•OURNALOF GEOPHYSICAL
RESEARCH,
SPACEPHYSICS
VOL. 73, NO. 5, MARCH 1, 1968
Studiesof the MagnetosphericSubstorm
Characteristics
of ModulatedEnergeticElectronPrecipitationOccurring
during Auroral Substorms
G. K. PARKS,
1 F. V. CORONITI,
1•. L. MCPHERRON,
ANDK. A. ANDERSON
Department of Physicsand SpaceSciencesLaboratory
University o• California, Berkeley, California 9472'0
The temporal,energy,and spatialcharacteristics
of energeticauroral-zoneelectronprecipitation in the 5- to 35-secperiod range are discussed.
The electronpulsationsof 5- to 10-sec
period occurduring 0200-1000local time, have a soft energy spectrumwith an e-folding
energy of about 15 key, and showspectrumhardeningat the peaks.The region over which
the electronpulsationsare occurringis about 100-150km. The electronpulsationsof 20- to
35-secperiod occur during 1000-1500local time, possessa hard energy spectrumwith an
e-foldingenergyof about30 key, and occasionally
showspectralhardeningat the peaks.The
scalesize of theseelectronpulsationsis often 100-150km, but occasionallyit is greaterthan
210 km. The electron pulsationsand the other forms of energeticelectron precipitation that
occur in the auroral 'zone are direct consequences
of worldwide disturbancecharacteristicsof
auroral
substorms.
INTRODUCTION
High-altitudeballoonmeasurements
of bremsstrahlungX rays resultingfrom the precipitation of energeticelectronshave shownthe presence of diversetemporal structure, from a few
milliseconds
to hundredsof seconds(for a review
paper of auroral-zone
X rays,seeBrown [1966];
Anderson[1966].
The purposeof the presentarticle is to discuss
in detail •he spatial,temporal,and energyspectrum characteristics
of auroral-zoneX rays that
exhibit pulsating features in the 5- to 35-sec
period range. In paper 2 (hereafter and in the
two followingpapers,this paper will be referred
to as paper .1,McPherremet al. [1968] as paper
2 and Co.roniti et al. [1968] as paper 3) the rel•tionship between energeticelectronsand geomagneticmicropulsations
is discussed.
Recently,
Parks et al. [19'66] have shown that certain
forms of energeti• electron precipitation and
directly related to auroral substorms[Alcaso/u,
1964]. Sincelocal time plays an important role
in the ultimate understandingof auroral-zone
electron precipitation and geomagneticmicropulsation phenomena,we have organized the
data in this article
in terms of local time.
The variousforms of structuredenergeticelectron precipitation phenomenathat occur in the
auroral zone during geomagneticallydisturbed
times have been classifiedinto varioustemporal
groups. We review briefly below the various
temporal structures in energeticelectronsas a
functionof the local time during which they are
most frequently observed.
1. Local time 2200-0200. Energetic electron
precipitation during this time is usually associatedwith auroral breakup. Until recently,the
precipitation is reported to have been intense
and featureless
over times of a few minutes.
However, rocket measurements[Evans, 1967]
magnetic micropulsationsin the 5- to 30-see and balloon-bornemeasurementsat 2 g/cm' atperiod rangewere associatedwith auroral sub- mosphericdepths [Parks et al., 1967] indicate
storms.In paper 3, it will be shownthat certain the presenceof 5- to 50-msecbursts of electrons
forms of energetic electron precipitation are during auroral breakup. These rapidly varying
bursts may be a persistentfeature of energetic
electronprecipitation during thesetimes.
2. Local time 0200-1000. Brown [1965] and
x Present address: School of Physics and AsBarcus et al. [1966] have reported numerous
tronomy, University of Minnesota, Minneapolis
55455.
observationsof X-ray pulsationsof 5- to 10-sec
1685
1686
PARKS, CORONITI,
McPHERRON,
periodsduring these local hours. These X-ray
pulsationsmay be closely related to pulsating
auroras [Rosenbergand Bjordal, 1967].
3. Local time 0600-1•00. In this region of
local-timemicroburstsare the predominanttype
of energetic electron precipitation [Anderson
etal., 1966]. The characteristicduration of microburstsis 0.1-0.5 sec.When groupsof microbursts occur,the spacingbetweenadjacent microburstsis typically 0.5 sec [Anderson and
Milton, 1964; Parks, 1967]. The groupsof microburstsare separatedby about 10 sec.
