Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm51b1805b&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM51B-1805
Physics
[2723] Magnetospheric Physics / Magnetic Reconnection, [2772] Magnetospheric Physics / Plasma Waves And Instabilities, [2784] Magnetospheric Physics / Solar Wind/Magnetosphere Interactions, [5443] Planetary Sciences: Solid Surface Planets / Magnetospheres
Scientific paper
The three Mercury flybys by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft occurred on 14 January 2008, 6 October 2008, and 29 September 2009. MESSENGER will be inserted into a polar orbit about Mercury on 18 March 2011. For all flybys the spacecraft followed similar equatorial trajectories, entering the dusk side of the magnetosphere downstream of the planet and exiting just sunward of the dawn terminator. The first flyby (M1) occurred during steady northward interplanetary magnetic field (IMF) and showed no evidence of reconnection or substorm activity. In contrast, the second flyby (M2) occurred during steady southward IMF, and intense reconnection was observed in the magnetotail and at the dawn-side magnetopause. The third flyby (M3) occurred during a period marked by intermittent northward and southward IMF. During M3, no magnetic field data were collected after closest approach (CA). Despite the different IMF geometry during M1 and M2, bursts of narrow-band waves at frequencies in the range between the He+ and H+ cyclotron frequencies (~1 Hz) were observed during both flybys. These bursts were seen almost continuously from a few minutes before CA to the dawn-side magnetopause crossing. Outbound from CA, trends in wave power, wave-normal angle, and ellipticity were similar for both flybys. Also, during both flybys a dawn-side "boundary layer" (BL) was marked by a step decrease in magnetic field strength but no change in field orientation. During both flybys an order-of-magnitude increase in wave power was observed in this BL relative to that before BL entrance. Overall wave power was about 4 times larger during M2 than during M1. For both flybys the wave-normal angles were generally greater than 45°, and power parallel to the local magnetic field was often greater than power in the perpendicular direction, suggesting a strong magnetosonic component for these waves. At lower frequencies, quasi-periodic (~20-30-s period) oscillations were observed near CA during M2. In contrast, no such oscillations were observed near CA during M1. These quasi-periodic oscillations were strongly compressional, with a fractional change in magnetic field magnitude, ΔB/B, of ~3%. The oscillation period was near and above the local Na+ cyclotron period, and these oscillations were strongly linear, with ellipticities near zero. We assess whether this low-frequency oscillation was more likely the result of Na+ pickup-ion instabilities, Kelvin-Helmholtz waves on the magnetopause, or quasi-periodic intense reconnection events in the magnetotail.
Anderson Benjamin J.
Boardsen Scott A.
Gloeckler George
Korth Haje
Raines Jim M.
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