JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 90, NO. A10, PAGES 9815-9823, OCTOBER
1, 1985
Electrical Measurements in the Atmosphere and the Ionosphere
Over an Active
Thunderstorm
1. Campaign Overview and Initial Ionospheric Results
M. C. KELLEY,
• C. L. SIEFRING,
• R. F. PF•, x P.M. KiNTNER,
• M. LARSEN,
• R. GREEN,
•
R. H. HOLZWORTH,
2 L. C. HALE,3 J. D. MITCHELL,
3 ANDD. LE VINE'*
The first simultaneouselectricfield observationsperformedin the ionosphereand atmosphereover an
active nighttime thunderstormare reported here. In the stratosphere,typical storm-relateddc electric
fieldswere detectedfrom a horizontal distanceof ~ 100 km, and transientelectricfields due to lightning
were measuredat severaldifferent altitudes.In the ionosphereand mesosphere,lightning-inducedtransientelectricfieldsin the range of tens of millivolts per meter were detectedwith rise times at least as fast
as 0.2 ms and typical duration of 10-20 ms. The transientshad significantcomponentsparallel to the
magnetic field at 150 km altitude. This implies that either considerable Joule heating occurs or a
collectiveinstabilityis presentbecauseof the high drift velocitiesinducedby the transientelectricfields.
Copious numbersof whistlerswere genratedby the storm and were detectedabove but not below the
baseof the ionosphere.
We presentherethe outlineof a newmodelfor directwhistlerwavegeneration
over an active thunderstormbased on these observations.The intensity of the observedtwo-hop whistlers implies that they were amplified along their propagation path and suggeststhat particles were
precipitatedin both hemispheres.
kHz. The electricfield detectorsconsistedof six sphericalsensors(1 foot in diameter) mounted on three setsof orthogonal
Lightning, and thunderstormsin general, involve a variety
booms (length 3 m) [Mozer and Serlin, 1969]. Conductivity
of fascinating physical processes.The experimental program
measurementsusingthe relaxation time constantmethod were
describedhere was designedto determinethe effectsof this obtainedevery 30 min. Similar instrumentationpackageshave
meteorologicalphenomenonon the ionosphereand also to been flown beforeand are describedby Holzworth [1981'! and
investigate electrical effects in the stratosphere and meso- Holzworth and Chiu [1982]. More details on the ballooon
sphere. To this end, a thunderstorm electric field campaign
systemare given in the companionpaper [Holzworth et al.,
was organized and carried out during an active nighttime
this issue],hereinafterreferred to as paper 2.
thunderstorm which moved through the rocket range at the
The Super-Arcusparachutepaylod carried a blunt-probe dc
NASA Wallops Flight Center, Wallops Island, Virginia (latielectricfield sensor(with a maximum frequencyof about 100
tude = 37.8ø,longitude = 284.5ø).
Hz) and a Gerdien condenserto measureatmosphericconducA 10-cm radar located at Wallops Island was used to moni- tivities. More detailed information on this measurement contor the backscatterintensitydue to hydrometeors.The radar, figurationcan be found in the works by Hale et al. [1981'! and
along with visual and RF observations,yielded a good indica- Mitchell et al. [1982] and in paper 2.
tion of thunderstormactivity. A ground-basedflat plate anBoth ballisticpayloadswere instrumentedwith dipole electenna also provided valuable diagnosticdata.
tric field detectorsemploying sphericalsensorsmounted on
Two high flying ballistictrajectorypayloadswere developed the end of extendablebooms. The ionosphericrocket, known
to make measurements
in the mesosphere
and ionosphereover as Thunder Hi, had two setsof booms of length 5.5 m mounta thunderstorm.One payload had an apogeeof 89 km, and ed orthogonally to each other (and to the spin axis of the
the other 154 km. To our knowledge these rockets were the
rocket) with a vertical separationof 1.5 m betweenboom sets.
first dedicatedto the study of the upward coupling of light- Thunder Hi also carried a fixed bias Langmuir probe operated
ning and thunderstormfrom electricfieldsinto the ionosphere.
in the electron saturation region to measure relative electron
In addition, a zero-pressureballoon was flown at an altitude
densities.The mesosphericrocket (Thunder Lo), which is also
of near 25 km, and a rocket-borneparachutepayload with an describedby Kelley et al. [1983], had two boom sets of
apogee of •75 km carried electric field sensors aloft to
lengths4.0 m and 5.5 m, oriented perpendicularto the spin
characterizethe electricfield signaturesat various altitudes in
axis along with a verticalprobe of length 1.3 m along the axis.
the atmosphere.
