Pulsating auroras in relation to proton and electron auroras1

F. CREUTZBERG, R. GATTINGER, F. HARRIS, A N D A. VALLANCE Jones Herzherg Insrirlire of Astrophysics, Nationcll Resecrrch Colit~cil of Cht~rrda, Olla\c~a, Otzt., Canada KIA OR6

Received December 12, 1980'

Observations of pulsating auroras in the magnetic zenith and proton and electron auroras along the magnetic meridian were conducted from three stations covering from 58" to 71" invariant during the pulsating aurora campaign in Saskatchewan in January and February of 1980. Pulsa- tions were observed only south of or on the southern edge of the proton precipitation region. This is in general agreement with other uncoordinated studies of proton and pulsating auroras in the late morning hours, but it also appears to be true for isolated events at other times as well.

Desobservationsd'aurorespulsantesauzenithmagnetiqueetd'auroresdeprotonsetd'electrons le long du meridien magnetique ont ete effectuees depuis trois stations couvrant I'intervalle de 58" a 71" de latitude invariante, au cours de la campagne des aurores pulsantes qui a eu lieu en Saskatchewan durant les mois de janvier et fevrier 1980. On a observe des pulsations seulement au sud ou sur le bord sud de la region de prkipitation de protons. Ceci est d'une f a ~ o n generale en accord avec d'autres etudes non coordonnees d'aurores de protons et d'aurores pulsantes, aux heures tardives du matin, mais semble aussi se verifier pour des evenements isoles se produisant a d'autres moments. Can. J. Phys., 59, 1124 (1981) [Traduit par le journal]

Introduction the relationship between auroral morphology and The morphology of pulsating auroras has been particle injection events. These studies have been

the subject of numerous investigations. Royrvik extended to cover a much broader latitude range by and Davis (1) described in some detail results "allance Jones et (9). obtained using television cameras and the DMSP However, coordinated studies of the instantane- imager. Their observations provide considerable OUS relationship between the morphologies of pul- insight into the highly variable nature of pulsating sating auroras and Proton auroras have not been auroras. The change of periodicity with latitude reported in the literature. For the pulsating aurora described by Thomas and Rothwell (2) (see also, campaign conducted in Saskatchewan in early 1980 Duncan et a l . ( 3 ) ) is yet another type of variation. meridian scanning photometers designed for ob-

Theories have been developed involving self- serving Proton and electron al~rora were a%'- modulated very low frequency (VLF) wave-elec- mented with multichannel photometers to view the tron interactions (4) and the interaction of micro- magnetic zenith for the Purpose of recording pulsations with waves (5). These theories do not pulsating auroras. Results obtained from the two predict a modulation of the precipitating protons. types of photometers are discussed in the following Eather (6) investigated this possibility experimen- sections with a view towards determining whether tally by observing the 4861 A H Beta and 5577 A 0 1 any persistent relationships exist between pulsat- emissions simultaneously. He concluded that there ing and Proton mroras. were no detectable pulsations in the proton emis- It appears that pulsating al~roras might occur sions arising from pulsating auroras except for only equatorwards of proton auroras, with little or those with periods of about 1 min or more. no spatial overlap, possibly to the extent that the

There have been numerous studies dealing with two types of auroras do not occur in the same lace the relationship between proton and electron at the same time- A very preliminary hypothesis aurora. Fukunishi (7) summarized photometric built upon these observations suggests that observations from Syowa station on the basis of pulsating and Proton auroras are mutually substorm time at various local times and arrived at exclusive. simplified schematic representations of the mor- Instrumentation and Analysis ph0logy of proton auroras during substorms. Meridian scanning photometers and zenith pho- Eather et a l . (8) compared their meridian scanner tometers were located at each of three stations data '0 A'S 5 article spectrograms to determine ( ~ ~ b l ~ 1) as part of the 1980 pulsating aurora

'NRCC No. 19423. campaign in Saskatchewan (10). The sites were ZRevision received February 23, 1981. selected to lie along a geomagnetic meridian.

0008-420418 1/08 1124-07$0 1 .00/0 @ 1981 National Research Council of Canada/Conseil national de recherches du Canada

Can

. J. P

hys.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV G

UE

LPH

on

12/0

2/14

For

pers

onal

use

onl

y.

