[1901 K. C. Yeh and C. C. Yang, “Mean arrival time and mean pulse-
VOI. 67, pp. 1261-1266,1977.
width of sign&propagating through a dispersive and random medium,” ZEEE Trans. Antennas Propagat., vol. AP-25, pp.
[ 1 9 1 ] K. C. Yeh and C. H. Liu, “Ionospheric effects on radio com- munication and ranging pulses,”ZEEE Trans. AntennasPropagat., [ 1921 C. H. Liu and K. C. Yeh, “Model computations of power spectra
for ionospheric scintillations at GHz frequencies,” J. A n o r [ 1 9 3 ) C. C. Yang and K. C. Yeh, “Temporal behavior of pulses after
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[ 1 9 7 ] M. R. Tucker, “A deterministic study of puke propagation in an electron bubble medium,” M S . thesis, Department of Elec- trical Engineering, University of Illinois at UrbanaChampaign, [ 1981 D. L. Knepp, “Multiple phase-screen calculation of the temporal
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behavior of stochastic waves,” presented at the North American Radio Science Meeting, paper B.9-6, Quebec, June 2-6, 1980.
[ 1991 R. L. Bogusch, F. W. Guigliano, D. L. Knepp, and A. H. Michelet, receiver tracking,”Proc. ZEEE,vol. 69, pp. 787-796, 1981.
“Frequency selective propagation effects on spread-spectrum [ 2 0 0 ] R.C.Dixon,SpreadSpecfrumSy~tems. NewYork: Wiley, 1975.
I2011 C. L. R h o , V. H. Gonzalez, and A. R. Hesing, “Coherence bandwidth loss in transionospheric radio propagation,” Radio [ 2 0 2 ] R. K. Crane, “Ionospheric scintillation,” Proc. ZEEE, vol. 65,
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[ 2 0 3 ] G. G. Getmantsev and L. M. Eroukhimov, “Radio star and satellite scintillations,” Ann. I S Q Y , vol. 5 , paper 13, pp. 2 2 9 - 2 5 9 , 1 9 6 7 .
[ 2 0 4 ] J. P. McClure, W. B. Hanson, and J. H. Hoffman, “Plasma bubbles and irregularities in the equatorial ionosphere,” J.
[ 2 0 5 ] M. C. Kelley and E. Ott, “Two-dimensional turbulence in Geophys. Res.,vol. 82, pp. 2650-2656, 1977.
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[ 2 0 6 ] C. M. Crain, H. G. Booker, and J.A. F e r g w n , “Use of refractive scattering t o explain SHF scintillation,” Rudio Sa.., vol. 14, (2071 A. W. Wernik, C. H. Liu, and K. C. Yeh, “Model computations of Radio Waves Scintillations Caused by Equatorial Ionospheric (2081 S. Basu and M. C. Kelley, “A review of recent observations of
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1031-1044,1931.
Global Morphology of Ionospheric Scintillations
JLES AARONS,
FELLOW, IEEE Invited PaperAbmct-Starting with post World W u I1 studies of fading of radio s t a r sources and continuing with fading of satellite signals of Sputnik, vast quantities of data have built up on the effect of ionospheric irregu- larities on signals from beyond the F layer. The review attempts to organize the available amplitude and phase scintillation data into equa- torial, mid&, and high4atitude morphdogies The effect of magnetic activity, solar sunspot cycle, and time of day is shown for each of these three latitudinal sectors.
The effect of the very high levels of solar flux during the past sunspot maximum of 1979-1981 is stressed During these years unusually hi@
levels of scintillation were noted near the peak of the Appleton qua- torial anomaly (- +15” away from the magnetic equator) as wen as over polar latitudes. New data on phase fluctuations are summarized for the auroral zone with its sheet-like irregularity structure.
Manuscript received October 19, 1981; revised February 2, 1982.
The author was with the Air Force Geophysics Laboratory, Hanscom AFB, MA 01731. He is now with the Department of Astronomy, Boston University, Boston, MA.
One m d is now available which will yield amplitude and phase predictions for varying sites and solar conditions. Other models, more limited m their output and
use,
are also available. The models are out- lined with their limitations and data bases noted.New advances m morphology and m understanding the physics of irre%uity development in the equatorial and auroral regions have taken place. Questions and unknowns in morphology and in the physics of heguhity development remain. These include the origin of the Beeding sources of equatorinl irreguluities, the physics of development of auroral irresulority patches, and the morphdogy of F-layer irregular- ities at middle latitudes.
A
I. INTRODUCTION
RADIO WAVE traversing the upper and lower atmo- sphere of the earth suffers a distortion of phase and amplitude. When it traverses drifting ionospheric irregularities, the radio wave experiences fading and phase fluctuation which vary widely with frequency, magnetic and U. S. Government work not protected by U. S. copyright
AARONS: GLOBAL MORPHOLOGY O F IONOSPHERIC SCINTILLATIONS 361
solar activity, time of day, season, and latitude. It is the pur- pose of this review paper to organize the experimental and theoretical studies which have been brought t o bear to isolate the variables.
Experimentally, the advent of beacons ranging from low- altitude satellites transmitting at 40 MHz to 3000 MHz and synchronous and very-highaltitude satellites transmitting in the UHF to microwave region have allowed geophysicists to take data over more than a solar cycle. The recent protracted high solar flux in 1979, 1980, and 1981 has shown unusual activity in the polar and the equatorial regions. We shall attempt to sort out the geographical and geophysical effects.
