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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, A11317, doi:10.1029/2008JA013367, 2008
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Reduction of electron temperature in low-latitude ionosphere
at 600 km before and after large earthquakes
Koh-Ichiro Oyama,1 Yoshihiro Kakinami,1 Jann-Yenq Liu,1 Masashi Kamogawa,2
and Tetsuya Kodama3
Received 3 May 2008; revised 8 August 2008; accepted 9 September 2008; published 25 November 2008.
[1] We examine ionospheric electron temperatures (Te) observed by HINOTORI
satellite during three earthquakes; M6.6 occurred in November 1981, M7.4 and M6.6 in
January 1982 over Philippine, respectively. It is found that Te around the epicenters
significantly decreases in the afternoon periods within 5 days before and after the three
earthquakes. The region of ionosphere disturbance extends to 80–120 degrees in
longitude. A tendency exists that duration of the disturbance becomes longer as the
increase of earthquake magnitude. F2 peak frequency, foF2 and virtual height, h’F from a
chain of 4 ionosonde stations located in the longitude zone of 120°E–130°E are used
together with electron density(Ne), that is observed simultaneously onboard HINOTORI
satellite to find possible cause mechanisms of the abnormal reduction of electron
temperatures. Behavior of HINOTORI Te/Ne and ionosonde foF2/h’F implies the existence
of westward electric field over epicentre. Our finding suggests that simple two plasma
instruments might be able to play a fundamental role to study ionosphere disturbance
associated with earthquake, if the constellation of small/mini satellites is organized and the
orbits are properly chosen.
Citation: Oyama, K.-I., Y. Kakinami, J.-Y. Liu, M. Kamogawa, and T. Kodama (2008), Reduction of electron temperature in lowlatitude ionosphere at 600 km before and after large earthquakes, J. Geophys. Res., 113, A11317, doi:10.1029/2008JA013367.
1. Introduction
[2] The possible precursor effects of the earthquake (EQ)
on the ionosphere have been reported by many scientists
[Pulinets and Boyarchuk, 2004]. Reduction of ionospheric
total electron content (TEC) produced prior to earthquake
occurrence has been reported by Liu et al. [2004]. Devi et
al. [2004] reported the reduction of TEC as well as increase
of TEC at the crest region of equator ionization. They also
found the bite out phenomena of foF2, which they claimed
appearing prior to a large earthquake. Depueva et al. [2007]
studied a strong earthquake which occurred near magnetic
equator on 15 August 1963 with magnitude M = 7.75 by
using Alouette-1 satellite as well as ionosonde data. They
found the reduction of F region density before and after the
earthquake more clearly in satellite data. At the same time
they stressed that the average effect on foF2 was very small
and was observed only in the daytime.
[3] In addition to the earthquake effects on low-latitude
ionosphere, there are several reports, which study the
middle latitude ionospheric foF2 and fbEs [Silina et al.,
2001; Ondoh, 2000]. Recently Pulinets et al. [2007] con1
Institute of Space Science, National Central University, Jhongli,
Taoyuan, Taiwan.
2
Department of Physics, Tokyo Gakugei University, Tokyo, Japan.
3
Earth Observation Research Center, Japan Aerospace Exploration
Agency, Tsukuba, Japan.
Copyright 2008 by the American Geophysical Union.
0148-0227/08/2008JA013367$09.00
ducted an intensive analysis on Irpinia earthquake which
occurred on 23 November 1980 by using seismic data, radon
emanation, hydrological anomalies, ground based ionosonde
network, thermal infrared irradiance, Intercosmos-19 satellite
topside sounding. They concluded that air lionization by
radon which was emanated during the earthquake preparation could explain all atmospheric and ionosphere parameters. However most of ionosphere researchers are still not
fully convinced with the existence of precursor effects of
earthquake [Rishbeth, 2007].
