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Observations of nighttime equatorial-upper-E-valley region irregular
structures from S~
ao Luı
´s radar and their occurrence statistics:
A manifestation of vertical coupling between E and F regions
E. Alam Kherani
n
, E.R. de Paula, R.Y.C. Cueva, L.A.P. Camargo
Instituto Nacional de Pesquisais Espaciais, Sao Jose dos Campos, Brazil
article info
Article history:
Received 13 January 2011
Received in revised form
11 August 2011
Accepted 29 August 2011
Available online 10 September 2011
Keywords:
Ionosphere
E-region irregularity
F-region plume
Plasma bubble
abstract
Radar observations of nighttime irregular structures in the equatorial upper-E-valley region are
presented. These structures are observed strictly during occurrences of an ascending irregular
bottomtype or bottomside F layer, during or prior to the appearance of a high-rising F region plume
and a momentarily intensification of E-region irregularities. The slope and the altitude coverage of
these structures are proportional to the ascending rate of the irregular bottomtype/bottomside F layer
and the altitude coverage of the plume respectively. These structures are found to consist of
substructures separated in times suggesting the presence of quasi-periodic striations within these
irregular structures. On the basis of occurrence statistics during October 2001–December 2008, the
occurrence rate of these structures is found to be 3.5%, indicating the nature of rare occurrence of these
structures over S~
ao Luı
´s. Moreover, their occurrence remains confined to the maximum solar-flux
period (2001–2003) and to the summer months (October–January). The presence of these structures is
a manifestation of the vertical coupling of E and F regions owing to the ambient electric fields of the
ionosphere and the fringe-field associated with the F region plume.
&2011 Elsevier Ltd. All rights reserved.
1. Introduction
On occasions, radar observations reveal the presence of irre-
gular structures in the equatorial-upper-E-valley region during
nighttime (Woodman and LaHoz, 1976;Kelley et al., 1981;
Woodman and Chau, 2001). These structures are observed strictly
during the occurrence of an overlying F region plume. In the radar
field of view, they first appear in the upper E-region near 120 km
and, with time, extend to the valley region and to the bottom-side
of F region. Later Chau and Hysell (2004) have reported the upper-
E-valley region irregular structures during twilight hours in the
absence of F-region plume.
The upper-E-valley region irregular structures in the presence
of the F-region plume are proposed to be generated by the
eastward polarization field (fringe field) associated with the
plasma bubble (Woodman and Chau, 2001;Kherani et al.,
2002). Using a numerical simulation model of the plasma bubble
that included the underlying upper-E-valley region dynamics,
Kherani et al. (2004) have shown that the fringe field of the
plasma bubble may penetrate down-to the 120 km altitude and
convects the E-region irregularities to the higher altitude, giving
rise to the irregular structures in the equatorial upper-E-valley
region. Moreover, the upward drift of the ambient ionosphere is
found to be an essential condition for the efficient penetration of
the fringe field. In the absence of F-region plume, these structures
are proposed to be generated by the interchange instability
triggered by the horizontal density gradient and ambient vertical
current in the valley region (Chau and Hysell, 2004).
The presence of irregular upper-E-valley region structures over
low-latitude ionosphere and simultaneous occurrence of an equa-
torial F region plume are also reported (Patra et al., 2002, 2005;
Yokoyama et al., 2005). These low-latitude observations are the
manifestation of the coupling along the geomagnetic field lines.
On the contrary, over the equator, the simultaneous occurrence of
the F region plume and irregular equatorial-upper-E-valley region
structures is the manifestation of the coupling across the geo-
magnetic field lines.
To date, the presence of irregular equatorial-upper-E-valley
region structures are reported from the Jicamarca radar. In the
present study, observations of these structures and their occur-
rence statistics are reported using the 30 MHz S~
ao Luı
´sradar.In
this study, the characteristics of irregular equatorial-upper-E-
valley region structures will be examined during observations of
plumes of varying nature and in two different local time duration
when ambient ionosphere varies differently. Moreover, based on
observations during October 2001–December 2008, the occurrence
Contents lists available at SciVerse ScienceDirect
journal homepage: www.elsevier.com/locate/jastp
Journal of Atmospheric and Solar-Terrestrial Physics
1364-6826/$ - see front matter &2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jastp.2011.08.017
n
Corresponding author. Tel.: þ55 12 32087 187.
