TIDE- Instrument Description

Science Objectives:
The Thermal Ion Dynamics Experiment (TIDE) and Plasma Source Instrument (PSI) have been developed in response to the requirements of the ISTP program for three-dimensional (3D) plasma composition measurements capable of tracking the circulation of low-energy plasmas throughout the polar magnetosphere. Large and relatively steady outflows of low energy ionospheric heavy ions (O++, O+, N+, and, to a lesser extent, N2+, NO+, and O2+) were identified by the Dynamics Explorer 1 Retarding Ion Mass Spectrometer (DE 1/RIMS) as originating in the auroral zone region [Waite et al., 1985; Moore et al., 1985; Lockwood et al., 1985; Pollock et al., 1990], particularly from the dayside. Relatively steady outflows of light ionospheric ions (H+, He+) were identified as originating from the entire high latitude region [Nagai et al., 1985; Chandler et al., 1991]. This plasma is in part lost into the downstream solar wind and in part recirculated within the inner magnetosphere, participating in the formation of the diamagnetic hot plasma sheet and ring current plasma populations [Moore et al., 1990; Delcourt et al., 1990], as well as the outer plasmasphere.

More recently [Giles et al., 1993] it has been discovered that the auroral zone is a significant source of m/q = 2 ions. Whereas such ions occur at an abundance relative to H+ of order 10-3 in the cold ionosphere (consistent with cosmic Deuterium abundance), the relative flux of such ions in auroral beams and conics is roughly two orders of magnitude higher. This strongly suggests that He++ is being created by the energetic electron precipitation of the auroral zone, calling into question the validity of source contribution inferences based on He++ composition. The fact that the ionosphere is a significant source of the major solar wind species introduces an ambiguity that can only be resolved by establishing unambiguously the transport of the ionospheric and solar plasmas within the magnetosphere.

The nominal 2 x 9 RE orbit of the POLAR spacecraft will project it high over the polar caps in the lobes of the magnetotail where it will be perfectly positioned to sample the continuation of the ionospheric outflows into the nightside magnetosphere as well as the entrainment of solar wind plasma into magnetospheric circulation. However, as noted above, the high altitude and low plasma densities present an observational challenge.

TIDE provides the sensitivity (seven channels having effective apertures approaching 1 cm2 each) and angular resolution required for this purpose. PSI produces a low-energy plasma locally at the POLAR spacecraft, providing the ion current required to balance the photoelectron current, along with a low temperature electron population, thus providing for the regulation of the spacecraft potential slightly positive relative to the space plasma.

Thus TIDE/PSI will address the following objectives:

(a) to measure the position-dependent density and flow velocity field of the polar wind, the heated auroral zone plasma outflows, and the low energy component of magnetosheath plasmas (< 500 eV) in the high polar cap and magnetospheric lobes.

(b) to quantify the extent to which ionospheric and solar ions are recirculated within the distant magnetotail neutral sheet or lost to the distant tail and solar wind.

(c) to investigate the mass dependent degree of energization achieved as plasma flows antisunward on merged flux tubes through the polar cap and into the lobes, by measuring the thermodynamic properties of these plasmas.

(d) to investigate the relative roles of ionosphere and solar wind as sources of plasma to the plasma sheet and ring current, by identifying the characteristic populations and distinguishing their behaviors using measurements of composition, flow, and thermodynamic properties.