Transcript Cross-Meridian Currents in Substorms and Sawtooth Events

Ground and Space-based
Magnetic Fields during a THEMIS
Double-onset Substorm
M. Connors1, C. T. Russell2, I. Voronkov1, E.
Donovan3, V. Angelopoulos2, S. B. Mende4, K.H. Glassmeier5, K. Hayashi6, E. Spanswick3, B.
Jackel3, H. Frey4, J. McFadden4
(1Athabasca U, 2UCLA, 3U. Calgary, 4UC
Berkeley, 5TU Braunschweig, 6U. Tokyo)
Cluster 15 Workshop, Tenerife
March 2008
Image: Mikko Syrjäsuo
Abstract (Main Points)
In support of THEMIS, ground-based auroral optical and magnetic detection in North
America has recently been greatly improved.
Magnetic data is now available from enough locations to support quantitative studies,
including techniques based on forward modeling.
We use Automated Regional Modeling (ARM) to specify the locations and strength of
electrojets and field-aligned currents (FAC).
On March 13, 2007, THEMIS was conjugate to central North America, clear weather
prevailed, and a double onset (5:08 and 5:36 UT) substorm took place.
Spacecraft data support the use of the Tsyganenko 89 tail model during periods near the
onsets.
The ground perturbations are well represented by a 3-D substorm current wedge
system.
Mapping changes can be studied with a combination of ground and spacecraft data.
A surge-like current system permits very accurate verification of the mapping of the
second onset, and its current is detected at the spacecraft.
1. Athabasca University Geophysical
Observatory (AUGO)
A comprehensive observatory ideally located for THEMIS conjunctions
54.72 N, 246.7 E
CGM (2005)
62.0, 306.5
L=4.55
Founded 2002
(UCLA mag 1998)
Will be moved in
2008 due to light
encroachment
AUGO’s Instrumentation
1. UCLA Fluxgate
2. THEMIS GBO Camera
3. KEO NORSTAR Camera
Guest instruments from
STELAB:
1.
2.
3.
Multispectal ASC
including Hβ
64 Hz induction coil
proton spectrometer
2. Ground Magnetometry
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
Often the locales are less agreeable than Tenerife (Kanji Hayashi
in LaRonge, Canada, mid-October 2004). This magnetometer was
critical to this study: wide and dense placement is essential!
Paddle Prairie
Athabasca
Edmonton
Red Deer
Calgary
Lethbridge
Sites
installed
fully and
data
available (2
Hz) since
Oct 4, 2005
Inuvik 2006
Athabasca University
has assisted or runs 16
sites in Canada (white
triangles and purple
dots in Western
Canada). Most data
available through
UCLA, STEP website,
or on request. PEA and
SFV hoped for soon.
New Polaris sites on E.
Coast of Hudson Bay
were installed in 2007.
Some THEMIS GBOs not
shown.
3. Optical Facilities
Red circles show the fields of view (FOVs) of THEMIS GroundBased Observatories (GBOs). Most have imager + mag. Small
blue circles show positions of U.S. subauroral magnetometers
(GEONS) whose data is available at themis.ssl.berkeley.edu
4. Data Interpretation for Ground-based
Magnetometers: 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
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 CAN be
observed from the
ground
AFM Apr 3 1997 red vectors are
model, black observed
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
Event Study March 13 2007
• A pseudo-breakup at 05:08 was followed by a
full onset at 05:36 on March 13, 2007
• THEMIS was in its initial string-of-pearls orbit
with all FGM turned on but only some plasma
sensing on THEMIS-A
• Four THEMIS spacecraft were very well
placed with respect to the activity
• Ground-based networks were also well placed
March 13, 2007
~0500 UT
Cluster
THEMIS
Cluster FGM
Generally positive BY
Moderately disturbed solar wind near ~5 UT onset time
Very stretched!
T89 Kp>6
GSM XZ
E
05:08 UT
A
B
D
C
GSM XY
THEMIS in early
orbit configuration
as “string of
pearls” less than
one month after
launch
66 seconds of imaging: every second image shown
Excellent conjugacy: T89 Kp 3to 5+ shown for E,A,B,D
Hbeta Proton aurora
630 nm Redline
Proton precipitation was intense in this event as shown by MSP, also
imaged by STELAB OMTI Imager at Athabasca (not shown). Onset arc
was poleward of the proton aurora.
Arc that brightens
Inner Edge of Plasma Sheet
Quantitative study of the onset arc
m~120
Brightness in Ewogram Bin
Brightness on Arc at Fixed MLONs
At ALL longitudes, the preonset arc faded measurably
before onset and then a
brightening took place in the
same region
0508
0536
THEMIS D
Bz
THEMIS B
Bx
THEMIS A
1e7,8
ESA Electrons
ESA Ions
THEMIS E
1e3,4
THEMIS superposed magnetic Bx and TH A low energy ions
Dipolarization seems to be plasma sheet recovery
THEMIS superposed magnetic Bx and TH A electrons
First onset at 05:08 was
marked by plasma sheet
recovery
Second onset at 05:36
showed a plasma
dropout and a strong Y
component (often a
field-aligned current
signature)
The optical data for
onset #2 is not as good,
what does magnetic
data tell us?
At 05:33, the pseudo-breakup
is fully developed. Its
perturbations are well
matched by a substorm
current wedge. Black =
observation. Red = model.
Pseudobreakup/Onset Comparison at
Maximum extent
Pseudobreakup
Onset
Observed
Model
Mapping to Space
One can in principle map precipitation regions to space
and hope to hit a spacecraft showing related particle
fluxes.
One can in principle map field-aligned currents derived
from regional modelling to space and see broader
effects.
However we have seen that diamagnetic effects can be
large and near the plasma sheet, dominant.
Need to examine discreet, recognizable features.
• To what
degree can we
do mapping?
• Lines in
panels a,b,e,f
show T89 B
levels for
different
stretching
• Also note in
panels g and h
the large Y
signature at
05:36 onset
FAC
The Y signature at
Fort Churchill can
originate from a
northward
ionospheric current
joining FAC sheets
to north and south.
The eastward
perturbation at the
THEMIS spacecraft
can arise from a FAC
sheet in space
passing over the
spacecraft.
T89 Model Study
The Y
perturbation
could arise
from a current
sheet moving
inward with its
foot moving
equatorward at
the time of
dipolarization.
This would
‘move’ the
spacecraft in
between two
current sheets.
There is some evidence of the equatorward motion of auroras at
onset;
in addition the sequence of Y perturbation at the E and A
spacecraft suggests inward motion of the field line. This suggests
that the modelling of this distinctive feature on ground and at the
spacecraft is basically correct, and that T89 with an adjusted
activity factor maps correctly.
Conclusions
• Ground enhancements for THEMIS have
greatly improved auroral monitoring, both
optical and magnetic, in North America
• For Cluster, optical does not benefit as much
as for THEMIS due to “NH summer” orbit
• We can quantitatively deal with magnetic data
• We can map into the tail with some confidence
based on studies of distinctive features
Acknowledgements
• Mark Moldwin, Andrei Runov (UCLA)
• Canadian Space Agency and University of Alberta for
CARISMA data, accessed through cssdp.ca; NRCan
for CANMOS data.
• This work funded by Canada Research Chairs,
Canada Foundation for Innovation, AU, and NSERC
• FMI IMAGE data used in making Slide 10
• A. Balogh, ICSTM, for Cluster FGM via CDAWeb
• N. Tsyganenko for availability of model software