Both payloads were designedto make dc, low-frequency, and
The stratospheric balloon was instrumented with both
broadband vector electric field measurements.The highestquasi-stati
c vector electric field detectorsand a broadband frequencyresponsefor one componentof the electricfield was
VLF electricfield detectorwith a maximumfrequency
of 100 48 kHz. The responsetime for a full vector measurementwas
INTRODUCTION
14 kHz
• Schoolof ElectricalEngineering,
CornellUniversity,Ithaca,New
York.
2 Space SciencesDivision, GeophysicsProgram, University of
Washington,Seattle.
3 Departmentof ElectricalEngineering,PennsylvaniaState University,University Park.
'• NASA GoddardSpaceFlightCenter,Greenbelt,Maryland.
Copyright 1985 by the AmericanGeophysicalUnion.
Paper number 4A8386.
0!48-0227/85/004A-8386505.00
9815
on Thunder
Hi and 8 kHz on Thunder
Lo.
The storm consistedof four major cells.Figure 1 showsthe
position of eachcell at the launch time of Thunder Lo (August
9, 1981, 0208:00 UT) and 15 s before the launch of Thunder
Hi. The locationsof Wallops Island and the stratosphericballoon are indicated along with the trajectoriesof the three
rocket payloads.Vertical radar scansof each cell just before
launch indicatedheavy precipitationreachingup to at least an
altitude of 13 km. Radiosonde data indicated that the tropopause temperature minimum was at 16 km, and we estimate
9816
KELLEYET AL.' ELECTRICAL
MEASUREMENTS
OVERA THUNDERSTORM,
1
MD
thunderstormcampaignis presentedbut with emphasisupon
the new and exciting observationsof lightning-relatedtransientsin the ionosphere.A detailed presentationof the "dc"
(quasi-static)electricfield data can be found in paper 2. Also
found in paper 2 is an analysiscomparing the quasi-static
electricfield data with earlier work. A more completetheoretical analysisof the ionosphericresultsis under way and will be
presentedin subsequentpublications.
EL
Balloon
DATA PRESENTATION
Simultaneous data from the two ballistic rockets, the
balloon-borneVLF sensorand a ground-based2-MHz receiver located at Wallops Island, are presentedin Figure 2 for a
6-s interval. The• smoothly varying sinusoidal signals in the
top panelsare due to the responseof the rotation dc coupled
antenna to the sum of the ambient electric field and the V x B
3•
electric field (the electric field induced by the motion of the
rocket moving at a velocity V acrossthe magnetic field B).
The sinusoidalsignalsare interruptedby nearly simultaneous
excursionsof the order of severaltens of millivolts per meter
r Hi
Thunder
measured on both rockets. For this event one rocket was lo-
catedat an altitudeof 142km, andtheotherat an altitudeof
88 km. Higher time resolution presentationsshow that the
signal at high altitudes is delayed from that detected on the
I o
I o
"
I o
lower payload by a time consistentwith a propagationspeed
76
76•
75•
75
74.
equal to the speedof light.
30'
30
30
The event just prior to 0210:40 consistedof a set of five or
Fig. 1. Map showing the location of the four thunderstorm cells
and the balloon at the launch time of Thunder Lo. The trajectoriesof six excursionsseparatedby about 0.05 s, each of which was
Thunder Lo, Thunder Hi, and the Pennsylvania State University detected on the three airborne platforms and the ground
parachute
payload(afterdeployment)
arealsoindicated.
receiver. Such a sequenceis characteristic of the multiple
strokeswhich make up a singlelightningflash.The magnitude
this to be the location of the cloud tops. It shouldbe noted
that there is a positivecorrelationbetweencloud heightand
elect,
rical activity[Shackford,1960].
