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Strange and unusual: Venus’ unique 557.7 nm OI oxygen green line aurora during solar minimum

Presentation #507.08 in the session Venus II (Oral Presentation)

Published onOct 23, 2023
Strange and unusual: Venus’ unique 557.7 nm OI oxygen green line aurora during solar minimum

Solar wind interactions with non-magnetic planets such as Venus and Mars gives way to unique auroral activity. Global diffuse aurora has been observed on both planets during intense solar storms, such as coronal mass ejections (CMEs). However, Venus also exhibits global auroral emission even during solar minimum. The Venusian 557.7 nm OI (1S – 1D) “oxygen green line” has been consistently detected not only after after large particle storms, but also during weaker particle events, such as the passage of a stream interaction region (SIR) or co-rotating interaction region (CIR).

Here, we seek to isolate the solar minimum conditions that are sufficient to induce Venusian aurora. We present the results of a monitoring campaign conducted at Apache Point Observatory (APO) using the Astrophysical Research Consortium Echelle Spectrograph (ARCES, R~35,000) spanning from 2018 – 2022. We compare solar wind conditions measured by NASA’s Advanced Composition Explorer (ACE) spacecraft by calculating the temporal offset needed for an observed SIR to rotate towards Venus by assuming a constant solar wind speed and solar rotation.

While this solar minimum was unusually weak, auroral emission was seen at the onset of a SIR passage over Venus and faded while Venus was embedded in the region. Several detections of aurora were found during strong, spurious 100 keV proton storms outside of flares, SIRs, and CMEs. The source of these solar wind proton events is currently an enigma. However, the most intriguing auroral detection was outside all such particle events. Strong emission was detected during a period when there was no increase or presence of high energy particles. Rather, strong magnetic field inversions were present, with a strength of 10nT (comparable to CIR passages). This results indicates that magnetic inversions alone are able induce aurora, likely by magnetic re-connections accelerating charged particles to the nightside of Venus. Of the 42 observing nights, emission was detected on 21 nights, suggesting the processes required to induce auroral emission on a non-magnetic planet are relatively common, even during solar minimum.

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