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The fading of radio waves reflected vertically from the ionosphere during magnetic storms
The fading of radio waves reflected vertically from the ionosphere during magnetic storms G. F. Cavendish
Fooss”
Laboratory,
Cambridge
&str&--Some features of the reffection of radio -awes from the ionosphere at vertica1 incidence during magnet,ic storms are described, and it is shou;n how correlation analysis can be applied to she fading a~ such times. The analysis shows that the irregularities in the diffraction pabtem on the groltnd it~e smsfleer and more nearly aligned with the earth ‘3 mngnetio field than they are for quiet. conditions. Drift. is a much more important source of fading than random chsnges in the pattern. The drift velocity is Irtrge, and is usually dkected ton;ard the West.
KHES a radio wave is reflected from the ionosphere, the irregularities in electron density that are usually present cause an irregular diffraction pattern to be formed on the ground. The echo from a pulse transmitter, in magnetically quiet conditions, usually takes the form of a single pulse that fades irregularly with a quasi-period of the order of a few seconds. If the amplitude is recorded at a small number of closely-spaced points, it is possible to find the velocity of drift of the diffraction pattern using the time-delay method of MITRA (IE149j and KT?AXYIW&IER {19~0). Further, the correlation analysis of Barons et nl. jt%X!j and PHILLIPS and SPESCEE ( 1955) can’ be applied to the records; this gives a value for the velocity of drift that is generally more accurate than that given b? the time-delay method. Also, correlation analysis gives the size, axial ratio and orientation of a “characteristic ellipse” that represents an average irregularity in the diffraction pattern; and finally it gives the value of a quant,ity P, which has the dimensions of a velocity and which is related to the rate at which the pattern undergoes random changes ss it moves. FOOKS and JOXES (1961) have given results obtained by these methods at Cambridge for magnetically quiet, conditions. In the observations described in this paper, the same methods of analysis wre applied to the fading during magnetic storms. The reflections are then often unusual in several respects, and corresponding changes in the methods of observation are of reflections from the ionosphere during magnetic necessary . The characteristics storms are discussed in Section 2. A brief description of the experimental apparatus and method is given in Section 3, and the results are presented and discussed in Section 4.
2. CHARACTERISTICSOF R~EFLECTKW~
DURIW
NAGSETTC
STORJB
The fading during magnetic storms can be very rapid; fading periods as short as occasionally. Also, previous observat,ions (reviewed by BRIGGS and SPESCEB, 1954) have shown that there is a tendency for the velocity of drift to be larger than for quiet conditions. There is evidence that these large velocities occur more frequentlp at night than during the day. BRIGG~ and SPESCER r/IO set are observed
* Xcnv
ac D.S.I.R.
Radio Research
Station,
Slough, Bucks. 43
G.
F. Fooss
give details of measurements of drift made during the magnetic storm of 26-27 Max 19.52: on that occasion the storm began at 0600 C.T. on 26 Ma?. but the drift velocities showed no unusual behaviour until about lSO0 hours on that day. There is a tendency in middle latitudes for spread-F echoes to occur during times of magnetic disturbance (K-RIGHT et aE.? 19.76). The different parts of a spread echo appear to fade independently of each other, so that in order to make useful records of the amplitude it is necessary to isolate a part of the spread echo by means of a narrotv strobing pulse, and measure the peak amplit~lde occurring within the strobe. Measurements of drift and results obtained by correlation analysis using such records IT-ill then refer only to those parts of the reflecting layer that contribute to the observed part of the spread echo. The validity of results obtained with this limitation is considered in Section 4. There is a tendency, again in the middle latitudes, for the critical frequency of the F2-layer to decrease at times of magnetic dist~~rbance (APPLETOS et a2.: 1937). In severe storms the critical frequency becomes very low, and observations are often impossible with the transmitters normally used for observations of fading and drifts at Cambridge. Also, there is frequently more absorption during magnetic storms than at other times: and observations are difficult with the available power from the transmitters.