On the theoretical side, conceptual advances in the instability mechanisms which could fit the data plus an extensive program in simulation have allowed physicists to develop theories of the formation of equatorial and auroral irregularities. We shall briefly touch on “accepted” concepts of the development of irregularities. These irregularities in the ionosphere introduce fading and enhancement of amplitude, phase fluctuation, and angle of arrival variations; collectively the effect is ionospheric scintillation.
The irregularities producing scintillations are predominantly in the F layer at altitudes ranging from 200 to 1000 km with the primary disturbance region for high and equatorial latitude irregularities between 250 and 400 k m . There are times when E-layer irregularities in the 90- to 100-km region produce scintillation, particularly sporadic E and auroral E ; we shall refer to these in the appropriate sections.
Several techniques have been used to study irregularities.
These include 1) ground, airborne, and satellite based HF swept frequency sounders studying electron density structure and observing both bottomide and topside F-layer irregular- ities; 2) in-situ measurements by rockets and satellites of electron and ion density irregularities, electric fields, and electron and ion flux; 3) coherent radar backscatter-VHF t o microwave; and 4) the scintillation technique which measures directly the perturbations of the radio signal as it transits the ionosphere. While we shall attempt to bridge the gap between sounders, radar backscatter, in-situ measurements, and scintilla- tions we shall concentrate on scintillation morphology which may differ considerably from the other data.
A . Global Morphology
From the global point of view there are three major sectors of scintillation activity (Fig. 1). The equatorial region comprises an area within *2O0 of the magnetic equator. The high-latitude region, for the purposes of the scintillation description, com- prises the area from the high-latitude edge of the trapped charged particle boundary into the polar region. We shall term all other regions “middle latitudes.”
In all sectors, there is a pronounced nighttime maximum.
At the equator, activity begins only after sunset. Even in the polar region, there appears t o be greater scintillation occurrence during the dark months than during the months of continuous solar visibility.
To order the geophysical occurrence and intensity of irregular- ities, reliance must be placed on a magnetic picture of the earth. While the sun’s role is ordered along geographical lines, the geophysics of irregularities is dominated by the tilted earth’s magnetic field. Motions of ionized particles are governed by the earth’s magnetic field with its northern pole near Thule, Greenland and its eccentric magnetic equator. The magnetic equator’s meanderings relative to the geographic equator will
Fig. 1. Global depth of scintillation fading during low and moderate solar activity.
SCINTILLATION AH0 FADE DURATION ANALYSIS
5 G N A L L E V E L
Fig. 2. Sample of intensity fading produced by signal passing through irregularities. Fade duration and cumulative probability density are also shown.
be illustrated. At the equator the earth’s magnetic field is parallel to the earth’s surface and is oriented magnetic N-S.
At Thule, the magnetic field is directed vertically and electrons spiral along the lines of force.
B. Scintillation Examples
The intensity fading and its characterization can best be characterized by the idealized example such as in Fig. 2. The signal is modulated by the passage through the irregularities so that the level instantaneously both increases and decreases.
In Fig. 2 the signal level at times is 3 dB above the mean sig- nal level and at other times fades below t h e 6 d B level. The number of fades and the fade duration for a typical 15-min length of signal from a synchronous satellite is shown in Fig. 2 along with the cumulative probability density function. In this example 9 1.7 percent of time the signal was above the 6dB fade level.
A slow speed recording of a transmission from Si Racha to Hong Kong via satellite is shown in Fig. 3 [ 1
1.
In this case the uplink was 6 GHz and the downlink was 4 GHz. The fading reached 8 d B peak to peak in this example from the disturbed equatorial region during a year of very high sunspot number.C. Signal CharacteTistics
The amplitude, phase, and angle of arrival of a signal will fluctuate during periods of scintillation. The intensity of the
- 4
-
31 :
3i
- 2:z - 4
I I I I I I
2000 2 100 2 200 2300 0000 0100
LOCAL TIME
Fig. 3. Slow speed recording of a transmission from Si Racha t o Hong Kong. Peak-to-peak fluctuations range t o 8 dB [ 1 ) .
scintillation is characterized by the variance in received power with the S4 index commonly used for intensity scintillation and d e f i e d as the square root of the variance of received power divided by the mean value of the received power [ 21.
An alternative, less rigorous but simple measure of scintillation index has been adopted by many workers in the field [3] for scaling long-term chart records.
The defiition is
SI = Pmax - Pmin Pmax +Pmin
where Pmax is the power level of the third peak down from the maximum excursion of the scintillations and Pmin is the level of the third peak up from the minimum excursion, measured in decibels [ 31 ,
The equivalence of selected values of these indices is indicated below.
s4
dB0.075 1
0.1 7 3
0.3 6
0.45 10.
Scaling of the chart records is facilitated by simply measuring the decibel change between the Pmax and Pmin levels. The phase variations are characterized by the standard deviation of phase u4.
