ther phenomena
and in normal solar geophysical environment.
In these perspectives of long-term trends, Jana &
Nandi [16-19] and Jana et al. [3,20], have performed the
analysis specifically based on computed long-term night
airglow intensity at 589.3 nm at some of Indian stations
and reported the close association of long-term change in
MLT Airglow intensities with depletion of long-term
trend in ozone. Similarly Vyas & Saraswat [21] on the
basis of observed data of Na-D airglow of the present
study site i.e., Kiso (Japan) reported a declining long-
term trend in ozone value and mesospheric temperature
or ozone depletion and mesospheric cooling phenomena.
The first critical review work on the long-term in meso-
spheric temperature trend on earlier reported observa-
tions were summarized and discussed by Beig et al. [22].
They established primary fact about the evidences of
long-term mesospheric cooling, based on ground, satel-
lite and rocket based experiments of the multi decade
period atmospheric data set. Sobral et al. [15], have in-
vestigated the ionospheric plasma bubbles climatology
based on 22 years of 630 nm airglow observations over
Thus, studies of long-term variation on mesospheric
airglow specifically based on experimentally observed
data and its possible causes are either rare or few [23-25].
Rajesh et al. [23], have reported the ionospheric plasma
depletion phenomenon over Kavalur i.e., Indian low lati-
tude station using the nighttime airglow intensity meas-
urements at 557.7 nm. Effect of seasonal, solar cycle va-
riation and determination of different range of periodi-
cities from a few days to less than one solar cycle on
nighttime airglow intensity of 557.7 nm over Kiso, was
also described by Das et al. [24,25]. But earlier workers
have not attempted to such rare studies on long-term
trend behavior on nighttime airglow emission intensity at
557.7 nm. Realizing these facts in mind, an attempt has
been made in the present paper to study the long-term
change on the basis of experimentally measured night-
time airglow intensities 557.7 nm, which were recorded
for the period 1979-1994 over mid latitude Japanese sta-
tions Kiso, Tokyo Astronomical Observatory (TAO),
University of Tokyo (35.79˚N, 137.63˚E; 1130 m), Japan.
Such long time series data are av ailable only on the stud y
site, so this particular station has been taken for the work.
2. Data Analysis
The basis of the present analysis is regular observations
of oxygen (OI) airglows brightness at zenith (3 degrees
diameter) on moonless clear night days. The present in-
vestigation is carried out from the analysis of data set of
hourly values of night airglow intensity at 557.7 nm re-
corded over Kiso, TAO, Japan. These daily hourly time
series have downloaded from the Solar Activity World
Data Center, Japan from the web site (http://solar www. during the period 1979-1994.
Such extensive data cover almost one and half solar cy-
cle and therefore provide an opportunity to carry out
long-term trend study analysis over mid latitude airglow
emission at 557.7 nm, which primarily refers to a meso-
spheric region [2].
To ascertain the long-term trend first, it is necessary to
remove hourly, seasonal and solar cycle variation of the
airglows intensities from the available raw time series
data. In this connection, following method has adopted in
the present course of work.
3. Method of Analysis
First, average monthly airglow intensity values have
computed for three hours intervals between 20:00 to
22:00 hours as Pre-midnight period and 23:00 to 01:00
Copyright © 2012 SciRes. AJCC
Copyright © 2012 SciRes. AJCC
have computed and plotted as a function of years. The
average values of such deviations are also depicted as
black dotted lines to visualize the overall lon g-term trend
in Mesospheric airglow intensities or departure from the
normal level.
hours as the Midnight period from their respective daily
values of each particular month of the years (1979-1994).