Pulses from a local transmitter, in the frequency range _“+O-4.0 MC/S were reflected from t,he ionosphere at vertical incidence, and were received on three crossed-loop aerials, arranged to accept only the “ordinary” circularly polarized component of the downcoming wat-e (PHILLIPS, 1951). The aerials were placed at the corners of a right-angled triangle; the sides containing the right angle lay Sorth-South and East-West, and were 130 m in length. Three similar receivers were used: together with a strobing arrangement to select the required echo or the required part of a spread echo. The strobed outputs of the receivers were displayed as horizontal displacements on three 1 in. cathode-ray tubes mounted side-by-side, and the variations in amplitude were recorded on a continuousI>- moving 33 mm film. To record the fastest fading TRI-S film was used, and was passed through t’he camera at a rate of about 40 cm/min. After processing, the parts of the film required for further analysis were enlarged opbically and traced onto paper. For correlation analysis, portions of record showing twenty or more cycles of fading were selected, and 240 to 360 equally spaced ordinates were measured on each of the three fading curves. The auto- and cross-correlation functions of the curves, for time displacements up to & 10 times the spacing of the ordinates, were calculated on an automatic digital computer. The graphical method of PHILLIPS and SPEXXR (1955) was used in the subsequent calculations to determine the parameters of the diffraction pattern on the ground. 4. RESULTS Table 1 gives the periods of magnetic disturbance during which observations were made, and the range of values of the magnetic K-index for the occasions when correlation analysis was applied to the records. 44
The fading of radio waves reflected vertically from the ionosphere during magnetic stoa
The records are far from complete for any of these periods. It was often impossible to obtain records with the available equipment. either because the crit.ical frequency was too low or the absorption too great. In addition, there were many occasions when the correlation between the fading at the three receivers was very low; at such times it was not possible to apply either correlation or time-delay analysis to the records. Useful records are available for rather less than half the total time during the disturbed periods mentioned above. Table 1. Correlation analysis during magnet,ic storms K
Date 2.5-26 September 17-18 December 13 July 19.59 17-18 July 31 March-1
19.55 19.58
1959 -April 1960
4-6 4-6 7 ~
6-7 f5--9
I 200
240
B-
L x E
2-
;
I 0
40
80
I20
I60
1
I 280
r-l 320
m
Fig. 1. Histogram for the semi-minor axis of the characteristic ellipse. For much of the time the correlation between the receivers was too small to measure, ancl so the maximum between 80 and 100 m is only apparent.
Correlation analysis was carried out on twenty-five records. Of these, five were of reflect.ions from t,he E-region and twenty from the F-region; three were taken during the day, and twenty-two at night. There were no significant differences between these various groups, and in this Section all the results are presented together. The results for quiet conditions quoted for comparison are those given by FOOKS and JOSES (1961). The lengths of the semi-minor axis of the characteristic ellipse are shown in histogram form in Fig. 1. Most of the values are considerably smaller than the median values obtained for quiet conditions; these were 235 m, 296 m and 149 m for the E-region, and for the F-region by day and night respectively. It should be 4.5
G. F. Fooss
noted that the storm results are for occasions n-hen measurements were possible with the aerial spacing of 130 m. The scale of the pattern was certainly smaller than 50 m for much of the t,ime, but such occasions were unsuitable for analysis because of the low correlations between the fading at the three receivers. If the ionosphere behaves as a diffracting screen that imposes phase variations much greater than 1 rad on an incident wave-front. the irregularities in the pattern on the ground will be smaller than the irregularities in t,he ionosphere (HEWISH,
Fig. 2. Histogram
for the axial ratio of the characteristic
ellipse.
Median = 2.0.
