JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A9, PAGES 19,859-19,869, SEPTEMBER 1, 1999
Effects of the vertical plasma drift velocity on the
generation and evolution of equatorial spread F
B. G. Fejer, and L. Scherliess
Center for Atmospheric and Space Sciences, Utah State University, Logan
E. R. de Paula
Instituto Nacional de Pesquisas Espaciais, Sail Jos• dos Campos, Sail Paulo, Brazil
Abstract. We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study
the effects of the F region vertical plasma drift velocity on the generation and evolution of
equatorial spread F. The dependence of these irregularities on season, solar cycle, and magnetic
activity can be explained as resulting from the corresponding effects on the evening and nighttime
vertical drifts. In the early night sector, the bottomside of the F layer is almost always unstable.
The evolution of the unstable layer is controlled by the history of the vertical drift velocity.
When the drift velocities are large enough, the necessary seeding mechanisms for the generation of
strong spread F always appear to be present. The threshold drift velocity for the generation of
strong early night irregularities increases linearly with solar flux. The geomagnetic control on the
generation of spread F is season, solar cycle, and longitude dependent. These effects can be
explained by the response of the equatorial vertical drift velocities to magnetospheric and
ionospheric disturbance dynamo electric fields. The occurrence of early night spread F decreases
significantly during equinox solar maximum magnetically disturbed conditions due to disturbance
dynamo electric fields which decrease the upward drift velocities near sunset. The generation of
late night spread F requires the reversal of the vertical velocity from downward to upward for
periods longer than about half an hour. These irregularities occur most often at -0400 local time
when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes. The
occurrence of late night spread F is highest near solar minimum and decreases with increasing
solar activity probably due to the large increase of the nighttime downward drifts with increasing
solar flux.
1. Introduction
Plasma irregularities in the nighttime equatorial F region,
generally known as equatorial spread F, have been the subject
of extensive experimental and theoretical investigations over
the last several decades [e.g., Fejer and Kelley, 1980; Kelley,
1989, Fejer, 1996]. These irregularities result from a
hierarchy of multistep nonlocal plasma processes involving
an interchange instability that includes the collisional and
collisionless Rayleigh-Taylor and ExB instabilities and drift
waves driven by coupled electrodynamic and neutral
atmospheric processes [e.g., Haerendel, 1973; Scannapieco
and Ossakow, 1976; Ossakow, 1981; Zalesak et al., 1982;
Keskinen et al., 1998, Sekar and Kelley, 1998; Basu and
Coppi, 1999].
The association of the rapid postsunset rise of the equatorial
F layer with the occurrence of spread F was suggested by the
initial observations of these irregularities [Booker and Wells,
1938]. Several studies have shown that the height of the
nighttime F layer is the single most important parameter
Copyright 1999 by the American Geophysical Union.
Paper number 1999JA900271.
0148-0227/99/1999JA900271 $09.00
controlling the generation of spread F [e.g., Farley et al.,
1970; Ossakow et al., 1979; Rastogi, 1980; Abdu et al., 1983;
Sastri, 1984; Kelley and Maruyama, 1992; Jayachandran et
al., 1993; Sultan, 1996]. This height is determined largely
by the equatorial vertical plasma drift velocity, which is
driven by the zonal electric field. The equatorial zonal electric
field affects the growth rate of the Rayleigh-Taylor instability
through the gravitational and electrodynamic drift terms and
by controlling the electron density gradient in the bottomside
of the F layer after dusk. In addition, the evening eastward
electric field can significantly decrease the E region Pedersen
conductivity [Hanson et al., 1986]. Therefore, as suggested
by Farley et al. [1970], the equatorial postsunset electric field
should play a dominant role on the variability of equatorial
spread F.
Studies of equatorial spread F using the 50-MHz radar at the
Jicamarca Observatory, near Lima, Peru (12øS, 76.9øW, dip
latitude 1 øN), have been carried out since the sixties. Farley et
al. [1970] used routine incoherent scatter observations,
whereas most of the following studies [e.g., Woodman and
LaHoz, 1976; Kelley et al., 1986; Hysell et al., 1990; Farley
and Hysell, 1996] relied on measurement techniques especially
designed for probing these 3-m irregularities. Recently,
Jicamarca spread F studies have used highly improved radar
observational techniques [Hysell and Woodman, 1997; Hysell
and Burcham, 1998; Kudeki et al., 1999; Kudeki and
Bhattacharyya, 1999], and have often been coordinated with
19,859
19,860 FEJER ET AL.: VERTICAL PLASMA DRIFT VELOCITY AND SPREAD F
optical and scintillation measurements [e.g., Basu et al.,
1996].
