RadioScience,
Volume35, Number6, Pages1437-1444,November-December
2000
IonosphericE region remote sensingwith ELF radio
atmospherics
Steven A. Cummer
ElectricalandComputer
Engineering
Department,
DukeUniversity,
Durham,NorthCarolina
Umran
S. Inan
Space,Telecommunications
andRadioscience
Laboratory,
StanfordUniversity,Stanford,California
Abstract. Radioremotesensingof the ionospheric
E regioncanbe difficult,particularlyin the
nighttimeE regionvalleywhichliesbetweentwo regionsof higherelectrondensity.We showthat
extremelylow frequency(ELF) electromagnetic
waveslaunchedfrombelowpenetratethisregion
because
of theirlow attenuation
andcanbe reflectedfromboththe D regionandthetop of the
E regionvalley.This doublereflection,alsocausedby sporadicE layers,produces
a measurable
effecton the subionospheric
propagation
of ELF waves,whichwe demonstrate
with numerical
Earth-ionosphere
waveguide
simulations.
Thissensitivity
opensthepossibility
of remotelysensing
theE regionanddetecting
sporadic
E layerswithbroadband
ELF propagation
measurements
using
lightningdischarges
asthesource.Fromnighttimeobservations
of ELF lightningradiationoverthe
samepropagation
pathon differentdays,we extractE regionelectrondensityprofilesthat show
significant
variabilitynotonlyfromdayto daybutoverthecourseof a singlenight.Thistechnique
is inherentlya path-integrated
measurement,
enablingtomographic
large-scale
ionospheric
remote
sensingwith multiplesourcesandreceivers.
1. Introduction
Becauseof its relativeinaccessibility
the lower ionosphere,consisting
of the D andE regions,is amongthe
leaststudiedregionsof the Earth'satmosphere.
The altitudesinvolved(•70-140 km) aretoo highfor balloons
tersareextremelylargeandexpensivebecauseof thelong
wavelengthsinvolved[Watt, 1967], making suchsounding difficult in practice. Nevertheless,man-madeVLF
transmittershavebeenusedvery successfully
to remotely
senseperturbationsin this regionof the ionosphere[e.g.,
Johnsonet al., 1999].
and too low for mostsatellites,makingin situ measureLightningis a powerfulbroadbandsourceof VLF and
mentsdifficult. Radio soundingdoesnot alwayswork
ELF energyand can be usedas a sourcefor sounding
becausethe electrondensitiesin thisregionareoftentoo
the lower ionosphere.Cummeret al. [ 1998] showedthat
low to reflect radio waves with ionosonde and incoherent
broadbandVLF radiationfrom lightningcan be usedto
scatterradar,particularlyat night. Rocketexperiments
measurean assumedD region electrondensityprofile
[e.g.,Mechtlyet al., 1967]havemeasured
thisregionbut
with an altitudeand scaleheightaccuracyof a few hunonly in limitedspatialandtemporalextents.
dred meters. In this work, we show that ELF radiation
Very low frequency(VLF, 3-30 kHz) andextremely
low frequency(ELF, 3-3000 Hz) electromagnetic
waves from lightningis partiallyreflectedfrom E regionaltiELF propagationmeasureare stronglyreflectedby the D and E regionsand can tudesandthat subionospheric
ments from lightning can be used to soundthe E rethereforebe usedfor radio soundingof thesealtitudes
[Sechrist, 1974]. However, at ELF and VLF, transmit-
Copyright2000by theAmericanGeophysical
Union.
Papernumber2000RS002335.
0048-6604/00/2000RS002335511.00
1437
gion, includingthe E regionvalley, which is difficultto
probefrom the ground. We also showtheoreticallythat
subionospheric
ELF propagationis very sensitiveto the
presenceof sporadicE layers(Es). The methodwe describemeasurespath-integratedelectrondensityprofiles
and therefore,with multiple sources(i.e., lightning lo-
1438
CUMMER AND INAN: E REGION REMOTE SENSING AT ELF
cations)andreceivers,
canbe usedto tomographically
measure
three-dimensional
ionospheric
structure
onlarge
spatialscales.
