Closed magnetic topology in the Venusian magnetotail and ion escape at Venus

Abstract

Venus, lacking an intrinsic global dipole magnetic field, serves as a textbook example of an induced magnetosphere, formed by interplanetary magnetic fields (IMF) enveloping the planet. Yet, various aspects of its magnetospheric dynamics and planetary ion outflows are complex and not well understood. Here we analyze plasma and magnetic field data acquired during the fourth Venus flyby of the Parker Solar Probe (PSP) mission and show evidence for closed topology in the nightside and downstream portion of the Venus magnetosphere (i.e., the magnetotail). The formation of the closed topology involves magnetic reconnection-a process rarely observed at non-magnetized planets. In addition, our study provides an evidence linking the cold Venusian ion flow in the magnetotail directly to magnetic connectivity to the ionosphere, akin to observations at Mars. These findings not only help the understanding of the complex ion flow patterns at Venus but also suggest that magnetic topology is one piece of key information for resolving ion escape mechanisms and thus the atmospheric evolution across various planetary environments and exoplanets.

Fig. 1. Overview of the PSP observations on 20 February 2021, during its fourth Venus gravity assist.

Panels (a–g) are the time series of: a the PSP trajectory in the Venus-Solar-Orbital (VSO) coordinates, b the PSP altitude (km) relative to the Venus surface, c omnidirectional electron differential energy fluxes (DEF, eVcm⁻²s⁻¹sr⁻¹eV⁻¹) and d normalized 132-eV electron pitch angle distributions measured by the SPAN-e instrument, e ion differential energy fluxes (DEF, eVcm⁻²s⁻¹sr⁻¹eV⁻¹) averaged over all look directions measured by the SPAN-i instrument, f the 1-s magnetic field vector and strength and g magnetic clock angles B_clk and cone angles B_cone in the VSO coordinates, measured by the FIELDS instrument. The two vertical dashed lines highlight a zoomed-in time period shown in Fig. 2. Panel (h) shows the PSP trajectory in the X_VSO − Y_VSO plane, with the black whiskers displaying the Bx_VSO and By_VSO components. The gray lines show the empirical bow shock and the induced magnetosphere boundary (IMB), in between is the magnetosheath (MS). The zoom-in period is indicated between the asterisk and diamond symbols.

Fig. 2. The PSP electron and magnetic field observations zoomed in between 20:08 UT and 20:15 UT on 20 February 2021.

The three panels are time series of omnidirectional electron differential energy fluxes (DEF, eVcm⁻²s⁻¹sr⁻¹eV⁻¹) (a) and normalized 132-eV electron pitch angle distributions (b) measured by the SPAN-e instrument, and c the 1-s magnetic field vector and strength in the VSO coordinates measured by the FIELDS instrument. The two vertical dashed lines highlight the time interval of two-sided loss cones.

Fig. 3. Selected examples of electron pitch angle and energy distributions.

Three selected times are 20:10:48–20:10:52 (panels a–c), 20:10:57–20:11:01 (panels e, f), and 20:11:11–20:11:15 UT (panels g–i), each averaging over 4-s SPAN-e observations. DEF stands for differential energy flux with a unit of eVcm⁻²s⁻¹sr⁻¹eV⁻¹. Three rows are the normalized pitch angle distributions by the averaged flux of each energy channel (panels a, d, and g), normalized pitch angle distributions by the maximum flux of each energy channel within 80–200 eV (panels b, e, and h), and the energy distributions of electrons at parallel (PA 0^∘–46^∘), perpendicular (PA 67^∘–113^∘), and antiparallel (PA 134^∘–180^∘) directions (panels c, f, and i).

Fig. 4. Schematics of the PSP locations and how PSP transverses the closed field structure.

Panels (a–c) show the magnetic field components measured by PSP in the VSE coordinates in three projections, a Bx_VSE – By_VSE in the X_VSE – Y_VSE plane, b Bx_VSE – Bz_VSE in the X_VSE – Z_VSE plane, c By_VSE – Bz_VSE in the Y_VSE – Y_VSE plane, blue and red for negative and positive Bx_VSE. The cartoon in panel (d) illustrates the tail magnetic topology for this PSP Venus flyby in the X_VSE – Y_VSE plane. In particular, the red lines show two scenarios of the closed topology: (1) a simple closed loop (dashed) and (2) a flux rope connected to the ionosphere (solid). The cartoon in panel (e) illustrates the ionosphere-closing flux rope in the Y_VSE – Z_VSE plane, with the PSP trajectory shown as the yellow line. The yellow dots in all panels mark the location of PSP encountering the closed topology.

Fig. 5. The PSP electron, ion, and magnetic field observations zoomed in between 20:00 UT and 20:20 UT on 20 February 2021.

Panels (a–e) are time series of: omnidirectional electron differential energy fluxes (DEF, eVcm⁻²s⁻¹sr⁻¹eV⁻¹) (a) and normalized 132-eV electron pitch angle distributions (b) measured by the SPAN-e instrument, ion differential energy fluxes (DEF, eVcm⁻²s⁻¹sr⁻¹eV⁻¹) (c) and ion mass spectra (in DEF, eVcm⁻²s⁻¹sr⁻¹eV⁻¹) averaged over all looking directions (d) measured by the SPAN-i instrument, and e the 1-s magnetic field vector and strength in the VSO coordinates measured by the FIELDS instrument. Between panels (b) and (c) are color bars for magnetic topology (red/green/blue for closed/open/draped, respectively), tail rays, and cold ion flow.