Search for solar wind?asteroid interactions at Eros

Abstract

The solar wind interaction with the asteroid 433 Eros is investigated using magnetometer data from the NEAR-Shoemaker spacecraft. From February 2000 to February 2001 NEAR-Shoemaker orbited Eros in nearly circular orbits having radii of 200, 100, 50 and 35 km. Since no signature of an intrinsic asteroidal magnetic field was detected at Eros, the obstacle to the solar wind flow is the size of the asteroid, 34×14 km 2 , smaller than ion scale lengths in the solar wind, so any electromagnetic solar wind interaction would be mediated by electron whistler waves. To search for interaction wake signatures the magnetic field data are organized by proximity to the expected location of a whistler wave wake. Distributions were constructed of average field magnitude and direction relative to the spacecraft-asteroid and sunward directions, fluctuation amplitudes parallel and transverse to the average field, and transverse fluctuation polarization. A weak (∼1%) enhancement in field intensity is suggested near the wake region but the signature is smaller than the statistical uncertainties of the observations. No signature of an average field orientation indicative of a bow wave is found. Fluctuations are primarily transverse consistent with Alfvenic turbulence, and display a small difference with proximity to the expected whistler wake region. However, the fluctuations are consistently slightly left handed, consistent with known properties of the IMF and opposite from expectations for whistler wave polarization. The results demonstrate that no whistler-mode interaction occurs in the absence of an asteroidal magnetic field.