•. Local time 1000-1500. For the first time,
electronpulsationsof 20- to 30-secperiod have
been observedduring these local times by the
Sparmo group [Ehmert etal., 1966]. In this
paper, data will be presentedconfirmingtheir
observations,and detailed properties of these
pulsationswill be discussed.
5. Local time 1600-2200. The energeticelectron precipitation during these hours generally
does not show any significant time variations
over several minutes. More data are needed to
better our understandingof the properties of
thistype of electronprecipitation.
There are observationsof other precipitation
forms with temporal variations of about 100
seconds[Evans, 1963; Barcus and Christensen,
1965]. These have only been observedrarely,
AND ANDERSON
ured, and energyspectruminformationwas obtained by use of pulse height discriminators.
Also,fasttime responsb
analogcount-ratemeters
of the type describedby Andersonand Milton
[1964] were employed. The true photon rates
were obtainedby useof infiight calibrators.The
data presented 'below were obtained from Flin
Flon, Manitoba, Canada (geomagneticcoordinates 63.6øN, 314.8øE).
METHOD OF DATA ANALYSES
To obtain the spatial propertiesof the X-ray
pulsations,the analog data correspondingto
photonsof energies20-45 kev o.btainedfrom the
three regionsof the sky were compared.The
ratios of the X-ray countrate from the different
detectorswerecalculatedand were comparedto
the theoretically calculated ratios expectedat
the balloon altitude for an isotropic and uniform X-ray sourcedistribution. The calculation
assumedthat the bremsstrahlung
energyspectrum resultedfrom an exponentialelectron energy spectrum. Corrections owing to photo-
electric and Compton scattering lossesand
differencesin the geometricalfactors of the
detectors have also been included. It should be
notedthat with the concentric
circulararrange-
ment of the detectorsystemthe sourcedistribution is not absolutelyknown.Thus, our analysis
and since their local time characteristics are not
pertains only to changesof a uniform and isoclearly understood,they have not beenincluded tropic X-ray sourcedistribution at the 100-km
X-ray production plane. The calculatedratios
in the abovelocal-time organization.
APPARATUS
The Compton mean scatteringlength for X
rays of energies20-150 kev is about 6 g/cm•. It
is important to minimize measurementsof scat-
tered photonsif accurateinformationregarding
the energy and spatial characteristicsof energetic electronprecipitation is desired.The data
to be presented in this article were obtained
from balloonsflown at 2-3 g/cm• atmospheric
depths.A singleballoondetectorassemblyconsisted of three NaI(T1) scintillators,7.6 cm in
diameter and 0.63 cm thick, all of which were
of the 210-km detector to the 70-km detector is
2.8, and the ratio of the 70-km detector to the
30-km detector is 4.0.
Informationon the kinetic energyof the electrons producingthe observedX rays was ob-
tainedusingthick targetbremsstrahlung
theory
and assumingan exponentialelectronenergy
spectrum.After correctingfor atmosphericabsorption and cosmic-ray contributions to the
detectorcount rate, the ratios of the photon
countrate in the differentenergyintervalswere
compared with the theoretically calculated
bremsstrahlungspectra for various electron e-
pointed in the zenith direction. The detectors
were physically collimatedwith an A1-Pb sand-
folding energies.The averageelectronenergy
obtainedin thismanneris goodto an accuracy
wich to give geometricalfactorsof 9.6 cm"'ster, of about30%. This accuracyhasalreadybeen
38.6 cm• ster, and 142 cm• ster to view concen- demonstrated
by Hudsonetal. [1965]. It should
tric circlesof diameters30, 70, and 210 km at be notedthat to obtainthe averageenergyof
the 100-km stopping heights for electrons.X
rays of energiesgreater than 20 kev were meas-
the electronsproducing the observedbrems-
strahlungX rays,the dete.
ctor energysettings
STUDIES OF MAGNETOSPHERIC
I
I
SUBSTORM, 1
I
1687
I
I
0400
90 ø WMT
9 SEPT. 1966
6000
800
...............................
Z•$000
400
--
:'" ...................................................
.
$000
400
20 0
'
0
50
I00
TIME
IN
150
200
SECONDS
Fig. 1. The analog X-ray count rate for a very periodic 5-second electron pulsation event.
Note the almost constant amplitude.
