Ionospheric signatures of Lightning
M. L. Hsu1 and J. Y. Liu1,2
1
Institute of Space Science; and 2Center for Space and Remote Sensing Research,
National Central University, Chung-Li 32054, Taiwan
1. Abstract
The geostationary metrology satellite (GMS) monitors motions of thunderstorm
cloud, while the lightning detection network (LDN) in Taiwan and the very high
Frequency (VHF) radar in Chung-Li (25.0°N, 121.2°E) observed occurrences of lightning
during May and July, 1996. Measurements from the digisonde portable sounder (DPS) at
National Central University shows that lightning results in occurrence of the sporadic Elayer (Es), as well as increase and decrease of plasma density at the F2-peak and E-peak in
the ionosphere, respectively. A network of ground-based GPS receivers is further used to
monitor the spatial distribution of the ionospheric TEC. To explain the plasma density
variations, a model is proposed.
2. Introduction
The ionosphere can be affected by many disturbances. There are considerable
evidences that transient disturbances occur in the ionosphere as results of weather in the
troposphere (Davies 1989). Recent observation of lightning associated upward electrical
discharge events including cloud-to-stratosphere, cloud-to-mesosphere, and lighting
induced airglow brightening in the lower ionosphere, such as red sprite, blue jet, and elve
(for example, see, Boeck et al., 1995; Wescoett et al., 1995; Winckler et al., 1996)
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demonstrate the importance of the electrical coupling existing between the troposphere
and the conducting ionosphere. However, these studies have generally been concentrated
with variations and mechanisms occurred below 90-120 km altitude of the lower
ionosphere. Since the ionosphere consist of a large amount plasma, it is interested in to
find how its electron density response to thunderstorms. This study examines concurrent
measurements of the geostationary metrology satellite (GMS), the lightning detection
network (LDN), the very high frequency (VHF) radar, the digital ionosonde, digisonde
portable sounder (DPS), the network of 7 ground-based receivers of the global positioning
system (GPS). The experiment was design to employ the GMS monitoring motions of
thunderstorm clouds; the VHF radar and the LDN detecting cloud-to-cloud (CC) and
cloud-to-ground (CG) lightning in the troposphere, and the DPS simultaneously observing
the electron density variation in the bottom-side ionosphere, from the E-region about 100
km altitude, the sporadic E layer around 110 km altitude to the F-peak 300-500 km
altitude, and the GPS network taking snap shots every 30 seconds for the ionospheric total
electron content. Thus, the 3-dimension structures of geo-space plasma – ionospheric
electron density - response to thunderstorms are simultaneously monitored by the five
different observations.
During May and August 1996, eight campaigns were carried out. One only The
observation of the GMS covers the whole Asia sector. To focus on the cloud motions
around Taiwan, the cloud maps between latitudes15°-35°N and longitudes 100°-130°E,
geographic, are presented in Fig.1. The Data of the LDN is set to cover an area of a radius
of 50 km at the center of National Central University (NCU), Chung-Li (25.0°N, 121.1°E).
Both VHF radar and DPS locate at NCU. The beam width and the side lope of the VHF
radar are respectively 7° and 50°, which result in the cloud-to-cloud lightning within about
60° field of view to be observed [Rottger et al., 1995]. The DPS beam width, which is a
function of the sounding frequency, varies from 60° to 120°. The combination of these
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four measurements allows us to observe cloud motions, CC lightning, CG lightning,
ionospheric electron density simultaneously.
Three typical thunderstorm events occurred on May 28 (Event A), June 14 (Event
B), and July 16 (Event C), 1996 in Taiwan area are investigated.
Figures 1a-1c
respectively show the cloud motions/locations of the three events observed from the GMS.
It can be seen that around 1132 LT of Event A, 1132 LT of Event B, and 0933 LT of
Event C the thundercloud appear near Chung-Li. Due to the limited radar operation hour,
the Chung-Li VHF radar was swished on, after the thunders were detected audiously and
by an oscilloscope; and turn off when they were no longer detected. The top panels
(panels a) in Figures 2-4 reveal that the CC lightning of the three events recorded by the
VHF radar occur during 1138-1419 LT, 1425-1605 LT, and 1345-1513 LT, respectively.
