Data obtained with rockets flown over New Mexico, U.S.A. and Manitoba, Canada have always shown the sporadic-E layer to be a thin layer with a large electron density gradient. The vertical electron density profiles and the horizontal uniformity of the sporadic-E layer are discussed herein. These layers have a strong tendency to form at preferential altitudes separated by approximately 6 km, and a striking correlation exists with wind- shears and magnetic field variations. In two cases where comparisons with ionograms were possible, the minimum frequency of the F-region echoes was found approximately equal to the plasma frequency of the sporadic-E layer reduced by half the gyrofrequency. On the other hand, the maximum frequency of the sporadic-E echoes as noted on ionograms was sometimes as much as 1 to 2 Mc greater than the plasma frequency

Seddon, J. Carl

English

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D-1043
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SPORADIC-E AS OBSERVED WITH ROCKETS
J. Carl Seddon
Goddard Space Flight Center
Greenbelt, Maryland
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON July 1961
https://ntrs.nasa.gov/search.jsp?R=19980228276 2020-06-15T23:24:45+00:00Z

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SPORADIC-E AS OBSERVED WITH ROCKETS
By
J. Carl Seddon
Goddard Space Flight Center
SUMMARY
Data obtained with rockets flown over New Mexico, U.S.A.
and Manitoba, Canada have always shown the sporadic-E layer to
be a thin layer with a large electron density gradient. The vertical
electron density profiles and the horizontal uniformity of the
sporadic-E layer are discussed herein. These layers have a
strong tendency to form at preferential altitudes separated by
approximately 6 km, and a striking correlation exists with wind-
shears and magnetic field variations. In two cases where com-
parisons with ionograms were possible, the minimum frequency
of the F-region echoes was found approximately equal to the
plasma frequency of the sporadic-E layer reduced by half the
gyrofrequency. On the other hand, the maximum frequency of the
sporadic-E echoes as noted on ionogramswas sometimes as much
as i to 2 Mc greater than the plasma frequency.

CONTENTS
Summary .................................. i
INTRODUCTION .............................. 1
GRADIENTS ................................. 2
UNIFORMITY OF THE E s LAYER .................. 2
HEIGHT PREFERENCE ......................... 7
GRADIENTS IN ELECTRON DENSITY ................ 10
COMPARISON WITH THE PULSE TECHNIQUE .......... 11
TIME VARIATION ............................. 11
References .................................. 12
Iii