The particular average monthly values of Pre-midnight
and Midnight hours have been computed to eliminate the
hourly, month-to-month as well as seasonal dependence
factors from the raw daily airglow data. Subsequently,
monthly average reference value (MARV) has also cal-
culated separately from the average monthly values of
Pre-midnight and Midnight hours for each specific month
of the entire selected study years. Such MARV value of
individual monthly value of airglow intensity is evalu-
ated to eliminate the solar cycle dependence of entire
specified period and is shown in the Table 1 and Figures
1 and 2. Therefor e, MARV ha s treat ed as refe rence v alue
for the specific month of observations of the chosen
study period.
Furthermore, such time series of deviation value of
MARV are subjected to statistical linear regression ana-
lysis to find the slope or yearly change and correlation
coefficient (R’) etc., of the linearly fitted line. The statis-
tically linearly regression fitted line as the red line is il
lustrated along with their observed trend exhibited by
dark black line in the same figures (Figures 1 and 2).
The slope, R’ and probability (P) values have also shown
in each figure, which gives the inference about the cha ng-
ing of airglow intensity per year with its statistical sig-
nificance, respectively. Figures 1 and 2 show graphical
representations of the long-term change in airglow inten-
sities 557.7 nm in each individual month of years from
The deviation of the individual average monthly val-
ues from corresponding MARV of the similar months
Figure 1. Long-term change in pre-midnight airglow intensity at 557.7 nm for different months.
Figure 2. Long-term change in midnight airglow intensity at 557.7 nm for different months.
1979-1994 for Pre-midnight and Midnight hours respec-
tively. The results obtained from the analysis have pre-
sented and discussed in the next section
4. Results
4.1. Mid Latitude Pre-Midnight Airglow
Figure 1 represents the year to year variation in devia-
tion values from MARV to its individual particular av-
erage monthly value of Pre-midnight green airglow
emission of Oxygen on 557.7 nm. It is quite eviden t from
figures as well as calculated slope, R’ and P values that
deviation values from the MARV airglow intensity
shows the maximum positive decadal change between 32
to 80 R1 during March to June and October to December
(with a maximum value of 80 R in October) but on the
basis of the statistical significance level, the good sig-
nificance level above 95% found during April to June,
October and November month. The significance level
below 95% found in March & December and in January
& February months only ab out 75% signif icance is found
during this period 28 R per decade change occurs in de-
viation in intensity from the mean value of the airglow.
While in rest of months like July to September, there is
an insignificant statistical change (below 50%) in yearly
variation in such airglow intensity. From the close look
to the nature of red and black line in the figures and
above discussed results, it implies th at the most prob able
deviation values from the MARV to individual average
value exhibit the increasing trend and its magnitude va-
ries between 25 to 80 R per decade in most of the months
with the statistical significance level of above 90% ex-
cept in July to September. While, MARV values are
found to be chang ed between 82 to 264 R with maximu m
and minimum in October and January months, respec-
tively. Thus, the results observed in the present work
1Unit of airglow intensity, Rayleigh photons/cm2/sec.
Copyright © 2012 SciRes. AJCC
Table 1. Monthly average reference values of the airglow
intensity of OI 577.7 nm and their standard error value in
pre-midnight and midnight hours of different months dur-
ing study period.
MARV during
Month Pre-midnight Midnight
82.41 ± 10.09
108.05 ± 11.65
136.68 ± 13.44
185.45 ± 27.14
168.51 ± 10.97
175.84 ± 9.09
133.38 ± 13.84
141.47 ± 15.28
222.57 ± 15.44
264.25 ± 18.84
150.93 ± 10.91
103.48 ± 13.34
116.67 ± 13.80
129.52 ± 12.57
151.78 ± 16.62
190.35 ± 25.91
186.43 ± 12.52
213.27 ± 10.23
156.69 ± 11.56
171.08 ± 18.04
195.01 ± 16.42
236.98 ± 19.39
168.51 ± 12.75
137.24 ± 14.95
like the variation of MARV follows the same trend as
well as comparable values to monthly airglow intensity
magnitude with the highest value of airg low intensity ob-
served in October and lowest in January retrieved from
several year observations over mid latitude sites by
Carovillano and Forbe [26]. They also reported the aver-
age value of night airglow of OI 557.7 nm of order 250 R
which are also in the same order as observed in the pre-
sent investigation of the average value of the MARV
value of all months.
