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Luminosity variations in several parallel auroral arcs before auroral breakup
Luminosity variations in several parallel auroral arcs
before auroral breakup
V. Safargaleev, W. Lyatsky, V. Tagirov
Polar Geophysical Institute, Apatity, 184200, Russia
Received: 18 March 1996 / Revised: 3 March 1997 / Accepted: 11 March 1997
Abstract. Variation of the luminosity in two parallel
auroral arcs before auroral breakup has been studied by
using digitised TV-data with high temporal and spatial
resolution. The intervals when a new arc appears near
already existing one were chosen for analysis. It is
shown, for all cases, that the appearance of a new arc is
accompanied by fading or disappearance of another arc.
We have named these events out-of-phase events, OP.
Another type of luminosity variation is characterised by
almost simultaneous enhancement of intensity in the
both arcs (in-phase event, IP). The characteristic time of
IP events is 10±20 s, whereas OP events last about one
minute. Sometimes out-of-phase events begin as IP
events. The possible mechanisms for OP and IP events
are discussed.
1 Introduction
Research into the dynamics and morphology of auroras
before auroral breakup may give information about
possible reasons for substorm triggering. There are two
well-known phenomena in auroras before the substorm
onset: pseudobreakup events and auroral fading.
The term ``pseudobreakup'' was introduced by Aka-
sofu (1964) for the description of local short-lived
activation of auroral arcs. McPherron (1991) has noted
that pseudobreakups are similar to the substorm
expansion phase including a burst of Pi2 micropulsa-
tions and weak enhancement of westward electrojet.
According to Koskinen et al. (1993) and Nakamura et al.
(1994) the main di€erence between pseudobreakups and
the substorm expansion onsets might be intensity and
size of the disturbance source.
Another prebreakup event is the auroral fading i.e.
decrease in the discrete auroral arc luminosity a few
minutes before an auroral breakup (Zaytseva et al.,
1976; Pellinen and Heikkila, 1978). Kauristie et al.
(1995) reported the ionospheric plasma depletion ob-
served by EISCAT radar together with the arc fading
just before the auroral breakup. Pudovkin et al. (1995)
suggested that a speci®c B
z
distribution in the vicinity of
the inner edge of the magnetotail plasma sheet hinders
the particle precipitation from the Earthward-moving
¯ux tube during the auroral fading.
Multiple auroral structures and their role in the
substorm development have been investigated by many
authors. The periodic (in longitude) arc structures at
separations of more than 150 km were studied by
Murphree et al. (1994). At a larger scale the observation
of double auroral oval at the recovery phase of
substorms was reported by Elphinstone et al. (1995b).
In another paper by Elphinstone et al. (1995a), the
possible role of periodic (in longitude) auroral structures
in evolution of multiple onset substorms is discussed.
Relatively little attention was paid to correlation of
the luminosity in the multiple auroral arc structures. In
some papers for example, Persson et al. (1994) and
Elphinstone et al. (1995b) the presence of several arcs in
the sky was used to map the plasma sheet boundaries in
the ionosphere and to de®ne a region in the magnetotail
where substorm onset takes place. Morse and Romick
(1982) noted that the sky does not fade as a whole when
more than one arc is observed, but some arcs dim while
others brighten. Recently, Safargaleev et al. (1996) have
reported an interesting case of observation of two
auroral arcs a few minutes before an auroral breakup.
They have found that activation of weak auroral arc
leads to fading of more powerful equatorward arcs, and
vice versa.
In the present study we extend this investigation,
including some cases of simultaneous observation of two
auroral arcs in the TV-camera ®eld of view just before
auroral breakups.
Correspondence to: V. Safargaleev
E-mail safar@pgi-ksc.murmansk.su
Ann. Geophysicae 15, 959±966 (1997) Ó EGS ± Springer-Verlag 1997 2 Method of investigation and description of events
In our study we used the data of routine TV-camera
registration of auroras at the Loparskaya observatory
(corrected geomagnetic coordinates 64.70 N, 116.33 E;
MLT=UT+3h). The TV-camera was pointed toward
zenith, the ®sh-eye lens gave a ®eld of view of
170 ´ 170°. We were interested in the intervals when
multiple auroral arc structures (MAAS) were seen in the
sky before a substorm onset. More than 60 h of TV data
near the local midnight, obtained during the observation
seasons of 1994±1995, were examined, but only three
MAAS intervals of about 5 min in duration have been
chosen for the further analysis. These data were digitised
at 2 s intervals with spatial resolution of about
0.6 ´ 0.6 km at an altitude of 110 km near the TV-
camera zenith. The samples along the geographic
meridian were used then to construct a keogram. Such
keograms indicate the temporal behaviour of auroral
luminosity as a function of latitude. The main features
of each of the MAAS intervals is described next.