The morphology of equatorial F region plasma drifts has
also been extensively studied using Jicamarca radar
measurements [e.g., Fejer, 1991, 1997]. These observations
have determined the dependence of the F region plasma drifts
on season, solar cycle, and magnetic activity. Maruyama
[1988] showed that the effect of the vertical drift velocity on
the growth rate of the Rayleigh-Taylor instability can explain
the winter-summer asymmetry of spread F occurrence over
Jicamarca during solar maximum. Kelley and Maruyama
[1992] presented case studies of storm-time electric field
effects on the generation of equatorial spread F over Jicamarca
during postmidnight hours and tested the assumption that the
penetration of eastward plasmaspheric electric fields initiates
the generation of these irregularities. However, prompt
penetration electric fields alone cannot explain the complex
dependence of equatorial spread F on magnetic activity [e.g.,
Fejer, 1996]. In addition, the equatorial vertical drift velocity
is strongly affected by both prompt penetration and
disturbance dynamo electric fields, and the characteristics of
the disturbance electric fields change considerably from
premidnight to postmidnight hours [Scherliess and Fejer,
1997; Fejer and Scherliess, 1997].
In this study, we use extensive F region incoherent scatter
radar measurements over Jicamarca to examine the effects of
quiet-time and disturbance vertical plasma drifts on the
generation and evolution of 3-m equatorial F region
irregularities for different levels of solar activity. These
measurements describe the ambient F region conditions prior
to and after the onset of spread F and the evolution of the
unstable layers. We will show that our results strongly
suggest that the day-to-day, seasonal, and solar cycle
variability of spread F occurrence over Jicamarca, and its
dependence on magnetic activity can be explained as due
mostly to the corresponding variability of the equatorial
vertical plasma drifts.
temperature measurements. In this case, electron density
profiles were derived from the backscattered power and Faraday
rotation, and spread F altitudinal profiles were obtained using
the modified range-time-intensity (MRTI) technique
[Woodman and LaHoz, 1976]. Plasma density, vertical drift
velocity, and spread F data obtained using this technique were
presented in a number of studies [Woodman and LaHoz, 1976;
Kelley and Maruyama, 1992]. Our most detailed data were
obtained from 1980 to March 1992 during a large number of
Word Day campaigns, which at Jicamarca were dedicated
mostly to drift observations. This database consists of
altitudinal profiles, usually from 200-600 km, of F region
plasma drifts, backscattered power, and pulse-to-pulse
correlation normalized to the signal power [e.g., Farley,
1971] for a typical time delay of 5-8 ms. The typical
integration time was a few minutes. In the unstable region,
these measurements describe the strength and the mean
velocity of the plasma irregularities.
The backscattered power from spread F irregularities varies
significantly with the level of development of the
inhomogeneities, local time, season, solar activity, and with
the transmitted power and pulse length. On the other hand, the
correlation values from F region irregularities are always
noticeably larger than from thermal fluctuations in the
incoherent scatter regions, independent from the experimental
parameters and geophysical conditions. Therefore it is more
practical to determine the height range of the unstable regions
using these correlation values. We have used correlation
values larger than 0.3 to define the altitudinal range of spread
F in the 1980-1992 data. Essentially identical results were
obtained for the unstable layer using somewhat larger
correlation values or the backscattered power profiles. It is
important to note, however, that the relatively large pulse
lengths used in the drift experiments result in an overestimate
of the vertical thickness of the unstable layers by typically
-15-20 km.
2. Measurement Techniques and Data
The results to be presented in this study were obtained using
the Jicamarca radar primarily for routine evening and
nighttime F region plasma drift measurements. Our database
consists of over 200 evening and nighttime periods from
April 1968 to March 1992. The experimental procedure for
measuring plasma drifts with the Jicamarca radar was described
by Woodman [1970]. These observations were typically made
over an altitudinal range of 200-600 km, with an altitudinal
resolution of 25-40 km, and with an integration time of-5
min. The plasma drifts used in our studies correspond to
average values generally between-300-400 km, where the
signal-to-noise ratios are greatest. The accuracy of the
vertical drift measurements is typically -1-2 m/s.
