We analyze tidal (diurnal, semidiurnal, terdiurnal, quarterdiurnal) phases
and related wind shear in the mesosphere/lower thermosphere as observed by
meteor radar over Collm (51.3∘ N, 13.0∘ E). The wind shear phases are
compared with those of sporadic E (Es) occurrence rates, which were
derived from GPS radio occultation signal-to-noise ratio (SNR) profiles
measured by the COSMIC/FORMOSAT-3 satellites. At middle latitudes Es are
mainly produced by wind shear, which, in the presence of a horizontal
component of the Earth's magnetic field, leads to ion convergence in the region
where the wind shear is negative. Consequently, we find good correspondence
between radar derived wind shear and Es phases for the semidiurnal,
terdiurnal, and quarterdiurnal tidal components. The diurnal tidal wind
shear, however, does not correspond to the Es diurnal signal.
Introduction
In the lower ionospheric E region, shallow regions of high electron density
are found, which are called sporadic E (Es) layers. Es layers consist
of thin clouds of accumulated ions. They occur mainly at middle latitudes,
and they are most frequently found during the summer season
e.g.,. Es are generally formed at heights between
90 and 120 km. Their occurrence can be described through the wind
shear theory . According to this theory, Es formation
is due to interaction between the metallic ion concentration, the Earth's
magnetic field, and the vertical shear of the neutral wind
e.g.,. If one neglects diffusion, the neutral gas
vertical wind component, and electric forces in the ion momentum budget
equation, the vertical ion drift wI becomes e.g.,:
wI=r⋅cosI1+r2U+cosIsinI1+r2V.
Here U and V are the zonal and meridional neutral wind components
pointing eastward and northward, respectively, and I is the inclination of the
Earth's magnetic field. The parameter r=ν/ω is the ratio of the
collision frequency ν between the ions and neutrals, and the gyro
frequency ω=eB0/mI, where e is the elementary charge, B0 is
intensity of the Earth's magnetic field, and mI is the ion mass. Since
r≫1 below ∼115 km , U is more efficiently
causing vertical plasma drift than V in the lower E region. Therefore, the
second term of Eq. () becomes small there, so that mainly the zonal
wind U is responsible for the vertical ion drift. Ion accumulation, i.e. the formation of Es, is consequently expected at those altitudes where the
vertical gradient of the vertical ion drift is negative (dwI/dz<0),
which in turn corresponds to the region of maximum negative vertical gradient
of the zonal wind, i.e. of maximum negative vertical zonal wind shear
dU/dz. Equation () is only valid for magnetic middle latitudes (about
20–70∘), because electric forces may be neglected there.
Correspondence between wind shear and Es had been obtained by
, who calculated ion flux divergence by using a model based on
the WACCM global circulation model, astronomically derived meteoric influx,
and winds from the Horizontal Wind Model HWM07 . They found
that wind shear theory satisfactorily explains the summer Es maximum,
although their metallic ion distribution also maximises in summer, which is
broadly in correspondence with qualitative results by
who compared meteor rates with Es occurrence rates (OR).
analyzed the global distribution of vertical ion
convergence based on GAIA Earth system model predictions. They showed that
the ion convergence distribution is broadly consistent with Es OR.
found correspondence between Es OR, calculated from
Global Positioning System (GPS) radio occultation (RO) data, and wind shear
measured with the TIDI instrument on the TIMED satellite.
The dynamics of the lower thermosphere at time scales up to one day are
mainly influenced by solar tides, with periods of a solar day and its
harmonics e.g.,. Their wind amplitudes usually maximize
around or above 120 km. In these regions, tidal amplitudes are of the
order of magnitude of the mean wind. Shorter period tidal waves often have
smaller amplitudes, so that, on a global scale, the major diurnal variability
of lower thermosphere winds is due to the diurnal tide
DT,, the semidiurnal tide SDT,, and, to a lesser degree, also to the
terdiurnal tide TDT,.
