Energy conversion at Saturn’s magnetosphere: from dayside reconnection to kronian substorms

1. Energy conversion at Saturn’s magnetosphere: from dayside reconnection to kronian substorms Dr. Caitríona Jackman Uppsala, May 22nd 2008

2. Outline • Introduction to Cassini • 2. The solar wind and interplanetary magnetic field • - Corotating Interaction regions • What happens when the solar wind reaches Saturn • - In situ evidence of magnetic reconnection • - Reconnection voltage • 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

3. 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… 4

4. The solar wind and CIRs Fast Slow Fast Slow The solar cycle: Jackman et al. [2004] 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. Corotating interaction regions (CIRs): Kunow, [2001]

5. Structure of solar wind upstream of Saturn HCS crossing 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 Effect of CME From Jackman et al., [2004]

6. Sketch of Saturn’s magnetosphere Courtesy E J Bunce

7. 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 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 Φ = Vsw B┴ L0 cos4(θ/2) Clock angle at Saturn: From Jackman et al., [2004a, b]

8. 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 From Jackman et al., 2004a

9. 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 Substorm cycle at Earth south north In situ examples of reconnection: Jackman et al., [2007, 2008]

10. In situ observation - plasmoid inward outward south • 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 north with rot. opposite rot. August 4th 2006 event, from Jackman et al., [2008]

11. The kronian substorm puzzle In situ Cassini observations (plasmoids) Titan position info (linked with SKR occurrence probability and substorms?) Kronian substorms Saturn Kilometric Radiation (SKR) measurements Solar wind data pre-SOI (input to the magnetosphere) SKR affected by solar wind conditions. Also found to be linked to tail reconnection!

12. Link between SKR and tail reconnection SKR burst Cassini data from Saturn Orbit Insertion SKR power & spectrogram From Bunce et al. [2005] Saturn Kilometric Radiation (SKR) emissions linked to planetary rotation, but respond to solar wind conditions Magnetic field strength Field ‘disturbance’ 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

13. Compare with theoretical work at Earth X(T) Burst lifetime TR2 TR1 Burst lifetime Time Burst lifetime PDFs: Freeman et al., 2000. 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?

14. Reconnection events and SKR SKR Event T=1e9 T=1e4 Probability distribution function of total SKR power for varying thresholds. 3 years of data included. 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]

15. 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