Incoherent scatter radar observations of spread-F producing ionospheric structures at arecibo

Journal of Atmospheric andTerrestrial Physics, 1972, Vol.54,pp.1119-1127. Pergamon Press.Printed inNorthernIreland

Incoherent scatter radar observations of spread-F producing ionosphere sectors at Arecibo J. 3). ~ATEEWS Department of Electrical Engineering and Applied Physics, Case Western Reserve University, Cleveland, Ohio 44106, U.S.A. and R. 3%. HARPER Space Science Dep&rtment, Rice University, Houston, Texas 77001, U.S.A. (Received

6 August

197 1;

in revised form 4 November

197 1)

Abstract-Range spreading type spread-F observed during the night of 7-8 January 1971 at Arecibo, Puerto Rico is shown to be associated with tilts in the ionospherewhich oocurredwhen a region of increasedtotal electron content drifted by in a southerly direction. This conclusion is reached by using the incoherent scatter radar to me&sureelectron densities, vertical and horizontal ion velocities, and total content. Also used for analysis of ionogramdata is a spread-F intensity index and a method of applying the time development of spread-F to estimate the vertical and horizontal velocity of the reflectorproducing the spread-F trace. It is also suggested that frequency spreading at Arecibo is due for the most part to overlapping of four or more range spreading type ionogram traces having different critical frequencies since each trace comes from different regions of the ionosphere. 1. INTRODUCTION THE PHENOMENON of spread-k’ has long been of interest and many explanations have been put forth (HEERMAN, 1966 and references therein; KING 19’70); but thus far no single explanation has proved significantly better than the others. Of the two general categories of explanations, ducting and specular reflection from tilted layers (see PITTEWAY and COHEN, 1961; and KING, 1970 respectively), this paper presents data supporting specular reflection from tilted layers as the cause of spread-P observed at Arecibo, Puerto Rico. The data presented in this paper were obtained by methods new to the study of spread-P; these include use of the incoherent scatter radar at Arecibo to provide electron number density profiles and vertical plus horizontal ion drift velocities. (For ion velocity measurement techniques, see WOODMAN and HAA~FORS, 1969; BEHNKE, 1970.) Also developed is a method whereby horizontal and vertical ‘reflector’ velocities can be estimated from the time development of range spreading type spread-F. Incoherent scatter radar techniques have been used to study equatorial spread-3 (PARLEY et al., 1970) at Jicamarca, but the technique suffers from the inability of that facility to measure horizontal ion velocities and from the fact that the strong presence of spread-$’ associated irregularities renders electron density and ion velocity measurements in the region of the irregularities impossible. The radar at Areeibo suffers no such difficuIties due to its higher operating frequency (430 MHz as opposed to 50&IHZ)and pointability. The principal purpose of this paper is to associate, for one night’s data, a particular ionospheric structure with spread-F. This structure is shown to have been a tilted layer due to the passage of an enhanced region of ionization travelling 1119

J. D.

1120

MATHEWSand R. M. HARPER

southward. Also presented are some early results which indicate that frequency spreading in spread-F is, at Arecibo, due to intense range spreading. Finally a spread-P intensity index is introduced and then employed to compare radar and ionogram data and to demonstrate the time evolution of spread-F. 2. PRESENTATION OF DATA The ionosonde used at Arecibo is a modified model C-3 with a peak power of kW, a pulse width of 50 psec, a ‘delta’ t,ransmitting antenna with essentially no directional characteristics, and a simple dipole receiving antenna. As a result of these characteristics the C-3 ionosonde, when compared to newer low powered sounders, can detect strong irregularities at large distances or weak irregularities in tlie local ionosphere. In Fig. 1 we present radar data, from the night of 7-8 January 1971, in a form The graph of radar useful in undcrsta’nding the geometries seen by the ionosonde. 10

I

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E”z ‘0 w

I

1900

5

2000

,

2100

I

2200

,

2300

,

0000

,

0100

TIME (LST) 1. The upper portion is a graph of radar derived constant plasma frequency contours (units of megahertz) plotted vs. time and altitude. The dashed contour is an ionogram derived virtual height (measured at 2 MHz) which is to be compared with the ~-MHZ plasma frequency contour. The lower portion of the graph is the spread-P intensity index (described in the text) plotted vs. time. This data is all Fig.

from the night

data time. beam data with

of 7-8 January

1971.