Note however, that at higher midlatitudes the DT amplitudes become small
compared with SDT ones during most of the year. Owing to its smaller
amplitude, the quarterdiurnal tide (QDT) has been analyzed less frequently in
the past , but more recently has attained increasing
attention .
Solar tides are a major source of the vertical wind shear, and the tidal
contribution to the overal wind shear is frequently larger than the one of
the background wind. Therefore, tide-like structures are also expected in
Es occurrence rates. Consequently, the SDT and DT are generally accepted
to be the major driver of Es , and they lead to the
downward moving tidal signatures in Es ionosonde registrations
e.g.,. Analyzing GPS Es
observations together with meteor radar wind measurements at Collm
(51.3∘ N, 13.0∘ E), could show that during one day
Es OR maximize when the zonal wind shear component due to the SDT is
negative. More recently, found a clear correspondence
between Collm zonal wind shear and Es OR for the 8 h component as well,
while showed from comparison of GPS Es RO and modeled
wind shear the similarity of the TDT in Es and wind shear on a global
scale. Similarly, recently showed that on a local scale
QDT phases agree with negative QDT wind shear phases, and also the global
distribution of QDT in Es is related to the one in wind shear.
modeled the connection of Es and tides, although
focusing at the equatorial region, where electric field effects become more
important than at middle latitudes.
The Collm meteor radar measures mesospheric and lower thermospheric winds at
altitudes of about 80–100 km since summer 2004 . These observations have already been used for
comparison with SDT, TDT, and QDT components in Es at these heights
, but for different time
intervals, and without providing an overview of all tidal components
together. In particular, the possible correspondence of the DT wind shear
over Collm with Es has not yet been considered. Therefore, in this paper
we analyze the DT, SDT, TDT and QDT seen in Es, obtained from GPS RO
measurements by the FORMOsa SATellite mission-3/Constellation Observing
System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) at the
latitude of Collm and compare Es phases with phases of negative wind shear
obtained from the local radar observations at Collm. Since the seasonal cycle
of Es and wind shear is different, comparison of amplitudes does not show
clear correspondence, so that we restrict ourselves to the discussion of
phase similarities. The remainder of the paper is organized as follows. In
Sect. the Es detection and the radar wind observations
are described. Results of tidal phase analysis and comparison with wind shear
phases are presented in Sect. .
Section concludes the paper.
Dataset descriptionSporadic E occurrence rates
We make use of ionospheric RO measurements by the FORMOSAT-3/COSMIC
constellation, which performs observations in both the neutral and ionized
atmosphere though a constellation of six low-Earth
orbiting (LEO) satellites. During each occultation, signals of the rising or
setting GPS satellites are received by a LEO satellite. When the signals pass
the atmosphere/ionosphere of the Earth, they are influenced in particular by
the ionospheric electron density, which cause refraction and degradation of
the GPS waves. This effect can be utilized to obtain information about the
ionosphere. Observation of the neutral atmosphere is also possible, but is
not the topic of this study. Detailed information on the RO technique
principles is provided by and .
Zonal and seasonal mean Es occurrence rates for (a) DJF,
(b) MAM,
(c) JJA, (d) SON. Data are averages over 2007–2016. Note the different
scaling for the summer OR. The grey contour lines show standard deviations
calculated from wind shear values for single years.
To derive information on Es from RO, we use the Signal-to-Noise ratio
(SNR) profiles of the GPS L1 phase measurements. The SNR is very sensitive to
vertical gradients of the electron density, which occur within Es layers.
