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Energy conversion at Saturn’s magnetosphere: from dayside reconnection to kronian substorms. Dr. Caitr í ona Jackman. Uppsala, May 22 nd 2008. Outline. Introduction to Cassini 2. The solar wind and interplanetary magnetic field - Corotating Interaction regions - PowerPoint PPT Presentation
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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