ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-36-1471-2018Variations of the 630.0 nm airglow emission with meridional neutral wind and
neutral temperature around midnightVariations of the 630.0 nm nightglow emissionChiangChih-Yujohnson@phys.ncku.edu.twTamSunny Wing-YeeChangTzu-FangInstitute of Space and Plasma Sciences, National Cheng Kung
University, Tainan 70101, TaiwanInstitute for Space-Earth Environmental Research, Nagoya University,
Nagoya 464-8601, JapanChih-Yu Chiang (johnson@phys.ncku.edu.tw)26October2018365147114819January20186February201831August201812October2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://angeo.copernicus.org/articles/36/1471/2018/angeo-36-1471-2018.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/36/1471/2018/angeo-36-1471-2018.pdf
The ISUAL payload onboard the FORMOSAT-2 satellite has often observed airglow
bright spots around midnight at equatorial latitudes. Such features had been
suggested as the signature of the thermospheric midnight temperature maximum
(MTM) effect, which was associated with temperature and meridional neutral
winds. This study investigates the influence of neutral temperature and
meridional neutral wind on the volume emission rates of the 630.0 nm
nightglow. We utilize the SAMI2 model to simulate the charged and neutral
species at the 630.0 nm nightglow emission layer under different
temperatures with and without the effect of neutral wind. The results show
that the neutral wind is more efficient than temperature variation in
affecting the nightglow emission rates. For example, based on our estimation,
it would require a temperature change of 145 K to produce a change in the
integrated emission rate by 9.8 km-photons cm-3 s-1, while it only
needs the neutral wind velocity to change by 1.85 m-1 s-1 to
cause the same change in the integrated emission rate. However, the emission
rate features a local maximum in its variation with the temperature. Two
kinds of tendencies can be seen regarding the temperature that corresponds to
the turning point, which is named the turning temperature (Tt) in
this study: firstly, Tt decreases with the emission rate for the
same altitude; secondly, for approximately the same emission rate,
Tt increases with the altitude.
Introduction
The atomic oxygen red line at 630.0 nm is the most prominent emission in the
nighttime ionosphere. It usually forms an emission layer in the F region at
altitudes of ∼200–300 km and can be easily observed from
ground-based observatories or satellites (Nelson and Cogger, 1971; Kelley et al., 2002; Thuillier et al., 2002). The
emission is related to O(1D), whose production in the nighttime is
mainly via the charge exchange and dissociative chemical processes listed as
follows:
O++O2→O2++O,O2++e-→O(1D)+O,O(1D)→O(3P)+hν(630.0nm).
Based on the [O+] ∼Ne (electron
density) approximation (Peterson et al., 1966; Link and Cogger, 1988) in
the F2 region, the intensity of the OI(1D) 630.0 nm spectral line
is usually used to identify the ionospheric electron density variations. From
a rich history in the literature, the intensity of OI(1D) 630.0 nm
airglow emissions is known as midnight brightness wave (MBW) (Herrero and
Meriwether, 1980; Herrero et al., 1993; Colerico et al., 1996; Colerico
and Mendillo, 2002).