Attempts have been made to model the observed amplitude PDF. Whitney et Q I . [ 41 and Crane [ 51 have constructed model distribution functions based upon the use of the Nakagami-m distribution (m = (S4)-2) and have shown that the empirical models provide a reasonable approximation to the calculated distribution functions. In addition, the Rayleigh PDF provided a good fit to the data under conditio? of very strong scintilla- tion (S4
>>
0.9). The Nakagami-m distribution approaches the Rice distribution as m approaches unity from higher values and equals the Rayleigh distribution for m = 1 (strong scintillation).D. Frequency Dependence
Observations [6] employing ten frequencies between 138 MHz and 2.9 GHz transmitted from the same satellite, show a consistent behavior of S4 for S4 less than about 0.6. The frequency dependence becomes less steep for stronger scintilla- tion, as S4 approaches a maximum value near unity with a few rare exceptions. When S4 exceeds 0.6 (peak-to-peak values
>
10 dB) the frequency dependence exponent decreases. If two frequencies are being compared and both experience strong scattering to the extent that each displays Rayleigh fading,then there is an effective X' dependence over the frequency interval. When strong scattering occurs but is not constant over the frequency interval, the wavelength dependence is difficult to determine. The [6] observations also show that the phase scintillation index varies as A under most condi- tions, a result also obtained by Crane [ 7] although at low frequencies this has not yet been shown. Phase fluctuations do not experience a variation in frequency dependence in the strong scattering region.
E. Fading Spectra
Radio waves from satellites encountering the ionospheric irregularities undergo spatial phase fluctuations. Intensity fluctuations develop as the wave emerges from the irregularity reaching their maximum intensity in the far field. Focusing effects can further increase intensity fluctuations.
The two-dimensional spatial spectrum of phase fluctuations is proportional to the integration of the three-dimensional irregularity spectrum along the propagation path. Thus if the power spectrum of the three-dimensional irregularity has a power-law slope of index p , the spatial phase spectrum will have a power-law index of p-1
.
The amplitude scintillations undergo Fresnel filtering.
Amplitude scintillations do not fully develop after traversing very large irregularities observed at distances very much shorter than the Fresnel zone radius
(XF
=6)
where z is the effective distance from the layer. Irregularities smaller than the Fresnel zone distance, according t o in-situ measure- ments of the intensity of electron and ion irregularities [ 81, [ 91 have low intensities with power-law behavior and therefore have a lesser effect. The Fresnel filter function thereforegenerates maximum intensity at a spatial wavelength of the Fresnel scale.
For weak scattering the spatial spectrum of intensity flu'ctua- tions is in effect a convolution of the phase spectrum with the Fresnel filter function.
A comparison of moderate scintillation levels (S, = 0.5) and very high scintillations indices (S4 = 0.94 (close t o Rayleigh fading)) is shown in Fig. 4(a) and (b) [ 101
.
The low frequency flat portion of the spectrum is extended in the strong scatter- ing case (Fig. 4(b)); the slope of the falling portion of the spectrum does not change significantly, keeping a spectral index of 3.For the synchronous satellite the spectra essentially include the velocity of the ionospheric drifts and the Fresnel wave- length. The spectra of phase scintillations however are not affected by Fresnel fitering.
The intensity spectrum changes as a function of drift speed, irregularity spectrum, and strength of scattering. Thus the morphology of spectra is in reality the interplay of these
AARONS: GLOBAL MORPHOLOGY O F IONOSPHERIC SCINTILLATIONS 363
dB
r
l o t
ANCON. PERU LES - 9 2 4 9 M H z
S 4 = 0 5 0 0111 U T
-2cL
-
(a)
- 6 0 1 , , ,
6 0 I20 180 ~ c - ~ I O - ' ,o0 10'
T I M E - s e c s FREOUENCY - Hz
ANCON, PERU LE5 - 9 2 4 9 M H z
S = 0 . 9 4 0214 UT
dB r
,
- 201
0 6 0 I 2 0 I &
' :::L
10-2 1c-1 ,oo,,'
T I M E - s e c s FREOUENCY - Hz
@)
Fig. 4. Intensity scintillation and frequency spectrum for both moderate (a) and very high (b) scintillation indices.
factors. In each of the geophysical areas where intense activity occurs, the three factors must be utilized to estimate the spectra of the scintillation intensity. We shall also refer to model computations of Wernik et al. [ 111 relative to non- stationary wedge-like electron density structures. In such cases the intensity scintillations exhibit spiky temporal varia- tions and fluctuations become nonstationary.
F. Geometrical Considerations
The intensity at which scintillations are observed depends upon the position of the observer relative to the irregularities in the ionosphere that cause the scintillation. Keeping both the thickness of the irregularity region and A N , the electron density deviation of the irregularity, constant, geometrical factors have to be considered t o evaluate data and t o predict scintillation effects at a particular location. Among these are:
a) Zenith distance of the irregularity at the ionospheric layer. One study [ 121 found the intensity of scintillation may be related approximately to the zenith path values by the secant of the zenith distances to 70'; below that an elevation angle dependence ranging between
3
and the first power of the zenith angles should be used.b) Propagation angle relative to the earth's magnetic field.