SPORADIC-E AS OBSERVED WITH ROCKETS
By
J. Carl Seddon
Goddm'd Spacv FliA, bl C(,nlcJ-
INTRODUCTION
Although sporadic-E has been intensively studied all over the world for the past two
decades, it is one of the most difficult of ionospheric phenomena to explain on theoretical
grounds. Beginning in 1946, tile sounding rocket provided a new means of obtaining
measurements. The most aceurate measurements of sporadic-E to date have been made
with a radio propagation technique devised by Seddon (Reference 1): Two CW harmonic
frequencies (either 4.2"/ or 7.75 Mc and the 6th harmonic) were radiated from the rocket
to ground stations nearly underneath the rocket, where the lower frequency was
frequency-multiplied by a factor of 6 and mixed with the higher frequency. The result is
simply adc voltage unless the rocket traverses a medium containing free electrons,
whereupon a low frequency ae voltage is obtained: the exact frequency depends upon the
frequencies radiated, the rocket veloeity, and the electron density. Certain assumptions
are necessary, the most important of which are: (1) the angle of arrival of the signals
does not change; (2) there is no time variation of electron density along the propagation
path; and (3) the index of refraction of the medium in the vicinity of the rocket does not
change rapidly in a distance of ' 2_ where _ is the vacuum wavelength. Both magneto-
ionic components of the lower frequency are utilized to provide a check on the results
obtained.
Except during radio blackout conditions, the ground ionograms taken during all but
one of the flights in these experiments showed sporadic-E reflections. In one of the flights
under blackout conditions, sporadic-E was probably present but the ionogram could not
show it. Nine flights made measurements of sporadic-E, but in three of these the meas-
urements were not detailed enough to provide much information. In two of these three
flights latter signal loss occurred because the plasma frequency of the sporadic-E considerably
exceeded the lower frequency used. Poor rocket attitude in the other flight made the data
discontinuousandlessaccuratethanusual. Theremailing six flights, however,provided
informationnotattainablebyothermeans.Of thesesi:: flights, threeweremadeover
NewMexicoandtheother threeover Fort Churchill, Manitoba,Canada.
GRADIENTS
In all cases, over both New Mexico and Fort Churchill, it was found that the
sporadic-E reflections observed on the ionograms wer_ due to thin regions with large
electron density gradients superimposed on the normal ionosphere profile. The altitudes
of these layers were identical with the measured virtual heights within the reading accu-
racy of the ionograms. In three out of six cases, the increase in electron density in these
layers above a smooth profile was approximately 10 to 20 percent; in one case, the
increase was a factor of 2. The layers in the E-region varied in thickness from 500 to
2,000 m and exhibited extraordinarily large electron density gradients, generally about
l0 s to 106 electrons/cm 3 per kin. The ionograms sho_ed constant-height reflections at
frequencies up to 2 Mc greater than the plasma frequenzy of the thin layer. This is not
unreasonable, for if it is assumed that the boundary of :he layer is sharp with respect to
the radio wavelength, the extraordinary ray 2 Mc highe • than the plasma frequency has a
reflection coefficient of about 5 percent. The highest fl equency E s echo noted during a
rocket flight was 8 Mc.
UNIFORMITY OF THE Es LAYER
One question which has remained unanswered is w! ether the sporadic-E layer is
uniform in electron density or whether it consists of m;tny pancake-shaped high-density
clouds. The rocket data have indicated consistently tha: the whole sporadic-E region
remains uniformly enhanced in density throughout. Ex_ ruination of the ionograms taken
at the same time at a station 10 to 80 km distant horizoltally reveals that the minimum
frequency of the F-region reflections reduced by one-h:df the gyrofrequency agrees fairly
closely with the plasma frequency measured by the rocl:et in the sporadic-E region. An
excellent example of this was described by Jackson and Seddon (Reference 2) for the flight
of Aerobee-50. The penetration frequency was 5 Mc wl:ich agreed with the electron den-
sity at the rocket within 17 to 30 percent. The 17 percent value would apply if the
penetrating ray was the Z mode, and the 30 percent value if it was the ordinary ray.
A previous flight over New Mexico (Reference 3) slowed that the rocket, immersed
in an electron density whose plasma frequency exceede(I the radiation frequency (but by
an amount less than half the gyrofrequency), was able to transmit the signal to the ground
by way of the Z-mode. Additional evidence was provided through the presence of a
reflected ray from the bottom of the sporadic-E region when the rocket was below it.
