GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L02101, doi:10.1029/2011GL050248, 2012
Observational evidence of ionospheric migrating tide modification
during the 2009 stratospheric sudden warming
J. T. Lin,1 C. H. Lin,2,3 L. C. Chang,4 H. H. Huang,4 J. Y. Liu,4,5 A. B. Chen,1 C. H. Chen,6
and C. H. Liu7
Received 3 November 2011; revised 14 December 2011; accepted 14 December 2011; published 19 January 2012.
[1] In this paper, modifications of the ionospheric tidal
signatures during the 2009 stratospheric sudden warming
(SSW) event are studied by applying atmospheric tidal
analysis to ionospheric electron densities observed using
radio occultation soundings of FORMOSAT-3/COSMIC.
The tidal analysis indicates that the zonal mean and major
migrating tidal components (DW1, SW2 and TW3)
decrease around the time of the SSW, with 1.5–4 hour
time shifts in the daily time of maximum around EIA and
middle latitudes. The typical ionospheric SSW signature: a
semi-diurnal variation of the ionospheric electron density,
featuring an earlier commencement and subsidence of EIA,
can be reproduced by differencing the migrating tides
before and during the SSW period. Our results also indicate
that the migrating tides represent 80% of the ionospheric
tidal components at specific longitudes, suggesting that
modifications of the migrating tides may be the major
driver for producing ionospheric changes observed during
SSW events, accounting for greater variability than the
nonmigrating tides that have been the focus of previous
studies. Citation: Lin, J. T., C. H. Lin, L. C. Chang, H. H. Huang,
J. Y. Liu, A. B. Chen, C. H. Chen, and C. H. Liu (2012), Observational evidence of ionospheric migrating tide modification during
the 2009 stratospheric sudden warming, Geophys. Res. Lett., 39,
L02101, doi:10.1029/2011GL050248.
1. Introduction
[2] A stratospheric sudden warming (SSW) is a meteorological event where the stratospheric temperature increases
rapidly in the winter polar region due to the abrupt slowdown or reversal of the polar westerly vortex by quasistationary planetary waves (QSPWs) [Matsuno, 1971]. An
SSW can last for several days or weeks as QSPWs with
zonal wave numbers 1 and 2 (PW1 and PW2), representing
zonal asymmetries in the polar vortex, grow in amplitude
1
Institute of Space, Astrophysical and Plasma Sciences, National Cheng
Kung University, Tainan, Taiwan.
2
Department of Earth Science, National Cheng Kung University,
Tainan, Taiwan.
3
Earth Dynamic System Research Center, National Cheng Kung
University, Tainan, Taiwan.
4
Institute of Space Science, National Central University, Chung-Li,
Taiwan.
5
Center for Space and Remote Sensing Research, National Central
University, Chung-Li, Taiwan.
6
Department of Geophysics, Kyoto University, Kyoto, Japan.
7
Academia Sinica, Taipei, Taiwan.
Copyright 2012 by the American Geophysical Union.
0094-8276/12/2011GL050248
in the high latitude region. Such QSPWs may propagate
vertically into lower thermosphere and interact with the
atmospheric tides that dominate the dynamics of that
region, resulting in a change of the global lower thermospheric wind patterns [e.g., Liu and Roble, 2002; Chang
et al., 2009; Pancheva et al., 2009]. Using NCAR
TIME-GCM, Liu et al. [2010] demonstrates that QSPW
activity associated with SSWs interacts nonlinearly with
pre-existing tides and modulates migrating and nonmigrating tides globally. Fuller-Rowell et al. [2011] simulates the
thermosphere and ionosphere electrodynamical response
to the 2009 SSW using the Whole Atmosphere Model
(WAM). Their simulation shows a substantial increase in
the amplitude of the 8-hour terdiurnal tide in the lower
thermosphere dynamo region (120 km altitude), which
has a significant impact on the diurnal variation of the
electrodynamics at low latitude ionosphere. The model
predictions coincide with recent observations of the equatorial upward E B plasma drift during the SSW, characterized by a morning enhancement and afternoon reversal
[e.g., Chau et al., 2009; Anderson and Araujo-Pradere,
2010]. Modification of the equatorial E B drift leads to
the earlier appearance and shorter duration (or earlier subsidence) of the EIA crests, resulting in electron density
enhancement in the morning/early afternoon and decrease
in the afternoon [e.g., Goncharenko et al., 2010a].
