Noise bands associated with the upper-hybrid resonance were used to provide direct evidence for the existence of regions of enhanced density in the equatorial magnetosphere near L = 2. Density enhancements ranging from several percent to as high as 45 percent are observed with radial dimensions of several hundred kilometers. The enhancement characteristics strongly suggest their identification as magnetospheric whistler ducts

Mosier, S. R.

English

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OBSERVATIONS OF MAGNETOSPHERIC
IONIZATION ENHANCEMENTS USING
UPPER-HYBRID RESONANCE NOISE
BAND DATA FROM THE RAE-1 SATELLITE
(NA`'A-, . -X 	145)
	
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CSCL 03P 63/93 34749
STEPHEN R. MOSIER
AUGUST 1975
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------ GODDARD SPACE FLIGHT CENTER
GREENBELT, MARYLAND
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.	 OBSERVATIONS OF MAGNETOSPHERIC IONIZATION
ENHANCEMENTS USING UPPER-IM3RID
RESONANCE NOISE BAND DATA
FROM THE RAF 1 SATELLITE
Stephen R. Mosier
Radio Astronomy Branch
Laboratory for Extraterrestial Physics
NASA/Goddard Space Flight Center
Greenbelt, MD 20771
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ABSTRACT
Noise bands associated with the upper-hybrid resonance have
been used to provide direct evidence for the existence of regions of
enhanced density in the equatorial magnetosphere near L = 2. Density
enhancements ranging from several percent to as high as 45 percent
are observed with radial dimensions of several hundred kilometers.
The enhancement characteristics strongly suggest their identification
as magnetospheric whistler ducts.
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Whistler studies from both ground stations and spacecraft have
long indicated the existence of magnetospheric ducts of enhanced
density which guide whistler waves between conjugate hemispheres.
The first evidence from spacecraft data that such ducts actually
v exist was derived from OGO 1 data by Smith and Angerami [ 19681. A
more detailed analysis of data from OGO 3 was performed by Angerami
[ 19701 in which estimates of the L-shell thicknesses, elongation in
longitude, and enhancement magnitudes for ducts were obtained from
whistler observations between L = 4.1 and L = 4.7. Cerisier [ 19741
observed ducted waves together with the irregularity responsible for
the ducting, using data from the rR 1 satellite at 750 km altitude.
However, at magnetospheric altitudes, there has been no direct
measurement of the relative density variations through a duct region.
This paper reports on the obserN ations of ionization enhancements
with the RAE 1 satellite at L-values near 2 at the equator, using
observations of noise bands associated with the upper-hybrid resonance
to measure the relative density fluctuations at the satellite. These
enhancements have the same characteristic dimensions and magnitudes
as those inferred from the study of whistler ducting, thereby supporting
the existence of whistler ducts of enhanced ionization.
Observations of noise band phenomena occurring below the upper-
hybrid resonance (UIIR) frequency and extending below the local
2
electron plasma frequency have been reported by several investigators.
The UHR frequency is given by the relation
22	 2'
W UHR — w pe + w ge
r i
where w UHR is the angular UHR frequency, w = (4n ne t/m)
1/2 is the
pe
angular electron plasma frequency, w = eB/mc is the angular	 j
ge
^^	 tl
electron cyclotron frequency, n is local electron density, m is electron
mass, a is electronic charge, and B is geomagnetic field strength
(gaussian units are used) . The most characteristic feature of these
naturally occurring noise bands is the sharp onset at, or just below, 	 j
the local UHR frequency.
i
This study utilizes observations of noise bands associated with
the UHR at frequencies of 540 and 700 kHz. At the orbit of RAE 1,i
the local UHR frequency occurs between these two frequencies as the
satellite traverses the equatorial region. Fine structure anticorrelations
of intensity between these two observing frequencies are observed over
short spatial distances and may be interpreted as local density
inhomogeneities along the satellite path. This paper discusses the
1	 inter :etation of the noise bands in terms of such inhomogeneities,
I
.	 or ducts, and the implications of the required scale sizes and
magnitudes.
3
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INSTRUMENTATION
The Radio Astronomy Exphrer (RAE 1) satellite was launched on
4 July 1968 into a 5850-km circular Earth orbit of 121-degree (retro-
grade) inclination. The instrumentation was designed to measure long-
wavelength radio phenomena emitted from the magnetosphere, the
solar corona and the Galaxy. The data used in this study were acquired
by fixed-frequency total power radiometers and a 37-meter tiprto-tip
electric dipole antenna; frequencies of 540 and 700 kHz were chosen
because of their relation to the UHR frequency at the equator. Additional
data from a step -frequency radiometer were used as an independent
system for establishing a local density scale from UHR noise bands
using the method discussed by Mosier et al. [ 19731 , hereafter referred
to as Paper I. The spacecraft utilizes gravity gradient stabilization
and gyroscopic forces maintain the dipole antenna alignment to within
13 degrees of the satellite velocity vector. A detailed description of
the RAE 1 satellite is given by Weber et al. [ 19711.
OBSERVATIONS
Figure 1 is a plot of the 540- and 700-kHz noise intensities for
an equatorial transit by the spacecraft. The basic features observed
on most such transits are the two noise bands at 540 kHz occurring 	 j
on either side of the dipole equator and the single noise band at
h,
4
	