In this paper an overviewof the experimentalresultsof the
•.o
w
of the electricfieldpulseat 142km (top panel)due to the first
stroke was comparable to the signal detected at 88 km. The
signalsat high altitude from subsequentstrokes of the same
flash were much smaller than those registeredin the meso-
142 km
Thunder Hi
oc
mmkm
Thunder_L.o
DC ELECTRICFIELB
za km
Balloon
BROAOBANO
o.o
-35.
o
•o.o
-40.0
:D
.,.
O.O
I
!
I
Q km
D
I
I
I
I
I
I
Grou_nd
I
I
3 MHzRECEIVER
0.0
I
UT 02:10:40
I
I
41
I
I
42
I
TIME
I
43
I
I
I
44
45
Fig. 2. Simultaneousdata from ionosphericrocket, the mesospheric
rocket, a VLF receiveron the stratospheric
balloon,and a ground-based
3-MHz receiver.Notice the signalsdue to lightningstrokesjust before0210:40UT and at
0210:44.5
UT.
KELLEYET AL.: ELECTRICALMEASUREMENTS
OVER A THUNDERSTORM,1
Flat
-I
+0
Plate
The Super Arcus parachute-borne payload measured the
vertical electric field from ~ 75 to ~25 km, but over a much
Balloon Verhcal
28
-046
Balloon
+ 47
-47
02
Horizontal
.
ii 00
UT
02
ß
12 O0
.........
..
ß
...
Thunder
Hi Horizontal
35'
Thunder
H• Horizontal
-351
02 II 20
02'11 30
02 II 50
T•me
02 I:> O0
Time
55
9817
Penn State
Vertical
0
02 II 50
02 12 O0
T•me
Fig. 3. (Top panels)Direct current electricfield data from the flat
plate antenna (located at Wallops Island), the balloon, and Thunder
Hi. Three major lightning flashesare indicatedby the arrows and
designatedeither intracloud(IC) or cloud-to-ground(CG). The other
arrowedeventis due to a relaxationconductivitymeasurementmade
by the balloon. (Botton panel) Vertical electric field measurement
made by the PennsylvaniaState Universitypayload at the sametime
asthe lasteventin the secondpanelfrom the top.
longer time span than the other rockets.The dc componentof
thesefieldsis treated in paper 2. Sixteentransient eventswere
observed,and the first two of these also correspondedto
events seen at the higher altitudes. At 0211:19.8 UT a downward electric field transient was seen at 50.5 km, with an
amplitudeof 3V/m; and at 0211:54 UT at 47.3 km, an upward
field change(cloud-to-groundflash) of 4.5 ¾/m was seen.The
rise time of the field changeswas of the order of 0.1 s, but it is
uncertainbecauseof telemetryfrequencyresponseand degradation during the flashesthat resultedin an inability to distinguish the individual strokes comprisinga flash. However,
after the flash the field persistedfor many seconds,with an
initial e-foldingtime of about 1 s and a "tail" of about 7 s time
constant.Many of the other transientsalso fitted this pattern,
which is describedin more detail by Hale [1983]. It is noted
that both of thesetime constantsare much larger than the
local atmospheric relaxation time, which from simultaneous
conductivity measurementswas determined to be about 0.1 s
at these altitudes.
The presentationsof Figures2 and 3 do ,aotfaithfully reproduce the temporal behavior of the high-altitude(Thunder Hi
and Thunder Lo) transientsbecauseof the long time interval
plotted and limited digitization rate (312 Hz) of those telemetry channels. Raw data from higher time resolution
channels are presented in Figure 4a. These ac-coupled
channelsrepresentthe vector electricfield in the rocket reference frame and had high-passfilters located at 14 Hz with
digitization rates of 5 kHz (Nyquist frequencyof 2.5 kHz).
Note that thesedata are during the same time span as one of
the events in the top panels of Figure 3. Since a band-pass
filter is used in theseac-coupledchannels,it is necessaryto
take into account the transfer function
of the electronics if we
wish to analyze the data. Figure 4b showsthe result of applying the inversetransferfunctionto the data. From this presentation we can see the basic characteristicsof the lightningsphere.The individual transient fields lasted for about 10-20
inducedtransientsin the ionosphere.Typically,the signalsrise
ms at 142 km and 15-35 ms in the mesosphereat 88 km. To
to a peak in lessthan 0.2 ms and have an amplitude of ~ 10
our knowledgethis is the first set of data showingsimulta- mV/m and a period of 10-20 ms. Unfortunately,another set of
neousmeasurements
of lightning-relatedelectricalphenomena data channelsusing even higher telemetry rates were often
extendingfrom ground level into the ionosphere.Another saturatedby the lightningevents,so that it is difficult to deterevent registeredon all four platforms occurred at 0210:44.5 mine an exactrise time. However,it is possibleto determinea
and can be seennear the right-handsideof Figure 2.
