Occurrence of high latitude ionospheric irregularities giving rise to satellite scintillation

Journnlof Atmospheric andTerrestrial Physics,1971,Vol. 23, PP.21-30. Pergamon Preaa.Printed in Northern Ireland

JON

I?RIIU(XEN

Norwegian Defence Research Establishment,

2007 Kjeller, Norway

Abstract-The variation of mean scintillation depth at 40 MHz with geomagnetic latitude, geomagnetic local time and K, hss been found. Scintillation peaks in two regions. A polar region above 80” geomagnetic latitude, and one resembling the amoral oval. Between these there is a nighttime scintillation trough. This trough may separate regions where different production mechanisms for ionospheric irregularities predominate. The regions expand especially on the dayaide with increasing R,. 1. INTROIXJCTION IN THE ionosphere, irregularities of electron density having a wide spectrum of sizes, shapes and intensities are known to exist. One set of irregularities are of thermal origin, fairly well understood and utilized in probing the ionosphere by incoherent baokscatter. Other irregula~ties that are produced by external forces or agents are observed by a variety of radio methods ranging from VHF aurora1 backscatter to LF backscatter, and by forward scatter from for example extraterrestrial sources. Choice of frequency and method decides partly what part of the ionosphere one will study and partly the parameters of the irregularities and which of these parameters may be studied. To obtain a reasonably complete picture of the irregularities in a pa~i~ular volume it is necessary to use a variety of methods able to view this volume simultaneously. Such coordinated measurements are however understandably very rare. In this paper only results of observations of the degree of amplitude scintillation of the 40/41 MHz beacons on Explorer XXII, as observed from Spitsbergen will be discussed. Satellite so~ti~ation is a rapid and irregular fading of the signal caused by scatter or diffraction by irreguhrritiea in the ionosphere. In principle one can by this method study irregularities with a minimum size of one wavelength up to very large sizes. In praotice the upper limit is set by the experimental conditions, and the lower perhaps by the smaller irregularities having to be much more intense than larger irregularities to give comparable effects. The general observation is that the irregularities typically giving rise to scintilI&ion are of the order hundreds of meters to a few kilometers and highly elongated along the magnetic field (SPENCER, 1955; JONES, 1960; KENT, 1960; AARONS et al., 1961;

TITHERIDOE,

1963;

KOSTER, 1963).

Orbiting satellites have made it possible to measure the height of irregularities giving rise to s~int~ation (FRIHAGEN and TR&M, 1961; YER and SWENSON, 1959; KENT, 1960; MCCLURE, 1964; LISZPA, 1964). The active irregularities are almost invariably observed to be located in the P-region say 250-500 km range. A notable exception is the mid-latitude, midday, weak E-region scintillation observed by MCCLURE (1964). Unpublished height measurements made on Spitsbergen in conjunction with the observations to be reported here give mean 21

Joa Fam.~ou~

22

heights of 400 km i-50 km for 10 cases properly reduced to date, and the same measurements gave correlation distances of the order 1 km in a transverse direction; evidence that they are elongated has been published by FRIHAGEN (1969). A brief consideration of the order of magnitude of electron density variations in the irregularities is in order. Let us consider highly field-aligned irregularities and two rays coming down from magnetic zenith, one inside an enhancement of electron density where the refractive index is it + An and one outside where the refractive.index is simply ‘it. The difference in phase when they reach a smoother ionosphere below is

At 40 MHz we have ~2=l-__._”

81N f2

and An

=--

(2)