1951; RATCLIFFE, 1956). This is almost certainly the case during storms, and at present it is not possible to say by how much the irregularities in the pattern on the ground are smaller than the structure in the ionosphere. Fig. _” gives a histogram for the values of the axial ratio of the characteristic ellipse. The median value (2.0) is not greatly different from the median values found for quiet conditions (l-5 for E-region, l-6 for F-region by day, 1.S for F-region by night), but there is a larger range of values, and on eight occasions the asial ratio was greater than 3.0. The directions of the major axis of the characteristic ellipse are shown as a polar histogram in Fig. 3. There is a clear tendency for the major asis to lie close to the magnetic meridian. Any elongation of the irregularities at the level of reflection in the ionosphere will be reproduced in the axial ratio and orientation of the characteristic ellipse of We conclude that during magnetic storms the diffraction pattern on the ground. the irregularities in t,he ionosphere tend to be aligned with the earth’s magnetic field. For quiet conditions this tendency is present only for waves reflected from the F-region b>- night, and then to a less marked extent. In all but six cases, the value of (V,),/V (as defined by FOOKS and JOSES, 1961) say that drift is much was not significantly different from zero. We may therefore more important in producing the fading during storms than are random changes in the diffraction pattern. This does not necessarily mean that random changes are completely absent; the accuracy in the measurement of ( v’e)r/T’ is poor, and when 46
N (true) N (magnetic)
\A
a-
\, \I ‘1
6-
6-
6-
IO-
12
Fig.
3. Polar
histogram
for the clirection
1 of the major
axis of the characteristic
ellipse.
this quantity has a value much less than 1-O the errors lie mainly in the determination of (V&-, i.e. in the estimate of the contribution of random changes to the fading (FOOKS and JOSES, 1961). In a spread echo it may well be that the parts of the layer contributing to the selected part of the echo are not all at the same height. If there were large differences in velocity between these parts of the layer, correlation analysis Tvould be expected to give an appreciable value of ( I’,),r/ V. On one occasion records were taken of the fading within a spread echo at three different virtual heights, all within a period of 5 min. The results of correlation analysis of the three records are given in Table 2. Measurements of the velocity and direction of drift are considered further below; in fact the differences between these quantities for the three records in Table 2 are probably not significant, and also the values of (V,),./V are not significantly different from zero. These results are consistent with the conclusion that there are 47
G. F. FOOKS Table
1. Correlation
Virtual lleiqht
analysis of three records taken 18 December 19%. Frequency
I-elocity of drift (m/set)
(km)
Direction of drift (‘E of S)
(I- ) -JG I*
between 00X 2.0 Jlc/s
Length of minor axis
*
and 0043 C.T..
-Axial ratio
(m)
Direction of major ask (‘E of S)
__400 5.50 700
I
277 224 "50
~
-730 '70 '70
130
0.3 0.2 0
90 10.5
~
--E 34 I..j
Iii 170 164
8
6
4
2
2
4
_
6
Fig. 4. Polar histogram for the directions of drift found by correlation analysis.
not large differences between the velocities of drift of the parts of the ionosphere at different heights contributing to a spread echo. Because of group retardation, the range of heights concerned is of course much less than the range of virtual heights from which the waves are apparently reflected. It appears, therefore, that a small part of a spread echo may be selected and used by itself for measurements of drift at times of magnetic disturbance. Fig. 4 gives a polar histogram for the directions of drift found by correlation analysis, On most occasions there was a large component of the velocity of drift toward the West. The accuracy of measurements of the velocity of drift is affected by the tendency for the direction of elongation of the pattern to lie close to the SS direction. The EW component of the velocity is more accurately determined than the SS component, whether the time-delay method or correlation analysis is used for measurement. Also, the directions found by the time-delay method are biased towards the 4s
The fading of radio W&VBJreflectecl vertically from the ionosphere during maeletic Storrm
