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Seasonal and Longitudinal Variations of Midlatitude
Topside Spread Echoes Based on ISS-b Observations
A. M. Mwene, G. D. Earle, J. P. McClure
William. B. Hanson Center for Space Sciences, University of Texas at Dallas
AUG-SEPT-OCTOBER
NORTH PACIFIC
Abstract
Procedure
A preliminary study of the seasonal and longitudinal
variations of spread echoes from the Ionosphere Sounding
Satellite (ISS) using the topside sounding data has been
undertaken. Significant longitudinal and seasonal variations
in midlatitude spread echoes are observed. The north
Atlantic region has the highest occurrence probability in the
winter solstice. The smallest occurrence is in the north
Pacific in the same interval. Occurrence probabilities of up
to about 30% are quite common.
Maruyama and Matuura [1980] describe the process of
inferring a simple index corresponding to spread echo
conditions from the ISS-b topside sounder data. Index values
greater than four correspond to widespread regions of spread
echoes.
McClure et al. [1998] offer a good overview of this
classification method, particularly as it applies to equatorial
spread F. We use the Maruyama index in our analysis to
identify regions at magnetic latitudes between ±20 and ± 50
degrees that have significant spreading. Table 1 shows the
breakdown of the various geographic regions, and Figure 1
shows these regions on a world map.
Instrumentation and Coverage
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Occurences in Logscale
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10
1
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10
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1
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10
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0
5
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1
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10
AUSTRALIA
1
SOUTH ATLANTIC
AFRICA
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1
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100
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0
5
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Spread F index
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Spread F index
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Conclusions
NORTH AMERICA
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SOUTH PACIFIC
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In summary, this analysis of the topside sounder data
from ISS-b leads to the following preliminary
conclusions:
 There is no apparent preference for midlatitude
spread echoes to occur over continental land masses.
 There are very large seasonal variations in the
occurrence probability of midlatitude spreading over
distinct geographic domains.
These seasonal
variations are largest over the oceanic regions.
 The highest occurrence probability for midlatitude
spread echoes is over the north Atlantic in the
November-January period. The smallest occurrence
probability is over the north Pacific, in the same
interval.
 Occurrence probabilities up to about 30% are quite
common at all locales.
ASIA
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NORTH ATLANTIC
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AUSTRALIA
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SOUTH ATLANTIC
AFRICA
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ASIA
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INDIAN OCEAN
EURASIA AND N.AFRICA
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NORTH AMERICA
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Spread F index
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Spread F index
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Fig. 4. Same format as Figure 2 for Nov-Jan.
NORTH PACIFIC
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1
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FEB-MARCH-APRIL
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Spread F index
EURASIA AND N.AFRICA
NOV-DEC-JAN
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The occurrence probabilities as a function of season and
geographic domain are presented in Figure 5.
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INDIAN OCEAN
NORTH PACIFIC
+20-50
+20-50
+20-50
+20-50
-20-50
+20-50
-20-50
+20-50
-20-50
-20-50
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EURASIA AND NORTH AFRICA
0
5
10
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Spread F index
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Fig . 2. Maruyama and Matuura’s [1980] spread
echo index variations for each region in Feb-Apr.
SEASONAL OCCURRENCE PROBABILITIES FOR SPREADING EVENTS
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FEB MAR APR
40
%
Figures 2-4 show logarithmically scaled histogram plots of
the Maruyama index values for each of the geographic
regions defined in Table 1. Each of the figures corresponds to
a different season; logarithmic axes have been used in order
to highlight the regions on each graph for which the index
value is greater than four. It is important to remember that the
regions defined in Table 1 correspond to very different
geographic areas (in km2). However, it is valid to compare the
seasonal variations for a given geographic area.
In Figures 2-4 the left column of histograms corresponds
to oceanic regions, and the right column corresponds to land
masses. The seasonal variations become more apparent when
the data from Figures 2-4 are presented as occurrence
probabilities. These have been calculated as follows for each
region:
Probabilit y  Number of events with index 5100%
Total number of observatio ns
NORTH ATLANTIC
AFRICA
Fig. 3. Same format as Figure 2 for Aug-Oct.
Data Presentation
1
This is surprising, since it might be expected that more
thunderstorms and subsequently more gravity wave seeding
for spreading would be expected over land masses, where
orographic features exist. The lack of such a correlation may
be due to the fact that gravity waves can be ducted over very
large horizontal distances, so that waves generated over land
masses may propagate for thousands of kilometers before
generating perturbations that lead to midlatitude spread
echoes.
