The dynamics of a steep onset in the conjugate auroral riometer absorption

THE DYNAMICS OF A STEEP ONSET IN THE CONJUGATE AURORAL RIOMETER ABSORPTION L. A. HA.lKOWC% Physics

Ikpartmcnl.

Univcraily

of Queensland.

St Lucia, Au\lralia

3067

sudden increases m aurora1 riometcr absorption (SAI events). for two conjugate stations : Macquarie Island (L-value = 5.2) and College (L-value = 5.5). located in the southern and northern aurordl zones. respectively. Additional riometcr data. from two more aurora1 stations (Syowa Base, L-value = 6.1 and Kiruna, L-value = 5.4). were used to deduce the drift velocities of the absorption evcnta. It is evident that the SAI events tend to occur closely in time at the conjugate stations before the local magnetic midnight. They propagate mainly westward with average velocities 2.7 4.5 km s ‘. The longitudinal position of the conjugate point to Macquarie Island was verified using absorption drift considerations. There arc appreciable hemispheric dificrcnces in the aurora1 absorption levels at College and Macquarie Island, consistent with the previous results on the relative aurora1 absorption and luminosity magnitudes at the conjugate locations. The cases of a complctc conjugacy breakdown, detected during the southern winter solstice period, appear to be related to a seasonal change In the latitude (l,-value) of the cotl.jugate point location Abstract-One

hundred

and sixty-one

recorded over three years. were compared

I. INTRODUCTION

Conjugate particle precipitation studies, using radio and optical methods, have been given considerable attention during the last three decades. This type of study provides a description of the substorm onset characteristics on a global scale. The importance of conjugate studies in understanding the magnctospheric source mechanism as well as the magnetic field lint geometry has been fully appreciated by a recent establishment of the Polar Anglo-American Conjugate Experiment, based on coherent radars fol investigation of the amoral ionosphere. There appears to be a shortage of information on the long term effect in the conjugate phenomena, particularly as applied to seasonal differences when a non-symmetrical position of the Earth’s magnetotail can affect the degree ofconjugacy. Also, little is known about the simultaneous extent and drift velocities of the conjugate particle precipitation regions. An instrument which is particularly suitable to study the latter phenomena is the riomcter; it is well known that absorption of extra-terrestrial cosmic noise, as measured by the riomcter, can be used to deduce the energy and intensity of aurora1 particle precipitation. It is generally accepted that the signature of a local substorm onset, according to the riometer obscrvations, takes the form of an abrupt increase in radio absorption, by several decibels within a few minutes, followed by a slow absorption decay. These are socalled sudden absorption increase (SAI) events and

should not be confused with the events which show both rapid increase and decrease in absorption, resulting in a spike-like feature in the riomcter data (Collis et ul., 1986). The SAI events can bc associated either with a local transit of the westward travclling surge (WTS) or with an amoral break-up within the fieldof-view of the riometer. The SAI cvcnts appear to have a close association with the aurora1 intensity fluctuations. Johansen (1965) analyscd a number of SAI events over 24 selected nights and compared them with the corresponding green line (557.7 nm) intensity fluctuation in the aurora1 spectrum. Hc found that the light intensity was proportional to the cosmic noise absorption. Thus, the results obtained from a study of the drift velocity of rapid absorption onsets can then be used to deduce the drift velocity of the visual aurora1 forms. Hajkowicr (1970) first attempted measurements of conjugate absorption drift velocity, using two stations in Alaska (Kotzebue and Collcgc) and a conjugate station in the southern aurora1 zone (Macquaric Island). He found that absorption events in both auroral zones tended to drift westward, before the local midnight, with a velocity of 2.2* 0.4 km s ‘_ He also deduced from the absorption drift study an average, longitudinal position of the conjugate point of Macquarie Island as being 170 + 60 km west of Kotzebuc. The present results, based on absorption data analysis from four aurora1 stations and over a period of three years, largely confirm his earlier results. Due to considerable uncertainties of this type of measurement

L. A. HAJKOWW

12x TAHLI. I. GCOMAGMTIC

COORDINATES ANII OTHER PAKAMETCKS ob THE AUKORAL Iii

THIS

Gcom. coord. (de&)

Invar. lat. (deg.)