direction perpendicular to the direction of elongation. i.e. roughly EIJY in this case. However, in view of the low value of ( Ve)r-/F7z the values of the E1V component found by the time-delay method should be reasonably accurate. The EW component of the velocity of drift was found by the time-delay method on twenty-nine occasions in addition to those for which correlation analysis was used; there was general agreement in the results obtained by the two methods. The values obtained showed great variability; the velocity in the ionosphere ranged from 100 to lSO0 m/set and TX-asusually directed toward the West,, though on two occasions large velocities toward the East were observed. There was corresponding variability in the X3 component., but the mean SS component xas small. These results for the velocity of drift are similar to those reported by BRIGGS and SPESCER (1954). Acknowledgements-The work described in this paper forms part of a programme of research on ionospheric irregularities, supported by a grant from the Department of Scientific and Industrial Research, and the author was in receipt of a maintenance grant from the same Department. Thanks are due to the Director of the Cambridge University Mathematical Laboratory for permission to use the digital computer EDSAC 2. The author also wishes to espress his thanks to the Computing staff of the Radio Section of the Cavendish Laboratory for their assistance with the analysis of the records, and to Mr. J. A. R;ITCLIFFE and Dr. B. H. BRIGGS for much helpful advice and discussion. REFERESCES
APPLETON E. I-., TAISMITH R. and Isc~~a~r L. .J. BRIGGS B. H., PHILLIPS C..J.and SH1SSD.H. BRIGG~ B. H. ~~~SPE~CER ;\I. Foam Cr.F. ancl.Jos~sI.L. HEWISH A. KRACTICRXMER J. MITRA S. S. PHILLIPS G.J. PHILLIPS G..J.and SPENCER >\I. RATCLIFFE .J.A. WRIGHT R. W., KOSTER J. R. and S~IXSER S. J.
1937
Phil.
Tmns.
1950
Proc.
Phy... Sot.
1954 1961 19.51 1950 1949 1951 19.55
Rep. Progr. J. Atmosph.
1956 1956
A 236, 191.
Phys. Terr.
Lond.
B 63, 106.
17, 245. Phys. 20, 229.
Proc. Roy. Sot. d 209, 81. Arch. Elektr. cbertr. 4, 133. Proc. Instn Elect. Engrs III 96, 441. Proc. Instn Elect. Engrs III 98, 237. Proc. Phys. Sot. Lond. B 68, 481. Rep. Progr. Phys. 19,188. J. Atmosph. Terr. Phys. 8, 240.
Fooss”
Laboratory,
Cambridge
&str&--Some features of the reffection of radio -awes from the ionosphere at vertica1 incidence during magnet,ic storms are described, and it is shou;n how correlation analysis can be applied to she fading a~ such times. The analysis shows that the irregularities in the diffraction pabtem on the groltnd it~e smsfleer and more nearly aligned with the earth ‘3 mngnetio field than they are for quiet. conditions. Drift. is a much more important source of fading than random chsnges in the pattern. The drift velocity is Irtrge, and is usually dkected ton;ard the West.
KHES a radio wave is reflected from the ionosphere, the irregularities in electron density that are usually present cause an irregular diffraction pattern to be formed on the ground. The echo from a pulse transmitter, in magnetically quiet conditions, usually takes the form of a single pulse that fades irregularly with a quasi-period of the order of a few seconds. If the amplitude is recorded at a small number of closely-spaced points, it is possible to find the velocity of drift of the diffraction pattern using the time-delay method of MITRA (IE149j and KT?AXYIW&IER {19~0). Further, the correlation analysis of Barons et nl. jt%X!j and PHILLIPS and SPESCEE ( 1955) can’ be applied to the records; this gives a value for the velocity of drift that is generally more accurate than that given b? the time-delay method. Also, correlation analysis gives the size, axial ratio and orientation of a “characteristic ellipse” that represents an average irregularity in the diffraction pattern; and finally it gives the value of a quant,ity P, which has the dimensions of a velocity and which is related to the rate at which the pattern undergoes random changes ss it moves. FOOKS and JOXES (1961) have given results obtained by these methods at Cambridge for magnetically quiet, conditions. In the observations described in this paper, the same methods of analysis wre applied to the fading during magnetic storms. The reflections are then often unusual in several respects, and corresponding changes in the methods of observation are of reflections from the ionosphere during magnetic necessary . The characteristics storms are discussed in Section 2. A brief description of the experimental apparatus and method is given in Section 3, and the results are presented and discussed in Section 4.