i
Ills;
low
700 kHz centered approximately at the equator. A self-consistent
fit of a local electron density scale to the characteristic frequencies
in the plasma was obtained by using noise band observations at several
closely-spaced frequencies (see Paper 1) . Then, using computed values
t	 of the geomagnetic field strength, the spacecraft orbit was plotted
on a C.MA diagram ( Ratcliffe, 1962 ] (also in rigure 1) . in this diagram,
scale lengths in the abscissa and ordinate are proportional to electron
density and to the square of the magnetic field strength, respectively.
Characteristic frequencies are represented by boundaries in this two-
L
.
dimentsional parameter space. As the spacecraft encounters increasing
density it crosses the cutoff for the x-wave of the extraordinary mode
(R = O in the notation of Stix [ 1962 ]) , the upper-hyLrid resonance
(S = O) , the electron plasma frequency (P = O) and the cutoff for the
z-wave of the extraordinary mode (L = 0), where
1
	
	 2
ape
R - 1
.0	
(2)
'	 ' ge)
2
wpe
L1--	 (3)
W ('V + U)ge)
;u 2
S = 2 (R + L) = 1 - w 2 - 
we 
2	 (4)
ge
W
2
P=1-	 (S)
W
S
do
y1	 J'-	
-1
For a discussion of the x- and z-wave notation commonly used in
ionospheric physics, see Ratcliffe [ 1962] . (The z-wave cutoff,
c'efined by L = O, should not be confused with McIlwain's [ 1961]
geomagnetic shell parameter L, discussed earlier) . The four
boundaries define five regions on the CMA diagram of Figure 1,
numbered from left to right.
As reported in Paper I, a noise-intensity increase of 10 to 20 dB
is often observed between R = O and S = O (region 2) . The most
intense noise bands (30 to 40 dB above background) were observed
in regions 3 and 4. Background intensity is not always attained by
the time the spacecraft reaches region 5 and there is often a further
intensity decrease of approximately 10 dB beyond L = O. When the
noise intensity at 540 and 700 kHz in Figure 1 exceeded a threshold
of 10 dB above background (indicated by horizontal bars under the
noise plots) , the CMA orbit in Figure 1 is plotted as a heavy line.
The most intense noise thus occurs in a band from about fz to fUHR'
This noise band appears in the V.vo channels sequentially as the
spacecraft traverses a smoothly varying plasma. In regions of the
orbit where the noise band appears only in the lower channel, an
anomalous increase in plasma density, if sufficiently large, can
displace the noise band out of the lower channel and into the upper one.
Similarly, anomalous decreases would be detectable in those regions
6
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r
where the noise appears only in the upper channel. In reality, both
i
enhancements and depressions are detectable in either region, but
the confidence level is low for the two cases not discussed above
since a signature would be present in only one channel. However,
the absence of the necessary signature is sufficient proof that a large
inhomogeneity is not present. Typical inhomogeneity signatures are
ti
seen in Figure 2, where an increase in noise intensity at one frequency
is accompanied by a simultaneous decrease in intensity at the other
frequency. These anticorrelated intensity features track quite well
as seen in Figure 3, where a single feature (on a different date) is
shown at increased resolution. At 2228:10 UT in Figure 3, the 540-kHz
noise intensity increases 26 dB in 13 sec, accompanied by a 21-dB
decrease at 700 kHz at the same time. In 13 sec, the spacecraft
travels 74 km. The duration of the "event" in Figure 3 is two minutes,
during which the spacecraft travels 685 km.
The choice of the 540- and 700-kHz observing frequencies limits
the observation of UHR noise to within approximately 30 to 40 degrees
of the geomagnetic equator. The anticorrelated fine-structure intensity
features P ie observed throughout this region and at all local times.
Assuming that the observed inhomogeneities are the signatures of
magnetospheric propagation ducts, then these data are in good
agreement with Cerisier [ 19741 who found that ducted propagation
7
u
!o, _
is a quasi-permanent feature of the VLF field between L = 1. 7 and 3.
The RAE 1 observations were strongly grouped at low values of the
geomagnetic activity Kp index. 	 This is probably due to the selection 1
s of data during periods =_@" low background noise activity for ease in
jdetermining local density scales and does not necessarily reflect a
}• correlation of the fine structure with low values of Kp.
DISCUSSION
From a study of the typical noise intensities observed in the
I
^` different CMA regions [ Paper I] , we conclude that when either the 9
540- or 700-kHz intense noise (i.e., 20 to 30 dB above background)
is cutoff, the observation cannot lie in region 3 or 4. 	 Therefore, at
L,l
{{ the
l^
onset of the event in Figure 3, the reduction of the 540-kHz signal
dd and the onset of the 700-kHz signal must correspond to a sudden
4	 I^ increase in electron density (a shift from region 3 or 4 to region 5 along
^.I the solid line orbital track and a shift from region 1 or 2 to region 3 or