E-mail addresses: alam@dae.inpe.br, alamhindi@gmail.com (E. Alam Kherani).
Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 64–70
characteristics of these structures with varying solar-flux and
season will be examined.
2. Observation
The S~
ao Luı
´s radar is the low-power coherent scatter radar
located at S~
ao Luı
´s equatorial station (2.31S, 44.21W, dip angle:
0.51)(de Paula and Hysell, 2003;de Paula et al., 2004). The
radar specification and other parameters used in the experiment
are given in Table 1. The radar principle beam is pointed vertical
and perpendicular to the magnetic field. The data are collected as
a time-series and the auto-covariance analysis is applied to obtain
the zeroth and first moments (Woodman, 1985). The zeroth
moment represents the intensity of backscattered echoes from
5-m irregularities inside the illuminated volume of ionosphere.
The first moment represents the line-of-sight or vertical velocity
of these irregularities. The data are presented in the form of RTI
(range–time–intensity) and RTV (range–time–velocity) maps. In
RTV maps, the positive velocity corresponds to the velocity away
from the radar.
3. Results and discussion
In this section, the irregular equatorial-upper-E-valley region
structure will be referred as IES.
3.1. Range–time–intensity maps
In Fig. 1a and b, RTI maps on two chosen nights, 08 November
2001 and 25 October 2001, are shown respectively. In Fig. 2a and b,
enlarged RTI maps in 90–200 km altitude region are shown for
these two nights respectively. It may be noted that:
(1) On 08 November 2001, an IES is observed during 19:30–20:00
LT in 120–200 km altitude region.
(2) On 25 October 2001, an IES is observed during 20:00–20:30
LT in 125–150 km altitude region.
(3) On both occasions, the IES is observed during the occurrence
of an ascending irregular-bottomtype-F-layer (on 08 Novem-
ber 2001) or an ascending irregular-bottomside-F-layer (on
25 October 2001).
(4) On both occasions, the IES is either observed during the
occurrence of a high-rise F-region plume (as on 08 November
2001) or observed just prior to the occurrence of a high-rise
F-region plume (as on 25 October 2001).
(By definition, the irregular-bottomtype-F-layer/irregular-
bottomside-F-layer is the irregular F layer observed prior to
the topside F-region plume, Hysell and Burcham, 2002.The
irregular-bottomtype-F-layer is defined as a thin single
back-scattering layer with the altitude spread no more than
50 km. The irregular-bottomside-F-layer is defined as a
thick multi-back-scattering layer with the altitude spread
coveringtheentirebottomsideandaplumewithinit.
Table 1
Radar specifications and parameters.
Radar location 2.31S, 441W, 1.31Sdip
Antenna half-power-full-beam-width (E-W) 101
Inter-pulse-period (IPP) 1400 km (9.34 ms)
Altitude coverage 87.5 km–1267.5 km
Altitude resolution 2.5 km
Coherent-integration 1
Velocity coverage 7250 m/s
Fig. 1. Range–time–intensity maps on two nights: (a) 08 November 2001 and (b) 25 October 2001. The altitude coverage is 87.5–1267.5 km. The color scale represents the
ratio ðSNÞ=Nin dB scale, where Sand Nare signal and noise strengths. (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.)
E. Alam Kherani et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 64 –70 65
The ascending/descending phase of these layers refers to
the positive/negative slope of the lower-envelope of these
layers.
By definition, a high-rise F-region plume is a plume verti-
cally elongated reaching beyond 800–900 km altitude.)
(5) On both occasions, the IES is observed at the time when the
intensity corresponding to the irregular E-region echoes
becomes momentarily large.
(6) On both occasions, substructures are identified within an IES,
which are separated in time as may be noted more clearly
from the modulation of upper envelope of the IES.
On these two nights, the nature of the irregular-bottomtype/
bottomside-F-layer and the F-region plume are very different.
How these differences reflect upon the nature of underlying IES
and may be studied qualitatively in the present study.
On both occasions, IESs are seen strictly during the time
when the slope of the lower-envelope of the irregular-bottom-
type-F-layer (on 08 November 2001) or the irregular-bottom-
side-F-layer (on 25 October 2001) is positive. This is more
evident on 25 October 2001 when IES was not observed before
20:00 LT when low-altitude-extended (up to 600 km) F region
plume was present within the irregular-bottomside-F-layer
but the slope of the irregular-bottomside-F-layer was negative.
