Comparison of radar backscatter and rocket probe measurements of ionospheric electron temperatur

Brace, L. H.

Carlson, H. C.

Mahajan, K. K.

English

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RADAR BACKSCATTER AND ROCKET
PROBE MEASUREMENTS OF ELECTRON
TEMPERATURE ABOVE ARECIBO
L. H. BRACE
H. C. CARLSON
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RADAR BACKSCATTER AND ROCKET PROBE MEASUREMENTS
OF ELECTRON TEMPERATURE ABOVE ARECIBO
L. H. Brace
H. C.. Carlson
K. K. Mahajan*
December 1968
Goddard Space Flight Center
Greenbelt, Maryland
NRC-NASA Post-doctoral Resident Research Associate.
`MNING PAGE BLAME: NoT
CONTENTS
Page
Introduction................................
	 1
Experimental Methods .........................
	 2
TheComparisons .............................
	 2
Acknowledgments ............................
	
5
References ................................
	 5
iii
RADAR BACKSCATTER AND ROCKET PROBE MEASUREMENTS
OF ELECTRON TEMPERATURE ABOVE ARECIBO
Tn+^^Ai i ^+; .,n
The two prime sources of information regarding ionospheric temperature
behavior are the various Hypes of probe measurements from satellites (and
rockets) and the radar backscatter measurements from ground-based installa-
tions. The satellites resolve the global structure of the ionosphere while the
radar resolves its attitudinal structure and diurnal behavior. Since these methods
are so complementary in the task of establishing a comprehensive description
of ionospheric behavior, it is clearly desirable to correlate measurements
from both methods to uncover and investigate any systematic errors which may
exist. This was the prime purpose of the launch expedition to Puerto Rico that
resulted in the electron temperature (T e ) comparisons presented here.
In this letter we compare the rocket profiles of T
.
 obtained at 0330 LT and
1530 LT on March 17, 1968 with temperatures derived nearly simultaneously by
the radar method at Arecibo Observatory. A later paper will treat these, and
other measurements made from the rocket payload, in greater detail. The other
measurements include electron concentration, ion composition, and molecular
nitrogen conc,entratior and temperature.
1
Experimental Methods
The cylindrical probe employed in these ejectable rocket payloads was
described fully by Spencer et al, (1965). The same instrument has been employed
on numerous rockets and satellites (Brace, Spencer, and Dalgarno, 1965) (Brace
and Reddy, 1965) (Brace, Reddy and Mayr, 1967) and (Brace, Mayr, and Carignan,
1969).
The Arecibo facility and its operation have been described by Gordon and
LaLonde (1961) and by Carlson (1965). The bulk of the temperature measure-
ments have been made with the autocorrelation technique developed by Perkl-is
and Wand (1965), using single pulse transmissions. The results discussed here
were obtained by the double-pulse method which has greater height resolution
(Perkins and Wand, 1966).
The Comparisons
The measurement sequence was arranged so that double-pulse radar observa-
tions were taken before and after each rocket flight. In the night flight an observa-
tion was also centered on the time of launch. During the daytime flight a 'plasma
line' radar run was made. The data from this run will be covered in the later paper.
Figures 1 and 2 show the resulting backscatter profiles and rocket profiles.
The rocket data in each case is based on the downleg data. Although there was
little difference between upleg and downleg data, the latter are less likely to be
disturbed by vehicle-borne contamination, since the instrument attains maxi-
mum separation from the vehicle during downleg.
2
The agreement is remarkably good in the daytime flight, the probe values of
T e falling approximately midway between the radar profiles taken about 25
minutes before and after the flight. Since the F 1 region temperatur^ decreased
by about 15% between the first and second radar runs, a really precise evalua-
tion of the agreement at this altitude will be feasible after the 'plasma line' data
are reduced. At altitudes below 180 kilometers and above 240 kilometers the
agreement is within 5%.
The nighttime agreement is not as satisfactory, however. The radar values
of T e are not yet available below 286 kilometers at the time of the flight, owing
to return echoes from the vehicle itself. The agreement above this altitude is
within 10%, with the radar values systematically lower. The pre-launch and
post-launch radar values were lower still.
The reason for this 10% difference is not clear. The three radar runs
show that T e was highly variable near 300 kilometers, the apogee altitude of
the flight. Thus it is possible that the difference arises from the different three
resolution of the two methods. The rocket profile is taken in only three or four
minutes while the radar employs a 20-minute integration time. However, we
should not ignore the possibility that the difference may reflect an as yet uni-
dentified systematic error in either or both methods of temperature measure-
