Characteristics of the mid-latitude trough as determined by the electron density experiment on Ariel III

Journal ofAtmoapherlc andTerrastrisl Physics, 1971, Vol. 33, pp. 1737 to 1761. Pergamon Press Printed in Northern Ireland

Characteristics of the mid-latitude trough as determined by the electron density experiment on Ariel III Y.

(KABASAKAL) TULUNAY and J. SAYERS

Department of Electron Physics and Space Research, University of Birmingham, Birmingham 16. England (Received26 April 1971; in rewired form 4 May 1971) Abstract-Criteria are established to identify mid-latitude troughs by computer analysis of Ariel III electron density data and are used to study the variations of such troughs with time of day, season and magnetic activity. The statistics presented are based on over 1000 troughs and the features found are compared with previoue work. 1. INTRoDOCTI~N IT IS well established

that there exists in the ionosphere a deep trough of ionization This structure is generally known as the along a circumpolar belt in mid-latitudes. mid-latitude trough and has been observed by top-side ionospheric sounding experiments and by ion traps carried aboard satellites (THOMAS and SADER, 1964; MULDREW, 1965; SHARP, 1966). The trough has also been observed using groundbased ionosonde equipment (STANLHY, 1966; BOWMAN, 1968). This mid-latitude trough has recently been the subject of much investigation and has been discussed by many authors including DAYHARSH and FARLEY (1965), THOMAS et al. (1966), CALVERT (1966), CARPENTER (1966), RYCROFT and THOMAS (1970) and LISZKA (1967). In this paper some of the previously reported results are discussed and compared with those obtained from the electron density experiment on board the Ariel III satellite which provided excellent coverage and a large amount of data in the regions concerned. Mathematical criteria similar to those used by previous workers have been used to define the trough-like phenomenon and these provide the basis for computer selection of these features from the electron density data recorded on digitised magnetic tapes. The results have been used to establish correlations between the appearance or structure of troughs and geomagnetic phenomena, The purpose of this paper is mainly to including seasonal and diurnal variations. present the results of these comparisons and correlations in graphical and tabular form. 1.1 Ariel III

data

The electron density data were obtained from an experiment on board the Ariel III satellite. The Ariel III satellite was designed and built in the U.K. and was launched by the U.S.A. (LADD and S~IIITH,1969). The electron density experiment on board has been described by WAGER (1968) and SAYERS et al. (1969). The data were recovered by two methods, high-speed real time data and low-speed tape recorded data. All the results described in this paper were obtained using the tape recorded data. This gave a measurement of electron density approximately every 210 km along the orbit or every l-7’ in latitude over the mid-latitude region. Areil III was launched in May 1967 and the tape recorder operated successfully for 7 months 1737

1738

Y. TULUNAY and J. SAYERS

and then intermittently for the next 4 months thus giving adequate seasonal coverage. The Ariel III orbit is near polar with an inclination of 80” and a period of 95 min. Successive orbits are spaced almost 24’ apart in longitude and the same longitude crossing at the Equator is repeated approximately every 22 days with the same local time repeated every 84 days. During the period of interest the orbit perigee decreased from 494 to 480 km and the apogee from 600 to 576 km. 1.2 Selection of trough-like phenomena A computer programme was developed to search for and select trough-like phenomena from the electron density data using the following criteria all of which had to be satisfied simultaneously. (i) The region of search lies between 30” and 70’ geographic latitudes, in both hemispheres. (ii) The ratio of electron density on either side of the trough minimum to that at the minimum is > 2 until 30” or 70’ latitude is reached on either side of the minimum. (iii) On either side of the minimum the logarithmic electron density gradients (to base 10) are greater than O-l/degree of latitude. Features which satisfy all the above phenomena are identified as mid-latitude troughs. In the Ariel III data:(a) the number of satellite crossings of the mid-latitude region is 22152 (b) the number of actual crossings of the mid-latitude region for which digitised data are available is 14794 (c) the number of trough transits detected by the computer is 1287 and (d) the number of troughs recorded is therefore 8.7 per cent of the mid-latitude transits. The electron density measurements were also plotted by computer as a function of geographic latitude and the 1287 troughs so presented were examined visually and it was found that by this somewhat subjective selection process 84 per cent of the computer selections could be considered as mid-latitude troughs as described by MULDREW (1965). The remaining 16 per cent either had L values which placed them in the high latitude category or exhibited irregular shapes, etc. which placed them in a doubtful category. Figure l(a) is a computer plot of electron density vs. latitude showing a typical computer selected mid-latitude trough and Fig. l(b) shows an example of the computer selected troughs which were rejected on visual inspection, in this case because the trough minimum has an L value 12% The auxiliary abscissae in Fig. 1 also give invariant latitude, geographic longitude, local time, magnetic local time and solar zenith angle. The day number and UT are shown at the top of each diagram. In Fig. l(a) the equatorial anomaly is quite obvious and from 30”s (geographical) the electron density varies uniformly in latitude to near 50”s where there is an abrupt decrease in electron density by approximately a factor of 8 followed at 63% by a very sharp recovery in density. Examples such as this may be called ideal mid-latitude troughs. In the following analysis, unless otherwise stated, the computer selected troughs have been used since they are selected by exactly defined mathematical criteria.