[4] Japanese Sun Observation satellite HINOTORI was
put into an equatorial orbit at the height of 600 km in
February 1981 with the inclination of 31°. Although the
satellite was dedicated to study solar physics, two unique
plasma probes which were developed in Japan [Hirao and
Oyama, 1970; Oyama et al., 1999; Oya and Obayashi,
1966] were accommodated to study ionosphere anomalies
triggered by solar events such as solar flare. These two
probes are resonance rectification probe for electron temperature Te, and impedance probe for electron density Ne.
The instruments have been flown in a number of sounding
rockets as well as satellites and their performances are well
established. Especially the resonance rectification probe
provides very accurate Te, which is even now difficult to
obtain [Oyama, 1976]. Data that were measured with these
two instruments on board HINOTORI were accumulated for
16 months, until the satellite operation was terminated in
June 1982 because of the battery problem. A set of the Te
data has been delivered to NSSDC, NASA and is open for
public users.
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Table 1. List of the Three Earthquakes, Which are Discussed in This Paper
EQ1
EQ2
EQ3
Date
Longitude
Latitude
Magnitude
Depth (km)
22 November 1981 15:05
11 January 1982 6:10
24 January 1981 6:08
120.8 E
124.4 E
124.3 E
18.8 N
13.8 N
14.1 N
6.7
7.4
6.6
24
45
37
[5] To study the effects of earthquake on ionosphere, we
adapt three procedures. As the first step, we need to grab
general features of Te/Ne, depending on local time, season,
solar flux, latitude and longitude [Su et al., 1996, 1997,
1998], as well as magnetic disturbance. Secondly, we need
to understand the physics of these various features, such as
Te in plasma bubble [Oyama et al., 1988], effect of electric
field on the morning overshoot [Oyama et al., 1996], Te
around equatorial ionization anomaly [Oyama et al., 1997;
Balan et al., 1997], annual behavior of Ne/Te [Bailey et al.,
2000], and effect of neutral wind upon Ne/Te regarding
tilted magnetic meridian [Watanabe et al., 1995; Watanabe
and Oyama, 1996; Oyama and Watanabe, 2004]. As the
third step, Te model [Oyama et al., 2004] as well as Ne
model [Kakinami et al., 2008], which describes normal and
average behaviors of the ionosphere are constructed. To
check the useful application of Te/Ne model, nighttime
increase of Te during geomagnetic disturbance was studied
[Oyama et al., 2005].
[6] On the basis of the high reliability of the models, we
tried to find out the earthquake effect on the ionosphere by
calculating the deviation of Te from the model value (DTe).
DTe during three earthquakes are examined. Table 1 summarizes geographic locations of the epicenter, the depths,
and the magnitude for 3 earthquakes (earthquake list is
provided by United State Geological Survey).
[7] Figure 1 illustrates the epicenters of three earthquakes
and locations of 4 ionosonde stations, which provide ground
based ionogram to be discussed in the later section.
collision with neutral gas is less. Owing to the low local Ne
and the resulting small heat capacity, Te at these heights is
raised rapidly with reducing solar zenith angle in spite of the
fact that the ionization of the atmosphere has changed very
little. When neutral density increases after the delayed
heating, energy of thermal electrons is dissipated through
the collisions between thermal electrons and neutral gas,
which results in a reduction of Te.
[10] Figure 2 displays that the morning overshoot peak is
higher in the winter hemisphere than in the summer. The
feature comes from lower Ne in winter than in summer. The
difference of Ne between two hemispheres is mainly caused
by meridional neutral wind [Lin et al., 2007]. Effect of
zonal wind appears on the Ne distribution in Asian as well
as American zones where magnetic meridional plane tilts
with respect to geographic axis [Watanabe et al., 1995].
[11] Afternoon overshoot, which shows more strong
latitude and seasonal variations, can be explained mainly
by heat flux conducted from the higher altitude along
magnetic line of force, although the heating due to photoelectrons travelling from below contributes. Another heat
2. Local Time Behavior of Te at 600 km
[8] To detect anomalies, typical variations of Te in
various local times, seasons, latitudes, longitudes, and solar
activities should be studied as a first step. Figure 2 illustrates one of the typical local time and geomagnetic variations of Te at 600 km during the northern winter months
(November, December, and January). It can be seen that Te
reaches the minimum value at 0400 LT, rises steeply and
yields a peak at 0900 LT (named as ‘‘morning overshoot’’). After the morning overshoot, Te reduces toward
noon, starts increasing at 1500 LT, and shows the second
peak at 1700 LT (named as ‘‘afternoon overshoot’’).