During occurrences of the MBW, increases in temperature are usually observed
around local midnight, which are termed the midnight temperature maximum
(MTM) effect. Harper (1973) and Spencer et al. (1979) reported the MTM
phenomenon first. The cases in their studies were observed by the incoherent
scatter radar from Arecibo and the NATE experiment aboard the Atmospheric
Explorer E (AE-E) satellite, respectively. The amplitude of the temperature
bulge was found to range from 20 to 200 K (Spencer et al., 1979; Burnside
et al., 1981; Colerico and Mendillo, 2002; Meriwether et al., 2008). In
addition, a number of studies about midnight brightness have reported the
relation between in situ temperature and neutral wind measurements (e.g.,
Herrero and Meriwether, 1980; Sastri et al., 1994, Colerico et al.,
1996; Colerico and Mendillo, 2002; Otsuka et al.,
2003; Mukherjee et al., 2006). Rajesh et al. (2009) showed the first
results of the limb image of 630.0 nm airglow using Imager of Sprites and
Upper Atmospheric Lightning (ISUAL) (Chang et al., 2012; Chiang et al.,
2013; Frey et al., 2016) onboard the FORMOSAT-2 satellite. Adachi et
al. (2010) also showed a 14-day time span of airglow observations obtained
from the Asian sector by ISUAL. On the basis of the observation time and
location, they suggested that the equatorial airglow probably corresponded to
the MBW which is in association with the occurrence of MTM. Furthermore,
Chiang et al. (2013) statistically investigated the global midnight
brightness according to seasons and found that the global midnight brightness
near the equatorial regions was controlled by different mechanisms. In the
study, the features and behavior of the 630.0 nm midnight intensity were
investigated by analyzing the optical images obtained by ISUAL. Cases of
enhancement of global midnight brightness were successfully categorized into
three types that were mainly due to the influence of temperature changes,
neutral wind and ionospheric anomaly.
Based on the previous studies, it is known that temperature and meridional
neutral wind are correlated and associated with manifestations of MTM. Thus,
we want to discuss these two effects at the same time. In this study, we
calculate the volume emission rates to understand the influence of neutral
temperature and meridional neutral wind on the 630.0 nm nightglow. We shall
discuss the sensitivities of the emission rates to the temperature and the
densities of several neutral and charged species. Moreover, some new
features will also be shown in the discussion section. And we also provide
ISUAL observation results to show that our calculation results are
reasonable and realistic.
Model features
Temperature changes and meridional neutral wind can influence the O(1D)
nightglow intensity through particle densities. The volume emission rate of
the 630.0 nm nightglow in the F2 region (Sobral et al., 1993) can be derived from the
chemical process of 630.0 nm nightglow (Supplement Sect. SI). It is shown as
follows:
I630=A1DμDγO2O+k1N2+k2O2+k3O+A1D+A2D,
where μD is the quantum yield of O(1D), which is about
1–1.3 (Torr and Torr, 1982); γ is the rate coefficient of
Reaction (R1) (St.-Maurice and Torr, 1978); k1, k2 and k3
are the rate coefficients of O(1D) quenched by N2,
O2 and O, respectively (Langford et al., 1986; Streit et al.,
1976; Sun and Dalgarno, 1992); and A1D and A2D are the transition
coefficients (Froese-Fischer and Saha, 1983). The formulas for the rate
coefficients (Vlasov et al., 2005) are listed in Table 1. The production
rate of O(1D) is contributed by the oxygen ion density
[O+] and the molecular oxygen density [O2] through the
linked Reactions (R1) and (R2). The major loss rates of O(1D) are
associated with the densities of molecular oxygen [O2], molecular
nitrogen [N2], and atomic oxygen [O], as reflected in
Eq. (1). The
densities [O+], [O2], [N2] and [O] and the rate
coefficients γ, k1, k2 and k3 all depend on
temperature. In addition, [O+] may change with the neutral wind
conditions. In order to determine I630 under different temperatures and
neutral wind conditions, one must first determine the densities of the
relevant species. In this study, [O+] and plasma temperatures under
various conditions are found by the SAMI2 model of the Naval Research Lab
(Huba et al., 2000). SAMI2 is a two-dimensional, first-principle model of
the comprehensive low- to mid-latitude ionosphere. SAMI2 code includes most
of the mechanisms that should be considered in the ionosphere. There are
photoionizations, chemical process, effects by the magnetic and electric
fields, plasma dynamics and the influence from the neutral atmosphere. The
input variables, neutral species, are specified using the empirical codes,
the Mass Spectrometer Incoherent Scatter model (NRLMSISE-00) (Picone et
al., 2002) for neutral densities and the Horizontal Wind Model (HWM-93)
(Hedin et al., 1996) for neutral wind. The continuity and momentum
equations of seven ion species (H+, He+, N+,
O+, N2+, NO+, and O2+) are
solved in the code.