Characteristics of mid-latitude

trough as determined by electron density experiment

DAY205

STARTTIME (UT)r 6.4463

10'1 IO

LAT -,I

INN

20

GE06 LAT. &,’

Jb

34

30

)o

60

’ ’ -30 -00

&

GEOC.LONC101

30

,‘w,

’

’

”

-60

140 MO

LOCAL TIM

0

mt1. SZA.

40 I’ ...L.,,. 60

.-

no

70

90

60

-.-

0 6 -___

0

w-.‘,,,.I’ loo 110 I20

60

b PO NO 100 90

I30

DAY 204 STARTTIME WT):20.9644

1”“. LAT. fl

10 10 30

50

40

30 60

00

60

60

60

70 30

JO

III?.

S7.A.

-,'a 110 L?o.no

,I--, 130 ha

110 100 90 90 70 69

39

,e

90

UI &I

00 II

30

ba

JbL

80

81

m so

Fig. 1. Computer plot of electron density vs. inv. lat., geog. lat., geog. long., local time, magnetic local time, solar zenith angle showing (a) a typical computer selected mid latitude trough and (b) an example of the computer selected troughs which were rejected on visual inspection.

1739

1740

Y. TULUNAY and J. SAYERS

2. ANALYSIS OF THE SELECTED TROUGHS 2.1 The relation between the occurrence of troughs and nmgnetic activity

Figure 2(a) shows a comparison between the number of troughs and the 3hourly sum of magnetic indices, 2 K,, as a function of day number throughout MayDecember 1967. Individual points are joined by straight lines for easy comparison of the variation of the number of troughs and the 2 K, index. It is immediately obvious that the occurrence of troughs varies in a similar manner to the magnetic activity, i.e. the higher K, is associated with the greater number of detected troughs. Assuming that the observations are independent, a cross-correlation coefficient of O-448 was obtained by simply considering the corresponding values of number of troughs and K, for each day. Considering the data of Fig. 2(a) as a sample record, the 95 per cent confidence limits for the population correlation coefficient are found to be 0.34 and 0.50 by using a one-tailed test of the normal distribution. The correlations of the detected troughs with 2 K, over a l-day period within the interval 4 days before to 4 days after the day of the trough measurement is shown in Fig. 2(b). Significant values are obtained only within the 2 day interval The K, index was next split into three groups around the day of observation. 0 Q K, < 3-, 3 Q K, < 5- and 5 G K, < 9, and the occurrence of troughs examined for each of these groups for the southern hemisphere and for the northern hemisphere using all the trough data. To ensure that the results were not influenced by the rate of occurrence of different K, groups, the number of troughs in each K, group as a percentage of the total number of troughs was divided by the percentage of K,‘s in that group to the total K,. The results are shown in Fig. 3 and it is clear that more troughs are detected when K, is high. A similar correlation analysis to that shown in Fig. 2(b) was carried out using D8, indices but no correlation was found. The probable duration of troughs was tested using the auto-correlation coefficients obtained for each day’s total number of troughs and graphically analysed. The half-width obtained is about 1 day indicating that this is the mean life time of troughs. 2.2 Geomagnetic control of the trough position Using polar geographic co-ordinates the distribution of troughs in the northern hemisphere is shown in Fig. 4(a) and superimposed are the curves of invariant geomagnetic latitudes at 600 km (after EVANS et al., 1969) which indicate the preference for geomagnetic alignment of the troughs rather than geographic. The southern hemisphere plots exhibit the same relationship. Since here the magnetic pole is displaced further from the geographic pole than in the northern hemisphere, then by including only selection in 30-70’ geographic latitudes a large sector was missed where the troughs occurred above 70’ geographic. Figure 4(b) shows an Trough min.

Mean

Standard deviation

Geographic Lat. Magnetic Lat. L shells Invariant Mag. Lat. a$ the satellite height

60.07’ 58.04’ 4.16’ (Earth radii) 58.87’

5-49 4.87 o-57 2.19

Characteristics

of mid-latitude

trough aa determined by electron density experiment

Avat13d a3133130 %4 = SHF)nOU~OK38Wi-lNlVlOl s3mvh do 30 wns Aitva

1741

1742

Y. TULUNAY and 5. SAYERS R,CORRELATlONCOEFFICIENT

I SHIFTS IN DAYS

Fig. 2. fb) The oorrelationof the troughs with 2 KB over & l-day period within the interval f4 days from the trough memurement. The peak correlation is statistically highly significantfor the number of detected troughs considered.