L-value

64.7 N, 257.0 E 61.0 N. 243.6 E 65.1 N. 115.9 E

64.6 N 64.1 N 65.5 N

5.45 5.22 5.17

64.7 S, 157.0 E

64.6 S

5.45

61.0 S, 243.6 E 69.X s, 7X.2 E

64.1 s 66.1 s

5.32 6.08

and noting 0lat both sets of data came from diffcrcnt parts of the solar cycle. the agrccmcnl between lhc two analyses (present and past) of conjugate absorption is very good. It follows that the results hcrcwith prcsentcd appear lo bc characteristic of the conjugate am-oral absorption, irrespective of the solar oyclc.

One hundred and sixty-one sudden absorption increases (SAI events). signiticd by a rapid onscl and slow decay ofionosphcric absorption (as classified by Ansari. 1964). have been compared for two aurora1 conjugate stations from January 1982 to Dccembcr1984. In addition. a number ol’ the corresponding SAI events were used for comparison for IWO more northern and southern aurora1 zone stations : Kiruna and Syowa Base. The geomagnetic coordinates of the stations and their conjugate locations arc given in Table I (cxtractcd from the Mastcr Station List by Buhmann P/ trl., 1974). 3.

STATIONS IJSHI

STLIIY

KESC L'I‘S

Examples of the con.jugate SAI (sudden absorption increase) arc given in Fig. I. The step-like onset in absorption was characteristic for a majority of the events. In the first cxamplc (a) thcrc are simullancous absorption onsets at both conjugate stations at 09:47 U.T., the initial absorption maximum, after the onset. was first at Collcgc (09:4X:0 U.T.) and then at Macquaric Island (09:4X:30 U.T.). Thus. the lag bctwcen the onset hcs at the stations. A(. = 0 min. indicating simultaneous absorption onsets al both stations. The time lag between the first absorption maxima is defined as AA4 = -0.5 min. the minus sign indicating that absorption was first recorded at College. then at Macquarie Island. The first absorption peak magnitudes are indicated : AA4 = 4.6 dB (Macquarie Island) and AC’ = 3.X dB (College). Finally. gcomagnetic K-tindices for both stations were: K = 7

(Macquaric Island) and K = 6 (College). The ncxl evenl (b) was again obtained during a highly dislurbcd period (K = 7 for both stations). Here, AC’ = ~ I .O min and Ail4 = -2.0 min, indicating that both cammcnccment and the first maximum of the SAI cvcnt occurred at Collcgc. The availability of the corresponding riomctcr data from Kiruna made it possiblc to dcfinc the time lag between onsets at Kiruna and College as AK = -35.0 min. indicating that the cvcnt occurred first at College and then al Kiruna. This is consistent with the negative delay for the ew~t onset f‘or Macquaric Island and Collcgc if it is assumed that the absorption cvcnt drifted westwards, from Collcgc to the Maccluaric Island conjugate. and then to Kiruna. The absorption levels for (b) were: .4h! = 3.3dB.AC= 4.XdBnndAK(Kiruna) = 3.OdB. The third cxamplc (c) again signifies a westward drift. this time using two aurora1 stations in the southern aurora1 lone. Macquarie Island and Syowa Base. The corresponding time lags arc : AC’ = -4.0 min, A;Cf = ~ 15.0 min and AS = -54.0 min (for Maccluaric Island and Syowa Base). The period was highly disturbed (K = X for both stalions) with the first absorption maxima: ilM = 3.5 dB. AC’ = I.9 dB and ,.l.S (Syowo Base) = 1.X dB. On a number of occasions both AC’ and AM wcrc positive which. according to the time lag convention, indicates an castward drift: SAI events were tirst rccorded al Kiruna. then at the Macquaric Island and finally at College. In the southern aurora1 oval the eastward drift was from Syoua Base to Macquaric Island and then 10 the College conjugate. On a few occasions. particularly during the southern winter solstice period. a SAI cvcnt at one station was not associated with any significant corresponding absorption increase at the conjugate station. Two such events are shown in Fig. Id. It can bc seen that a SAI event on 14 June 1982. at IO:17 U.T.. was recorded at Macquaric Island but not at College. A rcvcrsc situation is shown below. when H peak in the College absorption (at OX:07 U.T. on I3 June 1983) coincided

