3-D global hybrid simulations of magnetospheric response to foreshock processes

It has been suggested that ion foreshock waves originating in the solar wind upstream of the quasi-parallel (Q-||) shock can impact the planetary magnetosphere leading to standing shear Alfvén waves, i.e., the field line resonances (FLRs). In this paper, we carry out simulations of interaction between the solar wind and terrestrial magnetosphere under radial interplanetary magnetic field conditions by using a 3-D global hybrid model, and show the properties of self-consistently generated field line resonances through direct mode conversion in magnetospheric response to the foreshock disturbances for the first time. The simulation results show that the foreshock disturbances from the Q-|| shock can excite magnetospheric ultralow-frequency waves, among which the toroidal Alfvén waves are examined. It is found that the foreshock wave spectrum covers a wide frequency range and matches the band of FLR harmonics after excluding the Doppler shift effects. The fundamental harmonic of field line resonances dominates and has the strongest wave power, and the higher the harmonic order, the weaker the corresponding wave power. The nodes and anti-nodes of the odd and even harmonics in the equatorial plane are also presented. In addition, as the local Alfvén speed increases earthward, the corresponding frequency of each harmonic increases. The field-aligned current in the cusp region indicative of the possibly observable aurora is found to be a result of magnetopause perturbation which is caused by the foreshock disturbances, and a global view substantiating this scenario is given. Finally, it is found that when the solar wind Mach number decreases, the strength of both field line resonance and field-aligned current decreases accordingly.

Discovery of Alfven waves planetward of the Rings of Saturn

<p>Between April and September 2017 in the final stages of the Cassini Saturn Orbiter mission the spacecraft executed 22 orbits passing planetward of the innermost ring, the D-ring.  During periapsis passes on all these orbits oscillations were detected in the azimuthal magnetic field components on typical time scales from a few minutes to 10 minutes. We argue that the time-varying signals detected on the spacecraft are also primarily time-varying in the plasma frame.  Nonetheless, we show that nearly all signals exhibit a distinct spatial effect, namely a magnetic node near the effective field line equator.  The oscillations thus have a standing structure along the background magnetic field and it follows that they are field line resonances associated with Alfvén waves.  The form of the signals suggests that the local field line resonances are most likely pumped from global sources.  This is the first detection in a giant planet magnetosphere of a phenomenon known to be important at Earth.</p>