent even if the set of soundings is altered, while the secondary peaks tend to depend very much on specific soundings. The peak which persists for most of the sets has been taken as the principal peak and the corresponding time lag is taken to be representative of the actual situation. In the case of Point Barrow, a correlation peak is observed at H ..... 3 hr. However, this could partly be attributed to the very large temperature fluctuations in the 70-90 km region which are known to occur over this station, as observed by Nordberg caa) and Ramakrishna. (~4) It is quite conceivable that these temperature fluctuations, of the order

RESPONSE TO VARIATIONS IN GEOMAGNETIC ACTIVITY

1451

of 50°K or more, could cause the suppression of a genuine correlation peak and the enhancement of some other peak. As has already been indicated, the peak at Wallops is not sufficiently significant. However, from the general trend of the correlation curves, it is possible to identify the conspicuous peaks in each case, as marked by an arrow in Fig. 1. It is also worthwhile noting that at H - 3, H - 15 and H - 36, either a main peak or secondary peaks exist at all three stations. For uniformity, only the correlation values obtained over identical altitude regions have been plotted in Fig. 1 and referred to in the discussion so far. However, there are higher correlation values with correspondingly higher significance levels over slightly differing altitude regions for the three stations. Table 2 gives the correlation values with the highest significance levels along with the regression values at each station, for different layer thicknesses. TABLE2. CORRELATIONRESULTSFORVARYINGLAYERTHICKNESSES(T ANDK))

Station

Point Barrow 71°N

Fort Churchill 59°N

Wallops Island 38°N

No. of soundings

30

51

43

Time of occurrence of peak

Correlation Regression Levelof Layer (km) coefficient coefficient significance

H-- 3

62-91 67-91 72-91 77-91 82-91

0-41 0"40 0'38 0'38 0.39

4"44 5.41 6.49 7.82 9.19

H -- 15

60-89 65-89 70-89 75-89 80--89

0.51 0.50 0"49 0.45 0-42

8-76 9'91 11"36 11-62 11.46

0.1 0.1 0.I 0.1 2

H -- 36

61-90 66-90 71-90 76-90 81-90

0-27 0.29 0.32 0'35 0.37

2.58 3"12 3'85 4"64

30 25 20 10 10~

5.39

10~ 10~ 10 15 I0~

It can be seen that the peak has a time lag of 3 hr at Barrow, 15 hr at Churchill and 36 hr at Wallops (the peak at H -- 6 hr at Wallops does not persist over all the sets of soundings and therefore need not be considered here). The correlation at Churchill is found to be almost certainly significant (0.1 per cent) while that at Barrow is marginally significant. The value at Wallops is of doubtful significance and has to be considered only because it follows the general pattern. But it should be noted that this peak becomes marginally significant if only the higher altitude regions are considered (Table 2). An important feature to be noticed in Table 2 is the increasing value of the regression coefficient as we consider the higher layers. This confirms the conclusion that the heating is larger at the higher altitudes. The correlation coefficient, however, decreases slightly, due to the larger scatter at the higher altitudes as a result of larger errors in the temperature data at these altitudes. It has been observed that the regression value increases continuously from 0.19 for the 51-70 km region to 13.83 for the 76-95 km region at Churchill, for the direct heating effect. This feature of larger heating at higher altitudes is also evident at the other two stations.