Reactions and rate coefficients related to the volume emission rate
of the 630.0 nm airglow.
Note: Teff=0.67Ti+0.33Tn
(Teff: effective temperature, Ti: ion temperature,
Tn: neutral temperature) (St.-Maurice and Torr, 1978).
Oxygen ion density plotted in the latitude–altitude plane at
23:00 LT on 1 February (left panels) and 1 August 2007 (right panels) in the
Asian region (100∘ E longitude) from the SAMI-2 model:
(a) without neutral wind; (b) with the effect of normal
neutral wind, whose strength and directions are indicated by the arrows.
In order to understand the differences due to the meridional neutral wind, we
apply the SAMI2 model with and without neutral wind by changing the
multiplicative factor of neutral wind (tvn0) to see the differences between
two solstices. Thus, we simulate the cases of 1 February 2007 (northern
winter) and 1 August 2007 (northern summer). In the simulations, we suppose
that the solar and geomagnetic activities are in quiet conditions (F10.7
index = 60, Ap index = 7). The simulations are run for the altitude
range between 150 and 1000 km from -30 to +30∘ geomagnetic
latitudes. Inside this region, we use 100 geomagnetic field lines and 201
grid points along each field line. Our report of the results will focus on
the locations at -5 and +5∘ geomagnetic latitude (+2 and
+12∘ geographic latitude, respectively) along the 100∘ E
geographic longitude, which intersects these latitudes in the Asian region.
Figure 1 shows the O+ density along the magnetic lines with
altitudes between 150 and 315 km in the latitude–altitude plane at the time
and longitude described above. Figure 1a shows the results under the
condition that lacks neutral wind, and Fig. 1b shows the results with the
effect of normal neutral wind. The two left panels are for 1 February 2007
and the two right panels are for 1 August 2007. The arrows plotted in Fig. 1b
indicate the strength and directions of the meridional neutral wind.
Comparison of Fig. 1a and b clearly shows that meridional winds transport the
plasma along the magnetic field line and change the plasma density
distribution. And this change in the plasma profile could directly modify the
emission rate in Eq. (1). The dashed lines, which correspond to ±5∘ geomagnetic latitude, indicate the locations where the intensity
of the 630.0 nm nightglow is examined in detail in this study.
Results and analysis
Based on Eq. (1), I630 under different temperatures and different
neutral wind conditions is plotted in Fig. 2. The neutral wind conditions for
the results in Fig. 2 are the same as those for Fig. 1. The strength and
directions of the neutral winds are indicated by the arrows shown in Fig. 1.
The simulation results shown in the figure are for (a) 1 February and
(b) 1 August 2007, with the left and right panels, respectively,
corresponding to -5 and +5∘ geomagnetic latitude. The letters A,
B, C, D and E indicate the altitudes of 220, 230, 240, 250 and 260 km,
respectively. The dotted lines indicate the results with a normal neutral
wind effect; the solid lines indicate the results without a neutral wind
effect. Note that the temperatures of around 650 K, corresponding to the
leftmost points of the lines in the figure, were the initial neutral
temperatures obtained from the NRLMSISE-00 model at the various altitudes.
These neutral temperatures are input into the SAMI2 model, and we set up the
48 h data as a running loop to obtain the plasma data. In order to explore
the effects of temperature change, we modify the codes of SAMI2 by increasing
50 K per run as the inputs, and perform the simulations to calculate the
emission intensity values associated with different temperature conditions.
The results of the 630.0 nm emission rate at 23:00 LT at different
temperatures and under different neutral wind conditions for
(a) 1 February and (b) 1 August 2007: left and right
panels, respectively, for -5 and +5∘ geomagnetic latitude; the
letters A, B, C, D and E, for the altitudes of 220, 230, 240, 250 and
260 km, respectively; for normal neutral wind effect (black dotted lines)
and windless conditions (red solid lines). The neutral wind conditions of
Fig. 2 are the same as those shown in Fig. 1.
