In addition to spread F, evidence for field-aligned electron density irregularities is commonly observed on Alouette 2 topside sounder ionograms recorded near perigee (500 km). This evidence is provided by distinctive signal returns from sounder-generated Z mode waves. At low latitudes these waves become guided in wave ducts caused by field-aligned electron density irregularities and give rise to strong long-duration echoes. At high latitudes, extending well into the polar cap, these Z mode waves (and stimulated electrostatic waves at the plasma frequency) produce a series of vertical bars on the ionogram display as the satellite traverses discrete field-aligned density structures. The radio frequency (RF) noise environment to be expected in the 400 to 500 km altitude region from low to high latitudes was examined by analyzing perigee Alouette 2 topside sounder data. All observed noise bands were scaled on nearly 200 topside sounder ionograms recorded near perigee at low, mid, and high latitude telemetry stations. The minimum and maximum frequencies of each noise band were entered into a data base or compuer analysis. The signals of primary interest in the perigee study were found to be sounder-generated

Benson, R. F.

English

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A I
41(NASA-T11-86091) YIELD-ALIGNED ELECTRON
uEYSITY 1RhEGULA&ITIES NEAR 500 ka. EQUASOR
TO PULAR CAP TOPSIDE SOUNDER Z aODE
USSEEOATICNS (NASA) 7 p HC A02/11F COI
CSCL 04A G3/46
984-20277
Unclas
13954
NAM
Technical Memorandum 86091
FIELD-ALIGN6D ELECTRON
DENSITY IRREGULARITIES NEAR
500 KM — EQUATOR TO POLAR
CAP TOPSIDE SOUNDER Z MODE
OBSERVATIONS
Robert F. Benson
OF rM
r
m
o
APRIL 1984
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771
woo
ORIGINAL PAGE
OF POOR Q^UPJUTY
FIELD-ALIGNED ELECTRON DENSITY IRREGULARITIES HEAR 500 Kti - EQUATOR
TO POLAR CAP TOPSIDE SOUNDER Z MODE OBSERVATIONS
Robert F. Benson
Laboratory for Planetary Atmospheres
Goddard Space Flight Center
Greenbelt, MD 20771
ABSTRACT
Striking evidence, in addition to spread F, for field-aligned electron density irregularities
is commonly observed on Alouette 2 topside sounder ionograms recorded near perigee (500 ke). This
evidence is provided by distinctive signal returns from sounder-generated Z mode waves. At low
latitudes these waves become guided in wave ducts caused by field-aligned electron density
irregularities and give rise to strong long-duration echoes. At high latitudes, extending well into
the polar cap, these Z mode waves (and stimulated electrostatic waves at the plasma frequency)
produce a series of vertical bars on the ionogram display as the satellite traverses discrete
field-aligned density structures.
INTRODUCTION
This work is the outgrowth of an investigation to predict noise levels to be expected at
orbital altitudes covered during Space Shuttle missions. In particular, a major question in the
Waves in Space Plasmas (WISP) program concerns the RF noise environment to be expected in the 400-
500 km altitude region from low to high latitudes. WISP [Fredericks and Taylor, 19811 is one of
the prime experiments to be represented on the Space ShuttlejSpacelab flight designated as Space
Plasma Lab which is planned for a 1988 launch. WISP will include a highly flexible ionospheric
sounder with doppler and bistatic capability (the bistatic operation involves the use of a
subsateliite). In order to address the noise environment question, an investigation of perigee
(-500 km) Alouette 2 topside sounder data was initiated. All observed noise bands were scaled on
nearly 200 topside sounder ionograms recorded near perigee at low (Quito), mid (Ottawa) and high
(Resolute Bay) latitude telemetry stations. The minimum and maximum frequencies of each noise band
were entered into a data base for computer analysis in a manner similar to the recent investigation
of auroral kilometric radiation (AKR) using nighttime auroral zone ISIS I ionograms recorded near
apogee (3500 ka) [Benson, 19831. In contrast to the results of this AKR investigation, where the
signals of primary interest are of natural origin and are very common (see Figures 1 and 2), the
signals of primary interest in the perigee study are found to be sounder generated.
OBSERVATIONS
Ionogram examples revealing the presence of field-aligned electron density N e irregularities
in low, mid and high latitudes are presented in Figures 3a, 3b and 3c, respectively.
The series of sounder-stimulated electrostatic electron cyclotron harmonic wave resonances
extending up to WHO where fH is the electron cyclotron frequency, together with the electrostatic
plasma frequency resonance at fy near 14f H
 are very prominent in the low latitude example of Figure
3a. The large number of harmonic resonances following a weak fundamental in this ionogram is very
typical of low latitude Alouette 2 ionograms [Benson, 19721• These electrostatic features are more
dominant than the electromagnetic ionospheric reflection traces which are confined mainly to the
first 200 ke of virtual range in the 8-9 MHz frequency range. There is a very prominent feature
extending from 200 to 1000 km virtual range in the frequency range between the Z mode cutoff at fx
and the plasma frequency fp. This feature is attributed to sounder-generated Z mode waves which
are ducted along field-aligned Ne irregularities. Such Z mode ducting takes place in plasma
troughs in the frequency range just above the cutoff frequency f z
 and in plasma enhancements in the
frequency range just below the Z wave resonance at f N for propagation parallel to the ambient
magnetic field direction (W. Calvert, private communication, 1984). The lack of ordinary (0) and
extraordinary (X) wave ducting which are associated with Ne troughs and are common at higher
altitudes [see, e.g., Muldrew, 19691 indicate that the field-aligned irregularities near perigee
and near the magnetic equator are more suitable for Z mode ducting in enhancements than X. 0 or Z
N N, K !M
^t	 1 meet
,
,UOO
sac
1
Q 2000	 A 	 HARMONIC ©f
KRBAND
iUUU
U1	 Oh	 10	 15	 20	 30 AO 50 60
ORIGINAL i .PM iT
OF POOR QUALITY
Ff1EQUEf1CY IIIANal
Figure 1. M ISIS 1 ionogram revealing AKR together with sounder-stimulated electrostatic
wave plasma resonances (the dark stalactite-like signals tied to the zero time-delay baseline,
i.e., zero virtual range, at the top) and electromagnetic wave ionospheric echoes (the nearly
horizontal signals between 3000 and 4000 km virtual range and between the 1.5 and 4.0 MHz
frequency markers). The svmbols on this and later figures have the following meaning: N for the
plasma resonance at fN , H for the electrot, cyclotron resonance at f  (and nH, or simply n on
later figures, for those at of H ) and X for the extraordinary mode cutoff f x (and on later
figures. Z for the Z, mode cutoff f t ). (After Benson [1982).)
) MINE
2 --- —	 -
X
^—
JJJ
YX
Y	
xx X
h
X
xF J	 f T--1 ^1
F ffi	 -O
^'	 CCU''
	