Finally the peak disappears around 2000 LT. The morning
overshoot is not so sensitive to geomagnetic latitude,
comparing to the afternoon overshoot. In contrast, the
afternoon overshoot shows a clear latitudinal variation; its
peak is very low over the geomagnetic equator, but becomes
higher in higher latitudes.
[ 9 ] The morning overshoot peak at 0900 LT is
explained as following [Da Rosa, 1966]. Energy of thermal
electrons is dissipated through collision with other thermal
electrons and with neutral particles. Heating of the neutral
atmosphere is delayed because of the large heat capacity
and therefore the neutral density is low in the early morning.
Accordingly the energy loss of thermal electrons caused by
Figure 1. Epicenters of the three earthquakes and locations
of four ionosonde stations.
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Figure 2. Local time dependence of Te at the height of 600 km with respect to geomagnetic latitude for
northern winter for F10.7 < 200. Data include all longitudes. Red circles mark morning overshoot and
afternoon overshoot.
source might be provided from downward ExB plasma drift
near sunset, leading to the observed enhancement of Te
[Watanabe et al., 1995; Bhuyan et al., 2002]. Contrary to
the morning overshoot, which is mainly controlled by
neutral gas density, Te peak of the afternoon overshoot
largely depends on Ne. It is noted that the Te peak shows
the lowest value in equatorial region, where Ne is higher
than that in higher latitudes.
[12] We find that Te in the afternoon overshoot reduces
drastically and systematically before and after earthquake,
as we report below.
3. Reduction of Te in the Afternoon Overshoot
[13] Three examples of Te reduction in the afternoon
overshoot before and after the occurrence of earthquakes
are shown in Figures 3, 4, and 5. Figures 3, 4, and 5 consist
of two figures (a) and (b). Upper panel and lower panel of
figures (a) and (b) in the three figures provide Te and Ne
respectively. At the bottom of each 6 sub-panels, UT
(Universal time), local time (LT), longitude (LON), latitude
(LAT), and magnetic latitude (MLAT) are provided.
[14] Black and blue circles plot values from model and
observation respectively. Two thin black curves provide the
root square error of 500 K for Te and first and third quartile
(25%) for Ne in the model. If observed Te deviates more
than 500 K from the model, we take the feature as abnormal.
[15] Upper panel of Figure 3a shows Te behaviors near
afternoon overshoot two days prior to EQ1. The observed
Te do not increase when Te model expects the increase at
around 0921 UT at the longitude between 91.7° and 109.3°.
Reduction of Te continues during the expected afternoon
overshoot.
[16] In the upper panel of Figure 3b shows the behavior
of Te in the afternoon overshoot one day after EQ1. The
model illustrates that Te should start to elevate at about
0936 UT in the longitude 76° and shows the peak at
around 0947 UT, and disappears at 1000 UT at the longitude
close to 161.7°. On the other hand, Te values observed
remains constant during the period when Te is expected to
elevate. Upper panel of Figure 4a shows the case for 3 days
prior to EQ2. The panel (a) indicates that the deviation of Te
starts at 1126 UT at the longitude between 69.4 °and 87.5°.
Upper panel of Figure 4b shows the Te behavior 1 day after
the EQ2 occurred. At 0654 UT, Te should start to elevate
toward 0704 UT. The Te observed keeps constant value until
0702 UT, starts to elevate, and finally merges to the model
value at 0706 UT. The same features as those of Figures 3a
and 3b are noticed. However the disturbed region appears
much wider than that for EQ1.
[17] For EQ3, panels and panel b of Figure 5 are
presented for 6 days before and 2 days after the earthquake
respectively. Panel (a) shows the small depression of Te in
the afternoon overshoot, which should start at 0856 UT
according to the model Te. Panel (b) shows that even before
afternoon overshoot starts Te becomes lower than the model
value at 0626 UT.