FLIN
FLON
9 SEPT.
1966
FLIN
FLON
6 SEPT.
1966
(a)
4.8 SECS
2x104[
0400
0400 90 ø WMT •
7.0
IOO
(b)
90 ø WMT
6.::5
104
o
ß15
.20
.25
at_l_
.05
.50
FREQUENCY (CPS)
49 SECS
n-1500
FLIN FLON
6 SEPT. 1966
I000 90 ø WMT
•.•
(c)
0
u.
.L
.m_
.10
.15
y_---
11
.20
.25
FREQUENCY (CPS)
n-
FLIN
FLON
bJ
1200 9Oø WMT
8 SEPT.
1966
n-
(d)
37 SECS
• I000
•
,,o
I000
•
soo
500
•oo
.02
.05
.04
.05
FREQUENCY (CPS)
o
I
ß01
I
D2
I
D3
I,
.04
"1 ß
FREQUENCY (CPS)
Fig. 2. Results of a high-resoh•tionpower spectrum analysisfor the four events discussedin
the paper.
.05
1688
PARKS, CORONITI, McPItERRON, AND ANDERSON
must be taken into consideration. This amounts
i
I
i
i
0400
90" WMT
to addingthe energyvalueof the detectoredge "'
.,• 900- 9 SEPT 1966
settingto the electrone-foldingenergy.Therefore,the averageelectronenergycausing
X-ray I-- 800
Z
pulsations
is the e-foldingenergyplus20 key.
D
RESULTS
Local time 0200-1000. During the extended
regionof local time from 0200 to 1000, the elec-
-
700
-
0
600
-
,•500
• 400
with a typical e-foldingenergyof 15 kev. In this
section,two representativeevents will be dis-
2øøI
300
I
3.2 -
cussed.
the 70-kin detector was flown.
20<E<45KeV-
O
tron precipitation is often modulated with a
characteristicperiod of 5-10 seconds.The elec-f
trons generally exhibit a soft energy spectrum
Figure I presentsthe analagX-ray countrate
for an event that occurredon September9, 1966,
at 0400 (90ø WMT). The electron pulsations
occurredon top of a slightly increasedX-ray
backgroundof 5 photons/cm
' secster and have
a very regularperiodicityand amplitude.Figure
2(a) showsthe results of a power spectrum
analysisof this event. The singlepeak at 5 sec
demonstrates
the highlyorderedtemporalstructure of this event.A sequenceof data was digitized and is presentedalong with the ratio of
the X-ray count rate in the 20- to 45-key channel to that in the greater than 45-kev channel
in Figure 3. The averageelectrone-foldingenergy was about 17 kev. On comparingthe temporal changesof the energyspectrumwith those
of the count rate, it is seenthat the peaks of
the electronpulsationsare generallyharderthan
the backgroundby about 2 kev. It should be
pointed out that even though the method of
determiningthe energyspectrumyields values
with a possibleerror of _30%, the changesof
the energyspectrumduringthe electronpulsations are statisticallysignificant.No spatial informationfor this eventis availablesinceonly
i
I
RATIO
= CR(20<E<
45KeY) CR( E >45 KeV)
3o0-
-
'•2.6
2.4
(..9
2.2
•
2.0
Z
•
-
1.8
-
1.6-I
I
0
5
TIME
I
I0
IN
15
I20
SECONDS
Fig. 3. Digitized linear X-ray count rate and
the energy radio for part of the event shown in
Figure 1. The averageelectrone-foldingenergy
was 17 key. Note that the peak of the electron
pulsationsis harderthan the unstructured
precipitation.
Referringto Figure 2(b), we seethat the dominant spectral peaks were 7.0 and 6.3 seconds.
The digitized linear X-ray count rate, the
energy ratio of the count rate in the 20- to 45key channelto that in the greater than 45-kev
channel,and the spatial ratio of the count rate
in the 210-kin
detector
to that
in the 70-kin
The analog X-ray count rate for the second detector is presentedin Figure 5. 'Note that the
event that occurredon September6, 1966, at count rate in the various detectorswas high
0500 (90ø WMT) is presentedin Figure 4. The enoughso that the countingstatisticsgive only
backgroundrose to about 50 photons/cm
' sec 2-3% errorsin the ratios.The averagee-folding
ster in the energy channel 20-45 kev for 15 energy of the electronsduring this event was
minutes before the start of electron modulation.