0It.
Z O Z)b 19 Z
	 Z
Z re
7Z
Z
^Z
	 _Z'	 _
Z —
	
ZZ-7
	 L`
Z -,
L^Z 7-	 Z	 `	 --
`
Z^	
L	 -
^7- 'Z	 _ W
^l
0 O
.25	 .5
- X
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XX - _ }^ I T
_ XXx
O
9- - O r	 - N
O
0	 r -
-Oog -
Z	 - _Z l
Z_
- Z
2
Z Z -Z Z
W WW
W
.75 1.0 1.25
FN/FH
Figure 2.	 Normalized min[mnm frequencies of AKR of all intensities identified as
exrranrdinary (X), ordinar y
 (n), 7 or whistler (W) mode organized according to the value of
f N /f i
 at rhn pint ofj
 1 ^eivatiun.	 The horizontal dashes identify scaled or cr'cul - red va;•1es
Of f x , f 1• = (tp^ 
41
+ f ll - )	 -, t N , t 	 air. f z
 for each sonogram and are designated •_n 'he rib..,`•-"
side b• %, T, N,
	 and Z, resp^, etively.	 (Adapted from Benson [19B31.)
2
U1000
2000
3000
4000
0.1
02
0
1000
w
(3
Q 2000
Cr
Q 3000
Z)
OF PC.:)i-.
1 11213
H	 2	 3 4 56 78910 ZNX 14
II
tl	 I
0.5	 0.9	 1.25 1.5	 2.5	 4.5	 6.5	 8.5	 10.5
0.55	 1.6	 2.0	 7.0	 12.5
H
Z	 N	 T	 N T
R
r
j
w
,
r.
Cr
j 4000
0.1	 0.5
	