Dynamics

of steep onset in conjugate

aurnral riometer absorption

129

ONSET:0956U.T MAX :0956U.T

INSET 0947 UT MAX :09L85UT AM.L6dB K=7

ONSET:O955UT MAX 0956UT. AC.LBdB K.7

MAX 0753 u T AM=09dB K.6 DNSET .09L7 U 1 MAX : 0946 UT AC.36dB K.6

ONSET AK=30dB

1030 UT ONSET

K.4

ONSET : 1453 U T

07L6 UT

AC.33dB

K.7

2019182 FIG.

I.

EXAMPLES

ok

SAI

AIISOIWTION

EVENTS

wcowtx

\rUl) \ORlHI.KN

with relatively minor absorption at Macquaric Island. Such cvcnts were not useful for the drift study of absorption but nevertheless gave more insight into the conjugacy breakdown phenomenon. The positions of the aurora1 stations and the drift direction of SAI events arc shown in Fig. 2.

The time lag in the commencement of SAI events for Macquarie Islat&Collcge, AC showed a considcrablc scatter. The diagram in Fig. 3 shows that AC tended to be more negative than positive. which according to the definition of AC indicates that the prevailing drift direction of conjugate SAI events was westward. The onset of absorption tended to occur before the local magnetic midnight. This characteristic of the SAI events is similar to the general pattern of devclopmcnt of an aurora1 substorm : after an aurora1 break-up ;1 westward travelling surge (WTS) develops. It follows. from the close association of the SAI events with the visual aurora. that a westward travelling absorption event is associated with the WTS. Table 2 gives the average magnitude (with the standard deviation) of various paramctcrs associated with the drift of SAI events. The data were divided into three groups, depending on whether A<’ was negative, Lcro or positive. Thus, the drift direction is defined as wcstward (ACO); when A(‘= 0 there is an instantaneous absorption onset at College and Macquaric Island. In addition to the parameters

AT VARIOLS AIIROKAL

AUKOKAL

STATIONS

IN THE

SOITHIXN

ZO'iLS.

which have been defined in Section 3. I. the new ones arc introduced to describe more completely both the structure of SAI events as well as the cnvironmcnt in which they wcrc gcncratcd. The parameters RM and RC define the rate of absorption onset, from the commenccment to the first maximum. for Macquarie Island and College, rcspectivcly. The large value of RM or RC (in excess of I dB min ‘) indicates that the SAI cvcnts have a step-like structure. The gcomagnetic K indices. in three-hour periods. prcccding the onset (KQM and KQc3 and during the onset (KM and KC). are indicated for Macquarie Island and College, respectively. In general. geomagnetic disturbance levels were considerably lower in the period preceding the onset. In all the cases, there was littlc or no absorption before the onset of SAI cvcnts. The aurora1 clectrojet index (AI!?) for the hour following the onset indicates that both the extent and magnitude of substorms w’erc large at the time of occurrence of SAI events. Table 3 summarizes the results prescntcd in Table 2. It is evident that both the absorption magnitude and absorption incrcasc rate wcrc higher at College than at Macquarie Island. The ratios of AM and .4C‘. and RM and RC, are less than one. which indicates that the absorption magnitude at Macquaric Island is lower by about 10~30% than the corresponding absorption at College. The prevailing drift direction (almost half of all the cases) was westward. The absorption commencement time lag (AC’), rather than

AURORAL STATIONS USED IN THE STUDY OF DRIFT OF SAI EVENTS IN THE SOUTHERN AND NORTHERN AURORAL ZONES. 900 00

300

60°

9tp

L 1200

150-J

I 1800

I 2100

2400

EAST LONGITUDE

FIG. 2. THE WWKAPHI~AL.