The IES on this night was only observed after 20:00 LT when
the slope of the irregular-bottomside-F-layer was positive
followed by the occurrence of a strong and high-rise F-region
plume. During the occurrence of IES on 08 November 2001 (25
October 2001), the slope of the lower envelope of the irregular-
bottomtype-F-layer (the irregular-bottomside-F-layer) is
55 m=s(35 m=s). The slope of IES on 08 November 2001
and 25 October 2001 are 30 m=sand 20 m=srespectively.
Thus on both nights, the slope of the IES is smaller but
otherwise similar (here similarity means the sense of slope
which is positive) to the slope of the corresponding irregular-
bottomtype-F-layer (on 08 November 2001) and irregular-
bottomside-F-layer (on 25 October 2001). The slope of the
lower-envelope of the irregular-bottomtype/bottomside-F-
layer is considered as the representative of the ambient
vertical drift or zonal electric field of the irregular-F-layer
modulatedbywave-likeperturbation(Abdu et al., 2009;
Takahashi et al., 2010). Therefore, the similar slope of the IES
and the irregular-bottomtype/bottomside-F-layer is an indica-
tion of the role of modulated ambient zonal electric field to
determine the ascending rate of the IES in the upper-E-valley
region. The similarity in slopes also suggests that the ambient
F-region zonal electric field penetrates to the upper-E-valley
region without any significant alteration which is a known
characteristics from the study by Balsley et al. (1976). Based on
the slope of lower envelope of the irregular bottomtype/
bottomside-F-layer, it may be said that on 08 November
2001, the ambient zonal electric field is larger than its value
on 25 October 2001. This is also evident from the higher height
of the bottomtype-F-layer on 08 November 2001 as compared
to the lower height of the bottomside-F-layer on 25 October
2001. It is also noted that proportional to the strength of the
ambient electric field, the IES on 08 November 2001 is com-
paratively higher altitude extended than the IES on 25 October
2001. Similar slope of the irregular F layer and IES and the
proportional relation between the zonal electric field and the
altitude extension of IES suggest the presence of a dynamical
coupling between the ambient F-region dynamics and the
upper-E-valley region.
It may be noted that on 08 November 2001, the IES is
observed during simultaneous occurrence of an ascending
irregular-bottomtype-F-layer and a high-rise F-region plume.
On the other hand, on 25 October, 2001 the IES is observed
during the occurrence of an ascending irregular-bottomside-F-
layer but the high-rise plume is observed 20 min later.
This suggests that the upper-E-valley region IES may occur
with and without the presence of the F-region plume.
However, when it occurs in the presence of the F-region plume,
it may be extended to comparatively higher heights as found on
08 November 2001. This aspect suggests the presence of a
19:30 20:00
100
125
150
175
Altitude, km
19:30 20:00 20:30
−10
−8
−6
−4
−2
0
2
4
6
8
10
Local Time, LT Local Time, LT
Fig. 2. Range-time–intensity maps on two nights: (a) 08 November 2001 and (b) 25 October 2001. The altitude coverage is 87.5–200 km. The color scale represents the
ratio ðSNÞ=Nin dB scale, where Sand Nare signal and noise strengths. (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.)
E. Alam Kherani et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 64–7066
dynamical coupling between the F-region plume and the
upper-E-valley region.
It is interesting to note that on both nights, the IES first
appears in the radar field-of-view when the backscattered
intensity corresponding to the E-region irregularities becomes
momentarily strong. The intensification of E-region irregula-
rities implies the presence of a strong ambient vertical electric
field in the E-region (Fejer et al., 1975;Kudeki et al., 1982).
Therefore, it may be said that the IES is observed during the
time when the ambient vertical electric field in the E-region
becomes momentarily strong.
From RTI maps in Fig. 2a and b, it may be noted that there is a
discontinuity in the backscattered-intensity above 120 km alti-
tude that separates the IES from the E-region irregularities. This
discontinuity is more evident on 25 October 2001. This discon-
tinuity will also be evident from spectral characteristics
presented later in Fig. 4. This discontinuity suggests that irre-
gularities within the IES do not belong to E-region irregularities,
rather they are generated locally above 120 km in the upper-E-
valley region.
3.2. Range–time–velocity maps
In Fig. 3a and b, RTV maps corresponding to 08 November
2001 and 25 October 2001 are shown, respectively. It may be
noted that on 25 October 2001, the region of IES that lies below
135 km altitude, moves dominantly downward and the region of
IES that lies above 135 km altitude moves dominantly upward.
