Substorm-Associated Changes in the Particle Precipitation Pattern

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1NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA. ... of uniform electron precipitation which produces diffuse aurora (LUI and ANGER, 1973).
J. Geomag. Geoelectr., 44,1239-1249,1992

Substorm-Associated

Changes

in the Particle Precipitation

Pattern

Rumi NAKAMURA1 and TatsundoYAMAMOTO2 1Natl . Inst.of PolarRes.,Kaga,Itabashi-ku,Tokyo173,Japan 1NASA/Goddard SpaceFlightCenter, Greenbelt, MD20771,USA. 2Insttof SpaceandAstr. Sci.,Sagamihara,Kanagawa229,Japan (Received January9,1992;AcceptedMay8, 1992) Relationshipsbetween averagecharacteristicsof particle precipitationand substorm activityare investigatedinthe low latituderegion as wellas inthe highlatituderegionwithin the dusk-to-midnightsector. "Low latitude" is defined here as the region below the equatorwardboundary of the most equatorward electron BPS, while "high latitude" is definedas the region higher than that boundary.DMSPparticle data are dividedaccording to the phase of the substormand accordingto the longitudewith respectto the onset region byreferringto ground-basedmagnetogramsand DMSPauroralimages.At highlatitude,not only electronprecipitationbut also ion precipitationsignificantlyintensifiesin accordance with substorm activity. The energizationof these particlespresumably takes place in the near-tailregionwhere an unstabletail currentdevelopsand locallydivergesassociatedwith substorm onset. At low latitude, temporal and spatial changes of energetic precipitation wouldbe causedby injected,driftingparticlesfrom the substormonsetregions,particularly in the case of electrons. 1. Introduction A number of studies have investigated the characteristics of electron precipitation in the night side aurora! oval (e.g. WINNINGHAM et al., 1975; LUI and ANGER,1973; FELDSTEIN and GALPERIN,1985and references therein). Most of these studies identified two major regions in the electron aurora! oval: BPS and CPS. BPS consists of structured electron precipitation which produces discrete aurora, while CPS is located at the lower latitude side of the BPS and consists of uniform electron precipitation which produces diffuse aurora (LUIand ANGER,1973). This simple BPS/CPS pattern is not always present. A multiple BPS/CPS pattern was, for example, observed during a particular magnetic storm (SANDAHL et al., 1990). It is not yet fully agreed upon as to the relationship between the CPS/BPS precipitation pattern and different regions in the magnetosphere (FELDSTEIN and GALPERIN,1985).One of the key points here is the response of the precipitation pattern to substorms. WINNINGHAM (1975) suggested that the BPS exhibits dynamical change in average energy and intensity, as well as in location, while the CPS is a relatively stable region that experiences uniform increase and decrease in intensity and average energy. LYONSand EvANS(1984) suggested that discrete aurora, and therefore BPS, might be mapped along the outer boundary of the plasma sheet into the tail current sheet. The dynamical change in BPS characteristics is in fact consistent with the discrete aurora! activation during substorms (e.g. AKASOFU,1964). However, particle signatures at the boundary of plasma sheet were observed to be less affected by substorm activity than at the central plasma sheet (BAUMJOHANN,1991). The CPS, on the other hand, 1239

1240

R. NAKAMURA and T. YAMAMOTO

was

suggested

change

in

to connect the

however,

plasma

is not

is consistent

all

from

parison reported

In this electron

that

region

into

BPS. source

and

low

spatial

and

as the

equatorward

is defined

latitude

characteristics

of

arc

changes

both

of

high

the

precipitation 1988).

From

expansion

ion

com-

These

pattern.

distribution

divided

the

boundary

of the

onset.

precipitation

We The

their

which

magnetosphere.

in latitudinal

boundary and

1988),

NAKAMURA et al. (1992) expanding aurora, and the

the

substorms.

latitude.

regions

the

et al.,

before in

localized et al.,

auroral-type

observations, region of

also

LUI

a relationship

in

discrete

aurora!

phases

as

(LYONS

particle at the

transient

of both

precipitation

between

most

the

equatorward

relationships

to the

two

electron

magnetospheric

region.

2.

Data

Analysis

We

used

particle

ground-based

observatories.

cipitating

electrons

Auroral

imagery

magnetic

field

1985

to

The

64

the

so that

the

local

divided

according

instead

of the AE

regardless

of

Figure coordinates satellite

pass

of

11

are

of the

is that

size

of the the

and

satellite

1(b)

north-south

to

because

11,

shows

satellites keV

board and

Campaign

(HARDY

the

and

measured

pre-

et al.,

satellites.