POSITIONSOF .~HI: AUKOKAI. STATIONSUSEI)IN THE AIWKPTIO~~:DATA AKAL.YSIS

14

12 10

a 3 2 6 r .CL

80 i: -2 t5 -I# 5 g -6 s _* $ -10

-12

-14 U.T.

t t I 1I I I I iX

06

F~ti. 3. SCATTEK DIAWCAM OF KIOK THI: COP;JIJGAT~: STATIONS (IW The twm

12

16

20

THE ABSOKPTION : MAC-QUARK

24

‘~IMF. LAGS

ISLAYP

AC’

COLLEM

I'xu).

magnetic midnight inter\ al fix thcsc stations is indicuhA by the shaded arca.

the absorption maximum time lag (AM) was used to deduce the drift velocity ; the commenccmcnt time of the SAI events could be accurately defined, whcrcas the first absorption rn~xi~~~u~~~ was somctimcs dif~cult to identify for both stations due to the complexity of the events. The mean computed velocities are similar for the eastward and westward drift directions and arc very close to the v&cities of about 2 km s ’ computed by Hajkowicz (1970) for the same conjugate area, using a different set of the conjugate riomcter data and diffcrcnt techniques of the absorption data analysis. The relatively large range of the computed vcfocities. from about I .5 to 30 km s ‘, as wctl as the presence ofinst~nt~nct?us absorption onsets. suggests a possibility that the position of the conjugate point can change considerably for a specific substorm onset. This is supported by the results of other conjugate studies, as mentioned in the Discussion. The velocities of the absorption onsets in Table 3 were derived assuming that the Macquaric Island conjugate point was located at a longitude found from theoretical computations (Table 1). Since the thcoretical conjugate models were constructed for quiet geomagnetic conditions, this location does not ncccssarily apply to the disturbed conditions. charactcrixed

Dynamics of steep onset in conjugate auroral riometer absorption

131

by a K index of about six and AE index close to 600 nT (Table2). It follows that the real computed velocities might bc somewhat different from those shown in Table 3. In order to verify the magnitude of the conjugate drift velocities it was necessary to use riometer data from two other stations, positioned in the southern and northern aurora1 zones at approximately the same L-shell (to reduce the effect of a latitudinal drift in the absorption onset). Two such stations were found to bc suitable for the drift velocity verification : Kiruna and Syowa Base, in the Northern and Southern Hemisphcrc, respectively (Table I). The results of the true absorption drift velocity computations. associated with the corresponding time lags for the onsets of SAI events, wcrc obtained for two pairs of stations. Kiruna-College (northern aurora1 zone) and Syowa BascMacquaric Island (southern aurora1 zone). The number of corresponding onsets were considerably smaller than for the CollegccMacquarie Island pair, since both Syowa Base and Kiruna are positioned in a different diurnal sector and they can be, on occasions, well removed from the aurora1 oval where all magnetic activity takes place. A more suitable station (at least in the North) would bc Tixic Bay which has a running riometer, but no data would have been made available from this station. It was possible to use 46 pairs of events (for Kiruna College, Syowa Base -Macquaric Island and the corresponding events for College-Macquaric Island) to construct Table 4. All these events indicated a westward absorption drift: the number of events indicating an castward drift was small (seven). insufficient to give valid statistical results. It can be seen (from Table 4) that the true velocities are similar to the velocity computed from an assumed theoretical conjugate point location. This would indicatc that on the whole the theoretical model of the magnetic field conjugdcy is correct. The large deviation in the westward absorption velocity for the ColIcgeMacquarie Island conjugate indicates a change in the location of the conjugate point location, rather than a change in the velocity component. This is clearly evident from the range of velocities obtained for the fixed longitudinal points, which vary from 2.4 to 9.4 km s ’ as opposed to the range of 2.3 km s ’ to more than 30 km s ’ for Macquarie Island conjugatcccollege.

Figure Id illustrates that during seasonal periods centred on the southern winter solstice, a number of events were recorded at one conjugate station without the corresponding events being recorded at the other.

