Adjustable magnetospheric event-oriented magnetic field models

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Adjustable magnetospheric event-oriented magnetic
field models

• N. Yu. Ganushkina (1), M. V. Kubyshkina (2), T.
  I. Pulkkinen (1)
• (1) Finnish Meteorological Institute, Helsinki,
  Finland
• (2) University of St. Petersburg, Institute of
  Physics, St. Petersburg, Russia)

Event-oriented magnetospheric magnetic field
modelling - An accurate representation of
magnetospheric configuration for a specific event
(Ganushkina et al., JGR, 2002 Ganushkina
et al., AnnGeo, 2004)
ICESTAR Heliosphere Impact on Geospace,
February 5-9, 2007, Helsinki, Finland
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Magnetospheric magnetic field modelling
approaches Global and event-oriented
Global magnetospheric magnetic field models
- Most widely used (Tsyganenko 1987, 1989,
Tsyganenko 1995) Good representation of
average magnetospheric configuration, fine
structure of magnetic field during substorms and
large magnetic field changes during storms were
not accounted for. - Storm-time models
(Alexeev et al. 1996, Tsyganenko 2002) model
parameters for current systems fitted to entire
data set, model magnetic field defined by
assumed dependence on input parameters. Event-ori
ented magnetospheric magnetic field models
- An accurate representation of magnetospheric
configuration is of key importance for a
specific event ADJUSTABLE! - Study the
evolution of different current systems during
different storms and their relative
contribution to Dst Ganushkina et al., JGR,
2002 Ganushkina et al., AnnGeo, 2004
ICESTAR Heliosphere Impact on Geospace,
February 5-9, 2007, Helsinki, Finland
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Why do we need Event-Oriented Models?
Difference in mapping Concept of magnetic
conjugacy, crucial in interhemispheric comparisons
Magnetotail bending during substorm on Sep 14,
2004 10 deg to Sun-Earth line Large
interhemispheric asymmetry
Event-oriented model may provide the necessary
accuracy
ICESTAR Heliosphere Impact on Geospace,
February 5-9, 2007, Helsinki, Finland
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How to produce event-oriented model?

• Choice of existing magnetospheric magnetic field
  model to modify
• Any model easy to modify, simplest solution
• Tsyganenko T89, Kp 4 (for storm events)
• ? Replacing of T89 ring current with asymmetric
  bean-shaped ring current
• ? Varying the global intensity of T89 tail
  current
• ? Addition of thin current sheet
• ? Scaling of magnetopause currents
• Determining free parameters for each current
  system
• Collecting input data
• ? All available magnetic field measurements
  during the modelled event in magnetosphere
• SYM-H measurements on the ground
• Varying free parameters, we find the set of
  parameters that gives the best fit between model
  and all available in-situ field observations

ICESTAR Heliosphere Impact on Geospace,
February 5-9, 2007, Helsinki, Finland
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Additional measurements for better model accuracy

• Only several data points available.
• Best model input, if satellites are at different
  locations
• Measurements of point magnetic fields can be
  represented by different ways in models
• Require additional measurements!
• They can be
• ? Isotropic boundaries
• ? B-direction measured by LANL
• ? Pressure value measured at magnetospheric
  spacecraft

ICESTAR Heliosphere Impact on Geospace,
February 5-9, 2007, Helsinki, Finland
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Additional measurements for better model accuracy
(1)
1. Isotropic boundaries (IB) ? available almost
always, ? from IB to obtain curvature radius of
magnetic field line, ? parameter characterising
the field line, not the point measurement, ? in
the most variable region of transition between
dipole and stretched, tail-looking field
lines

ICESTAR Heliosphere Impact on Geospace,
February 5-9, 2007, Helsinki, Finland
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Additional measurements for better model accuracy
(2)

