Table Of ContentJournalofAtmosphericandSolar-TerrestrialPhysics86(2012)56–64
ContentslistsavailableatSciVerseScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics
journal homepage: www.elsevier.com/locate/jastp
Rocket borne in-situ Electron density and Neutral Wind measurements
in the equatorial ionosphere—Results from the January 2010 annular
solar eclipse campaign from India
G. Manjun, R. Sridharan1, Sudha Ravindran, M.K. Madhav Haridas, Tarun K. Pant,
P. Sreelatha, S.V. Mohan Kumar
SpacePhysicsLaboratory,VikramSarabhaiSpaceCentre,Trivandrum695022,India
a r t i c l e i n f o a b s t r a c t
Articlehistory: Oneoftherecentdevelopmentsintheformofanovelprobeforsimultaneousin-situmeasurementsof
Received29December2011 ElectrondensityandNeutralWind(ENWi)parameterswasextensivelymadeuseofduringtheannular
Receivedinrevisedform solar eclipse that passed right over the magnetic dip equatorial station, Trivandrum in India. The
20June2012 responseoftheequatorialionospheretotheannularsolareclipseofJanuary2010isinvestigatedusing
Accepted21June2012
the data from ENWi and other ground based instruments like digital Ionosonde, HF radar, magnet-
Availableonline30June2012
ometeretc.Significanteffectsontheneutralwindcomponentsandelectrondensitiesduringthepeak
Keywords: phaseoftheeclipsearebroughtoutbytheENWidata.Aclearreversalinthezonalneutralwindis
Annulareclipse observedduringtherocketflightsatthepeakphaseoftheeclipseaswellasintheflightconducted3h
Electrondensity
later.Thegroundmagneticfieldmeasurementsrevealedthereversalofelectrojetduringtheeclipse
Neutralwind
alongwith blanketing Es layers vindicating the earlier hypothesis that blanketing Es layers manifest
BlanketingEs
over magnetic equatorial regions only during periods of either weak or reversed electrojet (counter
Counterelectrojet
electrojet).TheHFradarresultsindependentlysubstantiatetheinsitumeasurementswithregardto
thelocationoftheirregularitiesandtheirDopplervelocities.Theregionalscalechangesthathavebeen
brought about by the Solar eclipse both in the neutral and ionized medium and the consequent
electrodynamicsarehighlighted.
&2012PublishedbyElsevierLtd.
1. Introduction etal.,2002;Chandraetal.,2007;Westetal.,2008;Leetal.,2009,
and the references cited in them). Nevertheless, the response of
Neutral winds and plasma densities are two important para- ionosphere over the magnetic equator and the adjoining low
meters, especially in the equatorial upper atmosphere, as they latitude regions has not been extensively studied for want of a
control/modulate all the major geophysical phenomena in this suitable event occurring right over the equator. Hence the rare
region.Theupperatmosphericstructure,energeticsanddynamics annular solar eclipse of 15 January 2010, with its maximum
are strongly influenced by the solar radiation and hence are obscuration at (cid:2)1315 IST provided a unique opportunity to
altered significantly during solar eclipses. The sudden cutoff of examine the eclipse induced effects on the equatorial electrojet
the solar insolation during a solar eclipse produces significant (EEJ)andassociatedphenomenaaroundlocalnoonwhenallthe
effects on the entire terrestrial atmosphere. These effects are equatorialelectro-dynamicalprocessesareexpectedtobeintheir
expected to be all the more significant in modulating the fullstrength.
equatorial electro-dynamical processes during a very rare phe- Sofar,directmeasurementsoftheneutralwindsinthelower
nomenon when the path of maximum obscuration falls right on thermospherecouldbemadethroughalkalivapourrelease/TMA
the magnetic equator that too during noon hours. Several release experiments (Larsen, 2002). This technique has the
researchers in the past, have investigated the response of the limitation of being restricted only to twilight and in the case of
ionosphere to solar eclipses (Klobuchar and Whitney, 1965; TMAcouldevenbeextendedtonighttimeconditions.Therehad
Afraimovich et al., 1998; Muller-Wodarg et al., 1998; Sridharan also been attempts to conduct Lithium releases during day time
conditions since the background continuum in the band of its
resonant scattering is significantly low and using special detec-
tiontechniquesonecouldimprovethesignal–noiseratiobutwith
nCorrespondingauthor.Tel.:þ914712562597;fax:þ914712706535.
little success from equatorial regions. Though the power of
E-mailaddress:[email protected](G.Manju).