132

L. A. HAJKOWICZ TAHLt 3. ABSOKI’TI~N ONSFT OKIF VtLO(‘lTlliS ANL)ASSOCIATEUI’AKAI1EEKS

AMAC’

RM!RC

0.X

0.8

0.9 0.9

0.9 0.7

Drtfi velocity Westward Eastward (km s ‘) (km s ‘)

Perccntagc occurr~‘ncc ( %r)

2.7(1.4 30.1) In~tantancous absorption onset 3.2( I .3~24.1)

49 20

,4 c (mill) -4.5*4.1 0.0 5.4*4.9

31

TABLE 4. COMPAKISON OP ~IEAN W~STWAKII I)KIFT VFI.OCITIFS(STA~DAKD IXVIAIKXXSOF THE TI~G I-AC; AND rHE C’OKK1:SPONL~INC; VELO(‘ITYKANGIS AKt- INUICATtlI) True velocity. points :

da-iced

from the absorption

time lags at the fined

Time lag

College Kiruna (30 events) Macquarie Island Syowa Base

-40.1 i21.4 - 33.0* 13.0

(16events)

Assumed velocity, found from the absorption time lags at : Macquaric Island conjugate College (24 cvcnts)

Southern _

seasons

Avcragc Macquaric

:

Summer (Nov Jan) Autumn (Feh Apr) Winter (May July) Spring (Aug ~Oct)

The

perccntagc

7153 0156 I 9X) 0458 I645 0715 IX36 0516

occurrence

bre:tkdown

from

is almost

Table entirely

winter:

31%

of the SAI

period

(mainly

June-July)

ber

of the

5, thitt limited

events

the

to the southern during

were “single-ended”

uncorrclated

events:

ten

Mac-

at

quarie Island and scvcn events at Collcgc). It should bc noted ing this

period

station.

southern the

continuous

that

the avcrago

is the longcsl

one

night-time

A possible

this in the con.iugacy breakdown

computed

conjugate

station

is in

implication

of in the

It is cvidcnl.

jugatc

absorption

in aurorul

frotn data

absorption

the analysis tends

locations

con.iugate

data analysis.

absorption

the analysis more

aurora1

:

of corresponding

(L-value

riomctor

rate:

the northern

larger

absorption

of SAI

close

confirming

to

to the

the

curlict

hemispheric

in

diffcrcnco

Icvel and in the absorption aurora1

station

onsets

has

than

the

stcepcr southern

events

breakdown.

occurring present

resulting in ;I num-

at one during

station

only,

is

the southern

wintct

drift

cvcnts

solstice. The prcscncc of the westward

at similat

has been

= 5.3). The

Nol-thcrn

combined data

dcducc

point

con-

that 21steep onset

to be present

times in the conjugate

clusions

is ;I consistent

almost cxclusivcly

of three-year

(SAI events),

conjugate

is on avcragc

point.

(d) The conjugacy ber Ar\iD I~ISCCISSION

was used to

of the

counterpart.

Discussion.

4. CONCLl~SIONS

’ along the

(I 970).

by Hajkowic/

onset

westwards.

km s

analysis

which

the con.jugatc absorption and

mainly

aurora1 ovnls.

drift

Island,

(c) Thcrc

travels

of 1.7-4.5

location

Macquaric

durfor the

will be given

onset

velocity

longitudinal

the

results

of the year

whcrcas the northern daylight.

an average

(b) The conjugate

this (num-

events

0 4 31 0

southern and northern

conjugacy

recorded

Percentage of uncorrected events

44 51 42 34

(a) The absorption with

4.5 (2.3 + 30)

Total number of events

I542 OX06 I909 050x Daytime I947 0404

of thcsc “single-cndcd”

3.7 (2.4-9.4) 3.5 (2.4 5.7)

-2.7k2.h

nigh-time (IJ.T.) Is. College

cvcnts is shown in Table 5. It is evident.

