Proton Temperature Anisotropies in the Plasma Environment of Venus

• Published in: Journal of geophysical research. Space physics 2019-05, Vol.124 (5), p.3312-3330
• Main Authors: Bader, A., Stenberg Wieser, G., André, M., Wieser, M., Futaana, Y., Persson, M., Nilsson, H., Zhang, T. L.
• Format: Article
• Language: English
• Subjects: Anisotropy; Cyclotrons; Distribution functions; Heating; Ionosphere; Magnetic fields; Magnetosheath; Magnetotails; Mass spectrometry; Maxwellian distribution; Protons; Solar wind; Velocity distribution; Venus; Venus Express (ESA); Wave generation
• ISSN: 2169-9380; 2169-9402; 2169-9402
• DOI: 10.1029/2019JA026619

Velocity distribution functions (VDFs) are a key to understanding the interplay between particles and waves in a plasma. Any deviation from an isotropic Maxwellian distribution may be unstable and result in wave generation. Using data from the ion mass spectrometer IMA (Ion Mass Analyzer) and the magnetometer (MAG) onboard Venus Express, we study proton distributions in the plasma environment of Venus. We focus on the temperature anisotropy, that is, the ratio between the proton temperature perpendicular (T⊥) and parallel (T‖) to the background magnetic field. We calculate average values of T⊥ and T‖ for different spatial areas around Venus. In addition we present spatial maps of the average of the two temperatures and of their average ratio. Our results show that the proton distributions in the solar wind are quite isotropic, while at the bow shock stronger perpendicular than parallel heating makes the downstream VDFs slightly anisotropic (T⊥/T‖ > 1) and possibly unstable to generation of proton cyclotron waves or mirror mode waves. Both wave modes have previously been observed in Venus's magnetosheath. The perpendicular heating is strongest in the near‐subsolar magnetosheath (T⊥/T‖≈3/2), which is also where mirror mode waves are most frequently observed. We believe that the mirror mode waves observed here are indeed generated by the anisotropy. In the magnetotail we observe planetary protons with largely isotropic VDFs, originating from Venus's ionosphere. Key Points We present maps of the perpendicular and parallel proton temperatures and their ratio in the plasma environment around Venus The largest perpendicular temperature anisotropy with a median temperature ratio of about 3/2 is found in the near‐subsolar magnetosheath The region with the largest observed temperature anisotropy coincides with observations of proton cyclotron and mirror mode waves