1CurrentlyCSIR-ESatPhysicalResearchLaboratory,Ahmedabad,380009. vapour release techniques have been amply demonstrated, the
1364-6826/$-seefrontmatter&2012PublishedbyElsevierLtd.
http://dx.doi.org/10.1016/j.jastp.2012.06.009
G.Manjuetal./JournalofAtmosphericandSolar-TerrestrialPhysics86(2012)56–64 57
methodisratherinvolvedandthepayloadsarebulkyinaddition driftvelocitywouldbeequalinthisregion.Itisthisprinciplethat
totherestrictionsonthelaunchtimings.Theothertechniqueviz., hasbeenusedforestimatingtheneutralwindvelocityfromthe
themeteorwindradarhasaratherrestrictedrangeof85–105km ENWi probe measurements. The photograph of the probe is
onlyandthemeasurementishamperedbythepresenceofstrong shown in Fig. 1a and the rocket payload assembly showing the
ionospheric irregularities in the crucial EEJ region. Further, position of ENWi payload is shown in Fig. 1. The probe has two
ground based airglow spectroscopy cannot give altitudinal pro- arms in the horizontal plane. Each arm has an outer tube
filesandaregenerallyrestrictedonlytomoonlessperiodsofthe maintained at ground potential. Insulated from this, is the field
night that too when the emissions intensities are favorable. freedrifttube,madeupofstainlesssteel(SS)withboththeends
Satellite based retrieval though is the best bet in the prevailing flared out. Two grids (with 496% optical transmission) are
circumstances; it is well known that it suffers from poor spatial welded to the flared-out ends of the SS tube. The drift tube
resolution. The above aspects are discussed in detail in Manju assembly is maintained at a potential of (cid:3)1V. Just behind the
etal.(2012)andreferencestherein.Inthisscenario,thesimulta- inner end of the drift tube is the Faraday cup collector which
neous measurements of neutral winds along with plasma den- collectstheincomingionsthatconstitutethecurrent.Thecross-
sitiesduringanoontimeannularsolareclipseoverthemagnetic sectionalviewofonearmofENWiisdepictedinFig.2a.
equatorial regions made for the first time using a rocket borne A negative accelerating potential ((cid:3)1V) is applied to the
Electron density and Neutral Wind (ENWi) probe are especially entrance grid so that the ambient positive ions are drawn into
important. ENWi measurements have been made at unprece- theprobe.Theseionsarethenguidedthroughthefieldfreespace
dented altitudinal resolution both on the eclipse day and on a inside the drift tube and finally reach the collector plate. The
control day in order to highlight the differences if any and to currents collected by the two collectors at the far inner ends of
understand the causative mechanisms. The present note, while thetwoarmsareindependentlymeasured.Theappliedpotential
demonstrating the power of the measurement technique, brings impartscertainvelocity(V)totheambientionsdependingupon
i
outthesignificantchangesthathavebeeninducedbythepassing its mass and charge irrespective of the orientation of the probe.
eclipseintheMesosphereLowerThermosphere(MLT)region.The When the ambient ions, which are singly charged and are
changesthathavebeenbroughtaboutintheneutral/ionizedparts molecular in nature ( with NOþ and Oþ being the dominant
2
of theupper atmosphere and the consequent equatorial electro- species) in the E-region, are already drifting along with the
dynamicshighlightthat,thelattercouldbeaffectedinmorethan neutral wind with a velocity v, the velocity imparted due to
i
one way that is generally believed to be applicable during quiet theappliedpotentialwouldgetaddedup.Whenorientedinthe
times.Whilemainlyreportingtheobservationsitisbelievedthat velocity direction, one arm of the probe would encounter ions
thepresentnoteopensupneweravenuesforpossibleinteraction with velocity of Vþv while the opposite arm would encounter
i i
between the neutral and the ionized components of the MLT ionswithV(cid:3)v.Themeasuredcurrentsatboththearmswould
i i
regionandtheconsequentelectrodynamics. thereforebedifferentandthisdifferencewouldbeproportionalto
theneutralwindvelocityv.Havinganideaonthedominantionic
i
species in the E-region, the number density of the charged
2. BasicprincipleofENWi particles and also on the applied potential, an estimate of the
neutral wind could be made at every instant and the altitude
IntheEregionoftheionosphere(90–120km),theionneutral profileoftheneutralwindscouldbebuiltupastherocketmoves
collisionfrequency(n)isverylargeascomparedtotheiongyro throughtheregion.
i
frequencyi.e.,ncO,whileontheotherhandtheelectronneutral The field of view of the instrument is the geometric field of
i i
collision frequency is dominated by the electron gyro frequency viewdecidedbytheinletapertureandthetraveldistanceforthe
(ne{Oe).Thatis, theions areessentiallycontrolledby collisions chargedparticlesbeforetheyreachthecollectorplate.Anarrow
with neutrals and they tend to move with the velocity of the fieldofviewof(cid:2)101hasbeendeliberatelychosensoastobeable
neutrals, whereas theelectrons are predominantly controlled by todemarcatethedirectionofmeasurementsandtoseparateout
the magnetic field. Thus the neutral wind velocity and the ion the zonal and meridional components unambiguously when the
Fig.1. (a)PhotographofENWipayloadand(b)photographofrocketpayloadassemblyindicatingthepositionofENWiprobe.