Previous theoretical studies have demonstrated the sen-
sitivityof ELF propagation
to E regionelectrondensities. Pappertand Moler [1974, 1978] showedwith full
160t
'/
6Oh"'
/ /;
wavemodelcalculations
thatEs andsharpelectrondensitygradients
around250km altitudeproduce
significant
attenuationmaximabelow 100 Hz. Barr [1977] showed,
againwithfull wavecalculations,
thatEs drastically
affectsELF attenuation
ratesby generating
a seriesof resonancesfrom 10 to 1000 Hz andthatthe Es characteristicsaffectthefrequencies
anddepthof theseresonances.
In thispaper,we showthatthenighttimeE regionvalley generates
weakerbutmeasurable
ELF resonances
that
depend
strongly
onthedetailsof theelectron
density
pro-
'-•
80'
1 '0
80
""
''
40
40
00
.........
1000
101
10
210
310
410
5
realpartofn
localskindepth(km)
Figure1. Indexof refraction
n andskindepthforvertically
incidentX- andO-modewavesin a modelnighttime
ionosphere
ELF propagation
experiments
havebeenperformed at 5 kHz and500Hz andforthe500Hz X modein a daytime
with man-made,single-frequency
transmitters
[Bannis- ionosphere.
Thelowerlosses
andsharper
n gradientat 180km
a sigter, 1975]andwithradioatmospherics
[Hughesand Gel- altitudefor thenighttimeX modeat 500Hz canproduce
file.
lenberger,1974]. Theseauthorsobserved
significant nificantE-F regionreflection.
variability in ELF attenuationrates, but the limited fre-
quencyrangesandspectralsmoothing
fromtemporalaveragingtendedto obscurethe resonances
which enable
theremotesensingdescribed
in thiswork.
cursundertheseconditions
at thehigherof the altitudes
where
fe2 = ffa andfe2 = f v aresatisfied.
The extraordinary
modeis only weaklyreflectedexceptby extremelysharpionospheric
gradients
andthere2. ELF PenetrationInto theE Region
forepenetrates
moredeeplyintotheionosphere.
It is this
The nominalreflectionaltitudeof electromagnetic modethatcanbe reflectedby E or F regiongradients
wavesin theionosphere
varieswithfrequency,
withlower substantiallyabovethe O-mode reflectionaltitudeif the
frequencies
usuallyreflectedby lowerelectrondensities frequencyis sufficiently
low. Figure 1 showsthe Oandtherefore
at loweraltitudes.In the F region,where and X-mode indexes of refraction n at 5 kHz and 500
electroncollisions
areinfrequent
enoughto berelatively Hz for vertical incidence as a function of altitude in a
unimportant,
verticallyincidentordinaryO andextraor- modelnighttimeionosphere
andtheX-moden at 500 Hz
dinaryX wavesarereflected
atthealtitudewherefe = f
for a daytimeionosphere.Expressions
for the indexof
andfe2 = f2_ ffB, respectively,
where
f isthewave refraction
in a coldplasma
aregivenbyBudden
[1985,p.
