Energy conversion at Saturn’s magnetosphere:

from dayside reconnection to kronian substorms

Dr. Caitríona Jackman

Uppsala, May 22nd 2008

Outline

1. Introduction to Cassini

2. The solar wind and interplanetary magnetic field - Corotating Interaction regions

3. What happens when the solar wind reaches Saturn - In situ evidence of magnetic reconnection

- Reconnection voltage

4. How the magnetosphere responds to solar wind interaction - Kronian substorms

5. Current and Future work - Link between substorms and Saturn Kilometric Radiation - Relative role of solar wind vs internal processes

4

Instrumentation – Cassini Spacecraft

Launch: October 1997. Saturn Orbit Insertion: July 2004

Nominal mission: 4 years. Extended mission: 2 years

Cassini sampled IMF upstream of Saturn during cruise phase…

Speed variations propagate radially

Fast follows slow – compression

Slow follows fast – rarefaction.

Expected structure:IMF consisting of two sectors per solar rotation. Sector boundaries (HCS crossings) embedded within two CIR compression regions.

The solar wind and CIRs

Slow

Slow

Fast

Fast

Corotating interaction regions (CIRs): Kunow, [2001]

The solar cycle: Jackman et al. [2004]

Structure over 8 solar rotations during Cassini approach to Saturn

Clear pattern in the IMF during the declining phase of the solar cycle:

Two compressions per solar rotation separated by rarefactions. Crossings of the HCS embedded within compressions

Thus, Saturn’s magnetosphere immersed in highly structured IMF

Structure of solar wind upstream of Saturn

From Jackman et al., [2004]

Effect of CME

HCS crossing

Sketch of Saturn’s magnetosphere

Courtesy E J Bunce

Simple empirical model for open flux production at Saturn’s magnetopause

Dayside reconnection voltage, Φ across the magnetosphere:

Φ = Vsw B┴ L B ┴ – strength of IMF perpendicular to velocity vector

VswB┴ – motional electric field in the solar wind

Φ = Vsw B┴ L0 cos4(θ/2)

Clock angle at Saturn: From Jackman et al., [2004a, b]

Effective length, L, width of solar wind channel in T-N plane that reconnects with planetary field:L = L0 f(θ)

For Earth:f(θ) = sin4(θ/2) L0≈5 RE

Adapted to Saturn:L0≈10 RS VSW=500 km s-1

Sketch of reconnection at Earth’s magnetosphere

From Jackman et al., 2004a

Solar wind conditions during Cassini approach phase

In situ evidence of reconnection at Saturn’s magnetopause: McAndrews et al. [2008]

Cumulative open flux of ~ 100 GWb per solar rotation.

This flux can be closed by reconnection in the magnetotail.

Assume each kronian substorm closes 20 GWb of flux: Five substorms per solar rotation!

Influence of solar wind adding flux to the magnetosphere can lead to periodic release on the nightside through reconnection

Survey of magnetotail data at Saturn• Surveyed all tail data from Cassini to date• Events appear as clear signatures, particularly in

theta (north-south) component• Events in the midnight or post-midnight sector

In situ examples of reconnection: Jackman et al., [2007, 2008]

south

north

Substorm cycle at Earth

In situ observation - plasmoid

• Magnetic field turns northward at ~16.50 UT. Distance of 48.96 RS downtail and local time of 23.67 h

• Theta becomes primary component – dipolarization!

• Field strength reaches >4 nT

• Evidence of angular momentum conservation

• Enhancement in spectrogram and subsequent dispersion

• Cassini further from Saturn than reconnection point – observing plasmoid passage

inward

outward

south

north

with rot.

opposite rot.

August 4th 2006 event, from Jackman et al., [2008]

The kronian substorm puzzle

Kronian substorms

In situ Cassini observations (plasmoids)

Saturn Kilometric Radiation (SKR) measurementsSolar wind data pre-SOI (input to the magnetosphere)

Titan position info (linked with SKR occurrence probability and substorms?)

SKR affected by solar wind conditions.

Also found to be linked to tail reconnection!

SKR power& spectrogram

Magnetic field strength

SKR burst

Field ‘disturbance’

Link between SKR and tail reconnection

Evidence of a compression region hitting the planet on the outbound pass of SOI. In situ evidence of compression-induced tail reconnection on the outbound pass - hot plasma injection. Disruption in SKR phase and intensity during compression

From Bunce et al. [2005]

Cassini data from Saturn Orbit Insertion

Saturn Kilometric Radiation (SKR) emissions linked to planetary rotation, but respond to solar wind conditions

Burst lifetime PDFs: Freeman et al., 2000.

Compare with theoretical work at Earth

For Earth, substorms identified by AE indices excursions from nominal baseline values.

Probability distribution functions (PDFs) of AE indices broken down into 2 components: exponentially truncated power law and lognormal.

Lognormal component - gives characteristic timescale of duration of about 100 min.

Can we reveal statistical evidence for substorms at Saturn in similar way with SKR?

TR1

TR2

Time

X(T)

Burst lifetime

Burst lifetime

Reconnection events and SKR

No AE indices available at Saturn! BUT, AKR is a proxy for the AE indices at Earth… And SKR is analogous to AKR…

Thus, apply thresholding analysis to SKR powers to look for characteristic substorm timescale.

Example reconnection event and associated SKR burst: Jackman et al., submitted [2008]

EventSKR

T=1e9

T=1e4

Probability distribution function of total SKR power for varying thresholds. 3 years of data included.

Summary and future work

Input to magnetosphere: Solar wind structure- Corotating Interaction regions (CIR)- IMF structure upstream- Quantify rates of dayside reconnection

Magnetospheric response- Kronian substorms- In situ plasmoid observation- SKR provides information on global dynamics

Further questions on Kronian substorms:- Characteristic timescale for substorm duration and recurrence?- Characteristic frequency bands associated with reconnection?- Compare solar wind input at Earth and Saturn – different driving conditions?

Kronberg et al., 2007