consists of constant plasma frequency contours plotted in altitude versus This data, as all other radar data to be presented, was taken with the antenna directed 15” from zenith and rotating but in such a manner that 10 min of is taken with the beam pointed north or south and east-west data is taken the beam rotating N-S or S-N at a maximum rate of 44 min per 90”. This

Incoherent

1121

scatter radar observations of spread-B’

data, taking procedure is necessary to determine vector ion velocities and the data is uncorrected in that the altitude should be scaled by cos 15’ (0.965). The fist four ionograms in Fig. 2 come from a series of seven ionogrems for the morning of 8 January 1971. They display spread-F typical of that observed at Arecibo and thus form a basis for further discussion concerning spread-F. Note the individual feature (identified by the arrows) which may be seen in each of the four. To aid in understanding the time evolution of spread-F a spread-P ‘intensity index’ was devised that consists simply of counting the total number of first hop I971 JAN S

0017

1971 JAN 8

1971 JAN t3

0028

1971 JAN

2357

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1971 MAR IO

2

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Fig. 2. The first four ionograms are from a series of seven on which the special feature, identified by the arrows, was seen. These ionograms display spread-P typical of Arecibo. The last ionogram (from 10 March) shows that frequency spreading in spread-P at Arecibo is associated with strong range spreading.

traces s,t 2 MHz (or any other frequency below which the 0 and X traces are superimposed for the entire time period in question) and plotting this number as a function of time. This procedure was followed for the event of interest and the results appear in the lower portion of Fig. 1. Note from the figure that relatively intense spread-P is correlated with ionospheric features seen at center times of 2120 hr, 2310 hr, and 0010 hr; in each case the features appear to be tilts (a region, in time, where layer height is changing) in the layer. Frequency spreading type spread-F did not occur on this particular night but on other nights frequency spreading was seen to occur when the intensity index was larger than four or five (see last ionogram Fig. 2). From this result we suspect that frequency spreading at Arecibo is due to the overlapping of traces at frequencies near the critical frequency and the fact that various traces exhibited different

J. D. MATHEWS

II”,”

a,nd R.

M. HaRPER

critical frequencies (presumably because of tilts and/or other irregularities) so that the net effect is a trace which is quite extended on the frequency axis. The dashed line also shown in Fig. 1 is the virtual height of the main ionogram trace, measured at 2 MHz. Before the spread-F occurs this line is about 10 per cent higher than the ~-MHZ plasma frequency contour due to the characteristic (group and deviative) delay of the ionosonde pulse. Note, however, that just before a tilt passes by the delay increases to nearly 20 per cent and after it passes by the delay decreases to a small unresolved value. We suspect this effect is due to t,he fact that 100

50 ;; : \ E

0

> 5 -5O r: w > -100

-150

-200 TIME

(LST)

Fig. 3. The radar derived north-south (dashed line and open circular data points) and vertica,l (solid line and open triangular data points) ion velocities and plotted vs. time for the night of 7-8 January 1971. The plot is such that upwards and north are positive. The three solid triangular data points are ionogram derived vertical velocitirs and the three solid circular data points are the corresponding horizontal velocities.

the 0 ray deviates northward and the X ray deviates southward so that the lack of symmetry in delay would indicate the predominance in return of one propagation mode over the other. (We have no evidence with which to decide the dominant mode.) It is important to remember this effect when further ionogram results are discussed. Figure 3 is a plot of both vertical and nort$hsouth ion velociCes obta,ined from the incoherent ba,ckscatter and gives substance to the idea that the ionosphere was The thin lines in motion and that the tilts in Fig. 1 were not due t,o recombination. in Fig. 3 are derived from a weighted running average of the individual dat,a points (also displayed) which are derived by averaging the data in Dhe three adjacent height channels with the best signal-to-noise ratios. The velocities t’hcn are not at constant altitude but tend to follow the F-layer peak. Figure 4 is a graph of the virtual height versus time for the special feature displayed in the ionograms of Fig. 2. These measurements were made at 3 MHz and constitute a mode of displaying ionogram data which is especially useful for

Incoherent scatter radar observations

3*otl0

1123

of spread-F

+

IO

20 TIME.

30

40

min

Fig. 4. Ionogram derived data, from the event displayed in Fig. 2. Virtual height is plotted vs. time and the solid line is a fitted curve. The error bars are the ionogram reading errors and are generally ~5 km.