example of detection of seven cases on day 314, northern hemisphere for a very quiet period. For these seven cases the positions of the trough minima have the above deviations from their means. Assuming that the Earth’s magnetic field can be represented by a centred dipole field, geomagnetic latitudes and longitudes were derived (LENHART,1968). The number of troughs for each 10’ geomagnetic latitude and longitude section (using normalised areas) was then obtained and the results for the northern hemisphere are shown in Fig. 5 for the different K, groups (using weighted .K,‘s as before). It is apparent that in the northern hemisphere troughs occur mainly in the 50” and 60” geomagnetic latitude region and as K, increases the percentage of troughs occurring at lower latitudes increases. A similar behaviour was also seen in the case of troughs in the southern hemisphere. If the troughs exhibit geomagnetic control then a similar relation should be obtained by considering their behaviour with respect to L value and some histograms were constructed showing the total number of troughs detected in each integral L; interval for different seasons and both hemispheres. However since the satellite spends more time at the lower L values then a statistical correction factor must be applied and the results obtained for different seasons in the northern hemisphere after this correction are shown in Fig. 6. From these it is seen that there are distinct maxima and minima in the distribution of troughs with L values. Since the purpose of this paper is to examine the mid-latitude through phenomena then further analyses are restricted to troughs occurring between _L = 1 and 7. However there appears to be a distinct pattern also at higher 2; values and these should be examined using a co-ordinate system which includes L, local time and distortions of the Earth’s magnetic field by the solar wind (HESS, 1963). From Pig. 6 the conclusion is that the mid-latitude troughs are most hkely to be detected at L values between 3 and 6 or invariant magnetic latitude at the satellite height 50’ .s A < 60’.

Characteristics

of mid-latitude

trough as determined by electron density experiment

1743

Kp GROUPS: s33 aII 45’ 5 SlII 69

0,cI

KP

S.H.

GROUPS N.H.

Fig. 3. The percentage of troughs in each K, group divided by the percentage of K,‘s in that group for southern hemisphere and northern hemisphere.

2.3 Seasonal and diurnal effects The number of troughs detected during the different seasons is shown in Fig. 7 for both hemispheres and it is apparent that there is greater occurrence during winter months. There are two factors which may slightly affect the shape of these histograms. Firstly the tape recorder did not function continuously during early 1968 (southern hemisphere, autumn) and so the data were somewhat incomplete for this period. Secondly playback of 75 per cent of winter data in the northern hemisphere and 78 per cent of winter data in the southern hemisphere was over northern hemisphere stations and therefore some possible data coverage must necessarily have been lost in the northern hemisphere. To investigate the dependence of trough behaviour on local time the histogram of number of troughs vs. local time was plotted and is shown in Fig. 8(a). This includes the period May 1967-April 1968 and covers both hemispheres and all longitudes. In the diurnal variation there is one distinct maximum in early morning and a subsidiary peak in late evening. When the data were subdivided into seasons

1744

Y. TULUNAY and J. SAYXIRS N.H.,WINTER ALL Lf

1800

*Kps3t xKp 2: 4-

I

N.H.,WINTER DAY 314(l967) LT. -02~~03~~ *Kp 6 J+

Fig. 4. (a) The distribution of the detected troughs in the northern hemisphere; superimposed are the curves of invariant geomagnetic latitudes at 600 km (after EVANS et al., 1969) (b) The position of p$w seven troughs for a very quiet period, 10 November 1967, northern hemisphere.

(b)

Characteristics

of mid-latitude trough as determined by electron density experiment

1745

HA

ObKp&-

3,cKpdS-

KP&

5

as z-

GEOMAGNETIC LATlTUDE(Deq.)

Fig. 5. Percentage of normalised troughs divided by the percentage of XV’s in that group as a function of geomagnetic latitude in northern hemisphere (normalised troughs are the number of troughs for each normalised 10’ ~orna~otic latitude and longitude section).

similar results were obtained for each individual season. The frequency distribution with magnetic local time shows a similar form in Fig. S(b, c) but the dependence is clearer in the latter two. To investigate the position of the trough as a function of local time it is necessary to take into account that the position also changes with R,. The variation of trough position with local time is shown in Fig. 9 for two R, groups thus keeping K, approximately constant in each group. It is apparent that as morning approaches the troughs move to lower L values. The line which is drawn really only serves as a demarcation between the two K, groups indicating the movement to lower latitudes as K, increases. A universal time dependence has also been observed and this can be seen in the histogram of Fig. lo(a) which shows the number of troughs vs. UT in the northern hemisphere winter. The maximum in the number of troughs occurs between 18 00 and 23 00 UT, the period which includes magnetic noon. It is clear therefore that the troughs respond to geomagnetic control and that there are local and universal

Y. TULUNAY and J. SAYERS

1746

N. Ii.

-

314GDAYSB

*‘*...

224sDAYf

-.-.-

131 s DAYSe

33 d

(1968)

308 215

1 L VALUES

(Earth

radii)

Fig. 6. The statistically corrected number of troughs in each integrel L interval for different seaeons, northern hemisphere.

time effects. In the southern hemisphere winter (Fig. 10(b)), the maximum occurs near 13 00 UT again indicating the same control although the maximum obtained is not as well defined as that in the northern hemisphere. Within the geographic limitations employed in the selection procedure there was no apparent variation with longitude in the number of troughs detected. 2.4

Shape of mid-latitude

troughs

(a) CALVERT(1966) and MULDREW (1965) reported that the low-latitude boundary of the main trough was extremely steep whereas SHARP (1966) indicated that the boundary on the high-latitude side of the trough is steeper. Figure 11 presents the results obtained from the present analysis. The gradient at each side of the trough was calculated between the trough minimum and the point where the electron density was twice that at the minimum. The histogram of Fig. 11 was then constructed by simply noting the number of occasions when the slope on the Equator side was

Characteristics of mid-latitude trough as determined by electron density experiment

1

N.H.