frequency,
fe is theelectron
plasmafrequency,
andfB is 74]. The left panelof Figure1 showstherealpartof n
theelectrongyrofrequency
[Budden,1985,p. 357]. The ona nonlinear
scaleto showbothmodes,whiletheright
O and X modes are the two characteristic waves that repanelshowsthelocalskindepth(or amplitude
e-folding
sultfrompropagation
in an anisotropic
plasma.Each depth)andreflectsthelossesof thewaves.At night,no
modehasits ownpolarization
andindexof refraction, dropsaround80 km for bothfrequencies,
andabovethis
andthepolarization,
whilefrequency
dependent,
is of- altitudethe lossesare veryhigh (skindepthsof 10 km
tencircular.As thewavefrequency
is reduced,
the O and less). Thus the O modeis only reflectednear 80
andX reflection
altitudes
movedownward
intoregions km. In contrast,thenighttimenx changesgradually,the
whereelectron
collisions
canbesignificant
andthesere- lossesare significantlylower,and thereforethe X mode
flection
heights
arenotnecessarily
valid.In thisregime, significantlypenetrates
the nighttimeionosphere.HowBudden[1985, p. 361] describes
the reflectionasoccur- ever, at 500 Hz the losses are much lower between 70 and
ring at a complexaltitude,but it is not clear how this 120km altitude,andthegradientin n at 180km is much
complexaltituderelatesto the real altitudebelow which sharper
thanat 5 kHz. ThustheX modeundergoes
some
mostof thewaveenergy
is confined.
Ratcliffe
[1959,p. reflectionin thenighttimeE-F region,andthecombina140]showsthatsignificant
ordinarymodereflection
oc- tionof gradients
andlosses
makesthisreflection
stronger
CUMMERAND INAN: E REGIONREMOTESENSINGAT ELF
at lower frequencies.During the day, however,X-mode
lossesare larger at 500 Hz, the sharpn gradientmoves
downto 90 km altitude,and the X modedoesnot penetrateto the E-F region.It is the nighttimeX modeE-F
regionreflection
thataffectssubi6nospheric
ELF propagation and forms the basisfor the remote sensingtechniquedescribedin this work.
3. E RegionEffect on Subionospheric
ELF
1439
300
250
.\
- •lect•'ons]
•' 200
• 150
Lq
• 100
50
Propagation
O' . ' . , . , ....
".
...... 012
102 100 102 104 106 108 10TM 1
Becauseof the secondary
ELF reflectionsfrom the E
collisionfrequency(s-])
region,subionospheric
ELF propagation
depends
on the
electron
densityat thesealtitudes.Thisdependence
can Figure3. Electronandioncollisionfrequency
profiles
usedin
bedemonstrated
numerically
withtheLongWaveProp- thesimulations
throughout
thispaper.
agationCode (LWPC) [Pappertand Ferguson,1986],
whichis commonlyusedfor VLF calculations
butis also
valid and has beenusedat ELF [Pappertand Moler,
presentas requiredfor chargeneutrality. The electron
1974].Figure2 showstwomidlatitude
electron
density
andioncollisionfrequency
profilesusedthroughout
this
profiles
fromthe1995international
reference
ionosphere
workarea concatenation
of D regionprofilesof Morfitt
[Bilitza,1997]anda thirdwitha modelEs layer.Of the
andShellman[1976]andE andF regionvaluesof Rishtwonon-Esprofiles,onecontains
a significant
E region
bethandGarriott[1969,p. 131]andareplotted
inFigure
valleydepletion,
andtheotherdoesnot. All of thepro- 3.
filesareidentical
below90 km.Theionospheric
magnetic In the Earth-ionosphere
waveguidethe unconfinedO
fieldin allthesimulations
istypicalofeast-to-west
propa- and X plasmawavescombineinto waveguidemodes
gationoverthecontinental
UnitedStates.Ionsplaya sigthatcontainbothwaves. The numberof propagating
nificantrolein subionospheric
ELF propagation
[Pappert modesdependson the frequencyrelativeto the waveandMoler, 1974],andwe assume
throughout
thiswork guide cutoff frequencies.We use the LWPC to calcu-
thatthepositive
iondensity
Nil-= Newhere
Ne> 102 latethewaveguide
propagation
constants
(phasevelocandthatN•- = 102where
Ne< 102,withnegative
ions ity andattenuation)
of the singlequasi-transverse
elec-
220
tromagnetic
(QTEM) modethatpropagates
at ELF (the
othermodes
areevanescent).
Figure4 shows
thesequantitiesasa function
of frequency
for thethreeionospheres
shownin Figure2 withthecollisionfrequency
profiles
in Figure3. The significant
variations
with ionosphere
demonstrate
theimportance
oftheE regionforELFprop-
:
agation.
