1. Drivers and Solar Cycles Trends of Extreme Space Weather Disturbances
Emilia Kilpua
University of Helsinki
@EmiliaKilpua)
What is the definition of extreme. Remind first that space weather is large context. There are dis rc, auroral regions and van allen betls that are not always decoupled. In addition, some storms might cause space weather hazards like power outages and satellite malfunctioning that is not always in accordande with

2. Key solar wind parameters
magnetic field magnitude and direction
dawn-dusk (driving) electric field
speed
dynamic pressure
density
Alfvén Mach number (bow shock compression, how plasma flows around the magnetopause, saturation)
First, what are solar wind conditions needed to drive the largest storms? Well established that the key requirement is Bs.
level of turbulence

3. Solar wind Alfvèn Mach number and saturation
highest MA
polar cap potential
Clear that response increases with increasing driving electric field. So fied has the major importance.
lowest MA
solar wind driving electric field (mV/m)
Myllys et al., 2016

4. Effect of dynamic pressure
polar cap potential
dynamic pressure (nPa)
Observed that dynamic pressure affects very clearly to saturation. When Pdyn is high saturation does not occur, but
solar wind driving electric field (mV/m)
Myllys et al., in preparation

5. Coronal Mass Ejections (CMEs) drive nearly all intense geospace storms
Drivers of geospace storms four Solar Cycles ( )
Storm intensity: Kp (NOAA scales)
G G G G G5
Richardson and Cane, 2012
Then what are solar wind structures driving the most extreme storms? It is clear if we think disturbances in the geomagnetic field, they are CMEs.

6. Two principal CME structures
Flux Rope
Sheath
Shock

7. Only flux ropes and sheaths have strong enough fields to drive extreme storms
Normalized occurrence distribution for events in
Flux rope
Sheath
CIR
Fast stream

8. Sheaths have clearly higher Pdyn and Alfvén Mach number than flux ropes
CIR
Fast stream

9. Which CMEs drive the most extreme storms?
intrinsic CME properties
modifications during the IP journey
pre-conditioning of the heliosphere
“PERFECT STORM SCENARIO”
(Liu et al., 2014; 2015)

10. Nov 20, 2003 (Strongest Dst storm of SC 23) driven by “isolated” CME
Example of an isolated CME. Not super-fast. Drove the largest Dst storm of solar cycle 23. But very rarely this storm is mentioned. Kp did not reach the highest value possible. No significant geospace hazards as far as I’m concernedf.

11. November 2001 storm and Halloween storms impulsive energy injection
V [km/s]
Pdyn [nPa]
> 60 nPa
Short main phase. High Pdyn, strong coincident southward IMF for a few hours enough!!!
BZ [nT]
Dst [nT]
Balan et al., JGR, 2014

12. CME-CME interactions
Liu et al., Nat Comm., 2014
prevents CME expansion  CME maintains high fields and speeds
turbulent and compressed regions (high Pdyn and MA)
shock merging (e.g., Lugaz et al., 2015) and shock compression of the preceding ICME (e.g., Lugaz 2016: Statistical analysis based on the Heliospheric Shock Database ipshocks.fi)
Mention that super-CMEs does not necessarily follow solar cycle.
Lugaz 2016: About half of the geoeffective sheaths are related to interacting ICME.

13. Interactions with the other large-scale solar wind structures (CIR, fast stream, HPS)
sheath  FR
B [nT]
“CME Sandwich” (March 17, 2015)
compresses sheath and FR
stronger storm than expected
BZ [nT]
Kataoka et al., GRL, 2015
V [km/s]
Fast stream compression enhances geoeffectivity of flux ropes with north-south rotating fields (see e.g., Kilpua et al., Ann. Geophys and Fenrich & Luhmann, GRL, 1998)
fast
stream
n [nPa]
HPS
Dst
4 days

14. Preconditioning of the heliosphere
low density
previous weaker CME
Liu et al., 2014
low density  minimal drag force  CME maintains high speeds
 large Ey and stronger field line draping in the sheath

15. Extreme Substorms
Extreme substorms  strong ionospheric currents without significant ring current (Dst) storm (Huttunen and Koskinen, 2004; Tsurutani et al., 2015)
During CME sheath regions (Huttunen and Koskinen 2004)
Triggered by pressure pulses?
(Tsurutani et al., 2015)
Strongest GICs occur during CME sheath regions (Huttunen et al., Space Weather, 2008)

16. Strong Van Allen belt enhancements: Fast streams are important
Kilpua et al., GRL,

17. Occurrence of extreme storms
Carrington storm
July 22 “super-CME”

18. Correlation between solar cycle size and storm occurrence
(14 cycles, SCs 11-23)
max SSN
Pearson correlation coefficients, confidence intervals calculated with the bootstrap method. From Kilpua et al., APJ, 2015
95%
confidence
mean SSN
 correlation between the storm occurrence and the solar cycle strength decreases with increasing storm magnitude

19. Solar cycle phases and storms
Kilpua et al., APJ 2015

20. Solar cycle phases and storms
Weaker storms occur predominantly in the declining phase
(see also e.g., Ruzmaikin and Feynmann, 2001)  poloidal field
 coronal holes  CIRs and fast streams
Stronger storms clustered close to maximum time  toroidal field  active regions  coronal mass ejection (also probability for CME-CME interactions increases)

21. Summary Extreme storms: strong (+ a few hours) BS, high V, Pdyn and MA
 CME sheaths and interacting CMEs
“ Perfect Storm Scenario”: Strong and fast CME(s), favorable modifications during IP journey, pre-conditioning
Extreme space weather may occur also during weaker solar cycles (probability of the next Carrington storm? Riley 2012: 10-yr occurrence probability 12%)
 smaller-scale dynamo and turbulent fields?
Stronger storms occur more near solar maximum