The magnitude and occurrence time of the CG lightning around Chung-Li of the three
events observed by the LDN are also presented in the top panels of the three figures. The
three panels show that Event A, B, and C have moderated (1310-1530 LT), no, and sever
(1240-1540 LT) CG lightning, respectively. Panels b, c, and d in the three figures present
the observed (asterisk lines), associated medians (solid lines), and associated means
(dashed lines), of the plasma frequencies of the F2 peak (foF2), the sporadic E (foEs), and
the E-peak (foE) from the DPS during the three events. Note that the associated medians
and means are calculated by data recorded from 15 days before to 15 days after the event
days. For each event, the agreement between the median and mean indicates that the two
are reliable references. Figures 2b-2d display that for Event A, foF2 (1530-1940 LT) and
foEs (1600-2100 LT) significantly decrease and increase while foE (1000-1300 LT and
1500-1700 LT) slightly decreases. Figures 3b-3d illustrate Event B that foF2 (1200-1800
LT) decreases, foEs (1300-1500 LT, after 1930 LT) intermittently increases, and foE
(1130-1400 LT) slightly reduces, respectively. For Event C, it can be found that foF2
decreases during 1330-1700 LT (see Figure 4b) and has no data recorded due to significant
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increase in foEs during 1000-1300 LT (see Figure 4c). Figure 4d illustrates that foE
slightly decreases during 1000-1300 LT. Table 1 summarize the appearance time of the
cloud observed by the GMS; and durations of the CC and CG lightning, respectively,
recorded by the VHF radar and the LDN as well as foE decrease, foF2 decrease, and foEs
increase observed by the DPS of the three events.
Table 1. The Summary of the Appearance Time in LT of the Cloud; and the Durations of
the Lightnings, and the Ionospheric Plasma Frequency Variation.
Event\obs.
GMS foE
VHF
LDN
foF2
foEs_____
A
1132 1000-1300
1138-1419
1310-1530
1530-1940
1600-2100
1425-1605
--
1200-1800
1300-1500
1500-1700
B
1132 1130-1400
1930--C
0933 1000-1300
1345-1513
1240-1540
1330-1700
1000-1300
1820-1940
3. Discussion and Conclusion
In Event A, the largest amount of thunderclouds, the longest duration of the CC
lightning, and the moderated amount of the CG lightning were recorded, while the greatest
changes in foF2 and foEs were observed. In Event B, the least amount of thunderclouds,
the shortest duration of the CC lightning, and no the CG lightning were observed, and
however, clear decrease in foF2 and increase in foEs were still detected. These indicate
that the CG lightning is not a necessary condition for the changes in foF2 and foEs. Event
C with moderated amount of the thundercloud yields the greatest values in the amount and
strength of the CG lightning among the three events. This suggests a certain amount of
thundercloud may be necessary to trigger the CG lightning. Nevertheless, results of the
three events show that after the CC and CG lightnings were detected, an obvious (25-50%)
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decrease in foF2 appeared.
It has been identified that thunderstorms are the dominant generator in the global
circuit, which connects the solid/liquid Earth, troposphere, middle atmosphere, ionosphere
and magnetosphere. Currents flow upward from the tops of thunderstorms maintains a
potential difference of 200 to 6000 kV between the Earth surface and the ionosphere
[Krider and Roble, 1986].
Scientists studied heating and ionization of the lower ionosphere by lightning
[Inan et al., 1991; Taranenko et al., 1993] and found that no substantial electron density
changes occur during daytime. Results of this study show that during the lightning period,
the plasma frequency (or electron density) at the E-peak (daytime at 109-113 km virtual
height) slightly decrease, which seems to agree with the daytime result of Taranenko et al.
[1993].
Hegai et al. [1990] based on the penetration of thundercloud electric fields into the
ionosphere of Park and Dejinakarintra [1973] theoretically studied the influence of
thundercloud electrostatic filed on the nighttime middle latitude ionospheric E-region.
They found that due to the thundercloud electrization, a cavity could be formed in the
upper part of the E-region above the cloud. Their profiles of electron density further
showed that the electron density slightly decrease at the E-peak but appreciably decrease
in the upper part. These generally agree with the slight decrease in foE and significant
decrease in foF2 observed in the three events.
Baginski et al. [1988] examined lightning-related transient electric fields and
conduction current density due to the redistribution of charge by the return stroke. They
suggested that the long-duration charge related fields could be responsible for triggering
lightning related magnetospheric events, which are whistlers, high-energy electron
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precipitation, etc. [Hale and Baginski, 1987].
