Answer:
The existence of a strong internal magnetic field allows probing of the interior through both long term changes of and short period fluctuations in that magnetic field. Venus, while Earth’s twin in many ways, lacks such a strong intrinsic magnetic field, but perhaps short period fluctuations can still be used to probe the electrical conductivity of the interior. Toward the end of the Venus Express mission, an aerobraking campaign took the spacecraft below the ionosphere into the very weakly electrically conducting atmosphere. As the spacecraft descended from 150 to 140 km altitude, the magnetic field became weaker on average and less noisy. Below 140 km, the median field strength became steady but the short period fluctuations continued to weaken. The weakness of the fluctuations indicates they might not be useful for electromagnetic sounding of the atmosphere from a high altitude platform such as a plane or balloon, but possibly could be attempted on a lander.
Introduction
The existence of a strong intrinsic magnetic field and the presence of time-varying fields can both be used to probe the electrical conductivity of a planetary crust1,2. Based on the Venus near-equatorial magnetic field surveyed down to 150 km altitude by the Pioneer Venus Orbiter (PVO), it is generally accepted that Venus has no significant global intrinsic field3,4. Although strong variable fields are present in the Venus ionosphere, these may be largely spatial variations, fossil fields impressed by slowly changing interplanetary magnetic fields “played back” at higher frequencies by the spacecraft’s rapid motion through the ionosphere. If so, then the fields at the surface of Venus resulting from the diffusion of the ionospheric magnetic field into the Venus atmosphere might be significantly weaker than in the ionosphere and much quieter5.
The Venus Express mission, designed to complement PVO, was inserted into an elliptical orbit with periapsis over north pole6,7. Initially, its periapsis was close to 78˚ latitude north and 250 km altitude. Later, the spacecraft periapsis moved to higher latitude over the pole and then to lower latitudes. Periapsis altitude was variable but typically remained above 160 km. A joint analysis of the Pioneer Venus and Venus Express measurements at low altitude within the ionosphere revealed that the magnetic field was horizontal on the dayside and expanded both into the wake at low altitudes and away from the wake at high altitudes8. During the last days of the Venus Express mission May – July 2014, an aerobraking campaign was performed. The changes of the spacecraft orbit allowed the periapsis to go as low as 129.7 km in altitude, which is well below the main peak ionosphere altitude of ~140 km9,10. Note that the lowest altitude measured by the PVO magnetometer was 150 km
Explanation:
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