Performing this calculation demands the use of an irregularity configuration and the consideration of a Gaussian or a power- law model for the irregularities. Sheet-like irregularities with forms of 10 : 10 : 1 have been found in recent auroral studies [ 131. For equatorial latitudes, this elongation along the lines of force may be of the order of SO to 100 [ 141.
c) The distance from the irregularity region to the source and to the observer (near the irregularities, only phase fluctua- tions are developed). As noted in [ 5
I
and [ 151 the theoretical scintillation index can be expressed in terms of the above factorswhen dealing with ionospheric irregularities represented by a Gaussian power spectrum.
hfikkelsen et al. [ 151 have attempted to determine the theoretical scintillation index S4 when the irregularities are described by a power-law power spectrum with a three- dimensional spectral index P = 4. This utilizes the coordinates of the radio ray in the local coordinate system with set values for the elongation of the irregularities along and perpendicular to the magnetic field lines.
Mikkelsen assumed the approximate dividing line between weak andstrongscintillationis -9 dB, with SI< 9 dB denoting the weak case. For this case, the geometric variation of S4 is given by
~4 0: d(z/ cos i) f($, @) where
i ionospheric zenith angle = angle between radio ray and irregularity layer
$ propagation angle = angle between the radio ray and the magnetic field direction
q5 azimuth of the radio ray in local coordinate system of z axis along the magnetic field and y axis in the magnetic east-west direction
f($, 9) = ay(y' cos' @ +sin2 @ +cosz $(cos' @
+
y2 sin' @)+
'a sin' $/[yz cos' $+
'a sin' $(y2 cos'+
sin241
' I 2-
z 1 )/z2 where z1 = slant range to irregularity layer, z2 = slant range to satellitea elongation of the irregularities along the magnetic field lines
y elongation of the irregularities in the magnetic east- west direction.
Z reduced slant range to irregularity layer = z1(z2
Using his irregularity formulation he found the Narssarssuaq observations of the orbiting satellite, Nimbus-4, at an altitude of 1000 km a best fit of irregularity configuration with 2.5 : 1.3 : 1 ; the first term is a, elongation of the irregularity along the lines of force of the magnetic field, the second is y, orthogonal to the elongation along the lines of force, being in the magnetic east-west dimension, and the last term is orthogonal to the other two planes. At high latitudes this last term would lie in the north-south meridian.
11. SPREAD F AND SCINTILLATIONS The term spread F is given to a type of F-layer backscatter signal taken by a vertically directed sweeping H F sounder. The returns from the F layer at each frequency are normally ob- served from that height at which the electron density reaches a value where the ionosphere acts as a reflector. When the re- turns from the F layer are observed from a series of '%eights"
rather than a single altitude we have a spread F condition.
When a wide range of frequencies shows returns from many ranges then the ionogram is said to exhibit "range spread
F."
When the spread in range is predominantly at the high end of the frequency sweep then the ionogram is said t o be of the
"frequency spread F" type. The major morphological studies of spread F [ 16
I
and [ 17I
have used predominantly frequency spread data to construct their maps of occurrence of spread F .The evidence from the correlation of scintillation occurrence and spread F [ 181 is that at equatorial and middle latitudes, range spread is associated with strong scintillation activity and frequency spread is not. Thus the available spread F maps
cannot be used for scintillation observations in these regions;
they are dramatically misleading in many cases. In the Mgh- latitude region no statistical study has been made to correlate types of spread F with scintillation activity.
scintillation have important differences (Equatorial Section 111). Ionosondes only observe reflections from the bottomside, from altitudes of the ionosphere lower than the level of
maximum ionization density; if sounders are in satellites, only > 1000 k m
-
Sfrom the topside. The reduced data are in terms of occurrence rather than amplitude of response or spread in range. In addi- tion, there is no indication as to the thickness of the irregular-
ities, their geometry, or the rate of fading. In their present THREE DIMENSIONAL form, spread F morphological studies are not useful even for PATCH MODEL
indications of scintillation occurrence. Wright et al. (1 977) Fig. 5. A magnetic equator cut through the g e n d form o f the e qua te
have found a means for converting ionograms into (ANIN) rial patch with typical dimensions shown.
rms by measuring the spread in frequency on "frequency
spread" ionograms. -I
It might be noted that even range spread occurrence and '00
111. EQUATORIAL SCINTILLATIONS In their intensity and their effect on transionospheric propagation, equatorial F-layer irregularities dwarf those of the high-latitude regions. Fluctuations from ionospheric
irregularities in the F layer have been reported at frequencies as high as 7 GHz. Fang has reported that over periods of time of the order of half an hour and longer, peak-to-peak fluctuations of 9 dB at 4 GHz may occur at elevation angles above 10' [ l l .
A . Patch Characteristics
Through theoretical considerations of instability mechanisms and through radar backscatter and rocket and satellite in-situ measurements, it has been established that nighttime iono- spheric equatorial irregularity regions emerging after sunset de- velop from bottomside instabilities, probably of the Rayleigh- Taylor type. The depleted density bubble rises into the region above the peak of the F 2 layer. Steep gradients on the edges of the hole help to generate the smaller scale irregularities with- in the patch which produces intense scintillation effects [ 191.
I ) Patch Development, Motion and Decay: A plume-like irregularity region develops, fmally forming a patch of irregular- ities which has been likened t o a banana or an orange segment.
A cut through the center of the "banana" is shown in Fig. 5.
The characteristics of the patch development, motion and decay can be summarized as follows:
1) A new patch forms after sunset by expanding westward in the direction of the solar terminator withvelocities probably similar to those of the terminator. It comes to an abrupt halt after typically expanding t o an east-west dimension of 100 to several hundred kilometers. It appears to have a minimum size of -100 km.
