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Journal of Atmospheric and Solar-Terrestrial Physics 169 (2018) 78–82
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics
journal homepage: www.elsevier.com/locate/jastp
Storm-time variations of atomic nitrogen 149.3 nm emission
Y. Zhang *, L.J. Paxton, D. Morrison, B. Schaefer
The Johns Hopkins University Applied Physics Laboratory, United States
A B S T R A C T
Net radiances of atomic nitrogen emission line (N-149.3 nm) from the thermosphere are extracted from the FUV spectra observed by TIMED/GUVI on dayside at sunlit
latitudes. During geomagnetic storms, the N-149.3 nm intensity is clearly enhanced in the locations where O/N2 depletion and nitric oxide (NO) enhancement are
observed. The N-149.3 nm intensity is linearly and tightly correlated with N2 LBHS (140–150 nm) radiance with a fixed LBHS/149.3 nm ratio of ~4.5, suggesting that
dissociation of N2 is the dominant source of the N-149.3 nm emission. In the regions without storm disturbances, the N-149.3 nm intensities are closely correlated with
solar EUV flux.
1. Introduction
The condition of the thermosphere strongly depends on the energy
inputs (solar EUV flux, particle and Joule heating, adiabatic heating, tides
and gravity waves from the lower atmosphere), cooling processes
(radiative cooling, such as CO2 and NO IR radiation, adiabatic cooling),
and transport due to neutral wind. Large neutral disturbances have been
observed during geomagnetic storms, such as temperature and total
density increase, composition changes (O/N2 decrease, NO enhancement). Bruinsma et al., (2006) reported a 300–800% increase in the
thermospheric density around 400–500 km on a global scale based on
CHAMP and GRACE measurements during the November 20–21, 2003
superstorm. By analyzing TIMED/GUVI limb data, Meier et al., (2005)
found the exospheric temperature was nearly doubled (~1 000 K versus
~1800 K) during the 2003 superstorm. Thermospheric O/N2 column
density ratios show significant depletion during storms (Zhang et al.,
2004) based on TIMED/GUVI imaging data. More recently, TIMED/GUVI
spectrograph data have been used to derive O/N2 ratio and NO column
density simultaneously (Zhang et al., 2014). In this paper, we report
storm-time atomic nitrogen emission (149.3 nm) enhancements at sunlit
latitudes that are likely due to storm-time increase in N2 density (major
source) and minor contribution from enhanced NO as well as enhanced N
transported from the auroral region.
2. Data
Under an imaging mode, TIMED/GUVI (Paxton et al., 2004; Christensen et al., 2003) measures intensities of thermospheric emissions in
five “colors”: 121.6 nm, 130,4 nm, 135.6 nm, LBHS (~140–150 nm) and
LBHL (~165–180 nm) from 2002 to 2007. Emissions intensities in the
“colors’ are useful and have been used to derived O/N2 ratio (Zhang et
al., 2004, Strickland et al., 2004; Meier et al., 2005) and auroral particle
information (Zhang and Paxton, 2008). However, it is not easy to separate different emission features from the radiances in the five colors
(Meier et al., 2015). TIMED/GUVI has been operating in spectrograph
mode since 2008. Using the GUVI spectrograph data, Zhang et al., (2014)
developed a method to extract nitric oxide (NO) ε band (172–182 nm)
intensity and derived NO column density. Comparison between simultaneous O/N2 and NO from GUVI indicates that they are anti-correlated
on a global scale during geomagnetic storms (Zhang et al., 2014). Using
the same spectra data from the NO processing, we further developed a
method to extract the net N-149.3 nm intensities.
Fig. 1 shows an example of binned GUVI spectra between 140 and
155 nm (black line) on July 16, 2016 (orbit 57 450). The scatter from
121.6, 130.4 and 135.6 was removed in the GUVI spectra (Zhang et al.,
2014). The blue dashed line is for simulated N2 LBH bands using AURIC
(Strickland et al., 1999). The simulated LBH spectra are further scaled by
matching the observed pure LBH bands in the wavelength ranges
(140.0–146.0 nm and 151.0–154.0 nm). By subtracting the blue line
from the black line, the net N-149.3 nm spectra are obtained (red line).
Note that the N-149.3 nm is a single spectral line. The broad spectra are
due to the instrument response of GUVI. To capture the true N-149.3 nm
intensity, the red line is integrated over a wavelength range of
147.8–150.8 nm.
Note that the N-149.3 radiances obtained through the above processing depend on solar zenith angle (SZA) which changes significantly
over a TIMED orbit. In order to make the radiances comparable between
different orbits, the radiances are corrected by multiplying a simple scale
* Corresponding author. The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 21073, USA.