58 G.Manjuetal./JournalofAtmosphericandSolar-TerrestrialPhysics86(2012)56–64
Fig.2. (a)Cross-sectionalviewofonearmofENWiand(b)samplemagneticsensorandvoltagedataforadurationofonesecond.
probe is mounted on a spinning platform like the top deck of a
soundingrocket.Weareeliminatingspineffectbyextractingdata
points only during the times when the probe is entirely in the
E–W and N–S planes (so that no further smoothing is required),
thedirectionbeingdeterminedusingspininformation.
Thedatafromthemagneticaspectsensorflownalongwiththe
payload is used to get the orientation of the probe in the
horizontalplane.Usingthespinrateinformationthewindvalues
corresponding to the zonal and meridional directions are
extracted. While the difference in the currents from both the
armsisproportionaltotheneutralwindvelocity,thesumofthe
currents from both the arms is directly proportionalto the total
ion density or in other words electron density (N¼N) since, in
i e
this case the background neutral wind component gets sub-
tracted. This situation is analogous to a DC probe. Since very
lowcurrentmeasurements areinvolved,thefrequencyresponse
oftheamplifierisverycritical.Forthis,anoperationalamplifier
havingverylowinputbiascurrenttypicallyoftheorderof75fA
andanoffsetvoltageof0.15mVisemployed.Theinputcircuitis
Fig. 3. Temporal variation of DH on the eclipse day (15.01.2010), control day
welldesignedkeepingalltheaboveinmind.Therocketspinrate (14.01.2010)andthemeanquietdaypattern.
is (cid:2)4rps and the signal frequency is restricted with a low pass
filter. We are not intending to look at small scale size irregula-
rities which would put in an impracticable demand on the seriesofquietdaysincludingcontrolday)isshowninFig.3.The
frequency response of the preamplifier. The output signal from upward arrow in this and the subsequent figures indicate the
one Faraday cup along with the magnetic sensor data for one time of maximum solar obscuration. Clearly there is strong
secondduration,isshowninFig.2. development of electrojet up to 0900h after which it showed a
Simultaneously obtained ground based digisonde data and decreasingtrendupto1400h,exceptforabriefincreasearound
SKiYMETmeteorwind radardataareusedto cross-calibratethe 1200h. A strong counter electrojet (CEJ) with DH reaching
ENWiderivedelectrondensityandneutralwindsrespectivelyat (cid:2)12nT manifested on the eclipse day after 1400h. The well
leastatsomeheights.Thecompletedetailsoftheprobewiththe established method of taking the difference in the DH values
first results and the experimental uncertainties such as photo- between a magnetic equatorial station and a station well away
electriceffect,wakeeffect,effectoflaunchelevation,ionPedersen fromtheequatorasrepresentativeoftheelectrojetstrengthhas
current effects and negative ion effects are discussed in Manju not been used for this event deliberately. The said approach is
etal.(2012). called for when one has to separate out local forcings from the
globalforcingslikethatoftheSqcurrentsandtheringcurrentsof
magnetosphere origin on the electrojet. On a day when no
3. Results significantringcurrentdevelopmentisevidentduringtheentire
duration of the eclipse, DH TRV itself is a good measure of the
3.1. Groundmagneticdata electrojetstrength.Thisconditionisvalidforboththeeclipseday
and the control day. Given the possibility of the generation of a
The ground based magnetic field data provide basic informa- counterSqcurrentsystemduringtheeventunderconsideration
tion on the prevailing geomagnetic conditions during the rocket assuggestedbySt.Mauriceetal.(2010),presumablyduetothe
experiments. The temporal variation of DH (the difference eclipseinducedchangesintheneutralatmosphericdynamics,itis
between the instantaneous horizontal component and the aver- felt that DH TRV itself could be used in our study. To make the
age night time value) from a nearby magnetic equatorial obser- point more explicit the comparison of the magnetic data is
vatory, Tirunelveli, for the eclipse day (15/01/2010) along with nowmadewiththe mean DH variationsfrom thesame location
that for the control day and the quiet day mean pattern (for a fortheperiodunderconsideration.The significantdepressionin
G.Manjuetal./JournalofAtmosphericandSolar-TerrestrialPhysics86(2012)56–64 59
magneticfieldbelowthenighttimelevelisverymuchevidentin decrease in densitydue to lack of production and the continued
relationtoquietdaymeanwhichdoesnotshowexcursionbelow loss due to recombination resulting in a decrease of electron
nighttimelevel.Itshouldalsobekeptinmindthatthemagnetic density by a factor of (cid:2)4 at 95km and more than 2, around
data is presented only to indicate the sort of changes the EEJ is 110km and (ii) formation of a ledge of ionization (cid:2)110km
undergoingandnoquantitativeassessmentofthestrengthofthe duringtheeclipse.
CEJisattempted.