ment. If such errors are present in the night flight, it is difficult to understand
the agreement found in the daytime flight where the temperature range encom-
passed the nighttime values.
3
It should be noted that the nighttime radar values were derived employing
preliminary measurements of mean ion mass from the ion spectrometer on the
rocket payload (Pharo, private coin munication). The daytime radar values of
T e
 were derived in the more usual way by assuming that the ion temperature
(T i ) was equal to a model gas temperature (T K) below 250 kilometers. Te
was then derived from the ratio of T, /T i obtained from the radar spectra.
In conclusion, it appears that the electron temperatures derived by the
cylindrical probes agree well generally with those derived by the backscatter
method employed at Arecibo. A small difference evident only at night may
arise from variability of the ionosphere, or this may reflect a real experimental
error in either or both methods.
These results are in conflict with a similar intercomparison of electro-
static probe data from Explorer XXXII and radar data from Jicamarca Observa-
tory in Peru. The Jicamarca comparisons revealed probe values of T  that
were approximately 70`Io greater than backscatter values both day and night
(Hanson, et al, 1969).
In statistical comparisons between the same probe experiment on Explorer
XVII and measurements at Millstone Radar, Evans (1965) found good agreement
in the daytime. At night, the probe values agreed well in about half of the cases
but were variably higher in the others. The average nighttime value of T
e 
from
the probe was about 20% higher than the Millstone radar monthly average value.
4
Unfortunately the measurements employed in the comparison were not simul-
taneous in the sense of the Arecibo comparisons presented here.
Further investigation of this question, now in progress, includes additional
comparisons of satellite probe measui-m--nts with simultaneous measurements
at all three backscatter stations.
Acknowledgments
This work was performed while one of the authors (K. K. M.) was a NRC-
NASA post-doctoral resident research associate.
References
Brace, L. H., N. W. Spencer, A. Dalgarno, "Detailed behavior of the mid-
latitude ionosphere from the Explorer XVII satellite", Planet. Space Sci., 13,
647-666, 1965.
Brace, L. H., B. M. Reddy, "Early electrostatic probe results from Explorer
=111, J. Geo;-hys. Res., 70, 5783-92, 1965.
Brace, L. H., B. M. Reddy and H. G. Mayr, "Global behavior of the ionosphere
at 1000-kilometer altitude", J. Geophys. Res., 72, 265-283, 1967.
Brace, L. H., H. G. Mayr, G. R. Carignan, "Measurements of electron cooling
rates in the midlatitude and auroral zone thermosphere ", J. Geophys. Res.,
74, Jan. 1969.
5
Carlson, H. C., "Ionospheric heating by magnetic conjugate point electrons as
observed at Arecibo", CRSR Report 212, Cornell University, Ithaca, N. Y.
(1965).
Evans, J. V., "A comparison of rocket, satellite, and radar determinations of
electron temperature at mid-latitudes ", J. Geophys. Res., 70, 4365-74,
(1965).
Gordon, W. E., P :id L. M. Lalonde, "The design and capabilities of an ionospheric
radar probe", I.R.E. Trani., Antenna Propag., 9, 17, (1961).
Hanson, W. B., L. H. Brace, P. L. Dyson and J. P. McClure, "Conflicting electron
temperature measurements in the upper F-region", (Subnutted to J. Geo-
phys. Res., 1968).
Perkins, F. W. and R. Wand, "Analysis of the ionospheric incoherent scatter
signal by digital methods ", CRSR Report 207, Cornell University, Ithaca,
N.Y., (1965).
Perkins, F. W. and R. Wand, "A double-pulse method for obtaining good height
resolution in the incoherent scatter spectra", (paper presented at the URSI-
USNC meeting, Wash., D.C., April, 1966).
Spencer, N. W., L. H. Brace, G. R. Carignan, D. R. Taeusch and H. Niemann,
"Electron and molecular nitrogen temperature and density in the thermo-
sphere", J. Geophys. Res., 70, 2665-98 (1955).
6
eARECIBO
MARCH 17, 1968
DAY FLIGHT
300
PRO 8EY	 15:? AST
w	 NO
RADAR
POST-LAUNCH
a1609 AST
RADAR
PRE-LAUNCH
1448 AST
100	 JID
0	 1000	 2000
Te (OK)
Figure 1-Comparison of daytime probe and backscatter T^ measurements. The measurements
are consistent at all altitudes. Each probe-derived point represents a mean of about 10 indi-
vidual measurements having a standard deviation of less than 100°K.
RADAR
AT LAUNCH
OA97 AST
Y
W0DH
PROBE
0330 AST
L
JQ
RADAR
POST LAUNC
0350 AST
RADAR
PRE-LAUNCH
0304 AST
ARECIBO
MARCH 17, 1968
NIGHT FLIGHT
	
loo?	 I
	
0	 500	 1000
Te (°K)
Figure 2-Comparison of nighttime probe and backscatter T. measurements. The agreement with
backscatter measurements is not as good as found in the daytime flight. The radar values taken
"at launch" (0327 AST) are b% lower than the probe values at 290 kilometers. Radar values be-
fore and after the launch are systematically (owe , by about 157o, an amount which approaches the
temperature variation between radar runs.
8


Radar backscatter and rocket probe measurements of electron temperature above Arecibo

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