2007
www.ann-geophys.net/25/483/2007/
© European Geosciences Union 2007
Annales
Geophysicae
Synchronous Nm</i>F2 and Nm</i>E daytime variations as a key to the
mechanism of quiet-time F2-layer disturbances
A. V. Mikhailov, V. H. Depuev, and A. H. Depueva
Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Troitsk, Moscow Region 142190, Russia
Received: 28 August 2006 Revised: 13 November 2006 Accepted: 15 February 2007 Published: 8 March 2007
Abstract.
The observed Nm</i>F2 and Nm</i>E variations were
analyzed for the periods of positive and negative quiet-time
F2-layer disturbances (Q-disturbances) observed in the mid-
latitude daytime F2-layer to specify the mechanism of their
origin. The noontime Nm</i>F2 and Nm</i>E deviations demon-
strate a synchronous type of variation which can be explained
by vertical gas motion in the thermosphere. This neutral gas
motion should result in atomic abundance variations, the lat-
ter being conrmed by the Millstone Hill ISR observations
for periods of positive and negative Q-disturbance events.
The analysis of the ISR data has shown that atomic oxygen
concentration variations are the main cause of the daytime
F2-layer Q-disturbances. The auroral heating which controls
the poleward thermospheric wind is considered to be the ba-
sic mechanism for the Q-disturbances, however, the specic
mechanisms of positive and negative Q-disturbances are dif-
ferent. Some morphological features of the Q-disturbances
revealed earlier are explained in the scope of the proposed
concept.
Keywords.
Ionosphere
(Ionosphere-atmosphere
interac-
tions; Ionospheric disturbances) Atmospheric composition
and structure (Thermosphere composition and chemistry)
1
Introduction
Our earlier morphological analysis (Mikhailov et al., 2004)
of the Nm</i>F2 quiet-time disturbances (Q-disturbances) has re-
vealed many interesting features in their occurrence. Positive
and negative Q-disturbances exhibit different morphologi-
cal patterns in diurnal, seasonal, and spatial variations and
this implies different mechanisms of their formation. Quiet
time F2-layer disturbances are closely related to the prob-
lem of the coupling from below: the so-called meteorolog-
Correspondence to: A. V. Mikhailov
(avm71@orc.ru)
ical control of the Earths ionosphere (e.g. Danilov, 1986;
Danilov et al., 1987; Khachikjan, 1987; Kazimirovsky and
Kokourov, 1991; Forbes et al., 2000; Rishbeth and Mendillo,
2001; Kazimirovsky et al., 2003; Lastovicka et al., 2003;
Vanina and Danilov, 2005). Quasi 2-day oscillations in the
ionosphere (Chen, 1992; Apostolov et al., 1995; Forbes and
Zhang, 1997; Forbes et al., 2000; Altadill and Apostolov,
2001; Rishbeth and Mendillo, 2001), which are seen in the
Q-disturbance occurrence, may also be attributed to the me-
teorological effects in the F2 region as they are not related to
geomagnetic activity. On the other hand, neither one can ex-
clude the high-latitude impact on the global circulation and
thermospheric composition. Goncharenko et al. (2006), for
instance, revealed a pronounced negative Q-disturbance ef-
fect using the Millstone Hill ISR and TIMED observations
and attributed it to IMF B
y
variations.
Very fruitful WINDII/UARS optical observations (Shep-
herd et al., 1999, 2002, 2004; Ward et al., 1997; Wang et
al., 2002) of the atomic oxygen and wind velocity variations
at E-region heights may be useful for studying the mech-
anism of the F2-region Q-disturbances formation, because
both ionospheric regions are related via thermospheric neu-
tral composition.
We will discuss here the physical interpretation of the Q-
disturbance morphology. We start with the daytime con-
ditions when the revealed seasonal variations are well pro-
nounced and the difference in the morphological pattern of
the two types of Q-disturbances is obvious (Mikhailov et al.,
2004). The formation mechanism of the mid-latitude day-
time F2-layer is well established and this should help us nd
out the pertinent aeronomic parameters and the processes re-
sponsible for the observed variations. The possibility to use
Nm</i>E variations which (along with the Nm</i>F2 data) may help
in understanding the phenomenon is an additional argument
in favor of considering the daytime conditions. So the aim
of the paper may be specied in the following way: to an-
alyze the observed Nm</i>F2 and Nm</i>E changes for the periods
Published by Copernicus GmbH on behalf of the European Geosciences Union. 484
A. V. Mikhailov et al.: Synchronous Nm</i>F2 and Nm</i>E daytime variations
Table 1.
List of stations used in the analysis, geodetic coordinates and invariant latitudes of the stations are given.