’ ‘1 AN w 2.5, lo-l*AN 2n f2

where N is electron density [m-“] and f is the radio frequency [Hz]. With

n-&f

=4OMHz

we find A+2.fO-YAN.S if AN = log rnc9 and the integration path S[m] (the length of an irregularity} is lo4 m we get Ad, = O-2 rad. which will give observable scintillation (RATCLIFFE,1966). Recent more complete calculations of AN made by SINGLETON(1970) show our value to be acceptable. This may be compared with the electron density in the ambient ionosphere which will be of the order 1010-1018m-s. The electron density deviation that is necessary is then in general terms of the order 10 per cent at night and 0.1 per cent during the day. The morpholo~ of satellite scint~lation at lower latitudes has been extensively studied. In general there is minimum scint~lation at middle latitudes and a strong increase towards equator and the aurora1 zone (AARONS et al., 1964). Diurnally scintillation maximizes strongly at night at all latitudes (BRKGGS, 1968; KOSTER, 1965; FRI~AGEN,1969). With increasing K, the degree of scintillation is increased in aurora1 to middle latitudes (Brnaas,1964; ALLENet al., 1964; STEWARTand TITHERIDGE,1969), but decreases at the equator (KOSTER and WRIQHT,1960). Seasonal variations are absent or weak in middle latitudes (BRIUQS, 1964; AARONS et al., 1964), at the equator there is an equinoctial maximum (KOSTE~, 1963). Little has been published about the morphology of scintillation within the polar regions. T~~ER~D~E and STEWED (1968, 1969) have pub~shed some results from the south polar region and FRIEA~EN (1969) has pub~shed some results from Spitsbergen observations.

Occurrence of high latitude ionospheric irregularities

23

2. OBSERVATIONS This paper is based on observations of the 40/41 MHz beacons on Explorer XXII. This satellite is in a near circular orbit at 1000 km height with an inclinaation of 80’. Amplitude recordings of the beacons have been made from Isfjord Radio on Spitsbergen 78.06 N 13.63 E geographic and 75 N 132 E geomagnetic. Normally 5-6 passes were recorded per day from June 1965 to May 1966. The recordings were scaled for scintillation depth every minute along the pass at Faraday fading maxima. The scintillation depth was given indices in the range O-9 by truncating the expression

A

- Amin max + Amin

SI = 10 /a=

to an integer. A,,, and Amin are, wherever reasonable, the level of the third highest fading maximum and the third lowest fading minimum, respectively, on a linear scale. This data was fed to a computer together with orbit information. The following output was obtained: (1) UT of observation. (2) Geographical coordinates at the ionospheric point i.e. the point at which the (line of sight) ray from the satellite to the observer passes the 400 km level. (3) Geomagnetic coordinates of the ionosphere point. (4) Local mean solar and geomagnetic time. (5) Azimuth and elevation of the satellite. (6) The angle between the magnetic field (computed by a spherical harmonic expansion) and the ray. This output was then sorted in a variety of ways as discussed below. 3. RESULTS The main objeotive of this paper is to try to establish how scintillation depth varies with local geomagnetic time, latitude, magnetic activity and season. Season, time and latitude are interdependent in a complicated way due to satellite orbit effects. Figure 1 shows the paths of the sub-ionospheric point for 6 consecutive passes. On any day parts of very similar passes are observed that will cover about 8 hr of geomagnetic time. Due to the precession of the satellite orbit the period of day that is covered will change slowly, moving anticlockwise, making a full revolution in about 6 months. For full diurnal coverage 4 months of data are necessary. In such a period any seasonal changes will be important. Figures 2(a-d) shows plots of contours of constant mean scintillation depth versus geomagnetic local time (MLT) and geomagnetic latitude (MLAT) for different K,. These plots are the result of sorting all the available data in boxes 1 hr MLT by 2’ MLAT. A priori these figures may be contaminated by angle of elevation and seasonal effects. Angle of elevation effects seem unimportant. Figure 3 shows the diurnal variation at 75’ MLAT for the complete set of data, and for data points with satellite elevation higher than 60’. The error bars are f standard deviation. The main

24

JON

fiIIW3EN

Fig. 1. The path of the subionospheric point on 6 consecutive passes. Coordinates we geomagnetic latitude and geomagnetic local time.

Fig. 2(a-d).

Contours of constant scintillation depth for different K,. are geomangetic latitude and geomagnetic local time.

Coordinates

Occurrence of high latitude ionospheric irregularities

Fig. 2(b).

Fig. 2(c).

25

26

/

KpS3

Fig. Z(d).

r

Kp =O 74 -76* __

-

1

00

GEOM.

LAT.

ELEVATION

>

ELEVATION

*

T

12

06 MLT

Fig.

-.

18

21

-

3, Diurnal variation of ~c~~t~l~tion index at 75O geoma@M& north for JG = 0. The broken line includes observations from dl sateBite elevations. The full line on& includes data from when the satellite ovas at greater than 60’ elevetion. The error bars are f 1 standard deviation.