E-mail address: yongliang.zhang@jhuapl.edu (Y. Zhang).
https://doi.org/10.1016/j.jastp.2018.01.023
Received 23 October 2017; Received in revised form 16 January 2018; Accepted 17 January 2018
Available online 1 February 2018
1364-6826/© 2018 Elsevier Ltd. All rights reserved.
Y. Zhang et al.
Journal of Atmospheric and Solar-Terrestrial Physics 169 (2018) 78–82
gridded (3 3 latitude and longitude bins) and interpolated and
smoothed in the longitude and latitude coordinates. Fig. 2 (top panels)
shows the SZA corrected N-149.3 nm radiances on March 16 (pre-storm
day), March 17 and 18 (storm days), 2013. On March 16, the radiances
are relatively uniform in the mid and low latitudes (e.g. jlatitudej < 30 ).
Enhanced radiances were seen at higher latitudes (jlatitudej > 40 ). On
March 17 (storm starting day), the N-149.3 radiances are enhanced at
latitudes (>30 or < -60 ) over most longitudes. On March 18, the
enhanced radiances in the Australia sector extended to latitudes near the
equator. However, the regions with enhanced radiances in the northern
hemisphere remain roughly unchanged. Statistical counting errors in the
original spectra were propagated to the final N-149.3 nm radiances. Fig.
2b shows the relative errors which vary from around ~2% at equatorial
region to ~10–15% at high latitudes. This latitude dependence is due to
high counts at low latitudes (low SZA) and low counts at high latitudes
(high SZA). The low relative errors at mid and low latitudes indicate the
storm-time variations in the observed N-149.3 nm radiances are
significant.
For a comparison, NO column density and O/N2 ratio maps are
plotted in the third and bottom panels in Fig. 2. The NO column density
was obtained from a table generated by AURIC (Strickland et al., 1999).
These maps were generated from the same GUVI spectral data and
represent simultaneous measurements of these parameters. NO
enhancement and O/N2 depletion were observed but limited to the high
latitude regions in both hemispheres on March 16. During the storm day
(March 17), NO enhancement and O/N2 depletion started to extend
lower latitudes. Such changes in NO and O/N2 are more significant on
March 18, especially in the Australia sector where significant N-149.3 nm
radiance enhancement was observed.
In addition to the intensity enhancement at high latitudes, the N149.3 nm radiances at equator and low latitudes also change over days.
Fig. 1. Observed GUVI spectra (black line), simulated N2 LBH bands (blue
dashed line) and net N-149.3 nm spectra (red line) between 140 and 155 nm
on July 16, 2012 (orbit 57 450). The method to determine the red line and
dashed blue line is described in the second paragraph of Section 2. (For
interpretation of the references to color in this figure legend, the reader is
referred to the Web version of this article.)
[1/cos(SZA)]. This simple correction may introduce some errors at high
SZA. But this study focuses on mid and low latitudes so the impact is
minimized. Following the same method used by Zhang et al., (2014),
data from along track pixels (5, 6, 7, 8, 9, 10, 11, 12) are used. Every 10
GUVI spectra are averaged. Spectral fitting process was performed for
each of these pixels using anaverageof 10 successive GUVI spectra. The
net N-149.3 nm radiances are obtained using the method described in
Fig. 1. Finally, the corrected radiances from the along track pixels are
Fig. 2. GUVI N-149.3 nm radiance (top row), relative errors in the N-149.3 nm radiances (2nd row), NO column density (3rd row), and O/N2 (bottom row) on
March 16 (left panels) and 18 (right panels), 2013. The solar zenith angle effect is corrected for N-149.3 nm radiances by 1/cos(SZA). The white oval indicates the
contamination of Southern Atlantic Anomay (SAA).
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Y. Zhang et al.
Journal of Atmospheric and Solar-Terrestrial Physics 169 (2018) 78–82
Fig. 3 shows the N-149.3 nm radiance maps on March 16, 21, and 28 in
2013 and 2015. It is apparent that the radiances decrease over the three
days in 2013 but increase in 2015 at mid and low latitudes. To help
understand these changes, the mean radiances with solar zenith angle
(SZA) < 60 at low latitude regions (15 < latitude < 15 ) and the longitudes (100 < longitude < 270 ) are plotted with solar EUV flux between 26 and 34 nm from SOHO/SEM (Judge et al., 1998) (see Fig. 4).
The selection of the longitude range (100–270) aims to avoid contamination from SAA which covers longitudes 300–360 and 0–100. The mean
N-149.3 nm radiances are correlated with solar EUV flux over the 30 days
(DOY 60–90) or about one solar rotation period. Specifically, over the
three days (March 16, 21, and 28 or DOY 75, 80, 87), the radiances
decrease with time in 2013 and increase with time in 2015, showing a
clear EUV control of the radiances.