The IES observed on 08 November 2001 also shows similar
velocity reversal during the first 15 min of its appearance. The
average upward velocity on 08 November 2001 and 25 October
2001 correspond to 35 m/s and 20 m/s, respectively. The slope of
IES, as noted from RTI maps in Section 3.1, is also similar to the
average upward velocity on both occasions.
In Fig. 4, spectral characteristics on 25 October 2001 are
presented. It may be noted that spectra corresponding to the
upper-E-valley region irregularities within the IES are much
narrower than spectra corresponding to the E-region irregulari-
ties within 100–110 km. Moreover, spectra corresponding to
upper-E-valley region irregularities within IES are comparatively
larger Doppler-shifted than spectra corresponding to irregulari-
ties in 100–110 km altitude. These aspects suggest that irregula-
rities within IES are different from E-region irregularities, rather
generated locally above 120 km altitude in the upper-E-valley
region and not caused by the convection of E-region irregularities
by the ambient zonal electric field and the fringe field.
Based on RTI and RTV maps and spectral characteristics, the
following picture may emerge: upper-E-valley region IESs are
strictly observed when ambient zonal and vertical electric fields
become strong. Moreover, IES is observed during presence of
F-region plume as on 08 November 2001 and may be observed
prior to the F-region plume as on 25 October 2001. These aspects
suggest that the appearance of IESs are closely associated with the
ambient electric field and may occur with and without the
F-region plume. Also, in the present observation, IES structures
are found to be disconnected from E-region irregularities, sug-
gesting that irregularities within the IES are generated locally in
the upper E-region-valley region.
On the basis of work by Chau and Hysell (2004),Woodman
and Chau (2001) and Kherani et al. (2004), the present observa-
tion of IES may be interpreted as the manifestation of mechanism
involving the interchange instability (Chau and Hysell, 2004) and
the fringe field (Woodman and Chau, 2001;Kherani et al., 2004).
Chau and Hysell (2004) have argued that, in the presence of the
vertical current and the horizontal density gradient, interchange
instability may be excited in the upper-E-valley region. In this
region, the vertical current may be driven by both vertical and
zonal electric field. In the present observation, close association
between the upper-E-valley region IES and ambient zonal/vertical
19:30 20:00
100
125
150
175
Altitude, km
19:30 20:00 20:30
−50
−40
−30
−20
−10
0
10
20
30
40
50
Local Time, LT Local Time, LT
Fig. 3. Range–time–velocity maps on two nights: (a) 08 November 2001 and (b) 25 October 2001. The altitude coverage is 87.5–200 km. The color scale represents the
vertical velocity in m/s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
E. Alam Kherani et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 64 –70 67
electric field is noted which favors the mechanism proposed by
Chau and Hysell (2004).Kherani et al. (2004) have shown that in
the upper-E-valley region, the fringe-field acquires both zonal and
vertical components and may also create the horizontal density
gradient. Thus, the fringe field may accelerate the interchange
instability mechanism proposed by Chau and Hysell (2004),by
providing additional vertical current and horizontal density
gradient. The IES on 08 November may fall in the category where
both ambient electric field and fringe field are driving interchange
instability mechanism in the upper-E/valley region. On the other
hand, 25 October 2001 may fall on category where ambient
electric field alone is driving the interchange instability proposed
by Chau and Hysell (2004).
The upper-E-valley region irregular structures are also
observed over low-latitude (Patra et al., 2005;Yokoyama et al.,
2005). These studies show that the low-latitude valley region is
coupled with the bottom-side of the F-region over the geomag-
netic equator, where coliisional interchange instability is excited.
The polarization electric field of the instability at the equator, and
not the fringe-field of the plume, maps along the geomagnetic
field lines and leads to the establishment of the coupling between
the equatorial F and the low-latitude valley region. The observa-
tions of the low-latitude upper-E-valley region irregular struc-
tures are the manifestation of such coupling. In the equatorial
region, on the other hand, the upper-E-valley region is coupled
with the overlying F-region across the geomagnetic field lines, not
along the geomagnetic field, by the fringe field of the plume. The
observed IES on 08 November 2001 in the present study is the
manifestation of such coupling.