2. CHARACTERISTICSOF R~EFLECTKW~
DURIW
NAGSETTC
STORJB
The fading during magnetic storms can be very rapid; fading periods as short as occasionally. Also, previous observat,ions (reviewed by BRIGGS and SPESCEB, 1954) have shown that there is a tendency for the velocity of drift to be larger than for quiet conditions. There is evidence that these large velocities occur more frequentlp at night than during the day. BRIGG~ and SPESCER r/IO set are observed
* Xcnv
ac D.S.I.R.
Radio Research
Station,
Slough, Bucks. 43
G.
F. Fooss
give details of measurements of drift made during the magnetic storm of 26-27 Max 19.52: on that occasion the storm began at 0600 C.T. on 26 Ma?. but the drift velocities showed no unusual behaviour until about lSO0 hours on that day. There is a tendency in middle latitudes for spread-F echoes to occur during times of magnetic disturbance (K-RIGHT et aE.? 19.76). The different parts of a spread echo appear to fade independently of each other, so that in order to make useful records of the amplitude it is necessary to isolate a part of the spread echo by means of a narrotv strobing pulse, and measure the peak amplit~lde occurring within the strobe. Measurements of drift and results obtained by correlation analysis using such records IT-ill then refer only to those parts of the reflecting layer that contribute to the observed part of the spread echo. The validity of results obtained with this limitation is considered in Section 4. There is a tendency, again in the middle latitudes, for the critical frequency of the F2-layer to decrease at times of magnetic dist~~rbance (APPLETOS et a2.: 1937). In severe storms the critical frequency becomes very low, and observations are often impossible with the transmitters normally used for observations of fading and drifts at Cambridge. Also, there is frequently more absorption during magnetic storms than at other times: and observations are difficult with the available power from the transmitters.
Pulses from a local transmitter, in the frequency range _“+O-4.0 MC/S were reflected from t,he ionosphere at vertical incidence, and were received on three crossed-loop aerials, arranged to accept only the “ordinary” circularly polarized component of the downcoming wat-e (PHILLIPS, 1951). The aerials were placed at the corners of a right-angled triangle; the sides containing the right angle lay Sorth-South and East-West, and were 130 m in length. Three similar receivers were used: together with a strobing arrangement to select the required echo or the required part of a spread echo. The strobed outputs of the receivers were displayed as horizontal displacements on three 1 in. cathode-ray tubes mounted side-by-side, and the variations in amplitude were recorded on a continuousI>- moving 33 mm film. To record the fastest fading TRI-S film was used, and was passed through t’he camera at a rate of about 40 cm/min. After processing, the parts of the film required for further analysis were enlarged opbically and traced onto paper. For correlation analysis, portions of record showing twenty or more cycles of fading were selected, and 240 to 360 equally spaced ordinates were measured on each of the three fading curves. The auto- and cross-correlation functions of the curves, for time displacements up to & 10 times the spacing of the ordinates, were calculated on an automatic digital computer. The graphical method of PHILLIPS and SPEXXR (1955) was used in the subsequent calculations to determine the parameters of the diffraction pattern on the ground. 4. RESULTS Table 1 gives the periods of magnetic disturbance during which observations were made, and the range of values of the magnetic K-index for the occasions when correlation analysis was applied to the records. 44
The fading of radio waves reflected vertically from the ionosphere during magnetic stoa
The records are far from complete for any of these periods. It was often impossible to obtain records with the available equipment. either because the crit.ical frequency was too low or the absorption too great. In addition, there were many occasions when the correlation between the fading at the three receivers was very low; at such times it was not possible to apply either correlation or time-delay analysis to the records. Useful records are available for rather less than half the total time during the disturbed periods mentioned above. Table 1. Correlation analysis during magnet,ic storms K
Date 2.5-26 September 17-18 December 13 July 19.59 17-18 July 31 March-1
19.55 19.58
1959 -April 1960
4-6 4-6 7 ~
6-7 f5--9
I 200
240
B-
L x E
2-
;
I 0
40
80
I20
I60
1
I 280
r-l 320
m
Fig. 1. Histogram for the semi-minor axis of the characteristic ellipse. For much of the time the correlation between the receivers was too small to measure, ancl so the maximum between 80 and 100 m is only apparent.