"slew rate" or dE/dr for the initial rise in the transient field
Although the balloon broadband receiver detected the
using thesechannels.A typical dE/dt for the initial rise is at
sphericassociatedwith the two eventsin Figure 2, no clear least0.03 mV/s, which is equivalentto 30 V/s.
dc electricfield changewas registeredon the balloon (some
The presentationthus far has been in the rocket reference
100 km horizontal distance).The balloon registeredonly 13 frame. Several of the lightning penetration eventshave been
lightningflasheswith dc electricfield changesduringthe flight analyzedin detail for the ionosphericcase.In general(and in
of Thunder Hi, althoughthe Thunder Hi payload registered the caseillustratedin Figure 4) the primary componentof the
about 70 electricfield "spikes."In fact, there was a frustrating electricfield was vertical and upward. Ground-basedobservaminute or so with very little lightningactivity. When heavy tions do not clearlyidentify all of the lightningflashesseenby
lightning activity resumed after 0211 UT, the Thunder Lo
the rocket as either cloud-to-ground or intercloud flashes;
payload was no longer taking data. Between the times of
however,we have associatedthe upward vertical electricfields
0211:00 UT and 0212:00 UT, three very clear major events with cloud-to-groundstrikes.The upward electricfield signawere registeredon the balloon dc electricfield instrument and
ture suggeststhat there was a significant component of the
on the flat plate antenna. The charactersof these eventsare
electric field parallel to the magnetic field, which was also
illustrated in Figure 3. The balloon dc electric field data as mostly vertical(dip angleof 70ø) over Wallops Island. Figure
5 shows data from the vector broadband receivers which have
well as the flat plate antenna are shown,and insetsare usedto
show the data (for 10-s periods)from Thunder Hi and the been transformedto a geomagneticcoordinatesystemusing
parachute-borne
payload(data are only shownfor the second the attitude information from an on-board gyroscope.From
cloud-to-groundstroke).We note that there is very little acthis plot we can see that indeed the parallel electric field is
tivity seenfrom the intracloudflashon the high-altitudepay- comparableto and sometimes
largerthan eitherof the perpenload while the secondcloud-to-groundflash causesa large dicular components.
amount of activity at the higheraltitude.
The VLF characteristicsof the lightning event illustrated
9818
KELLEY ET AL.' ELECTRICALMEASUREMENTS
OVER A THUNDERSTORM,1
12.0
ward of the storm. The nominal L value associated with the
storm was L- 2.6, but the whistler path is associatedwith
L - 2.8. The whistlerwas very intensewith a broadband electric field exceeding1.0 mV/m and was accompaniedby other
VLF emissionsindicatedby the arrows in the sonogram.
It is interestingto note that no whistlerswere detectedby
instrumentson the mesospheric
rocketor on the stratospheric
balloon. In fact, as shown in Figure 7, even though copious
numbersof intensewhistlerswere detectedduring the 4-min
ionosphericrocket flight, none of thesewere detectedby the
receiversbelow the ionosphere.It is considerednormal for
undueted whistlersto be detectedabove the ionosphereand
0.0
-12.0
12.0
0.0
-12.0
12.0
not below. However, similarities between ducted whistlers de-
tected at Palma, Antarctica Oustminutesbefore the launch),
'.%
and the rocket whistlers shown here indicate
.
.
ß
-12.0
02'11'53.8
UT
11.54.0
TIME
12.0
that these were
indeed ducted events (D. L. Carpenter, personal communication, 1983).
