Constraining Substorm Onset from Space- and Ground-Based Observations

Constraining Substorm Onset from Space- and Ground-Based

Observations

Department of Space & Climate Physics

Mullard Space Science Laboratory

A. P. Walsh1, A. N. Fazakerley1, I. J Rae2, I. R. Mann2,, K. R. Murphy2, M. Volwerk3

!7th Cluster Workshop, University of Uppsala, 15th May 2009

1. Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Surrey, UK

2. Department of Physics, University of Alberta, Edmonton, Alberta, Canada

3. Space Research Institute, Austrian Academy of Sciences, Graz, Austria

Introduction – 2 Substorm Models

• With this event we probably can’t work out which of these is right.

• Will concentrate on near-Earth features of the substorm, particularly the formation of the Substorm Current Wedge.


Sketch : UCLA

Near-Earth Observational Signatures


• Include, in the ionosphere:– Geomagnetic bays (= ionospheric current)– Pi1 & Pi2 pulsations– Auroral emissions

• Include, in the magnetotail:– Dipolarisation of the magnetic field– Formation of Substorm Current Wedge (FACs)– Injection of energetic particles near geosynchronous

orbit

1 October 2005 Event: auroral imaging from space


• IMAGE-FUV data (southern hemisph.)

• Images every 2 mins; time stamp is at the end of the 2 min interval during which image is collected

• Frey list onset at 04:21:20 (Frey & Mende, 2006) 04:19-21

• Site of auroral expansion seems to be a few deg’s east; first brightening at 04:21-23; expansion starts 04:23-2504:21:20 UT

04:23:26 UT 04:25:31 UT 04:27:36 UT

1 October 2005 Event : ground magnetometers


• Canadian magnetometers (northern hemisphere)

• The onset of Pi1/2 pulsations can be timed and localised using a wavelet transform (Milling et al., 2007, Murphy et al., 2009).

• Experience shows that wave power arrives a few minutes before auroral intensification, co-located within 1 or 2 degrees

• Onset of Pi1/2 activity at 04:16UT at GILL (Cf 04:19-21 at IMAGE)


• Experience shows that wave power arrives a few minutes before auroral intensification, co-located within 1 or 2 degrees

• Onset of Pi1/2 activity at 04:16UT at GILL (Cf 04:19-21 at IMAGE)

• A Pi1/2 onset/intensification is seen centred between ISLL & PBQ which starts no later than 04:22:24 (Cf 04:21-23 at IMAGE)

• Locations are consistent with the two separate IMAGE onsets.



• Compare magnetometer time history for GILL and PBQ

• GILL shows a weaker bay (weaker ionospheric current overhead) consistent with main substorm ionospheric current being located east of GILL, nearer PBQ, consistent with IMAGE evidence that the eastward event is associated with stronger activity

GILL

PBQ


1 October 2005 Event : Near Earth Spacecraft


• Blue radial line represents longitude of Churchill Line magnetometers used here.


• GOES12 & Double Star TC-2 are close to magnetically conjugate with the 2nd IMAGE onset, apparently.

• LANL90 & LANL94 bracket the onset region

• LANL spacecraft see evidence of a particle injection initiated near TC-2 /GOES12

GSE XY positions of Near-Earth spacecraft

TC-2 at (6.0, -2.2, 1.0) RE GSE

1 October 2005 Event : Near Earth Spacecraft

Double Star TC-2 DataMullard Space Science Laboratory

• TC-2 in Northern hemi-sphere, in plasma sheet at time of 2nd IMAGE onset.

• Large deflections in dBY,

pulsations in dBZ and enhanced T|| / T┴ observed around the time of onset.

(i.e. FACs and Pi1/2s)PA0

PA90

PA180

T// /

T


• TC-2 in Northern hemi-sphere, in plasma sheet at time of 2nd IMAGE onset.

• Large deflections in dBY,

pulsations in dBZ and enhanced T|| / T┴ observed around the time of onset. (i.e. FACs and Pi1/2s)

• Sees dipolarisation and more energetic electron population ~4 mins afterwards.

PA0

PA90

PA180

T// /

T

TC-2 & GOES12 Data


• GOES-12 sees deflections in BY, but in the opposite sense to TC-2. Timing matches IMAGE eastern onset.

• Consistent with the spacecraft begin located either side of the Western FAC of the Substorm Current Wedge, as it formed.

• Pi1/2 pulsations at TC-2 start at same time as dBY deflections. (waiting for higher time resolution GOES data)

• GOES12 observes the dipolarisation before TC-2. What does this say about SCW evolution?

j (SCW)

Earth

IMAGE Eastern Onset


• Pi1/2 oscillations start at 04:20:20 UT. After GILL, but fits with PBQ.

• A lower energy population of field-aligned electrons are present during Pi1/2 pulsations. These disappear when the dipolarisation starts.

• Earthward flow is seen with dipolarisation, but not as early as Pi2 waves.

• Pulsations have frequencies of ~50 to 100mHz. How are these related to electrons, FAC appearance and IMAGE onset?

PA90

PA180

Par-Perp

PA0

Conclusions


• Ionosphere: Pi1/2 pulsations are detected on the ground at virtually the same time as, and at a location consistent with, the main auroral substorm onset.

• Magnetosphere: Similar pulsations at TC-2 start at the same time as the spacecraft sees evidence of the appearance of FACs which we suggest are part of the Substorm Current Wedge.

• M-I link?: Timing evidence consistent with suggestion that these signatures are connected; spacecraft and ground observations magnetically linked? Can we use the data to investigate the onset of cross-tail current diversion?

• Pi1/2 origin: Possibility that the spacecraft and ground pulsations are a signature of local plasma flow is not supported in this case.

Future Work


• Characterise the TC-2 pulsations properly, compare with candidates for current disruption:– EMIC drift (e.g. Yoon et al., 2009)– Kinetic Ballooning (e.g. Liu et al., 2008)

• What is the role of the electrons?• Put into the wider context of the substorm (Cluster

data – Cluster was not in the central plasmasheet at this events onset time…).

LANL Energetic ParticlesMullard Space Science Laboratory

LANL Energetic ParticlesMullard Space Science Laboratory

e-

p+

LANL90

LANL94

Constraining Substorm Onset from Space- and Ground-Based Observations

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