Global characteristics of magnetic flux ropes in the Venus ionosphere

Abstract

In this paper we examine the global characteristics of the magnetic flux ropes in the Venus ionosphere. These structures are the most common magnetic feature of the ionosphere, being observed on more than 70% of the orbits of the Pioneer Venus orbiter passing through the dayside ionosphere. The spatial occurrence of flux ropes maximizes at altitudes near 165 km; they may occupy more than 50% of the ionospheric volume at this altitude. At higher altitudes the fractional volume occupied by ropes decreases below 20%. Characteristic radii of the ropes vary between 6 km at 160 km altitude and 15 km at 500 km altitude, and scale roughly with both the local ion gyroradius and ion inertial length. Ropes above 200 km altitude in the low zenith angle regions (χ < 45°) appear to have quasi‐horizontal orientations, while those below 200 km tend to be quasi‐vertical. The high zenith angle cases (χ > 45°) tend to be horizontal above 300 km and randomly oriented below 300 km. Ropes may tend to be more tightly ‘twisted’ at low altitudes than at high, and more so in the χ < 45° regions than nearer the terminator. Rope field strengths are highest near the altitudes where their occurrence maximizes, i.e., near 165 km, and scale with the square root of the ambient thermal pressure. Finally, the global polarities of flux rope field‐aligned currents appear to be random and do not clearly support a steady, nonturbulent global formation mechanism.