Velocity (km s ‘)

(mln)

from

stations, resulted in the following

with two con-

rtomctcr

rcportcd

from

I~cmisphcrc. data

oral stations

from

21 number Ranta

cxpnnd

CI (I/.

in the

( 19X3) used

in large number of northern

aur-

: hc dcduccd that after a substorm onset

near the magnetic midnight may

in SAI

of studies

westward,

the aurora1 prccipilation

with

a sharp

onset

(SAI

Dynamics

of steep onset in conjugate

events) at the front. The westward-expanding absorption activity correlated welt with local magnetic variations, as is the case in the present study. Ranta c’t nl. (1981) found that the median velocity of Ihe westward drift was 6 km s- ‘. A relatively large variation in the velocity of the conjugatc SAI events (Table 2) can be partly atlributod to a change in the conjugate point location rather than to a change in the speed of the absorption onsets. This is supported by an carlicr work on the magnetic conjugacy between Macquaric Island and the conjugate stations (including Cottegc) in Alaska by Wescott and Mathcr (1965). They found cnscs 01 dcfinitc departure from the conjugacy and conctudcd that the conjugate area varies in size. shape and its location. but it is elongated along a tine of constant L-vatuc. In gencrat two types of conjugacy should be distinguished : the lime and magnitude conjugacy. Hajkowicz (1970) anatyscd sotne 100 aurora1 absorption events al three stations, Macquaric Island. Kotzcbuc and College. Hc found that the absorption increase features were similar at the soulhcrn station and at the Iwo northern stations. The most pronounced diffcrcncc was in the time lag or the corresponding absorption features: this time tag was about six times larger Tar Macquaric Island-Cottegc than for Macquaric Istand~Kotzebue, the latter pait being a close conjugate pair. It follows that SAI cvcnts appear and drift simullaneousty only for the ctoscty conjugate stations. The prcsencc of larger and steeper absorption onsets al Coltcgc than at Macquaric Island has been noted by Hajkowiczand Hunsuckcr (19X5). They anatyscd 32 simuttancous conjugate SAI events at five Alaskan stations with different L-values. and at two stations in the Southern Hemisphcrc (Macquaric and Campbell Island LIsland, L-vatuc = 5.3 value = 4.0). It was noted that the average absorption onset rate and magnitude were dccrcasing rapidly below the L-value = 5.5 which is the L-value of Coltegc : Macquaric Island, with a lower L-value. showed about 25% lower absorption than Cottcgc. This is consistent with the present results. The diflerencc in the aurora1 absorption may also be rctatcd to 21decpcr cause as suggcstcd by Hajkowicz (1970) and Stenback et ~1. (1973). H>ijkowicz (1970) noted that the conjugate absorption events at Kotzcbuc (Alaska) tended to bc higher (up to 40%) in magnitude than the corresponding events at Macquaric Island although both stations have the same L-value. He suggested that this diffcrencc may bc related to a difference in the shapes of the northern and southern aurora1 zone. The difference applies not only to absorption but also to

aurora1

riometcr

absorption

133

luminosity intensity of aurora1 forms. Conjugate attsky camera data obtained in I9 flights through the aurorat zone al the College meridian show hemispheric differcnccs in aurora1 frequency and intensity (Stenback ct d.. 1973). Conjugate auroras arc about 30% brighter in the northern than in the southern aurorat zone. The hcmisphcric difference was atlributcd to the X000-nT diffcrencc in magnetic tield strength bctwccn the conjugate areas. A separate and hitherto largely unknown topic is the conjugacy breakdown. Hajkowicz (1970) reported some casts of “single-ended” absorption evcnls rccordcd tither at Kotzcbue (Alaska) or Macquaric Island. In most cases (65% of all the cases) the uncorrelated events occurred close to the nautical sunset or sunrise for both stations. The relatively tow corrctation of SAI events in the southern winter may bc linked with a seasonal change in the latitude (L-value) of the conjugate points. For examptc. Ecktund and Hargreavcs (1968) used a system of five riotnctcrs at Bait St Paul (L-value = 4.0) and one al the southern conjugate point. Eights (L-value = 3.9). They found cvidencc that thcrc was a seasonal movcmcnt of Ihc conjugate point to Eights 0.3 L-vatuc southward in the southern winter. If this is applied to the conjugate point of Macquaric Island then it wilt be shifted by 0.5 L-value towards the equator. away from the Lvalue of College. Such a large difference in the Lvalue would result in a number of “single-cndcd” absorption events at these two stations. Although much has been learned about the tongitudinat locations of the aurorat conjugate points little is known about the latitudinal drift (in L-value) of the step-like absorption onset. The study of such a drift Would give a full experimental evaluation of Ihc conjugate point locations. Since a change in I,-vatuc of the conjugate point appears to bc associated with the conjugacy breakdown, such a study would also give valuable information on the asymmetric position 01 the aurora1 particle source in the tail of the magnctozphcro [cf. Mcng and Akasofu ( 196X) for ;I more detailed discussion of this topic]. An experimental solution of this problem, cxtcnsivety discussed by Hajkowicz and Hunsuckcr (I 9X5), is to USC ofT-vertical riotncter systems at possibly the world’s best conjugate pair: Macquarie Island~Kotzebuc. Ack,low,/~,~~c,m~,~t.r--The riometcr data were supplied by the C&physical Institute, University ofAlaska; the Department of Science and Technology, Kingston. Australia: Kiruna Geophysical Institute. Sweden ; and the National Institute of Polar Research, Japan. I am grateful to Mrs D. J. Dearden of the Department of Physics. and Mr M. Bourkc of St Laurence‘s College. Brisbane. for their assistance in the riometer data analysis. I

