Bepi Colombo

BRIEF DESCRIPTION

1. Scientific Objectives

Mercury is still one of the least explored planets of our solar system. No spacecraft has visited Mercury since Mariner 10 made three flybys past the planet in 1974 and 1975. Although the plasma scientific payload of Mariner 10 was very limited, it made a very important discovery that Mercury possesses an intrinsic magnetic field, whose intensity was in a very intriguing range in terms of comparative planetary magnetospheres. The dominance of the dipole term in the spherical harmonic expansion of the Mercury´s magnetic field suggests that the interaction between the solar wind and Mercury´s magnetosphere should be „Earth-like“, in contrast to the cases of Mars and Venus where the planetary magnetic fields have negligible intensity or have only local effects on the interaction. On the other hand, because of the smallness of its size and gravity, Mercury has very different environmental characteristics compared to the Earth. First, the tenuous atmosphere and the possible dominance of the heavy elements supplied from the surface may provide a unique opportunity to assess the relative importance of the solar wind source and the mixing mechanisms of magnetospheric plasma. This is difficult to do at the Earth where the proton component of the ionospheric plasma cannot be distinguished from the solar wind. Second, the tiny nature of the Mercury´s magnetosphere requires that totally new ideas be applied to the basic concepts that have been applied to the Earth´s magnetosphere. The large gyro-radii of energetic and thermal plasma components (relative to the magnetospheric size) suggests that ideal MHD may no longer apply; we anticipate that the scale-coupling between the microscopic and macroscopic plasma processes, which have been highlighted by a recent theoretical work, has global and dominant importance in the Mercury´s magnetosphere.

As Mariner 10 observations showed, the Mercury´s magnetosphere is rich in energetic particles of the keV order and higher. Exploration of the particle environment of Mercury is very promising since the spatial distribution and temporal evolution of charged particles within the magnetosphere provide important information on the dynamics of the Mercury´s magnetosphere. Mariner 10 observations suggested that acceleration events reminiscent of the Earth´s „substorms“ occur in the Mercury´s magnetosphere and electrons could be accelerated to higher than 35keV within a few seconds in this mini-magnetosphere. A detailed analyze of particle acceleration events by MMO will place significant constraints on some of the acceleration mechanisms discussed so far to explain particle acceleration in the Earth´s magnetosphere, particularly in terms of magnetic reconnection.

In the Earth´s magnetosphere, substorm is thought to be a physical process in which the energy stored in the magnetotail is explosively converted into particle kinetic and thermal energies. The storage of energy in the magnetotail is a natural consequence from reconnection between the interplanetary magnetic field (IMF) and the Earth´s intrinsic magnetic field. The magnetic field lines cannot be accumulated unlimitedly in the magnetotail, thus magnetic reconnection takes place and stored energy is released. These processes are thought to exit in any magnetosphere embedded by the solar wind. Mariner 10 data have been interpreted as an evidence for occurrence of substorms in Mercury. There are many outstanding questions about this topic. Since the concept of storage and sudden release of the energy is likely to be a universal one, efforts in answering the following questions enable us to examine ubiquitous problems in magnetized plasmas.

What controls magnetic reconnection between the IMF and the planetary magnetic field? In-situ observations of reconnection in the sunward side of Mercury as well as the occurrence frequency of substorms in the anti-sunward side provide an efficiency of reconnection between IMF and the planetary magnetic field.

Are there enough magnetic fields in the tiny magnetosphere for the prolonged intense southward IMF Bz? In such solar wind conditions, there might be severe erosions and there might be no space for the sunward magnetosphere.

What controls the onset of substorms (large-scale reconnection in the magnetotail)? Is there any typical time scale of the energy storage in the magnetotail? What is the nature of the threshold for magnetic reconnection, if exists?

What is plasma instability leading to magnetic reconnection? Can we understand the growth rate of the instability? Furthermore, the solar wind near Mercury has a high particle density and high magnetic field intensity in comparison with those near Earth. This would alter conditions for plasma instabilities.