0.9
	
1.25 1.5
	
2.5	 4.5	 6.5
	
8.5 10 5
0.2	 0.55
	 1.6
	
2.0	 7.0	 12.5
0--
1000
2000
3000
4000
0.1	 0.5	 0.9
0.2	 0.55
H	 N	 T
(I
I	 ^	 I
i	 ll	 i	 I	 I	 ^(C)
	
1.25 1.5	 2.5	 4.5	 6.5	 8.5 10.5
1.6
	 2.0	 70	 12.5
FREQUENCY (MH-,
it,±ure 3. Alouette 2 ionograms recorded near perigee at Quito ( a), )ttawa (b) and Resolute
Bay (c,. See captions for Figures I and 2 for symbol defint I ions . The time and position
information for these ionograms are as follows: 20 June 196 8 2349 UT, 1842 magnetic local time
(MLT), 519 km altitude, 18.8 0 invariant latitu- _ A in (a); 14 November 1967 0307 UT, 2054 MLT,
525 km and 62.1 0 A in ( b); 13 October 1966 0435 UT, 1730 MLT, 520 km and 84 . 30 A in (c).
3 s
2.
t:.
3.00
WNW
1.00-
ddr
dF
f
0.00
6.00
	 1.00	 FWFM	 2.00	 3.00
Figure a. Normalized minimum frequencies of long-duration echoes from sounder-generated Z
mode signals (Z) and plasma resonance signals (N). The vertical lice above each symbol repre-
sents the normalized frequency bandwidth for the corresponding signal. See Figure 2 for a
description of the background grid.
I .9'0
" 1.00
N
N
Z
Zz
Z Z  Z 
z
N	 h.
ZZ
NZ
0 so
` 10.00 20.00 30.00 44.00 50.00 00.00 70.00 80.00 90.00
INVARIANT LATIT= tCEGMS)
Figure 5. The minimum frequency of the Z and N signals of Figure 4 with f /f > 1
normalized by f  (rather than f H) plotted against the invariant latitude of Alouette N it the
Point of observation.
4
tode ducting in troughs. (It is unlikely that the differences are due to temporal changes in the
-1 see fs to fx sweep time since examples such as the one in Figure 3a are very common in this
region.)
The add-latitude sonogram of Figure 3b is from a series of ionograms where large Ne
fluctuations were observed from sonogram to ionogram. The double recording of fN and fT resonance;
indicates severe Ne changes along the satellite path during the2 recording of this particular
ionogram (the upper hybrid frequency fT is given by fT . fN + fH ). In particular, the average
Ne
 change from the fT resonance near 1.6 NHz to the fp resonance near 1.9 MHz was 18%/km. The
sounder-generated Z mode waves between fN and fT, which are so common on high latitude ionograms
(see, e.g., Figure 14 of Muldrew 119691 and Figure 9 of James (19791) highlight these frequency
domains and also indicate (by their continuous presence) that there were no severe Ne changes
during the fN to fT sweep time.
Field-aligned Ne irregularities with a much smaller spatial separtion are indicated in the
high latitude ionogram of Figure 3c. Here the sounder-generated Z mode waves are not continuous
over the fN to fT frequency domain. In addition, a series of 'vertical bars" are observed between
fH
 and fN. This bar phenomenon, and its relation to N irregularities, has been discussed by
Muldrew and Hagg (1970). The solid black bars are most liekely due to fN resonance signals whereas
the lighter ones resemble thin frequency samples of Z mode signals between fN and fT. In either
case, if the spacing on the frequency axis is interpreted in terms of the satellite motion at -8
km/sec during the 0.1 to 0.4 sec sweep time between bars, the satellite is crossing a series of Ne
fluctuations (of 10's of percent) over the polar cap with typical spacings of hundreds of meters to
a few kilometers. Similar Ne structures in the topside auroral ionosphere have been vividly
demonstrated in the ISIS 1 satellite /Chatanika Radar correlative investigation of Muldrew and
Vickrey 119821.
The distribution of Z mode signals which cover this entire ionogram virtual range scale, and
solid black vertical bars attributed to f N resonances, of the type illustrated in Figures 3b and 3c
are presented in Figure 4 using the same format as in Figure 2. Unlike the case of Figure 2,
however, the data represented in Figure 4 correspond to sounder-generated signals rather than the
naturally occuring signals. (Very few signals of natural origin were observed in the present data
set.) The vertical lines above each symbol in Figure 4 designate the normalized frequency
