Introduction

In spite of significant advances in our knowledge of the solar-terrestrial system and changes in the elements of the GGS mission since the Science Plan [Redbook, 1984] was written, the central scientific goals of the mission have not changed. That is to perform a global assessment of the mass, momentum and energy flows through the geospace system and to improve our understanding of the underlying physical processes involved. Specifically these include the transfer of solar wind energy and momentum to the magnetosphere, the interaction between the magnetosphere and the ionosphere, the transport processes that distribute plasma and energy throughout the magnetosphere, and the interactions that occur as plasmas of different origins and histories mix and interact. The TIMAS investigation is relevant to each of these.

During the past two decades, it has become increasingly clear that ion composition measurements of the hot plasmas within the earth's magnetosphere are essential for understanding the origins, transport, and acceleration of plasmas and their interactions with spacecraft systems. On past missions (e.g. S3-3, ESA/GEOS-1,-2, SCATHA, ISEE-1, DE-1, AMPTE/CCE) spectrometers used for this purpose performed serial measurements in the multi-dimensional angle-energy-mass parameter space with only limited coverage of the full 4 p solid angle. As a consequence of the rather poor time resolution ( ~ minutes) and limited field-of-view, the information acquired was incomplete and susceptible to sampling aliases. The need for much higher time resolution and full solid angle coverage arises from the fact that space plasmas are generally anisotropic, spatially inhomogeneous, and inherently time varying on scales of seconds or less. As a component of the ISTP GGS mission, POLAR has the objective of studying the global transfer of mass, momentum and energy through the sun-earth system. In particular, the temporally and spatially variable regions of the magnetosphere, such as the dayside boundary layer, the auroral zone, and dayside cusp, must be accurately characterized. This requires a new type of ion mass spectrometer capable of much faster time resolution and wider solid angle coverage than was possible with previous mass spectrometers.

The Toroidal Imaging Mass-Angle Spectrograph (TIMAS) instrument described here addresses these objectives by providing essentially the full three dimensional (3-D) velocity distribution function of all ion species within 1/2 of a satellite spin period. The TIMAS is a spectrographic imaging instrument that simultaneously measures all mass/charges (M/Q) from 1 AMU/e to greater than 32 AMU/e over a 315 deg x 10 deg instantaneous field-of-view for one energy/charge setting within ~ 20 ms. Conceptually, the TIMAS (Figure 1) may be viewed as a rotationally symmetric generalization of earlier instruments [Shelley et al., 1985; Ghielmetti and Young, 1987]. However, the geometry adopted here relies on the nonconventional "poloidal" direction of ion motion in toroids [Ghielmetti and Shelley, 1990; Young and Marshall, 1990], and thus requires a different mathematical treatment. The TIMAS ion optics are configured for first order double focusing (angle-energy), thus providing an achromatic ring-shaped image on the annular microchannel plate (MCP) detector with the mass spectrum dispersed radially and incident direction dispersed in azimuth. A list of the key instrument characteristics is provided in Table 1.

The 3-D ion distributions are measured with approximately 11 degree angular resolution over the energy per charge range of 15 eV/e to 32 keV/e. The full energy per charge range is spanned in 28 discrete steps which are spaced approximately logarithmically. Below 2 keV/e the ratio of successive energy steps is approximately 1.33 and the energy passbands are contiguous or slightly overlapping due to the preacceleration of the ions. Above 2 keV/e the ratio of successive energy steps is approximately 1.24 and the gaps between adjacent energy passbands increases due to the diminishing effects of preacceleration. At 32 keV/e the passband approaches the internal analyzer resolution of approximately 8%.

In order to handle the large quantity of data generated by the TIMAS within the telemetry allocation of 4.1 kbps, the 3-D distributions described above are integrated to varying degrees, log-count compressed and then further compressed by a lossless process [Rice and Lee, 1983; Rice, 1979]. The instrument control and data processing is carried out by a pair of SA-3300 microprocessors, each operating at 4 MHz.

The TIMAS electrostatic analyzer system requires six independently stepping, fast settling high voltage (up to 5kV) sources. In order to achieve rapid slewing at minimum power, the system uses several optically controlled current sources in a series-shunt regulator configuration operating from a few fixed high voltage converters.

Last modified February 1996 by Bill Peterson bill.peterson@lasp.colorado.edu