time evolution studies of spread-F. Two additional events similiar to the one displayed in Figs. 2 and 4 occurred on the night in question; curiously most such events at Arecibo show only diverging traces; presumably this means that most tilts which pass by Arecibo come from the same direction and the reason they appear on the ionosonde as diverging traces is one of preferred propagation paths. This observation correlates with the one made earlier concerning the asymmetrical delay seen on the Z-MHz virtual height contour in Fig. 1. If we note from Fig. 3 that the ion velocities are southward during the appropriate period we then conclude from Given that we see only the earlier discussion that the tilts are drifting southward. diverging traces in events such as shown in Fig. 2 and that the extraordinary ray deviates southward it seems likely that the X mode is the dominant return in such cases. At this point we find any explanation of why delays differ in the Z-MHz ionogram contour in Fig. 1 impossible; except to say again that mode switching seems likely. 3. DISCUSSION To more clearly link spread-F with a particular ionospheric geometry, we use the time development of the three ionogram features discussed in the last paragraph to estimate the horizontal and vertical velocities of the ‘reflectors’. To accomplish this we assume that the ionosphere for the time interval in question is described by

J. II.

1124

MATHEWS

and

R.

M. HARPEK.

just two velocities, oh and vZ, the horizontal and vertical velocity components. Then considering time equals zero and altitude equals &, when the reflector is overhead we see from Fig. 5 that r = [(h, +- VJ)” + w,z tall

(1)

recombination effects where r is the range; t,hat is, we have ignored electron-ion and propagation effects and consider the problem exactly as one would a radar echo.

h, + v,t

HORIZONTAL Fig.

6. Schematic

DIRECTION

representation

of equ&on

(1) in thr tnxl

.

After choosing h,, equation (1) was least-squares fitted to the appropriate ionogram data (see Fig. 4; solid line is fitted); the results appear in Table 1 and in Fig. 3. The velocities obtained from the ionograms in Fig. 2 appear in Table 1 for the time 0017 hr. Perhaps the greatest significance of Table 1 is that the signs of the r~~dar-ionogram velocities are consistent; but beyond this we feel that the magnitudes agree sufficiently to say that the reflectors were moving with the whole Table 1. Comparison of radar and ionogram derived vertical and horizontal velocities. The radar velocities are for ions and the ionosonde velocities are representative of features in the ionosphere at the same time Radar

Time MT) 2252 2350 0017

Ionosondc P’requency

V, (mjsec) -.. -5 +25 122

.-

J,‘:v&n/sec) -125 -140 -145

V,(m/sec) -33 +12 -j-30

17,(m/set) .--_I____.. -190 -97 -72

(MHz) - .--_- .__

-

2 2 3

ionosphere and not with the neutral winds. Thus we again conclude that only a tilt in the layer could satisfy the necessary reflection conditions for returning rays over time periods of greater than 4 hr and still be a relatively localized region of the ionosphere. When we look at Fig. 1 for the times listed in Table L (i.e. 2252, 2350, 0017 hr) we see features which are the tilts we expected. The three geometries are probably as Fig. 7 suggests; but the radar would have to be used at greater resolution to make this determination.

Incoherent scatter radar observations of spread-F

1125

It seems most likely to us that the irregular ionosphere was a region of enhanced ionization drifting by with velocities which are shown in Fig. 3. This conclusion is arrived at by citlculating the total (line of sight) content as a function of time; this is accomplished by integrating the radar profile information of Fig. 1. This information appears in Fig. 6; the data points represent lower bounds because the electron density profiles were measured to only 524 km thus truncating the integration, The single error bar shown in Fig. 6 is a maximum error for the whole period and shows conclusively that the total content did increase by a factor of 2 I

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Pig. 6. The upper portion of the figure is the radar derived tots,1content plotted vs. time in units of loll electrons/cm2; and the lower portion is the ionogram derived critical frequency plotted vs. time.

from the relative m~imum at 2250 hr. This increase is probably due to the shift in neutral winds; during the hours after P-region sunset while the neutral wind was northerly ionization was blown down the field lines (dip angle 50’) as manifested in negative vertical ion velocities (see Fig. 3) before the wind reversal. The reversal causes ionization to be blown up the field lines as manifested in positive vertical ion velocities in Fig. 3; the result, due to the fact that the reversal occurs at lower altitudes earlier (the data marginally supports this), is that the ionosphere is compressed from both directions along the field lines and the total content increases. During the same time the critical frequency (also shown in Fig. 6) increased from 3-l MHz to between 5 and 6 MHz (probably 5.8 MHz). Additionally after the winds at all altitudes are reversed the layer height is increased as seen in Fig. I.