These localized electron density variations lead to phase fluctuations of the
GPS signal, which are observed as changes of the received signal strength
. The basic signal power differs among ROs; therefore the SNR
profiles are normalized before they are analyzed. In those cases when no
ionospheric disturbances occur, the SNR value is almost constant above
35 km altitude. We analyze the vertical profile of the standard deviation
of the SNR. If the SNR exceeds an empirically found threshold of 0.2, the
profile is considered to be disturbed. If large standard deviation values
occur that are concentrated within a layer of less than 10 km thickness,
we assume that the respective SNR profile disturbance is owing to an Es
layer. The height where the SNR value most strongly deviates from the mean of
the SNR profile is taken as the Es layer altitude, as has been been
validated by comparisons with ionosonde observations . The Es OR is simply calculated by the number of Es
registrations in a given time, height, and latitude interval divided by the
number of RO in that time and latitude interval. The method to derive Es
OR from RO records was described by .
Figure shows 2007–2016 mean diurnal cycles of Es OR
within a 10∘ latitude window centered at 51.3∘ N, and within a
10 km height window each. Data from all longitudes are used. We show
seasonal means for December–February (DJF), March–May (MAM),
June–August (JJA) and September–November (SON), with the respective
standard deviations calculated from the OR data of single years added as grey
contours. Maximum OR are found at altitudes slightly above 100 km. OR
maximize in summer, which is thought to be owing to increased meteor influx
during that season , although claimed
that wind shear theory gives an explanation for this maximum, too. We note
that the diurnal variability of Es mainly contains a diurnal and
semidiurnal component. The quarterdiurnal and terdiurnal components, however,
although being weaker, are also included in the spectrum as was shown by
and .
Seasonal mean zonal wind shear over Collm for (a) DJF, (b) MAM, (c) JJA, (d) SON.
Data are averages over 2007–2016. The zero line is
highlighted. Note the different scaling of the respective panels. The grey
contour lines show standard deviations in ms-1km-1 calculated from
wind shear values for single years.
The tidal components of Es OR are calculated with a modeled least-squares
fit including mean Es0, 6, 8, 12, and 24 h
components:
EsMod(t)=Es0+∑i=14aisin2πPit+bicos2πPit,
with t as the time and Pi as the above mentioned periods. The
coefficients ai and bi are determined through minimizing
∑(Es(t)-EsMod(t))2. The phases Ti of Es OR are derived
from ai and bi as
Ti=Pi2πarctanaibi.
Collm mesosphere/lower thermosphere wind shear
At Collm (51.3∘ N, 13.0∘ E), a SKiYMET meteor radar is operated
since summer 2004. The radar operates on 36.2 MHz in a quasi all-sky
configuration, and the main parameter observed are the MLT radial winds
determined from the Doppler shift of individual meteor trails. Details of the
radar system and the radial wind determination principle can be found in
, , and . During
2015 the radar was upgraded by increasing the peak power, and replacing the
until then used Yagi antennas by crossed dipoles, while maintaining the
transmit frequency . The heights of the individual meteor
trail reflections vary between about 75 and 110 km, and the maximum
meteor count rate is found at an altitude slightly below 90 km
e.g.,.
The data are binned here in 6 different not overlapping height gates that are
centered at 82, 85, 88, 91, 94, and 98 km. The hourly mean
reflection height may slightly deviate from the nominal heights due to the
uneven height distribution of meteors within each gate ,
and in particular the real mean height of the uppermost height gate is close
to 97 km rather than 98 km so that we use this height as reference
altitude for the uppermost gate. Hourly mean horizontal wind values are
calculated from the individual radial winds within one height gate using a
least squares fit of the horizontal wind components to the raw data. This is
done under the assumption that vertical winds are negligibly small
. As in , , and
hourly values of the zonal wind shear are calculated from
adjacent height gates. The reference height for shear values is attributed to
the center between the nominal heights of the height gates.
Figure shows 2007–2016 seasonal mean diurnal cycles of
zonal wind shear over Collm. Again, the figure shows means for DJF, MAM, JJA,
and SON, with the respective standard deviations (in ms-1km-1)
calculated from the shear data of single years added as grey contours. The
zonal wind shear maximizes at more than ±8ms-1 in
winter, similar to the values shown by for January 2007.
