molecules are quite small, resulting in large rotational energy splittings. Hydride molecules are extremely important for astronomy for several reasons. First, the large energy difference between the rotational levels makes them efficient coolants in dense gas. Also, because hydrogen is the most abundant element, hydride compounds are common in diffuse and dense clouds. Moreover, hydride species are the basic building blocks of interstellar chemistry. Understanding their abundances and distribution is key to chemical modeling. But not all hydride species are known interstellar molecules primarily because their exact transition frequencies have not yet been measured.

Future research in this largely unexplored spectral region is likely to yield additional spectral transitions and continuum bands of interest to the passive services. Administrations are urged to protect the passive services from harmful interference, particularly those bands to be used by ALMA (275-375 GHz, 385-500 GHz, 602-720 GHz, and 787-950 GHz).

Table 2.1 lists some important submillimeter molecular tracers, their frequencies, and their significance for astronomy.

Earth Exploration-Satellite Service

EESS (passive) currently uses spectrum in the range between 275 and 3000 GHz for several important measurements focusing on improving our understanding of the atmosphere and providing information needed by policy makers. A list of a few of these uses is given in Table 2.3. Table 2.2 gives a corresponding list of representative bands associated with these measurements. The list in Table 2.2 is not exhaustive. As in the case of radio astronomy, these measurements cannot be made in other bands because pressure-broadened transitions of different atmospheric constituents are being observed. In light of this and recent advances in relevant technologies, EESS use of this portion of the spectrum is expected to increase significantly. It is therefore important to protect EESS use in this region of the spectrum.

TABLE 2.3 Typical EESS Uses of Spectrum from 275 to 3000 GHz

Use/Measurement/Target Significance

Mapping of ozone, polar stratospheric clouds, chlorine sources Three-dimensional (3D) mapping of ozone in the stratosphere to understand current ozone distribution and mechanisms for its depletion
Cloud ice and frozen precipitation Key variable in the understanding of the water cycle, Earth’s energy budget, and the effect of cloud feedback on the climate, viewed in window regions around absorption features from gaseous constituents
Upper troposphere and stratospheric water vapor Key aspect of the water cycle and important for determining climate feedback effects on radiative forcing in the presence of increasing greenhouse gases. Because multiple bands are used with varying sensitivity to water vapor, this use has varying applicability as a function of instrument scan type (nadir vs. limb) and water vapor distribution
Stratospheric temperature Three-dimensional (3D) mapping of stratospheric temperature for understanding atmospheric dynamics
Upper tropospheric pollution Understanding of distribution and transport of pollutants in the upper troposphere
Trace gases Three-dimensional (3D) mapping of key atmospheric constituents (e.g., CO, SO2, HCl, BrO, N2O) tied to carbon cycle, global climate, pollution, and atmospheric transport

NOTE: This list presents some significant uses of the spectrum but is not exhaustive.

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