Polar cusp ion drift studies through high‐resolution interferometry of O⁺ 7320‐Å emission

Abstract

High‐resolution Fabry‐Perot interferometric measurements of Doppler shift in the O+(²P°3/2 → ²D°5/2) emissions from daytime polar cusp auroras over Longyearbyen (Λ = 75°N), Svalbard, are presented to illustrate some temporal aspects of polar F region ion drifts. These measurements show some of the features expected of a pass under the throat region of a twin cell convection pattern. The dayside limits of the morning and evening convection cells can be discerned from the Doppler shift observations, but the expected strong poleward flow near local noon is not present. Ion convection velocities are found to vary from 100 to 800 ms−1. The interferogram contains profiles of the OH (8, 3) P1(2) rotational line at 7316.4 Å (completely resolved from the O+(²P°3/2 → ²D°5/2) 7320.2‐Å line), and the O+(²P°1/2 → ²D°5/2) 7319.4‐Å line which shows up on the short‐wavelength shoulder of O+(²P°3/2 → ²D°5/2) 7320.2‐Å line; all these lines appear in different orders. Most of the time the interferogram yields the 7320.2‐Å/7319.4‐Å intensity ratio value equal to the statistically weighted ratio of Å coefficients for the ²P1/2.3/2→ ²D5/2 transitions, though at times enhancement of the 7319.4‐Å line with respect to the 7320.2‐Å line is observed. The OH emission is generally weaker than the O+ 7320‐Å line, a result confirmed by simultaneous spectrophotometric observations which record maximum O+(²P) doublet intensities in the range of 20–230 R. Spectroscopic observations also confirm the absence of the N21P (5, 3) band or any other auroral emission in the cusp auroral spectra around 7320 Å, which may complicate interferometric studies of the ion line; the nearest OH rotational line is at 7316.4 Å. Additionally, the O+ 7320 Å doublet is at least an order of magnitude more intense than the OH 7316.4‐Å line in long‐rayed auroral forms occurring in the cusp region prior to and after local magnetic noon; during local magnetic noon the average energy of precipitating particles is much less than the energy of maximum ionization cross section for O. Temporal variations in the O+ 7320–7330‐Å emissions, observed with the spectrometer, indicate an effective lifetime of O+(²P) in the range 2.4±0.4 to 5±1 s; these time constants correspond to peak O+ emission heights above 200 km.

Determining rotational temperatures from the OH(8-3) band, and a comparison with OH(6-2) rotational temperatures at Davis, Antarctica

“…This value is consistent with our measurement. However, Smith et al (1982) list in their Abstract a separation of 0.08 nm for these O + lines. This may not be a value that they have measured, and it is not supported by our measurements, which indicate a separation of 0.105 ±0.005 nm.…”

“…The upper state is activated by low energy auroral electrons or sunlight. Collisional deactivation limits intense O + emissions at λ732 nm and λ733 nm to altitudes above 200 km (Rusch et al, 1977;Smith et al, 1982). The high altitude of these ionic emissions means they can still be activated by sunlight after the hydroxyl layer at ∼87 km is in darkness.…”

“…Further O + emissions at λ732.97 nm ( 2 P o 1/2 → 2 D o 3/2 ) and at λ733.07 nm ( 2 P o 3/2 → 2 D o 3/2 ) are blended with P 2 (3) at λ732.92 nm. These O + lines result from a metastable state with a theoretical radiative lifetime of ∼5 s (Smith et al, 1982). The upper state is activated by low energy auroral electrons or sunlight.…”

“…2 yields wavelengths of λ732.02 nm and λ731.915 nm and a relative intensity of 1:0.28. Smith et al (1982), who observed this region with a Fabry-Perot spectrometer, use a wavelength of λ732.02 nm for the O + ( 2 P o 3/2 → 2 D o 5/2 ) emission. This value is consistent with our measurement.…”

First direct optical observations of plasma flows using afterglow of in discrete aurora

“…In this case the oxygen red line at 630 nm is used (Hernandez and Killeen, 1988). Smith et al (1982) measured polar ion drifts with a Fabry-Perot interferometer by observing the Doppler shift of the O þ 2 P 3=2 -2 D 5=2 emission at 732.0 nm. This method is limited in the ability to deduce horizontal components of plasma drifts, because the flow vector is obtained by combining multiple lineof-sight measurements at different times and in different look directions.…”