Seasonal, diurnal and magnetic dependence of ionospheric scintillation at 64° invariant latitude

Planet. Space Sci. 1972, Vol. 20, pp. 957 to 964. Pereamon Press. Printed in Northern Ireland

SEASONAL, DIURNAL AND MAGNETIC DEPENDENCE OF IONOSPHERIC SCINTILLATION AT 64” INVARIANT LATITUDE Radio Astronomy

J. AARONS, J. P. MULLEN and H. E. WHITNEY Branch, Air Force Cambridge Research Laboratories, Bedford, Massachusetts

U.S.A.

aad F. STEENSTRUP Danish Meteorological Institute, Copenhagen,

Denmark

(Received 8 November 1971) Abstract-F layer irregularities at the invariant latitude of 64” were studied by observing amplitude titillations at 137 MHz of the s~chronous satellite, ATS-3; observations were made from Narssarssuaq, Greenland. In the 2 yr of data analyzed (196%1970) consistent seasonal effects were noted. Quiet day winter records showed little diurnal variation. Quiet day summer daytime values showed extremely low occurrence of scintillation. These seasonal patterns must now be integrated into the model of the high latitude irregularity region. The occurrence of high amplitude fluctuations correlated sensitively with the magnetic index. Nightly means of scintillation index showed a positive correlation of 0.46 with night means of KS,. The autocorrelation function of the nightly scintillation index reveals that a long time constant of several days exists for the ~rre~l~ity pattern. During magneticstorms the time of maximum occurrence of scintillation shifts from the quiet day peak of 2200 to a peak between 0300 and OS30 when K, = 4-9. The morphoiogy of the irregularity region is becoming more evident with the addition of the seasonal pattern and the long term consistency of the irregularities. INTRODUCTION

The F layer irregularity structure has been studied by the scintilla~on technique prestudies at auroral dominancy from mid-latitude and sub-aurora1 positions. Morphologic and polar latitudes have been done using radio stars (Little et al., 1962), and low altitude satellites (Frihagen, 1971; Liszka, 1969; and Titheridge and Stuart, 1968). In the experimental studies of Frihagen and of Titheridge and Stuart, use was made of the 40 MHz transmissions of S-66. Since trans-ionospheric signals are frequently fully scintillated at high latitudes, (i.e. the signal fades down to noise level or an arbitrary low level near sky noise) it was difficult to determine what would have been the depth of fade at a higher frequency. EXPJ!XIMENTALAND ANALYTICAL GREENLAND

SETUP OF NARSSARSSUAQ, EXPERIMENT

In order to obtain long term records of scintillations on a path through the lower edge of the aurora1 oval a cooperative program was set up between Air Force Cambridge Research Laboratories and the Danish Meteorological Institute. Amplitude records were taken on the 137 MHz signal from ATS-3. The antenna used was a pair of yagis phased 90” for circular polarization. These were connected to a receiver where detector output fed a paper chart recorder with a time constant of 1 sec. The records were reduced by the method given by W~tney et al. (1969) into 15 min scintillation indices. In this system a scintillation index of 20% represents an increase in signal above mean quiet level of O-8 dB and a negative fade of 1 dB; 40% represents + 1.5 dB to -2.2 dB; 60% represents +2 dB to -4 dB. The data used was taken between September 1968 and May 1970 (data from December 1970 was also included); approximately 350 days were completely analyzed. During this 1

957

958

J. AARONS,

J. P. MULLEN,

H. E. WHITNEY and F. STEENSTRUP

period of time the satellite was moved from a sub-satellite position of 45” West to 90” West. However, the sub-ionospheric latitude of the propagation path (at 350 km) remained near 64” invariant latitude. It might be noted that the sub-ionospheric time changed during this period; this variation is at most 4 hr and the mean local sub-ionospheric time correction is not significantly different from the 45” West meridian local standard time used in this paper. The diurnal pattern and its variation with K,,of half of the data utilized was used in an earlier paper on the scintillation boundary (Aarons and Allen, 1971). DIURNAL

PATTERN:

THE VARIATIONS

WITH MAGNJZTIC! INDEX

In order to determine the diurnal pattern of scintillation, the data was sorted by K,. The diurnal pattern is shown in Fig. 1 for the percentage of occurrence of scintillation index >40 for K,,= 0, 1, Kg = 2,3 and K, = 4-9.From Fig. 1 we can determine that the per-

I,

I

Fm. 1.

NOTE

cb3JRRENCE

CHANGE

I

I

I

0400 MIDNiGHT

0000

IN

OF TIME

I

I

I

0600

SCINTJLLATION OF

MAXIMUhl

I

I

II

1200 09’00

INDICES TOWARDS

11

UT LST

”

1600

GREATER LATER

” “‘I 2opo 1700

”

THAN

HOURS

40 AS

2400

AS A FUNCTION

THE

K

INDEX

OF

K,,.

