Transcript Cross-Meridian Currents in Substorms and Sawtooth Events

Automated Forward Modelling and
its Results for Substorms and
Sawtooth Events
Martin Connors1, Gordon Rostoker2, R. L.
McPherron3, Tung-Shin Hsu3, Jason Ponto1
1Athabasca
University, 2University of
Alberta, 3UCLA
CAP-DASP Saskatoon, June 2007
Image: Mikko Syrjäsuo
Ground Magnetometery: In a Sun-to-Mud approach, we
are in the mud…
EDMO UCLA magnetometer installed by Martin Connors (Tom Sawyerlike technique applied to astronomer Brian Martin) in December 2004
And you thought the RAIN was bad… (Kanji Hayashi in
LaRonge, mid-October 2004)
Athabasca University
has assisted or runs 16
sites in Canada (white
triangles and purple
triangles in Western
Canada). Most data
available through
UCLA, STEP website,
or on request. Polaris
sites should be
installed in Quebec
soon. With recent
vigourous activity by
AU and University of
Calgary, Canada is
ready for THEMIS!
What can ground magnetic data tell us?
Regrettably, a single
magnetogram often
tells nearly nothing
and even multipoint
measurements from a
meridian chain are
difficult to interpret,
needing…
“geomagic”…
Array Interpretation from a distributed region is even
more difficult, complicated by problems of
nonuniqueness. An inversion procedure is needed.
• On the ground, one
detects primarily the
magnetic effects of the
Hall currents associated
with the auroral oval
electric field
• FAC effects are not dealt
with in this talk but CAN
be observed from the
ground
AFM Apr 3 1997 red vectors are
model, black observed
Even clear magnetograms from a meridian chain need
interpretation, often done minimally or lumped into an index
(such as AE)
April 10 1997 X solid, Y dashed, Z dotted
When was the real onset time; what was the current?
Automated Forward Modelling (AFM) can help.
For meridian data, AFM adjusts current and
borders
The method is however, much more general and
includes field-aligned currents in realistic 3-d
configurations. Midlatitude perturbations can be
included as can a Dst-like parameter.
Inversion tells us more by giving simple parameters
extracted from several ground stations
April 10 1997
Ability to match input data is best near the middle of the chain
(although often not in Z due to electrojet structure)
Note: different event and stations
Independent confirmation: Comparison of optical borders from
meridian scanning photometer and inversion results for growth
phase (Feb 22 1997); also confirmed by FAST FAC detections
FAST
Two electrojet model
results are shown
superposed on 557.7
nm optical meridian
scan data from
Gillam. The growth
phase arc is poleward
of the evening sector
eastward electrojet.
Note that the method
is sensitive to initial
conditions
ALert
• The AL (or AE) index can
be misleading
• Here the AFM results are
extremely clear for a
substorm with strong
growth phase
• AL or even the inverted
current mislead as to
onset time
• AL pre-onset shows
Alaska conditions, postonset shows Churchill
Statistical Properties of Substorms
• A large-scale inversion project was undertaken for
1997 Churchill meridian data
• Baselining the data is essential yet was challenging
• Approximately 65 onsets were found to be very
robustly inverted, comparable to the number of events
in some other statistical studies
• We have studied internal relations of parameters and
not yet relation to external parameters such as solar
wind in any great detail
Frey et al. (2004) found a distribution of
Image FUV onsets skewed toward the
evening sector. Our onsets straddle
midnight. FUV onsets are due to bright
evening sector auroras – the currents are in
fact roughly symmetric around midnight.
Our results indicate a westward
electrojet at time of onset of about 0.1
MA and also show the latitude at onset
to increase with lesser current. The
former is a quantitative measure but
likely an overestimate; the latter is a
well-known result made quantitative.
Post-Onset
AVERAGE
Behaviour
The current increases rapidly (20 min)
to about 0.45 MA (an overestimate),
black dots AFM, open dots AE in MA,
curve Weimer (1993)
The electrojet poleward border rises
rapidly by about 5º (black dots AFM,
open dots Frey et al., 2004). The
equatorward border does not move.