27

Occurrence of high latitude ionospheric irregularities

I

(351

I

1641

I

(671

I

I I GEOM LAT 09 -15 MLT

I

1571

730-77*

(79)

Il2Sl

(1611

I

(175)

I

(124)

up:3

Kp=2

Kp : 1

Up = 0

00

I

I

I

I

100

20*

300

400

ELEVATION

1

I

I

I

60*

70*

600

go*

I SO“

-

Fig. 4. Soint~~tion index vs. satellite elevation for different K, during daytime at 7&’ geo~~et~c north. Error bars are f 1 standard deviation. The total number of observations in each 10' step of elevation is shown in brackets.

features are similar on the two curves. A further study was made by sorting the data for limited ranges of geomagnetic time and latitude, in elevation. One set of results are shown in Fig. 4. Very little systematic variation is seen. BRIGGSand PARKLN(1963) have published a rather comprehensive theory of angle of elevation effects based on weak diffraction by field aligned axially symmetric irregularities. A strong maximum in scintillation is expected in magnetic zenith. Away from this direction the scintillation rapidly drops to follow the curve for isotropic irregularities quite closely, thus giving a broad shallow minimum of scinti~ation in the zenith. As we shall see, the ma~mum in magnetic zenith is also evident in our data. On sorting the data in elevation this maximum will be distributed over a fairly wide range in elevation, and contribute towards reducing the expected broad minimum in the zenith. An attempt to find any seasonal variation was made by sorting the data in 8 month intervals, K, and narrow geomagnetic latitude and time intervals. NO systematic long term variation was found. 4. D~scossron The main results of this study are:

(I) The very strong R, dependence of scint~ation depth, specialIy on the dayside. (2) The ‘trough’ in scintillation at about 75’ geomagnetic latitude at night, separating ‘auroral’ and ‘polar’ maxima in scintillation.

28

JONFRIEA~EN

Both these phenomena have their greatest interest in that they put strict requirements on possible production mechanisms of F-region irregularities. The strong K, dependence is perhaps somewhat surprising when compared to the lack of K, dependence of the occurrence probability of topside spread-F north of 30” (CALVERTand SCHMID,1963). Both spread-F and scintillation are generally thought to be caused by irregularities, and a good correlation has been observed at middle latitudes (BRIGGS,1965), while in the aurora1 zone (OWRENet al., 1964) the correlation is not clear. Some difference might be explicable by spread-F being scaled as present or absent, while scintillation is scaled in intensity. Also spread-F might require more intense irregularities (SINGLETON,1970) than can give measureable scintillation. These might well, however, be real differences in the occurrence patterns of spread-F and scintillation that need being looked into. In our data the midnight period has mainly been sampled round midwinter. In this period, December-February, the trough is not merely a result of statistical handling of the data but actually is observable on practically all passes of the satellite that cross the trough. The presence of the trough may be an indication that at least two different mechanisms are at work in producing irregularities. One decreases equatorwards from 80” and may or may not go appreciably beyond the aurora1 oval. The other maximizes in the oval. Whether subauroral irregularities are caused by one or the other is at present arbitrary. A number of mechanisms that might produce the aurora1 maximum have been suggested. Many of these are naturally connected with energetic particle precipitation. YEH and SWENSON(1964) have proposed a two-stream instability between the precipitating particle flux and the ambient ionosphere. This mechanism might be active in a relatively narrow zone with a well defined northern boundary at the cut off latitude of the active particles. FARLEY (1963) has suggested that two-stream instabilities caused by the aurora1 electrojet may be active. This mechanism would also be restricted to a narrow region. DESSLER(1958) has considered that hydromagnetic waves might produce irregularities of maximum intensity in the aurora1 oval and weaker irregularities for some distance away. SINGLETON(1964) has also proposed that these weaker irregularities might be amplified by the MARTYN(1959) process to become strong, under certain circumstances, thus explaining the equatorial irregularity region. It is doubtful whether this mechanism would also take place in the polar area. BOWHILL(1964) has considered the effect of possible spatial variations in the protonspheric heat flux into the ionosphere. The resulting temperature differences will produce irregularities, if they exist. REID (1968) proposes that aurora1 and equatorial irregularities can be produced by an instability caused by the presence of an electric field together with gradients in electron density. We do not know which, if any, of these mechanisms may partake in producing the observed aurora1 zone irregularities. Few of them are likely to be effective at higher latitudes. FRIHAGEN(1969) has noted the rapid flux variations observed in low energy ( ~1 keV) electrons at high latitudes (HOFFMAN,1970; BURGH, 1968). If these are spatial variations to only a small degree they are