4 along the dashed line orbital track on the CMA diagram in Figure 1).
^j
Using noise resonance data at several other frequencies combined with
^I
the RAE 1 capacitance probe data, a value of 25 percent has been
x,
ti	 ^! derived for the minimum density enhancement required to explain the
structure in Figure 3.	 This minimum value reflects uncertainties
in the ambient density determination and allows for a possibly greater
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enhancement magnitude.
	 ;n defining the enhancement magnitude,
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Ll
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A
the highest density attained is that which shifts the 700-kHz orbit
plot into region 3. The background densit ,  from which the enhancement
Is measured is dependent upon the position of the orbit plot for the
700-kHz background noise intensity. In order to better define the
enhancement magnitudes, a study of 244 cases was performed using
UHR noise band data and the dipole antenna capacitance probe on
RAE 1 to determine the local electron density for each event just
before the onset of intense noise at 700 kHz. The resulting data
must be treated statistically due to the uncertainty associated with
density determinations by capacitance probe techniques (fora discussion
of the calibration of the RAE 1 capacitance probe using UHR noise bands,
see Mosie •- and Kaiser C 1975 3) . Prom a preliminary statistical study
of these data, it appears that the observed density enhancement
magnitudes range from less than 10 to 15 percent to as great as 45
percent. These enhancements occur typically within cross-L-shell
distances of the order of hundreds of kilometers. The suggested geometry
of the density enhancement in relation to the satellite trajectory is shown
in Pigure 4_
The physical picture shown in rigure 4 which we derive from
the data is of a field-aligned ionization enhancement, or duct, of as
much as 45 percent of the ambient density and of several hundred
9
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RV
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kilometers spatial extent radially at the equator near L = 2 at all
local times. This is in good agreement with Angerami C 19 70 ] , who
derived duct thicknesses of 220 to 430 km radially at the equator
near L = 4 from whistler observations with OGO 3.
Furthermore, the UHR data present evidence to support the long-
standing hypothesis that whistler ducts are enhancements, rather
than depressions, of ionization. Although some density depressions
arg observed, they are generally of much smaller magnitude than are
the enhancements and are quite often superimposed on enhanced
density levels; dramatic depressions of density have seldom been
observed. Using a model for whistler ducting, Angeraml G 1970] con-
cluded that the minimum density enhancement required for supporting
guided whistlers is between 4 and 8 percent. He further estimated
that enhancements generally lie between 6 and 22 percent and rarely
exceed 33 percent. The UHR data reported here confirm AngeramVs
estimates but extend the upper limit of enhancement magnitude to over
40 percent.
Park and Helliwell L 19711 suggested a possible mechanic{m to
explain irregularities in electron density involving E X B convection
cells in which tubes of ionization are mixed. They showed that a
0.1 mV/m electric field in the equatorial plane can produce electron
10
4
IIj
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x
w
ij
i
density enhancements and depressions of the order of 5 percent at
L = 4 in about one-half hour. In about one hour, multiple. peaks and
valleys start to appear, and at later times the scale size of the
irregularities decreases and the amplitude increases slightly to near
0 percent, with dimensions between peaks and valleys of the order
of 50 km. It is interesting to note from rigure 3 that there is considerable
structure within the density enhancement regior as observed by RAE 1.
The cross-L-shell dimensions for individual structures within this
region are of the order of tens of kilometers, which are consistent with
the E X B convection model of Park and Helliweli L 19711 . However, the
RAE-1 data do not support the existence of density depressions and
enhancements of equal magnitude, large depressions being rarely
o o_- *ed. It is possible that an electric-field-driven mechanism
such as that of Park and Helliwell is operating at the RAE 1 orbit but,
if so, it must certainly be a minor effect.
In summary, regions of enhanced density are obser . I between
L = 1.9 and L = 2.5 within 30 degrees of the geomagnetic equator.
The peak fractional density increases range from a few percent to as
much as 45 percent of the normal ambient density and the structures
have overall radial dimensions of several hundred kilometers, with
smaller structure observed over distances of the order of tens of
kilometers. The similarity of these enhancements to those derived from
whistler data suggests their identity as whistler ducts.
11
GY
	