Frey’s FUV width is wider than AFM
gives.
AE and AFM match on average and
can be cross-calibrated. Weimer’s
ate-bt+c
parametrization is very good on
average.
Sawtooth Events
• Several sawtooth events were selected from a list
supplied by Joe Borovsky from LANL injections. Our
final selection was then based on inspection of
CANOPUS magnetometer data
• It is hard to determine to what degree this sample
may be biased toward large events
• Inspection of the CANOPUS stackplots already
makes clear the large latitudinal extent of sawtooth
events. AE will be biased downward in such cases.
• Large currents make sawtooth events good for AFM
October 3-4 2000 Event in CANOPUS Churchill
Meridian X Component (quiet time in middle is day)
Oct 3-4 2000: Oct 3 is not
discussed much here but note
good Image WIC data. ACE
shows extended periods of -BZ.
Quiet time at CANOPUS likely
due to +BZ when on dayside
Inversion Results
for Oct 4 2000
• Early UT hours are quiet,
during +BZ
• Growth-like signature ca.
4 UT accompanies slow
southward turning
• Onsets are like substorms
but width of electrojet
very large
• Currents of up to 2.5 MA
are several times those of
average substorms
• Current density in
electrojets may not be
extreme: AL proxy
Inversion
Verification
Black is X
(north), dots for
model, solid for
observation
Blue is Z
(downward)
Z is small at N
edge likely due to
current wedge
width being
chosen too small.
Other Inverted
Cases
Several other cases were
inverted with generally
similar results: very wide
electrojets and very large
currents. This case of Nov. 8
2004 did not invert well due
to lack of stations far enough
south. Nevertheless there are
indications of currents of 7
MA, comparable to the largest
ever seen to cross a meridian
(Hallowe’en storm 2003).
This is one of few cases found
with good ground-based
optical data.
IMF Properties – already shown to be essential to understanding
Nov 3-4 2000 storm and here examined with a large sample
(quartiles shown)
Dst
-50
-100
-150
20
Epoch Time (days)
Bmag
15
10
5
0
10
0
-50
Dst
0
IMF Properties during Sawtooth Storms
-100
-150
20
10
5
0
10
Epoch Time (days)
0
Bz
Bz
-10
sigBz
3
2
1
0
5
Epoch Time (days)
0
-4
-3
-2
-1
0
1
Time (days)
2
3
4
5
-20
4
Epoch Time (days)
3
sigBz
B and BZ
larger but
not greatly
so
Epoch Time (days)
2
1
0
5
Ey
-20
4
Ey
Epoch Time (days)
0
-10
-5
-5
Epoch Time (days)
15
Bmag
Dst roughly
twice as
large in
sawtooth
storms
IMF Properties during All Storms
Epoch Time (days)
0
-5
-5
-4
-3
-2
-1
0
1
Time (days)
2
3
4
5
Conclusions
• Automated Forward Modeling (AFM) is a new technique for
inverting currents in near-Earth space and especially effective
on meridian chains
• Electrojet border behaviour mirrors that of optical data
• Quantitative aspects of substorms include peak currents of
about 0.45 MA cross-meridian and latitudinal extent (width) of
about 7º
• Sawtooth events have currents about ten times this and widths
of twice or more
• The quantitative parameters of AFM would be useful for
correlation with IMF but this has been done only to a very
limited extent so far
• Nevertheless it is clear that Sawtooth events are driven by
extreme solar wind conditions
Acknowledgements
• FMI – Image magnetometer array data
• CDAWeb; S. Mende (Image FUV), N. Ness (ACE)
• Canadian Space Agency and University of Alberta –
CANOPUS/CARISMA data
• Collaborators including Kanji Hayashi, Brian Martin,
UCLA magnetometer team, and James Weygand
• This work primarily funded by Canada Research
Chairs and Canada Foundation for Innovation