[3] Day-to-day variation of the ionospheric migrating and
nonmigrating tidal components have been examined using
global ionospheric map TEC (GIM-TEC) and electron density profiles retrieved by FORMOSAT-3/COSMIC (F3/C)
satellites. Pedatella and Forbes [2010] extracted tidal components from GIM-TEC, resolving an enhancement of the
semi-diurnal westward wave-1 (SW1) nonmigrating tide due
to nonlinear interaction between PW1 and the migrating
semidiurnal tide (SW2) during the 2009 SSW. Pancheva
and Mukhtarov [2011] reported that, using F3/C data at the
mid- and low-latitude ionosphere, the zonal and time mean
of ionospheric foF2 and the DW1 amplitude of electron
density at fixed height show negative responses to the SSW
temperature pulses.
[4] Although modification of the migrating and nonmigrating tides during the SSW have been examined individually by previous studies, the relative importance of the
migrating and nonmigrating ionospheric tidal responses in
generating the observed SSW ionospheric variability have
not been studied in detail. In this study, we apply tidal
decomposition to global observations of ionospheric peak
electron density (NmF2) obtained by F3/C. The modifications of ionospheric migrating and nonmigrating tidal signatures during this time are shown, and their relative
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Figure 1. Day-to-day variations of (a) zonal and time mean NmF2 and (b) amplitudes of diurnal westward wavenumber 1
(DW1), (c) semidiurnal westward wavenumber 2 (SW2), (d) terdiurnal westward wavenumber 3 (TW3) and (e) semidiurnal
westward wavenumber 1 (SW1) during DOY 001-055 2009. Solid lines overplotted in each frame are averaged stratospheric
temperature between 60–90° latitudes at 10 hPa potential height obtained from radio occultation soundings of F3/C.
importance in producing ionospheric variability during the
2009 SSW is examined.
2. FORMOSAT-3/COSMIC Data Analysis
[5] The 2009 SSW presents unique conditions for studying stratosphere-ionosphere coupling due to a particularly
strong and long lasting warming effect during deep solar
minimum conditions. It is known that SSWs can last for
several days or weeks, Goncharenko et al. [2010b] found
that perturbations in the EIA were observed for up to 3 weeks
after the peak in high-latitude stratospheric temperatures.
Similarly, observations from ground-based GPS-TEC and
electron densities retrieved from F3/C indicate that the
signature of ionospheric response to 2009 SSW event
occurred during DOY 022-047 (Lin et al., Observations of
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Figure 2. Amplitudes of DW1, SW2 and TW3 migrating tides (a, b, c) before SSW on DOY 009 and (d, e, f) during SSW
on DOY 028. Superposition of migrating tides (g) before and (h) during SSW and (i) their difference.
global ionospheric responses to the 2009 stratospheric sudden
warming event by FORMOSAT-3/COSMIC, manuscript
submitted to Journal of Geophysical Research, 2011 hereinafter referred as Lin et al., submitted manuscript, 2011).
For the F3/C satellites, one needs to accumulate around
20-day observations to obtain the full global and local time
coverage required for unambiguous tidal retrieval. In this
paper 20-days of ionospheric peak electron density (NmF2)
data (with storm days removed) are binned into a 5° 5°
grid in geographic longitudes and magnetic latitudes
between 40 magnetic latitude (MLAT) with the time
interval of one hour in local time. The 20-day window is
then moved through the time series with steps of one day
in order to obtain the daily values.
[6] The NmF2 are further analyzed in terms of their various migrating and nonmigrating tidal components. Similar
to analysis of solar atmospheric tides [e.g., Zhang et al.,
2006; Pedatella and Forbes, 2010], the NmF2 are fit to the
following harmonic functions by using the least-squares
method:
X ðtLT ; lÞ ¼ X þ
3 X
4
X
An;s cos nWtLT ðn þ sÞl þ qn;s
n¼1 s¼4
þ
4
X
A0;s cos ðsÞl þ q0;s :
ð1Þ
s¼1
[7] Where X denotes the binned observational data, X the
zonal and time mean, n is subharmonics of a solar day, W is
the rotation rate of the Earth (2p/day), tLT the local time, s
the zonal wavenumber propagating in the eastward (positive values) or westward (negative values) direction, and l
the longitude, while An,s, qn,s and A0,s, q0,s are amplitudes
and phases of migrating/nonmigrating tides and stationary
planetary wave, respectively. The n = 1, 2, 3 components
are referred to as diurnal, semi-diurnal, and ter-diurnal
tides, respectively. The fit is performed for n = 0, 1, 2, 3
and s = 4, 3, … 3, 4 and a nearly full spectrum of
migrating/sun-synchronized (n + s = 0) and nonmigrating
(n + s ≠ 0) tidal components and stationary planet waves
for a range of latitudes are obtained.