Occurrence of high latitude ionospheric irregularities

29

sufficient to produce irregularities. Even though these particles also show a ‘polar cavity’ (~AE~Lu~, 1968), there may be sufficient flux and spatial variation to produce irregularities up to the pole. Considering the rapid and strong variations in soft electron precipitation that have been observed it seems difficult to escape that this mechanism must produce irregularities from the aurora1 zone, or below, up to the pole. This does require that the spatial and temporal variation is such that the integrated flux over a time corresponding to the lifetime of free electrons against loss and diffusion varies suficiently from point to point. REFERENCES AARoNs J. et al. AARONS J., MULLEN ALLEN R. S., Amom ~HITi?EY H. BOWH~L S. A.

J. and BASU

S.

J. and

1961 1964 1964

Planet. Space ~5%.5, 169. J. geophys. Res. 69, 1785. IEEE Trans. m&it. Electron. &X&&146.

1964

I% Spread P and its E$ecta upon Radio Wave Propagation and Communications Aga~dog~ap~ 95 (Edited by P. NEWMAN), Teobnivision. J. Atmosph. Tew. Physs, 12, 89. J. Atmoaph. Tew. Phys. 26, 1. J. Atmosph. Tern. Phys. 25,339. J. geop~ys. Res 73, 3585. J. geophys. Res. 69, 1839. J. geophys. Res. 63, 507. J. geophys. Res. 68, 6083. J. Atmo~~. Tew. Phys. 20,215. J. Atmcsph. Tew. Phys, 81, 81. The Polar ionosphere a.& Magnetospheric P~oceases (Edited by G. SEOVIZ), Gordon & Breach, New York. J. Atmosph. Tew. Phys. 1@,26. J. Atmosph. Tew. Phys. 19, 272. J. geophqs. Rea. 88, 2579. J. geophys. Res. t&2303. Ark. Geofvs. 4,211. Proc. Instn Radio Engrs 47, 147. J. geophys. Res. 69, 2775. J. geophys. Res. 73, 3459. Spread F a& iti EfJect uptm Radio Wave Propagati~ aped~~~~~~~~~~ Agardograph 95 (Edited by P. NEWMAN), Technivision. Rep. Prog. Phys. 19,188. J. geopys. Res. 73, 1627. Spread F and its E’e’ecta upon Radio ?Vave Propagation and Communications, Agardo~aph 95 (Edited by Y. NEWMAN), Technivision. J. Atmosph. Tepr. Phys. 32, 187. Proc. Phys. Sot. 883, 493. J. geophys. Res. 88, 3399. J. Atmosph. Tew. Phys. 30,85.

~RIWS B. S. BRIIXS B. S. BRIGI~SB. S. and PARKIN I. A. BURGH J. L. CALVERT W. and SCHMID C. W. DEE~@~RA. S. Fmmu D. T. FRIHA~EN J. and TRSIX J. FI~IHAGENJ. NOFXXAN R. A.

1968 1964 1963 1968 1964 1958 1963 1961 1969 1970

JONES I. I;. KENT G. S. KOSTER J. R. Komm 5. R. and WRIOKT R. W. H Lrs!mA L. MARTYN D. F. MCCLURE J. P. MAEHLUM B. N. O~REN L. FREXOW E. J. and

1960 1960 1963 1960 1964 1959 1964 1968 1964

RATCLIFFE J. A. REID G. SINGLETOND. G.

1956 1968 1964

SINGLETOND. G. SPENCERM. TITEERID~E J. E. TIT~~D~E J. E. and STEWARTG. F.

1970 1955 1963 1968

30 TITHERIDGE J. E. and hEWART G. F. YEH K. C. and SWENSON G. W., JR. YEH K. C. and SWENSON G. W., JR.

JON FRIHAGEN 1969

J. A&mph.

Tern. Phys.

31, 905.

1959 1964

J. geophys. Rec. 84, 2231. &we& P and its Effecta upon Radio Wave Propagation and Conmunications Agardograph 95 (Edited by P. NEWMAN), Tecbnivision.