3.3. Electrondensityirregularities
3.2. Electrondensitymeasurements
TheENWidatahadalsobeenusedtoextractthecharacteristic
featuresofthesmallscaleirregularities.Itiswellknownthatthe
TheENWiprobewasmountedalongitsaxisonthetopdeckof
equatorialionosphereisrepletewithplasmadensityirregularities
the sounding rocket. With the effective launch elevation of
due to the variety of plasma instability processes operating
85deg, the probe axis is nearly in the horizontal plane. The
therein.TheE-regionoftheionosphereintheequatorialelectrojet
deviationsifanywouldgetreflectedinthewindestimates.With
region during daytime is known to show two distinct types of
itsnarrowfieldofviewof (cid:2)10deg,astherocketspins,theprobe
irregularitiesviz.,typeIandtypeII,theformergenerateddueto
wouldbesamplingdifferentlookangles,enablingonetochoose
two stream instability and the latter due to gradient drift
the zonal and meridional components with ease, with reference
instability.Thesehavebeenextensivelydiscussedintheliterature
to the magnetic aspect sensor data. Detailed description of the
(Fejer and Kelley, 1980; Balsley et al., 1976; Farley and Balsley,
experiments is provided by Manju et al. (2012). As mentioned
1973; Farley et al., 1981; Prakash et al., 1972). Characterizing
earlier,thesumofthecurrentsfromboththearmsatanyinstant
these irregularities enables one to understand the basic drivers
wouldbeproportionaltothetotalchargeparticledensitywhich,
and theoverallelectro-dynamicalprocesses.Extensive workhas
in this case, is the electron density. The altitude profile of the
beendoneintheselinesfromIndiaaswellasfromotherlocations
electron density is derived by summing the currents from both
(Prakashetal.,1972;Subbarayaetal.,1983).
the arms. These are depicted in Fig. 4 for the eclipse day along
A detailed study during specific events like the present one
withthecontroldaypatternandthequietdaymeanpattern.The
helps us to understand the response of the main drivers to the
quiet day mean electron density profile is a typical profile
changesoccurringduringasolareclipse.Asafirstcutexercisethe
obtained at Thumba. It is obtained by averaging the profile
entireelectrondensityprofileisobtainedbasedonsinglearmdata
obtained on 14 January 2010 and other available profiles from
and the electron density data were available at around 120m
Thumbaundersimilarsolaractivityconditionsaroundnoontime
intervals.Astherocketmovesclosertotheapogee,itslowsdown
(Subbarayaetal.,1983).Ontheotherhand,theprofileduringthe
and for the same spin rate; the altitudinal resolution would
peak of the annular eclipse shows significant differences. As
steadily increase. In order to emphasize medium scale irregula-
expected, large decrease in the electron density is observed in
ritiesof several hundred meters tokilometer scale sizes, that are
theEregionduringthepeakphaseoftheeclipse.Thisdecreaseis
responsible for affecting the radio communication, the electron
duetothesuddencutoffofthesolarinsolationwhichresultsin
densitydatawereinterpolatedsothattheyareequallyspaced,at
thedecreaseofionizationduetodominanceofchemicalrecom-
every200m.Thisputsalimitonthesmallerscalesizesthatcould
bination.Thedensitieswerefoundtosteadilyincreaserightfrom
beextracted;inthiscaseitis 4400m.Thedatawerede-trended
95km,upto110km.Averyinterestingobservationisthesharp
so that scale sizes greater than 1km were removed. Thereafter,
noselike structurefollowedbya negativegradientat (cid:2)110km.
waveletanalysiswascarriedoutwhichyieldedthepowerspectral
Theprofileshowedatendencytoincreasefurtherabove.Forma-
output,forallthethreeflights(twoontheeclipsedayandoneon
tion of a ledge of ionization around 110km and the negative
thecontrolday),inthescalesizesof0.4–1km(Fig.5).
gradient that followed have important messages to convey and
The significant differences in the altitude distribution of the
theseaspectsarediscussedinthelatersection.Onthewhole,the
powerlevelsbetweenthetworocketflightsoneontheeclipseday
significant differences between the two profiles are (i) Overall
and another on the control day highlights to what extent the
fundamental physical processes got altered during the eclipse. An
independentcrosscheckoftheresultsfromENWihasbeenmadeby
comparing them with the ground based coherent HF radar results
for the same day and duration. The HF radar that operated at
18MHz,givesinformationonthe8.3mscalesizeirregularities.The
MLTregionofinterestissampledin16rangebins.Thoughthescale
sizescurrentlydelineatedfromENWiprobeisintherangeof400–
1000m, the basic process that is responsible for their generation
andthe8.3mscalesizesrecordedbytheradaristhesameandone
would expect to see good one to one correlation in their altitude
profiles.Fig.6depictsthenormalizedpoweroftheirregularitiesin
therangeof400–1000mforENWiprobeandthatcorrespondingto
8.3mscalesizeinferredfromtheHFradar.Thealtituderesolution
ofHFradaris3km.Dataineachpanelisnormalizedwithrespectto
thepeakpowerconsideringallthethreeflights.