[18] In the lower panel of Figure 3a, Ne observed is nearly
equal to the model value at the beginning of Te reduction.
At 0930 UT, Ne starts to deviate from the model value.
[19] The lower panel of Figure 3b shows the similar
features; Ne follows the model value until 0948 UT and it
suddenly drops. The similar features are recognized in the
lower panels of Figures 4a, 4b, 5a, and 5b.
[20] The features, which we described above for Te and
Ne data, are common for all the three earthquakes.
[21] Figure 6 are shown to stress the spatial distribution
of DTe around earthquake days for three EQ events (EQ1,
top panel; EQ2, middle panel; and EQ3, bottom panel). DTe
is plotted in geographic longitude –latitude coordinate along
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Figure 3. Examples of the deviation of Te and Ne from the model for EQ1 in the afternoon overshoot
for EQ1. 2 days before EQ1 (a) (satellite pass, 4065), and 1 day after (b) (satellite pass 4110). Ne is shown
for each panel at the lower panels. Black and blue dots show the model and observation respectively. Two
thin lines show the upper and lower values of standard deviation. Lower panel shows the same but for Ne.
Location of the spacecraft is listed at the bottom. From the top, UT, LT, longitude, latitude, and
geomagnetic latitude.
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Figure 4. Same as Figure 3 but for EQ2. (a) 3 days before EQ2 (satellite orbit 4799). (b) 2 days after
EQ2 (satellite orbit 4856).
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Figure 5. Same as for Figure 3 but for EQ3. (a) 6 days before EQ3 (satellite orbit 4947). (b) 2 days after
EQ3 (5065/5066).
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Figure 6. Spatial distributions and daily variations of deviation of Te in the longitude -latitude
coordinates for three earthquakes. Satellite orbit data are shown for three periods: before EQ, during EQ,
and after EQ. D in each figure indicates the earthquake day. Stars in the middle of each panel show the
epicentre locations. Note that Te deviation spreads more than 50 degrees in longitude toward both west
and east from epicentre. Bold line in each figure indicates geomagnetic equator.
each satellite orbit. In the north of the epicentres, data
acquired by HINOTORI satellite was scarce. During the
access (about 10 minutes) with HINOTORI satellite, over
Kagoshima Space Centre (131.08°E, 31.24°N), the time
was used to retrieve the data stored in a data recorder and not
sufficient to retrieve the real time data. Red Stars in the middle
of the figures (22 – 26 November 1981, 11 – 15 January
1982, and 24– 28 January 1982) for three earthquakes mark
the epicentres. For each earthquake, three panels are
provided before earthquake (left), during the earthquake
(middle), and after the earthquake. The middle panels of
Figure 6 show that Te deviation from the model reaches the
maximum around earthquake days. The figures also show
that near epicentre the reduction of electron temperature is
most intense, although the maximum reduction does not
always coincide with the latitude of epicentre such as
the cases of EQ2 and EQ3. It is noted that the region of
large Te deviation ranges from more than 60° to the west and
40° to the east from the epicenter, respectively. Although
the data beyond 160° in longitude was not available for
EQ2, the region might extend more than 160° in the East.
The spatial distributions are nearly symmetric in longitude
with respect to the epicenter. Comparison of the disturbed
region of EQ2 with those of other two earthquakes appears
to suggest that EQ2 (the largest earthquake among the
three) has wider region than other two. It seems that the effect
of the Pacific Ocean does not exist as far as we examine the
EQ2, which has data up to the longitude of 170° east.
[22] As we mentioned that the largest deviation of Te with
respect to latitude does not match with the epicenter. This
suggests that only Te measurement is not enough to identify
the location of earthquake epicenter. However identification
of the epicenter might be possible by combining equator
and polar orbiting satellites, which provide other physical
parameter such as Ne and O+.