1747

S. H.

L

SD 4s . 40 ' 3s 30 . 2s ' 20 . 15 IO. 5

L

Fig. 7. The number of troughs detected during the different seasons for both hemispheres. BOTH HEMISPHERES, ALL LONWUDES MArl967-A~lL I968

LOCAL

TIME,

hr

Fig. 8. (a) Histogram of number of troughs as a function of local time, May 1967-April 1968.

Y. TULVNAY and J. SAYERS

1748

N.H.,WINTER ALL Up

I-

I J-J r

0~[1.~I.‘.“““.‘..,..,., IO

I,

I.2

I,

I,

I5

I6

I,

I6

IO

10

*I

22

II

0

MAGNETIC LOCAL TIME,

I

2

J

I

I

6

I

6

0

10

hr

(b)

s.H.,WINTER. ALL K,

MAGNETIC LOCAL TIME,

hr

Fig. 8. Histogram of number of troughs as a function of (b) Magnetic local time, northernhemisphere,winter; (c) Magnetic local time, southernhemisphere,winter.

S. H., WINTER.

4

I5

lb

,I

lb

19

20

21

2.2

LOCAL

25

I

0

TIME,

x

OcKps3+

o

4---CKp,c9

2

5

4

5

1

b

hr

Fig. 9. Variation of trough position in L with local time for two K, groups, southern hemisphere (the line drawn serves as a demarcation between two regions).

N.H.WINTER (Equal covrroge

,

I

I

in Longitude,

I

1

Local time)

1I

I

I-

I -

) 1 1 .

I -

, 1 -

I I-

l-

10

II

I2

0

I4

Is

I6

I7

I8

I920

UNIVERSAL Fig.

10. (a) Number

of troughs

2122

TIME,

250

1I

9.

14

5

4

7

8

9

hr

vs. universal 1749

time for northern

hemisphere.

Y. TTJLTJNAY and J. SAWERS SM.WINTER (Equalcoverage in L~gi&u~e,Lo~l time)

I

I

I

UNIVERSALTIME,

hr

U-4 Fig. 10. (b) Number of troughs

time for southern hemisphere, winter (I& vs. UT for winter is also shown). VS. universS;t

greater than that on the poleward side and vice versa. These occasions are shown opposite the abbreviations ek and pole respectively. The results are also presented for the four separate seasons and it is imme~a~ly clear that the steeper edge occurs mainly on the poleward side of the trough. (b) The trough width is defined as being the width between the two points on each side of the minimum where the electron density is twice the minimum value, Then by considering the width of the trough, the local time corresponding to the minimum value and KS as variable and treating each as a function of the other two the various linear, multiple and partial co~elation eoefbcients were computed. These show that at a significance level of @05 the theoretical population coefficient differs from zero and some typical examples are given in Table 1. It is found that as K, increases, the widths of the troughs begin to decrease and this is illustrated in Fig. 12 together with the dependence of width on looal time (divided into 6-hr periods) and it is apparent that during early morning and late evening the trough width is greatest. (c) The logarithmic electron density gradients on either side of the trough minimum were computed and are found to be about O-1 degree agreeing well with CALVERT’S (1966) results.

Characteristics

of mid-latitude

trough

as determined

AIJTUWN

0"

90.

g

80.

f

TO-

z

by electron

density

experiment

WINTER

S?RlNC

60 -

1

* *k

90,*

rk

901,

Fig. 11. The histogram of the number of troughs vs. calculated trough gradient (the gradient is calculated between the points of trough minimum and where the E.D. is twice the minimum. The occasions when the slope on the Equator side was greater than that on the poleward side are shown opposite the abbreviation ek. and vice versa), left hand of each pair is for N.H. and the other is for S.H. Table Northern hemisphere LT: 1 00-2 00

summer:

1

Sample

size:

19, All K, 1355 f 8.81 -0.663

Average width f standard deviation RI2 = Linear correlation coef. between width and K, ignoring LT R m1.13 = Linear multiple correlation coef. of width on K, and LT R 911.3 = Linear partial correlation coef. between width and K, keeping LT constant Northern hemisphere winter: Sample 6 00-7 00 LT: Average width f standard deviation

size:

RI2 R ?n1*3 R 9128

-0513

Q 3 10.98 i 3.81 -0.367 0.420 -0.347

R,s R WI153 R me3 22 00-2 00 Sample size: 13, K, LT: Average width f standard deviation

25, K,

0.667

< 3

10.56 + 5.63 -0.843 0.286 -0.826

1751

1752

Y. TULUNAY and J. SAYERS

OsKp
Kpa5

l5- 1.s

M - 1,s

6 HR-LOCAL

m-195

TIME GROUPS,

195-H

hr

Fig. 12. Trough width es a. function of local time groups for three different K, groups (trough width which is defined as being the width between the two points on each side of the minimum where the electron density is twice the minimum value). 3. SIMILARITIES IN BEHAVIOUR OF