14 kev. On comparingthe changesof the energy
It is seenthat, unlike the previousevent, the
electron pulsationsoccurredirregularly, often
with a few cyclesof modulationseparatedby
spectrumwith thoseof the electronpulsations,
it is seenthat the spectrum always hardenson
the rise of the pulsation and softenson the deunstructuredprecipitation.The ,amplitudeof cline. The changein the e-foldingenergyfrom
the pulsations,however,was roughly constant. the peak to the valley is about 2 kev.
STUDIES
OF MAGNETOSPHERIC
SUBSTORM,
1689
1
0400
90 ø WMT
6 SEPT, 1966
-- ' ........':...... •:-'-:-:'-:::'"',"',:-'
................
:,:":-'-:-:
....•,'":'•uj'x.•-.'-•.'- - .•
•__
6000
([• 800 '•; '; ....
.... i'; ....................
3000
_'
'•--:-..:..-'-•
_.._ ' ' "......... ••".7-..7•;;;';
4OO
1200
4.00
.......................
I
-- '_-_
.'•'•_'sJ_¾..'.•..•
¾
'.'•
I
1200
800
4.00
,-
,,,, •
,•.
--
--
ß -.
---
I
",,.• -'• -
--
x
r,.w
............. .-73.. ;-¾7Z;;;- ;;'--.¾•-.--......
I
• ,:3000
I
:-
..•
'•-0¸ '-- i!,E>45KeV
I
n,' 800
-...... .••-
':....;:'::t:.;;:
..... ...............
800
6000
'-" '
-
.......•--- - --
-- '1
..................
!.................
i........ : ...... 20<
E<
45KeV-============================
ß......
•.? .... • ...........
300
• ...................
i-•"'•i;"'•
..........
Z .....
400
TIME
. .................
500
IN
E >45KeV
600
SECONDS
Fig. 4. The analogX-ray countrate for a 6- to 10-secperiodelectronpulsationvent. Note
the rise in the backgroundlevel before the start of the pulsationsand the irregularnature of
the modulation.
The last part of Figure5 showsthe changes preflight and inflight calibration,the spatial raof the ratio of the count rate in the 210-km
detector to that in the 70-km detector. Assum-
tios of the 210 to 70 km
and 70 to 30 km
of the spatialratio changes
with the X-ray pul-
plane.
sationsshowsthat the ratio decreases
during the
It would appear, then, that sincethe 30-km
detectorwas seeingsucha high flux, this could
be interpretedin terms of a•scalesize of 30-50
detectorswere checkedduringthe unstructured
ing an isotropicflux at the 100-kmX-ray pro- electronprecipitationthat occurredjust before
duction plane, the ratio of the 210-km de- and after this event. In both cases,the spatial
ratios were those characteristicof an isotropic
tector to the 70-km detector on the basis of the
theoreticalcalculationwouldbe 2.8. Comparison distribution at the 100-km X-ray production
rise of the pulsationand increases
duringthe
decline.At the peaksof the pulsations,
the ratio
is 2.6,whereas
at the valleyit is 3.4.The fact
that the ratio at the valley is higherthan the
calculatedisotropicvalueof 2.8 is probablydue
to the pulsationregionbeingcenteredoff axis.
The ratio of the count rate in the 70-km detec-
tor to that in the 30-km detector (not shown)
showedthe samesystematicchanges
as the ratio
of the 210 km to 70 km. However, the average
ratio of 70 km to 30 km was 2.3, whereasthe
calculatedratio for an isotropic distribution is
4.0. To determine whether the detectors and
electronicswere functioningaccordingto the
km in diameter for the region over which the
electron pulsationsoccurred.This result, however, is not unambiguous.
For example,a possible interpretationof the high flux in the 30-km
detectorwouldbe an auroral form of comparable
dimension that
was centered over the 30-km
detectorduring this event. This could substantially changethe flux of the 30-km detector
without greatly affectingthe 210-or the 70-km
detectors.Other spatial distributionsthat would
reproducethe aboveresultsare alsoconceivable.
Another possibilityis that the regionof precipi-
1690
'
PARKS, CORONITI, McPHERRON, AND ANDERSON
I I goI øIWMTI I I i I i [
0400
.ooo1--
I
-I
7000
6000
detector and none in the 30-km
detector. In all of the electron pulsationsanalyzed to date, no such casehas even been observed.Our conclusionis that the spatial region
of the electron pulsationsis probably of the
order of 100-150
km in extent. We thus tend
to confirm an earlier result using two balloons
by Barcusei al. [1966] and Brown ei al. [1965]
that the pulsatingregionis of the order of 100
5000
4000
km.