[23] Figures 7 and 8 are presented to show detail diurnal
variation of the deviation of Te (D Te) from the model for
three earthquakes respectively. The longitude and latitude
ranges are limited between 20° and 220°, and between 0°
and 32°, respectively. Dst and Kp indices, which illustrate
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Figure 7. Deviation of Te observation from model value for November 1981 (EQ1). Top panel shows
Dust (blue) and Kp indices (black). Bottom panel shows the Te deviation with respect to date. Depending
on the local time when the data were obtained, the data points are colored from dark blue (early morning)
to deep red (midnight) evening. Red arrow shows the days when earthquake EQ1, occurred. Please note
that points plotted by orange color, which correspond to afternoon overshoot, show the largest deviation
of Te. Deviation, which shows minimum around 11 November is considered to be caused by magnetic
disturbance.
magnetic disturbance at low and equatorial regions, are
plotted at the top of the figures. Vertical color scales at
the right in three figures show the local time when DTe is
observed. At the bottom panel, deviation of Te (DTe) is
shown with earthquake day marked by a star. Figure 7 shows
the EQ1. Magnetic disturbance occurs on 23 November
1981 with the lowest Dst values of 75 nT. The deviation
of Te from the model starts to increase on 18 November.
Two days data gap exist, and no measurement is available
on earthquake day. It appears that the largest deviation of Te
in the afternoon overshoot is found around 23 November
1981. Large negative deviations of Te from the model
are seen in the morning overshoot from 6 November to
13 November, which might be attributed to magnetic
disturbance. It is noted that deviation of Te from the model
scatters in both positive and negative values during morning
overshoot period for magnetic disturbance.. It is also noted
that daytime Te varies in antiphase with Ne for magnetic
disturbance [Oyama et al., 2005]. While for earthquake
disturbance deviation of Te is only biased to negative value,
and shows gradual steady reduction. Antiphase relation
between Te and Ne is not found.
[24] In Figure 8, EQ2 and EQ3 cases are shown. Dst
value is above 50 nT except 31 January. Kp indices are
below 4 most of the days. Two red stars in the horizontal
axis at the bottom panel indicate the days when EQ2 and
EQ3 occurred. Before 8 January 1982, no data were
acquired because of New Year holidays in Japan. Further
data were not obtained on 10, 17, and 24 January 1982
because of the non-operation of the satellite (one day off
every week).
[25] Te deviation shows the two minima around 11, 12,
and 13 January, and around 24, 25, and 26 January 1982.
These days are close to the days when EQ2 and EQ3
occurred. Although we cannot acquire firm conclusion, all
three cases indicate that Te deviation appears to reach its
maximum about 2 days after the earthquakes. It is noted that
the recovery to non-earthquake state is about one day slower
for EQ2 than EQ1 and EQ3.
[26] Findings obtained from the analysis of HONOTORI
data are summarized as followings.
[27] Reduction of Te in the afternoon overshoot is found
prior to and after earthquake in low/middle latitude region.
[28] Deviation of Te in the afternoon overshoot starts
about 5 days prior to the earthquake and recovers to the
original value after about 5 days. Deviation of Te tends to
appear earlier and the recovery is slower as the magnitude
of the earthquake increases.
[29] The disturbed region is nearly symmetric in longitude
with epicentre and ranges from 40°– 60° to the west and 40°–
60° to the east in longitude from the epicentre. A tendency
exists that larger earthquake has larger disturbed area.
[30] In latitude wise the range of deviation extends from
north to south with steep reduction in high latitude.
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Figure 8. Same as for Figure 7, but for January 1982 (EQ2, and EQ3). Two red arrows show the days
when earthquake EQ2, and EQ 3 occurred. Please note that orange color data points show the largest
deviation of electron temperature.
[31] Ne does not change or shows small increase at the
beginning of Te reduction, but later Ne reduces drastically.
The reduction of Ne is not because of the local time variation,
but because of the satellite location in higher latitude.
that EQ2 is the largest earthquake among three. Detail
feature of foF2 at these two stations will be described later
regarding the mechanism of the Te reduction.