THE MACJNETOSPHERIC

KNEE AND THE MID-LATITUDETROUGH

The possible link between the plasmapause and the trough has been suggested by CARPENTER(1963), GRIN~AUZ(1963), CARPENTER(1966) and AN~ERAMIand CARPENTER (1966). By generalising his results, which were based on whistler data, Carpenter proposed a model of thermal ionization in the magnosphere which involved a dense inner region and a tenuous outer region separated by a sharp field aligned boundary, the plasmapause. It was found that the plasmapause moved from a position of L = 6 during quiet magnetic periods to L = 3 during storm times (CARPENTER,1967; TAYLORet al., 1968) and since then the similarities and dissimilarities between the behaviour of the trough and plasmapause have been discussed extensively. THOUS and ANDREWS (1968), RYCROFTand THOMAS(1967) and ANUERAMIand THOMAS(1964) proposed a link between the plasmapause and trough along the magnetic field lines on the basis of diffusive equilibrium existing along the field line and supporting this using data from the topside sounder Alouette I. TYHOMAS and ANDREWS(1969) used the

Characteristics of mid-latitude trough as determined by electron density experiment

1753

low-latitude edge of the trough as the termination of the mid-latitude ionization after THOMAS and DUFOURS’ (1965) suggestion that the trough could be considered as a demarcation between solar produced ionization at lower latitudes and aurorally produced ionization at the higher latitudes. RYCROFT and !CHOMAS (1970) have also used whistler data and found that for quiet magnetic conditions the centre of the night-time trough appears to fall on the same geomagnetic field line as the centre of the knee. They also found the following relations as the best fit equations for the position of the trough and the centre of the knee:

LT = 5.64 - (1.09 f 0*22)1/K, R, = 5.64 - (0.78 & 0+12)2/K,. Using the data from the local time groups (listed below) for each four seasons and for each hemisphere a regression analysis using a least-squares criterion was carried out to find whether the behaviour of the trough minima could be described best by a linear, parabolic, or an inverse relationship. Linear relationship between trough minima and a generalised invariant latitude (O’BRIEN, 1963; RYCROFT and THOMAS, 1970) at the satellite height as well as K, values were also established. In addition the same analysis was also performed for magnetic local time. Hour Time Groups:

(1) 0 00-l 00, 100-2 00 . . .23 00-O 00; (2) 0 00-2 00, 2 00-4 00 . . . (3) 100-3 00, 3 00-5 00 . . . .

For each above mentioned curve the correlation coefficient between the two variables were obtained. The percentage of the r.m.s. deviation of the observations explained by the relation, together with its significance in the form of the F ratio (the F distribution is the distribution which the F ratio has when the null hypothesis is false) were also computed. When the data were grouped by considering the magnetic local time the results obtained were distributed more homogeneously. As an example Table 2(a) gives a summary of the results of this analysis for northern hemisphere winter, three K, groups and for 2-hr magnetic time groups starting at (0 00-l 00) MLT. The results would probably also have been improved by using a more critical definition of trough and by using local K, indices. It is interesting to note that the inverse correlation between L values of the trough minimum and K, indices holds not only for the quiet period (0 < K, < 3f) but also in some cases for the higher K, levels. Although it seems that the variability is least with KS2 and with invariant magnetic latitude, the resultant increase in the percentage of the deviation is not sufficiently great to make this result physically more justified. Table 2(b) shows the ‘X’ indicates where no reasonable relationship was results for both hemispheres. obtained for a magnetic local time sector. It is also interesting to note that with the present method of analysis in most cases no reasonable fit can be obtained for data of MLT = 2 00-4 00 and 18 00-20 00 perhaps implying the presence of another physical phenomenon governing the trough behaviour. For example, neutral-air winds are predicted to last from 16 00 LMT until midnight at medium to low latitudes and corresponding westward wind, between 02 00 and 06 00 hr LMT (CHALLINOR, 1969). An investigation of these phenomena will be the subject of future work.

1754

P. TULUNAY and J. SAYERS Table 2(a)

Northern hemisphere winter: Magnetic local time

No. of data

(br)

points

All KS Linen fit L=a+

bK,

25

R F .l..m.s.

-0.25

0, 1

47

R F r.m.ci.

-0~50

2, 3

23.87 34.7

4, 5

47

R F r.m.s.

6, 7

23

16, 17

Ptwaboiic fit

Parabolic fit I;=+ bdlc,

.L=af

bKsa

Hyperbolic I fit z=a+

bK,

Linear fit h=af

bK,

0.29

0.29

-0.53 17.3 27.6

0.64 31.08 40.9

0.62 28.6 38.9

-0.64 31.07 40.8

--0*56 20.25 31.04

-0.54 IS.00 28.7

0.56 2006 30.8

0.55 19.06 29-8

-0.56 20.1 30-o

R F p.m.*.

-0.37 3.39 13.9

-0.31 229 9.82

0.47 5.97 22.15

0.41 4.31 17.0

-0.47 501 21.9

7

R F r.m.s.

-0.80 8.75 03.6

-0.70 8.38 62.6

0.81 9.67 65.9

0.81 9.3 650

-0.81 9.84 66.3

--050

0.46

0.40

13

R F r+m.s.

-0*50

IS.19

0.02

0

17

R F r.m.s.

0.05

20,21

22, 23

35

R F r.m.s.