bandwidth of the sounder-generated signal. If there were no significant horizontal Ne gradients,
the sounder-stimulated Z mode signals extending over the full ionogram virtual range scale would be
confined to the frequency domain between fp and f T when f./f.> 1 and there would be no entries for
"vertical bars' (other than for those along the slanting distribution corresponding to the fN
resonance). The abundance of Z mode signals outside t ye fN to fT range and of N's removed from the
fN distribution when fN /fH > 1, however, indicates the common occurrence of N e gradients along the
satellite path which are attributed to field-aligned N e irregularities.
In order to obtain information on the location of these field aligned irregularities, the
minimum frequencies of the Z mode and fN signals when fN/fH > 1 of Figure 4 were normalized by fN
and plotted against the invariant latitude of Alouette 2 corresponding to the time when the fH
resonance was recorded. The results are presented in Figure 5. If the ionosphere contained no
horizontal N gradients, all of the Z's and N's should fall on the f/f N - I line. The results of
Figure 5 indicate that this is not the case, however, and that field-aligned irregularities are
common in the polar cap. As mentioned earlier, these irregularities have separations of ?00's of
meters to a few kilometers. Their w:.de distribution over the polar cap could be the resu!t of
plasma instabilities operating at the e3ges of larger scale irregularities ()- 10 km) which can
convect over the polar cap (Kelley et al., 19821.
SUMMARY
Sounder-generated Z mode waves can be employed as a sensitive diagnostic tool for high
latitude field-aligned irregularities with scale sizes in the range from 100's of meters to several
kilometers. An inspection of Alouette 2 perigee data (-500 km) indicate that such irregularities
are n common feature of the polar cap. In addition, field-aligned irregularities capable of pro-
ducing ducted Z mode echoes near 500 km are common at low magnetic latitudes.
Acknowledgements. I am grateful to M. Posey for assistance with the data reduction, to
N. Schur for assistance with the data presentations of Figures 2, 4 and 5 and to the National Space
Science Data Center at the Goddard Space Flight Center for photographic reproductions of the
topside sounder ionograms used in Figures 1 and 3.
REFERENCES
Benson, R. F., - Ionosphere Plasm Resonances: Time Durations vs. Latitude. Altitude, and fNlfN,
Planet.. Space Sci., 30, 683-746 9 1972.
Benson, R. F., Charaeteristies of Auroral Kilometric Radiation as a Function of Source Region
Plasma Parameters, 5088s 64, 270, 1983.
Fredricks, R. H., and W. it. L. Taylor, Haves in Space Plasmas Program, in The Effects of the
Ionosphere on Radiowave Svsese. Proceedings of the 1981 IRS Symposium jointly sponsored by
Naval Research Laboratory; the Office of Naval Research, and the Air Force Geophysics
Laboratory in Alexandria. Virginia from 14-16 April, 1981.
Janes, H. G., Have Propagation Experiments at Medium Frequencies between Two Ionospheric Satellites
3. Z Mode Pulses, J. GeophZe.Res., 84. 499-506, 1979.
Kelley, M. C., J. F. Viekrey, C. H. Carlson, and R. Torbert, on the origin and spatial extent of
high-latitude F region irregularities, J. Geop s. Res. 87. 4469-4475, 1982.
Muldrew, D. B., Wonvertical Propagation and Delayed-Echo Generation Observed by the Topside
Sounders, Proc. IEEE, 57. 1097-1107, 1969.
Muldrew, D. B. and E. L. Hagg, Stimulation of Ionospheric-Resonance Echoes by the Alouette II
Satellite, in Plasma Haves in S ace and in the Laboratory. NATO Advanced Study Institute,
Rdroa, Norway, April 17-26. 1968, Proceedings, , Vol. 2, Edited by J. 0. Thomas and B. J.
Landmark, pp. 55-68, Edinburgh University Press, Edinburgh, 1970.
Muldrew, D. B., and J. P. Vickrey, High-latitude F Region Irregularities Observed Simultareously
with ISIS I and the Chatanika Radar, J. Geophys. Rest 8.7a $263-$272, 1982.
k
6


Field-aligned electron density irregularities near 500 km. Equator to polar cap topside sounder Z mode observations

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