3.3. Quasi-periodic striations within IES
As mentioned under point (6) above, on both occasions,
modulation of upper envelope of an IES is observed which
suggests the presence of substructures or quasi-periodic
striations within an IES. The equatorial quasi-periodic (EQP)
striations within an IES is reported from Jicamarca radar
observation (Woodman and Chau, 2001). These EQP striations
were observed with a negative slope having a period of
1:5 min and an altitude spacing of 20 km. On contrary,
the EQP striations in present observation are found to be
vertical oriented (or slightly positive slope) having a period of
10215 min and an altitude spacing of 20240 km. There-
fore, the characteristics of EQP striations observed over S~
ao Luı
´s
are different from EQP striations observed over Jicamarca. It
may be pointed out that S~
ao Luı
´s radar may not able to resolve
the striations with period 1:5 min owing to the radar’s wide
beam width 101and lower temporal resolution. Therefore, it
is possible that striations with period 1:5min are present,
but the present observation may not register them. In terms of
period and altitude spacing, the EQP striations in present
observation are similar to the upper-E-region QP striations
observed over low-latitude/mid-latitude (Yamamoto et al.,
1994;Tsunoda et al., 1998;Choudhary and Mahajan, 1999;
Chau and Woodman, 1999;Yokoyama et al., 2005).
3.4. The occurrence statistics of upper-E-valley region irregular
structures
S~
ao Luı
´s radar is operational since October 2001 and during
October 2001–January 2008, it is operated on 581 nights with the
range coverage of 87.5–1267.5 km (de Paula et al., 2011). Obser-
vations are more or less evenly distributed from 2001 to 2008
with coverage of either all four months or at least two months
within October–January period. IESs were observed on 20 nights
during 581 nights of observations, i.e. occurrence rate of IES is
3.5% revealing the nature of rare occurrence of these structures
over S~
ao Luı
´s. The two examples of IES presented in this study are
indicative of the typical characteristics of IES observed on rest of
18 nights. It may also be pointed out that February–September
20:03
Altitude, km
157.5147.5137.5127.5117.5107.597.587.5
20:05 20:08 20:10 20:13 20:15
−171 0 171
Doppler Velocity, m/s
20:18 20:20 20:23 20:25 20:28 Local Time
Fig. 4. The spectra on 25 October 2001 during the observation of IES. The Doppler velocity range is 7171 m/s.
E. Alam Kherani et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 64–7068
months are excluded from the statistics since never a single IES
event is observed during these months.
It is also found that the occurrence of IES remains confined to
years 2001–2003 and to the months of October–January. In other
words, IESs are observed strictly during years of maximum solar-
flux and during summer months over Brazil. These are years and
season of maximum occurrence of F-region plume over S~
ao Luı
´s
and also years and season of largest upward drift of the ambient
F-region (Batista et al., 1986;de Paula et al., 2011). However,
the occurrence rate of IES is much smaller than the occurrence
rate (95%) of F-region plume, indicating that the occurrence of a
F-region plume and an ascending irregular-bottomtype/bottom-
side-F-layer are necessary but not sufficient conditions for the
occurrence of irregular upper-E-valley region structures.
It may be pointed out that the upper-E-valley region IESs are
observed more frequently at Jicarmaca (Chau and Hysell, 2004)
and less frequently at S~
ao Luı
´s. The low occurrence rate over
S~
ao Luı
´s may be an important clue to understand the genera-
tion mechanism responsible for the upper-E-valley region IESs.
Based on the proposed mechanism by Chau and Hysell (2004),
the low occurrence over S~
ao Luı
´s indicates the less favorable
condition for the interchange instability. The instability relies
on two ambient conditions: the horizontal density gradient and
the vertical current (or ambient vertical/zonal electric fields).
Abdu et al. (2005) have studied the evening-time ionospheric
dynamics over the Brazilian and Peruivian sector. According to
them, the large horizontal density gradient and subsequent
strong ambient electric field are developed when the magnetic-
meridian and the sunset terminator are aligned. Owing to the
large declination over Brazil, this alignment condition is not as
frequent as over Peru, leading to the less frequent occurrence of
strong horizontal gradient and strong electric fields over Brazil
or less frequent favorable condition for the interchange
instability in the upper-E-Valley region. Thus the low occur-
rence of the upper-E-Valley region over Brazil may be explained
consistently on the basis of interchange instability mechanism
proposed by Chau and Hysell (2004).
