MESSENGER Observations of Transient Bursts of Energetic Electrons in Mercury’s Magnetosphere

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Before orbit insertion, MESSENGER completed three flybys of Mercury in 2008 and 2009. During these flybys, EPS detected no increases in particle intensity above instrumental background at any time during MESSENGER’s near-equatorial magnetospheric passages (15, 21). Fluorescent x-ray events seen by MESSENGER’s X-Ray Spectrometer (XRS) during the flybys indicated the presence of suprathermal plasma (tens of kilo–electron volts) electrons inside Mercury’s magnetosphere below the EPS energy-detection threshold (21).

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The |B|,L system (9) for Mercury is well described by the offset dipole model (6), which gives |B| = M{3(Z_M/R)² + 1}^1/2/R³ and L = R/{1 − (Z_M/R)²}, where $$R=\sqrt{X^2+Y^2+Z_{\mathrm{M}}^2}$$ is the radial distance from the center of the magnetic dipole in a Cartesian coordinate system (X,Y,Z) co-centered with the planet; the dipole moment M is 195 nT – R_M³ (where R_M is Mercury’s radius); the vertical component Z_M of the radius vector from the dipole is Z – Z₀, where Z is the component of the radius vector from the center of the planet normal to the planetary orbit; and the dipole offset Z₀ is 484 km northward. Because of the small size of the magnetosphere and the distortion of the magnetic field from that of a dipole, the L parameter represents the equatorial crossing point for only L ≲ 2 but provides a useful indication of the approximate magnetic latitude of the field line threading the spacecraft even for L > 2.

The proximity of the largest events to Mercury’s magnetic equator could account for the fact that EPS detected no increases in particle intensity above instrumental background during MESSENGER’s three Mercury flybys in 2008 and 2009. All three flybys followed trajectories with closest approach on the nightside of the planet near the geographic equator, 484 km south of the magnetic equator.

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