-0.51 11.83 26.4

-0.23

0.07

-0.50 10.9 24.8

Northern hemisphere winter: Megnetio local time

No. of data

W

points

og Linear fit &=a+

054 13.8 29.5

I&<

Parabolic fit air,

L=a+

62/z

-@28

-0.47

0

-0.56

O-56 f&89 31+06

1429 30.22

3+ Parabolio fit &=a+

bKss

Hyperbolio fit ;=a+

bK,

Linear ,ti = :+

O-1

0.1

-0.38 6.0 147

0.55 15.2 30-3

O-5 11.7 25

-0.55 15.0 39.1

-0.37 5.4 14.1

-0.35 4.7 12.5

o-35 4.6 12.0

0.35 45 12.2

-0‘35 4.7 124

-0.24

-0~10

0.44

0.34

-0.44

-0.24

-0.23

O-24

0.25

-0.24

-0.50

- 0.50

0.46

0.49

-0.47

-0.0

18

R F r.m.9.

-0.06

0, 1

2, 3

37

R F 2.rn.8.

- 0.46 8.97 20.04

4, 5

35

R F r.m.8.

6. 7

20

R F

--ol

l!Al.S.

16, 17

4

R F 2.lil.S.

l&7,19

13

R F r.m.13.

bK,

Characteristics

of mid-latitude

trough as determined by electron density experiment

1755

Table 2(a) Northern hemisphere winter: Magnetio looal time

No. of data

(h)

points

20,21

22, 23

o+
Parabolio fit bK,

R F r.In.*.

-0

6

35

R F r.nl.*.

-0.37 4.02 13.9

3+

L=a+

Parabolic fit

ad/K,

L=a+

0

3+
Linear A = :+

0

0.39 4.94 15.23

0.36 3.65 12.7

- 0.40 4.85 16.25

9+ Hvoerbolic “-fit

04

points

0, 1

7

R F r.nl.e. R F r.m.s.

-0.1

-0.1

0.1

0.1

-0.1

10

-0.39

0.39

0.33

-0.34

12

R F r.m.s.

-0.39

4, 5

6, 7

3

R F I.xn.8.

16, 17

3

R F r.m.e.

18, 19

0

R F r.ln.*. R F r.m.s.

012

0.07

0.06

-0.09

11

8

R F r.In.s.

-0.81 11.4 65.5

0.82 12.7 67.9

0.83 13.0 68.5

-0.82 12.2 66.9

20,21

22, 23

bK, 0.55

0.11

-0.81 11.3 65.2

0.55

bK,

0

No. of data

L=a+

Parabolic fit L=a+ al/K,

bK,

Magnetic locfd time

2, 3

Linear fit

;=a+

0

-0.35 3.4 12.08

Northern hemisphere winter:

bK,,”

Hyperbolic fit

Parabolic

1

fit L=a+

bK92

-0.56

x=“+

bKp

-0.57

Linear A=:+

bK, 056

Although useful information about the general behaviour of the troughs can be obtained by this method, too much emphasis should not be placed on the exact mathematical relationships obtained as a result of the regression analysis. Firstly it is obvious from the results that the same exact mathematical formula will not suffice for every case. Secondly a problem which arises in regression analysis, even when the assumptions are close to being satisfied, is that the assumptions may not be independent of each other. The result is that it is only possible to conclude with certainty that the positions of the trough minima move to lower L values as K, increases.

1756

Y. TULUNAY and J. SAYERS Table 2(b) 2 hr MLT groups Seasons

Northern hemisphere (All K,) Winter Spring Summer Autumn Southern hemisphere (All K,) Winter Spring Summer Autumn

0, 1

x -

-

2, 3

2/

X

x---x

4, 5

6, 7

8-15

;:I

2/xXx-

X

d-

;,g--

x -

2/

x x

x

-

16, 17

18, 19

20, 21

d

-

x

x

-

X

d _

x

_ _

X

-

d

_

22, 23

Gi _

X

d

g

2/ 2/

X

_

-

-

._

1/: Correlation is significant. X: No reasonable relationship obtained. -: No data. THOMAS and ANDREWS (1968) explained the systematic differences which occur between the position of the lower latitude trough edge and that of the plasmapause by allowing for field line distortions caused by the solar wind when projections along the field lines are attempted. FELDSTEIN and STARKOV (1970) using the Alouette 2 data obtained the boundary of the stable trapping region and thus the limit of the closed geomagnetic field lines (4~). According to their report the equatorward boundary of the aurora1 oval is closely related to the position of the region in which the geomagnetic field lines change from closed to open regardless of the degree of magnetic activity. The values of #c on the day side of the Earth change with universal time and they suggested that this change was caused by the variation in orientation of the geomagnetic axis with respect to the streaming solar wind around the magnetosphere. Figure 13 shows their results as well as the average positions of the trough minima from the present analysis (for 1 hr UT groups in the 10 00-18 00 LT range). The curve which indicates the changing of the boundary of the closed field lines with UT is given for quiet magnetic conditions. However, the experimental points give the average position of the trough minima for all K, values. The data for K, < 3 also follow the same pattern within the range of computational and experimental error.