Localtime1000-1500.
Themodulated
elec-
RAT•=CR(20<E<45
KeY)
4.5
3.5
we would expect to see an occasionalpulsation
in the 210-km
-
tron precipitation in the region of local time
from 1000 to 1500 is characterizedby longer
periods,20-40 seconds,and by a harder energy
spectrum,e-foldingenergyabout 30 kev. The
temporal structure is quasi-periodicand even
irregularat times,and the amplitudeof the pulsations is more variable
than in the 5- to 10-
secondcase.In this section,two eventswill be
discussed;the first is a very intenseprecipita4.0
DA•_
CR(210
Kin)
•'
'•-- CR(70 •)
tion event, and the secondis a more typical
event that occurredon iop of a low background.
The analogX-ray count rate recordfor the
first event that occurredon September6, 1966,
at 1000 (90 ø WMT) is presentedin Figure 6.
3.0
At the start of the event, the photon flux was
400 photons/cm' sec ster in the energy
2.5
- about
channel
20-45 kev. Note that the electronprei
I
i
i
i
i
I
i
I
i
5
20
35
50
cipitation was unstructured during this most
TIME
IN
SECONDS
intense part of the event. The photon flux then
steadily decreasedto about 200 photons/cm'
Fig. 5. Digitized linear X-ray count rate, the
energyratio, and spatial ratio for part of the event secster, and at this point, the precipitationbeshown in Figure 4. The average electron e-folding came modulated. This event suggeststhat durenergy was 14 key, and the peaks are systematically harder than the unstructured precipitation ing very intenseelectronprecipitation,the modby about 15%. The spatial ratio showsthat during ulation mechanismis ineffective. Referring to
an electron pulsation the 70-km detector seesmore
the power spectrumresult in Figure 2(c), it is
flux than the 210-km detector, indicating a preseenthat there were severalbroadpeaksat 23,
cipitation region of about 100 km in extent.
32, 42, and 49 seconds.
A sectionof digitizedlinear X-ray count rate,
tation couldbe in systematicmotion. A prelim- the energy ratio of the count rates in the 20- to
inary analysisof the widthsand risetimesof the 45-kev channel to that in the greater than 45pulsationsin the three detectorsindicatesthat kev channel,and the spatial ratio of the count
large-scalemotionsof the precipitation region rate in the 210-km detector to that in the 70sweepingacrossthe field of view of the three km detectorare presentedin Figure 7. The averdetectorsare not occurring.The temporal reso- age electrone-foldingenergyfor this event was
lution of the analysiswas sufficientto eliminate about 25 kev. The peaks of the electronpulsathe possibilityof velocitieslessthan a few hun- tions are systematicallyharder than the valleys
dred kilometersper second.
by 2-3 kev. The spatial ratio decreasedduring
A further argument against the conclusion the riss of the pulsation and increasedduring
that the spatial extent of the pulsating reg'on the decline. The ratio of the count rate in the
is 50 km in diameteris that, if it were this small, 70-kin detector to that in the 30-km detector
STUDIES OF MAGNETOSPHERIC SUBSTORM, 1
1691
wasabout 5.5 and was constantduringa pulsation. This somewhathigher ratio for the 70 to
to Figure 2(d), the power spectrum analysis
showedpeaks at 20, 25, and 37 seconds.These
30 km and the low ratio 2.6 of the 210 to 70 km
periodswere presentthroughoutthe event, thus
probably meansthat the pulsationregionwas illustrating the quasi-periodicnature of the
centered off axis with respect to the three de- longer-periodelectronpulsations.It shouldalso
tectors.As before,the spatial changesimply a be notedthat the amplitudeof the electronpulscalesize for the electronpulsationregion of sationswas more variable during the event than
about 100-150 kin.
was the amplitude of the 5- to 10-secondelecFigure 8 presentsthe analogX-ray countrate tron pulsations.
for part of an eventthat occurredon September
The resultsof the energyspectrumand spatial
8, 1966, at 1300 (90ø WMT). The entire event sizeanalysisshowedthat the longer-periodeleclastedmore than onehour, althoughthe electron tron pulsationshad variable characteristics.In
pulsationsshowedseveralbreaks in continuity what follows,we will presenttwo digitized secduringthis time.The pulsations
occurredon top tions of data that demonstrate this fact. The
of a low backgroundof about 20 photons/cm
• digitized linear X-ray count rate, the energy
secster in the 20- to 45-key channel.Referring ratio, and the spatial ratio are presented in
moo0
90 ø WMT
6 SEPT. 1966
20
<E
<45
Kev
! ...................
i'
:::
'.:::
:':":::
:.:-•
ß ..................
i '•A'•
' RATION
.................