With consideration of above reported average nominal
value of night time airglow intensity of 557.7 nm order
of 250 R, observed change in MARV of individ ual mon-
th and decadal fluctuation rang e of 25 to 80 R, it is impe-
rative that increasing decadal change in the Pre-midnight
airglow intensity is seen up to 30% of the average refe-
rence nominal value. Furthermore, the year to year fluc-
tuations in the deviation value of the airglow intensity is
also attributed the inter-annual month ly nature as evident
from the black dotted line.
4.2. Mid Latitude Midnight Airglow
The Midnight yearly behavior of departure values from
the MARV to each specific monthly average value are
plotted with years in Figure 2. The positive deviation v a-
lues per decade are found to be lying in the range be-
tween 30 to 88 R (statistically significant at least 90% or
above significance level), which is in the same order as
observed for the Pre-midnight period. However, the li-
near regression trend line (shown by red colour) and av er-
age deviation line (shown by dotted black line) exhibit
enhancement trend in the highest range of 60 to 88 R per
decade during March, October and November and the
lowest range of 30 to 40 R per decade in Feb ruary, April
to July. However, in remaining months there is no sig-
nificant long-term variation in the same night airglow
intensity. The MARV of the different months are also
found to be varied from 116 to 236 R with a peak value
of 236 R in October and the least value in January as
shown in Table 1 and Figure 2. Similar monthly varia-
tion in airglow intensity is also observed in Pre-midnight
hour case in the present investigation and also reported
by several researche rs on ot her stations [3-5,10 ,11] .
On the basis of the above discussed results, it is obvi-
ous that there is a remarkable inclination of long-term
increasing trend in Pre-midnight as well as Midnight
mesospheric airglow intensity of 25 to 80 R in some of
months with good statistical significance level above
90%. This rate of increase the airglow of OI 557.7 nm
line intensity per decade is in the magnitude of 10% to
30% with the observed MARV as well as average nomi-
nal of 250 R. Furthermore, the yearly fluctuations in the
difference values of specified month are also illustrating
the nature of inter-annual monthly behavior in the air-
glow intensity.
5. Summary and Discussions
The long-term behavior of Pre-midnight and Midnight
airglow intensities concerning to mesospheric layers are
described in the present work. The present results reveal
that there is positive increasing decadal change in Mid-
night and Pre-midnight mesospheric airglow intensity of
the magnitude in the range of 25 to 88 R, which is the
order of 10% to 30% of the observed MARV and aver-
age night airglow intens ity values of OI 557.7 nm of 250
R as reported by Carovillano & Forbes [26]. It has been
well recognized and well established fact that each air-
glow emission intensity and its width primarily depends
on the large number of ionospheric parameters like elec-
tron density, temperature profile, the nature of chemical
species of the concerned height range of airglow emis-
sion lines, where it takes p lace. On the basis of the above
argument, it may be stated that consequence of the in-
creasing of mesospheric airglow intensities may be re-
sponsible to alter the mesospheric thermal structure, wh ich
further seems the cause of reduction of mesospheric
electron density. Therefore, such long-term trend in air-
glow mesosphere intensity can be further linked to the
observed reduction of electron density, shrinking of the
ionosphere or in term of negative varying nature of its
decadal change in temperature and electron density etc.,
of concerned ionospheric D-region [23-25].
Copyright © 2012 SciRes. AJCC
6. Acknowledgements
We the authors are grateful to Dr. G. Beig, IITM, Pune,
India for scientific advice and suggesting the views for
the present carried research work. The authors are also
thankful to Head, Department of Physics, M. L. Sukhadia
University, Udaipur, India for providing necessary en-
couragements during investigation.
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