These reflections occurred immediately upon penetration of the sporadic-E region on the
descent, and continued during the remainder of the descent. It was calculated from the
geometry that the reflection point must have moved 20 km horizontally along the bottom
of the layer. During this time there was a small variation in amplitude but never any
cessation of the reflected signal. Further, as a result of differential refraction, the
ordinary and extraordinary rays would have a horizontal separation at the bottom of the
E-region of as much as 1 km during the rocket descent. If, then, there were clouds 1 km
or so in vertical thickness present in this layer, with lower electron densities between
them, there would have been a substantial difference between the electron densities near
the rocket computed from the two magneto-ionic components due to the different elec-
trical path lengths. There seems to be no definite evidence that this was happening. A
possible explanation, however, is that there were numerous small clouds very close
together, which would tenci to average out this sort of effect and so maintain the agree-
ment between the results of the two magneto-ionic components.
Still further evidence of the approximate continuity of the ionization can be found in
the V2 flight of January 22, 1948 (Reference 1). The 4.274 Mc signal abruptly disap-
peared at an altitude of 100 kin. For the remainder of the flight, there were only five
short occasions when a very weak signal could be detected. These signals were received
during only a few seconds of the time the rocket was above the layer (about 4 minutes).
They could have been due to a modest decrease in tim electron density in the layer, which
apparently had a plasma frequency slightly greater than the signal frequency. Compari-
son with measurements made on the ascent could not be made because the rocket record
was very irregular at that time, apparently as a result of rocket out-gassing. (This
situation has been considerably improved in recent rockets by using propellant shut-off
valves.) However, an interesting comparison can be made with measurements made by a
probe technique over Australia, as reported by Massey (Reference 4). Figure 1 shows
the similarity between sporadic-E over New Mexico at mid-day with that over Australia
at night. The difference in altitude is only about 1/2 kilometer.
The data obtained from a rocket flight at Fort Churchill, Canada at night, along with
the corresponding ionograms, are shown in Figure 2. The grid, placed over the ionogram
at the ionosphere station, is placed incorrectly, nearly 10 km too low. This could be ver-
ified from other ionograms in which multiple echoes were present. The electron density
was quite low up to an altitude of about 170 kin, with no trace of an E-region except for
sporadic layers. The critical frequency of the lowest layer was approximately 1-1/2 Me,
but the ionogram shows sporadic echoes up to 3-1/2 Mc at this same altitude.
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JUNE 19 1958
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Figure | - Comparison of sporadic-E at noon over NewMexicc, U.S.A. with that over Woomera,
Australia at midnight
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Table 1
Altitudes of High Electron Density Gradients
Rocket
V2 No. 34
Viking 5
Viking I0
Aerobee-Hi
NRL- 50
Aerobee-Hi
IGY NN3.08F
Aerobee-Hi
IGY NN3.10F
Aerobee-Hi
IGY NN3.11F
Date
Jan. 22, '48
Nov. 21, '50
May 7, '54
June 29, '56
July 4, '57
Feb. 3, '58
Feb. 4, '58
Launch
Time
(hrs-LMT)
1314
1018
1000
1209
1216
1202
0017
Place
White 100
Sands
White
Sands
White 100
Sands
White
Sands
Fort
Churchill
Fort
Churchill
Fort
Churchill
i01
98
Altitudes (km)
I
b-a
0
(Reference 13) shows large wind-shears at altitudes of 9: l, 100, 107, 110, 115 and 130km.
Thus there are grounds for strong suspicion that the higt electron density gradients are
associated with wind-shear. This condition has been exa_nined theoretically by Whitehead
(Reference 14).
Accurate but unpublished measurements of the earth's magnetic field with a proton
precessional magnetometer over Fort Churchill (by Mer,:dith et al., Reference 15) show
a number of maxima between 70 and 140 km. The avera_ e separation between these
maxima is approximately 6.5 km. The altitudes at which the magnetic field was a mini-
mum correspond to the altitudes of sporadic-E.
Temperature measurements are difficultin the E-r_ gion, but the pitot-staticmeas-
urements of Ainsworth, Fox and Lagow (Reference 16) slow definite minima at 73 and 92
km. At lower levels,between 30 and 70 kin, there are slaallvariations in the temperature
profile with approximately 6 km separation between the Ieaks except for one case of 13
km separation. Ainsworth (Reference 17) found that the vector wind gradient Idv/dhl had
200
I
1. SEPT 9 1957 AT 19h 50 LOCAL TIME
18h 50 MOSCOW TIME
2. AUG 25 1957 AT 06h 23 LOCAL TIME