":'.'.
Ld
6OOO
$000
8OO
ß
:'Z-i!"
E,>45 Ke,V
....
0
moo
m
Iz
$000
$000
800
2:00
300
I
20 <E <45KEY
'.
0
...............................
ß
......
$000
$000
800
.
,
'i.....
' ' "':!:'"'' ' ' ' ' ' ' " ' ' ' ____-_
_• '
v
E>45KeV
:::'" :':':' "i': '
.....................
'
...................
,
.
•.
!
•
•
ß
:
......................
...
400
500
i
600
I
I
$000
$000
800
$000
$000
800
E> 45 KeY
CALIBRATION
I
I
I
900
800
7O0
TIME
'
IN
I
I000
SECONDS
Fig. 6. Analog X-ray count rate for an 18- to 25-sec electron pulsation event. Note the
very intenseprecipitationlevel at the start of the event and that electronpulsations
began
Mter this high level had decreased.
1692
PARKS, CORONITI, McP'HERRON, AND ANDERSON
"'
1000
90•
6 SEPT 1966
n,' 9000
Figure 10 presentsthe digitizedlinear X-ray
I
7000
X
other hand, showsno systematicchangeduring
an electronpulsation.For this part of the event,
6000
i
.
1,40
0
1.35 . ]•
D^'r'.'•CR(20<E<
45KeY)
<:• 1.30
1.2:0
I,I
Z
1.15
b.I
1.10
2.8
2.7
then, the scalesize of the regionof the electron
pulsationswas greaterthan 210 km in extent.
I)ISCUSSIOAI
1.25
•c3
ratio of a sectionof data from a later part of
the event.The energyspectrumis again very
hard with an average e-folding energy of 30
kev. In this case, however, the peaks of the
electron pulsations are systematically harder
than the valleyswith a changein e-foldingenergy of about 5 kev. The spatial ratio, on the
• 8000
•
count rate, the energy ratio, and the spatial
--
__,CR(210
Kin)
2.6
Beforewe discussthe significance
of the data
presented,the salientfeaturesof the X-ray pulsations are briefly summarized.In Figure 11
(left curve), we presenta compositeof the 5to 10-secelectronpulsationsthat typically occur between 0200 and 1000 local time. The aver-
age electrone-foldingenergy is about 15 kev.
The energyspectrumalwayshardensat the peak
:>.4
of the pulsationswith a typical changein the
e-foldingenergy of 2 key. The temporal structure of theseelectronpulsationscan showa very
2.2
0
40
80
120
regular periodicity but can also occur in an
TIME
IN SECONDS
irregular manner. Our generalimpressionfrom
analyzingmany electronpulsationeventsis that
Fig. 7. Digitized linear X-ray countrate, the the pulsationsare more periodicwhen the backenergy ratio, and spatial ratio for part of the groundof unstructuredelectronprecipitationis
event shownin Figure 6. The averageelectron
e-foldingenergywasabout25 key, and the peaks low. The amplitude of the electron pulsations
for both casesis roughly constant for a time
of the electron pulsationsare harder than the unstructuredprecipitationby 2-3 key. The spatial interval long comparedwith the period of the
ratio showsthat, duringan electronpulsation,the pulsations.The 5- to 10-secelectronpulsations
2.5
2ø5
70-kin detector sees more flux than the 210-km
have a characteristicspatial dimensionof about
100-150 kin, whereasthe unstructured electron
precipitationbackground
appearsto be isotropic
over 210 km. This confirmsthe previousresults
Figure 9 for an early sectionof the event.The of Barcus et al. [1966].
averagee-foldingenergywas28 key. There isn't,
The longer-periodpulsationsof 20-35 sectyphowever,a systematicchangeof the energy ically occur between 1000 and 1500 local time
spectrumduringan electronpulsation,and both and exhibit complexenergy and spatial charspectralhardeningand softeningat the peaks acteristics.These electronpulsationsare quasioccur. The spatial ratio of the count rate in the periodicin nature with a broad rangeof periods
210-km detector to the 70-km detector decreases
occurringthroughoutan event. The amplitude
during the rise of the electronpulsationand in- is alsohighly variable. We alsonote that in the
creasesduring the decline.The 30-km detector very intenseevent of September6, 1966, the
was not used,owing to its poor countingsta- modulationof the electronpulsationsdepended
.detector,
indicating
that the precipitation
region
zs about 100 km in extent.
