Stratospheric Electric and Magnetic Field and Conductivity Measurements Above Thunderstorms:
Implications for Sprite Models
Jeremy N. Thomas (jnt@u.washington.edu) , Robert H. Holzworth (bobholz@ess.washington.edu), and Michael P. McCarthy (mccarthy@ess.washington.edu), Dept. of Earth and Space Sciences, University of Washington, Box 351310,
Seattle, WA 98195-1310, USA Osmar Pinto Jr. (osmar@dge.inpe.br), Instituto Nacional de Pesquisas Espaciais, INPE, Ave. dos Astronautas, 1758 CEA/DGE, Sao Jose dos Campos, SP 12227-010 Brazil
Mitsuteru Sato, (msato@pat.geophys.tohoku.ac.jp), Department of Geophysica, Graduate School of Science, Tohoku University, Aramaki-Aoba, Sendai, 980-8578 Japan
II. In-Situ Electric and Magnetic Field Measurements Above Thunderstorms with Remote ELF Magnetic Field Measurements
Abstract
Electric and magnetic fields and conductivity were measured in the
stratosphere (32-34 km altitude) above thunderstorms as part of the
Sprite Balloon Campaign 2002-2003 in southeastern Brazil. During the
two balloon flights, the payloads measured hundreds of electric field
transients, some as large as 130 V/m, correlated with lightning events.
Also, conductivity variations over thunderstorms up to a factor of two
different from the fair weather values were measured. Since optical
verification of sprites was not achieved during these flights and recent
studies (Hu et al. 2002) have found that the probability of sprite
production is proportional to the charge moment of the corresponding
positive cloud-to-ground (+CG) stroke, we rely on charge moment
estimates from remote ELF observations in Onagawa, Japan and
Syowa, Antarctica to indicate possible sprite events. Here we present a
detailed study of several nearby (< 60 km) positive +CG strokes
indicated by the ELF charge moment estimate to be probable sprite
generators. By using a quasi-static electric field model (Pasko et al.
2000), we show how these large field changes correlated with +CG
strokes, along with varying conductivity over thunderstorms, may
provide the necessary conditions for sprite development.
I. The Sprite Balloon Campaign 2002-2003 Overview
•Balloon payloads were launched
from Cachoeira Paulista, Brazil
approximately 200km northeast
of Sao Paulo, on Dec. 6, 2002
and March 6, 2003.
II.A. Event 1: A +CG stroke at ~00:00:09.19 Dec 7, 2002 about 34.4 km horizontal distance from the balloon payload (altitude ~34 km) with peak current of 53 kA and charge moment of 436 C-km
Remote Data: ELF (Extremely Low Frequency) Magnetic
Field Data from Syowa, Antarctica with the perturbation due
to the +CG stroke (event 1) circled in red (Courtesy of M.
Sato). From this ELF data the charge moment for event 1 was
estimated to be 436 C-km (10-20% likelihood of being a sprite
generator according to Hu et al. 2002).
In-Situ Data: (A) Vertical DC Electric Field. (B) One Component of
the Horizontal DC Electric Field (horizontal direction (X+) points
about 46 degrees from event 1). (C) Optical Lightning. The second
dashed vertical line represents the time of event 1 as recorded by the
Brazilian ground-based detection network (the first dashed vertical
line is +15 kA stroke that precedes event 1).
In-Situ Data: (A) Vertical and Horizontal (One Component) AC
Electric Field (horizontal direction (X+) points about 46 degrees from
event 1). (B) Optical Lightning. The second dashed vertical line
represents the time of event 1 as recorded by the Brazilian groundbased detection network (the first dashed vertical line is +15 kA stroke
that precedes event 1 and the three additional dashed lines are three
strokes after event 1).
In-Situ Data: VLF Magnetic Field Data in stroke centered
cylindrical coordinates for event 1. (A) Vertical VLF Magnetic
Field. (B) Azimuthal VLF Magnetic Field. (C) Radial VLF
Magnetic Field. The dashed vertical line represents the time of
event 1 as recorded by the Brazilian ground-based detection
network.
II.B. Events 2 and 3: A +CG stroke (event 2) at ~00:16:03.20 Dec 7, 2002 about 50.7 km horizontal distance from the balloon payload (altitude ~34 km) with peak current of 42 kA and charge moment of 390 Ckm. A +CG stroke (event 3) at ~00:16:03.585 Dec 7, 2002 about 47.4 km horizontal distance from the balloon payload (altitude ~34 km) with peak current of 56 kA and charge moment of 1111 C-km.
•The balloon payloads measured
DC to VLF vector electric fields,
VLF magnetic fields, X-rays, and
optical lightning at a float altitude
of 32-35 km.
•The
electric and magnetic
signature of hundreds of lightning
events were measured by the
payloads.
I.A. The Sprite Payload Configuration, Nov. 2002
Remote Data: ELF (Extremely Low Frequency) Magnetic
Field Data from Syowa, Antarctica with the perturbations
due to the +CG strokes (event 2 and 3) circled in red
(Courtesy of M. Sato). From this ELF data the charge
moments for events 2 and 3 were estimated to be 390 and
1111 C-km (>90 % likelihood of being a sprite generator, Hu
et al. 2002) respectively.