From Fig. 2, we can see the influence of temperature and neutral wind on the
nightglow emission. Note that the neutral wind conditions are as in Fig. 1:
Fig. 1a for zero wind condition and Fig. 1b for normal wind condition.
The influence of the temperature variations on I630 is usually less than
3 photons-1 cm-3 s-1 at the heights of 220 to 260 km. The variation of
I630 with temperature, however, is not monotonic; there is a maximum in
the intensity as the temperature changes. In terms of height, as I630
depends on the local neutral and charged particle densities in accordance
with Eq. (1), the emission is the strongest at 230 km, except for the
condition of very weak emission (<1 photons-1 cm-3 s-1) that
occurs at +5∘ geomagnetic latitude in August with normal wind
effect (right panel of Fig. 2b).
As for the influence of the neutral wind on 1 February 2007 (Fig. 2a),
both locations (±5∘ geomagnetic latitude) clearly feature
significantly smaller I630 under this effect. We suggest that this is
due to the meridional neutral wind blowing equatorward in both hemispheres
(see Fig. 1) and pushing the plasma upward along the field lines, reducing
the local charged particle densities and consequently the emission rates as
well. On 1 August 2007, as shown in Fig. 2b, the neutral wind causes the
intensity at +5∘ geomagnetic latitude to decrease significantly
for the same reason as the wind direction is locally southward
(equatorward). This southward neutral wind, however, has an opposite effect
on the intensity at -5∘ geomagnetic latitude; being locally
poleward, the wind pushes the plasma downward along the field lines,
increasing the local charged particle densities and consequently the
emission rates as well.
From Eq. (1), we can see that I630 is related to the densities of
several neutral species as well. In order to find out how the temperature
affects the overall chemical process that leads to the 630.0 nm emission, a
few relevant parameters are shown as functions of temperature in Fig. 3,
based on the condition at 230 km altitude and -5∘ geomagnetic
latitude on 1 February 2007. In Fig. 3a, we plot [O], [N2] and
[O2] in dotted, dashed and solid lines, respectively. Then the
corresponding loss rates of these neutral species are shown in Fig. 3b. In
Fig. 3c, [O+] with and without neutral wind effect are plotted with
dotted line and solid line, respectively. The values of
γ[O+][O2], which are related to the production
rate and in the numerator of Eq. (1), are plotted in Fig. 3d. The dotted line
represents the normal neutral wind condition, and the solid line the windless
condition.
The profiles of neutral and charged species versus temperature which
are involved in Eq. (1) at 230 km altitudes and -5∘ geomagnetic
latitudes on 1 February 2007. (a) [O], [N2] and
[O2] versus temperature. (b) The loss rate terms of
k1[O], k2 [N2] and k3 [O2] versus
temperature. (c) [O+] versus temperature with/without the
neutral wind effect. (d) The production-rate-associated term of
γ[O+][O2] versus temperature with/without the
neutral wind effect.
When the neutral temperature increases from 600 to 900 K, the rate
coefficients k1 and k2 decrease by 5.8 % and 3.7 %,
respectively, and k3 increases by 7.4 % as calculated from Table 1.
The rate coefficients k1, k2 and k3 do not change
significantly. However, in the same temperature range, [O], [N2]
and [O2] show prominent increases of 253 %, 363 % and
171 %, respectively, as shown in Fig. 3a. Therefore, the atomic and
molecular densities dominate the changes in the loss rates (Fig. 3b).
Discussion
From Fig. 1a, we can see that along the field lines, the O+ density
is maximum around the geomagnetic equator when there is no neutral wind,
whether it is in the summer or winter season. But the [O+] maxima tilt
to the winter hemisphere in the presence of summer-to-winter neutral wind at
the geomagnetic equator, as shown in Fig. 1b. Therefore, we suggest that
the low-latitude emission enhancement in the winter hemisphere be achieved
by plasma accumulation brought about by the summer-to-winter neutral wind.
From the results that include the normal wind effect as shown in Fig. 2, the
intensities on opposite sides of the geomagnetic equator are very different.