Along with the whistlers, signalsfrom two VLF transmitters, NAA located in Cutler, Maine (17.8 kHz), and NSS in
Annapolis,Maryland (21.4 kHz), were also detected[Cornish
et al., 1981]. In the gray-scaleformat of Figure 6 both VLF
transmittersappear as horizontal lines. In the stratosphere
(not shown) and in the mesospherethe trasmitter signalsare
linearly polarizedas expected.In the bottom panel of Figure 6
this can be seenas a modulation in intensityas the spacecraft
spins;that is, the horizontal line correspondingto the transmitter signal is modulated. However, in the ionospherethe
transmittersignalsare circularlypolarized,and no spin modulation is observed.A similar phenomenonwas reported by
Kintner et al. [1983] based on data from rockets flown over
the Siple transmitterin Antarctica and clearly showsthe transitionsfrom "vacuum"propagationto the whistlermode.
• ..'..';%,,½
:/,:./..,.•..v•.,.,•,.:,,,,.,
0.0
ß
.
..,
-12.0
12.0
ß
-12.0
DISCUSSION
12.0
Direct
o.
o
'*"'"'.
•'*••;•!.•"..
.
-12.0
02:11:53.8
UT
11:54.0
Current
Electric
Fields and
ConductivityMeasurements
This sectionbriefly reviewsthe resultsdescribedin paper 2
as they are relevant to the overall experiment.The stratosphericballoon data, which were continuousbeginningnearly
TIME
SOUTH
12.0
Fig. 4. Electric field data from the ionosphericrocket (Thunder
Hi). The three panelscorrespondto three orthogonaldirections:two
perpendicularto the rocket spin axis and the third along the spin
axis. (a) Raw data from Thunder Hi. Notice the electricfield transientswhich are inducedby lightning strokes.(b) Electricfield data
that have been corrected(phaseand amplitude)to accountfor the
effectsof the band-passfiltersusedin the data channels.Notice that
the lightning-induced
electricfield signalsrise rapidlyto a peak and
then decayin an exponentialmanner.
-12.0
EAST
12.0
0.0
-12.0
previouslyin Figure 2 are shownin the frequency/timespectrogram of Figure 6. The plot is a gray-scalerepresentationof
the intensity and frequencycharacteristicsof the signal as a
function of time detectedin the mesosphere(bottom panel)
and in the ionosphere(top panel). Each of the five strokesis
seen as a broadband signal in the two detector outputs, a
signaturecharacteristicof lightning-generatedVLF measurementsmade in the atmosphereand termedspherics.On Thunder Hi, an intensesubprotonospberic
(SP) whistlerwas detected along with an intenseclassicalwhistler.The delay time and
nose frequencyof the classicalwhistler indicated a propagation path to the southernhemisphereand back slightly pole-
PARALLEL
12.0
'•
0.0
ß
-12.0
02-11' 53.8 UT
11' 54.0
TIME
Fig. 5. Ionospheric electric field data which have been transformed into geomagneticcoordinates.Notice that the lightningtransientshave a significantelectric field parallel to the earth's magnetic
field.
KELLEYET AL.' ELECTRICAL
MEASUREMENTS
OVERA THUNDERSTORM,
1
9819
9820
KELLEY ET AL.: ELECTRICALMEASUREMENTS
OVER A THUNDERSTORM,1
o
Oo
KELLEYET AL..'ELECTRICALMEASUREMENTS
OVER A THUNDERSTORM,
1
9821
2 hours before the rocket flights, indicated that the rockets
were launched into the waning stagesof a seriesof thunderstorm cellswhich startedshortly after midnight on August9,
1981. The dc electricfield in the stratosphereat a distanceof
nearly 100 km pointeddirectly away from the most activecell,
the storm cell which the rockets passedover (see Figure 1).
The conductivity measurementson the balloon and bluntprobe payloadswere lower than thoseusedin earlier theoretical studies.This stronglyaffectsthe amplitude predictionsof
electricfield mapping into the ionosphere.Using the conductivity and electricfield measurements,
it is shown in paper 2
ever, that the ionosphericfields had a significantcomponent
parallel to the magneticfield. The classicalJoule heating rate
at 60 km over the storm and 40 km from the storm zenith.
energyinputwill be muchlessthan o'oE•,
• because
of the
aoE•,
2 forsucha transient
fieldisquitelarge,andthenetheat
input very significant,even though the temporal duration of
the field is short. However, we agree that it is likely that the
heatingrate is actuallymuchlessthanaoE•,
:. Assuming
a
mean free path of 0.2 km for 100 km altitude, an electron in a
10-mV/m electric field will be acceleratedto approximately 5
times the electronthermal velocity beforesufferinga collision.
