NmF2 VARIABILITY at Equatorial and Low Latitude Stations: A Review
The relative variability of peak electron density, NmF2 at equatorial and low latitude stations has been studied fairly well by workers in the past decade. A review of these studies is carried out in this work. The various methods used by workers, their merits and demerits are discussed. The results with respect to diurnal influence, seasonal effect and influence of solar cycle epochs, latitudinal dependence and effect of geomagnetic activity are synthesised and discussed. Areas requiring further investigation are mentioned.
February 28, 2010; Accepted: April 08, 2010;
Published: July 17, 2010
It is well known that there is a lot of variations in the ionosphere due to
the effect of solar, meteorological and geomagnetic activities. Apart from solar
cycle variation, seasonal effect, latitudinal dependence, there are day-to-day
and hour-to-hour variations. The variations concentrated upon most by researchers
is the day-to-day variation. This is because day-to-day variation or variability
is greater than other variations. Thus an understanding of day-to-day variation
will help radio scientists and engineers to know to a large extent the expected
deviations from monthly mean values. Bilitza (2000)
pointed out the need for the variability of mean values of ionospheric parameters
of which the electron density peak (NmF2) is the most frequently studied being
the most easily accessible characteristics of the ionosphere (Mansilla
et al., 2004).
The subject of this review paper is variability of equatorial and low latitude NmF2. In this study the different methods used in describing variability or VR is discussed in section 2.0. Results obtained by various workers are then reviewed with respect to diurnal, seasonal, solar cycle, latitudinal and geomagnetic influence.
This review is suppose to complement the efforts of the International Reference Ionosphere (IRI), an international project sponsored by the Committee on Space Research (COSPAR) and International Union of Radio Science (URSI) to model the Ionosphere.
MATERIALS AND METHODS
Three major methods used to determine ionospheric VR are:
||The percentage ratio of standard deviation, σ, to mean,
of ionospheric parameters, i.e., VR= σ/x100
(Bilitza et al., 2004; Somoye,
2009a, b; Rishbeth and Mendillo,
2001; Akala et al., 2010a)
||The ratio of interquatile range (q75-q25)
to median, M i.e.,
This method was employed by Kouris and Fotiadis (2002)
||The ratio of interdecile range (q90-q10)
to median, M i.e.,
The merit of the methods in (ii) and (iii) is that of easy interpretation of
probability since the interquatile range covers 50% of all data points (Bilitza
et al., 2004) and by extension the interdecile range covers 80% of
all data points.
Their demerit is that 50 and 20% of the data is ignored in the second and third
method, respectively. This is not the case in the use of standard deviation
whose demerit is that of interpretation in terms of probability. Another method
is that of ratio of monthly range to monthly median at each hour. Kouris
and Fotiadis (2002) used ratio of the difference of hourly daily values
from monthly median to hourly monthly median.
It should be mentioned that the foF2 and the corresponding NmF2 values used
by authors are obtained mostly from Union Radio Mark 2 recorder type or DPS-4
digisonde. A Union Radio Mark 2 recorder type ionosonde has been described by
Somoye (2009a) while the description of a typical digisonde
is given by Huang and Reinisch (1996).
NmF2/foF2 VR Results
Results of different aspects of NmF2/foF2 are given below:
It should be mentioned that results of NmF2 VR and foF2 VR are used interchangeably
just as have been done by many workers (Zhang et al.,
2004; Abdu et al., 2004; Mansilla
et al., 2004), the latter being proportional to the square of the
former. This implies that foF2 corresponds to NmF2 (Abdu
et al., 2004). The correspondence between the two parameters is clear
from Fig. 1a and b which shows relative
VR of NmF2 and foF2-obtained from the ratio of their standard deviation to mean-
for both low and high sunspot number periods. The similarity in the plots during
both low and high sunspot number periods justifies the use of one for the other
Diurnal NmF2/foF2 Equatorial VR
The day-to-day variability of NmF2 at equatorial and low latitude stations
is high during the night, i.e., between sunset and sunrise with the major peaks
about an hour pre-sunrise (Bilitza et al., 2004;
Zhang et al., 2004; Araujo-Pradere
et al., 2005; Somoye, 2009b; Akala
et al., 2010a).
|| Relative variability of NmF2 and foF2 during a 1958 and b
||Annual NmF2 relative variability of different LT-bins for
1965 and 1973, respectively. (Somoye, 2009b)
NmF2 relative VR is reported to be between 5-15% during the daytime by these
authors. Occasionally, however, daytime VR is higher than 15% during the Winter
of periods of low solar activity and lower than 5% during September equinox
of 1973 (Fig. 2a, b).