Clearly, the main contribution to zonal wind and wind shear variability in
winter is due to the SDT. However, during the equinoxes the diurnal cycle
below about 90 km is more dominated by the diurnal component. In summer,
the wind shear is mainly positive, which is due to the strong positive
gradient of the zonal prevailing wind in combination with small tidal
amplitudes .
Therefore, considerable summer tidal amplitudes are only observed at greater
altitudes and negative wind shear with a semidiurnal diurnal cycle can be
seen only above ∼93 km. Therfore, ion accumulation and thus Es
formation is not supported below ∼93 km, which can be see in
Fig. , where summer Es are present at greater altitudes
than Es during the other seasons. This is in accordance with results,
e.g., by who also showed the higher summer Es altitudes.
Zonal and seasonal mean Es DT phases at 51∘ N for (a) DJF,
(b) MAM, (c) JJA, (d) SON. Data are averages over 2007–2016. Collm wind
shear phases acccording to Eq. () are added in red. Solid symbols
denote oscillations significant at the 5 % level according to a t test.
The error bars show standard deviations calculated from phases for single
years.
The tidal components of zonal wind shear are calculated with a modeled
least-squares fit according to Eq. (). The wind shear phases are:
Ti=Pi2πarctanaibi+Pi2,
similar to Eq. (), but since here we define the phases as the local
time of maximum negative wind shear, the Pi/2-term is added to the right
hand side of Eq. ().
Results
One can clearly see from Figs. and that the
seasonal cycle of Es amplitudes does not correspond with the one of zonal
wind shear below 100 km. Es maximize in summer, while the largest shear
amplitudes are found in winter. The discrepancy arises because Es OR
amplitudes are not only influenced by wind shear, but also by background
Es OR, which are dependent on meteor influx and background ionization in
the course of a day. Therefore, in the following we only compare phases of
the 24, 12, 8, and 6 h components of the Es OR and
the negative wind shear in order to check whether ion accumulation actually
take place at the convergence nodes of vertical ion drift forced by vertical
wind shear. Note that the results for the QDT phases have already been
presented in , but are repeated here for the sake of
completeness.
The radar observations of wind shear are only available at altitudes up to
about 95 km. Therefore, direct comparison between wind shear and Es
phases is not possible above that height. However, if we find a
correspondence between Es and wind shear at lower altitudes for some tidal
components, this may be considered as an indication that above 100 km the
Es diurnal cycle is also connected with tidal wind shear, although this
cannot be considered as a real proof for such a hypothesis.
Diurnal tide
Ground-based and satellite observations as well as model results show that
the migrating DT amplitudes maximize at or equatorward of 30∘
latitude .
Poleward of 50∘ the DT wind amplitudes are usually much smaller than
the SDT ones, which also has been frequently shown by radar observations
. Therefore it is not expected that the
relatively strong DT signature in Es as is seen in Fig. is
soleyly due to a strong wind shear DT. One may rather assume that the diurnal
cycle of background ionization may enhance the daytime Es OR and therefore
leads to a DT signature in Es.
As in Fig. , but for the SDT.
Zonal and seasonal mean Es phases at 51∘ N of the diurnal component
for four seasons are shown in Fig. . Data are averages over
2007–2016. Collm DT wind shear phases according to Eq. () are
added in red. Solid symbols denote oscillations significant at the 5 % level
according to a t test. The error bars show standard deviations calculated
from phases for single years. During most seasons, the Es phase is largely
constant with height. This would be consistent with the diurnal maximum of
Es owing to the general diurnal change of ionization. The Es phases do
not agree with wind shear ones during most of the year, except for summer at
altitudes below about 95 km. However, it is not clear whether this
correspondence may be due to coincidence. Although this does not exclude a
wind shear effect on Es, during most seasons a clear difference of the
phase structure shows that there is another effect on the diurnal Es
signature. The general disagreement between Es and wind shear phases
therefore leads to the conclusion that the main part of the diurnal cycle of
Es is most probably not due to the diurnal wind shear cycle and not
related to the wind shear DT.