• 2. ?B angle measured by LANL spacecraft
• angle between distribution symmetry axis and spin
  axis of the spacecraft
• determines the B-direction
• symmetry axis of electron
• temperature matrix T aligned with
• local magnetic field
• ? Available if Tpar/Tperp far from 1,
• for anisotropic plasma

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Ring current representation (1)
Symmetric ring current eastward westward
Asymmetric ring current partial closing
Region 2 field-aligned currents
Local time asymmetry gives rise to field-aligned
currents (calculated numerically)
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Ring current representation (2)
Free parameters
mean radius max current density width of
J distr. anisotropy index constant duskward
shift angle for partial RC
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Addition of a new tail current sheet (1)
Global changes intensification of the tail
current (T89) as a whole with amplification
factor (1ATS). Local changesAdding a new thin
tail current sheet.
Two vector potentials, similar to T89
With truncation factors
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Addition of a new tail current sheet (2)
Subtracting
gives a thin current sheet with finite x-scale,
zero outside 25 Re.
Free parameters
thin current sheet intensity X0 - location of
steepest decrease of W(x,y) D0 -
half-thickness of current sheet
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Storm-time magnetic field modelling
Magnetopause currents
Scaling factor AMP?3 for the magnetic field of
Chapman-Ferraro currents at magnetopause,
determined from solar wind pressure variations
PSW
Parameter RT , characteristic scale size of
magnetotail, defined by solar wind parameters
30 RE - parameter RT in T89 Kp4, ZT, Shue -
magnetopause position given by Shue et al 1998
model dependent on PSW and IMF Bz
at X - 20 RE , Y 0, ZT, T89 - magnetopause
position given by T89 Kp 4 model
at X - 20 RE , Y 0.
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February 5-9, 2007, Helsinki, Finland
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Event-oriented magnetospheric magnetic field
modelling
Baseline model Tsyganenko T89 Kp4
? 0.8 A 1
Dst0 40 nT
Varying free parameters, we find the set of
parameters that gives the best fit between model
and all available in-situ field observations,
for example, by GOES, Polar, CLUSTER, LANL
satellites and Dst (SYM-index) measurements.
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Storm on Oct 21-22, 2001
Magnetic cloud arrival Oct 21, 2001, 1645
UT Cloud sheath region driver for storm main
phase At cloud leading edge IMF Bz 0 At cloud
trailing edge IMF Bz lt 0 Wind, ACE measurements
almost identical despite large distance between
s/c
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Storm on Oct 21-22, 2001 Magnetotail behavior
Strong sawtooth-like injections at
geosynchronous Dipolarization events
simultaneous at GOES and POLAR and with
injections
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October 22, 1000-2000 UT Magnetic field during
saw-tooth event
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Event-oriented magnetospheric magnetic field
modelling Advantages and disadvantages
Allows to play easily with current systems,
their location and parameters, to get better
agreement with data Good representation
of smaller scale variations in magnetic field
substorm-associated, saw-tooth events etc.
Good representation of local magnetic field
variations (observations at a specific
satellite) To get detailed magnetic field
variations for a specific event, time period,
magnetospheric region ? use event-oriented
model - Only for specific events, when
magnetic field data are available at least at 3
satellites in different magnetospheric
regions - Requires some work for
determination of model parameters To get
magnetic field quickly, for several storms, over
a large region in magnetosphere, good in average
? use T01s model
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Usefulness of isotropic boundaries concept for
event-oriented modelling and event-oriented
modelling for interhemispheric studies
? Isotropic boundary position depends only on
magnetotail magnetic field for a given
particle. ? Isotropic boundary provides a method
of remote-sensing of magnetospheric magnetic
field by low-latitude spacecraft
measurements. ? Isotropic boundary is an
additional input into event-oriented model
construction. ? Isotropic boundary can be used
as an indirect indicator of the accuracy of
magnetospheric magnetic field models. ? Accurate
magnetic field model is highly needed for
interhemispheric studies.
ICESTAR Heliosphere Impact on Geospace,
February 5-9, 2007, Helsinki, Finland