INCREASES.

centage of occurrence of SI > 40 increases with magnetic index at all times of day. In addition the time of both peak and null shifted; the high magnetic index data has two peaks, a minor peak at 2000 and the major peak at 0300 local time, while the quiet magnetic data has a single peak before midnight at 2000-2100. These effects have been discussed in Aarons and Allen (1971). The lowest occurrence of scintillation takes place before noon local time, probably near 1000. By subdividing the quiet magnetic indices into K,,= 0,K, = 1, and K, = 2 it can be seen that the irregularity pattern correlates sensitively with magnetic disturbance. Figure 2 shows the variation in the occurrence of SI > 20 with unit changes in K index. In order to determine statistically the correlation between magnetic and scintillation activity the following technique was used. Hourly indices were averaged over the local time period 1700-0800; magnetic indices (K,)for this period were also averaged. A correlation coefficient of 0.46 was found between these nightly values.

MAGNETIC DEPENDENCE

I

t

0000

t

1

I

,040o

I

MIDNIGHT

Fro. 2. THE CQNDlTXONS.

OCCURRENCE

EXCEPTFfBR A

t

II f 0800

OF IONOSPHERIC

11

1 ’ 1 1200 UT OdOO

’

1’

I600

LST

959

SCKNTILLATION

”

”

2qoo

‘1’

2400

1700

OF SCZNTKLATION INDICFZS OVER 20 UNDER QUIET PERIOD AROUND NOON, SClNTiLLAl’ION -NCR SENSITIVELY WITH K INDEX.

MACNiTIC CHANGES

From Figs. 1 and 2 and the measured correlation coefficient it can be seen that the irregularity structure as determined from a site at the lower edge of the midnight quiet auroral oval varies in intensity both as a function of time and magnetic index. The hypothesis advanced earlier (Aarons, 1971) is that the aurora1 oval is the center of the irregularity region (and probably its most intense area). However, the irregularity region extends far lower than the aurora1 oval indicates. The aurora1 oval during quiet magnetic conditions has 63” as its lower boundary at midnight and 76” at noon (Feldstein, 1963). It should be noted that scintillation index at a particular frequency is quantitative; the occurrence of Spread F fails to show the intensity of the irregula~ties. It is hoped that with the accumulation of scintillation data more quantitative results on the variation of electron density with the irregularities at high latitudes would allow for the construction of a statistical model of electron densities in the irregularities at high latitudes. SEASONAL

PATTERN

The data being reported was taken over a little less than two years; only two sets of data are available for each season (with the exception of summer where only one set is avaiIable). In order to determine if the seasonal variations were consistent, we have plotted in Fig. 3 the raw data for two autumns (autumn is taken as Aug., Sept. and Oct.). The data is fairly consistent when one holds Kg constant (in this case K, = 0, 1) and Fig. 4 plots smoothed diurnal curves of percentage occurrence for the other three seasons. It can be seen from these two figures that during low magnetic activity, night maxima are higher at the equinoxes than during both summer and winter. Winter data shows very low occurrence of night-time deep scintillation. In addition winter afternoons show a higher occurrence of scintillation than do the other seasons. Daytime scintillation indices reach a low during the summer.

960

J. AARON& J. P. MULLEN, H. E. WHITNEY and F. STEENSTKUP

X= 9/68 - IO/68 .* E/69 - IO/69 - i SMOOTHED MEAN

. . .

.

l

I

I

I

0000

I

I

II

I

I

0400 WDN’IGHT

I

I

1

L

’

1

12,oO UT 0900 LST

0800

’

1 ” 1600

” “‘I .2opo 1700

2400

FIG. 3. THE OCCURRENCE OFINDICES GREATERTHAN 40 WHRN THE MAGNETICINDEX IS Low (K,,= o&IS SHOWN IN THIS FIQURE POR AtJTLRdNAL VALUES. TV.'0AUTUMN SARESHOWNAS ~~ASASM~~D ~ANTOS~OW~E~NS~NCYOF~O~~RVA~ONSOF~O YEARS, 100

. .. . . . . . . . . .

Wf,,fER

-

SPRING

-----

SUMMER

oI

0000

I

I

a

I

0400 MIDN’IGHT

I

I

II

0800

II

11

”

”

l2,oOUT 0900

LST

”

1600

”

”

20,oo

”

2400

1700

Fm.4. SMOOTfXED MEANS FOR WINTER,SPRING AND SUMMER OCCURRENCE OF SCINTILLATION INDICES GREATER THAN 40 ARE SHowN FOR Q~@T MAGNETIC CONDITIONS. WINTER SHOWS LITTLE DIURNAL VARIATION. MATRICES ARE NOTED DURING ~~~0~.