As in Fig. , but for the TDT.
Semidiurnal tide
Globally, the SDT extends more into the northern winter hemisphere than the
DT , and the SDT is the
strongest tidal component at 50–60∘ latitude. Its amplitude at
90–100 km maximises during winter and autumn, which was frequently
shown from ground based and space borne observations , but in the thermosphere the SDT amplitudes are large in
summer as well . Zonal and seasonal mean phases at
51∘ N of the Es semidiurnal component for four seasons are shown in
Fig. . Data are again averages over 2007–2016, together
with Collm SDT wind shear phases according to Eq. ().
Es phases broadly agree with wind shear ones in winter and especially in
spring. In particular, their vertical gradients agree with each other. This
is in agreement with who found from comparison of the diurnal
cycle of Es occurrence and wind shear derived from the empirical HWM07
model that these are broadly in agreement for the SDT. Note that the SDT
phases do not vary strongly from year to year, so that comparison of an
empirical climatology and observed Es phases will provide reasonable
results.
There is a small but significant difference in winter. This might be due to a
nonmigrating SDT component which could lead to a difference of local and
zonal mean phases. However, nonmigrating SDT amplitudes are not large, as was
shown by from TIDI satellite observations. Longitudinal
phase differences are of the order of 1 h, but the
Collm SDT phases had been shown to be rather close to the zonal mean
. Still, however, this effect may explain the difference
between wind shear and Es phases in Fig. a.
In summer there is correspondence between SDT wind shear and Es phases
only at ∼95 km, and in autumn this is only the case at altitudes above
90 km. However, in autumn at lower altitudes Es 12 h oscillations are
not significant anyway, which may be due to the small SDT zonal wind shear
amplitudes. Also did not observe good correspondence between
Es and wind shear at lower altitudes in summer. Figure shows
that below about 90 km the autumn diurnal wind shear is dominated by the
DT, but not by a semidiurnal component. In summer, SDT wind amplitudes are
small at heights below 90 km, and the vertical wavelength is large
. Therefore, the SDT wind shear amplitude at these heights
is generally small in summer. Taking into account that the background wind
shear is strongly positive in the upper mesosphere, wind shear effects on the
Es diurnal cycle, if any, are expected only for the upper radar height
gates. Indeed, Fig. shows negative wind shear only for the
uppermost height gates, which corresponds to the similarity of Es and wind
shear phases at about 95 km, but not below that height, which is seen in
Fig. c.
Terdiurnal tide
Hints for terdiurnal signatures in Es were first reported by
, in a case study by in
ionosonde critical frequencies at Milos (36.7∘ N) and Rome
(41.9∘ N), and by in a more systematic study.
Radar observations at Collm and other sites at similar latitude had shown
that the TDT maximizes at equinoxes .
Consequently, maximum TDT Es amplitudes at 50∘ N derived from GPS
RO are also found during these seasons . During summer,
the Es TDT is weaker, although found a TDT only
during that season.
As in Fig. , but for the QDT. These phases have also
been shown in .
Zonal and seasonal mean Es OR phases at 51∘ N of the TDT for four
seasons are shown in Fig. . As in Figs. and
the data are averages over 2007–2016, and Collm wind
shear phases according to Eq. () are added in red. The data represent
an update of the results for 2007–2010 that were presented by
. Note that the TDT maximizes near equinoxes
, and correspondingly the Es
TDT are more significant then.