Hence it is likely that a collectiveinstability will develop becauseof the high electron drift velocities.The collectiveinstathat verticalcurrentdensities
as highas 120pA/m2 wereseen bility will causean anomalousresistivityand implies that the
This is well above the nominal "electrosphere"referred to in reduced conductivity [Siefring and Kelley, 1983]. We point
out in passing that laboratory experiments dealing with
many atmosphericelectricalstudies['cf.Chalmers,1967].
It should be noted that the measured conductivity (see anomalous resistivity often induce the desired parallel electric
Figure 3 of paper 2) is somewhatdifferent from the conduc- field component by dischargingan external coil I-cf.Hambercreatesan electrotivities used in theoretical calculations by Park and Dejna- ger and Jancarik, 1971]. The resultingt?B/t?t
karintra [1973] (on the mapping of dc thunderstorm electric magneticpulse not unlike the transientsignal associatedwith
fields)and by Dejnakarintraand Park [1974] (on the propaga- lightning.
The perpendicular componentsof the electric field trantion of lightning-inducedelectricfields).The theoreticalconductivity and the measuredconductivityare equal at about 33 sientsmay also have important effectson the ionosphere.One
km; however,they differ by 2 ordersof magnitudeat 70 kin. possibleimplication has already been treated [Kelley et al.,
The conductivityscaleheight usedby Park and Dejnakarintra 1984] in the context of the explosive spread F phenomenon
was 6 km while the experimentaldata show a scaleheight of 8 first reported by Woodmanand LaHoz [1976] and more rekm below 40 km and 11 km above 40 kin. The conductivity cently studied by Woodman and Kudeki [1984]. The latter
and the conductivityscaleheight are very important in deter- paper shows a good correlation between the rapid onset of
mining how the dc and transient electric fields map in the 50-MHz backscatterfrom the F region and the detection of
spherics.Kelley et al. [1984], using propertiesof the transient
atmosphereand ionosphere.
Direct current electric fields of the order of 0.1 V/m were fieldsreported here, showedthat a modified two-streaminstameasuredby the balloon at a distanceof nearly 100 km. This bility should operate and result in a 10- to 30-dB increasein
the 3-m (50-MHz) backscatter signal strength within some
is somewhat surprising since Park and Dejnakarintra [1973]
tens
of milliseconds(a characteristic of explosive spread F).
predictfieldsof lessthan 10-: V/m for an averagethunderstorm at this distance.It may be that the differencesbetween These data and theoretical studiesmay also explain data presentedby lsted [1954] which showeda relationshipbetween
these data and the theoretical calculations are due to the conductivity model usedin thesecalculations.It is also interesting lightning and ionosphericdisturbances.
Detailed theoreticalcalculationsof upward lightning penethat the Pennsylvania State University payload detected dc
storm-relatedfields up to an altitude of 70 km. More detailed tration do not yet exist at the frequenciesof interest here.
discussionof the dc electricfields measuredduring the storm Dejnakarintraand Park [1974] studiedthe upward couplingof
electric fields in the frequencyrange from dc to 10 Hz. Burke
can be found in paper 2.
[1975] has also made calculationsin the ULF range. Although the data are not directly applicable to the full freElectric Field TransientsDue to Lightning
quency range of the present observations,we can conclude
It is difficult to make a comparisonbetween the lightning that the data are in qualitative agreementwith the theory in
transientsmeasuredin the ionosphereand mesosphereand the the following sense.The calculationsshow an attenuation of
transients measured in the lower atmosphere.In the lower the field strength which is quite severeat dc but which deatmospherethe recoverytime for the electricfield was longer creaseswith increasingfrequency.The dc field measurements
than the time betweenlightning return strokes.Thus the low- made in the ionosphereon Thunder Hi indicate that the value
frequencyfield changesseenin the lower atmospherewere due is lessthan the instrumentthresholdof a few millivolts per
to the entire lightning flash. This is different from the iono- meter [Holzworth et al., this issue].This is consistentwith the
sphericand mesospheric
caseswherewe have found that dis- theoreticalestimates,sinceDejnakarintraandPark [1974] pretinct field changesare associatedwith each return stroke. We dicted an ionosphericdc field of lessthan 1 mV/m for a large
should note that it is possibleto measure the electric field thunderstorm cell while Burke's [1975] estimates were even
responseto each stroke in the lower atmosphereand on the lower.
ground.However,the frequencyresponseof our instruments
did not allow this. So far, we have not attempted to identify
the higher-frequencyelectricfield signaturesin the ionosphere
and mesospherewith any of the severalwell-studiedlightning
processes(e.g., the step leader), as Beasleyet al. [1982] and
othershave done for ground-basedobservations.However, we
do plan to pursuethis in the future.