NmF2/foF2 nighttime relative VR is found higher than daytime relative VR being
12-45%. Bilitza et al. (2004) observed nighttime
relative VR of 15-30% with post-midnight peak and post-sunset peak of 30-40
and 15-30%, respectively for Ouagadougou (12°N, 1.8°W, 15.9 dip),
Zhang et al. (2004), working at Hainan (dip 22.8°N), China, obtained
12-40% nighttime variability. Nighttime VR is found to be 18-45% from the diurnal
curve by Somoye (2009a, b) at
Ibadan (7.4°N, 3.9°E, 6°S dip). Akala et
al. (2010b) reported nighttime VR of 15-35% with post midnight peaks
of 35-55% and a post sunset of 18-38%. The two peaks of relative VR reported
by workers fall between the hour 0200 to 0500 and 2000 to 2300 h with the former,
in most cases greater than the latter.
Nighttime equatorial and low latitude NmF2/foF2 relative VR is greater than
that of daytime partly because of the low values of NmF2 mean values at night
(Bilitza et al., 2004). Another reason given
in literature is the decay in NmF2 mean values which not only leads to reduction
but also to irregularity in the magnitude of NmF2 (Somoye,
2009b). The irregularity in the decay of NmF2 (Rishbeth
and Garriott, 1969) leads to variability. Figure 3 which
illustrates the absolute variability (standard deviation) of NmF2 for the March
equinox of 1958 shows that absolute VR is high during nighttime than daytime.
Seasonal NmF2/foF2 VR
Daytime NmF2/foF2 does not show much of seasonal variation except for stations
located in the neighbourhood of the crest of equatorial anomaly i.e., ±20°
latitude. For such stations, June solstice or summer and the equinox have higher
VR of about 5% than December solstice or winter (Bilitza
et al., 2004; Somoye, 2009a). This difference
may be latitudinal. Zhang et al. (2004), however,
working at Hainan (dip 22.8°N) in the neighbourhood of the equatorial anomaly
maximum reported daytime NmF2 VR of less than 15% for Summer, Winter and the
equinox with lowest values at the equinox. The difference in the result of Zhang
et al. (2004) and those of Bilitza et al.
(2004), Somoye (2009a) and Akala
et al. (2010a) may be due to longitudinal effect. The longitude of
Hainan is 109°E while those of Ouagadougou and Ibadan are 1.8°W and
3.9°E, respectively. Nighttime VR of NmF2/foF2 is found higher in June solstice
and September equinox than the other two seasons by Akala
et al. (2010a) which is the same as reported by Bilitza
et al. (2004).
||Diurnal NmF2 (absolute variability) SD for March Equinox.
Somoye (2009b) also got the same result for NmF2 VR
of 1958, a year of high sunspot number. He reported that NmF2 VR of 1965 and
1973, years of moderate and high sunspot numbers, respectively are greater at
the equinox. The least nighttime NmF2 VR at summer observed by Zhang
et al. (2004) is possibly due to the same longitudinal difference
in daytime VR.
Comparison of Low Latitude Stations with Mid Latitude Stations
Comparison of NmF2/foF2 relative VR at low latitude stations and mid latitude
stations indicates that relative VR is higher at equatorial and low latitude
stations than at mid- latitude stations. Rishbeth and Mendillo
(2001), Araujo-Pradere et al. (2004) and
Rawer et al. (2003) observed higher relative
VR at low latitude stations than at mid- latitude stations. Somoye
(2009b) compared the relative VR of Ibadan and Singapore, two low latitude
stations with that of Slough-a mid- latitude station. He obtained results which
show that relative NmF2 VR is higher at the low latitude station at night during
both periods of low and high solar activity. No latitudinal difference is observed
by him in daytime relative VR. During high solar activity, Somoye
(2009b) reported that daytime relative VR is higher at Slough than the other
two stations. Rishbeth and Mendillo (2001) pointed out
that variability is smaller at most mid-latitude stations than equatorial stations.
Latitudinal dependence of foF2 VR is also reported by Ezquer
et al. (2004).