AUSTRALIA
SOUTH ATLANTIC
1
Table. 1.Definitions of the regions of interest.
SOUTH PACIFIC
ASIA
1
NORTH ATLANTIC
Mag Latitude
225-2850
345-600
60-1400
10-500
110-1550
140-2250
155-2800
285-3450
315-100
50-1100
North America
Eurasia
Asia
Africa
Australia
North Pacific
South Pacific
North Atlantic
South Atlantic
Indian Ocean
1
SOUTH PACIFIC
1
Occurences in Logscale
Geog Longitude
Region Name
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Fig . 1.Satellite coverage map showing regions of interest.
10
1
Occurences in Logscale
The topside sounder instrument from the ISS-b satellite is
used as our diagnostic tool. The satellite provided useful data
from August 1978 through December 1980, with intermittent
tape recorder outages and data dump intervals resulting in
roughly a 30% duty cycle. The satellite was inserted into a 70
degree inclination orbit, with apogee and perigee at 1220 km
and 972 km, respectively. The 150 W topside sounder
instrument used for this study covered the frequency range
from 0.5-14.8 MHz in 0.1 MHz steps, with a receiver
bandwidth of 6 kHz.
Figure 1 shows the satellite coverage over the course of
one season. The points on the map correspond to the locations
at which topside ionograms were obtained. Midlatitude
coverage is relatively good for all seasons except for the
May-July solstice period. We have therefore omitted this
interval from our analysis.
100
20
0
60
AUG SEPT OCT
40
Future Work
%
Ionosonde signatures of spread echo conditions are not
strictly limited to regions near the magnetic equator. A
number of radar and satellite studies have shown that radio
scintillation and large scale density irregularities in the F
region plasma also occur at midlatitudes, although less
frequently. Fukao et al. [1991] observed spread F type
ionograms quite far from the magnetic equator, and Hanson
and Johnson [1992] observed mid-latitude density
perturbations at dip latitudes as high as 40 degrees using the
AE-E satellite. Our focus in this work is to determine
whether midlatitude spread echoes have any statistically
significant seasonal or geographical variability.
100
20
0
60
NOV DEC JAN
40
%
Introduction
NORTH AMERICA
20
0
Npacific
Spacfic
Natlantic
Satlantic
Indianocean
Africa
Namerica
Eurasia
Asia
Australia
Fig. 5. Topside spread echo occurrence
probabilities as a function of season and
location.
Discussion
With reference to Figure 5, there are very large seasonal
differences in occurrence probabilities for midlatitude spread
echoes in the north Atlantic, south Atlantic, and north Pacific
regions. Somewhat less striking seasonal variations are
evident in Asia and Europe. The other geographic domains
have much less pronounced seasonal variations. The
occurrence of spread echoes over the north Atlantic region is
particularly variable. This region shows the highest
(November-January) and second lowest (August-September)
occurrence probabilities. The overall occurrence probabilities
for MSF are quite large when classified using the Maruyama
and Matuura [1980] index. This may be caused by incursion
of high and/or low latitude irregularities into the midlatitude
domain. In general there are no differences between the
number of spreading events occurring over land masses and
over oceans.
It may be interesting to compare the statistics we have derived
here to global weather patterns. For example, the existence of
monsoon zones in the equatorial zone in southeast Asia can be
expected to launch copious quantities of gravity waves, which
might in turn be expected to trigger outbreaks of spreading
events.
It may be fruitful to compare satellite observations of
midlatitude gravity waves at F region heights to the occurrence
probability plots shown here. We have begun a study of this
nature using DE-2 data, but the results are not yet ready for
such a detailed comparison.
References
[1]Fukao, S., et al., Turbulent upwelling of the mid-latitude ionosphere:
1.Observational results by the MU radar, J. Geophys.Res., 96, 3725, 1991.
[2]Hanson, W. B. and F. S. Johnson, Lower midlatitude ionospheric disturbances and
the Perkins instability, Planet. Space Sci., 40,1615, 1992.
[3]Maruyama, T., and N. Matuura, Global distribution of occurrence probability of
spread echoes based on ISS-b observation, J. Radio Res. Lab., 27, 201, 1980.
[4]McClure, J.P. S. Singh, D.K. Bamgboye, F.S. Johnson, and H. Kil,Occurrence of
equatorial F region irregularities: Evidence for tropospheric seeding, J.
Geophys. Res., 103, 29,119, 1998.
Acknowledgments
We thank Dr. T. Maruyama for the ISS-b data. The first author
thanks Patrick Roddy for assistance. This work was supported
by NASA grant NNG04WC19G