Electron Heating at Saturn's Magnetopause

Background: The magnetopause (MP) boundary is formed by the solar wind plasma flow interacting with a planetary magnetic field. Magnetic reconnection is an important process at this boundary as it energises plasma via release of magnetic energy. Reconnection of the IMF and internal magnetic field of the planet produces an “open” magnetosphere allowing solar wind and magnetosheath particles to directly enter the magnetosphere. At Saturn, the nature of MP reconnection remains unclear. Masters et al. (2012) hypothesised that viable reconnection under a large difference in plasma β across the MP also requires a high magnetic shear (i.e. magnetic fields either side of the boundary close to anti-parallel).

Objective: The current study uses bulk electron heating at MP crossings (‘events’) as a reconnection signature to test the following hypotheses, suggested by the study of Masters et al. (2012). 1) Events where the boundary is locally closed would have essentially no observed temperature change, whereas most events with locally open boundary should have observed change close to the theoretical prediction. 2) Events with evidence of plasma energization should be in the ‘reconnection possible’ regime, whereas those without such evidence should be in the ‘reconnection suppressed’ regime.

Methods: We analysed 70 MP crossings made by the Cassini spacecraft from April 2005 to July 2007, previously reported by Masters et al. (2012). These 70 events have a determined plasma β on both sides of the MP. Magnetic field and particle data were used to characterize the crossings. The bulk temperature was determined using three different methods, related to properties of the observed energy distribution (including methods from Lewis et al. 2008). We compared the observed heating of magnetosheath electrons with the prediction based on reconnection, using the semi-empirical relationship proposed by Phan et al. (2013) which relates the degree of bulk electron heating to the inflow Alfven speed.

Results: Plots of observed versus predicted electron temperature change for all 70 crossings showed that there is positive correlation between the two when the 1d moment method (Lewis et al. 2008) was used to calculate heating (Figure 1). We do see a tendency of better agreement with prediction for the locally ‘open’ boundary cases based on the threshold  Bn/B >= 0.1 for the minimum variance component of the magnetic field. For the case of locally ‘closed’ boundary (Bn/B < 0.1), we observe a cluster of points near dT=0, but also numerous cases of significant heating. We find five cases where the observed heating exceeds prediction significantly (>3eV). These results suggest that although a large portion of events fit our hypothesis 1 within uncertainty, there are some which do not. Based on the magnetic shear measured locally by the spacecraft either side of the MP, we find 81% of events with no energisation were situated in the ‘reconnection suppressed’ regime, and 43% of events with energization lay in the ‘reconnection possible’ regime (Figure 2). These results support hypothesis 2 to some extent.

Conclusion:

A statistical study of observed and theoretical electron bulk heating was performed at the magnetopause based on 70 magnetopause crossings detected by the Cassini spacecraft. Our results support both hypotheses 1 and 2 to some extent. One reason why some events do not fit our hypotheses is because we are assuming local conditions to be indicative of the putative reconnection site. However, the spacecraft could be quite distant from this site, and still magnetically connected to it. Another reason is temporal variability in the near-magnetopause environment. We plan to analyse the dataset further in future work, by taking these aspects into account.

       

Figure 1 (Left): Observed against predicted bulk electron temperature change for all 70 crossings. First column: Heating based on 3d moment method for full energy distribution. Second column: Heating based on 3d moment method for the cold energy distribution. Third column: Heating based on 1d moment method for the peak of the energy distribution. Top panels: locally ‘closed’ boundary based on threshold Bn/B < 0.1. Bottom panels: locally ‘open’ boundary based on threshold Bn/B > 0.1. Red markers: Steady transitions with field rotation. Green markers: Turbulent transitions with field rotation. Blue markers: Transitions without field rotation.

Figure 2 (Right):  Assessment of diamagnetic suppression of reconnection using the 70 MP crossings. Colour represents observed heating using 1d moment method. The solid curve corresponds to a current sheet thickness L = 1 d_i, and the dashed curves on the left and right of it correspond to L = 0.5 d_i and L = 2 d_i, respectively. ‘x’ markers: Steady transitions with field rotation. ‘o’ markers: Turbulent transitions with field rotation. ‘v’ markers: Transitions without field rotation.