The information on the F region plasma inhomogeneities
observed by the Jicamarca radar was obtained using three
different sources. From 1968 to 1979, the presence of spread
F was determined mostly from the vertical plasma drift
velocity records, as done previously by Farley et al. [1970],
who used the 1968-1969 data. From 1969 to 1972, on several
occasions, the radar was operated with one half of the antenna
pointed perpendicular to the Earth's magnetic field for vertical
drift and spread F observations, and the other half was pointed
3 ø away from perpendicularity for plasma density and
3. Results and Discussion
In this section, we describe the effects of the vertical
plasma drift velocity on the generation and evolution of F
region coherent radar backscatter layers over Jicamarca and
their dependence on solar and magnetic activity. Since these
effects change significantly from early to late night hours, we
will consider these two cases separately.
3.1. Spread F During Premidnight Hours
Figure 1 shows typical examples of the temporal variation
of the vertical drift velocity (positive upward), height of
maximum backscattered power from thermal fluctuations
(Hmp), and spread F layers during equinox low solar flux
conditions. The decimetric solar flux indices were about 75 (in
units of 10 -22 Wm -2 Hz-1), and the average Kp indices were
-1.5. Since the received radar power is inversely proportional
to the square of the backscattering altitude, the heights of
maximum power are slightly lower than the electron density
peak heights. When spread F occurs over a large range of
altitudes sampled by the radar, the incoherent scatter technique
cannot be used for measuring the ambient plasma drift
velocity. Figure 1 illustrates the control of the vertical drift
velocity on the height of the F layer after-1800 LT, and on
the generation of spread F echoes. The onset of these
FF_JER ET AL.: VERTICAL PLASMA DRIFF VELOCITY AND SPREAD F 19,861
J I CAMARCA
I I I I I I I
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16 18 20 22 24
LOCAL TI ME
Figure 1. Vertical drift velocities (circles), heights of
maximum backscattered power (x), and spread F scattering
layers (vertical lines) during March 1985. The maximum
height probed by the radar was 650 km.
80
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E 80
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Figure 2. Same as Figure 1, but for September 1987. The
maximum height probed by the radar was 550 km.
irregularities generally occurs close to the time of reversal of
the evening drift velocity when the F layer reaches its highest
altitude and the growth rate of the Rayleigh-Taylor instability
is maximum. For a given season and solar flux value, larger
evening drift velocities lead to the generation of wider and
longer lasting regions of strong echoes, which often give rise
to radar plumes. The power backscattered from relatively
narrow unstable regions below the F region peak, following
small evening upward drifts, are significantly smaller (by tens
of decibels) than from higher altitude wider scattering layers
and radar plumes, which is consistent with results from earlier
studies [e.g., Woodman and LaHoz, 1976; Hysell and
Burcham, 1998].
Figure 2 presents another set of solar minimum equinoctial
radar data. In this case, the daily decimetric solar flux indices
were-80, and the average Kp values were 2.9, 2.9, and 4.5,
respectively. Figure 2 illustrates again the occurrence of broad
unstable regions, often covering the entire range of
observations, after large upward evening drifts, and the
generation of a shorter lived and narrower low altitude unstable
layer following smaller upward evening drift velocities. The
thickness of thin and low-altitude spread F layers is
overestimated in these data owing to the relatively large radar
pulse lengths used in the drift experiments. These weak low-
altitude scattering layers are not easily detected using other
ground-based techniques. The third panel illustrates the
absence of unstable layers associated with an early downward
reversal of the vertical drift velocity. The results above
support the conclusions of Basu et al. [1996], who suggested
that postsunset upward drift velocity enhancements of the
order of 10-20 m/s are necessary for the occurrence of spread F
over Peru during equinox solar minimum.
The evening upward drift velocities and the early night F
region peak heights increase significantly from solar
minimum to solar maximum. Figure 3 shows measurements
from equinoctial periods when the solar flux indices were 24 2,
275, 265, and 204, and the average Kp values were 1.2, 1.2,
3.1, and 2.2, respectively. The two upper panels show
relatively broad spread F layers of strong radar backscatter
signals, which occasionally reached the highest altitudes
sampled by the radar, following very large upward drifts and
layer heights. The third panel show a very short lived and
very weak (only -3 dB stronger than the incoherent scatter
signals) at-2100 local time, and the fourth panel show no
irregularities in spite of the relatively large values of the
evening upward drifts.
The results presented above highlight the importance of the
evening upward drift velocities on the generation of spread F.