II.C Summary of DC Electric Field Changes
Associated with Events 1-3
vector search coil
In-Situ Data: (A) Vertical DC Electric Field. (B) One Component of
the Horizontal DC Electric Field (horizontal direction (X+) points
about -29 degrees from event 2 and -100.5 degrees from event 3. (C)
Optical Lightning. The two dashed vertical lines represents the time
of event 2 and 3 as recorded by the Brazilian ground-based detection
network.
In-Situ Data: (A) Vertical and Horizontal (One Component) AC
Electric Field (horizontal direction (X+) points about -29 degrees from
event 2 and -100.5 degrees from event 3). (B) Optical Lightning. The
two dashed vertical lines represents the time of event 2 and 3 as
recorded by the Brazilian ground-based detection network.
III. Comparison with Sprite Models
In-Situ Data: VLF Magnetic Field Data in stroke centered
cylindrical coordinates for event 2. (A) Vertical VLF Magnetic
Field. (B) Azimuthal VLF Magnetic Field. (C) Radial VLF
Magnetic Field. The dashed vertical line represents the time of
event 2 as recorded by the Brazilian ground-based detection
network.
In-Situ Data: VLF Magnetic Field Data in stroke centered
cylindrical coordinates for event 3. (A) Vertical VLF Magnetic
Field. (B) Azimuthal VLF Magnetic Field. (C) Radial VLF
Magnetic Field. The dashed vertical line represents the time of
event 3 as recorded by the Brazilian ground-based detection
network.
IV. Conductivity Measured During Sprite Flight 1, Dec. 7, 2002
•Events 1(red dot) and 2 (blue dot) are within
one order of magnitude of the predicted vertical
electric field value, which is in reasonable
agreement with the model since these events
occurred at r=34.4 and 50.7 km respectively.
Event 3 is more than two orders of magnitude
different from the model, which is less
reasonable since it occurred at r=47.4.
HV Probes
Vertical quasi-static electric field (r=0) vs. altitude for +CG with
charge moments of 100 and 1000 C-km (adapted from Pasko et. al,
2000). The red, (event 1, 436 C-km) , blue (event 2, 390 C-km) and
green (event 3, 1111C-km) dots are the vertical dc field changes
measured by the balloon payloads (note that the profile is for r=0
while our measurements were for r=34-51 km)
low voltage probes
•We can use the modeled vertical electric field
altitude profiles (such as this one from Pasko et
al. 2000) to extrapolate our stratospheric
measurements to higher altitudes. Thus, for
event 1 our stratospheric data with the altitude
profile shown suggest that the vertical field
change at 60 km was about -15 V/m.
Positive (blue +) and negative conductivity (red dots)
measurements for Sprite Flight 1. The dashed horizontal lines
are averages over all the measurements and the dotted lines
are averages during a period of nearby (<100 km) lightning
activity (blue for positive and red for negative). Note that
both the positive and negative conductivity decreased during
the period of nearby lightning.
Total conductivity (blue squares) for Sprite Flight 1. The
blue dashed horizontal line is an average over all the
measurements and the blue dotted line is an average
during a period of nearby (<100 km) lightning activity.
Note that the total conductivity decreased during the
period of nearby lightning.
I.B. Balloon Trajectory and CG Lightning
IV.A. Comparison to Conductivity Profiles
  0.2s
53 kA
+CG
Flight Path
+ = + CG
= - CG
Yellow : CG<50km
Orange: 50km<CG<100km
Red: CG>100km
Theoretical (Park and Dejnakarintra, 1973) and experimental
(Holzworth et al., 1985) single polarity conductivity profiles with
altitude. The average single polarity conductivity (average of
positive and negative conductivity) for Sprite Flight 1 is shown by
the red dot. Using the experimental profile from Holzworth et al.
1985 to extrapolate in altitude, the ambient time constant at 60 km
above is estimated to be tau=0.2 s.
V. Conclusions:
•We have measured large vertical and
horizontal dc electric field changes (up to
69 V/m per component) in the stratosphere
associated with large +CG strokes with
time scales longer than the local relaxation
time.
•Events 1 and 2 reasonably agree with
vertical electric field profile in the sprite
model by Pasko et al 2000. Event 3 is does
not agree well with this model.
•Large horizontal electric fields and
appreciable vertical magnetic fields
suggest that horizontal currents might play
a significant role in large +CG strokes.
•Conductivity variations occur above
thunderstorms which could increase the
ambient time constant at sprite altitudes.
•Since conductivity is an input parameter,
these conductivity variations are important to
sprite models.
•Extrapolating our stratospheric
measurements for a 436 C-km +CG gives an
vertical electric field change of about -15
V/m at an altitude of 60 km and a horizontal
distance of 34.4 km with an ambient time
constant of 0.2 s.
•These large quasi-static electric fields along
with conductivity variations may provide the
necessary conditions for breakdown to occur at
sprite altitudes.
•However, comparison with various sprite
models (quasi-static field, runnaway
breakdown, streamer breakdown, fully
electromagnetic) at exactly the location of our
in-situ data is needed provide a better test of
these models.
Acknowledgements
J.N.T, R.H.H., and M.P.M were supported by the National
Science Foundation grants ATM-0091825 and ATM-9987684.
O.P.Jr. was supported by FAPESP grant 02/01329-1. M.S. was
supported by the 41st and 42nd Japanese Antarctic Expedition
Research Programs.