Asexpectedthealtituderegionoftheirregularitiesduringthe
controldayflighton14thJanuary2010istypicalofanormalday
andisrestrictedtotheregionof95–100kmcorroboratedbythe
HF radar. It should be borne in mind that the HF radar has a
poorer altitude resolution due to its finite pulse width. On the
otherhand,whenwelookattheeclipsedayresultsoftheENWi
probe corresponding to the maximum obscuration of the sun at
Fig. 4. Electron density profiles obtained from ENWi on the eclipse day
(15.01.2010),controlday(14.01.2010)andthemeanquietdaypattern. 1305h, theregionofirregularitiesisshifted to107–113kmand
60 G.Manjuetal./JournalofAtmosphericandSolar-TerrestrialPhysics86(2012)56–64
Fig.5. Irregularitypowerspectrumforthemediumrangescalesizes(0.4–1km)forthreeflights;topleft:duringthemaximumobscurationphaseoftheeclipse,topright:
correspondingtothecontrolday(14.01.2010),andbottom:1600hflightontheeclipseday.
Fig.6. Altitudeprofileofthenormalizedpowerforthethreerocketflights(leftpanel)andthecorrespondingHFradarnormalizedbackscatteredpower(rightpanel).
theHFradartoorevealsexactonetoonecorrespondencewiththe understanding of the equatorial processes in general, and the
ENWiresults.Thethirdflightthatwasconductedat1600honthe electro-dynamicalprocesseslikeEquatorialElectrojetinparticu-
eclipsedayrevealednoirregularitieseitherintheENWiortheHF lar, simultaneous measurements of both the neutral and iono-
radar. The distinctly different irregularity profiles indicate sig- sphericparametersarenecessaryanditisbelievedthatthenew
nificantchangesbroughtaboutbytheeclipseandthesewouldbe ENWiprobehasthepotentialtofillupanimportantgaparea.The
discussedlater. zonalandmeridionalwindcomponentshavebeenderivedforthe
eclipse and the control days from the probe data and are
3.4. Neutralwindcomponents presented below. Considering the sources of error in measure-
ments mentioned in Manju et al. (2012), the measurement
The significant advantage of the ENWi probe had been in its uncertainties pertaining to the instrument alone is estimated. It
ability to provide simultaneous measurements of the neutral is seen that such errors amount to less than 10m/s. For further
windcomponentsalongwithplasmadensitiesinacriticalregion quantification of the uncertainties, we plan to undertake more
with unprecedented altitude resolution. For a comprehensive ENWi flights along with complementary experiments like TMA
G.Manjuetal./JournalofAtmosphericandSolar-TerrestrialPhysics86(2012)56–64 61
releases so that a comprehensive estimate of the errors may be 105–115km.Thus,fromthebehavioralpatternofthezonalwind
arrivedat. components,itcouldbeseenthat,thatthereisaneclipseinduced
effectintheform of reversalof thezonal winds,which couldin
turnhaveimportantramificationsaswouldbediscussedlater.
3.4.1. Zonalwindcomponent
The control day, as expected on a typical day, showed zonal 3.4.2. Meridionalwindcomponent
winds in the westward direction with a least velocity of There had been hardly any change in the magnitude of the
(cid:2)50ms(cid:3)1 around 100km which monotonically increased to meridionalwindsfromthecontroldaytotheeclipseday.Onboth
large values of (cid:2)250ms(cid:3)1 around 110km, which is the apogee the occasions the maximum of the wind had been 50ms(cid:3)1 and
of the sounding rocket (Figs. 7 and 8). When compared with directed southward. There had been a shift in the altitude of
satellitederivedwinds,themagnitudesinferredfromENWidata maximum velocity from 105km on the control day to 102.5km
appeartoberatherlarge.Theexperimentaluncertaintiesinclud- ontheeclipseday.Ontheotherhand,thesecondrocketflighton
ingthevariouspotentialsourcesoferrorsandtheseeminglylarge theeclipsedayat1600hregisteredapolarityreversalallthrough
values for the zonal components on the control day have been thealtitudeofmeasurements.