1452

S. RAMAKRISHNA and R. SESHAMANI

It can also be seen from t:ig.l that there is a positive correlation peak for Churdfiii at H + 18 and for Wallops at H F- 24, indicating that here the temperature mcre~,.~,~: precedes enhanced geomagnetic field variations by 18--24 hr. Similar behaviour has :dso been reported by Hicks and Justus (~) in their analysis of the relationship between ~,~ :tnd wind velocities in the 90-140 km altitude region over Eglin, Florida (30°N). They reported a positive peak in the correlation at t f 21. This is an indication thal the temperature perturbations and the geomagnetic field variations are both caused by circulation disturbances in the mesosphere and lower thermosphere. These circulation disturbances are probably caused by some other phenomenon which occurs first, followed by a temperature and wind velocity change and later, by a / ~ change. At Barrow, a strong negative correlation is present at H + 39 (Fig. 1), This indicates a temperature variation preceding geoma~letic field variations by 39 hr, but with the two varying out of phase. (b) Correlation with A,, In the case of A~, there is a high correlation at D -- 8 and D -- 5 for the same altitude region (61-90 kin) as for K~, at Churchill (Fig. 2). There is also high correlation at Wallops at D -- 9 and D -- 5. The correlation peaks are listed in Table 1. The correlation analyses for Churchill and Wallops, between temperature and A~, indicate that long term disturbances set up in the upper mesosphere and lower thermosphere, lasting over a few days, cause higher mesospheric temperatures about 5-10 days after a magnetically disturbed day. Scherhag ~15~has also observed heating of the order of 5°K at the 10 mb level over Berlin (50°N) occurring about 5-10 days after days of high magnetic disturbance as characterised by high values of A~. In the present T -- A~ analysis, the heating effect at Churchill is of the order of AT := 0.97Av, 5 days after the day of high A~. The effect, however, is not observable at Barrow though it is quite pronounced at Churchill and less so at Wallops. This indicates that circulation disturbances of the long term type may not be similar at different locations, ('~) even though these may be approximately at the same geomagnetic latitude. A secondary peak can also be noticed around the D day confirming the suggestion of a direct heating effect with a lag of a few hours, as evidenced by the K~, analysis. At Barrow there is a positive correlation peak at D + 7 which could be interpreted in the same manner as the K~ peaks for positive values of % as discussed above. This effect is also observed at Churchill at the same time lag but not at Wallops. These results indicate circulation disturbances persisting over several days, caused by magnetic activity. At Wallops, on the contrary, there appears to be a broad negative correlation spreading from D + 2 to D q- 8. This behaviour was also obtained by Ramakrishna (14) in an earlier analysis for Wallops for the years 1961-1967. In that analysis too, a negative correlation between A~ and temperature at the 80 km level was suggested. The present analysis indicates that this effect is also present up to 90 km at Wallops, with a temperature decrease in the mesosphere preceding high magnetic activity. Another interesting feature is the oscillation in the regression values as well as the correlation values which change from negative to positive and vice versa with a period of a few days, at Barrow and Wallops (Fig. 2). This oscillation is probably the cause of the negative correlations for negative ~- values. Shefov (~) has observed a similar oscillatory phenomenon in the relationship between magnetic activity and the rotational temperature of the OH emission.

RESPONSE TO VARIATIONS IN GEOMAGNETIC ACTIVITY

1453

CONCLUSIONS The present analysis has demonstrated two types of interrelationship between mesospheric temperatures and geomagnetic activity. The first is the direct heating effect, which is found to be present in the mesospheric region. This was also observed at higher altitudes experimentally, using satellite observations. The temperature change is found to lag behind the magnetic disturbance by about 12-15 hr at Churchill, where this effect is predominant and is given by A T = 9.7Kj,. The second feature is the temperature changes which precede geomagnetic activity, indicating a source lying in the circulation disturbances caused by solar electromagnetic radiation effects which themselves precede corpuscular radiation effects. The correlation results are almost certainly significant at Churchill and marginally significant at Barrow. The correlations at Wallops follow the general trend and some of these are also marginally significant. REFERENCES 1. L. G. JACCHIA,J. SLOWEYand F. VERNIANI,J. ffeophys. Res. 72, 1423 (1967).

2. 3. 4. 5.

L. L. DEVRIES,E. W. FRIDAYand L. C. JONES,Space Research, Vol. VII, 1173 (1967). M. ROEMER,Space Research, Vol. VII, 1091 (1967). J. E. HICKSand C. G. JUSTUS,J. geophys. Res. 75, 5565 (1970). N. N. SHEFOV,Planet. Space Sci. 17, 797 (1969). 6. W. S. SMITH,A. AZCARRAGA,J. F. CASEY,J. J. HORVATtt, L. B. KATCI'IEN,P. SACItER, P. C. SWARTZ and J. S. 3~EON, NASA Tech. Repts. TR-R-211 (1964), TR-R-245 (1966), TR-R-263 (1967), TR-R-288 (1968), TR-R-316 (1969), TR-R-340 (1970), TR-R-360 (1971). 7. R. A. MINZNER,P. MOR~ENSTERNand S. M. MELLO,NASA Tech. Rept. CR-R-85565 (1967). 8. H. C. EULER,NASA Tech Rept. TM-X-64526 (1970). 9. S. RAMAKRISHNAand R. SESHAMANI,XV COSPAR Symposium (1972). 10. E. T. FEDERIGHI,J. Am. statist. Ass. 54, 383 (1959). 11. H. G. MAYRand H. VOLLAND,Planet. Space Sci. 20, 379 (1972). 12. L. L. DEVRIES,NASA Tech Rept. TN-D-6518 (1971). 13. W. NORDBERG,Annals of the IQSY, Vol. 5, 53 (1969). 14. S. RAMAKRISHNA,Indian J. pure appl. Phys. 9, 526 (1971). 15. R. SCHERHAG,Annals of the IQSY, Vol. 5, 33 (1969).