The GUVI design has sufficient spectral resolution to extract the
149.3 nm emission from the underlying N2 LBH emission. GUVI has 14
independent along track pixels. Fig. 5a show GUVI spectral line-shape for
along tack pixel 8. The line-shape allows us to determine its Full Width
Half Maximum (FWHM) to be 1.3 nm. On the other hand, the GUVI detector's sample resolution is ~0.42 nm (see the data points or squares in
Fig. 5b). This indicates that the GUVI's system capability to resolve
spectral features are mostly determined by its FWHM which is larger than
the detector's resolution. Note that the GUVI central along track pixels are
well focused. But the pixels at both ends of the 14 pixels have a relatively
larger FWHM and this is why the data from along track pixels 5, 6, 7, 8, 9,
10, 11, 12 were used in this analysis.
Fig. 4. SOHO/SEM solar EUV flux (26–34 nm) between DOY 60 and 90 in
2013 (blue line) and 2015 (red line). The blue and red diamonds are for GUVI
N-149.3 nm intensities for 2013 and 2015, respectively, in the low latitude
region (15 < latitude < 15 , and 100 < longitude < 270 ). (For interpretation of the references to color in this figure legend, the reader is referred to
the Web version of this article.)
Fig. 3. GUVI N-149.3 nm radiance map on March 16 (top), 21 (middle) and 28 (bottom) in 2013 (left panels) and 2015 (right panels).
80
Y. Zhang et al.
Journal of Atmospheric and Solar-Terrestrial Physics 169 (2018) 78–82
Fig. 5. GUVI spectral line-shape (a) and sample spectra (b) for along track pixel 8. The Full Width Half Maximum (FWHM) is 1.3 nm. The detector spectral
resolution is ~0.42 nm (see the squares in the spectral data).
(R6, R7) dissociative recombination of NOþ and Nþ
2 , (R10) reaction be4
tween Nþ
2 and O. The mechanism (R5) just produces ground state of N( S)
which has no contribution to the N-149.3 nm emission that requires
N(2D). R8 is the same as equation (2). Other mechanisms (R6,7,10) can
produce N(2D). However, the relative contributions of (R6, 7, 10) to the
N-149.3 nm emission remain to be determined.
To further quantify the relation among the various radiances, the net
N-149.3 nm radiance, coincident LBHS and NO ε bands radiances and O/
N2 column density ratio from GUVI on March 17–19, 2013 are plotted in
Fig. 6. The scatter plots are for a region with 30 < Longitude <180 and
50 < Latitude <0 (sub-auroral latitudes) where significant storm-time
changes were seen in the parameters. The LBHS and N-149.3 nm radiances show a clear and tight linear correlation. A linear fitting is applied
to the LBHS and N-149.3 nm radiances. The fitting formula are (a) LBHS
¼ 4.5 *R149.3 nm 3 9.2, (b) LBHS ¼ 4.5*R149.3 nm þ 10 6.5, and
(c) LBHS ¼ 4.5*R149.3 nm þ 12 6.7 for March 17, 18, and 19, 2013,
respectively, where LBHS and R149.3 nm represent the radiance of LBHS
and N-149.3 nm emissions. The slopes remain unchanged (4.5) over the
three days. The fitting line LBHS intercepts (3, 10, and 12 Rayleigh) are
only about 1% or less of the LBHS radiances (>1 000 R). This indicates
that the LBHS/N-149.3 nm ratio is truly fixed at 4.5, regardless storm
(March 17) or storm recovery (March 18–19). This feature strongly
suggests that N-149.3 nm radiance traces N2 density since N2 LBHS is
proportional to N2 density.
On the other hand, Fig. 6 also shows that N-149.3 is positively and
negatively correlated with NO ε band and O/N2 ratio, respectively. Note
the peak NO ε band radiance changed from ~2 kR on March 17 to ~7 kR
3. Discussion
To understand the variations in the observed N-149.3 nm radiances, it
is important to briefly review the key sources of the emission. Meier et
al., (1980) listed three major processes that lead to the N-149.3 nm
emissions:
0
N þ e→N 2 D →Nð2 PÞ þ 149:3 nm
(1)
N2 þ e→N þ N þ e→149:3 nm
(2)
N2 þ hν→N þ N→149:3 nm
(3)
where equations (1)–(3) represent the processes of electron impact on
atomic nitrogen, N2 dissociation due to electron impact, and photodissociation of N2, respectively. The electrons in the equations are mostly
photoelectrons at sub-auroral latitudes. Simulations by Meier et al.,
(1980) further revealed that the process (equation (3)) produces most of
N-149.3 nm emissions followed by the process of equation (2). The
process (equation (1)) generates only about 20% of that of equation (3)
according to the simulations. At sub-auroral latitudes, the atomic nitrogen density is also contributed by N transported from auroral region and
photodissociation of nitric oxide (NO) (Engebretson and Mauersberger,
1983; Bailey et al., 2002). Note that the NO column density may increase
by a few times during storms at sub-auroral latitudes (Zhang et al.,
2014). In addition to equations (2) and (3), Bailey et al., (2002) listed a
few other mechanisms on production of N: (R5) photo dissociation of NO,
Fig. 6. Scatter plots between net N-149.3 nm radiance and coincident LBHS and NO ε bands radiances and O/N2 column density from GUVI on March 17–19,
2013 (a–c) in a region with 30 < Longitude < 180 and 50 < Latitude < 0 where significant storm-time changes were seen in the parameters.