2) It is composed of field-aligned elongated rod or sheet irregularities. The vertical thickness of the patch is 50 to several hundred kilometers. The patch has maximum intensity irregularities in a height region from 225 to 450 k m , with irregularities to over 1000 k m .
3 ) Its north-south dimensions are of the order of 2000 km or greater.
4) Once formed, the patch drifts eastward with velocities ranging from 100 to 200 m/s.
5) The patch duration as measured by scintillation techniques
401
I I ' I \ I f4
L
1
L E S - 9 . AFGL AIRCRAFT 249YHz1
i Y.-^'.
20
c
I f "N
"I I19 20 21 22 23 00 01 L S T
Fig. 6. Fading rates and scintillation observations made by the AFGL aircraft on March 19-20, 1977 illustrating the slowing down of the patch after 2350 LST.
is known t o be greater than 2 1/2 h; individual patches have been tracked by airglow techniques up t o 3 h where they have maintained their integrity [ 201. Effects have been seen over 8 h.
6 ) The life history of a few patches has been studied in years of low and moderate solar flux [21]. The decay of patches in the midnight time period was of the order of 1 h after local midnight in years of low sunspot activity. Aarons e t al. [ 221 have also shown weak 3-m size irregularities on backscatter contours coupled with low or no scintillation activity. Fig. 6 [ 221 gives evidence for slowdown of the velocity of the patch by means of aircraft data. The fading rate when the patch was decaying (2350-0050 LST) showed the same rate whether the aircraft was flying against the patch motion (W) or with it (E), indicating a slowdown of irregularity velocity at the time when scintillation indices were low.
For an observer of synchronous satellites in the equatorial region, the eastward nighttime plasma drift moves these patches of irregularities through his beam. An encounter with one of these patches and the amplitude fading produced by them can best be illustrated by the severe case shown in Fig. 3 where an uplink signal from Si Racha at 6 GHz was retrans- mitted to Hong Kong at 4 GHz [ 1 1. The resulting scintillation activity is probably predominantly a t 4 GHz from the downlink path.
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS 365
NOS1
Fig. 7. Horizontal profde of ionospheric F-region plasma density indi- cated by electron (le) and ion ( l i ) currents on Rev. #2177 of S3/4.
S l / I is relative irregularity intensity [ 231.
t x ~ ' ~ a t / r n ' ~ TEC ASCENSION ISLAND
SIR10 SATELLITE 26-27FC0.1970
2000 2100 2200 2500 oooo UT
Fig. 8. Two depletions in total electron content from an assumed quiet background observed on Feb. 26-27, 1978 at Ascension Island. The close assodation with the Occurrence of amplitude scintillation should be noted.
2 ) Patch In-Situ Measurements: In-situ measurements within the F layer (at 225 km, for example) measure irregularity in- tensity as a function of electron density by measuring electron and ion currents. In one example [231 (Fig. 7) the electron and ion densities in the S3-4 data showed severe depletions.
Exact correspondence has been found between the in-situ depletions and scintillation activity [24] and between the scintillation activity and depletions as shown by optical air- flow measurements [ 251
.
3 ) Polarization Fluctuations: Patches show both depletions and polarization fluctuations, the latter effect is noted by a variation in total electron content as seen on Faraday rotation records [ 2 6 ] . While the total number of electrons depleted may be only of the order of 20 percent in some cases, the de- pletion a t certain altitudes is on occasion of the order of one or two magnitudes.
To illustrate Total Electron Content (TEC) and scintillation observations, Klobuchar and Aarons [27] recorded these two parameters at Ascension Island at a dip latitude of -16's for the 350-km intersection point. Continuous measurements of the Faraday rotation have been converted to equivalent vertical TEC in a standard manner using the longitudinal magnetic field intensity and zenith angle at a mean height of 420 km. Fig. 8 shows an evening period when two clearly evident depletions in TEC occurred. Note that depletions from an assumed quiet background TEC, indicated by a dashed line, are up to 10-1 5 percent. In addition, the close association with the occurrence of amplitude scintillation is indicated by
the start and stop times of amplitude scintillation. This figure shows the intimate association of TEC fluctuations with rapid, severe amplitude scintillations observed along the same path [281.
B. Variation of Scintillotion Activity
I ) Longitudinal Variations: Spread F measurements have shown that there is a clear longitudinal difference in F-layer irregularity occurrence as a function of day of the year. The differences may be due to the displacement of the magnetic pole vis-&vis the geographical pole, to the seasonal pattern of lower atmospheric triggering activity (thunderstorms, for example) as a function of longitude, or t o global wind systems.
Spread F soundings have been separated into longitudinal sectors for purposes of summarizing data. Scintillation data taken at a common frequency for a common period and re- duced in a similar manner are, however, sparse. We shall attempt to illustrate longitudinal differences with the available data.
The dip latitude 8 used in this paper is based on the formula tan 8 = 1 /2 tan I where l i s an inclination or dip of the magnetic field from the horizontal. At the dip equator the magnetic field is parallel to the earth's surface.
2 ) Data Comparison: Comparison was made of scintillation activity at 250 MHz at a variety of observatories with data taken over the same time period [29]. One set of data was taken at Huancayo, Peru; Natal, Brazil; and Accra, Ghana with all observations made at elevation angles greater than 20'
GEOGRAPHIC LATITUDE
30.N L~:N---
'> GUAM 0
YAG. EP.