Scandinavia

region.

northern

the

1984).

The from

activity

the

respect

first

hemisphere

GADC

December

during

The

advantage at the

data the

analysis

GADC

in the

of using

substorms

of the

the

particle

satellite

to

part

observatories

determined. The

with For

ground-based be

of the

relationships

onset

substorm.

of the

sec

the

line.

(•`19•K).

zone

time

The

local

In this

zone

period

auroral

sets

zone

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magnetogram

location

can

data

be determined

of the sector.

line,

whereas

we

concentrated

study,

aligned

in corrected

lines).

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represents

distribution

north-south

stations

(solid

number

premidnight

north-south

are

auroral

auroral

local

the the

they

on

Alaska

satellites

1988).

could

in the

of 380

along

radiometer

30

F7

Dynamics

DMSP and

and

substorm.

within

almost

eV

F6

Aurora!

the

Temporal

near

the phase

tracks

located

30

in the

distribution

a length

to the 06

phase

shows

are

region

to the

between

to the

orbits

activity

1(a)

aligned

slanted from

the

side

the

(1)

DMSP

Global

board

et al.,

respect

geomagnetic

Figure

them

F7 night

index

with

(hour).

All

with

the

Canada,

parts:

by

on

scanning

(OGuTI

relationships

traversed

the

in Northern

of two

DMSP

at

energies

by

taken 1986

obtained

obtained

with

provided

31,

data

experiments

consists

satellite

image

data

ions

were

January

selected

when

field

was data

Spatial

auroral

Particle and

analysis

(2)

and

magnetic

(GADC)

time

temporal at different

well

with

is expected

The 1984;

to identify

observation

at the

1977).

BAKER,

as

correlate

dependence

regions:

discuss

to

et al., (e.g.

altitude

satellite

precipitation

examine

regions

We

at low

found

(LUI

It is important

aurora and DMSP ion precipitation

precipitation

two

sheet substorm

CPS.

S3-3

ion

we

ion

precipitation

we

been

substorm

study,

and

a stable

polar-orbiting

energetic

suggest

plasma during

observations

has

the

of such

results

and

these

between ground-based enhanced energetic

absence

15,

with

precipitation

obtained

central

particles

consistent

with

Ion

to the sheet

and

same

passes later

two on

geomagnetic corresponds

its approximate

The the

line

shown

four

passes passes

located

and

such

as

within

in Fig.

passes 03

the

to

a

universal

from 05 the

are

1(a). 06

later

coverage

to

rather four, of

the

in

the

observatories. For northern within

the

second

part

hemisphere the

DMSP

of

from image

data.

the

analysis

January Particle

to

we

selected

March data

sets

1986 were

32 when divided

DMSP an

F6

and

expansion

according

F7

orbit

aurora to the

passes was

longitude

detected relative

Substorm-Associated

Fig.

1.

(a)

(•`19•K).

The

number

DMSP

passes

the

on January

expansion We

structured

electron that

of

most

the

intended

to

the

keV

electrons

slightly

from

between

two

such

the

image

and

to a satellite

of each

pass.

(b)

1241

satellite

tracks

pass

with

a length

Local

time

distribution

in the

of 380

sec

of the

data.

"low

latitude". and

the

observed

the

region

effect

where

CPS/BPS by

the

SANDAHL

with

such

energy

a field an

effect

al.

"high

at

least

to

discrete

aligned is

as

potential negligible.

drop

BPS

the

boundary arc.

is

We

dominant

classification

we within

the

Latitude

auroral

This

example, being

latitude" all

1keV.

corresponds

For (1990)

at including

of

equatorward

classification. et

and

latitude

therefore

most of

latitude" zone

boundary

to

the

"low

auroral

peaks

The

hence

where the

as

spectral

BPS

of

at

latitude" with

from

as

time

Pattern

coordinates

corresponds

universal

to

"high

region

that

geomagnetic

line

characteristics

precipitation

separate

BPS

approximate

referring

identify

is called

corrected

Each

Precipitation

(a).

by

equatorward

in the Particle

in

lines).

precipitation

differs

region.

a few

in

stations

(solid the

shown

we

region

zone

1986

represents

aurora

Here

below

auroral 13,

examined

separately.

for

of the

zone

same

to

Distribution

auroral

Changes

included the

high

CPS latitude

1242

R. NAKAMURA and

3.