5. Mechanism of Te Reduction
4. Ionosonde Data at Manila and Taipei
[32] Figures 9 and 10 show ionogram data obtained at
Taipei and Manila for EQ1, and EQ2 and EQ3, respectively.
At the top of the panels Dst and Kp indices are added to
make it easier to refer geophysical situation. The second and
third panels show daily variation of foF2 and h’F for Taipei.
Two black thin lines indicate first and third quartile of
15 days. Blue lines illustrate the 15 days median values.
Red lines show observation data for each day. The fourth
and fifth panels provide foF2 and h’F at Manila. Colors for
lines are the same as for Taipei.
[33] In Figure 9, h’F at Taipei does not indicate a clear
reduction associated with earthquake. While at Manila, h’F in
the afternoon shows clear reduction of about 100 km, and
small reduction of about 5 – 10 km in the early morning
between 18 and 25 November. These features are also seen in
Figure 10 for EQ2 and EQ3 before and after the earthquakes.
It is noted that reduction of h’F in Manila around 11 January
1982 when EQ2 occurred, is clearer than other two cases.
[34] Variations of foF2 due to earthquake effect are
difficult to be identified from Figure 9 and 10. However
at Manila, small increase of foF2 is found in the late
afternoon on 7, 9, 12, and 13 January 1982. Clear reduction
of foF2 is seen between 7 and 16 January in the early
morning when foF2 shows the minimum. It is noted again
[35] Our main finding from HINOTORI satellite is that
the afternoon overshooting of Te totally disappears or
partially disappears. Ne, which is very close to model value
or slightly higher than the model value at the beginning of
Te reduction, starts to decrease some time after the Te
reduction, which means that Ne depletion, occurs in higher
latitude. Ionosonde data show reduction of ionosphere F
region height of about 100 km at Manila. However foF2
does not change during this phenomenon. This feature is
very puzzling, because it is a well-known fact that during
daytime when Ne reduces (increases), Te shows increase
(reduction). Two components might play the role. One is
related to heat input from higher altitude, another from
lower altitude, as we described in section 1. For both cases,
a region of enhanced Ne should exist between h’F height
and 600 km height. This region might be produced as a
combination of eastward dynamo electric filed (which is
originally generated by neutral wind) and westward electric
field that is generated prior to earthquake, and remains even
after. Near equator the westward electric field weakens the
eastward electric field, and as a result, crests of the equator
anomaly shift to the equator ward, which has been reported
before [Liu et al., 2001, 2002]. In addition to this movement, h’F reduces due to the weakening of the eastward
electric field as Manila ionosonde shows clearly. Above F
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Figure 9. Ionogram data from Taipei and Manila for EQ1. From the top, first panel shows Dst and Kp
indices. The second and third panels show foF2, and h’F for Taipei respectively. The fourth and fifth
panels provide foF2 and h’F for Manila respectively. Two black thin, blue, and red lines in the second to
fifth panels show first and third quartiles of 15 days, 15 days median value, and observed foF2 and h’F,
respectively.
region height, the westward electric field drives ionosphere
plasma downward under earth magnetic field, and as a
result, a region is formed where electron density becomes
denser and this dense electron region remains late in the
afternoon.
[36] Heating of thermal electron due to the heat conducted from higher altitude reduces when enhanced Ne
region exists below 600 km and Te both at lower height
and at the height of 600 km decreases because of high-heat
conductivity along magnetic line of force.
[37] Photoelectrons, which are mainly generated in upper
E region and F region, and travel along magnetic line of
force, might also contribute to the behavior of Te at the
height of 600 km. Generally the photoelectrons continuously
heat thermal electrons up to the height of 600 km. However,
photoelectrons that travel through the density-enhanced
region dissipate their energy there, and as a result Te reduces
along magnetic line of force.
[38] In the higher latitude, a region of depleted Ne exists
because of the downward motion of plasma. This situation
is shown in Figure 11. Left and right panels are provided for
non-earthquake and earthquake case respectively. In both
panels direction of the natural eastward electric field is
shown by circled dot (pointing vertical from the paper). In
the right panel, westward electric field (vertical to the paper,
from the front toward back) is indicated by a crosssurrounded by a circle and three red vertical arrows show
the epicenters of 3 earthquakes. Dotted line shows the
altitude of HINOTORI satellite. A slant large arrow shows
the direction of plasma drift produced by the westward
electric field.
