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Figure
1 |
Figure 1
(based on Anderson et al., JGR, p. 15,757, 2000): IMF measurements
from three spacecraft indicating that steady, strongly southward,
IMF impacted the magnetosphere during the man phase of the October
19, 1998 storm from ~0440 to ~1510 UT. During this period of
enhanced steady convection, no substorms were observed until
1325 UT, indicating that substorms generally do not occur when
the IMF driver is known to be strongly southward and steady.
After ~9 1/2 hours of steady IMF driving, a substorm did occur.
However this occurred shortly after a change of Bx with respect
to x was observed in the solar wind, implying significant y (or
z) variation of By (or Bz), and when a northward turning was
observed by Geotail when it briefly entered the magnetosheath
(See Figure 2). |
|
Figure
2 |
Figure 2:
Bz, measurements from Geotail as compared to the IMF Bz measured
by Wind on October 19, 1998. Geotail was mostly within the magnetosphere
but occasionally entered the magnetosheath; the measurements
are shown as a dark black line during intervals when Geotail
was identified as being within the magnetosheath. These measurement
show that the 1325 UT substorm may have been the result of a
northward turning (reduction in negative Bz,) of the IMF that
occurred in the vicinity of the Earth, despite this northward
turning not being seen in the IMF measurements shown in Figure
1 by three spacecraft in the solar wind. |
|
Figure
3 |
Figure 3
(Lyons, L. R et al., New Perspectives on the Earth's Magnetotail,
AGU, p. 241, 1998): CANOPUS meridian scanning photometer observations
illustrating the difference between the poleward expanding regions
of intense aurora associated with substorms (which initiate in
the equatorward part of the aurora oval) and the much more frequent
, but small-scale, auroral poleward boundary intensifications
which initiate near the poleward boundary of the auroral oval. |
|
Figure
4 |
Figure 4
(based on Lyons et al.,. JGR, p. 349, 2001): Ground X and Pi
2 and geosynchronous energetic electron observations showing
that geosynchronous injection (and presumably dipolarizaton0
initiated ~2 1/2 minutes prior to onset |
|
Figure
5 |
Figure 5
(Lyons, JASTP, p. 1087, 2000): Representative IMF measurements
from multiple monitors during time intervals surrounding substorm
onsets identified with CANOPUS photometer and ground magnetic
field data. All times have been corrected to an estimated magnetopause
contact UT assuming magnetic structures are aligned at 45°
in the x,y plane. Notice that large differences occur between
simultaneous measurements of the IMF from satellites that are
often used as IMF monitors. Also notice that substorms can appear
to occur during a steady IMF (e.g., 2/19/96) or in association
with a southward turning of the IMF (e.g., 4/15/96) based on
one monitor, but appear to be triggered by a northward turning
of the IMF based on other monitors located closed to the Earth-sun
line. |