Temporal

We The

data

sector

Change

first

present

and

were

taken

from

(pass

auroral

06 pass

zone

from

top

to

bottom.

071

SUT

Barrow

(BRW)

activity.

The

Fig. 1)

a quiet

next and

took Pass

spectra during

the

boundary

of

of

and

high

latitude

latitude electron

for

the

flux

ions,

energetic

left

between

for

are

second

the

energy

and

the line,

with

structured are

can

precipitation

see

that

the

which

indicated

to the

YLK

flux

and

JN for

Pass

10

the

line

is indicated

by

commenced west

of

the

at

YLK

College

(COL)

occurred

during

region

number in each to

more

our

and

conventional

of

right. the

high

at lower

ions

indicates

definition.

BPS

to the intense

flux) panel

is at the

precipitation

be

one The

farther

and

five

lines.

onset

YLK

corresponds to

place the

according

with

boundary tends

took

latitude

electron

broken

orbit

before

dotted

consistent

by

for

western

at BRW.

region low

most

(SHM)

activity.

3. The

premidnight north)

the

satellite

place

activity

the

precipitation.

in the

Shamattawa

observatories

latitude

while

precipitation one

are

took

two

latitude

to

disturbance 06

in Fig.

low

station

and

of the

the

shown

of the

region

the

recovery

(JE

passes

DMSP nearest

Pass

during

eastern

Another

low

of DMSP (geomagnetic

(YLK)

0900UT.

the

place

total four

is at the

high

As

intense

the

diffuse

respectively. an

and

of

observatory

located

during

and

crossings

most

crossings

the

by

was

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to •`1230UT.

11 took

region the

with

08,

from

at Yellowknife

recovered

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Energy

region

from

latitude

zone

2 showsXMcomponents

zone

disturbance

electrons

examples

auroral

of high

auroral

plotted

from •`1030UT

period.

latitude

are

obtained

and

high

1). Figure

The

and

examples

11 in Fig,

A localized

around

four consecutive

They

trace

an arrow.

discuss

Pattern

four

stations.

magnetogram

(see

in Precipitation

T. YAMAMOTO

The

For

low

these

latitude and

CPS,

latitude

end

latitudes.

Fig. 2. XM components (geomagnetic north) of five auroral-zone magnetograms for the interval when the DMSP passes took place (indicate by broken lines). The magnetogram traces are plotted from the most eastern station to the most western one from top to bottom. PDB, Poste de la Baleine; SHM, Shamattawa; YLK, Yellowknife; COL, College; BRW, Barrow.

Substorm-Associated

Changes in the Particle Precipitation

Pattern

124:

Fig.3. Spectrumsandtotal flux (JEfor the energyflux and JNfor thenumberflux) of ions andelectrons. Thedotted line in each panel indicatesthe boundaryof high latitudeand low latituderegion(see text for its definition).

The first and the third panels are obtained during relatively quiet periods while the second and the fourth panels correspond to disturbed periods (see Fig. 2 for geomagnetic activity) . The energy flux of electrons both at high latitude and at low latitude significantly intensifies associated with substorms. The response of the ion precipitation to the substorm is somewhat different between the two latitude regions. The low latitude ions are quite stable and exhibit little change in energy flux level associated with substorms. On the other hand, the high latitude ions significantly enhance associated with substorm activity and the latitude where the maximum flux is observed correlates with that for the electrons. We further examined 64 DMSP events when the satellite passed near one of the GADC auroral zone stations in order to obtain more general relationships between the precipitation

1244

R. NAKAMURA andT. YAMAMOTO

pattern and the substorm phase. We defined six different categories of the geomagnetic activity levels as illustrated in Fig. 4. They are quiet time (q), less than one hour before the bay onset (b), bay increasing (i), maximum or prolonged activity (m), bay recovery (r), and less than one

Fig. 4. Illustration of a typical magnetogram trace (XM component) which represents the six categories of the geomagnetic activity level: quiet time (q), less than one hour before the bay onset (b), bay increasing (i), maximum or prolonged activity (m), bay recovery (r), and less than one hour after recovery (a). The number of the events for each category is given below.

Fig. 5. Frequency of occurrence when the energy flux exceeds certain values plotted as a function of the activity level for the electron precipitation at high latitude (upper left) and at low latitude (lower left) and for the ion precipitation at high latitude (upper right) and at low latitude (lower right).

