Longitudinal and Seasonal Variation of Lunar tide strength
Endawoke Yizengaw, Edgardo E. Pacheco, and Cesar E. Valladares
Institute for Scientific Research, Boston College, Chestnut Hill, Massachusetts, USA
Introduction
Lunar tide, which happens twice a day (semidiurnal tides), occurs due
to the gravitational attraction between the Earth and the moon. When
the sun and moon are aligned (see Figure 1), there are exceptionally
strong gravitational forces, causing very high and very low tides
which are called spring tides. On the other hand, when the sun and
moon are not aligned, the gravitational forces of Sun and Moon cancel
each other out and the tides are not as dramatically high and low
which is known as neap tides. It has been known since many decades
that lunar tide, which manifests itself as a semi-diurnal wave that
precesses through all local times within one lunar month, has been
identified as a significant force that can produce a notable influence
on the equatorial electrojet (EEJ) and longitudinal equatorial density
distribution. However, there has never been a comprehensive study,
except some statistical analysis using the satellite based in-situ
observations, of the lunar tide effect on a global scale. This left several
questions, like the longitudinal and solar flux dependence of the lunar
tide, the tidal wave strength difference between new and full moon
phases, remained unanswered. The proximity of the Earth/Moon
system to the Sun, proximity of a New/Full Moon to one of the nodes
of the lunar orbit, and the proximity of a New/Full Moon to the
perigee of the lunar orbit are among the main factors that can affect
the seasonal variability of lunar tide strength.
remove the ring current contribution of the magnetic field
disturbance, we subtracted the Dst index from the
magnetometer H-component, and (3) finally to remove the
solar tide component, we subtracted the 90-days average
(average taken after the Dst index removed). Finally the
residual variations are plotted as function of local time
and day of the year as shown in Figure 2. In this typical
example, a clear longitudinal difference of lunar tide
impact is clearly visible.
Figure 2 Signature of lunar tide strength obtained from
equatorial H-component magnetic field. The bold and
dashed vertical lines represent Full and New Moon periods.
Solar Cycle variation of lunar tide
Figure 1. Spring lunar tides occur when the Earth, the Sun, and the
Moon are in line which happens during the Full and the New Moon
Equatorial bubbles and lunar tide
GPS TEC depletions, which consist of abrupt decreases
in the TEC value that last from 10 to 60 min followed by
a recovery to the TEC value preceding the depletion, are
considered to be produced by plasma bubbles drifting
across the line‐of‐sight between the GPS receiver and
the satellite. The TEC depletion or bubbles show
significant seasonal dependence with stronger during
equinox and weaker during northern summer as shown
in Figure 4. The figure also shows that the occurrence
rate of the bubbles has a clear longitudinal dependences,
with the maximum bubble formations follow the solar
terminator and magnetic field alignment path, indicated
black curves in Figure 4. Moreover, the correlation
between the occurrence frequency of equatorial plasma
depletions (bubbles) and the lunar tide strength have
been investigated. It was found that the occurrence
frequency of the bubbles appeared to be stronger during
New and Full Moon periods as shown in the top panel of
Figure 5. This is consistent with the lunar period when
stronger lunar tide amplitudes are often observed as it is
shown in the bottom panel of Figure 5 which is obtained
from Jicamarca magnetometer data.
With the indications of lunar tide influence on the strength
of dayside EEJ (enhancing its strength shortly after New
and Full Moon), we utilized the fourteen years (1998 –
2012) ground-based magnetometer data and investigated
the various aspects of lunar tide effects during different
solar activities at three different longitudinal sectors. As it
is clearly shown in Figure 3, the lunar tide has a stronger
tidal amplitude during solar maximum than solar
minimum periods.
Longitudinal difference of Lunar tide strength
It is well known that, due to the enhanced ionospheric conductivity at
the magnetic equator, the horizontal component (H) of the
geomagnetic field experience stronger solar and lunar tides at the
magnetic equator compared to those at middle-to-low latitudes. Thus,
in order to understand the seasonal and longitudinal difference of the
lunar tide impact, we analyzed magnetometer data located at the
geomagnetic equator in different longitudinal sectors. The
magnetometers located at Jicamarca for American sector, Addis
Ababa for African sector, and Tirunelveli for Indian sector are used
for this comprehensive study. We applied the following steps to
extract out only the lunar tide component from the disturbance of Hcomponent magnetic field data: (1) to see the disturbance of the
magnetic field due to external phenomena, including lunar tide, the
daily variation, H, of the horizontal component of the magnetic
filed, is determined by subtracting the nightside average value, (2) to
Figure 3 As for Figure 2 but for different years of data.
Figure 4. TEC depletions detected from equatorial
GPS receivers located at different longitudinal sectors
during 2009-2011. Top panel show the TEC depletion
magnitude in TECU and bottom panel indicates the
TEC depletion time gap in minutes.
Figure 5. The number of bubbles observed in the Peruvian
sector plotted as a function of local time and day of the year
(top panel). The bottom panel shows the signature of lunar tide
obtained from Jicamarca magnetometer data. The bold and
dashed vertical lines represent Full and New Moon periods.
Conclusion
Using the 14 years of ground-based magnetometer data the
statistical influence of lunar tides on EEJ and density
distributions have been investigated at different
longitudinal sectors. The longitudinal and seasonal
difference of lunar tide strength have also been
investigated. This is the first comprehensive study, using
ground-based observation, that address the various aspects
of the lunar tide impact on the global density distribution
and on dayside EEJ. Some of the important findings of our
comprehensive study are:
• Most of our 14 years observations show the lunar tides
strength have a clear longitudinal difference, with
stronger tidal amplitude in the American sector
compared to the African and Indian sector.
• Our comprehensive study show a clear seasonal
differences (not shown here) in lunar tide impact with
stronger tidal amplitudes during November – February
and weaker around June solstice.
• Solar activity also has significant impact on lunar tide
amplitude; and thus during solar maximum period the
lunar tide have stronger amplitude whereas during solar
minimum the lunar tide has a much clearer feature but
weaker in amplitude.
• Finally, lunar tide has significant impact on density
distribution (not shown here) and on the formation of
density irregularities or bubbles.