CREUTZBERG ET AL. 1125

TABLE 1. Observing stations (units in degrees)

Geographic Scanner data limits Zenith photometers Invariant

Station Lat. Long. Lat. Elev. angle Invariant lat. Elevation Azimuth Field of view

Rabbit Lake 58.2 103.7 67.8 16s-14N 64.6-71.3 80.9 199.5 1 La Ronge 55.2 105.3 64.6 15s-14N 61.2-68.1 78.4 199.0 4 Saskatoon 52.2 107.2 61.3 18s-16N 58.3-64.5 75.7 198.7 4

The scanning photometers monitored numerous emissions including 0 1 5577 A and H Beta 4861 A and associated background channels to remove contributions from underlying features. The raw data from each of the original 30s meridian scans were converted to absolute brightness units, cor- rected for extinction and van Rhijn effects, and then transformed to invariant latitude coordinates with an assumed auroral altitude of 110 km, and with the scanner data limits given in Table 1. The assumed altitude was found to produce good corre- spondence in overlap regions when the data from the three stations were merged together to produce an intensity profile from 58.3" to 71.3" in lat. The 30 s time resolution inherent in the original merged scans was intentionally degraded by averaging a number of successive scans for the purpose of producing the compressed intensity-latitude-time displays discussed in the next section. A surround- ing heavy solid line was used to indicate the latitude and time periods covered by the scanning photometer data.

Auroral pulsations in the magnetic zenith were monitored by multichannel photometers sensitive to selected N,+ first negative emissions and 0 1 5577 A as well as numerous other auroral features. The photometer viewing directions (Table 1) were centered on the 100 km altitude points of the field lines through the stations. This resulted in the center line of the field being aligned to within about 0.5" of the tangent to the field line throughout the major auroral emission region. The presence or absence of pulsations was determined by simple inspection of the zenith photometer records.

The grey scales in the upper panels of Figs. 1 to 3 are linear in brightness with white representing less than 1 kR and 20 R and black greater than 15 kR and 300R for I(5577) and I(4861) respectively. The ratio 1(5577)/1(4861) is considerably higher than values expected in pure proton auroras (Eather et al. (8) and references therein), so the 0 1 5577 A emission arises mainly from electron precipitation.

Results and Discussion The results are presented in what might be

described as an increasing order of complexity. Initially, the simple post midnight case, of regular northward motion of the proton precipitation re- gion, with early morning pulsations is considered. This is followed by a case involving two isolated displays with one near midnight and one in the early morning hours. Finally, a complex case involving multiple events from early evening through to the late morning hours is discussed.

Case 1 The first case (Fig. 1) from the night of Feb. 15,

1980 might be called a classical case. A strong electron precipitation event (01 5577 A panel) occurs near magnetic midnight (about 0800 U T for La Ronge) and then slowly changes into a pulsating or patchy aurora in the morning hours as in the models developed by Akasofu (1 I). The northward expansion of the proton precipitation region (Hp 4861 A panel) during the midnight electron precip- itation event is as described by Fukunishi (7). In the late morning hours the proton precipitation region drifts northward just as was observed by Fukunishi (7) and Wiens and Vallance Jones (12). This relatively simple case provides a very good starting point for determining whether there might be some relationship between proton and pulsating auroras.

The montage in the lower panel of Fig. 1 portrays the development with time of a pulsating aurora in the magnetic zenith a t La Ronge as seen in the N2+ first negative 3914A channel. Very weak pulsa- tions were apparently present as early as 0845 U T but they did not become well defined until about 0940 UT. Pulsations were present from that point onwards right up until dawn.

If a pulsation onset time of 0940 U T is adopted it can be seen that the proton precipitation region had moved almost completely north of the La Ronge magnetic zenith before pulsations commenced. When charge exchange spreading of the proton beam (13) is taken into account, it seems most likely that the precipitating proton beam originated from field lines located north of La Ronge. This apparent separation of the proton precipitation region and the pulsating auroral region is the point

Can

. J. P

hys.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV G

UE

LPH

on

12/0

2/14

For

pers

onal

use

onl

y.

CAN. 1. PHYS. VOL. 59, 1981

Frc. 1. Meridian scanner 01 5577 A and H Beta 4861 A plots (upper panel) and La Ronge zenith photometer ou examples from February 15, 1980.

of major interest in the three case studies presented here.

The sequence of events as seen from Rabbit Lake was very similar. Pulsations in the magnetic zenith commenced at about 1200 U T by which time the proton precipitation region had largely moved to the north of Rabbit Lake. The meridian scanning photometer at Saskatoon did not detect any au- roras near the zenith but did observe some near the northern horizon.

In this first case the gradual northward motion of the proton aurora coupled with the gradual onset of a pulsating aurora make it difficult to establish the extent to which the two phenomena are mutually

exclusive. This is clarified somewhat in the next two cases presented.