We note a correspondence of Es OR phases and Collm zonal negative wind
shear, especially concerning their changes with height. During winter, at
95 km Es and wind shear phases fit within their standard deviation,
while there are some differences below, however, at these heights the Es
TDT is not significant. During spring, there is a time shift visible between
wind shear and Es phases above 90 km. In the lower part the phases are
the same, but the Es TDT is not significant there. We conclude that
notwithstanding the larger TDT amplitudes in spring, a clear correspondence
of TDT wind shear and Es is not seen then, apart from the vertical phase
gradients that are similar.
As with the summer SDT, the Es and wind shear TDT phases during summer
only correspond at 95 km. This can again be explained by the positive wind
shear provided by the background wind, similar as with the SDT. Note,
however, if one extrapolates the wind shear phases, they would fit the Es
ones also slightly above 95 km. During autumn, there is a correspondence
between Es and wind shear TDT phases. However, in autumn the vertical
phase gradient is close to zero, i.e. the wave is evanescent as has been
shown already by . Therefore the wind shear TDT amplitudes
are small and the correspondence with Es phases may be due to coincidence.
Nevertheless, given the overall similarity of wind shear and Es phases we
may conclude that the wind shear mechanism plays a role in forming the TDT in
Es, although the correspondence is not that clear than the one of the SDT
components.
Quarterdiurnal tide
There are not many reports on a 6 h component in Es. Some publications
report that no QDT have been found in ionospheric records e.g.,
Cyprus, 35∘ N, 33∘ E,. However, 6 h tidal
signatures have been observed by incoherent scatter observations in lower
ionospheric Es parameters over Arecibo 18.3∘ N,, and QDT signatures were found in summer ionograms performed by
at 24.3∘ N. A study based on GPS RO has recently been
performed by .
Zonal and seasonal mean Es phases at 51∘ N of the QDT for four
seasons are shown in Fig. . Again, data are averages over
2007–2016, and wind shear phases are added. The data are the same as
were shown already in , so that they are only briefly
discussed here. Phases of the Es OR QDT agree very well with wind shear
QDT phases during each season, and given the generally small QDT amplitudes
the correspondence is striking, particularly in summer
when the TDT is not significant. We may conclude that the wind shear
mechanism is most probably responsible for the 6 h oscillation in Es.
Conclusions
Tidal phases from lower thermosphere meteor radar zonal wind shear
measurements at 51.3∘ N are analyzed together with Es OR phases at
the same latitude. The seasonal cycle of Es amplitudes is influenced by
the Es seasonal cycle with maximum in summer. Consequently, there is no
correspondence between tidal amplitudes in Es and wind shear tidal
amplitudes, and our analysis is limited to the comparison of phases.
Phases of SDT, TDT and QDT fit reasonably well during most seasons. This
means that tides seen in Es are probably at least partly due to neutral
zonal wind shear in the presence of a northward component of the Earth´s
magnetic field. The diurnal component of Es OR diurnal variability,
however, does not fit to zonal wind shear phases during most seasons, and is
therefore probably not primarily owing to the neutral wind DT.
Data availability
Radio occultation data are freely available from UCAR on
http://cdaac-www.cosmic.ucar.edu/cdaac/products.html, last access:
26 September 2018. Collm radar wind shears
are available from the corresponding author on request.
Author contributions
CJ performed Collm radar wind
measurements and analyses, as well as the tidal analyses based on GPS Es, which were analyzed by CA. The paper was written jointly by CJ and CA.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Kleinheubacher Berichte 2018”. It is a result
of the Kleinheubacher Tagung 2018, Miltenberg, Germany, 24–26 September 2018.
Acknowledgements
The provision of FORMOSAT-3/COSMIC data by University Corporation for
Atmospheric Research is gratefully acknowledged. Christoph Jacobi acknowledges
support through the Deutsche Forschungsgemeinschaft (DFG) under grants JA 836/30-1
and JA 836/34-1. Christina Arras acknowledges support by the DFG
Priority Program DynamicEarth, SPP 1788.
Review statement
This paper was edited by Ralph Latteck and reviewed by Laysa Resende and one anonymous referee.
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