Ionosphere & induced magnetosphere investigations
Given that Venus possesses no intrinsic dipole magnetic field, the solar wind plasma can easily approach the planet and interact directly with the upper atmosphere, leading to the formation of an ionosphere and a whole induced magnetosphere that surrounds the planet.
Simulations in Figure 1 are obtained for average solar wind condition and by means of the BATR-US code publicly available here, and it describes the most probable configuration expected during the BepiColombo flyby (to be confirmed and updated when the data will be available and analyzed). Left panel shows the proton density as it increases in the subsolar region above the planetary surface, and along a wide shell around the planet that elongates in the anti-solar direction forming a wake. It also shows BepiColombo trajectory during the flyby (in red the portion lying above the X-Y plane): it will cross the extended region of the different interactions occurring between the solar wind and interplanetary magnetic field on one side, and the planet Venus on the other. It will come from the outer unperturbed solar wind, through the bow shock, the compressed proton density region in the magnetosheath, the ion composition boundary, ionosphere and magnetotail. Simulation is completed also by the expected morphology of the planetary induced magnetic field in the X-Y plane (Figure 1, right panel).
The occurrence of the BepiColombo flybys at Venus on October 15th 2020, together with the presence of a suite of instrumentation (onboard both MMO and MPO) fully devoted to the magnetospheric studies, can allow a detailed study of the extent, interactions, magnetic and density discontinuities, of the many interactions occurring between the solar wind and the environment of Venus.
In fact, as it can be noted from the timeline below (Figure 2), all instruments (apart from PHEBUS) will be on during the bow shock crossing and the ion composition boundary:
The BepiColombo instruments involved for the Magnetospheric investigations at Venus are:
- MAG: the magnetometer (continuously operating during the cruise phase) will be run in high resolution mode (at 128 Hz) for the 48 hours around the closest approach, with the intent to measure the discontinuities of bow shock and other boundaries crossings; to measure the draped dayside magnetic field, low frequency wave activity inside the ion composition boundary, potential measurements of flux ropes and possible confirmation of the tail lobes with opposite magnetic field polarity;
- SERENA ion sensors PICAM and MIPA will be able to measure density and velocity of the ions populating the different regions crossed by the spacecraft trajectory (solar wind, bow shock, magnetosheath, ion boundary composition…). In particular, exactly for this purpose, MIPA will operate down to 72 hours after closest approach, to be able to detect also the exit crossing of the magnetotail;
- SIXS-X may monitor the flare activity on the Sun, which could affect the atmosphere and exosphere of Venus. In addition, its observations could be used to search for the weak charge exchange induced X-ray emission from the interaction between the solar wind and the Venus exosphere;
- BERM can provide info on the radiation environment through its electron, proton and heavy ions detectors;
- In addition, onboard MMO, the 5 sensors of MPPE are expected to provide measurements of electron shielding effects, atmospheric pick-up ions and energetic neutral atoms as derived from solar wind backscattering and/or ion sputtering over the exobase; they could help to relate the ionized and neutral components of the Venus environment, hence the induced magnetosphere with the atmosphere themselves. The 2 sensors of PWI and MMO/MGF magnetometer will also be operative and provide additional measurements of the magnetic and electric fields around Venus (with possible detection of upstream foreshock waves, proton cyclotron waves, magnetosheath turbulence…);
- Finally, also ISA may measure possible acceleration effects derived from the crossing of bow shock and other boundaries, as already happened during the Earth flyby.
The collection of all these measurements would then be of great interest to determine the position and extent of the bow shock (depending on the solar wind and interplanetary magnetic field condition at the moment of the flyby) and the other boundaries; to provide additional information on the electron and ion populations at different energies, and on the occurrence of some of the interactions mechanisms known to exist inside the induced magnetospheres.
Hence, the listed measurements, apart from being interesting investigation by itself, will also be important complement for:
- the space weather studies, as related to the interplanetary magnetic field and solar wind and radiation propagation and interaction with the planet Venus;
- the atmospheric studies (i.e. atmospheric escape and ionosphere dynamics).
NEWs! FIRST DATA from jaxa
On November 4th a press release by JAXA showed a combined view of the trajectory and measurements by the Mercury Plasma Particle Experiment (MPPE) and the Plasma Wave Investigation (PWI) instruments onboard Mio. They imaged the particles and magnetospheric environment of Venus.
Full text can be found at JAXA site here.
In the composite image above, the crossing of the different boundaries of the magnetospheric tail are shown, as measured by MPPE (MEA and MIA sensors). In fact both ions and electrons detected show a distinctive pattern that changes in correspondance to the bow shock crossing (shortly after 04:00 UT), the entrance in the ionotail (before 05:00 UT) and the plasma sheet crossing (at about 06:00 UT).
In addition, the two sensors of PWI (OFA and SORBET) identified the moment of the entrance in the Venus bow shock by a correspondant end of plasma wave emission related to the undisturbed solar wind.