Statistical characterisation of the growth and spatial scales of the substorm onset arc
N.M.E.
1
Kalmoni ,
I.J.
1
Rae ,
2
Watt ,
C.E.J.
K. R.
3
Murphy ,
C.
1
Forsyth ,
b)#
C. J.
nadine.kalmoni.13@ucl.ac.uk
1
Owen
1Mullard
Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, RH5 6NT, UK
2Department of Meteorology, University of Reading, Reading, UK
3NASA Goddard Space Flight Center, Greenbelt, ML, USA
a)#
e)#
b)#
c
x103 Counts
es
rate
d
duration
04:58:18
04:58:30
05:00:00
Auroral beads along onset arc on 2011-10-02 observed by Gillam ASI
•  An auroral substorm can be observed in the
ionosphere as a brightening and rapid poleward
expansion of a quiet auroral arc.
•  Auroral beads form azimuthally along the onset arc
in the minutes leading to substorm onset.
•  Auroral arcs are usually aligned with constant
geomagnetic latitude.
•  The temporal and spatial evolution of the beads can
be characterised before they expand poleward.
Probability
4.InstabilityComparison
a)#
Linear fitting
algorithm to define:
•  Start time
•  End time
•  Growth rate
•  Growth duration
a)  Keogram- slice through ASI field of view perpendicular to arc
c)#
orientation. Observe auroral brightening and poleward
expansion.
b)  E-W Keogram. Slice along the auroral arc to track auroral
intensity as a function of longitude and time. Can see
eastward propagation of beads.
d)#c)  Power spectral density of auroral arc. Spatial FFT of E-W
keogram to show spatial frequency of beads. Higher power is
a)#
localised at low klon,i .
d)  Duration of exponential growth of individual spatial scales
detected by linear-fitting algorithm.
e)  Growth rate vs klon
Conclusions
klon,m ( x 10-6 m-1)
Some wavenumbers
exhibit:
e)#
•  Higher growth
rates
•  Higher power
•  Longer duration
of growth
CFCI:%Near*Earth%
CFCI:%Mid*tail%
Growth rate (s-1)
f)#
e)#
d)#
04:57:57
UT
klon,i ( x 10-4 m-1)
klon,m ( x 10-6 m-1)
c) Normalised growth rates as a function of
magnetospheric k
•  Spatial scales mapped to the plasma sheet
•  Normalised to the maximum growth rate for each
event.
•  There is a peak in growth rates at
klon,m = 2.5 - 3.75 x 10-6 m-1.
•  Rates are larger to 98% certainty.
•  Most unstable spatial scales corresponds to
λ = 1700 – 2500 km in the magnetosphere.
PSD (Int2 mHz-1)
klon,i ( x 10-4 m-1)
11)) -1)
log(PSD)))(Int
log(PSD)
(Int22)mHz
mHz
c)#
klon,m ( x 10-6 m-1)
Growth rate (s-1)
d)#
klon,i ( x 10-4 m-1)
Image courtesy, NASA
c)#
Earth’s magnetosphere
04:57:27
Growth rate (normalised)
x103 Counts
Geomagnetic
Latitude (°)
b)#
Aurora
map to the
plasma
sheet
04:57:39
c)#
a)#
1.Introduc0on
b)#
17 auroral substorm and pseudo breakup events.
Spectral analysis of auroral beads observed by Gillam ASI on
2011-10-02.
Geomagnetic
Longitude (°)
•  Signatures of waves are frequently observed in
substorm aurora.
•  Auroral brightness grows exponentially across a
wide range of spatial scales.
•  The most unstable spatial scales with the highest
growth rates agree spatially and temporally with
those predicted by the Shear Flow Ballooning
Instability [Voronkov et al. 1997].
a)#
3.Sta0s0cs
2.CaseStudy
Growth rate (normalised)
Keypoints:
CFCI:%Near*Earth%
CFCI:%Mid*tail%
Crossb)#
Field Current
Instability.
•  Growth rates peak at
lower klon,m than
observed.
•  Growth rates for CFCI
peak at 0.28 – 0.4 s-1.
•  CFCI cannot explain
observations.
- Less than
20 points per bin
b)#
Shear-Flow Ballooning
Instability.
•  SFBI predicts maximum
growth rates of 0.2 s-1.
•  Peak at the same
spatial scales as
observed for a shear
flow width of 600-700
km.
5.Conclusions
We have demonstrated that:
1.  Exponential growth of specific spatial scales of the
substorm onset arc provides evidence that auroral
beads are driven by an instability.
2.  The most unstable azimuthal wavelength of
magnetospheric instability is at least λ ≈ 1700 - 2500
km.
3.  The most unstable spatial scales have growth rates
ranging over one order of magnitude from
0.03 - 0.3 s-1.
4.  The SFBI with a localised shear flow region of
~650 km and magnetic field strength of 40 nT can
explain our observed results.
Please see Kalmoni et al. [2015] for more information
Defini0ons&Abbrevia0ons:
ASI- All Sky Imager
Keogram- Auroral intensity along a North-South slice
through the field of view of an ASI vs. time
E-W Keogram- Along-arc auroral intensity vs. time
klon, i - longitudinal wavenumber measured in the
ionosphere
klon, m - longitudinal wavenumber measured in the
magnetosphere
γ- Growth rate
SFBI- Shear Flow Ballooning Instability [Voronkov et al.
1997]
CFCI- Cross-Field Current Instability [Lui 2004]
Acknowledgements&
References
We acknowledge NASA and V. Angelopoulos for use of data from the
THEMIS Mission. Specifically S. Mende and E. Donovan for use of the
ASI data and the CSA for logistical support in fielding and data retrieval
from the GBO stations.
Kalmoni, N. M. E., I. J. Rae,
C. E. J. Watt, K. R. Murphy, C. Forsyth, and C. J. Owen (2015),
Statistical characterization of the growth and spatial scales of the
substorm onset arc, J. Geophys. Res. Space Physics.
Lui, A. (2004), Potential plasma instabilities for substorm expansion
onsets, Space Sci. Rev.
Voronkov, I., R. Rankin, P. Frycz, V. T. Tikhonchuk, and J. C. Samson
(1997), Coupling of shear flow and pressure gradient instabilities,
J. Geophys. Res.