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REFERENCES
Angerami, J.J., Whistler duct properties deduced from VLF observations
made with the OGO 3 satellite near the magnetic equator, j..
	
4
	
Geophvs. Res., 75, 6115-6135, 1970.
,N
Cerisier, J.C. , Ducted and partly ducted propagation of VLF waves
through the magnetosphere,.I. Atmos. XW. Phvs., 36, 1443-1467,
1974.
McIlwain, C.E., Coordinates for mapping the distribution of magrintically
trapped particles,.• . Geophvs. Res., 66, 3681-3691, 1961.
Mosier, S.R., and M.L. Kaiser, Calibration of a cylindrical RF
opacitance probe, 1. Geophvs. Res., ^, 702-703, 1975.
Mosier, U.R., M.L. Kaiser and L.W. Brown, Observations of noise
banns associated with the upper hybrid resonance by the IMP 6
radio astronomy experiment,.,. Geophvs. Res., 78, 1673-1679, 1973.
Park, C.G. and R.A. Heiliwell, The formation by electric fields of
field-aligned irregularities in the magnetosphere, Radio Sci. ,
6, 299-304, 1971.
Ratcliffe, J.A., The Magneto-ionic Theory and Its Application to tho
Ionosphere, p. 59, Cambridge University Press, London, 1962.
k
Smith, R.L., and J. J. Angerami, Magnetospheric properties deduced
from OGO 1 observations of ducted and nonducted whistlers,
I, Geophvs. Res., 73, 1-20, 1968.
J
40	
`JIB
" +
.
Stix, T.H., The The 	 of klasma Waves, pp. 27 ff, McGraw -Hill, New
York, 1962. 
Weber, R.R., J. K. Alexander, and R. G. Stone, The Radio Astronomy
Explorer satellite, a low-frequency obzervatory, Radio Sod ,
1085, 1971.
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FIGURE CAPTIONS
Figure 1 - UHR noise bands observed at 540 and 700 kHz and the CMA
diagram orbit plots for these frequencies. The horizontal
bars under the noise peaks indicate those times for which
the noise intensity exceeds 10 dB above back ground. The
CMA orbit for these times is plotted as a heavy line.
Figure 2 - UHR noise bands with superimposed anticorrelated intensity
structure.
Figure 3 - Detail of anticorrelated intensity structure.
Figure 4 - Geometry of density enhancement region in relation to
satellite trajectory.
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Figure 4


Observations of magnetospheric ionization enhancements using upper-hybrid resonance noise band data from the RAE-1 satellite

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