With K,, = 2, 3 (Fig. 5) the winter night-time occurrence of high scintillations remains low relative to other seasons and fairly high 130~2100. Summer daytime indices are quite low from 0900 to 1400 local time with no hourly occurrence of SI > 60 during this time period. During magnetic storms (& = 4-9) a diurnal pattern exists but due to saturation effects it is difficult to see seasonal differences. However it might be noted that for two hours

MAGNETIC

DEPENDENCE

OF IONOSPHERIC

SCINTILLATION

961

w **-

x iii

UC

-. . . . . . . . .._.

2 60:: I&.

-

0

!402 w ::

AUTUMN W,NTER

-

SPRING

-----

SUMMER

-

k!20-

OI

I

I

I

I

0400

0000

I

I

I

I

0000

MIDNiGHT

Fra.

I,

I

1200 osbo

I

I

UT LST

f

I

1600

I

I

I4

I

2000 I

t

I

I

2400

7bO

FOUR SEASONS WITH Kp = 2, 3 AND WITH Sf > 60. 0900 LOCAL TIME ARE LOW FOR ALL SEASONS. WINTER SHOWS TJB LEAsT DIURNAL VARIATION.

5. Sh@JOTH8D

VALUES AROUND

I

MEANS FOR THE

(0900-1000) zero occurrence of SI > 60 was recorded for the summer even at these high magnetic indices. The choice of months to place into the seasonal sort was based on astronomical considerations. In a plot of occurrence of SI > 40 in Fig. 6 for KB = O-3 at midnight local

t 80 w Y u i!i ifi ::

60-

8 g

40-

? i5 P ii!

20-

FIG. 6. MONTHLY VALUES OF THE OCCURRENCE OF SI ORRATER THAN 40 WITH K, VALUES RANQING PROM 0 TO 3. THB PERCENTAGB OF OCC URRENCR AT MIDNlGEiT Is SHOWN WEEi PEAKS AROUND APRIL AND SEPTEMBER AND MINIMA IN No~ER-D~~ AND JULY.

J. AARONS, J. P. MULLEN, H. E. WHITNEY and F. STEENSTRUP

962

time, minima are noted in December with low values near July; peak months are April and May. The conclusion to be drawn is that a seasonal pattern clearly exists at the intersection latitude. It is of course known that Spread F at high latitudes has a seasonal pattern. (Penndorf, 1962; Tao, 1965). From Penndorf’s curves we have scaled the values for the Narssarssuaq intersection point. These are shown in Fig. 7 for July 1957 and September 1957 (1957 was a high sunspot number year). The curve corroborates the summer decrease of our curves. They do not show the very low readings found for our observations but our readings of Penndorf’s curves (at low spread F occurrence levels) are rough.

‘0°$ I

I

I

0000

I

I

0400

I

I

I

I

06.00 0600

I

1957

I

LST

I

11

1200

UT

11

11 ISPO 13bo

1

’

1) 2000

“1

2400

FIO.

7. SPREADFOCCURRENCEASSHO~NBYF%NNDORP(~~~~)PORTHEINTERSE~TIONLATTWDE AND LONGITUDE TO ATS3. HATCHED AREA IS BEU~W 10 PER CENT AND NOT EASILY hGWWRABL.E PROM PUBLISHED CURVES. HIGHER OCCURRENCE WAS NOTED DURING SEPTEMBER THAN DURING JULY WHICH AGREES WITH THE DATA SHOWN IN FIG. 6.

Another comparison was made with spread F occurrence, i.e. that above Narssarssuaq as determined by ionosoundings. This data, as reported by Olesen and Jepsen (1964), consists of ionospheric observations approximately 5” higher than the ATS-3 intersection. The curves culled from the Olesen and Jepsen sunspot minimum data are shown in Fig. 8. They show a low winter nightly spread F intensity (instead of occurrence which is tabulated in Penndorf’s papers). The prenoon summer daytime minimum of SI can also be noted in the spread F data. Autumn records show low values of spread F intensity. In his analysis of 1964 spread F occurrence, Tao found no high latitude midnight maximum in winter months (at the Narssarssuaq intersection latitude). Only by extending the scintillation data base can we determine if the seasonal pattern of scintillations is a function of the intersection latitude or is an overall change in electron density in the irregularities at high latitudes. AUTOCORRELATION

FUNCTION

OF NIGHTLY MEAN OF SCINTILLATION

INDEX

The series of nightly means were compared to magnetic index (as previously noted) and to solar flux at 2695 and 4995 MHz (a correlation coefficient of 0.16 was found with these two parameters) and the autocorrelation function was computed.