The data presentedhere show the first observationsof transient electricfield penetrationinto the ionosphere.At 150 km
altitude the verticalcomponentof the transientswas primarily
upward. This in itselfis not surprising.It is remarkable,how-
VLF
Measurements
In this experimentwe can directly comparethe intensity of
the sphericin the ionospherewith the whistler it creates.Furthermore, we can perform this comparison at virtually the
same altitude and in virtually the same plasma conditions.
Such a comparisonrevealsthat the signalstrengthsare similar
over the entire frequencyrange of the whistler (i.e., 1 kHz to 8
kHz), even though the whistler wave packet has traveled some
30,000 km. The peak of the intensity for the returning whistlers was always in the frequencyrange of 4-6 kHz. In this
9822
KELLEYET AL.: ELECTRICAL
MEASUREMENTS
OVERA THUNDERSTORM,
1
frequency range the whistler was typically greater than its
associatedsphericintensity by 10 to 15 dB. To get a conservative estimate of the lossesalong the path (i.e., from the northern hemisphereto the southern hemisphereand back) we cal-
servedfor NAA, NSS, and other VLF transmitters[Imhof et
al., 1983; Goldberget al., 1983].
culate the collisional attenuation of the ¾LF waves, but we
out the launch of three rockets and a balloon over an active thunder-
ignore path lossesdue to spreading of the VLF waves. The
storm relied upon the dedicationand technicalexpertiseof a number
of individuals.These includeD. Detweiler and R. Long of the Wallops Flight Center, who managed the Cornell payloads, M. Silbert,
who was the balloon launchmanager,and J. Brown, who handled the
Super Arcus launches.We would also like to thank D. L. Carpenter
for his helpful insightsin interpretingthe whistler data. The work at
Cornell University was sponsoredunder NASA grant NSG-6020, at
University of Washington under NASA grant NAG5-604, and at
PennsylvaniaState University under NsG-6004.
The Editor thanks the refereefor his assistance
in evaluatingthis
attenuation
is calculated
in a manner
similar
to Helliwell
[1965].We usean E regiondensityof 2 x 10'• cm-3 (asmeasuredby the rocket) and an F region model [Helliwell, 1965]
with foF2 of 7.5 MHz (as measured by an ionosonde at
Wallops Island). We assumea reflectionheight of 95 km in the
southern hemisphere and no attenuation above altitudes of
1500 km. This analysisyields an attenuation which is proportional to the squareroot of the frequencyand is equal to
7.4 dB at 4 kHz. The experimental observationsthus indicate
that at least a 17- to 22-dB amplification must have occurred
along the path (in the 4- to 6-kHz range).
These whistler observationshave also given rise to a new
model for generationof whistlersin situ above a thunderstorm
cell. The interpretationwe presenthere and plan to quantify
in a future publicationis that the area of the ionosphereabove
an active thunderstormcan be viewed as an aperture antenna.
The spatial and temporal variations in electric field on the
aperture are prescribed by the penetrating transient electric
fields from the lightning discharges.This antenna generates
waveswhich propagatein the whistlermode with a variety of
wave normal angles.If a duct is illuminated at the base of the
protonosphere,it will trap some of the energy,guide it to the
other hemisphere,and provide a return path for the signal.
The returing signal is in the whistler mode and hence,because
of its relatively short wavelength,will not be readily transmitted acrossthe baseof the ionosphere.In this model there is no
need to make a "leaky waveguide"sourcefor magnetospheric
whistlers,sincethey are producedin situ in the ionosphereby
the transient fields we report here.
The
VLF
emissions
which
"surround"
the classic whistler
Acknowledgments.The successwith which we were able to carry
paper.
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9823
(ReceivedOctober22, 1984;
revisedFebruary 12, 1985;
acceptedApril 18, 1985.)