05h 23 MOSCOW TIME
3. MAY 16 1957 AT 06h 14 LOCAL TIME
(EUROPEAN PART OF THE USSR, MIDDLE L#TITUDE)
180
v 160
i---
140
<_
120
l O0
I I I
10 s 2 x10 s 2.8× 10 s
ELECTRON DENSITY (EL/CC)
Figure 4 - Russian results using a CW technique (from Doklady Akad. Nauk S.S.S.R., and Proc. IRE, February 1959)
12
below the rocket, or because of inhomogeneities in the ionosphere in a horizontal direc-
tion which result in changes in the electron content alon,; the propagation path with
horizontal motion of the rocket. Some error can indeed be introduced by these mecha-
nisms. However, two stations 10 kilometers apart give data that agree within about 5
percent. Moreover the ordinary and extraordinary com:_onents at each station are in
similar agreement with each other. Still further, the cc,mputation of the virtual height at
various frequencies shows good agreement with the ionograms taken at the same time.
In order to account for the high electron densities in the sporadic-E layer measured
during the flight of Aerobee-50 where large electron densities were observed, we might
consider the error introduced by having all the ionizatic,n between 75 and 100 km double
in value in a period of 1-1/2 seconds, the transit time of the rocket through the layer.
This extremely improbable situation is still a factor of about 5 below that needed to
account for the observed results. In addition, it can be pointed out that even when these
thin, high gradient layers do not increase greatly in electron density, sporadic echoes are
invariably found on the ionograms at these exact height_,_. The rocket results obtained
thus far on sporadic-E by the CW propagation technique simply cannot be explained either
by time variation of electron density or by interception of irregularities through the hori-
zontal motion of the rocket.
.
.
.
.
.
,
,
REFERENCES
Seddon, J. C., "Propagation Measurements in the Ionosphere with the Aid of Rockets,"
J. Geophys. Res. 58(3):323-335, September 1953
Jackson, J. E., and Seddon, J. C., "Ionosphere Electron-Density Measurements with
the Navy Aerobee-Hi Rocket," J. Geophys. Res. 63(1): 197-208, March 1958
Seddon, J. C., "Electron Densities in the Ionospher _," J. Geophys. Res. 59(4):463-
466, December 1954
Massey, H. S. W., 1958; to be published in Annals cf the IGY, Pergamon Press,
London
Prenatt, R. E., "Ionospheric Structure Above Fort _hurchill, Canada, from Faraday
Rotation Measurements," J. Am. Rocket Soc. 30(8):763-765, August 1960
Pfister, W. H. and Ulwick, J. C., "The Analysis of Rocket Experiments in Terms of
Electron-Density Distributions," J. Geophys. Res. _3(2):315-333, June 1958
Gringauz, K. L., "Raketnye Izmereniia Elektronnoi Kontsentratsii v Ionosfere pri
Pomoshchi Ultrakorotkovolnovogo Dispersionnogo ]nterferometra," Doklady Akad.
Nauk S.S.S.R. 120(6):1234-1237, 1958
2M
0
13
8. Seddon, J. C., Pickar, A. D., and Jackson, J. E., "Continuous Electron Density
Measurements up to 200 Kin," J. Geophys. Res. 59(4):513-524, December 1954
9. Milhnan, P. M., "Visual and Photographic Observations of Meteors and Noctilueent
Clouds," J. Geophys. Res. 64(12):2122-2128, December 1959
10. Greenhow, J. S., and Neufeld, E. L., "Measurements of Turbulence m the 80- to 100-
Kin Region from the Radio Echo Observations of Meteors," J. Geophys. Res. 64(12):
2129-2133, December 1959
11. Blamont, J. E., "Nuages Artificiels de Sodium: Vitesse du Vent, Turbulence et
Densite de la Haute Atmosphere," Compt. Rend. 249(14):1248-1250, October 5, 1959
12. Manring, E., Bedinger, J. F., Pettil, H. B. and Moore, C. B., "Some Wind Determina-
tions in the Upper Atmosphere Using Artificially Generated Sodium Clouds," J.
Geophys. Res. 64(6):587-591, June 1959
13. Manring, E., Bedenger, J. F., Knaflich, H., and Lynch, R., Geophys. Corp. of
America Tech. Report 61-l-N, Bedford, Mass., 1961
14. Whitehead, J. D., "The Formation of the Sporadic-E Layer m the Temperate Zones,"
J. Atmosph. Terr. Phys. 20(1):49-58, February 1961
15. Meredith, L. H., Davis, L. R., Heppner, J. P., mid Berg, O. E., "Rocket Auroral
Investigations," IGY World Data Center A, Rockets and Satellites, IGY Rocket Report
Series No. 1:169-178, July 30, 1958
16. Ainsworth, J. A., Fox, W. F., and Laaow, H. E., "Measurement of Upl)er-Atmosphere
Structure by Means of the Pilot-Static Tube," NASA Technical Note D-670, February
1961
17.
18.
19.
Ainsworth, J. A., Private Communication 1961
Stroud, W. G., Nordberg, W., Bandeen, W. R., Bartman, F. L., and Titus, P..
"Rocket-Grenade Measurements of Temperatures and Winds in the Mesosphere over
Churchill, Canada," J. Geophys. Res. 65(8):2307-2323, August 1960
Gallet, R. M., "Aerodynamical Mechanisms Producing Electronic Density Fluctua-
tions in Turbulent Ionized Layers," Proc. I.R.E. 43(10): 1240-1254, October 1955
NASA - Langley Field, Va.



Sporadic-E As Observed with Rockets

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