114

L. A.

am also grateful to Dr G. Bowman on some aspects of this report.

HAJKOWIC7

for his helpful discussion

RISXRENCES Ansari, Z. A. ( 1964) The aurornlly associated absorption oi cosmic noise at College. Alaska. J. yco~h,x. Rcr. 69. 4493. Buhmann. R. W., Rocderer, J. D.. Shea. M. 0. and Smart. D. F. (1974) Master station hst for solar tcrreslrial physic5 data at WDC-A for solar terrestrial physics. Report (JAG-38. World Data Center A. NOAA. Boulder. Coorado. Collis, P. N.. Kirkwood, S. and Ilall. C. M. (1986) n-region sipnaturcs of substorm growth phase and onset observed by II/SC’.-f I: P/U/K’/. .s/ww SC,;. 48. x07. Ecklund. W. L. and Hargrcux L‘S,.I. K. ( I96S)Somemcasurcmcnts of iiut-oral ahaorplion slruclurc over dislancch 01 about 300 km and of abxxp~~on col-relation bct~~ccn cow jugalc regions. J. ~/mos. Iwr. /‘/fv.v. 30. 265. HajkowicL, L. A. (1970) Conjugalc riomclcr studies of auroral zone cosmic noise absorplions. ilu.v/. J. Plr>x. 23, I X7. Hajkowicr. L. A. and Hunbuckcr. R. D. (1985) Conjugate

absorption study using directional riometer antennas. A proposal for the National Science Foundation. Gcophysical Institulc, Iinivcrsity of Alaska. Johansen. 0. E. (1965) Variations in cncrgy spectrum ol aurora1 electrons detected by simultaneous observation with photometer and riometcr. Pltrrrc~t. Sp~x~~.Sc,i. 13. 225. Mcng. C.-I and Akasofu, S.-I. (196X) Polar magnetic substorms in the conjugate points. Rrxlro Sci. 3. 751. Ranla. H.. Ranla. A., Collia. I’. N. and Harercaves. J. K. (1981) Devclopmcnt of the aurora1 absorptl’on substorm : studies of the prc-onset phase and sharp onset using an cxtensivc riometer network. P/r,lr/. S~xrcx, Sci. 29, 1287. Ranla. A. Ranta, II., Rosenberg, T. J., Wedcken. U. and Statming, I’. (1983) Devclopmcnt of an aurora1 absorption substorm : studies of hubstorm related absorption cbcnts in Ihc ;ilic~-noon early e\ enins scclor. P/rm,/. .Spow SC,;. 31, 1415. Stenback, H. C.. Wescott, E. M., Davis, T. N. and Pelcrben. R. W. (1973) Differences in aurora1 intcnsit al conjugate points. J. qcoph~v. Rc.v. 78. 659. Wescott. E. M. and Mather, K. B. (1965) Magnetic conjugncy from L = 6 lo I. = 1.4. I, aurora1 Tone. J. ,qc~~~/l,\.,,. Ret. 70. 43.