The weaker emission is in the summer hemisphere, and brightness of higher
intensity appears in the winter hemisphere. In previous studies, Rishbeth and Setty (1961)
found that NmF2 was larger in winter than in summer, and they first
suggested the possibility of composition change being the cause of the
winter anomaly. Rishbeth (1972) and Torr and Torr (1973) suggested that the anomaly might be due
to transequatorial neutral wind blowing from the summer hemisphere to the
winter hemisphere. Therefore, the enhancement of the emission at the low
latitudes of the winter hemisphere should be the results of plasma
accumulation caused by the neutral wind effect.
Quantitative results for how (a) the neutral temperature
and (b) the neutral wind affect the 630 nm airglow intensity.
Figure 2 shows the influence of temperature and neutral wind on the
nightglow emission rates. We estimate the intensity change under different
neutral wind conditions based on the location at 230 km altitude and
-5∘ geomagnetic latitude on 1 February 2007. In this situation,
the emission would be reduced by the wind flow, and the average change is
about 0.690 photons-1 cm-3 s-1 for every m-1 s-1 of the wind speed. In
comparison, the change due to temperature variation is just 0.015 photons-1 cm-3 s-1 for every K. The ratio of the two numbers is 46.
Consideration of other conditions, such as those cases shown in Fig. 2, may
reduce the corresponding ratio, but it should still be at least 20.
According to earlier studies, the neutral wind speed is generally 0–300 m-1 s-1 in the F region (Dyson et al., 1997), while the amplitude of the temperature
bulge due to the MTM effect has been found to range from 20 to 200 K
(Burnside et al., 1981; Colerico and Mendillo, 2002). Even if one assumes the maximum wind speed is just
60 m-1 s-1 as in the simulations in this study, it would require a temperature
change of 1200 K to match the same change in emission intensity caused by
the neutral wind. Such a large temperature change is not realistic in
comparison with the maximum observed difference of 200 K. Thus, the emission
rate of nightglow, realistically, is influenced more by the neutral wind
than temperature change when the former mechanism is clearly present.
Plots of the emission rates against the turning temperature between
220 and 260 km altitudes.
The densities and some of the rate coefficients are temperature dependent, as
given in Eq. (1). We analyze the change with temperature of the individual
terms in Eq. (1). In Fig. 3b and d, we plot the terms in the numerator and
denominator on the right-hand side of Eq. (1) and find that all these terms
increase with temperature. However, if we consider the derivative of the
terms with respect to temperature, which characterizes how sensitive the
terms are to temperature change, we notice that the derivatives for
k1[N2] and k3[O] increase with temperature, while those
for k2[O2] and γ[O+][O2] decrease,
as shown in Fig. 3b and d. How the variations of these terms affect the
dependence of I630 on temperature can now be understood from the
right-hand side of Eq. (1). In particular, the numerator, which characterizes
the production rate of O(1D) and is proportional to γ[O+][O2], increases with temperature while featuring a
relatively large increase at lower temperatures (less than ∼750 K). On
the other hand, the denominator, which characterizes the total loss rate of
O(1D) and is dominated by k1[N2] as Fig. 3b
indicates, features a relatively large increase at higher temperatures
(larger than ∼750 K). Upon division of the numerator by the
denominator, the plot of I630 versus temperature is thus characterized
by quasi-parabolic lines with the presence of a local maximum – or a turning
point in the curve – as shown in Fig. 2. We refer to the temperature that
corresponds to such a local maximum as the turning temperature
(Tt). Below Tt, I630 increases with temperature,
meaning that the increase in the production of O(1D) associated
with a rise in the temperature is more efficient than the increase in its
loss. In contrast, I630 decreases with temperature above Tt,
meaning that the increase in the production of O(1D) associated
with a rise in the temperature is less efficient than the increase in its
loss. Thus, Tt has the significance of being the temperature at
which the production and loss rates of O(1D) are equally sensitive
to a temperature change.
Four observation cases by ISUAL in February and August 2007 (the
same periods as shown in Fig. 1).