4. CONCLUSIONS The troughs investigated here were limited to those between 30’ and 70” geographic latitude region and 1 Q L G 7, primarily to facilitate a comparison with other published results. As stated before, these limits are being modified to magnetic latitude and incorporated in future analysis in the hope of providing additional information on the behaviour of the troughs. Because of the greater number of troughs examined, the reported results are more generalised and their behaviour in relation to other phenomena examined closely. The trough minima align along lines of constant L value, which suggests outer

Characteristicsof mid-latitude trough as determined by electron density experiment

2

a

4

6

lo

12

UNIVERSAL

H

16

TIME,

hr

I8

10~ <

LT <

0

Kp<

3

x

ALL

KP

20

22

1757

ISo0

n

Fig. 13. Average positions of trough minima vs. universal time. Limit of closed geomagnetic field lines & vs. UT is also shown on the top (after FELDSTEIN and STARKOV,1970) (average position is determined for I-hr UT groups in the 10 00 to 18 00 LT period).

From the example given in Section 2.2 the quiet day LThe position of trough minima depends on planetary magnetic activity index K,; in fact, trough positions move to lower L values as K, increases. As an example, the reaction of the trough minima to changing magnetic activity is shown in Fig. 14 by a sequence of passes in the southern hemisphere for magnetospheric

control.

shell value is 4.16 f

0.57.

Y. TULUNAYand J. SAYERS

1758

days 145-147, which occurred before, during and after a magnetic storm. The first pass (25 May) is at the start of a 9-hr period of low activity (K,: 2-, 2, 1). There is a trough at L = 6.6. The second pass (26 May) takes place during a high activity (K,: 9, 5-) and the trough is very deep with the position of minimum having moved inward to L = 2.3. The day following this high active period (27 May) has a depletion of ionization in this local time region and trough minimum is at L = 4.5, moving outward from L = 2.3. The relationship between trough minimum position and K, was examined and it was statistically verified that there is an inverse correlation between K, and trough minima positions. It is important to note that the K, index is a measure of solar radiation based upon the intensity of geomagnetic activity caused by the electric currents produced in the ionosphere by such radiation (MATSUSHITAand CAMPBELL, 1967). A possible explanation of the plasmapause has been given in current models of the magnetosphere by NISHIDA (1966) and by BRICE (1967). Both authors have used the concept that the knee location represents an equipotential in the convection plus rotation potential system and the decreased electron density beyond the knee UT = Oloo LT = ISo0 l6Oo

LONG= - Isboo--117’ LAT= - 7S00--19 ALT

.-610

km

146,26May,Kp= 9

l45,25May,Kp

147,2i’May,

f

,l’t , , , , ,, , , iZJ4SbT

8

1

VALUES

(Earth

(

, ,

,

,

9

D

I2

I3

II

= 2-

Kp = 4

, , ‘

radii)

Fig. 14. Response of the trough minima to changing magnetic activity for southern hemisphere, for 25-27 May 1967.

Characteristics

of mid-latitude

trough as determined by electron density experiment

1759

is due to loss of plasma flowing out along the open tail field lines assuming the driving force for convection was viscous interaction of the type proposed by AXFORD and HINES (1961). Although the assumption by these authors of a steady-state convection pattern is an oversimplification, the inward movement of the plasmapause and trough at times of high magnetic activity is consistent with a convection pattern that penetrates deeply into the magnetosphere at such times (AXFORD, 1969). In both models this convective force was associated with electric fields. NISHIDA (1967) has indicated the region in which the plasma escape via the open field lines influences the ionospheric electron concentration by providing another loss mechanism in addition to ion chemical processes, i.e. the ionospheric projection of the plasmapause is suggested to be the low latitude edge of the trough. From Fig. 9 it is inferred that there is a general trend for trough minima to occur at smaller L values at dawn compared to those at dusk. This fact supports the above proposed models since the field lines inside the region will be spread through larger L values inside the dusk sector because of the combined motion of corotation and convection. The convection equation is: E+V,xB=O. The associated electric field strength can be estimated from the cross L movement of the trough minima for a constant local time for two different K, values. Thus with the data of Fig. 9, on the average, at LT:

16 00;

Em

163 mV/m

and at LT: 04 00;

E M 110 mV/m.

Also, for the storm of 25-26 May 1967, E M 170 mV/m which gives a corresponding value projected to the equatorial plane E,, M 4-l mV/m. BEHANNONand NESS(1966) using data obtained from the Imp I satellite reported that there were positive correlations between an increased tail field strength and the planetary magnetic index, K,. This increase is represented by an increased convection electric field which changes the convection pattern to smaller L values. This can explain the movement of the trough minimum towards small L values as K 9 increases. From the histograms presented in this paper it is seen that most troughs are detected in winter and before sunrise. Thus it would appear that solar control of the magnetosphere is a major factor in governing their behaviour. MAEHLUM(1968) reports the UT variations in the low energy electron fluxes at high latitudes and concludes that they must also be related to the variations in the geomagnetic field distortions caused by the change of the geomagnetic axis with respect to the solar wind direction. It has also been shown (THOMASand ANDREWS, 1969; DUNCAN, 1962; KING et al., 1968)that the maximum electron densities in the P-region are observed near 06 00 UT in the southern hemisphere and near 18 00 UT in the northern hemisphere. From Fig. 10 it seems that the occurrence of troughs also exhibits a similar UT dependence. Figure 13 shows that trough minima depends on UT in a similar way as the limit of closed geomagnetic field lines given by FELDSTEIN

1760

Y.