81
Y. Zhang et al.
Journal of Atmospheric and Solar-Terrestrial Physics 169 (2018) 78–82
traces N2 density. This supports that dissociations of N2 are the key
sources of N-149.3 nm emission. On the other hand, NO related photochemical processes have little contribution to the N-149.3 nm intensity.
Simultaneous observations of N-149.3 nm, N2 LBHS, NO ε band emissions and O/N2 ratio by TIMED/GUVI provide a unique way to monitor
variations in storm-time thermospheric composition, part of important
space weather effects and serve as validation data sources for physics
based models.
(March 18) and ~4 kR (March 19). The peak N-149.3 nm radiance
changed from 270 R to 315 R and 270 R over the same period of time.
The relative changes in peak NO ε and N-149.3 nm radiances are ~250%
and 17% (March 18 versus 17) and 100% and 0% (March 19 versus 17).
The very different magnitudes of relative change indicate that NO is not a
dominant contributor in the N-149.3 nm radiance.
Considering all of the above factors related to the N 149.3 nm emissions, the enhanced N-149.3 nm emissions over limited regions during
storm-time, such as these shown in Fig. 2 (top panels), were mostly due to
enhanced N2 density. Other minor contributions could be related to
enhanced NO density (and associated photo-chemical processes) and
atomic nitrogen transported from the auroral region in the thermosphere
(Engebretson and Mauersberger, 1983) and excited by photo electrons
(see equation (1)). This interpretation is consistent with observed O/N2
depletion and enhanced NO column densities that are caused by the
heating (Joule and particle) and particle precipitation. As a matter of fact,
the O/N2 depletion is contributed by both O column density decrease and
N2 column density increase though the latter has a smaller magnitude of
change. Note that the O/N2 ratios were reduced from undisturbed values
~0.8 (north of Australia) to ~0.35 (over Australia), representing a ~55%
depletion (Fig. 2) on March 18, 2013. The N-149.3 nm radiance is
~270 R over Australia and ~240 R over the northern low latitude regions
in the Australia longitudes on March 18, 2013 (Fig. 3). This represents
about ~10% increase in the radiance in the storm-disturbed region
(Australia) over undisturbed region. Since the N2 density variations can
be traced by the changes in the N-149.3 nm radiance or N2 LBHS radiance
and considering that the O/N2 ratios depend on both O and N2 densities,
the above simple estimation indicates that the O density decrease is
~50%.
On the other hand, the photo electron density and N2 photo dissociation rate (related to the dominant processes in equations (2) and (3)) are
proportional to the solar EUV flux. This explains the close correlation
between the N-149.3 nm radiances in the low latitudes (away from the
regions with storm-time disturbance) and solar EUV flux (as shown in
Fig. 4).
Despite multiple sources for the N-149.3 nm emissions, observations
of the emissions provide a way to monitor the relative changes in stormtime N2 (the dominant source) density and variations in solar EUV flux.
Simultaneous data products of thermospheric O/N2 ratio, NO column
density and N-149.3 nm radiances from GUVI provide an opportunity to
study the thermospheric composition changes during geomagnetic
storms as well as provide data sources for validation of physics based
models or input for assimilative models.
Acknowledgements
This work is partially supported by NASA grants (NNX14AC13G,
NNX14AK74G2 and NNX15AB83G) and NSF grant (1681-206-2009506).
The AURIC software was provided by Doug Strickland and Scott Evans of
Computational Physics Inc. Discussion with Sam Yee is appreciated.
Appendix A. Supplementary data
Supplementary data related to this article can be found at https://doi.
org/10.1016/j.jastp.2018.01.023.
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4. Summary
Net N-149.3 nm emission intensities from the thermosphere are
determined using the TIMED/GUVI spectrograph data. During geomagnetic storms, the N-149.3 nm radiances at sunlit latitudes were enhanced
in the regions with O/N2 depletion and NO enhancement. On the other
hand, the N-149.3 nm radiances in the low and equatorial regions (undisturbed by geomagnetic storms) are strongly correlated with solar EUV
flux. A close quantitative comparison indicates that the N2 LBHS radiance
is proportional to N-149.3 nm radiances with a fixed ratio of ~4.5 at subauroral latitudes. This clearly suggests that the N-149.3 nm radiance
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