In order to quantitatively describe the effects of neutral temperature and
meridional neutral winds, we calculate the 630 nm airglow intensity by
integrating the volume emission rate along the altitude. Thus, the change in
the integrated emission rate (ΔST) over a fixed altitude range
h1 to h2 due to a change in temperature from T1 to T2 can be
written as
ΔST=ST2,W-ST1,W=∫h1h2I630T2,W,zdz-∫h1h2I630T1,W,zdz,
where S is the integrated emission rate from height h1 to h2 as a
function of temperature and neutral wind speed W. Similarly, the change in
the integrated emission rate (ΔSW) over a fixed altitude range
h1 to h2 due to a change in the neutral wind speed from W1 to
W2 can be obtained as
ΔSW=ST,W2-ST,W1=∫h1h2I630T,W2,zdz-∫h1h2I630T,W1,zdz,
Combining the changes in both temperature and neutral wind, one may express
the change in the integrated emission rate over the altitude range as
ΔST,W=ST2,W2-ST1,W1=∫h1h2I630T2,W2,zdz-∫h1h2I630T1,W1,zdz.
One can show that to the leading order, the above equation reduces to
ΔST,W=ΔST+ΔSW,
with ΔST in Eq. (2) evaluated at W=W1 and ΔSW in Eq. (3) evaluated at T=T1. Based on Eq. (4), we
calculated I630 for different temperatures and neutral wind conditions,
and then, according to the integrals in Eqs. (2) and (3), integrated the
emission rates over the major altitudes of the 630.0 nm nightglow emission
layer, ranging from 150 to 315 km altitude. Figure 4a and b show how the
integrated emission rates vary with the increases in the neutral temperature
and neutral wind speed, respectively. Figure 4a shows the result regarding
the integrated emission rate as affected by neutral temperature (at
-5∘ geomagnetic latitude on 1 February 2007). The curve in red is
fitted as a second-order polynomial:
ΔST=0.1354±0.0069ΔT-4.6835±0.2652×10-4ΔT2,
where ΔST (km-photons-1 cm-3 s-1) is the change in integrated
emission rate and ΔT (K) is the increase in neutral
temperature, compared with the standard conditions of 650 K neutral
temperature and zero neutral wind. Figure 4b shows the result regarding the
integrated emission rate as affected by neutral wind. The results are
obtained based on the same standard conditions as those considered in Fig. 4a. The curve in red fits an exponential function:
ΔSW=64.8883±0.7772×1-exp-0.0885±0.0041ΔW,
where ΔSW (km-photons-1 cm-3 s-1) is the
change in integrated emission rate and ΔW (m-1 s-1) is
the change in neutral wind velocity. Therefore, according to Eq. (4), we
combine the results of the two fitting functions to approximate the overall
change in the integrated emission rate due to the two effects:
ΔST,W=0.1354ΔT-4.6835×10-4ΔT2+64.88831-exp-0.0885ΔW.
Based on the function, we can quantitatively compare the neutral temperature
effect with the neutral wind effect. From Fig. 4a, increasing the neutral
temperature by about 145 K leads to the maximum change in the integrated
emission rate of 9.7859 km-photons-1 cm-3 s-1. In contrast,
to get the same change in the emission rate by varying the neutral wind, it
just requires a change in neutral wind velocity by 1.85 m-1 s-1
(Fig. 4b). With the above estimation, the neutral wind effect would certainly
be larger than that of the neutral temperature for this case.
ISUAL data in the specific regions and seasons considered in the
simulations: the nightglow bright spots on valid observation days during
(a) January–February and (b) July–August.
Figure 5 shows a plot of Tt versus the emission rate I630 at
specific altitudes. The results include all the cases shown in Fig. 2, with
different symbols indicating different altitudes. Two kinds of tendencies can
be seen from the plot: firstly, Tt decreases with I630 for
the same altitude; secondly, for approximately the same emission rate,
Tt increases with the altitude. This is the first result to show
these tendencies of the turning temperature.