TULUNAY and

SAYERS

J.

and STARKOW(1970). From these similarities along with the general trough behaviour it is tempting to think that between the greater electron densities at lower latitudes and the polar peaks there exists a single region of low electron density which is identified as the mid-latitude trough. Quoting from NISHIDA(1967)-“It seems more appropriate to consider that there is only one trough that extends from 50’ or so to the vicinity of the pole. This trough sometimes appears to be divided into smaller parts because aurora1 crests several degrees wide are superposed on them”. During storm time aurora1 crests are seen at 66” magnetic latitude in the midnight meridian (NISHIDA,1967) and perhaps the reason that there are more troughs as K, increases is partly related to the movements of the aurora1 crests toward smaller latitudes. Acknowledgements-The

research described here forms part of the Space Research programme supported by the Science Research Council. The UK-3 instrumentation which acquired the data presented here was provided by J. H. WAGER, the Project Engineer, with the advice of Dr. J. W. G. WILSON, Project Physicist. We would also like to record our thanks to W. R. PIGGOTT, Miss P. ROTHUTELLand Dr. M. J. RYCROFT for reading the text and for their valuable comments. REFERENCES ANDREWS M. K. and THOMAS J. 0. ANGERAMI J. J. and CARPENTER D. L. ANGERAMI J. J. and THOMAS J. 0. A~ORD W. I. and HINES C. 0. AXFORD W. I. BEHANNON K. W. and NESS N. F. BOWMAN G. G. BRICE N. M. CALVERT W. CARPENTER D. L. CARPENTER D. L. CARPENTER D. L. CHALLINOR R. A. DAYHARSH T. I. and FARLEY IV W. W. DUNCAN R. A. EVANS J. E., NE~VKIRE L. L. and MCCORMAC B. M. FELDSTEIN Y. I. and STARKOV G. V. GRINGAUZ K. I. HENS W. N.

1969 1966 1964 1961 1969 1966 1968 1967 1966 1963 1966 1967 1969 1965 1962 1969 1970 1963 1968

KING J. W., KOHL H., PREECE D. M. and SEABROOK C. LADD A. C. and SMITH J. F. LENHART K. B. LISzEA L. MAEHULM B. N. MATSUSHITA S. and CAMPBELL W. H.

1969 1968 1967 1968 1967

MULDREW D. B. NISHIDA A. NISHIDA A. O’BRIEN B. J.

1965 1966 1967 1963

1968

Nature, Lond. 221, 223. J. geophys. Res. 71, 711. J. geophya. Res. 69, 4537. Can. J. Phys. 39, 1433. Rev. Geophys. 7, 421. J. geophys. Res. 71, 2327. Planet. Space Sci. 17, 777. J. geophya. Rea. 72, 5193. J. geophys. Res. 71, 3665. J. geophys. Res. 68, 1675. J. geophy8. Rea. 71, 693. J. geophys. Res. 72, 2969. Planet. Space Sci. 17, 1097. J. geophya. Rea. 70, 5361. J. geophya. Res. 67, 1823. Lockheed Palo Alto tion DASA 2347.

Res. Lab.

Publica-

Planet. Space Sci. 18, 501. Planet. Space Sci. 11,281. The Radiation Belt and Magnetosphere, p. 63, Blaisdell, U.S.A. J. Atmosph. Tern. Phys. 30, 11. Proc. R. Sot. Lond. A311, 479. ESRO

Scientific Memorandum

ESDAC.

J. Atmosph. Tern. Phys. 29, 1243. J. geophys. Rea. 73, 3459. Beomagnetism, Vol. 11, p. 1203. Academic Press, New York.

J. J. J. J.

geophys. geophya. geophys. geophys.

Res. Rea. Res. Res.

70, 2635. 71, 5669. 72, 6061.

68, 989.

Characteristics of mid-latitude

trough as determined by electron density experiment

RYCROFT M. J. and THOMAS J. 0. SAYERS J., WILSON J. W. G. and LOFTUS B. SHARP G. W. STANLEY G. M. TAYLOR H. A., JR., BRINTON H. C. and PHARO III M. W. THOMAS J. O., RYCROFT M. J., COLIN L. and CHAN K. L. THO~US J. 0. and ANDREWS M. K. THOMAS J. 0. and ANDREWS M. K. THOMAS J. 0. and DUFOUR S. W. THOMAS J. 0. and SADER Y. WAGER J. H.

1970 1969

Planet. Space Sci. 18, 65. Proc. R. Sot. Lond. A311,

1966 1966 1968

J. geophys. J. geophys. J. geophys.

1966

Proc. NATO

1968 1969 1965 1964 1968

J. geophys. Res. 73, 740. Planet. Space Sci. 17, 433. Nature, Lond. 296, 567. J. geophys. Res. 69, 4561. Radio and Elect. Engmg 1, 55.

1761

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Study, Norway.