---
15.5 HUANCAYO
1 k . E ) 60.W 3O.W 0.
LONGITUDE
Fig. 9. Map of equatorial regions using the 1975 epoch of the DMA magnetic indination map. X marks subionospheric intersection.
and with distance between the most separated stations about 70' of longitude; a map of both geographical and magnetic coordinates is shown on the right side of Fig. 9.
The occurrence percentages are shown in Fig. 10(a) and (b).
For this longitude region (-0-7OoW) the lowest scintillation occurrence takes place from May to July. The period August t o October shows similar occurrence rates at all observatories with little dependence on magnetic activity.
It might be noted that Accra and Natal, though both south of the dip equator, are almost equidistant from the geographic equator, one north and the other south so that June is the center of summer for Accra and winter for Natal (and Huancayo).
Therefore, local summer and winter at a station do no t play a role in scintillation occurrence. The intersection point of the Huancayo path was north of the dip equator; that of the Natal propagation path south of the dip equator, but the patterns were similar.
A second comparison of data at 250 MHz was made between observations from Huancayo and from Guam. The data are shown in fig. 1 1 ; activity minima occur from May-July in Hu-
ancayo and from November-January in Guam. The conclusion is that the occurrence patterns are longitudinally controlled.
Guam viewing of MARISAT was slightly north of the dip equator as was the intersection point of the Huancayo path, yet their patterns differed considerably.
While local summer at the observation site cannot be a factor as shown by the similar patterns of Huancayo and Accra in May, June, and July (each on opposite sides of the geographic equator) the pattern of seasonal electron density variations at the ends of the equatorial field-aligned patches may play a role.
It should be noted that in general maximum intensity occurs in the equinoctial months. This can best be illustrated by the occurrence of L-band 1500-MHz activity at Huancayo, Peru.
That evidence is shown in Fig. 12 [30]. L-band activity at Huancayo does not suffer from strong scattering or from
saturation (as do 136-MHz and 250-MHz data on occasion);
the data show clear equinoctial maxima.
3 ) Geomagnetic Control of Scintillations: From available data it appears as if geomagnetic control of the occurrence of scintillation differed with longitude. The generalization can be made that increased magnetic activity inhibits scintillation activity before midnight-except during those months with
very low scintillation activity (May-July for the region (!- 7OoW) and November-January in the Pacific longitudes (135
-
180'E)). After midnight the scintillation activity in general in- creases slightly with the presence of magnetic storms. The data shown in Fig. 1 O(a) and (b) are for a year's observation in each case. The complexities of the magnetic control of scintillation occurrence are illustrated by the variations in the curves of occurrence at each station in each season. For further details see Mullen [3 1
1.
C. In-Situ Data
The larger data base of continuous observations from ground station measurements has been utilized to establish the features of the major m a t i o n regions. However, this is uneven in longitudinal coverage and unavailable over ocean surfaces.
Satellites carrying out in-situ observations of irregularity param- eters such as electron density variations do providea mapping technique.
One example of data collected and organized [32] is shown in Fig. 13. It should be pointed out that this figure was o b tained over a period of two months (November, December
1969), for a relatively high level of sunspot activity, and is valid for the time period 19-23 LT. It is illustrative of mapping which can be done a t various altitudes. Scintillation intensity is a function of both AN and the thickness of irregularity layer. In-situ measurements do not measure thickness and its variations or orientation of the irregularities. Therefore, a model must be developed to utilize these data.
Basu and Basu [33] have developed a model from in-situ, theoretical, and scintillation studies. In their morphological model of scintillations, measurements of irregularity amplitude h N / N as computed from T seconds of data are utilized in con- junction with simultaneous measurement of electron density N . A combination of ANIN and N data provides the required A N parameter as a function of position and time. In case the satellite altitude is much lower than the height of maximum ionization, proper allowance should be made in deriving AN estimates. The in-situ measurements of irregularity spectrum and phase scintillation measurements with the 1000-km high inclination Wideband satellite indicate that the outer scale at
F
region heights is large, probably on the order of tens of kilom- eters. In view of this, the spatial length corresponding to T- seconds time interval when projected in the direction of shortest
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS 361
- _ _ _
K . 0 - I ' K = 3 - 9FEE). - APR
60 257MH2 ; i
ACCRA, GHANA 40
M 20
HUAMCAVO. PERU
20 20
(a)
_ _ _
K : O - I ' K = 3'-9_ _ _
K = O - I *-
-
I( i 3-9 YAY - JULY AUG - 0 C T6 0
"1
, ~ ACCRA,GHAN; ,20
257MHZ
60 2 5 7 MHZ
ACCRA, GHANA
1
I2 I8 24 6 I 2 LT 12 18 24 6 I2 LT
m
(D 6 60r
9 _j
, - NATAL, BRAZIL , ~ ,/?i
NATAL. BRAZIL2 m
Y 0 2 w
18 24 6 12 LT 42 18 24 6 12 L T
2 12
x
D
6or
r AHUANCAYO. PERU 4 0 1
p k
HUANCAYO. PERU(b)
Fig. 10. (a) Seasonal patterns of occurrence o f scintillation activity
> 6 dB (S, = 0.3) for very quiet (Kp = 0 - 1') and for disturbed (Kp = occurrence of scintillation activity > 6 dB (S, = 0.3) for very quiet 3+
-
9) magnetic conditions for Nov.-Apr. (b) Seasonal patterns of for May-Oct.(Kp = 0 - 1') and for disturbed ( K p = 3'
-
9) magnetic conditionsFig. 11. Comparison of seasonal patterns of occurrence of scintillation 0
-
1') and disturbed (Kp = 3* - 9) magnetic conditions.activity >10 dB for Guam and Huancayo under very quiet (Kp =
PERCENT OCCURREWE OREATER THAU 2 d 0 IS, = ,131
SUNSET l 3 M k m l SVNRlSE
t i
I I II I 1
4
SEPI 1
IS 21 03 09 15 LT
HUANCAYO I 5 4 G H z APRIL 76 - O C T 7 7
Fig. 12. Percentage occurrence of 1.5-GHz scintillation 3 2 dB during Apr. 1976-0ct. 1977.