3. Results and Discussion
[8] Figure 1 shows the day-to-day variations of the
retrieved zonal and time mean ( X ) and amplitudes of
migrating (DW1, SW2 and TW3) and SW1 nonmigrating
tide during DOY 001-055 2009. The zonal and time mean
shows a clear reduction and hemispheric asymmetry during
DOY 020-040 in both northern and southern EIA regions
(1540° MLAT), before returning to a larger value symmetrically in both hemispheres. Similar reductions during
DOY 020-040 are seen in DW1 amplitude at EIA regions
and TW3 around the magnetic equator. Reductions in the
zonal and time mean and the DW1 are consistent with those
reported by Pancheva and Mukhtarov [2011]. It is noted that
the decrease of TW3 amplitude occurred only during the
SSW period when inspecting the TW3 variations during
DOY 305 2008 - DOY 055 2009 (figure not shown). The
SW2 shows apparent reductions in the southern EIA region
after DOY 020, whereas intensifications of SW2 in the
northern EIA region appear after DOY 030. The SW1 nonmigrating tide weakens as early as DOY 015 followed by
major intensifications in both hemispheres after DOY 030.
The intensifications coincide with amplitude enhancement
of SW2. Appearance of SW2 reduction before the peaked
stratospheric temperature may be related to the reversal of
the zonal mean zonal wind (from eastward to westward).
The strong westward wind in the MLT region could decrease
the strength of westward propagating tides prior to the SSW
[Pedatella and Forbes, 2010].
[9] Electron density observations indicate that the ionospheric SSW effect is characterized by a morning enhancement and afternoon reduction [Goncharenko et al., 2010a].
Lin et al. (submitted manuscript, 2011) show that during the
2009 SSW the global ionosphere reveals a strong electron
density reduction for several hours in the afternoon period,
while the morning enhancement occurs for a shorter duration. It is also known that the usual EIAs become well
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Figure 3. (a) NmF2 map at 120°E geographic longitude on DOY 028 observed by F3/C, (b) the reconstructed 9 zonal
wavenumber harmonic fits to the F3/C data, and (c) superposition of the migrating tides, DW1, SW2 and TW3.
developed and the electron densities therein are stronger
during afternoon period than morning. Therefore, the SSW
related short-period morning enhancement and prolonged
afternoon reduction in the electron densities may be reflected
in the decrease of the zonal and time mean. The effect may
also decrease the amplitudes of DW1, SW2 and TW3.
Unlike the DW1 and TW3, SW2 intensified after DOY 030,
after the stratospheric temperature reaches its maximum on
DOY 024. Similar results were reported by Pedatella and
Forbes [2010], who attributed the SW2 enhancement to
enhanced eastward mean winds in the post-SSW period due
to forcing by PW1 upon the middle atmosphere mean flow
[cf. Liu et al., 2010]. Enhancement of the nonmigrating SW1
may result from nonlinear interaction between PW1 and
SW2, which generates SW1 as a byproduct [Chang et al.,
2009]. It is noted that while SW1 has comparable amplitudes to TW3, it is only 10% and 50% of DW1 and SW2
amplitudes, respectively.
[10] Fuller-Rowell et al. [2011] show that the 2009 SSW
related modifications of thermospheric SW2 and TW3
become prominent on DOY 024 in the Whole Atmosphere
Model (WAM), while Goncharenko et al. [2010a] show that
the low-latitude TECs are modified largely on DOY 027.
The reductions of zonal and time mean, migrating and
nonmigrating tides shown in Figure 1 coincide with these
studies. In Figure 2, we compare the local time variation
produced by individual migrating tidal components before
(DOY 009) and during (DOY 028) the SSW. Clear amplitude reductions and phase shifts are seen in each of the three
migrating tides. DW1 has a daily maximum occurring at
13.5 LT in the northern hemisphere (14.5 LT in the
southern hemisphere) before the SSW. During the SSW,
the DW1 maximum appears at 12 LT in the northern
hemisphere (13 LT in the southern hemisphere), approximately 1.5 hours earlier than before the SSW. Similarly,
there are 2.5–3.5 and 4 hour time shifts for SW2 and TW3
(around EIA and middle latitudes), respectively. Comparing
the superposition of the three migrating tides before and
during the SSW, a feature of semi-diurnal variation similar
to GPS-TEC variation reported by Goncharenko et al.