Correlation analysis was carried out on twenty-five records. Of these, five were of reflect.ions from t,he E-region and twenty from the F-region; three were taken during the day, and twenty-two at night. There were no significant differences between these various groups, and in this Section all the results are presented together. The results for quiet conditions quoted for comparison are those given by FOOKS and JOSES (1961). The lengths of the semi-minor axis of the characteristic ellipse are shown in histogram form in Fig. 1. Most of the values are considerably smaller than the median values obtained for quiet conditions; these were 235 m, 296 m and 149 m for the E-region, and for the F-region by day and night respectively. It should be 4.5
G. F. Fooss
noted that the storm results are for occasions n-hen measurements were possible with the aerial spacing of 130 m. The scale of the pattern was certainly smaller than 50 m for much of the t,ime, but such occasions were unsuitable for analysis because of the low correlations between the fading at the three receivers. If the ionosphere behaves as a diffracting screen that imposes phase variations much greater than 1 rad on an incident wave-front. the irregularities in the pattern on the ground will be smaller than the irregularities in t,he ionosphere (HEWISH,
Fig. 2. Histogram
for the axial ratio of the characteristic
ellipse.
Median = 2.0.
1951; RATCLIFFE, 1956). This is almost certainly the case during storms, and at present it is not possible to say by how much the irregularities in the pattern on the ground are smaller than the structure in the ionosphere. Fig. _” gives a histogram for the values of the axial ratio of the characteristic ellipse. The median value (2.0) is not greatly different from the median values found for quiet conditions (l-5 for E-region, l-6 for F-region by day, 1.S for F-region by night), but there is a larger range of values, and on eight occasions the asial ratio was greater than 3.0. The directions of the major axis of the characteristic ellipse are shown as a polar histogram in Fig. 3. There is a clear tendency for the major asis to lie close to the magnetic meridian. Any elongation of the irregularities at the level of reflection in the ionosphere will be reproduced in the axial ratio and orientation of the characteristic ellipse of We conclude that during magnetic storms the diffraction pattern on the ground. the irregularities in t,he ionosphere tend to be aligned with the earth’s magnetic field. For quiet conditions this tendency is present only for waves reflected from the F-region b>- night, and then to a less marked extent. In all but six cases, the value of (V,),/V (as defined by FOOKS and JOSES, 1961) say that drift is much was not significantly different from zero. We may therefore more important in producing the fading during storms than are random changes in the diffraction pattern. This does not necessarily mean that random changes are completely absent; the accuracy in the measurement of ( v’e)r/T’ is poor, and when 46
N (true) N (magnetic)
\A
a-
\, \I ‘1
6-
6-
6-
IO-
12
Fig.
3. Polar
histogram
for the clirection
1 of the major
axis of the characteristic
ellipse.
this quantity has a value much less than 1-O the errors lie mainly in the determination of (V&-, i.e. in the estimate of the contribution of random changes to the fading (FOOKS and JOSES, 1961). In a spread echo it may well be that the parts of the layer contributing to the selected part of the echo are not all at the same height. If there were large differences in velocity between these parts of the layer, correlation analysis Tvould be expected to give an appreciable value of ( I’,),r/ V. On one occasion records were taken of the fading within a spread echo at three different virtual heights, all within a period of 5 min. The results of correlation analysis of the three records are given in Table 2. Measurements of the velocity and direction of drift are considered further below; in fact the differences between these quantities for the three records in Table 2 are probably not significant, and also the values of (V,),./V are not significantly different from zero. These results are consistent with the conclusion that there are 47
G. F. FOOKS Table
1. Correlation
Virtual lleiqht
analysis of three records taken 18 December 19%. Frequency
I-elocity of drift (m/set)
(km)
Direction of drift (‘E of S)
(I- ) -JG I*
between 00X 2.0 Jlc/s
Length of minor axis
*
and 0043 C.T..