However, the evolution of the unstable layer can be strongly
affected by the drift velocities also after their evening
reversal. Figure 4 shows an example of a rapid downward
motion and weakening of an unstable layer due to unusually
large downward drift velocities driven by westward disturbance
electric fields.
We have examined the relationship of the amplitudes of the
peak prereversal velocity enhancements and the occurrence of
weak and strong spread F echoes in the premidnight sector.
Since the backscattered power from the irregularity regions
changes noticeably with solar flux due to the change in the
ambient plasma density and also with the transmitted power
19,862 FFJER ET AL.: VERTICAL PLASMA DRIFF VELOCITY AND SPREAD F
J I CAMARCA
'• III SpF • 14 MARCH 1991 -
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LOCAL TIME
Figure 3. Examples of vertical drift velocities, heights of
maximum backscattered power, and spread F scattering layers
during equinox solar maximum conditions. The maximum
height probed by the radar was 650 km.
and pulse length, we classified the spread F events in terms of
the thickness of the unstable layers. Strong radar echoes are
always associated with relatively wide scattering layers at
higher altitudes, whereas weak echoes correspond to lower
altitude and narrow layers [e.g., Woodman and LaHoz, 1976;
Hysell and Burchain, 1998]. .For the 1980-1992 data, we
consider spread F to be strong whe.!! the unstable layers cover
J I CAMARCA
E 80 III SpF 22 FEBRUARY 1990 800
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16 18 20 22 24
LOCAL TIME
Figure 4. Vertical drift velocities, heights of maximum
power, and spread F scattering layer during a disturbed solar
maximum period. The Kp values were 5-, 5-, and 3-, and the
decimetric solar flux index was 216.
altitudinal ranges in excess of 100 km, which occasionally
extend over most of the altitudinal range sampled by the radar.
The irregularities in these layers have relatively large eastward
drift velocities indicative of dominant F region dynamo
effects. These layers correspond essentially to the bottomside
layers and plumes studied in detail by Hysell and Burchain
[1998]. With this criterion, Figures 1-4 show strong spread F
events during March 20, 1985, September 23, 1987, and
March 14 and 18, 1991. We identify weak spread F with lower
altitude and narrower (thickness smaller than ~100 km)
scattering layers. The irregularities in these layers have either
westward or relatively small eastward drift velocities
(indicative of relatively strong E region dynamo, or of
disturbance dynamo effects) and therefore have generally the
characteristics of bottomtype irregularities [Hysell and
Burchain, 1998]. For most of the observations before 1980,
we do not have detailed information on the strength and/or
altitudinal range of spread F echoes. In this case, following
Farley et al. [1970], we consider that strong spread F occurs
when incoherent scatter drift measurements are not possible
owing to these coherent echoes and that normal ambient drift
measurements in the presence of coherent echoes are
indicative of weak irregularities.
Figure 5 shows the relationship of the amplitudes of the
prereversal velocity enhancement and the occurrence of spread
F. The negative values correspond to downward drifts at the
typical times of the prereversal peaks. The evening and early
night drift velocities are highly variable near solar minimum.
In this case, weak spread F can occur even on nights when the
drift velocities are downward near dusk provided that they
reverse later to upward •lrifts for periods longer than about half
an hour. Figure 5 shows that equatorial spread F is almost
always observed near dusk at all seasons and levels of solar
activity and the occurrence of strong scattering layers when
the solar cycle dependent evening drift velocities are large
enough. This suggests that the seeding mechanisms necessary
for the generation of strong irregularities are present
essentially all the time. Kudeki and Bhattacharyya [ 1999]
showed that the prereversal velocity enhancement constitutes
an important seeding mechanism for the generation of F
region irregularities, but there are also other seeding
mechanisms such as gravity waves. For a given season and
solar flux level, strong echoes are generally associated with
larger drift velocities. Over Jicamarca, strong spread F is
common between September and April and is rare near June
solstice. As the solar flux index increases from 70 to 250, the
threshold velocity for the generation of strong echoes
increases from ~5-10 m/s to ~50-60 m/s. This result is
consistent with HF Doppler radar and ionosonde measurements
from India which showed that the threshold vertical drifts
decreased from ~30 to ~20 m/s as the mean decimetric solar
flux index changed from 120 to 70 [Jayachandran et al.,
1993].
Figure 6 shows the prereversal velocity enhancements and
the occurrence of weak and strong spread F following at least 6
hours of magnetically quiet and disturbed conditions. We do
not present the results for June solstice since the occurrence of
weak spread F does not change much from quiet to disturbed
periods, and strong echoes are rarely seen during that season.