Burke [1991] further showed due to
lightening electric field, precipitation of high-energy electrons producing secondary
ionization, which could result in occurrences of the Es layer. The plasma frequency (or
electron density) at the Es-layer (daytime at about 113-125 km virtual height) significantly
increases, which agrees with the result of Burke [1991]. However, the L-value 1.2 at
Chung-Li, which means the field at magnetic equator to be 1,200 km altitude, indicates
instead of the magnetosphere related that the significant increase in foEs of current study
results from electrons along the magnetic filed line transporting from the upper ionosphere
around the magnetic equator to the lower one in the off-equator.
Table 1 shows that when the cloud appears around Chung-Li, the foE starts
decreasing, then the lightening occurs, later the foF2 decreases, and finally foEs increases.
Based on previous studies and the results shown in Table 1, a model is introduced to
explain the changes of the three plasma densities in the ionosphere. Figure 5 is a sketch of
the electric conductivities and Earth’s magnetic field for the Chung-Li daytime
ionosphere. The typical conductivity values Johnson [1961] suggests that the Pedersen P
and Hall H conductivities the E region can be modeled with a slab geometry. It can be
seen that the two conductivity profiles change shapely and yield their maximums at about
110 km altitude. For a symmetric slab, the bottomside and topside altitudes are about 60
and 170 km, respectively. Figure 5 illustrate inside of the conducting slab P and H>>0.
Therefore, currents flow upward from the tops of thunderstorms results in a vertical
upward E-filed between the Earth surface and the ionosphere, which induces the
buttonside of the slab to be negatively charged and topside positively charged. In the
buttonside, the induced negative charges then further push local electrons together with
associated positive ions away from the thunderstorm region, which possibly causes the
slightly decreases in the foE. The direct conductivity 0 value is much greater than those
of P and H. When electrons along the same magnetic field lines in higher altitude, such
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as the F region of geomagnetic low latitude or equator, see the induced positive charges on
the slab topside, then travel down along the magnetic field line, and finally bombard the
upper E-region (see Figure 5).
Note that precipitation of high-energy bombarding
electrons producing secondary ionization, which could result in occurrences of the Es
layer. Of course, the high-energy electron precipitations can also triggered by whistlers,
which are also shown by waves in Figure 5, as suggested by [Hale and Baginski, 1987].
The continuous electron precipitations from the F-region evenly result in decreases in the
foF2.
Based on the model in the paper, the foE should slightly decrease first, and then the
foEs starts increasing and the foF2 decreasing. Table 1 shows that soon after (within 30
minutes) the thunderclouds appear, the foE decreases, within 3.5 hours the CC and CG
lightning occur, and within 4.5 hours the foEs and foF2 increases and decreases,
respectively. The agreement between the model and observation suggest that during
daytime, the electrostatic filed generated from thunderstorms could significantly disturber
the plasma density in the ionospheric F region.
Acknowledgements. This research was partially supported by National Science
Council of ROC Grants NSC85-2612-M008-005 AP5 and NSC85-2612-M008-001 AP5
to National Central University. The author thanks Prof J. R Liu at Center for Space and
Remote Research of NCU for the GMS imagines, Prof. C. J. Pan at Institute of Space
Science of NCU for the VHF radar lightning observations, and Taiwan Power Research
Institute for data of the network lightning detection system. Mr. S. Y. Bai at Institute of
Space Science of NCU preformed data processing and graphing.
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Figures
Figure 1. The GMS imagines observed at 0933 LT (a), 1132 LT (b), 1332 LT (c), 1632
LT (d) on 28 May 1996 (Event A); at 0933 LT (e), 1132 LT (f), 1332 LT (g), 1632 LT (h)
on 14 June 1996; and at 0933 LT (i), 1332 LT (j), 1432 LT (k), 1632 LT (l) on 16 July
1996.
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Figure 2. The observations of VHF radar and LDN (a), foF2 (b), foes (c), and foE (d)
during Event A.
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Figure 3. The observations of VHF radar and LDN (a), foF2 (b), foes (c), and foE (d)
during Event B.
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Figure 4. The observations of VHF radar and LDN (a), foF2 (b), foes (c), and foE (d)
during Event C.
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Figure 5. A sketch of the ionospheric thunderstorm model.
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