Sunday, December 30, 2012

A newly discovered magnetic state for the Venusian ionosphere

We have known since 1962 (thanks to Mariner II) that Venus does not possess a magnetic field of its own (Sonett 1963).  In the late 1970's, the Pioneer Venus Orbiter's magnetometer detected large-scale, steady magnetic fields in the Venusian ionosphere  during solar maximum, when the solar wind dynamic pressure was great enough to overcome the ionosphere's thermal pressure.  These magnetic fields are mainly oriented horizontally, and often exceed 100 gammas (Luhmann et al. 1980).

Even when the ionosphere was not dominated by the solar wind, the Pioneer craft detected small-scale magnetic features interpreted as flux ropes, structures that resemble a helix of woven magnetic fields, not unlike a rope.  This led researchers to conclude that the ionosphere's magnetic state could be categorized in two forms: magnetized, and unmagnetized with some small-scale flux ropes (Luhmann et al. 1980).

A recent publication aims to change this long-standing picture of the Venusian ionosphere, as a result of observations made by the Venus Express spacecraft since its orbit was modified such that it now reaches periapsis at a latitude of 90 degrees (over the North pole) and a height above Venus of only 170 km (well into the ionosphere).

T. L. Zhang and colleagues (2012) report that during solar minimum, Venus Express observed "giant flux ropes" embedded within a magnetized ionosphere.  They determined these woven structures to be on the order of 100 kilometers in diameter, 10 times the size of the flux ropes detected by Pioneer in an unmagentized ionosphere.  The magnetic flux contained within the giants ropes measures near 1000 Webers, much greater than the 2 to 3 Weber fluxes in the small ropes.

It is notable that in over 100 orbits when the Venus Express magnetometer detected these enhanced magnetic strength structures, they detected only one such giant rope during passage through periapsis. In addition, while the giant rope structure is seen as a single event by Venus Express only near 90 degrees latitude, the small flux ropes observed by Pioneer occurred throughout the dayside atmosphere.

These researchers propose that the Venusian ionosphere has a third known state, in addition to the two mentioned above (Zhang et al. 2012).  The possible states are:
  • Magnetized (during solar maximum and high solar wind pressure)
  • Unmagnetized with small-scale flux ropes (during solar minimum)
  • Magnetized with embedded giant flux ropes (during solar minimum)
And what do they believe is the cause of these large-scale magnetic helixes?  It remains "unknown and speculative."

It is interesting that Venus is not the only rocky planetary body in our solar system to display magnetic flux ropes.  The Mars Global Surveyor spacecraft has detected flux ropes in the ionosphere of Mars, and Cassini has detected twisted magnetic fields in the lower ionosphere of Titan that resemble the flux ropes observed on Venus and Mars (Wei et al. 2010).


Luhmann, J., Elphic, R., Russell, C., Mihalov, J., & Wolfe, J. (1980). Observations of large scale steady magnetic fields in the dayside venus ionosphere. Geophysical Research Letters, 7, 917-920. (pdf)

Sonett, C. P. (1963). A summary review of the scientific findings of the mariner venus mission. Space Science Reviews, 2(6), 751-777.

Wei, H., Russell, C., Zhang, T., & Dougherty, M. (2010). Comparison study of magnetic flux ropes in the ionospheres of venus, mars and titan. Icarus, 206(1), 174-181. (pdf)

Zhang, T., Baumjohann, W., Teh, W., Nakamura, R., Russell, C., Luhmann, J., . . . Du, A. (2012). Giant flux ropes observed in the magnetized ionosphere at venus. Geophysical Research Letters, 39(23), L23103.