Metastable 2D Research Articles

The N I (5200 Å) dayglow

The production and loss mechanisms of metastable (2D) nitrogen atoms are studied on the basis of Atmosphere Explorer satellite's measurements of 5200-A emission by the visible airglow experiment, complemented by simultaneous measurements of the ion and neutral composition of the atmosphere and the photoelectron flux. The relative yield of (2D) metastable and (4S) ground-state nitrogen atoms in the creation and destruction processes is examined. The results are used to calculate NO densities, making allowance for photochemical and vertical diffusion processes.

Observations and theory of the formation of stable auroral red arcs

A population of protons with energy of some tens of keV, called the ring current, is found near the equatorial region of the magnetosphere at several earth radii. During the main phase of geomagnetic storms the ring current shifts toward lower L values into the region of the plasmapause, which is characterized by steep gradients in the plasma density. This interaction together with an anisotropic pitch angle distribution leads to ring current instability and the growth of ion cyclotron wave turbulence. As wave energy is dissipated in the ambient electron gas by Landau damping, the plasmapause electron temperature is raised to a few electron volts, and a substantial temperature gradient is created with respect to the ionosphere. The energy transferred to the ionosphere by pitch angle scattering in the low collision frequency region and by heat conduction in the collision‐dominated regime raises the ionospheric electron temperature to several thousand degrees. Therefore an appreciable number of electrons in the high‐energy tail of the Maxwellian distribution, i.e., electrons with energy greater than 2 eV, exist in the F region of the ionosphere at about 400 km, where atomic oxygen is the dominant neutral gas constituent. Two eV is the threshold for excitation of oxygen atoms to the metastable ¹D level, and these O(¹D) atoms emit 6300‐Å radiation, the signature of stable auroral red (SAR) arcs. Although the energy input rate required to produce electron temperatures sufficient to cause average SAR arcs is less than 0.1 erg cm−2 s−1, the energy radiated in the red line is only about 0.003 erg cm−2 s−1. Thus an SAR arc is an optical manifestation of a slow release of energy from the magnetosphere during a geomagnetic storm. Energetically it is small in comparison with high‐latitude auroral processes.

AN ATTEMPT TO DETECT THE PRESENCE OF METASTABLE ATOMS IN ACTIVE NITROGEN BY LIGHT ABSORPTION

In order to explain the nitrogen afterglow, Cario and Kaplan have assumed the presence of metastable 2D and 2P atoms in active nitrogen. The nitrogen emission lines at 1492 Å and 1742 Å have these metastable states as lower states, and should therefore be absorbed by active nitrogen. However, no absorption has been found by the writers. This leads to 1/6000% as an upper limit for the stationary concentration of metastable atoms in active nitrogen, which is compatible with the experiments. In order to account for the observed intensity of the afterglow, on the basis of Cario-Kaplan's theory, a lifetime of the metastable 2P atoms [Formula: see text] has to be assumed and a concentration of metastable molecules greater than 1/330%.

Photodissociation of Nitrous Oxide

THE action of light on nitrous oxide has been studied by several workers in recent years. It was found by Wulf and Melvin1 that N2O, when irradiated by light of 2350 A., broke up into NO and N. Later on, Dutta2 studied the absorption spectrum, and found that light was continuously cut off from a long wave-length limit 2750 A., and showed that the energy equivalent of this wave-length accounts for the dissociation of N2O into NO and N, both in the normal states. I extended this work3 in the Schumann region, and two more continuous absorptions were found beginning from the long wavelength limits 1850 A. and 1580 A., separated by a region of transparency ; it was shown that these further absorptions are responsible for raising N, one of the products of dissociation, to the metastable states 2D and 2P respectively. In short, the absorption of light by N2O takes place in the following steps.