discussedintheliterature(Manjuetal.,2012).Eclipsedaypattern
(Fig. 7), on the other hand, depicts a large reversal to eastward 3.5. Irregularityphasevelocityestimates
direction in the entire altitude range above 98km. The wind
magnitude had remained close to zero up to 97.5km beyond The HF radar located in Thumba, Trivandrum, operating at
which, there had been a buildup up to (cid:2)105km where in the 18MHz,hasabeamswingingcapabilitytothreedifferentdirec-
magnitudehadreachedavalueof (cid:2)70ms(cid:3)1.Higheraboveithad tionslikethewest,zenithandeastand,asmentionedearlier,was
remained centered to this value up to the apogee of the rocket. alsooperatedtocontinuouslymonitorthechangesbroughtabout
Fig. 7 also shows the zonal wind profile from another flight by the annular eclipse. Using the standard procedure (Tiwari
conducted at 1600h on the eclipse day with an aim to study et al., 2003; Manju et al., 2005), by making use of the Doppler
the persistence of the eclipse effects if any. Incidentally, during information of the type II irregularities monitored by the radar,
this flight too the zonal winds had remained similar to the the line of sight phase velocities (Vy) of the irregularities in the
p
maximum obscuration time. The polarity of the wind had been electrojethasbeenderivedontheeclipsedayandthecontrolday
eastwardandvariedfrom50–100ms(cid:3)1inthealtituderegionof (Fig.9).Westward(eastward)driftofirregularitiescorrespondsto
an upward (downward) polarization electric field (Manju et al.,
2005). The dotted lines in figure correspond to the absence of
signal or non-availability of data. Vy increased to a value of
p
(cid:2)60ms(cid:3)1 (westward) in the morning hours, but started to fall
off to reach low values of (cid:2)10ms(cid:3)1 (westward) close to noon
time. Though the data are intermittent for want of sufficiently
intense returns from the irregularities, the trend could be seen
veryclearly.By1245h,itisclearthatVyhasreversedtoeastward
p
direction attaining a maximum eastward value (30ms(cid:3)1) at
1305h, coinciding with the rocket experiment. Correspondingly
theverticalpolarizationelectricfieldalsowouldhavereversedto
downward direction. Later on by 1320h Vy reverted back to its
p
typical daytime polarities. It turns out that there was a clear
reversal in the polarity of Vy during 1245–1320h, close to the
p
peak eclipse phase, unlike on the control day, when no such
polarity reversal is observed. Under normal circumstances over
the magnetic equator, modeling studies have revealed that the
Fig.7. ZonalwindsderivedfromENWiprobeoneclipseday(15.01.2010)and verticalelectricfieldwouldincreasewithaltitudeupto (cid:2)120km
controlday(14.01.2010).
Fig.8. MeridionalwindsderivedfromENWiprobeoneclipseday(15.01.2010) Fig.9. Timevariationoflineofsightphasevelocityofirregularitiesoneclipseday
andcontrolday(14.01.2010). andcontrolday.
62 G.Manjuetal./JournalofAtmosphericandSolar-TerrestrialPhysics86(2012)56–64
blanketing Es layer is formed in the ground based ionosonde
andthemaximumfrequencyreturnedfromthislayerwithtimeis
alsoshowninthefigure.Itisveryinterestingtonotethatasthe
radarechoesgrewinstrengtharound1300h,blanketingE (E )
s sb
is observed in the Ionosonde, exactly coinciding with the strong
echoes. The E echoes were observed from (cid:2)1240 to 1325h
sb
whilethemaximumobscurationoccurred (cid:2)1315h.Theintense
manifestation of E had been rather short lived and lasted only
sb
for (cid:2)20min in agreement with the field reversal duration.
However,themaximumcounterelectrojetrelatedmagneticfield
depression was observed much later (cid:2)1430h. The location of
irregularities and the associated sharp layer of ionization that
manifestedasblanketingE atahigherheightviz.,110kmitself
s
are rather unusual. The few available measurements during
counter electrojet periods show that, customarily, the formation
ofaledgeisencounteredaround92–94kmwiththecorrespond-
ing blanketing Es being detected by the ionosonde at those
heights(ShirkeandSridharan,1979andthereferencedtherein).
Only in one other case, reported by Manju and Viswanathan
Fig.10. TemporalvariationofthealtitudeofpeakelectrondriftandfbEsonthe (2007), blanketing E relatedradar echoes were observed during
s
eclipseday.
CEJ at higher altitudes. The presence of a ledge of ionization
providing negative gradients is a must for the gradient drift
and later steadily would decrease as the Pedersen mobility instability to operate, when the vertical field is pointed down-
approachestheHallmobility(Reddy,1977).However,2-Dmod- wards.However,itiswellestablishedthatthough,theformation
elingstudiesbyAnandaraoetal.(1977)didshowthattheelectric ofsuchsharpledgesofionizationcouldeasilybedoneoverlow
fieldtendstomaximizeonlyattheheightofmaximumvelocity. andmidlatitudesbywindshears,overthemagneticequatoritis
Whenwecriticallyanalyzethealtitudevariationofthepeakdrift not possible due to the magnetic field lines being horizontal.
velocityduringtheeclipse, itisseenthatthetypeIIirregularity Such restructuring of the ionization has to be necessarily done
drift velocity maximized around 110km on the eclipse day as due to electrodynamical processes. It could be seen that the
shown in Fig. 10 which depicts the temporal variation of the eclipse time measurements have revealed significant changes
altitudeofthepeakelectrojetdriftvelocityasderivedfromtheHF broughtaboutintheMLTregion,theramificationsofwhichneed
radaratTrivandrum.Thoughattheoutsetitmayappearthatthe tobeunderstood.