Station
Lat
Lon
Inv. Lat
Station
Lat
Lon
Inv. Lat
Kiruna
67.8
20.4
64.4
Slough
51.5 0.6
49.8
Loparskaya
68.2
33.1
64.0
Ekaterinburg
56.7
61.1
51.4
Sodankyla
67.4
26.6
63.6
Kaliningrad
54.7
20.6
51.2
Lycksele
64.7
18.8
61.5
Moscow
55.5
37.3
50.8
Arkhangelsk
64.6
40.5
60.1
Kiev
50.7
30.3
46.5
Uppsala
59.8
17.6
56.6
Lannion
48.4 3.3
47.0
St. Petersburg
59.9
30.7
55.9
Poitiers
46.6
0.3
45.1
Gorky
56.1
44.3
51.4
Rostov
47.2
39.7
42.3
Table 2.
Average Nm</i>E ±SD value, along with the Student t-parameter, and the correlation coefcient between Nm</i>F2 and Nm</i>E, along
with the Fisher F-parameter for negative and positive F2-layer Q-disturbances.
Disturbance
Average ± SD
t-parameter
Corr. coeff
F-parameter
Negative
0.947±0.067
5.95
0.26
3.98
Positive
1.064±0.060
10.66
0.22
2.59
of F2-layer Q-disturbances, to make quantitative estimates of
the governing aeronomic parameter variations, and to discuss
possible processes which could provide such variations.
2
Data analysis
The analysis was made using the daytime (11:0014:00 LT)
fo</i>F2 and fo</i>E observations available for the periods of posi-
tive and negative F2-layer Q-disturbances. The list of the Eu-
ropean ionosonde stations used is given in Table 1. One can
nd the information on the process of the Q-disturbances ex-
traction from the routine Nm</i>F2 observations in Mikhailov et
al. (2004). Here we repeat briey for the sake of convenience
the main idea. The (Nm</i>F2/Nm</i>F2
med
1)×100% hourly de-
viations exceeding 40% are considered as a Q-disturbance,
if all 3-h a
p
indices were 7 for the preceding 24 h. The
27-day Nm</i>F2 running median centered to the day in ques-
tion rather than the usual monthly median is used for the
Q-disturbances specication. Only long lasting, 3-h (4
successive hourly Nm</i>F2 values), disturbances are used in
our analysis. The same procedure is applied to the Nm</i>E
hourly variations, but the priority is given to the Nm</i>F2 dis-
turbances, that is, we select positive and negative F2-layer Q-
disturbances and take the corresponding Nm</i>E deviations as
they are. The deviations =Nm/Nm
med
for Nm</i>F2 and Nm</i>E,
averaged over the 11:0014:00 LT interval, are considered in
our analysis.
Mikhailov et al. (2004) showed earlier that the daytime
Q-disturbances were relatively rare in occurence, so we had
to put together the data for all levels of solar activity to in-
crease the statistics. The negative Q-disturbances are less
frequent as compared to the positive ones, but they exhibit
a pronounced seasonal variation pattern in the wide latitu-
dinal range, with the maximal occurrence around the winter
solstice (DecemberJanuary). The seasonal variation pattern
for positive Q-disturbances depends on latitude, but for the
European stations considered in this paper, the occurrence
frequency has maxima around equinoxes. So, in the case
of positive Q-disturbances, we conne our consideration ac-
cording to the equinoctial periods only.
3
Synchronous Nm</i>F2 and Nm</i>E variations
The analysis has shown that Nm</i>F2 and Nm</i>E deviations
demonstrate, to some extent, a synchronous type of variation
during the periods of the F2-layer Q-disturbances. For check-
ing this effect, 58 negative and 101 positive Q-disturbances
were analyzed. We checked rst whether the type (posi-
tive/negative) of Nm</i>F2 and Nm</i>E deviations is the same in
a statistical sense. This can be done comparing Nm</i>E with
unity (i.e. with a median) for the selected F2-layer distur-
bances. One can estimate the statistical signicance of this
difference using the Student t-criterion. The results are pre-
sented in Table 2 which shows that the Nm</i>E difference from
the unity is signicant at any condence level for both neg-
ative and positive Q-disturbances. It means that under neg-
ative Q-disturbances in the F2-layer we have negative devi-
ations in the E-layer, whereas positive F2-layer disturbances
are accompanied by positive deviations in Nm</i>E. So they ex-
hibit in-phase variations in a statistical sense.
It would be interesting to check if there is a point-to-point
correlation between Nm</i>F2 and Nm</i>E deviations during
Ann. Geophys., 25, 483493, 2007
www.ann-geophys.net/25/483/2007/ A. V. Mikhailov et al.: Synchronous Nm</i>F2 and Nm</i>E daytime variations
485
such events. Table 2 shows that the correlation is not high
but it is signicant at least at the 90% condence level, ac-
cording to the Fisher F-criterion. On the other hand, the
correlation may be much better for individual, strong events.
For instance, for the 610 April 1973 positive Q-disturbance
event (Mikhailov et al., 2004, their Fig. 8), the correlatio