Case 2 The second case (Fig. 2) from the night of Jan.

17, 1980 is somewhat more complex. There is again a strong electron precipitation event near magnetic midnight, but this is followed by another one in the morning hours. However, the behaviour of the proton aurora is very similar to that of Case 1; there appears to be no simple correspondence between the morning electron event and the proton aurora.

For this night the pulsations commenced in the magnetic zenith at La Ronge, at approximately

Can

. J. P

hys.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV G

UE

LPH

on

12/0

2/14

For

pers

onal

use

onl

y.

CREUTZBERG E T A L

l(3914) IN MAG. ZEN - 64 6' IN LAT.

FIG 2. Meridian scanner 01 5577 A and H Beta 4861 A plots (upper panel) and La Ronge zenith photometer output examples from January 17, 1980.

0720 UT - about halfway through the midnight of the proton aurora near 0720 UT was followed event. Weak pulsations were present for a brief almost immediately by the onset of pulsations at La period near 0715 UT, but well defined pulsations Ronge. The minimum time period required for this did not occur until about 0725 UT. The pulsations transition appears to be no longer than a few continued, although somewhat erratically, through minutes. If the mutual exclusivity hypothesis is to dawn as is evident in the lower part of Fig. 2. correct, then the minimum duration of this transi-

Pulsations were not observed at Rabbit Lake tion period from proton aurora to pulsating aurora until about 1030 UT by which time the proton might be an important point to bear in mind when precipitation region was largely north of Rabbit developing models to describe pulsating auroras. Lake. An aurora was observed only near the northern horizon from Saskatoon. Case 3

Again, as in Case 1, the proton aurora was The third case (Fig. 3) is from the night of Feb. located distinctly north of the pulsating aurora. 16, 1980 which was the most active night during the However, an additional effect was observed here - pulsating aurora campaign. There was an isolated the sudden northward motion of the southern edge early evening electron event, and then almost

Can

. J. P

hys.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV G

UE

LPH

on

12/0

2/14

For

pers

onal

use

onl

y.

C A N . J. PHYS. VOL. 59, 1981

! ..... 1 . L............... L.-- I

L. .--..L___:

0 2 0 0 0 4 0 0 0600 08 0 0 1000 1200

UNIVERSAL TIME

robs I

lp04

l(4278) IN MAG. ZEN. - 61.3O IN. LAT.

FIG. 3. Meridian scanner 0 1 5577 A and H Beta 4861 A plots (upper panel) and Saskatoon zenith photometer output examples from February 16, 1980.

continuous electron precipitation from midnight to auroral oval, the zenith photometer a t Saskatoon dawn. The proton aurora exhibited many north- was used to search for pulsating auroras. Selected ward and southward motions throughout the night. short periods from the output of the N,+ 1N 4278 A This type of motion of the proton aurora provides a channel are presented in the lower part of Fig. 3. good test of the mutual exclusivity hypothesis The output is on a logarithmic scale, but this is not a developed in the description of the first two cases. serious problem for the type of qualitative analysis

Because of the expanded configuration of the discussed here.

Can

. J. P

hys.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV G

UE

LPH

on

12/0

2/14

For

pers

onal

use

onl

y.

CREUTZBERG ET AL. 1129

The first occurrence of pulsations was during a short period of time of about 4min at about 0250 UT. At this time the major proton precipitation region was north of Saskatoon, but there was some H Beta emission up to about 1" south, presumably due to charge exchange beam spreading. This brief burst of pulsations occurred shortly after the time when the gradual evening southward motion of the proton precipitation region had been interrupted by a pronounced electron event, and had moved northwards. The pulsations ceased as the proton region started moving southwards once again.

The next pulsation period occurred between 0640 and 0650 U T which was immediately after a sudden brightening and northward motion of the H Beta emission region. Here again the major portion of the proton precipitation region was north of the pulsating region, although there was appreciably more H Beta emission to the south of the pulsations than for any other event described up to this point. The source of this emission to the south could be beam spreading in the easterly or westerly direc- tion from a strong proton aurora just off the magnetic meridian. Further observations with an all-sky H Beta imager with a time resolution of at least 1 min are required to check this possibility.

Pulsations appeared most of the time throughout the period from 0740 to 0825 UT. Here again the main proton precipitation region was well north of the pulsating region. However, weak H Beta emis- sions were observed much further to the south with a brightness about one-tenth of the peak brightness on the meridian. These weak emissions could arise as a result of scattering by atmospheric haze, an effect which is very difficult to assess quantita- tively.