drift velocities have shown a tendency to maximize at higher
altitudes,itisnotpossibletounequivocallymakethisstatement 3.6. Discussion
becausethedriftvelocityestimatesarebasedontheradarreturns
from 8m irregularities that get generated when the conditions Itcouldbeseenfromtheabovepresentationthatthesimulta-
arefavorablei.e.,whentheelectricfielddirectionandtheplasma neous measurements of neutral winds, plasma densities, ioniza-
density gradients are in the same direction. During the eclipse, tion irregularities, and ground based radio probing by HF radar
since the electric field had already reversed its polarity, the andIonosondealongwithgroundmagneticdata,asapart ofan
irregularities could be generated only in the negative gradients intense campaign to understand the effects induced by the
oftheelectrondensityprofilewhichinthiscaseislocatedaround annular solar eclipse in the equatorial ionosphere/thermosphere
110km. Though the magnitude of the drift velocity had been system have yielded rich dividends. The unique event with an
ratherlarge((cid:2)30m/s),duetowantofirregularitieslowerbelow obscuration of 484%, with its path of maximum obscuration
one cannot make an assessment of the velocities. It is seen that lyingjustoverthemagneticequator,thattooduringnoonhours
the back scatter returns come from the typical electrojet height when all the equatorial geophysical processes would be in their
regionof94–107kmpriorto1230hwhileclosetothepeakphase fullswing,providedauniqueopportunitytostudytheimpactof
of eclipse, strong signal returns are observed from altitudes the eclipse. The promise shown by the newly developed ENWi
centered (cid:2)110km. From Fig. 6 it could also be noted that the (Manju et al., 2012) probe also comes to the fore based on the
receivedpowerfrom110kminthiscaseiscomparabletothatat results obtained from it. When it comes to the plasma densities
theEEJmaximumaltitudes.Thiscouldmeanthattherehadbeen and the density irregularities, the probe had shown distinctly
verysignificantchangesintheEEJparametersduringtheeclipse. differentdensityprofileswhichcouldbeunderstoodbasedonthe
In the absence of complete information one has to necessarily chemical recombination of the dominant molecular ions in the
infer based on the general picture. Normally the drift velocities near absence of the ionizing solar radiation. The recombination
maximize around noon ((cid:2)50–60m/s) and the typical velocity effectswould besignificantlylarger inthe lowerheightsand its
during similar conditions is (cid:2)20m/s around 1300h. On the effect is expected to decrease with altitude. As one moves to
eclipse day when one encounters a magnitude of 30m/s under higher altitudes the transport effects would become dominant.
fieldreversedconditions,eitheritcouldbethemaximumvalueor The next critical observation pertains to the plasma density
lowerbelowitcouldevenbelarger.Intheabsenceofanydirect irregularities. As one would expect, on the control day, the
measurementsnofurtherinsightcouldbeobtained. medium scale irregularities that are primarily generated due to
Itistobenotedthat,inthealtituderegionjustabove110km, thegradientinstabilityarelocatedatthepositivegradientwhen
theN profileontheeclipsedaydepicts,aclearnegativeelectron theverticalelectricfieldisdirectedupwardmakingitconducive
e
density gradient. In the presence of downward directed electric fortheinstabilitytooperate.Thisisatypicalnoontimecondition
field which has been substantiated by the radar results, the overthemagneticequator.TheHFradarconfirmsthepresenceof
conditionforgenerationofGradientdriftinstability(DN(cid:4)E40) 8.3mirregularitiesexactlyinthesamealtituderegionasdetected
(Prakash et al., 1972; Fejer and Kelley, 1980) is satisfied only in bythe ENWiprobe.Ontheother hand, duringtheeclipse when
the region of 110–112km. Around this time, a very strong the ground magnetometer had clearly shown the presence of a
G.Manjuetal./JournalofAtmosphericandSolar-TerrestrialPhysics86(2012)56–64 63
counterelectrojet,theprimaryfieldwouldhavereversedresult- justoppositeofwhathadbeenshownearlieronmanyoccasions
ing in the reversal of the vertical field to point downward. Both (Vineethetal.,2007)whereinthemesosophericregionshadbeen
theconditionsviz.,thegradientwhichalsohadbecome shallow shown to become cooler during CEJ. These differences highlight
and also the field direction in the region where normally these thepointthatthecausativemechanismsfornormalCEJandthe
irregularitiesarelocated,arenotfavourableforthegenerationof reversal experienced during this eclipse may be totally different
irregularities.ThemeasurementsbothbyENWiandtheHFradar andcallsforanindepthstudy.Aspresentedearlier,themeasured
confirm this aspect. However, at 110km where a ledge of zonal wind component has reversed from normal westward to
ionization is detected, one encounters a narrow height region eastwardwindsmakingitsimilartowhatoneencountersduring
withanegativealtitudinalgradient.Kato(1973)attemptedaone night time conditions. On the other hand, it is noticed that the
dimensional theory invoking electric fields generated due to meridional components on both the control day and the eclipse