.a. 60
I
10ø
......... ß : :':'::':':::
'•'::-:':::::': :.:.i,
..............................
:: i:
101
102
i_iii!!iii:
103
104
105
' 10
electrondensity(cm-3)
Figure2, Threenighttimeelectrondensityprofileswith differentE regioncharacteristics
andonewithan Ez layer.The
profilesare identicalbelow 90 km.
ThisspecificEs layercreatesa strongattenuation
maximumat 400 Hz. The depthandfrequency
of thismaximum dependon the altitudeand maximumNe of the
Es layer[Barr, 1977]. However,eventhe E regionvalley profilewithoutEs produces
distinctattenuation
maxima at approximately
250, 520, and750 Hz and attenuation minima at 400, 630, and 900 Hz. In both cases,a
resonance
phenomenon
appearsto be responsible
for the
variations,
asexpectedfroman ionosphere
thatproduces
two distinctreflections.An oversimplified
but instructivemodelof thisprocess
is asfollows.Let therebe two
ionospheric
reflectionaltitudeshi andh2, with h2 > hi.
The regionbetweenthe reflectionaltitudeshasan index
1440
CUMMER
AND INAN: E REGION REMOTE
tion from lightningdischarges.For the remainderof the
paperwe focusprimarilyon non-Esionospheres.
• 1'0I
.....
•--no-valle•
/\,,
•- -valley
.:.,.
'•
•
SENSING AT ELF
/ ..... valley
w/Es
4. QuantitativeE RegionRemoteSensing
o.
0 ' 660 ' 880 lOOO ELF propagationdependenceon the E
26
frequency(Hz)
o
ß
• -10
•
novalley I',..,'
-15/].........
......
:"vnaø11•
Iley
I• /
J
-20
0
- valley
w/EsI !ti.[
i
i
200
i
¾
i
i
400
600
frequency(Hz)
i
.
i
800
"••
. J
i
1000
Figure 4. QTEM modecharacteristics
(phasevelocityand attenuationrate) computedwith LWPC for the threeprofilesin
Figure2. The differencesdemonstrate
the importanceof the E
regionon ELF propagation.
of refractionnh >> 1, and the regionbelow h] is free
space.For any angleof incidencebelowh l the refracted
wave aboveh] will have a nearlyverticalwave normal
anglebecausenh >> 1. Thus the phasedifferencebetweenthe two reflectedwavesin the free spaceregionis
q•h= wnh(h2-- hl)/C, wherec is the speedof light. The
two waveswill interferedestructively
if q5
h = (2m - 1)zr
and constructively
if qbh= 2mzr. The nearlyconstant
spacingbetweenadjacentminima and maxima (Af m
110) impliesthat nh(h2 -- hi) - c/(2Af) = 1360 km.
Reasonable
E region
valley
parameters
(Ne= 500cm-3,
v = 6000s-], andfb = 1.5MHz)giveanX-modeindexof refractionof --,20 at 500 Hz, givingh2 - h• m 68
km. This simpleestimateis quitecloseto the heightdifferencebetweenthe undersideof the D region(--,80 km)
andthe top of the E regionvalley(,-, 160 km), suggesting
that the double-reflectionprocessis responsiblefor the
subionospheric
ELF attenuationrate and phasevelocity
variations.The processis similar with an Es layer that
generatesan evenstrongersecondaryreflectionandreso-
region ionosphereopensthepossibilityof remotelysensingthe electron densityin thisregion.As discussed
in section1, the
value of this ELF techniqueis that it is sensitiveto E
regionvalley parametersthat are difficult to measureby
othertechniques.
However,we needto quantitatively
understandthis sensitivityin orderto probethisregionwith
ELF.