correlation distance of electron density deviation sets the apparent outer scale length 4 0 . The outer scale wavenumber is, therefore, K O = 2n/qo. For the equatorial scintillation model that they developed from the OGO-6 in-situ observations, the time interval was T = 3 s and the outer scale length was con- sidered t o be 20 km corresponding t o an outer scale wave- number of K O = 0.3
km-' .
D. Sunspot Cycle Dependence
From the viewpoint of electron density variations the equa- torial region around the magnetic equator displays a complex pattern. During the day an increase in maximum electron
density occurs away from the equator. The electron density contours display a distinct trough of electron density in the bottomside and topside ionosphere at the magnetic dip equator with crests of ionization at f1S0-200 north and south dip latitudes; this is the Appleton anomaly with the region within
*So dip latitude of the magnetic equator termed the electrojet region.
From the solar cycle minimum in 1974 and maximum in 1969-1970, Aarons 1341 found that there was a higher occur- rence of deep scintillations during a year of high solar flux than during a year with low solar flux for observations at both Accra, Ghana and Huancayo, Peru.
Recent observations of L-band scintillations during the period of maximum solar flux (1979-1981) [37] haverevealed that scintillation intensities maximize in the Appleton anomaly region rather than near the magnetic equator.
At Calcutta, India, which is situated close t o the northern crest of the Appleton anomaly in the Indian longitudinal sector, a remarkable increase in the Occurrence of VHF scintillation was observed between 1977 and 1980 when solar flux in- creased [35].
The contrast between scintillation levels with the path t o the satellite in the electrojet region and with the path in the anomaly region can best be seen with the aid of the map in Fig. 9 and the contrast in data between Natal, Brazil and Ascen- sion Island, both observing the L-band beacon of MARISAT at approximately the same longitude. Natal data show no incidence of scintillations beyond 8 dB, Ascension Island records show scintillation activity of the type shown in Fig. 14, i.e., peak-to-peak fades of 27 dB for hours. Fig. 15 illustrates the percentage Occurrence for a two month period during this year of very high solar flux.
Fang [ 1 ] has presented similar results of high scintillation intensity observing from Hong Kong. He recorded fluctuations to 9 dB on the 4-GHz COMSAT downlink with paths through
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS 369
KWAJELEIN HUlWCA.70 ACCRA
PERU THHUIIRA H0N6
GHANA INDIA KON6 QUAM
G E O G R A P H I C L O N G I T U D E
Fig. 13. Percentage occurrence of scintillations >4.5 dB at 140 MHz
(19-23 LMT, Nov.-Dec. 1969. 1970, A p < 12) using scintillation data and O W - 6 obser~ations.
A S C E N S I O N I S L A N D 26 DEC 1979
Fig. 14. Sample of both UHF and Lband data recorded at Ascension both UHF and L-band channels.
Island during December 1979-January 1980. Note excursions on
ASCENSION ISLAND JAN
-
FEB 19801541 MHz
M I D N I G H T LOCAL
Fig. 1 5 . Percentage occurrence of L-band scintillations 2 2 0 dB at into quiet (Kp = 0-3) and disturbed (Kp = 3+-9) magnetic conditions.
Ascension Island during Jan.-Feb. 1980. Observations are segmented
the anomaly region. Recordings have been shown earlier in Fig. 3.
Older data have been reviewed [ 3 6 ] , i.e., results from Ascension Island on an S-band transponder on the moon.
Scintillations as large as 20 and 25 dB on the two-way path, ground to transponder and return were noted. Canary Island observations also through the anomaly taken simultaneously between November 1969 and June 1970, a period of high solar flux, showed similar scintillation activity.
The conclusion in the study [371 is that the intense scintilla- tion activity during years of high solar flux are due to two factors:
1) The equatorial anomaly has considerably higher electron density values in high sunspot number years than in years of low solar activity.
2) The occurrence of maximum electron density for anomaly latitudes is near sunset in the years of high sunspot number and in the afternoon in years of low solar activity. Thus the post sunset irregularity patches form high m l e v e l s in the years of high solar flux. Data from ionosondes and from total electron content measurement corroborated the extremely high levels of electron density and the lateness of the appearance of a maximum of electron density during 1979 and 1980.
IV. MIDDLE-LATITUDE SCINTILLATION The middle-latitude scintillation activity is not as intense as that encountered at equatorial, auroral, or polar latitudes.