tistics.For thiscase,ihen,thedimension
of the
precipitatingregionwas about 100 kin.
on the total precipitatedelectronflux, possibly
indicatinga saturationeffect.The averageelec-
STUDIES OF MAGNETOSPHERIC SUBSTORM, I
1693
tron energyspectrumof thesepulsationsis very
hard with a typical e-foldingenergyof 30 key.
12), showingthe results of several hypothetical
balloonexperimentsat various local times. The
Some of the electronpulsationsshow energy varioustemporal structuresof energeticelectron
spectralhardeningat the peaks; however,oc- precipitation at various local-time zonesare incasionallyno systematicspectral changesare terpreted to represent the various instabilities
observed.The resultsof the spatial analysisin- operating at their respective local times during
dicatethat the extentof theseelectronpulsating the worldwide disturbance that affects the enregions is often of the order of 100 km and
tire magnetosphere.Thus, the local-time region
sometimesgreater than 210 kin. These features between 2200 and 0200 shows the characteristic
havebeensummarized
in the composite
diagram auroral breakup impulsive electron precipitashownin Figure 11(b) (right curve).
tion of several-minuteduration and the rapid
That the varioustemporalfeaturesof auroral- 50-msec bursts. From 0200 to 1000 local time
zone energeticelectronprecipitationoccur pri- the balloon-borne detector would measure 5- to
marily during times when an auroral substorm 10-sec electron pulsations.In the overlapping
is in progresswill be demonstrated
in paper 3. regionbetween0600 and 1400 local time, microFurthermore,it will be shownin paper 2 that bursts
withtheircharacteristic
temporal'features
geomagnetic
micropulsations
and energeticelec- would be detected. The 20- to 35-sec electron
tron precipitationare temporallyand causally pulsationsof the type discussedin this paper
related. To further illustrate the different forms
would
be detected
in the local time
zone of
of energeticelectronprecipitationthat occurin 1000-1500. In the dusk region, to the best of
the auroral zone during an auroral substorm, our knowledge,the electronprecipitation would
we have constructed
a schematicfigure (Figure be featureless over several minutes.
1500
6000
3000
LLI 800
I-<1:
n"'
400
200
_
--
9O"
WMT
8 SEPT. 1966
.
---
.........................................................................................................
--
........
:................
:.:
I-- 3000
6000 - '.
ß
.
ß
ß
'
....:..........20<E<45KeV
ß
. ..............
--
Z• 8oo
----::.•
........
:".:;"
':.....
:......
i.......
:':
.........
!..............
'"'.::::":':::
.....
::::::
........
::'"'
40O
o
(,.)
0 ...........
200 ' '
400 ....... E> 45KeV
<:•6000_
_
ooo ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
::::::::::::::::::::::::::
800
I• 400
x
200
6000
3000 .............
ß
.
::...................... ':....:'":................. :.......... ...::'": ....... ß......
•
800
--
....
e '"'
8oo
TIME
E > 45 KeY
,ooo
IN
,200
SECONDS
Fig. 8. The analogX-ray countrate for part of a 20- to 35-secelectronpulsationevent.The
entire event lastedover one hour. Note the quasi-continuous
nature of the pulsations.
-
1694
PARKS, CORONITI, McPHERRON, AND ANDERSON
,• i[ ' 1500
90ø
WMT
JI • ,9oo
8SEPT.
,966
I.l.I
'
'
'
'
'
'
'
'
]•1
I
I
I
I I I I I -•[
n,80o
r
8
SEPT,
1966
-I
•
1800
•z 1600
1300
(•) 1400
•.• 1700
0 1200
• 1600
• I000
90 ø WMT
J
• 1500
n-'
• 800__,,
: ', : ', ', ", ', ',:, •:•1400
1.3•
........
I X• 1200
,300
•
......