A broadbandELF sourceis ideal for this application
becausetheresonances
canbe easilydetectedwith a completeELF spectrum.Naturehasprovidedus with sucha
sourcein lightningdischarges.We usethe LWPC model
to simulate,for a varietyof ionospheres,
the ELF spectra of the signal radiatedby a lightningdischargeand
received
2000km away. We assume
thatthelightning
discharge
is an impulsive
sourcefor frequencies
up to
1 kHz, a valid assumption
sincethe risetimesand fall
timesof lightningcurrentareusuallylessthana fewhundredmicroseconds.
Everylightning
discharge
generates
quasi-static
electricfieldsin the mesosphere
that drive
slowlyvaryingrelaxation
electriccurrents
thatgenerate
theirownelectromagnetic
fields[Greifinger
andGreifinger, 1976]. Theserelaxation-drivenfields, which are not
implicitlycalculated
by modetheorysimulations,
canbe
significant
andtherefore
canaffectourassumption
of an
impulsivesource.Recentsimulations
haveshown,how-
.;'
.... ß::..:!..5../:'
.
•
ß. ;.!.-•.i.i'i.•
'valieY:"11':
: ,,,.'"51:5
' '::..
ß
[ '?':' !'•a:iiey
i--;(;•'::.D__r_e_g!gni":ill
! 5::'....
nance.
The abovesimulationsshow that Es and ambient vari-
ationsaffectnighttimesubionospheric
ELF propagation.
On the basisof the simplifiedmultiple reflectionanalysis, the observablequantitiesthat containinformation
aboutthe E regionare the frequenciesand depthof the
amplitudeminima and maxima. We explorehow these
quantitiesdependon quantitativeE regioncharacteristics
and whether these characteristics can be extracted from
measurements
of subionospheric
broadbandELF radia-
h;½-11'!'!'!';
i i i!;.i.i!-:;.
;
101
102
103
104
105
electrondensity(cm'3)
Figure 5. Five-parameter
ionospheric
modelusedto calculate
thesensitivity
of ELF propagation
to ionospheric
variability.
CUMMER
AND INAN: E REGION
ever, that theserelaxation-drivenfields are insignificant
700 km from the source above 50 Hz [Cummer, 2000].
Thus,for our propagationdistances(> 1000 km) andfrequencies(>50 Hz), the lightning sourcecan be treated
as impulsive.Furtherdetailson modelingELF radiation
from lightningare givenby Cummerand lnan [2000].
Figure5 showsa five-parameterionospherewhichis a
slightlysmoothedpiecewiseexponentialelectrondensity
profilethat we useto quantitativelyinvestigatethe effects
of ionospheric
changeson theELF spectrum.The parametersHval, Emin,and Drnaxdescribethe heightof the E
regionvalley,the minimumNe in the valley,andthe peak
D regionelectrondensitybelow the valley,respectively.
REMOTE
SENSING AT ELF
1441
•, 4xld5
••
'<'•"......
"•?>-,,.........
/
[
Emin=200
/
Emin=400
•,,,,
Hval=125
•.......... Hva,=1!
.....;•
Dmax=1000
,....•
.fir'./,
'-
................
Dmax=2000
The D regionparameters
h' andfl arethealtitudeandinversescaleheight of an exponentiallyincreasingmodel
electrondensityprofile originallydescribedby Waitand
Spies[1964] and commonlyusedin D regionmodeling.
The D regionelectrondensityprofileis givenby Ne(h) =
1.43
x 107exp(-0.15h')exp[(• - 0.15)(h- h')]cm-3.
We simulatetheeffectof perturbations
froma basicnight-
,.,:.."•...
/.•./"-
/
and maxima.
,..
h=85
...............
I
I
I
I
./'•,.
timeionosphere
(h' = 85km,fl = 0.5km-1, Hval= 130
km, Emin= 400 cm-3, andDmax= 2000cm-3) in
all parameters
individuallyto understand
whichmightbe
measurablefrom broadbandELF spectra.