Substorm-Associated

Changes

in the Particle

Precipitation

Pattern

1245

hour after recovery (a). The number of the events for each category is also given in the figure. We compared the intensity of the ion/electron precipitation at the high/low latitude among these defined activity levels. Figure 5 shows the frequency of occurrence when the energy flux exceeds certain values plotted as a function of the activity level defined in the previous figure. The two upper panels are for the high latitude precipitation, while the two bottom panels are for the low latitude precipitation. The electron precipitation in the high latitude region (upper left panel) is always more energetic than that in the low latitude region (lower left panel). In the high latitude region the possibility to get more energetic electron precipitation increases and decreases in accordance with substorm development and recovery. These electron signatures at the high latitude are consistent with the discrete auroral activity observed from the ground in the premidnight sector. That is, stable arcs exist even during very quiet time, while discrete aurora activate during substorms (e.g. AKASOFU,1964). The low latitude electrons, on the other hand, also energize in association with substorms. The intensification persists even after the recovery. The development and the decay of the low latitude electrons occurs with some time delay relative to substorm phases. In contrast to the electrons, the ions are always more energetic in the low latitude region (lower right panel) than in the high latitude region (upper right panel) for all activity levels. The low latitude ion precipitation becomes somewhat energetic prior to the onset and reduces already at the maximum of geomagnetic activity. Hence, this precipitation seems to be less affected with substorm activity. This is consistent with proton auroral signatures at the equatorward edge of the duskside auroral oval which were observed during various geomagnetic activity conditions (ONOet al., 1987). On the other hand, the high latitude ion precipitation significantly intensifies in accordance with the substorm activity as well as with the change in high latitude electron precipitation. 4. Precipitation Pattern Relative to the Expansion Aurora Associated with the substorm expansion onset, the aurora expand not only poleward but also westward (and somewhat eastward), and form a bulge (e.g. AKAS0FU,1964).A bulge (or an expansion aurora) generally consists of several different types of aurora as shown in Fig. 6(a), which is an image taken from DMSP 6. At the western edge of an expansion aurora there is a bright surge at the north of a discrete auroral arc, while the eastern part consists of more thin auroral structures (the eastward propagating aurora) and diffuse aurora at the low latitude side. Bright N-S-aligned aurora can be seen in between. The auroral break up develops from the region between the surge and the N-S aurora. Clearly, the precipitation characteristics are expected to change in the east-west direction relative to the onset region. In order to examine this spatial relationship of the precipitation characteristics and the expansion aurora, we selected DMSP events when the relative longitude of the pass could be determined from the auroral image. The data sets were divided into four longitude sectors as illustrated in Fig. 6(b). They are west of the surge head (denoted as WW), surge region (W), central part with N-S aurora (C), and eastern part of expansion aurora (E). Figure 7 shows the frequency of occurrence when the count level of 3keV- and 30keVdetectors for electrons/ions exceeds 1 count plotted for the four longitude sectors, which were defined in the previous figure. Here, the 3keV particles represent precipitating particles that have an energy comparable to the field aligned potential, whereas 30keV particles represent

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R. NAKAMURA and T. YAMAMOTO

Fig.6. (a)Anexampleofan expansion auroratakenfromDMSP6. (b)Illustrationoftheexpansionauroraandour definitionofthefourlongitudesectorsrelativetotheaurora;thesurgehead(denotedasWW),surgeregion(W), centralpartwithN-Saurora(C),andeasternpart of expansionaurora(E).

particles whose energization would take place in the magnetospheric source region. At high latitude, the 3 keV electron precipitation is detectable throughout the longitude (see the upper left panel). This is expected from the discrete auroral activity. The 30keV electron precipitation is enhanced at the surge and at the central region of the bulge. This is consistent with previous observations of the electron precipitation near the surge (MENDet al., 1978). Most energetic electron precipitation at the high latitude, therefore, is located where the auroral breakup starts. The high latitude ion precipitation (upper right panel) significantly reduces to the west of the surge head for both 3keV and30 keV. From Figs. 5 and 7 it is suggested that the ion precipitation at the high latitude is also spatially as well as temporarily confined to the substorm activity region. At low latitude, on the other hand, the longitude of the breakup region is not where the most energetic precipitation occurs. The low latitude electron precipitation (lower left panel) tends to reduce west of the surge head and to enhance as one goes to the east of the expansion aurora. This tendency is particularly clear in the 30keV precipitation. As for

Substorm-Associated

Changes

in the Particle

Precipitation

Pattern

1247

Fig: 7. Frequency of occurrence when the count level of 3 keV- and 30 keV-detectors exceeds 1count in the electron precipitation at high latitude (upper left) and at low latitude (lower left) and in the ion precipitation at high latitude (upper right) and at low latitude (lower right). Differential flux level corresponding to 1count for each energy channel is also given in the figure.