Another possibility is that the increase in total content was due to downward As can be calculated from diffusion of ionization from the protonospher~. :Fig. 6, the average time rate of change of total content from 2300-0140 hr was 2.8 x lo8 (cm2 . see)-l and two calculations of downward flux at 450 km yields 3.0 x lo8 (cm2 . set)-l at 2308 hr and 2.8 x 10s (cm2 . see)-l at 0051 hr. These simple calculations ignored all recombination effects but certainly leave open the possibility of protonospheric contribution to the increased total content. Most, probably both the reversal of the neutral winds and a downward AU of ionization Nevertheless, theeffect was associ-from the protonosphere contributed to theeffect. ated with a tilt in the ionosphere which drifted by. It is also of some importance tha,t the electron-ion temperatures remained about constant during the increase in total content and although this o~)servatioll is ~onlp~icated by the possibilit,~~ of ~changing composition (hydrogen comin g down) we would espect the ete&ron-ion kempemture ratio to increase if downward diffusion of a presumably hot,ter plasma were important. The slight leveling off of the total content versus time plot in E’ig. 0 centered at ~2120br appears to be associated with the tilt centered at 2120 hr in Pig. 1 and with increased spread-F activity seen at the same time. The conclusion is that a small ‘blob’ came by or was formed before the main event. 4. c0xcL.us10xs the night of 7-S January 1971 the ra,nge spreading t,ype spread-F observed found to be caused by tilts (see Pig. 7) in the ionosphere. The tilts were due

For WAS

PLASMA FREQUENCY CONTOURS

MAIN TRACE

I

w

HORIZONTAL

to the drifting

DIRECTION

southward of a region with enhanced t,otal content; this region was composed of a small, almost unresolved, blob of enhanced ionization which produced some spread-F and the nmin event during which total content increased by a factor of 2 and the spread-F was intense and prolonged. Three observational factors were used to determine that tilts were the cause of the observed spread-F: (1) the correlation between spread-F intensity and those jonospheric features visible in a plot of constant plasma frequency contours in altitude and time (see Fig. 1); (2) the comparison of radar and ionogram derived velocit,ies, which agreed reasonably well; (3) the time origin of the three event,s for

Incoherent

scatter radar observations

of spread-p

1127

which ionogram derived velocities could be obtained all corresponded to tilts as seen in Fig. 1. We also showed that the tilts were caused by the passage of a region of enhanced ionization which nearly doubled both the total content and critical frequency (see Fig. 6). Finally we suggest that, although range spreading type spread-F is the most common at Arecibo, when frequency spreading does occur it is apparently due, by association, to intense range spreading in spread-F. That is when the spread-F intensity index reaches 4 or 5, the traces are so overlapped at high frequencies (near the critical frequency) that frequency spreading seems to occur. Additionally since each trace comes from a different region of the ionosphere (or at least s, different propagation path) it seems likely that the critical frequency of each trace would be different thus causing additional frequency spreading. Acknowledgement-The authors would like to acknowledge the help of Dr. B. S. TANENBAUM at Case Western University in preparing this paper, and of the ionospheric staff at the Arecibo Observatory in obtaining and analyzing this data. The work of John Mathews was supported by National Science Foundation Grant GA-10717. The Arecibo Observatory is operated by Cornell University under contract with the National Science Foundation and with additional support from the Advanced Research Projects Agency. REFERENCES FARLEY D. T., B_GSLEY B. B., WOODMAN R. F. and MCCLURE J. P. HERMAN J. R. KING G. A. M. PITTEWAY M. L. V. and COHEN R. WOODMAN R. F. and HAGFORS T. Reference

is also made to the following

REHNKE R,. A.

J. geoph.ys.

1966 1970 1961 1969

Rev. Beophys. 4, 255 J. Atmosph. Tern. Phys. 32, 209. J. geophys. Res. 66, 3141 J. geophys. Rea. 74, 1205

unpublished 1970

Res.

75, 7199

1970

material: Ph.D. Thesis, Rice University, Texas.

Houston,