Figure 6 showsthe effect of variationsin the five
ionosphericparametersdescribedaboveon the broadband ELF spectrumobserved2000 km away from the
lightningdischarge(or any impulsivesource). The top
panel showsthat reducingEminincreasesboth the frequencyand depth of the ELF resonances.This is precisely what is predictedby the approximateanalysisin
section3. DecreasingEminreducesthe averageindex of
refractionnh betweenthe two reflectinglayers,whichincreasesthe resonancefrequencies.DecreasingEminalso
reducesthe attenuationof the fields which penetratethe
firstreflectingaltitudeandreachthe secondreflectingaltitude, which increasesthe amplitudeof the secondreflectionandthereforeleadsto deeperinterferenceminima
h'=82
'"-'•.•........
/
0
•
200
...................
400
600
800
.... ----•-m
::--
....
1•=0.4
•
1•=o.5
1000
1200
frequency(Hz)
Figure 6. Variationin ELF spectrumwith the ionospheric
parametersin Figure 5.
increasesthe index of refraction nh betweenthe two re-
flectionaltitudes,whichreducesthe resonance
frequenciesjustasdoesincreasing
Ah. The D regionaltitudeh'
canbe inferredindependently
usingVLF lightningpropagationmeasurements[Cummeret al., 1998], but the simi-
larityof theeffectsof Hva•andDrnaxonELF propagation
makes it difficult to measure both with this method.
The lastpanelof Figure6 showsthatthe D regioninversescaleheightfi doesnot changethe resonancefrequencies,but doesstronglyaffectELF attenuationabove
•,300 Hz. The D regioninversescaleheightfi canalsobe
measuredfrom VLF observations[Cummeret al., 1998],
but the strongELF dependence
providesan independent
andperhapsmoresensitivemeasurement
of thisparame-
The middlethreepanelsof Figure6 showthat variationsin Drnax,Hval,andh' all havevery similareffects
on theELF spectrum.IncreasingHva•,increasingDrnax,
anddecreasing
h' all reducetheresonant
ELF frequencies
withoutchanging
theresonance
sharpness.
All of these
effectsarepredicted
by theapproximate
analysisin sec- ter.
Givenhowionospheric
parameters
affecttheELF spection 3. Decreasingh' reducesthe firstreflectionaltitude
hi, and increasingHva•increases
the secondreflection trum, the followingproceduremay be usedto determine
altitudeh2, which in both casesincreasesthe reflection the probableionospheregivenan ELF spectrum.AssumVLF
altitude difference Ah = h2 - hi and thereforereduces ing that h' is alreadyknownfrom the broadband
the resonance
frequencies.IncreasingDrnaxeffectively spectrum[Cummer et al., 1998], select Emin to match
1442
CUMMER
AND
INAN:
E REGION
REMOTE
SENSING
AT ELF
thedepthof the observed
resonances,
selecteitherDmax The generallygood agreementbetweenthe simulations
or Hva]to matchthe resonancefrequencies,and select andobservations
meansthatthe inferredionospheres
do
explaintheobservations,
subjectto thenonuniqueness
is/5 as neededto matchthe upperELF (500-1500 Hz)
in section4. The ELF spectrum
of thelast
fieldamplitude.Thereis inherentnonuniqueness
in the suesdiscussed
deducedionosphere
becauseof the similarityDmaxand time periodcouldnotbe reproducedwith anyreasonable
parameterization
in Figure5,
Hva]. This resultis not surprising,sincewe are only variationof the ionospheric
matching
a three-parameter
measurement
(resonance
frequency,
resonance
depth,andupperELF amplitude)
to a
.0
. ..• ....
://..'•i..... i.... !" Jul•
22,
(•700-07S0
UT
four-parameter
model. To resolvethis,in section5, we
1.51
ß
fix Dmax= 2000cm-3 andonlyvaryHva!tomatchthe
1.2
observations.
-':•.,
'.•,,•.'•.'
' '--'i•','X"i
..... i"
":•'"...,.."i"
'//'
•-
observed
-..........modeled
0.
