Shocklets and Short Large Amplitude Magnetic Structures (SLAMS) in the High Mach Foreshock of Venus

Abstract

Shocklets and short large‐amplitude magnetic structures (SLAMS) are steepened magnetic fluctuations commonly found in Earth's upstream foreshock. Here we present Venus Express observations from the 26th of February 2009 establishing their existence in the steady‐state foreshock of Venus, building on a past study which found SLAMS during a substantial disturbance of the induced magnetosphere. The Venusian structures were comparable to those reported near Earth. The 2 Shocklets had magnetic compression ratios of 1.23 and 1.34 with linear polarization in the spacecraft frame. The 3 SLAMS had ratios between 3.22 and 4.03, two of which with elliptical polarization in the spacecraft frame. Statistical analysis suggests SLAMS coincide with unusually high solar wind Alfvén mach‐number at Venus (12.5, this event). Thus, while we establish Shocklets and SLAMS can form in the stable Venusian foreshock, they may be rarer than at Earth. We estimate a lower limit of their occurrence rate of ≳14%.

Plain Language Summary

We discover that Venus, like Earth, also has magnetic structures called Shocklets and short large‐amplitude magnetic structures (SLAMS) in its foreshock region, which is the area upstream of the planet where the interplanetary magnetic field is connected to its bow shock. Shocklets and SLAMS are common in the foreshock of Earth. However, Shocklets have not been observed at Venus before, and SLAMS have only been seen once, and then only during a large disturbance of the space near Venus. Thus it is unknown if SLAMS and Shocklets can form in the foreshock of a planet as close to its star as Venus. We used observations from the European Space Agency's Venus Express orbiter (2006–2014) to identify these structures in the Venusian foreshock. The structures were found to be present during periods of high solar wind activity, and a lower limit on how often they occur is at least 14% of the time. These findings provide new insights into the space environment around Venus and may help us understand the differences in the space environments of different planets.

Key Points

• Shocklets and short large‐amplitude magnetic structures (SLAMS) can form in the steady‐state foreshock of Venus despite the magnetosphere being 1/10th the size of Earths

• The Venusian Shocklets and SLAMS had comparable magnetic signatures to those reported near Earth, but may be rarer

• Analysis of the solar wind at 0.72AU suggests Shocklets and SLAMS occur during high Alfvén mach‐numbers with a lower limit on occurrence rate of ≳14%