Observations have found cases that are consistent with our simulation results
regarding the influence of the neutral wind. Figure 6 shows four cases
observed by ISUAL in the Asian region at 23:00 local time during the two
months considered in our studies: two cases in February shown on the left
side and two cases in August shown on the right side. Figure 6a would be for
the condition of no wind or weak wind, while Fig. 6b would correspond to the
normal wind condition. We can see from Fig. 6a that a bright spot of
nightglow was observed at the geomagnetic equator during both months. As the
volume emission rate, according to Eq. (1), is proportional to the
O+ density, the observations were supportive of the simulation
results of density variations in Fig. 1a. Similarly, the two cases in
Fig. 6b, which featured nightglow bright spots in the winter hemisphere,
suggested that the density variations shown in Fig. 1b are realistic.
Previously, Chiang et al. (2013) examined the occurrence rates of global
midnight brightness observed by ISUAL. In order to verify the enhancement of
the emission intensity in the winter hemisphere by the neutral wind, we
examined the ISUAL data that correspond to the specific regions and seasons
considered in our simulations, and the results are shown in Fig. 7a and b. We
found that among the 22 valid observation days during January and February,
∼77 % of the days featured the appearance of nightglow bright spots
in the low-latitude region of the winter hemisphere (Fig. 7a). Furthermore,
∼83 % of the 30 valid observation days during July–August also
featured nightglow bright spots at low latitudes in the corresponding winter
hemisphere (Fig. 7b). Thus, statistical results regarding the location of
nightglow bright spots agree with the simulation results that demonstrate the
crucial role of the neutral wind in affecting the location of high-intensity
nightglow regions.
Rajesh et al. (2014) showed their simulation results and claimed that using merely
the background meridional winds could reproduce the observed brightness.
They selected a few cases of ISUAL image data and compared those data with
the simulation results by the SAMI2 model. Nevertheless, using such a method
by Rajesh et al. (2014), one should be very careful about the details when
it comes to physical insights or conclusions drawn from the study. This is
because ISUAL only provided optical data and there was not any instrument on
the satellite to directly observe the relevant conditions (temperature, wind
field, etc.) in the environment. Without such observations to provide
constraints for modeling, one can easily reproduce similar-looking results
of selected short-period data by adjusting modeling parameters in
simulations. However, images seemingly similar to that of an ISUAL
observation could be produced from simulation results using considerably
different parameter values, which may correspond to different dominant
mechanisms. Thus, when there are few constraints for the parameter values,
roughly comparing a short-period case of ISUAL image data with simulation
results without paying attention to details may lead to an interpretation of
brightness production mechanisms that is different from the real situation.
Observations of the movement of MTM temperature bulge and that of nightglow
have led to postulations of an association between pressure bulge and
nightglow intensity (Colerico et al., 1996; Colerico and Mendillo, 2002;
Meriwether et al., 2008). However, the high intensities of the observed
nightglow have not been successfully reproduced using existing models
incorporating the MTM effect, such as the NCAR
thermosphere–ionosphere–electrodynamic general circulation model (TIEGCM),
as pointed out by Colerico and Mendillo (2002) and Meriwether et
al. (2008). Note that temperature was not included as a varying quantity in
traditional ionospheric models. Thus the simulation study of temperature
effect upon nightglow intensity is lacking. Our simulation results have
demonstrated the unexpectedly non-monotonic dependence of the intensity of
nightglow on the neutral temperature, with the turning temperature
Tt that arises from the dependence implying a limitation for the
growth of the emission rates. As the temperature increases above
Tt, the emission rates do not continue to grow. In fact,
temperature change such as in the case of heat transfer is affected by the
density, which controls the heat capacity. At the same time, temperature
change may generate pressure difference and lead to transport that changes
density profiles. As nightglow intensity depends also on particle densities,
its non-monotonic variations with temperature are in fact due to the
combination of temperature and density. While our study suggests that neutral
wind is the dominant driver of the I630 variation, its influence,
however, is via transportation of plasma and neutral particles, in which case
consideration of the effect of temperature on the density is essential.
