1. Johns Hopkins Applied Physics Laboratory, Laurel MD, USA
EGU 2016
Solar and Interplanetary Causes of Extremely Intense Substorms During Superstorms
Bruce T. Tsurutani
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA
Rajkumar Hajra
Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E), CNRS, Orléans, France
Ezequiel Echer
Instituto Nacional de Pesquisas Espaciais, Sao Jose dos Campos, SP, Brazil
Jesper Gjerloev
Johns Hopkins Applied Physics Laboratory, Laurel MD, USA

2. Supersubstorm/SSS events
Extremely intense substorms with SML peak intensity < nT
Identified by the negative bay development in the westward electrojet index – SML
SML is SuperMAG AL index
SuperMAG consists of > 300 ground-based magnetometers including not only the standard AL sites but also higher and lower latitude stations

3. Why Study Extremely Intense Substorms?
It is found that supersubstorms (SSSs) are different than regular substorms during magnetic storms. SSSs can be externally triggered.
Supersubstorms may cause power outages during storm intervals.

4. Supersubstorm onsets Two shocks 24 November 2001 What is this?
Is this a ICME
Loop?
Tsurutani et al., AnGeo Comm., 33, , 2015

5. The Magnetic Perturbations at the Ground During the Two Supersubstorms
Tsurutani et al., AnGeo Comm., 33, , 2015

6. SSS seasonal dependence
Prominent annual variation
Not a single year with two seasonal peaks
“Semiannual” variation – artifact resulting from superposition of years with a dominant annual variation with opposite equinoctial maxima

7. Solar cycle phase
Years
# SSSs
SSS/year
Ascending
, , 9
1.5
Maximum
1981, , 22
3.1
Descending
, , 34
3.8
Minimum
, ,
0.9
Total 74
2.3

8. SSS event Relationship to Magnetic Storms
Geomagnetic condition
# SSSs
% of SSS
Magnetic storm main phase 64
86.5
Magnetic storm recovery phase 7
9.5
Quiet time 3 4
SSSs do not coincide with geomagnetic storm peaks

9. No strong correlation of SSSs with geomagnetic storm intensity

10. Superposed Epoch Analyses
Enhanced solar wind density and ram pressure leading to SSS
SSS peak

11. Precursor IMF Bsouth
Enhanced dayside reconnection rate
SSS peak

12. Interplanetary “precursor” for SSS events
Peak VswBs at 1 h prior to SML peak is the most pertinent interplanetary precursor of the SSS strength

13. Average solar wind/interplanetary conditions during SSS events

14. Average (median) energy budget during 1 h prior to the peak SSS intensity – 24 events during SC 23 ( )
Total energy (1016 J)
Magnetosphere Eɛ
4.0±2.9 (3.2)
Joule heating EJ
2.2±1.5 (1.3)
Ring current (ER)
0.09±0.07 (0.08)
Energy dissipation rate (%)
Joule heating EJ/Eɛ
46.4±24.2 (46.1)
Ring current ER/Eɛ
3.6±2.3 (3.8)
Joule heating is the dominating energy dissipation mechanism during SSS events

15. Main results
We identified 74 SSS events occurring from 1981 through 2012 using measurements from the SuperMAG network of ground-based magnetometers.
The SSS events may occur during all phases of the solar cycle, with the highest occurrence rate in the descending phase.
The number of SSS events showed an annual variation, with maximum altering between spring equinox in SC 22 and fall equinox in SC 23. Possible ionospheric control on the substorm triggering was suggested.
Overwhelm majority (86.5%) occurred in the main phase of geomagnetic storms. About 9.5% occurred in the storm recovery phase and 4% in geomagnetically quiet.
On average, ~46.4% of the magnetospheric input energy goes into Joule heating and only ~3.6% into ring current for the SSS events.
The precursor IMF southward fields were provided by the magnetic clouds in 46% of the times and sheath fields in 54% of the times.

16. Thanks Form more details, contact:
Bruce T. Tsurutani, JPL, NASA
Rajkumar Hajra, LPC2E, CNRS, France