4. Summary
In this study, characteristics of irregular-equatorial-upper-E-
valley region structures (or IESs) are examined during observa-
tions of the plume, irregular-bottom-F-layer and E-region irregu-
larities of varying nature. Based on observations of IESs on two
nights (08 November 2001; 25 October 2001), it is found that
these structures are observed strictly during the occurrence of an
ascending irregular-bottomtype/bottomside-F-layer. The observa-
tion also presents a case when the IES is not observed during a
descending irregular-bottomside-F-layer, despite the presence of
a F-region plume. The slope of the IES is found to be proportional
to the slope of the ascending irregular-F-layer (or ambient zonal
electric field). Moreover, the height extension of IES is propor-
tionally related to the strength of the ambient zonal electric field.
Similar slope of irregular F layer and IES and the proportional
relation between zonal electric field and the altitude extension of
IES suggest the presence of a dynamical coupling between the
ambient F-region dynamics and the upper-E-valley region.
On one night (08 November 2001), the IES is observed
simultaneously with the F-region plume. On another night (25
October 2001), the IES is observed prior to the F-region plume.
This suggests that the upper-E-valley region IES may occur with
and without the F-region plume. It is also found that when IES is
observed in the presence of the F-region plume, it is extended to
higher heights. This suggests the presence of a dynamical cou-
pling between the F-region plume and the upper-E-valley region.
On both nights, it is found that IESs are observed strictly
during the time when the backscattered intensity corresponding
to E-region irregularities becomes momentarily strong, i.e. when
the ambient vertical electric field in the E-region becomes
momentarily strong. These aspects bring an important character-
istic into consideration that the vertical electric field may also be
contributing to determine the dynamics of the IES.
Observations on both nights demonstrate that IESs are not
originated from the E-region, but from a region near 120–130 km
altitude. The observed spectral characteristics reveal that the
spectra corresponding to the upper-E-valley region IES are less
broadened and more Doppler shifted compared to the spectra
corresponding to E-region irregularities. These aspects suggest
that the irregularities within IES are generated locally in the
upper-E-valley region and they are not a result of convection of
E-region irregularities by the ambient zonal electric field and the
fringe-field of the F-region plume.
Chau and Hysell (2004) have proposed the irregularity gen-
eration mechanism in the upper-E-valley region relies on the
vertical current and horizontal density gradient driven inter-
change instability. In the upper-E-valley region, the vertical
current may be driven by both vertical and zonal electric field.
In the present observation, close association between the upper-
E-valley region IES and ambient zonal/vertical electric field is
noted which favors the mechanism proposed by Chau and Hysell
(2004). The fringe field of the F-region plume may accelerate this
mechanism by providing additional vertical current and horizon-
tal density gradient. It is suggested that the IES on 08 November
may fall on the category where both ambient electric field and
fringe-field are driving interchange instability mechanism in the
upper-E-valley region. On the other hand, 25 October 2001 may
fall on the category where the ambient electric field alone is
driving the interchange instability proposed by Chau and Hysell
(2004).
The quasi-periodic (QP) striations are found to be present
within IESs, having period of 10215 min, vertical spacing of
20240 km and mainly vertical oriented. These characteristics
of QP striations are different from characteristics of QP striations
observed from Jicamarca but are very similar to the character-
istics of upper-E-region QP striations observed over low-latitude/
mid-latitude ionosphere.
The statistics of the occurrence of IESs from S~
ao Luı
´s radar
reveal that the percentage occurrence of these structure is 3.5%
indicating the nature of rare occurrence of these structures over
S~
ao Luı
´s radar. Moreover, their occurrence remains confined to
years of maximum solar-flux (2001–2003) and to the summer
months over Brazil. Based on the interchange instability mechan-
ism proposed by Chau and Hysell (2004), the low occurrence of
the upper-E-Valley region over Brazil may be explained consis-
tently as owing to the large declination angle over S ~
ao Luı
´s
leading to the less frequent alignment of magnetic meridional
and sunset terminator during evening hours. The two examples
presented in this study are representative of all/a majority of the
events captured from S~
ao Luı
´s and therefore the conclusions of
the paper are robust.
Acknowledgements
EAK wish to acknowledge the supports from FAPESP through
the process 07/00104-0. We wish to acknowledge the support
from FAPESP through the processes 99/00026-0, 04/01065-0.
Authors wish to acknowledge Dr. B. Fejer for his valuable com-
ments and encouragements during the accomplishment of this
work.
E. Alam Kherani et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 64 –70 69
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