Solar Cycle Effect of VR
Solar cycle effect of daytime relative VR is not significant at equatorial
and low latitude areas. Diurnal plots of relative VR at Ibadan (7.4°N, 3.9°E,
6°S dip) during 1958 (high sunspot number period), 1965 (low sunspot number
period) and 1973 (moderate sunspot number period) by Somoye
(2009b) shows that daytime relative VR is less than 15% during these epochs
of solar cycle. Comparison of diurnal plots of relative VR of the month of July
for Ibadan (7.4°N, 3.9°E, 6°S dip), Singapore (1.3°N, 103.8°E,
17.6°S dip) and Slough (51.5°N, 359.4°E, 66.5°S dip) during
1970 (high sunspot number year, Rz) and 1971 (moderate sunspot year) by him
reveals that while daytime VR of the other two latitude stations is similar
during both years, it is not the same for Slough.
Generally nighttime VR is higher during periods of low solar activity than
periods of high solar activity though this is not strictly a low latitude feature
(Ezquer et al., 2004; Bilitza
et al., 2004; Kouris and Fotiadis, 2002;
Rishbeth and Mendillo, 2001; Aravinda
and Iyer, 1990; Jayachandran et al., 1995;
Mosert et al., 2003). From the diurnal plots of
relative VR by Bilitza et al. (2004) and Akala
et al. (2010a) for Ouagadougou, post midnight VR peak of 40% during
1978 obtained by Akala et al. (2010b) is slightly
higher than the post midnight VR peak of 38% during 1985 observed by Bilitza
et al. (2004). While 1978 is a high sunspot period (Rz = 93), 1985
is a low sunspot period (Rz = 18). Somoye (2009a) obtained
results that shows that nighttime VR of NmF2 during 1973, a period of moderate
sunspot number (Rz = 38) is sometimes higher than that of 1965 (low sunspot
number period). He also used the plot of correlation coefficient, r, between
NmF2 and Rz to show that relative VR of NmF2 is inversely proportional to Rz
at some hours of the night while at other hours it is directly proportional
with Rz. More work needs to be done by workers to ascertain the increase or
decrease of NmF2 relative VR with sunspot number, Rz, at different times of
Effect of Geomagnetic Activity on Equatorial/Low Latitude VR
Variability is reported to increase during geomagnetic storms at all latitudes
(Mansilla et al., 2004; Araujo-Pradere
et al., 2004, 2005; Rishbeth
and Mendillo, 2001). Araujo-Pradere et al. (2005)
mentioned that variability tends to increase with geomagnetic activity in Winter
and equinox but remain fairly constant in Summer at low latitudes. Araujo-Pradere
et al. (2005) reported that VR at low latitudes during equinox is
as low as 10% because of weak inter hemispheric flow at this season of year.
Araujo-Pradere et al. (2004) and Akala
et al. (2010b), however, observed similar high variability during
both perturbed and unperturbed conditions at low latitude areas. The variability
of foF2 is observed to show post-midnight and post-sunset peaks during six different
geomagnetic storms of 1989 except one (that of 20-22 October,1989) studied by
Akala et al. (2010b) which is a more or less
similar result to that of unperturbed condition(Akala et
al., 2010a). In the day-to-day variability, the contribution of geomagnetic
and meteorological sources are said to be greater than that of solar ionising
flux during the day and the night (Forbes et al.,
2000; Rishbeth and Mendillo, 2001). Of about 20%
VR during daytime 13 and 15% are ascribed to geomagnetic and meteorological,
respectively by Rishbeth and Mendillo (2001) since the
sum of the squares of these latter values makes the total relative VR to be
about 20%. These values are for mid-latitude stations. For low latitude stations,
contributions from geomagnetic and meteorological sources may be greater since
VR is greater at the low latitude than at mid latitude. Effect of geomagnetic
activity on NmF2/foF2 needs to be investigated further as reports of few workers
are available in this regard.
The review of NmF2 relative variability at equatorial and low latitude stations has been carried out. The various methods of obtaining relative variability is described. NmF2/foF2 relative VR is found (1) generally greater during the night than during the day. (2) by some workers to increase with decrease in Rz in the night and to decrease with increase in Rz during the day. Some others, however, found VR alternating between increasing and decreasing with Rz during the day and during the night. More work needs to be done to ascertain the true position. (3) greater at equatorial and low-latitude station than at mid-latitude stations. The reverse is the case during the day. (4) dependent on geomagnetic activity. (5) to show little or no seasonal variation during the day except for stations located around the equator anomaly crest. Nighttime NmF2/foF2 VR is reportedly higher during June Solstice and September equinox.
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