[39] Figure 12 shows the latitude distributions of foF2
around equatorial ionization anomaly with respect to LT for
EQ1, EQ2, and EQ3. Every 1 hour foF2 data from Manila
(121.1°E, 14.7°N), Okinawa (127.8°E, 26.3°N), Taipei
(121.2°E, 25.0°N), and Yamagawa (130.6°E, 31.2°N) are
used. Data between stations are calculated by linear interpolation. Top, middle, and bottom of the left panel show
15 days median values during 5 – 9 November 1981 for
EQ1, 24 December 1981 to 7 January 1982 for EQ2, and
8– 22 January 1982 for EQ3. In the right panel, top panel
shows foF2 on 20 November 1981 (2 days before earthquake). Middle panel shows foF2 on 8 January 1982 (3 days
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Figure 10. Same as for Figure 9 but for EQ2, and EQ3. It is noted that observed foF2 shows clearly the
value lower than the lower quartile, starting from 7 January to 16 January.
before EQ2). Bottom panel shows foF2 on 23 January 1982
(one day before EQ3).
[40] The panel shows three features on the day of the
earthquake: (1) Ne start increasing late in the morning, (2)
equatorial anomaly crest moves toward equator, and
(3) dense electron density region extends to even 20 LT at
the latitude of 20° to 25°N. The panel strongly supports the
mechanism that we proposed.
Figure 11. Cartoon, which shows the energy dissipation of photoelectrons, which travel through high
plasma density region below 600 km. Three arrows indicate epicenters of EQ1, EQ2, and EQ3,
respectively. Left panel and right panel are supposed to be for no-earthquake and earthquake days,
respectively. Direction of original dynamo electric field and westward electric field, which might be
associated with earthquakes are drawn by a circled cross and circled dot, respectively.
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Figure 12. Latitude-local time map of foF2 (top panel, EQ1; middle panel, EQ2, and bottom panel;
EQ3). Left figures in each panes show foF2 averaged over 2 weeks. Right figures in each panel provide
foF2 2 days, 3 days, and 1 day before EQ1, EQ2, and EQ3 respectively. Common features for three cases
are that (1) high plasma density region stays late in the afternoon and (2) EIA cleft moves toward lower
latitude.
[41] In the above we tried to explain our findings by
electric field. The reduction of Te in the afternoon overshoot
sometimes appears at magnetic conjugate point (for example, EQ on 9 October 1981, Lat/Long: 10.0/162.1, M = 6.5).
Ruzhin et al. [1998] reported similar conjugate point effects
for NmF2. We found that atomic ion density measured by
DE-2 shows the conjugate point effect. Although reduction
of Te in the afternoon overshoot can also be explained by
neutral wind at topside ionosphere [Watanabe et al., 1995],
relation between Ne and Te is in antiphase. It is also noted
that the depletion of Te, which we think as due to earthquake effect, is not detected beyond 30 degrees in
geomagnetic latitude. This feature is also favorable to
electric field.
[42] Therefore our next question is on the origin of
electric field. Although we do not want to speculate the
generation mechanism of electric field at this moment, when
information is scarce to get conclusion, we close this section
by only showing two plausible mechanisms below.
[43] The strong electric field might possibly be generated
at the heights where cloud is formed prior to and after
earthquakes. The westward electric field which shows a
symmetric feature both side of the epicentre suggests the
existence of a narrow region of very intense electric field
near epicentre along north – south direction; positive at
eastside and negative at west side of the epicentre. The
east/west width of the strong electric field region might be
less than 100 km, so that, this small-scale electric field
cannot be seen at the ionosphere height. This mechanism
assumes the direct penetration of electric filed into topside
ionosphere from cloud height.