Figure 6 indicates that during equinox and December solstice
quiet time conditions, the F region over Jicamarca is highly
unstable during the early night period, with the frequent
occurrence of strong spread F. The variability of the
FEJER ET AL.' VERTICAL PLASMA DRIFF VELOCITY AND SPREAD F 19,863
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J I CAMARCA 1968'1992
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Figure 5. Seasonal and solar cycle dependence
postsunset maximum upward drifts and spread F occurrence.
of
250 300
prereversal velocity enhancements generally decreases with
solar activity. Figure 6 shows that sometimes the points of
strong and moderate/weak spread F are very close. This is
partly due to the fact that the prereversal drift peak does not
fully account for the time dependent effect of the vertical drift
velocity. In addition, the distinction between moderate and
strong spread F is sometimes somewhat subjective. The
results for magnetically disturbed periods are discussed below.
The role of the vertical plasma drifts on the generation and
evolution of these irregularities can be determined from their
effects on the time integrated growth rate of the Rayleigh-
Taylor instability [e.g., Maruyanta, 1988; Sultan, 1996]. We
can estimate these effects by examining the evening and
nighttime Jicamarca average quiet time vertical drifts
presented in Figure 7. These low and high solar flux curves
were obtained by averaging the quiet drift values for Sa < 80,
and Sa > 150, respectively. In this case, the average low solar
flux indices were ~75; the high solar flux values were ~195 for
June solstice and equinox, and 174 for December solstice. The
evening upward and the nighttime downward drifts have small
average values and are also highly variable for all seasons near
solar minimum [e.g., Fejer, 1981]. In this case, spread F can
be easily excited whenever the drift velocities after sunset are
upward for periods longer than about half an hour due to either
their quiet time variability or to upward disturbance drifts.
This is consistent with the occasional occurrence of weak
irregularities even when evening drifts are initially downward
(see Figure 5) and also with the large range of the onset times
of strong spread F near solar minimum [e.g., Hysell and
Burchant, 1998].
The evening upward drifts and layer heights increase
noticeably with solar activity, but so do the nighttime
downward drifts, the ion-neutral collision frequency, and the F
layer Pedersen conductivity. Near solar maximum, when the
evening upward drifts are large enough, spread F onset
typically occurs again near the time of the drift reversal. In
this case, however, large downward drifts after sunset rapidly
decrease the layer height and the instability growth rates
during equinox and June solstice. Therefore the evolution of
spread F during these seasons is a race against time where the
kilometer scale irregularities in the low-altitude layers need to
grow in amplitude, reach the strongly nonlinear regime, and
rise into the topside F layer under the effect of internal
polarization electric fields, before they are damped as a result
of the large downward drifts. Figures 5 and 6 show that the
equinoctial upward drifts are generally large enough for
exciting strong spread F. On the other hand, the small upward
drifts and early reversal times during June solstice do not allow
for enough wave growth prior to the damping due to the large
downward drifts so that the narrow low-altitude unstable layers
rarely develop into wide high-altitude strong scattering layers.
During December solstice, the large evening upward drifts, the
late reversal time, and the small early night downward drifts
provide the most favorable conditions for the generation of
fully developed strong and longer lasting scattering layers
over Jicamarca, as shown earlier by Maruyama [1988].
However, when there are unusually large downward drifts in the
early night period (due to westward prompt penetration electric
fields, for example) during this season, the unstable layers
rapidly weaken and disappear, as illustrated in Figure 4. The
results above are fully consistent with the frequency of
occurrence of spread F irregularities observed over Jicamarca.
The effect of geomagnetic activity on F region irregularities
can be directly related to the response of the equatorial zonal
electric field to these disturbances. Several studies have
shown that the onset of magnetic activity in the late afternoon
increases the occurrence of spread F, amplitude scintillations
of VHF and UHF beacons, and large-scale plasma depletions i n
the premidnight sector, while the onset of strong magnetic
activity near the noon sector decreases the occurrence of these
irregularities and depletions [e.g., Aarons, 1991; Abdu et al.,
1995; Sobral et al., 1997; Sahai et al., 1998]. These effects
can be explained as due to eastward prompt penetration and
westward ionospheric disturbance dynamo electric fields,
respectively. The large prereversal velocity enhancement and
strong spread F during June solstice (solar flux index of ~ 170)
shown in Figure 5, for example, was associated with a large
increase in high-latitude convection near dusk.