An Interaction Region Near the Top of the Ionosphere Observed at Mars and Venus
The European Space Agency (ESA) currently operates spacecraft at both Mars (Mars Express - MEx) and at Venus (Venus Express - VEx). On both MEx and VEx is the Analyzer of Space Plasmas and Energetic Atoms (ASPERA) experiment, which measures the electron spectrum with the Electron Spectrometer (ELS) and the ion spectrum with the Ion Mass Analyzer (IMA). The MEx spacecraft also contains the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) which can derive the thermal electron density and magnetic field magnitude from its ionograms. The VEx spacecraft also contains a magnetometer (MAG) experiment. At the top of the ionosphere of Mars and Venus is a region where there is mixing between plasma from the ionosphere and plasma from the solar wind. ASPERA-3 data shows high energy strahl from the solar wind penetrating through the bow shock and magnetosheath, and into the dayside ionosphere. At the same time, plasma showing electron peaks generated by the ionization of carbon dioxide and atomic oxygen by solar He 30.4 nm penetrates into the magnetosheath. This overlap region is located where the thermal electron density at Mars derived by MARSIS decreases. This region marks the beginning of ion acceleration as ions begin to flow down the tail. This region is also observed at Venus. All three experiments show turbulence near and through the ionosphere/solar wind interaction region. In this presentation we will show examples of this transition region from both Mars and Venus.
Heavy Ion Effects on Resonant Absorption at Mercury's Magnetosphere
When an incident compressional wave propagates across an Alfven velocity gradient in multi-fluid plasmas, the compressional wave can couple with the Alfven resonance for lower frequency and/or the ion-ion hybrid (IIH) resonance for higher frequency. Recently, a wave simulation in electron-hydrogen-sodium plasmas suggested that the field line resonance at Mercury is expected to occur when the IIH and/or Alfven resonance conditions are satisfied. However, the relative efficiency of wave energy absorption at these resonances has not been studied in the context of Mercury's magnetosphere. To understand the efficiency of wave absorption, we evaluate absorption coefficients at the IIH and Alfven resonances for variable concentrations of sodium, ηNa = NNa / Ne, where Nj is number density for particle species j, and azimuthal wave number, ky. When the compressional wave encounters a single resonance (the Alfven or IIH resonances), wave absorption at the Alfven resonance occurs in wide range of ηNa and ky RM ≥ 1, where RM is Mercury's radii, but the absorption at the IIH resonance occurs in narrow range of 0.15< ηNa < 0.55 and ky RM ≤ 1. We also present the absorption when the compressional waves from low magnetic field side encounter both the IIH and Alfven resonances and compare the results with for single resonance cases.
Comparison of Ultra-Low-Frequency Waves at Mercury under Northward and Southward IMF
The trajectories of the two MESSENGER flybys of Mercury on 14 January 2008 and 6 October 2008 were very similar with respect to planetary magnetic latitude and local time of day. During the first flyby, however, the vertical component of the interplanetary magnetic field (IMF) was northward and the magnetosphere was quiet, while during the second flyby the IMF was southward and the magnetosphere was highly disturbed. These flybys thus present an opportunity to investigate similarities and differences in ultra-low-frequency (ULF) wave activity between quiet and highly active magnetospheric conditions. Observed outbound from closest approach (CA) during both flybys was a "boundary layer" (BL) whose beginning was delineated by a step decrease in magnetic field strength but no change in orientation. There was a strong increase in ULF wave activity at frequencies greater than the He+ cyclotron frequency just before closest approach that persisted almost continuously up to the outbound magnetopause crossing during both flybys. During the first flyby a frequency drift was observed in these waves from just above the He+ cyclotron frequency to just below the H+ cyclotron frequency between CA and the inner edge of the BL. However, no such drift was apparent during the second flyby. Overall these waves exhibited lower coherence during the second flyby, and their bandwidths were larger, than during the first flyby. Within the boundary layer, larger-amplitude ULF waves were detected under both magnetospheric conditions, but the ULF wave power was four times larger during disturbed conditions than during quiet conditions. At longer periods, a quasi-periodic 20-to-30-s oscillation was observed throughout the second flyby. We consider whether this low-frequency oscillation is more likely due to Na+ pickup-ion instabilities or alternatively reflects signatures of quasi-periodic intense reconnection events.