CR(20<E<
45 KeV)
0
/.•
,•
1.2
0
•
I.I
n- 1.2
I
i
I
I
I I I
I
I
I
1.3
I.I
I i I I I I • -, i[ [ • • 0.9I I I
[
I
o
0 2.5
•,• 2.4
•
•
•
[ •
I [ [ I, ,
13::
2.2
0
50
[00
•50
200
TI ME IN SECONDS
Fig. 9. Digitized linear X-ray count rate,
energy ratio, and spatial ratio for part of the
electron pulsation event shown in Figure $. The
averageelectrone-foldingenergywas28 key, but
there were no systematicenergyspectrumchanges
during this part of the event. The spatial ratio
indicatesthat the precipitatingregionfor the pulsations was about 100 km in extent.
•.e
=
,oo
TIME
IN
SECONDS
F•g.zO.:D•g•U•,ed
]•,•e•r
X-r• cou,n
r•e e,•-
ergy ratio and spatial ratio for another section of
the eventshownin Figure $. The averageelectron
e-foldingenergywas30 key, and the peaksof the
electronpulsationsare systematicallyharderthan
the unstructuredprecipitationby about5 key. The
spatialratio showsno systematicchanges
during
this part of the event,implyingthat regionover
which the pulsationswere occurringwas greater
than 210 km.
STUDIES OF MAGNETOSPHERIC SUBSTORM, 1
1695
----30SEC•-1
r..-Io
SEC•
Eo=30 KeV
•_••//'Eo=$O
KeV
Eo=17KeV Eo=35 KeV
l > 210 Km
L > 210 Km
Lß210
Km
•
L,-,100 Km
L~ 100 K m
Fig. 11. A compositesummaryof the temporal, energy, and spatial characteristics
of the
5- to 10-secand 20- to 35-secperiod electronpulsationstypically occurringbetween0200 and
1000and 1000 and 1500local times, respectively.
SCHEMATIC
AURORAL
ENERGETIC
ELECTRON
PRECIPITATION
2200-
0200
ZONE
PATTERN
ELECTRON e - FOLDING ENERGY m I0-15 KeV
SPATIAL DIMENSIONS m STRUCTURED SIMILAR
LOCAL TIME
TO AURORAS.
,•j.•l•, 50Mfilisecond
Bursts
,,,/<_
MiNUTEs•pUl
....Precipitation
0200-
I000
LOCALTIME
K-- I o SEC.---N
5-10 Second
ELECTRON
e-FOLDING
SPATIAL
DIMENSION
ENERGY
SPECTRUM
m 100-150
ELECTRON
SPATIAL
LOCAL TIME
K-•25 SECONDS
•
20-30
Second
Pulsations
Krn
HARDENS
0600-'4J?
0LsOEcC.
AITlMEjM,crobursts
1000-1500
ENERGY m 15 KeV
Pulsations
e-FOLDING
AT PEAKS
ENERGY m 25 KeV
DIMENSION OF INDIVIDUAL
ELECTRON e-FOLDING
MICROBURSTS m lOOKm
ENERGY m :30 KeV
SPATIAL DIMENSION m I00 Km , SOMETIMES
GREATER
1500-2200
LOCAL TIME
THAN
210 Krn
FeaturelessOver
Minutes
ELECTRONe-FOLDING ENERGY
SPATIAL
DIMENSION
m ?
TIME
Fig. 12. A summary of the local-time characteristicsof the various forms of energetic
electronprecipitation.Each form occursat its respectivelocal time during somephaseof an
auroral
substorm.
Acknowledgments. We would like to thank
Professor R. R. Brown for many helpful discus-
This work was partly supportedby the National
Aeronautics and Space Administration under
sions. We also wish to thank the various members
grant NsG-243-62. The balloon instruments and
of the balloon support staff for their genero,us balloon flight support were obtained under grant
assistance.
GP-5583
from
the National
Science Foundation.
PARKS, CORONITI,
1696
McPHERRON,
Administrative and logistic sapport was provided
by the Office of Naval Research Skyhook Program.
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and
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Rosenberg, T. J., and J. Bjordal, Balloon obserCoroniti, F. V., R. L. McPherron, and G. K.
vation of pulsating X-rays and auroras (abParks, Studies of the auroral substorm,3, Constract), Trans. Am. Geophys. Union, 48, 72,
cept of the magnetosphericsubstorm and its
1967.
relation to electron precipitation and micropul(Received August 4, 1967,
sations,J. Geophys. Res., 73(5), 1968.
revised November 29, 19,67.)
Ehmert, A., G. Kremser, G. Pfotzer, K. It. Sarger,