2.1 January 30, 1995, 1727:15-1730:35 UT
This interval is the most simple for analysis. The
geomagnetic and aurora activity for the period is
presented in Figs. 1, 2 where the MAAS interval is
shown with either horizontal bars or black arrows. The
development of the substorm is shown in Fig. 1a, b
where variations of the horizontal component of the
geomagnetic ®eld at Lovozero observatory (corrected
geomagnetic coordinates 64.09 N, 115.47 E; MLT=
UT+3h) are presented. The data from Kiruna and
Sodankyla observatories are shown in Fig. 2, bottom
right, the location of Kiruna, Sodankyla, Lovozero and
Loparskaya may be seen in Fig. 2, bottom left.
As seen from the magnetograms in Figs. 1a and 2, the
substorm expansive phase begins near 1725 UT as a
strong negative excursion of the north component of the
magnetic ®eld in Lovozero, Sodankyla and Kiruna. At
1733 UT the next negative deviation takes place in
Lovozero and Sodankyla. Both activations were accom-
panied by trains of Pi2 pulsations in Lovozero (see
Fig. 1b). A few minutes before the second activation,
during the relatively quiet period approximately 5 min in
duration, the multiple auroral arc structure appears in
the sky in Loparskaya.
Development of the auroras during the interval
1727±1730 UT is shown in Fig. 2 (upper panel) as a
sequence of all-sky images. The interval is characterised
by appearance of a new arc approximately 70 km
equatorward of a pre-existing arc. Appearance of the
new arc was accompanied by fading of the poleward arc.
Unfortunately, there is a data gap between 1730:35±
1733:15 UT, and we cannot de®ne the onset of the
auroral breakup correctly. In accordance with the rapid-
run magnetogram in Fig. 1b, its beginning may be
de®ned close to 1733 UT. The auroras for this moment
of time are shown in Fig. 2, bottom left (negative
representation).
30 January 1995 Lovozero
H
100 nT
16:00
17:00
UT
18:00
19:00
20:00
11 February 1994 Lovozero
H
100 nT
18:00
19:00
UT
20:00
21:00
22:00
30 January 1995 Lovozero
H
10 nT
17:20
17:25
UT
17:30
17:35
17:40
11 January 1994 Lovozero
H
b
10 nT
19:55
20:00
UT
20:05
20:10
20:15
a
Fig. 1a, b. Variations of the H-component of geomagnetic ®eld in
Lovozero. a Standard magnetograms showing the level of geomag-
netic activity. b Rapid-run magnetograms. The intervals of observa-
tion of multiple auroral arc structures (MAAS) are shown with
horizontal bars
960
V. Safargaleev et al.: Luminosity variations in several parallel auroral arcs before auroral breakup In more detail, the variations of the mean value of the
arc luminosity are presented in Fig. 3 (upper panel).
Also shown are the keograms obtained along the
(Z)enith, (E)astward and (W)estward pro®les (lower
panel). These pro®les are indicated in Fig. 2 by white
vertical lines on the ®rst all-sky image. The keogram for
the Z-pro®le indicates the temporal behaviour of sky
luminosity along the geographic meridian of Lopars-
kaya, Z° means the zenith angle (positive to the north).
The width of the Z-pro®le is about 25 km at an altitude
of 110 km near the zenith. The keograms for the W- and
E-pro®les are presented to show the sky luminosity in
the zenith vicinity. For convenience, we use the same y-
axes as for the Z-pro®le. It is seen from these keograms
that the arc fading as well as the arc brightness occurs
almost simultaneously eastward and westward of the Z-
pro®le and, hence, seems not to be caused by propaga-
tion of any small-scale irregularity along the arc.
Two steps of auroral development can be seen during