What determines the time scale of substorms? In Mercury, the observed particle acceleration events had the time scale of 1 minute and at least three to four events occurred within 15 minutes. Each Mercury event might correspond to each onset in multiple-onset substorm in Earth. Does steady reconnection exist or is reconnection always bursty? If the time scale of 1 minute is found to be really the time scale of a whole substorm process, plasma measurements would have to answer a fundamental question that is „how magnetic reconnection starts and ends“?

What carry field-aligned currents, if they exist?

How is all stored energy dissipated in magnetic reconnection in the tail? Plasma bulk motions, plasma temperature, plasma number density, and the volume of the plasma sheet remain unknown. If ionospheric contributions were zero, energy dissipation mechanism in Mercury would be totally different from that in Earth. Furthermore, a pure state in magnetic reconnection (no feedback of the ionosphere-magnetosphere coupling) would be explored.

2. MEA Electron Analyzer

- To impose at least three reflections to UV photons inside the analyzers.

- To provide a uniform coverage of the total 4π steradian field of view with a good angular resolution (11.25° x 22.5°).

- To have a high sensitivity and large dynamic range (> 106) to support high time resolution measurements over the wide range of plasma conditions to be encountered in the Bepi Colombo mission.

- To have the ability to routinely generate the fundamental plasma electron parameters on-board with half a spin or one-spin time resolution. These parameters include the density (n), velocity vector (V), pressure tensor (P), and heat flux vector (H).

-  To have a wide energy range from ~ 10 eV to 30 keV.

- To have versatile and easily programmable operating modes and data processing routines to optimize the data collection for specific scientific studies and widely varying regimes.

- To rely as much as possible on well-proven designs by basing sensor designs on those successfully flown on the AMPTE, GIOTTO, INTERBALL, CLUSTER, STEREO missions.

To satisfy these criteria, the Mercury Electron Analyzer (MEA) consists of two sensors located 90° apart on the satellite in order to cover the 4π steradian solid angle in a quarter of the spin period.

The sensors cover the energy range between 0.01 and 30 keV/charge. To cover the large dynamic range (~106) required for accurate measurements in the low-density plasma of the Mercury’s magnetotail on the one hand and the dense plasma in the solar wind, magnetosheath/cusp/boundary layers on the other hand, MEA employs two different sensors, each one has a variable geometrical factor. The minimum number of counts in a distribution needed for computing the basic plasma parameters is about 100. Such total count must be accumulated in ¼ spin (1 s) to provide the necessary time resolution. This corresponds for example to a measurement of the tenuous cold plasma in the lobes with T=30 eV and N=0.005 cm-3. In this case, the detector count rate at the maximum of the energy flux is ~ 500 s-1 (see Fig. 1.). On the other hand, the maximum count rate which can be handle is ~ 5 106 counts/s. This means the dynamic range achievable with a single sensitivity is only ~ 104. MEA must cover at several times 1010 (cm2s sr)-1 to tail lobe electrons of a few times 1042s sr)-1, requiring of larger than 106. This can only be achieved if MEA incorporates sensors with variable geometrical factors and with intrinsic sensitivities differing by a factor of ~ 20. This last task is accomplished by reducing the transmission of one of the two identical sensors to approximately 5% using a grid located inside the collimator. Figure 1 shows that the low GF sensor therefore can cover solar wind/magnetosheath/plasmasheet in the most cases. Each sensor is furthermore designed in a way that allows us to reduce electronically the energy-geometry factor down to 1%.

The launch of the mission is scheduled for 2012 but we would like to use the time prior the launch for an intensive study of the electron component of the solar wind and Earth magnetosphere. This study will use the data already gained by other missions. At present, we have in our disposal all electron and ion measurements from the Interball project that cover the solar wind, bow shock, magnetosheath, magnetopause, and outer magnetosphere. A disadvantage of these measurements is limited temporal resolution of the measurements but this problem can be solved by an implementation of corresponding measurements of CLUSTER II mission satellites. The data are further complemented with the measurements of the LEO spacecraft like Active or Apex. Consequently, we have the data from all regions that are expected to be found at Mercury. Since a significant part of these data was never processed for scientific purposes, we expect to achieve many new scientific results during their processing.

Principal Investigator  :   Jana Šafránková