Io plasma torus electrons: Voyager 1

The analysis of in situ plasma electron observations in the Io plasma torus by the plasma science experiment during the Voyager 1 encounter with Jupiter is presented in terms of two components: a thermal ( c ) Maxwellian component and suprathermal ( H ) non‐Maxwellian component of the electron distribution function. Average electron temperatures are T e < 1 eV in the cold torus ( L < 5.5), with T e ≃ 5–6 eV in the hot torus (5.5 < L < 7.6); T e rises abruptly to T e ≃ 30 eV just outside the hot torus ( L > 7.6) and then continues to rise to T e > 100 eV at r > 12 R J . In the cold torus the density ratio of the suprathermal component n H to that of the cold component n c was <10 −4 ; but in the hot torus, n H /n C ∼ 10 −3 was observed, and outside the torus, n H /n c can exceed 10 −1 . We present evidence that suprathermal electrons are locally produced in the hot torus. Throughout the hot torus the electron temperature T e is a factor of 10 less than the thermal ion temperature. A large difference in the hot electron pressure P H is observed between the inbound and the outbound data which is interpreted as a latitudinal gradient with P H being a maximum at the magnetic equator. If one imposes the theoretical and observational constraint that ( T ⊥ / T ∥ ) EQ ≤ 2 for the hot electrons, then one requires the presence of a parallel electric field E ∥ > 2.5 µV/m which exceeds the ambipolar electric field E ∥ < 1 µV/m produced by the centrifugally confined ions. However, if unacceptable charge imbalances in the thermal plasma are not to occur from this larger E ∥ , then sufficient wave turbulence in the plasma must be present to adequately scatter the thermal electrons. We infer the presence of a neutral corona around Io from the observed decrease and symmetry with respect to Io of T c . The energy input to the torus by charge exchange and ionization in this neutral corona followed by pickup is ∼2 × 10 11 W, substantially less than the EUV luminosity. In the hot torus, suprathermal electrons contribute significantly to the ionization of the more highly ionized ions (O + , O 2+ , S 2+ , and S 3+ ).