Powerpoint template for scientific poster

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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
Procedure 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.
Abstract 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.
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.
Fig. 3. Same format as Figure 2 for Aug-Oct.

• Conclusions
• 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.

Introduction 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.
Table. 1.Definitions of the regions of interest.
Data Presentation 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 The occurrence
probabilities as a function of season
and geographic domain are presented in Figure 5.
Fig. 4. Same format as Figure 2 for Nov-Jan.
Instrumentation and Coverage 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.
Future Work 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.
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.
References 1Fukao, S., et al., Turbulent
upwelling of the mid-latitude ionosphere
1.Observational results by the MU radar, J.
Geophys.Res., 96, 3725, 1991. 2Hanson, W. B.
and F. S. Johnson, Lower midlatitude ionospheric
disturbances and the Perkins instability, Planet.
Space Sci., 40,1615, 1992. 3Maruyama, T., and
N. Matuura, Global distribution of occurrence
probability of spread echoes based on ISS-b
observation, J. Radio Res. Lab., 27, 201,
1980. 4McClure, 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
Fig . 2. Maruyama and Matuuras 1980 spread
echo index variations for each region in Feb-Apr.
Fig . 1.Satellite coverage map showing regions
of interest.