-Axial ratio
(m)
Direction of major ask (‘E of S)
__400 5.50 700
I
277 224 "50
~
-730 '70 '70
130
0.3 0.2 0
90 10.5
~
--E 34 I..j
Iii 170 164
8
6
4
2
2
4
_
6
Fig. 4. Polar histogram for the directions of drift found by correlation analysis.
not large differences between the velocities of drift of the parts of the ionosphere at different heights contributing to a spread echo. Because of group retardation, the range of heights concerned is of course much less than the range of virtual heights from which the waves are apparently reflected. It appears, therefore, that a small part of a spread echo may be selected and used by itself for measurements of drift at times of magnetic disturbance. Fig. 4 gives a polar histogram for the directions of drift found by correlation analysis, On most occasions there was a large component of the velocity of drift toward the West. The accuracy of measurements of the velocity of drift is affected by the tendency for the direction of elongation of the pattern to lie close to the SS direction. The EW component of the velocity is more accurately determined than the SS component, whether the time-delay method or correlation analysis is used for measurement. Also, the directions found by the time-delay method are biased towards the 4s
The fading of radio W&VBJreflectecl vertically from the ionosphere during maeletic Storrm
direction perpendicular to the direction of elongation. i.e. roughly EIJY in this case. However, in view of the low value of ( Ve)r-/F7z the values of the E1V component found by the time-delay method should be reasonably accurate. The EW component of the velocity of drift was found by the time-delay method on twenty-nine occasions in addition to those for which correlation analysis was used; there was general agreement in the results obtained by the two methods. The values obtained showed great variability; the velocity in the ionosphere ranged from 100 to lSO0 m/set and TX-asusually directed toward the West,, though on two occasions large velocities toward the East were observed. There was corresponding variability in the X3 component., but the mean SS component xas small. These results for the velocity of drift are similar to those reported by BRIGGS and SPESCER (1954). Acknowledgements-The work described in this paper forms part of a programme of research on ionospheric irregularities, supported by a grant from the Department of Scientific and Industrial Research, and the author was in receipt of a maintenance grant from the same Department. Thanks are due to the Director of the Cambridge University Mathematical Laboratory for permission to use the digital computer EDSAC 2. The author also wishes to espress his thanks to the Computing staff of the Radio Section of the Cavendish Laboratory for their assistance with the analysis of the records, and to Mr. J. A. R;ITCLIFFE and Dr. B. H. BRIGGS for much helpful advice and discussion. REFERESCES
APPLETON E. I-., TAISMITH R. and Isc~~a~r L. .J. BRIGGS B. H., PHILLIPS C..J.and SH1SSD.H. BRIGG~ B. H. ~~~SPE~CER ;\I. Foam Cr.F. ancl.Jos~sI.L. HEWISH A. KRACTICRXMER J. MITRA S. S. PHILLIPS G.J. PHILLIPS G..J.and SPENCER >\I. RATCLIFFE .J.A. WRIGHT R. W., KOSTER J. R. and S~IXSER S. J.
1937
Phil.
Tmns.
1950
Proc.
Phy... Sot.
1954 1961 19.51 1950 1949 1951 19.55
Rep. Progr. J. Atmosph.
1956 1956
A 236, 191.
Phys. Terr.
Lond.
B 63, 106.
17, 245. Phys. 20, 229.
Proc. Roy. Sot. d 209, 81. Arch. Elektr. cbertr. 4, 133. Proc. Instn Elect. Engrs III 96, 441. Proc. Instn Elect. Engrs III 98, 237. Proc. Phys. Sot. Lond. B 68, 481. Rep. Progr. Phys. 19,188. J. Atmosph. Terr. Phys. 8, 240.