[2010a, 2010b] is obtained (Figure 2i). This result suggests
that modification of ionospheric migrating tides alone can
produce the typical ionospheric response to the SSW.
Fuller-Rowell et al. [2011] report that the migrating tides in
the lower thermosphere dynamo region are substantially
modified during the 2009 SSW, where the TW3 showed a
substantial increase at the expense of the more typical SW2.
It is noted that the ionospheric tidal components may not
exactly correspond to the components of thermosphere. The
thermospheric SW2 and TW3 perturbations in the E-region
could affect the dynamo processes leading to perturbations
in diurnal vertical E B drift and ionospheric electron densities. The thermospheric results reported by Fuller-Rowell
et al. [2011] and the ionospheric results obtained from
F3/C indicate that variations of the migrating tide during the
SSW may be the major driver responsible for the ionospheric
electron density variations during this time. The importance
of migrating tidal variations responsible for the SSW ionospheric effect is further elucidated in Figure 3, which shows
that the superposition of the migrating tides alone represent
more than 80% of the ionospheric local time variation
resulting from all (both migrating and nonmigrating)
tidal components.
[11] The earlier time shifts of the three migrating tidal
maxima similar to those shown in Figure 2 can be derived
from the initial phases after applying the harmonic fitting.
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Figure 4. The day-to-day variation of occurrence times of maximum amplitudes of (a) DW1, (b) SW2 and (c) TW3 derived
from their initial phases using equation (2) in the text.
For migrating tides, each of their cosine function reaches its
maximum when
tLT
MAX
¼
qn;s
qn;s 24
¼
nW
n 2p
ð2Þ
From this, the occurrence time of first daily maximum of
each migrating tidal component can be obtained. Figure 4
shows the day-to-day variations of occurrence times of
maximum amplitudes of DW1, SW2 and TW3 derived from
their initial phases using (2). The phase shifts appear around
DOY 020-050, and become more prominent around DOY
028-040 at EIA latitudes. The largest time shifts for DW1
and SW2 at EIA latitudes are 1.5 and 4 hours, respectively.
The largest time shift for TW3 occurred around EIA and
middle latitude is about 4 hours. It is noted that the maximum amplitude of TW3 appear around the magnetic equator, however the time shift is not obvious therein. It is also
interesting that the most prominent time shifts of migrating
tides are seen around DOY 035 (Figure 4), several days
after the stratospheric temperature maximum at DOY 024.
The time delay of ionospheric response is consistent with
other observation results [e.g., Goncharenko et al., 2010b;
Pedatella and Forbes, 2010] as well as theoretical model
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LIN ET AL.: IONOSPHERIC MIGRATING TIDES DURING SSW
prediction [e.g., Liu et al., 2010], although the mechanism is
not clear.
[12] In conclusion, the day-to-day variations of amplitudes
and phases of ionospheric tidal components (Figures 1 and
4) can be applied to study the ionospheric electron density
modification and time shift of daily maximum related to the
2009 SSW. The ionospheric tidal components extracted
from F3/C electron density observations indicate that the
variations of migrating tidal components represent the
majority (80%) of the ionospheric SSW effect (Figures 2
and 3), as opposed to the nonmigrating tides, which play a
secondary role. In the future, the observational results presented here could compare with tidal signatures of ionospheric electron density output from whole atmosphere
models and further correlate with neutral atmospheric tides
therein to verify relative importance of SSW related tides.
[13] Acknowledgments. The authors are grateful to Arthur D.
Richmond, Wenbin Wang and Hanli Liu of NCAR-HAO for their useful
suggestions and comments. This work is supported by the Taiwan National
Science Council under project numbers NSC 100-2628-M-006-002-MY3
and 100-2119-M-006-013. Part of the work is supported by NSPO under
NSPO-S-100011. The authors wish to thank the reviewers for their pertinent and constructive comments.
[14] The Editor thanks two anonymous reviewers for their assistance in
evaluating this paper.
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L. C. Chang, H. H. Huang, and J. Y. Liu, Institute of Space Science,
National Central University, Chung-Li 320, Taiwan.
A. B. Chen and J. T. Lin, Institute of Space, Astrophysical and Plasma
Sciences, National Cheng Kung University, Tainan 70101, Taiwan.
C. H. Chen, Department of Geophysics, Kyoto University, Kyoto
606-8502, Japan.
C. H. Lin, Department of Earth Science, National Cheng Kung
University, Tainan 701, Taiwan. ([email protected])
C. H. Liu, Academia Sinica, Taipei 115, Taiwan.
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