the ions, the low latitude precipitation (lower right panel) can be seen in the entire region independent of the location relative to the expansion aurora. 5. Discussion and Conclusion We have shown that the enhancement of electron precipitation in the high latitude region during substorms is accompanied by enhancement of ion precipitation both in time (Figs. 3 and 5) and in space (Fig. 7). The region where maximum energy flux of ion precipitation is observed coincides with the region of the strongest electron precipitation. Such an energetic ion precipitation region was found to coincide with the poleward expanding auroral arc from simultaneous all-sky TV and satellite observations (NAKAMURA et al., 1992). That is, these ions are observed at a latitude region where a field aligned potential above the satellite causes acceleration of electrons but deceleration of ions. The acceleration of such ions therefore should occur either at the southern conjugate region or in the magnetotail region. We interpret the source region to be the magnetotail because the observed ions have about a factor of ten more energy than the usual field aligned potential drop of a few kV. LYONSet al. (1988) reported on a general association between the discrete aurora and

1248

R. NAKAMURA andT. YAMAMOTO

isotropic ion precipitation and suggested that these ions thread the tail current sheet at distances where ion motion violates the guiding center approximation. From the observed strong temporal relationships between substorm activity and energetic precipitation for both ions and electrons, we suggest that the energization of these particles is related to the substormassociated processes. In the late growth phase and early expansion phase an extremely thin current sheet was observed to develop close to the earth tail region where ions are scattered to become isotropic (BAKERand MCPHERRON,1990). We interpret the energization of the observed ions and electrons to take place in such a substorm-associated near-earth current sheet. Such a thin current sheet is unstable and leads to its local disruption (MITCHELLet al., 1990). A field aligned current system is produced and the electron precipitation, causing auroral breakup, is also expected to take place in that region. The energetic particle precipitation, which we observed mostly at the breakup region is therefore consistent with this interpretation. After the substorm expansion onset when the field is changing from a tail-like to a Bipolar configuration both ions and electrons are further accelerated due to the inductive electric field (HEIKKILAand PELLINEN,1977; WILLIAMSet al., 1990), causing probably further enhancement in energetic particle precipitation. At low latitude, the electron precipitation dynamically changes associated with substorm activity. The CPS therefore seems not to be a stable region which merely shifts its latitude as was suggested by WINNINGHAM et al. (1975). Important characteristics are that the enhanced precipitation lasts until after the recovery (Fig. 5) and the region of the electron precipitation, particularly that of energetic particles, is predominantly to the east of the expansion aurora (Fig. 7). We interpret these precipitation characteristics as follows: the observed electrons at the low latitude consists of electrons which are injected at the substorm onset into the inner magnetosphere. This region would expand due to the earthward and eastward drift of the injected electrons. Furthermore, during multiple expansion events, successive expansion which might predominantly tend to occur west of the previous ones would also cause the expansion of this precipitation region. This eastward extension of the energetic electrons from westward moving source regions can explain the observed temporal and spatial relationship of the low latitude electron precipitation to the substorm activity. The characteristics of the low latitude ion precipitation can only be partly explained by the injected and westward drifting ions from a westward moving source region as discussed for the electron precipitation. The ions seem to have more permanent structure where they tend to be more energetic with decreasing latitude, independent of the substorm phase (Fig. 3). This is consistent with an adiabatic compression of the plasma sheet (HARDYet al., 1989). The low latitude ion precipitation would also be controlled by such a mechanism which has a longer time scale than the substorm expansion. These different behaviors of electron and ion become crucial when we infer from the precipitation pattern its conjugate area of the onset region in the magnetosphere. Wegratefullyacknowledgeuseful discussionswith T. Hirasawaand D. N. Baker.We are greatly indebtedto those members of the GeophysicsResearch Laboratory,University of Tokyo who were involvedinconstructingthe instrumentsand collectingthemagneticdata.The DMSPdatawas provided by WDC-Afor Solar TerrestrialPhysics through WDC-C2for Aurora (National Institutefor Polar Research).We would like to thank H. Nakajimafor his help in constructing computerprogramsfor DMSPdata plot. The work of R. Nakamurawas supportedby the fellowshipsof the JapanSocietyfor the Promotionof Sciencefor JapaneseJunior Scientists.

Substorm-Associated Changes in the Particle Precipitation Pattern

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