MHD simulation of Kronian
magnetosphere with highresolution solar wind databy Cassini
Keiichiro FUKAZAWA1, 2, 3,Raymond J. WALKER4, Stefan ERIKSSON5
1. Research Institute for Information Technology, Kyushu University
2. International Center for Space Weather Science and Education, Kyushu
University
3. JST, CREST
4. IGPP/UCLA
5. Laboratory for Atmospheric and Space Physics,
University of Colorado at Boulder
April 26, 2012

Context
Simulation of the Kronian magnetosphere using the solar
wind data observed by Cassini
Introduction• Previous simulation results of turbulent magnetosphere• Motivation

Simulation Setting• Solar wind condition from Cassini

Simulation Results• Initial simulation results

Summary1

Introduction | Turbulent magnetospheric convection
In our early simulation
results, the vortex andturbulent convection
are appeared in theKronian magnetospherefor three IMF (no /southward / northward)
cases from the early
simulation results.

Fig.1. The temperature and flow vectors in the equatorial plane for the simulations
with no IMF (a), southward (b) and northward IMF (c) [Fukazawa et al., 2007a]2

Introduction | Vortex at dawn in the observations by Cassini
2006_FGM_KSO_1M
6.00

BX[nT]

4.00

These variations
may indicate theconfiguration ofvortex.

2.00
0.00
-2.00
-4.00
6.00

BY[nT]

4.00
2.00
0.00
-2.00
-4.00
2.00

BZ[nT]

0.00
-2.00
-4.00

BTOTAL[nT]

-6.00
8.00
6.00
4.00
2.00

0.00
DOY:7619:00
2006-Mar-17

22:00

01:00

04:00

07:00

10:00

13:00

X -2.30
Y -26.72
Z
8.80
LOCAL HOUR
5.70

-1.84
-26.04
8.72
5.70

-1.38
-25.34
8.62
5.80

-0.91
-24.61
8.52
5.90

-0.45
-23.85
8.41
5.90

0.01
-23.078.29
6.00

0.48-22.25
8.15
6.10

Fig. 1. One minute averages of Cassini magnetic field observations in
KSO coordinates (X – Saturn to Sun, Z-upward normal to Saturn’s
orbital plane, Y – completes a right handed system) on March 17 and18, 2006 [Walker et al., 2011].

Masters et al. [2009]
studied Cassini
magnetic field andthermal plasma
observations at thedawn magnetopause toinfer tailward
propagating surface
waves on theboundary andsuggested they werecaused by the K-H
instability.

3Introduction | Simultaneous observation of HST and Cassini
Cassini observed the upstream
solar wind of Saturn and HST
took the aurora of Saturn
Enhancement of aurora
emission at dawn side occurredin the CIR hitting themagnetosphere
Is this phenomena related tothe disturbed convection andvortices of Kronian
magnetosphere?

Fig.3. Comparison between HST images and solar
wind conditions propagated to Saturn for the period
25–30 January 2004 [Crary et al., 2005].

4

Introduction | Vorticity on equatorial plane
Recently we can simulate with the fine resolution thanks to thelarge scale simulation project in Japan.
Vortices are formed along the both dawn and dusk magnetopause

Fig.4. The vorticity parallel to the magnetic field in the equatorial plane at t = 10, 12, and 16 h. Vorticity with blue
represents the clockwise motion, and red represents the anticlockwise motion [Fukazawa et al., 2012].

5

Introduction | FACs on polar southern ionosphere
Patchy and spot like feature is appeared
due to the vortices

Fig.5. The distribution of field-aligned currents (FACs) in the southern polar cap mapped
along the magnetic field lines from the simulation results to the southern ionosphere at t= 10, 12, and 16 h [Fukazawa et al., 2012].
Fig. 6. Pseudoimages obtained with the FUV channel of the Cassini‐UVIS
spectro‐imager on DOY 239 (26 August) of 2008 [Grodent et al., 2011].

6Motivation
Simulation of Kronian magnetosphere using the solar winddata observed by Cassini
We want to understand the real relationship between the solar
wind effect and response of the Kronian magnetosphere.

In February 2008 Cassini observed the solar wind upstream theSaturn and HST observed the Kronian aurora simultaneously.
Recently we obtain the computer resources and can perform thesimulation with fine resolution.
7

Simulation Setting
Fine resolution (Grid interval 0.1RS )• We can obtain the small FACs configuration with this grid spacing

Grid size• (nx, ny, nz, nmhd) = (1800, 1200, 1200, 8) → about 84GB
• Use 7 times larger memory (600GB) than the grid size in the calculation

Time scale• Calculate for 12hours in the real time
• To create the initial data (quasi-steady state), we calculated for 40hours.

Simulations run on the supercomputer• Sugoka X86 cluster (Fujitsu PRIMERGY RX200S6) at the Research Institutefor Information Technology, Kyushu University• 768 cores is available for 2160hours (CPU time) as this collaboration research

Parallelization by MPI• Three dimensional decomposition8

Difficulty of Large Scale Computing
Long calculation time is needed• It takes one year to run this simulation if including the job waiting time.

Large storage is required• One time step data size is 84GB and we have 300 sampling data then the all data
size is 84×300=25TB!!
• In addition for the initial simulation data, we have 100 sampling data.
→Now I use the Gfarm storage on NICT which is the Grid type storage.
→This storage is connected by network thus the bandwidth is narrow.

Hard to analyze the simulation results• To do with the simulation data, it need over 256GB shared-memory computer.
→Now I use supercomputer not only for simulation but for analysis.
→This is not effective because not interactive operation.

9

Solar Wind Data from Cassini
Cassini located at(XKSM, YKSM, ZKSM) =(24.5–26.7RS, -1.3–3.1RS, 7.4–13.0RS).

→Cassini was almost upstream
the magnetosphere.
Polarity reversal

Enhancement ofdynamic pressure
10

Simulation Results | Movie of equatorial plane
The position ofmagnetopause isvaried dynamicallythen themagnetospheric
convection becomesdisturbed.
The big vortices areformed when theshock coming andthey move into thetail.
11

Simulation Results | Configuration of polar ionosphere
When the shock coming, theenergy flux expand to lower
latitude around 12LT, then its
expansion travel through thedawn and dusk to the midnight.
Shock makes the strong FACs
and pacthy like configuration
around 12LT. After 22:20,
strong upward FAC is formed
between 80° and 85 °.

12

Observation Results in February 2008

Fig.8. Sample UV images of Saturn’s south pole in February 2008 with quiet anddisturbed conditions [Clarke et al., 2009].

Fig.7. Total auroral power from Saturn’s southpolar region, best fit auroral oval radius, and SKR
emission spectrum compared with propagatedsolar wind velocity and dynamic pressure inFebruary 2008 [Clarke et al., 2009].

This period iscorresponding to thesimulation period.

HST has justobserved the UV
image during thisperiod.

13Comparison of Simulation with ObservationEnhancement of By
after the decline

1206

Upward FAC
around high latitude
may becorresponding to thebrightening ofobservation
06
Enhancement ofdynamic pressure
just before
becoming 2/1370°
80°

121880°70°
00
14

Summary
Simulation of Kronian magnetosphere using the solar
wind data observed by Cassini
 We are interested in the disturbed convection and vortex of
Kronian magnetosphere.
 Recently we can obtain the small scale configuration ofFACs due to the vortex configuration.
 Using the solar wind data by Cassini, we can get thesimulation results of dynamically changing magnetopause
and large vortices.
 A snapshot of simulation results of FACs looks like theimage of HST observation, however we should examine
more in detail.
15