Moreover, it has not been established that MTM is affected by the wind
primarily. The combination of temperature and density, which has been shown
to cause non-monotonic results in this study, may very well be an important
factor in the study of MTM. Thus, if one wants to fully reproduce the
observation results, we suggest other extra factors associated with
temperature variations should also be considered, such as different tidal
modes from the lower atmosphere (Akmaev et al., 2009). Our findings of the
turning temperature tendencies can help as a guide for choosing the
background temperature in future modeling attempts to obtain intensities of
nightglow brightness comparable to those observed from ground or from space.
Shepherd (2016) investigates the possible extent of the MTM at ∼20–40∘ N, considering O(1D) airglow volume emission
rates, Doppler temperatures, and neutral wind (zonal and meridional)
observations by the Wind Imaging Interferometer (WINDII) experiment onboard
the Upper Atmosphere Research Satellite (UARS). Their results provide us with
the relations of the zonal wind to the O(1D) emission rate and of
the meridional wind to the temperature. Such relations potentially guide us
to design a more extensive future study in simulation so as to reproduce the
observation and statistical results by Shepherd (2016).
Conclusions
Previous studies of the MTM effect have pointed out that the temperature
anomaly influences the nighttime behavior of the thermosphere. And the
neutral wind also plays a key role in causing the intensity variations in the
nighttime ionosphere. Based on our simulation results, both temperature
change and meridional neutral wind could cause the 630.0 nm nightglow
intensity to vary, while the latter is more effective. A temperature change
of 145 K is shown to result in an integrated emission rate change of
9.8 km-photons-1 cm-3 s-1. However, it only requires a
neutral wind velocity change of 1.85 m-1 s-1 to produce the same
change in the integrated emission rate. And the simulation results may
successfully explain most of the observational results by ISUAL. An
unexpected aspect of the results is the non-monotonic dependence of the
emission rate on temperature, featuring a turning point as the temperature
changes. The temperature Tt at which the turning point occurs
corresponds to a balanced condition between the production and loss of
O(1D). Thus, our results help understand how the overall chemical
process of nightglow is affected by the variations of neutral temperature and
neutral wind. Two kinds of tendencies can be seen regarding the turning
temperature Tt. One is the higher Tt corresponding to
higher altitude at the same emission rate; the other is the higher
Tt corresponding to a lower emission rate at the same altitude.
Our findings of these turning temperature tendencies can guide future
modeling attempts to match the observed nightglow brightness intensities.
The ISUAL image data are provided by the NCKU ISUAL team
(http://sprite.phys.ncku.edu.tw/joomla3/index.php?option=com_content&view=article&id=235&Itemid=441,
last access: 24 October 2018). The SAMI2 model can be downloaded from the
U.S. Naval Research Laboratory website
(https://www.nrl.navy.mil/ppd/branches/6790/sami2, last access:
24 October 2018).
The supplement related to this article is available online at: https://doi.org/10.5194/angeo-36-1471-2018-supplement.
CYC took the lead in writing the manuscript.
SWYT contributed to the research plan and to the manuscript. TFC carried out
data collection and data analysis. All authors read and approved the final
manuscript.
The authors declare that they have no conflict of
interest.
Acknowledgements
The authors acknowledge the FORMOSAT-2/ISUAL science and operator team for
providing image data
(http://sprite.phys.ncku.edu.tw/joomla3/index.php?option=com_content&view=article&id=235&Itemid=441,
last access: 24 October 2018). The work by Chih-Yu Chiang and Sunny Wing-Yee
Tam is supported by Taiwan Ministry of Science and Technology grant MOST
107-2111-M006-003. Tzu-Fang Chang acknowledges support by the Ministry of
Education, Taiwan R.O.C., from the Aim for the Top University Project to
National Cheng Kung University. Edited by:
Keisuke Hosokawa
Reviewed by: Yuichi Otsuka and one anonymous referee
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