MAGNETIC

DEPENDENCE

OF IONOSPHERIC

SCINTILLATION

963

z v, 2.0 z i2 - 1.5ll.

---

SEPTEMBER . . . . . . . . . . . . . . . DECEMBER -

P w I.02 VJ

__--

\

MARCH ____

J”NE

‘4 -7.y--__

035 -

jr

o.oII

0000 Fm.8.

I

I1

Ifi

0400

11

0000

II

LMT

11

1200

I”““““’

1600

2000

2400

SPREADF INTENS~~YO~ERNARSSARSSUAQDURINGAYBAROFLO~SUN~P~TNUMBER, 1953.

s,,l 0

‘I

3 DELAYhI

FIG. 9. A~TOCORR~LATION

DAYS

4 DAYS

FUNC!TION OF MEAN NIGHTLY SCINTILLATION INDIGEN AND MEAN NIGHTLY K,, VALUES.

The data is shown in Fig. 9. With one day’s separation the coefficient falls only to O-516, two days to O-416. The autocorrelation of the average K,, for the same nights falls more rapidly with a value of O-38 with one day’s separation, O-19 the second and essentially zero on the third day. The autocorrelation function indicates the existence of not only magnetic storm effects which are known to last for several days but also the existence of very low scintillation activity during long lasting magnetically quiet periods. Data from two periods in 1969 shown in Fig. 10 illustrate the long time constant of the system. The lower latitude boundary of the irregularity region may move considerably above its mean position during long periods of low magnetic activity. The decrease to negligible scintillation activity at this latitude appears to come only after several days of magnetic quiet. The data clearly indicates that the irregularity structure has a long time constant in its behavior, changing its configuration very slowly.

964

J. AARONS,

J. P. MULLEN,

H. E. WHITNEY and F. STEENSTRUP

----

MEAN

Kp

-

MEAN

51

CONCLUSION

Within the aurora1 oval a high occurrence of deep fading occurs which varies as a function of time and magnetic conditions. At a particular site (64”) the intensity increases during magnetic activity. The form of the diurnal variation also changes with magnetic activity (time of peak and minimum occurrence of deep scintillation). Nightly means of scintillation index correlate with nightly K, means with a coefficient of O-46. The autocorrelation of the nightly scintillation index reveals a long time constant of several days for the irregularity region intensity. The seasonal pattern of the scintillation also revealed that winter nighttime occurrence during quiet magnetic periods is low. These data point to large scale magnetospheric activities producing the behavior pattern of the irregularity region. Acknowledgements-We would like to thank Dr. A. Lundbak for organizing the cooperative observations program, Mrs. A. Penney for her work in reducing the data and Mr. C. Cantor for programming the reduction of the data. REFERENCES AARONS, J. and ALLEN, R. A. (1971). Scintillation Boundary During Quiet and Disturbed Magnetic Conditions J. geophys. Res. 76, 170-177. AARONS, J. (1971). Satellite Scintillations in the High Latitude F-layer Irregularity Region, in Radar Propagation in the Arctic, AGARD Symposium, Lindau, Germany. To be published. FELDSTEIN, Y. I. (1963). Some Problems Concerning the Morphology of Auroras and Magnetic Disturbances at High Latitudes. Geomugn. Aeron. 3, 183-187. FRIHAGEN, J. (1971). Occurrence of High Latitude Ionospheric Irregularities Giving Rise to Satellite Scintillation. J. atmos. terr. Phys. 33,21-30. LISZKA, L. (1969). Scintillations of Satellite Signals, In Low Frequency Waves and Irregularities in the Zonosphere (Ed. by N. D’Angelo) p. 192-206. Springer-Verlag, N.Y. LIT-IZE,C. G., REID, G. C., STILTNER,E. and MERRIT, R. P. (1962). An experimental investigation of the scintillations of radio stars observed at frequencies of 223/Mc/sec and 456/Mc/scc from close to the aurora1 zone. J. geophys. Res. 67, 1763-1784. OLESEN,J. K. and JEPSEN,S. B. (1966). Some Characteristics of Spread F in Very High Latitudes (Ed. by P. Newman), pp. 127-136. Technivision. PENNDORF,R. (1962). Geographic Distribution of Spread Fin the Arctic. J. geophys. Res. 67,2279-2288. TAO, K. (1965). Worldwide Maps of the Occurrence Percentage of Spread F in Years of High and Low Sunspot Numbers. J. Radio Res. Lab. 12,317. TITHERIDGE,J. E. and STUART,G. F. (1968). The Distribution of Irregularities in the Antarctic Ionosphere (1). J. atmos. terr. Phys. 30, 85-98. WHITNEY,H. E., AARONS,V. and MALIK, C. (1969). A proposed index for measuring ionospheric scintillations. Planet. Space Sci. 17, 1069-1073.