The next occurrence of pulsations was at about 0935 UT, which was just after the sudden north- ward motion of the proton precipitation region from a location mainly south of 60" invariant lat. near 0900 UT. Pulsations were absent for almost 1 h centered on 0910 UT spanning the period when the major parts of the proton and electron auroras were either overhead or south of Saskatoon.

The pulsations continued from about 0935 to 0945 UT during which time the proton precipitation region was well north of Saskatoon. From about 0945 to about 1000 UT the proton precipitation increased in the zenith and south of Saskatoon and the pulsations were absent. During this period the electron aurora was well north of Saskatoon.

From 1000 UT to dawn the proton precipitation region was well north of Saskatoon and pulsations were observed on numerous occasions. In this late

morning period the most pronounced pulsations appeared at the southern edge of the electron aurora as noted by Cresswell (14).

Pulsations were also observed in the zenith at La Ronge for several brief periods centered on about 0735 and 1045 UT. The later occurrence does not conflict with the mutual exclusivity hypothesis because there was very little H Beta emission south of La Ronge at that time. For the earlier event at 0735 UT, the meridian scanners detected a consid- erable amount of H Beta emission south of La Ronge. However, the scanners also detected a sudden decrease in the auroral brightness centered on about 0735 UT from slightly north of La Ronge to well south of Saskatoon. This again suggests that the observed H Beta emission could have arisen from bright displays just off the meridian. Inspec- tion of the La Ronge all-sky film indicated that there indeed were bright active auroras both east and west of the La Ronge meridian. Displays of this type are often accompanied by widespread H Beta emission, but with such a high level of auroral activity it becomes very difficult to estimate the effects of proton beam spreading. The sky was overcast at Rabbit Lake for most of the night so no definitive results were obtained from that station.

Further observations are necessary using sensi- tive all-sky proton aurora monitors as well as instruments designed to detect atmospheric haze to remove the uncertainties involved in the discus- sion, especially for Case 3. As it stands, the mutual exclusivity hypothesis remains in a verv prelimin- ary state of development.

Conclusions The main conclusion, although somewhat tenta-

tive, is that auroral proton precipitation does not occur coincident with nor equatorwards of pulsat- ing auroras. Also, the transition time from a rapidly poleward moving proton aurora to a pulsating aurora can be as short as a few minutes. Converse- ly, pulsating auroras can give way to equator- ward moving proton auroras in a few minutes.

This suggests that energetic protons (perhaps above a certain threshold) in the parent plasma might suppress the processes which cause pulsa- tions in the precipitating electrons.

1 . 0. ROYRVIK and T. N. DAVIS. J. Geophys. Res. 82, 4720 (1977).

2. R. W. THOMAS and P. ROTHWELL. J . Atmos. Terr. Phys. 41, 1179 (1979).

3 . C. N. DUNCAN, F. CREUTZBERG, R. G A ~ I N G E R , F. HARRIS, and A. VALLANCE JONES. Can. J . Phys. This issue.

Can

. J. P

hys.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV G

UE

LPH

on

12/0

2/14

For

pers

onal

use

onl

y.

1130 C A N . J . PHYS. VOL. 59, 1981

4. G. J. DAVIDSON. J. Geophys. Res. 84, 6517 (1979). 5. F. V. CORONITI and C. F. KENNEL. J. Geophys. Res. 75,

1863 (1970). 6. R. H. EATHER. Ann. Geophys. 24, 525 (1968). 7. H. FUKUNISHI. J . Geophys. Res. 80, 553 (1975). 8. R. H. EATHER, S. B. MENDE, and R. J . R. JUDGE. J .

Geophys. Res. 81, 2805 (1976). 9. A. VALLANCE JONES, F. CREUTZBERG, R. GATTINGER, and

F. HARRIS. In preparation.

10. D. J . MCEWEN and C. N. DUNCAN. Can. J . Phys. This issue.

11. S-H. AKASOFU. Polar and magnetosphere substorms. D. Reidel Publishing Co., Dordrecht, Holland. 1968.

12. R. H. WIENS and A. VALLANCE JONES. Can. J. Phys. 47, 1493 (1969).

13. G. J . DAVIDSON. J . Geophys. Res. 70, 1061 (1965). 14. G. R. CRESSWELL. J. Atmos. Terr. Phys. 34, 549 (1972).

Can

. J. P

hys.

Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV G

UE

LPH

on

12/0

2/14

For

pers

onal

use

onl

y.