[44] Recently Immel et al. [2006] showed that a newly
discovered 1000-km scale longitudinal variation in ionospheric density might be explained by consideration of the
dynamo interaction of the tides with the lower ionosphere
(E– layer) in daytime. England et al. [2006] published a
paper that seems to support above idea.
[45] Hagan and Forbes [2002] investigated mesospheric
and lower thermospheric migration and nonmigrating tidal
components that propagate upward from the troposphere,
where they are excited by latent heat release associated with
deep tropical convection. Pulinets proposes the ionization
produced by the radon emanating from the Earth’s crust. As
a result, hydrated ions are produced, which finally release
the latent heat of evaporation [Pulinets et al., 2007]. The
second mechanism to modulate lower E region by atmospheric waves seems to be most plausible.
6. Concluding Remarks
[46] On the basis of our firm confidence that the data of
Te and Ne which were measured by Japanese scientific
satellite HINOTORI are reliable, we have studied the
behavior of Te in the afternoon overshoot for three earthquakes larger than 6.5, which occurred around equator
ionization anomaly. 3 events show common features. Continuous systematic reduction of Te in the afternoon overshoot can be found prior to and after the big earthquakes.
We presume that a weak electric field, that is generated
associated with earthquake, is playing a fundamental role in
the region. The electric field is the order of 1 mV/m or less
and it should have very slow time variation of the order of
10 days starting from about 5 days before earthquake.
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OYAMA ET AL.: REDUCTION OF TE BEFORE AND AFTER EQ
[47] Although the cases, which we have shown here,
are for low-latitude earthquake of large earthquake,
the detectability of smaller earthquake or high latitude
earthquake depends on the accuracy of the model, which
can be made with repeated accurate measurements by
satellites.
[48] Our result suggests that even a small satellite,
which carries two simple reliable plasma probes, can play
unexpectedly significant role for the study of precursor
phenomena associated with earthquake. Height of the
satellite is one of the crucial factors for the findings.
International collaboration among countries that are suffering from earthquake disasters should be established urgently
to launch small satellites from these countries.
[49] The reduction of Te in morning overshoot, which
appears to be associated with earthquake, is also found in
HINOTORI satellite data, but not so clear. It is also stressed
that the cases exist where Te in the afternoon overshoot
shows the deviation from model when no earthquake is
reported in USGS list. The effects of large meteorite shower
seem to be not negligible. Cases also exist where clear Te
reduction is not found even in the existence of large
earthquake. This case seems to happen in high latitude
earthquake as well as in ocean earthquake. Although the
road to understand the precursor effects is still steep, these
issues will be summarized in the separate paper, as the
purpose of the present paper is to inform the high possibility
of detecting precursor effects of big earthquakes and
encourage scientists of various fields to work together as
early as possible.
[50] Acknowledgments. Prof. S. Watanabe, Hokkaido University,
originally constructed Te and Ne models. The authors are grateful to Prof.
I. Kutiev, Bulgarian Geophysical Institute, Bulgaria for his contribution at
the initial stage of this work. Suggestions by Dr. S. Abe of Japan Aerospace
Exploration Agency and Dr. T. Maruyama of National Institute of Information and Communications Technology regarding meteor shower events
were informative. The manuscript was completed while one of the authors
(K.-I. Oyama) was staying at National Central University as a visiting
Professor. We also express our gratitude to Mr. T. Izumida of Toshiba
Electronics Engineering Corp. for drawing Figures 9 and 10 and Mr. H. K.
Jhung of Institute of Space Science, National Central University, Taiwan for
drawing Figure 12. This work was partially supported by Japan Aerospace
Exploration Agency.
[51] Amitava Bhattacharjee thanks the reviewers for their assistance in
evaluating this paper.
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Y. Kakinami, J.-Y. Liu, and K.-I. Oyama, Institute of Space Science,
National Central University, Jhongli, Taoyuan, Taiwan. ([email protected].
ncu.edu.tw)
M. Kamogawa, Department of Physics, Tokyo Gakugei University, 4-1-1
Nakuikitamachi, Koganei City, Tokyo, Japan.
T. Kodama, Earth Observation Research Center, Japan Aerospace
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