gravity wave winds, to explain the stratifications seen in the day had been comparable. The effect of these changed wind
eveningandnighttimeequatorialionosphere.A2-Danalysisby patternsisexpectedtohaveasignificanteffectontheequatorial
Anandarao et al. (1977) demonstrated that the efficiency of the electrodynamicsingeneralandtheequatorialelectrojetinparti-
gravity wave winds is significantly less than what has been cular. St. Maurice et al. (2010) in one of their studies have
inferred by the 1-D calculations. In spite of this finding, the proposed that the relatively cooler temperatures along the path
suggested mechanism by Kato (1973) could still be effective of the eclipse could trigger a low pressure system and result in
duringeveningandnighttimeconditionswhentherecombination somesortofalocalizedneutraldynamoalteringthemainglobal
rate is slower when compared to the rate of convergence. The scaledynamosignificantlyintheregionoftheeclipse.Changesin
presentmeasurementsduringaneartotaleclipsewouldtakethe thermosphericmeridionalwindpatternintheequatorialandlow
E-regionoftheionospheremoretowardstheeveningconditions latitudeFregion,fromequatorwardduringpeakeclipsephaseto
implyingthepossibilityofgravitywavewindscouldhavehada strong poleward in post eclipse phase have been reported by
say in the formation of the ledge of ionization around 110km. Madhav Haridas and Manju (2012). Our observations do show
More detailed model calculations are called for to explain the significant changes in the neutral wind and plasma density
observations. Further studies would address this aspect. Coming distribution patterns. While the present paper deals with the
back to the negative gradient shown by the ledge of ionization, observed changes in the MLT region associated with the eclipse
interestinglytheirregularitiesaredetectedexactlyinthatheight more detailed analysis is called for by taking these measured
regionbyboththetechniques.Thiswouldimplythattheoverall parametersasinputstotheelectrojetmodelsasappliedtoIndian
backgroundandalsotheelectro-dynamicalconditionshavegone longitudes.Furtherworkontheselinesisunderprogress.
throughmajorchangesduringtheeclipsewhichmeansthatthe
maindriverfortheelectrodynamicsviz.,theneutralwindsshould
haveundergonesignificantchanges.Undernormalcircumstances
4. Summary
the effects of neutral winds on EEJ over Indian longitudes have
been studied in detail by earlier workers. Anandarao and
The ENWi probe had been flown on multiple rockets during
Raghavarao (1987) have shown that the effects of horizontal the recent annular solar eclipse event on 15 January 2010 that
winds and shears in them are felt only beyond 2deg on either
passedthroughtherocketrangeduringafternoonhourswhenthe
side of the magnetic equator while there had been very less EEJintensityisusuallyhighmakingthefirstofitskindsimulta-
change in the centre of the electrojet. While the zonal winds
neous measurements of neutral winds and electron density
could alter the width of the electroject, the meridional winds during an annular solar eclipse at the equatorial region. The
could at the most shift the centre of the electrojet. Further,
analysis of the ENWi data in association with the HF radar data
RaghavaraoandAnandarao(1980)havedemonstratedusingtheir highlight the eclipse induced changes in both the neutral and
model that vertically downward winds, presumably of gravity
electrodynamics that resulted in the occurrence of counter
wave origin, are capable of reversing the electrojet. Another electrojet and blanketing Es layers at elevated altitudes over
independent study by Somayajulu et al. (1993) had shown very
equator.
largechangesinthezonalwindmagnitudesassociatedwithCEJs.
Ontheotherhand,studiesontheoccurrencepatternoftheCEJs
hadshownthatthereexistsaclosecouplingbetweenthesudden
Acknowledgements
stratospheric warming in the polar regions to the periodic
occurrencepatternoftheCEJsovertheequator,makingitintoa
The solar Eclipse campaign was supported by the Indian
globalphenomenon(Steningetal.,1996;Vineethetal.,2009).All
national programunder CAWSES-INDIA.The activeparticipation
theaboveonlyindicatethecomplexityofthephenomenonofCEJ
by the ATVP division and the TERLS rocket range personnel and
and suggest that there couldbe morethan one way by which it
the keen interest shown by the VSSC in making the program a
couldbeaffected.Thepresentresultsdiscussedinthepaperbring
success is duly acknowledged. The active participation of the
out one more dimension to the problem of the reversal of the
Atmosphere Technology Division of SPL is gratefully acknowl-
electrojet.TheENWiderivedneutralwindsdoshowinparticular
edged. R. Sridharan duly thanks CSIR for the emeritus scientist
thatthepolarityandmagnitudeofthezonalwindshavechanged
position. The work is supported by the Dept. of Space, Govt.
significantly during the eclipse. In all probability this could be
ofIndia.
localised and has a modulating effect on the normal Sq current
system. The sudden cooling along the path of the eclipse right
over the magnetic equator and the consequent changes brought
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