For the engineer, however, activity may reach levels, primarily at VHF and UHF, which will increase error rates of systems with low fade margins.
The difficulty with describing middle-latitude scintillation activity is that at times what takes place at middle latitudes is an extension of phenomena at equatorial and auroral lati- tudes. For example, scintillation activity in 1979-1 98 1, years of high sunspot number, was observed to be high in data from Hawaii and from Japan; the effects were possibly caused by equatorial phenomena during years of high sunspot number.
The depletion regions which originate at equatorial latitudes do move to higher altitudes but these irregularities would have to be >2000-km altitude. The perturbing effects of these regions and the higher electron densities during high sunspot number years might combine to provide effects along the lines of force thus extending equatorial activity to the ''lower'' middle latitudes.
At high latitudes, there is a motion of the irregularity bound- ary equatorwards during years of high sunspot number and during magnetic storms. Auroras have been noted in the southern U.S., for example, along the 70°W meridian. Scin- tillation activity is present at these times at these lower lati- tudes where optical aurora are seen.
A second complicating factor in middle-latitude scintillation morphology is the effect of sporadic E. Several studies have shown that intense sporadic E produces scintillation. The behavior of sporadic E is totally different from the morphology
KOKUBUNJI
1977 1976
SCINTILLATION
TIME OF MAXIMUM AMPLITUDE I "
NOV - DEC
+
I1
GUAMI
I
i
O J F M A M J J A S O N D
1979
Fig. 16. (a) Percentage Occurrence of scintillation 3 3 dB seen at Koko- bunji at midnight in 1977-1978. (b) Scintillation Occurrence >10 dB in hours seen at Guam during 1979.
of F-layer irregularities. Thus two independent variables pro- duce the fading phenomena. .At middle latitudes, there is a high Occurrence of daytime sporadic E resulting in a second maxi- mum of scintillation. Nighttime sporadic E adds to the effects of F-layer irregularities.
A . Results from Longitudes in the Western Pacific
Measurements of scintillation activity have been taken in Japan primarily from Tokyo which observes a synchronous satellite at its longitude through a 350-km ionospheric inter- section of 36'N, a dip latitude of 27'N.
At the VHF frequency of 136 MHz, observing ETS2, Sinno and Kan [38] found a maximum of scintillation activity at night and in the May-July time period. We have reconstructed their data to show the percentage of occurrence of 3 d B scin- tillation in 1977 and 1978 (Fig. 16(a)). We have also placed in Fig. 16(b) the occurrence of scintillation activity in Guam for the following year [391 t o allow the comparison of various months of the year. The monthly pattern of the Japanese data follows somewhat the pattern of equatorial scintillation except for August and September. The lack of exact corre- spondence of observation dates makes the comparison tentative.
By observing ETS-2 from Taiwan at 25'N, Huang [40]
found similar results, i.e., the same nighttime maximum in the May-July period and a summer daytime maximum of lower level fluctuations.
Observations of severe ionospheric scintillations primarily in the 4-GHz range have been reported by Tanaka (198 1) [80]
for paths primarily at higher latitudes than the equatorial anomaly region. For periods of time of 30 min t o a few hours, on a few occasions, scintillations of the order of a 2-4 dB were noted after sunset. The hypothesis advanced is that during ionospheric storms the positive phase produces high electron densities to latitudes above the anomaly. The dis- turbing wave traveling from the equator to higher latitudes, triggers plasma instabilities which affect the ambient high electron densities during this phase of the storm.
M A R - A P R
+
I caseYAY-JUN
JUL - A M
SEP
-
D C T16 2 0 2 2 0 2 4 6 L T
Fig. 17. Histograms of the times of maximum scintillation index for from Ramey, Puerto Rico.
each 2-month period throughout the observations made at 136 MHz
B. Results from Longitudes in the Americas
With transionospheric propagation data taken in 1976 from sites in Puerto Rico and Florida, Kersley et al. [42] found that scintillation activity at Ramey, h e r t o Rico occurred between 2100 and 0230 LT with maximum levels in the post midnight period (Fig. 17).
The general level of scintillation at 136 MHz was in the 2-8 dB peak-to-peak range with occasional increases to 12 dB peak t o peak. For these observations maximum occurrence was noted in July with minima in the equinoxes. The seasonal pattern along with other factors indicated that the low-latitude scintillation activity was not related t o equatorial irregularities.
By performing simultaneous incoherent scatter radar measure- ments and scintillation observations Basu et al. [43] demon- strated that the scintillation maximum is associated with the midnight descent or collapse of the F region.
The general pattern of what might be termed "upper" middle latitudes, i.e., from 30"-45' dip latitude is that two diurnal maxima exist, one at midday and the other at midnight. The midday maxima are associated with sporadic E and appear primgily during the summer. The nighttime maxima appear in all seasons and are predominantly associated with spread F although high values of f o E , were noted during nights of high scintillation activity [44]. MacDougall, in observations made from southern Ontario in 1977-1 978 (private communication), showed a midday maximum in the summer.
C. Effect of Magnetic Index on Middle-Latitude Scintillation
At latitudes below the auroral oval, various sets of data have yielded behavior indicating little correlation with magnetic conditions. Evans [45] found no correlation of the ionospheric