5. Comparisonof Observationsand
Simulations
1.2 [
Lightning-generated
broadband
ELF spectrado show
significant
variabilitythatcanbeattributed
toionospheric
E regionvariations.We usenighttimehorizontalmagnetic field measurements
recordedby the StanfordUni-
versityELF-VLF Radiometer
Experiment[Chrissan
and
Fraser-Smith,1996] in July 1996. Usinga procedureessentiallyidenticalto that of Cummeret al. [1998], we
temporallyaveragetransientradioatmospherics
(sferics)
launchedby lightningdischarges
in a smallgeographic
region(asmeasured
by theNationalLightningDetection
[
3 o.}
•1
•
•'
[-
'"i
....
".
;'-''
"'/'•'•'-i ....
'"// -'•' •'
"• 1'2I F
- 0.8
?"
?"
IJuly
26,0400-0430
UT
---- observed
iii'•'";:
""i'"I ...........
mødeled
I
Network [Cumminset al., 1998]) in a 30 min time period, whichis usuallylongenoughto accumulate
enough
sfericsfor a goodsignal-to-noise
ratio. We implicitlyas0.4
....
sumethatthe ionospheredoesnot vary significantlyover
0
thisperiod,andwe do notincludewaveforms
fromlarge
0
400
800
1200
1600
positivedischarges
thattendto havesignificant
continua.
frequency
(Hz)
ing currents[Uman, 1987, p. 200] andare thereforenot
an impulsiveELF source.The ELF portionof the spec200
trum of the resultingaveragewaveform,whichincludes
.... July
24,0400-0430
UT
180
the effect of a single-pole,high-passfilter at 420 Hz in
......... July
26,0400-0430
UTI
-the receiver,is the measuredELF spectrum.
160
The solidlinesin Figure7a are measuredELF spectra
from July22 (0700-0730 UT), July24 (0400-0430 UT),
and July 26 (0400-0430 UT and 0845-0915 UT). The •140
propagation
distancefrom sourceto receiverfor these
120
spectra
is •2000 km. Themeanamplitudes
differslightly
betweenperiodsbecausethe averagelightningdischarge
lOO
' ß ::'i'i'i:i
- '":':'
:
'":':' x
':'i'!':'
strengthis differentin eachperiod. The detailsof the
spectra,includingthe resonance
frequencies,
vary more
8o
thancouldbe causedby just the simultaneous
D region
variationsmeasured
usingthetechniqueof Cummeret al.
[1998], whichwe appliedto the simultaneous
VLF data.
b.
electron
density
(cm
'3)
This indicatessignificantE regionchangesbetweenthe
four periods.For the firstthreetime periodsthe dashed Figure 7. (a) ObservedbroadbandELF spectraand, in three
linesin Figure7a aremodelELF spectrafoundby using cases,model ELF spectrathat agreewell. (b) The threeionothe iterativematchingproceduredescribedin section4. spheresinferredfrom the observations.
...
CUMMER
AND INAN: E REGION REMOTE SENSING AT ELF
1443
servations.In general,the agreementwas good, and we
foundthatin mostcasesthevariationsin thespectracould
behighlyvariable,andmatchingall of thedatarequiresa be explainedby normalE regionvariability.
The utility of the single propagationpath technique
broaderionospheric
parameterization.
that
we havedemonstrated
can be extendedsignificantly
Figure7b showsthe three D and E regionelectron
with
multiple
receivers
and
multiple source locations.
densityprofilesthatreproduced
the observations
in the
Since
only
a
single
mode
propagates
in the frequency
first threetime periods. The ionospheric
variabilityis
significant
but not extraordinary,
highlighting
the sensi- range of interest,a WKB-type solutionis valid in the
tivity of ELF spectrato the D andE regionionosphere. presenceof evenrelativelysharp(with respectto waveThusthe
Thoughnoneof theobserved
spectracontained
evidence length)horizontalionosphericinhomogeneities.
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