D. J. McEwen

Tidal oscillations of the Arctic upper mesosphere and lower thermosphere in winter

The upper mesosphere and lower thermosphere of the northern polar cap was investigated using Fabry-Perot interferometer measurements of the horizontal wind velocity in the lower thermosphere, meridian scanning photometer observations of atomic oxygen green line brightness in the lower thermosphere, and Michelson interferometer records of the infrared Meinel OH bands brightness and rotational temperature in the upper mesosphere. Tidal airglow oscillations in the absence of local solar heating were studied using a superposed epoch analysis of observations obtained around three new-Moon intervals (November 9-22, 1993, December 7-20, 1993, and January 4-17, 1994) in Eureka (80°N). Tidal harmonics were uncovered with an iterated least chi-squared fit, and their existence in the data tested using a goodness-of-fit probability against a null hypothesis of no oscillations. No single dominant tide was found at all times and altitudes of the winter Arctic upper mesosphere and lower thermosphere. The horizontal wind velocity of the lower thermosphere exhibited all the first three harmonics of the tide, with the largest-amplitude oscillations shown by the diurnal component of the meridional wave speed. The atmospheric layer near the mesopause that contributes to the OH airglow emission experienced terdiurnal oscillations throughout the winter season. Theoretical polarization relations for evanescent tides, considerations of energy density propagation with altitude, and a day-by-day analysis of the zonal and meridional wind speed and temperature all indicate that the terdiurnal tide observed on November was an evanescent zonally symmetric tide. Other data indicate propagating tides, migrating tides, or a mixture of both.

Temperature and airglow brightness oscillations in the polar mesosphere and lower thermosphere

Abstract Large amplitude oscillations in airglow brightness and OH rotational temperature were observed on Dec. 31, 1993, 00 UT to Jan. 1, 1994, 18 UT over Eureka (80.0°N). The airglow brightnesses of atomic oxygen O[1S] emission at 5577 A and sodium Na ( 2 P 3 2 , 1 2 ) emission at 5890 and 5896 A were measured by a multi-channel, meridian scanning photometer. The Meinel OH (3,1) band brightness and rotational temperature were monitored by a Michelson interferometer. The period of the observed oscillations was 8.4±1 h. Supporting evidence for a non-migrating tide (zero zonal wave number) explanation of these oscillations are an observed period close to 8 h, a large vertical wavelength as derived from the airglow emissions, and wave persistence for five complete cycles. On the other hand, the small amplitude and negative phase of Krassovskys ratio are not consistent with a tidal source for the observed oscillations. Similar 8.5±1 h variations were observed on Dec. 21, 1993. The case for a non-migrating tide as a source for these oscillations is further weakened by a 1.5 h difference in local time when these oscillations were observed to peak during the two events (which occurred 10 days apart). Explanation of the above observations in terms of an inertiogravity wave is favored by the relatively infrequent appearance of these oscillations at all altitudes monitored, the incoherency between the two events, and the fact that temperature variations lead OH airglow brightness variations, in qualitative agreement with gravity wave theory. The observed lower limit to the horizontal wavelength of the wave compares with its value predicted by the dispersion relation of inertio-gravity waves. Forced planetary waves, observed to peak in amplitude on Dec. 21, 1993, and Jan. 1, 1994, as well as the stratospheric warming of the middle of December and end of December/beginning of January, were likely driven by the same tropospheric disturbance or directly related to the source of the inertio-gravity wave observed over Eureka on Dec. 21 and Dec. 31-Jan. 1, respectively.

Polar cap study during northward interplanetary magnetic field on 19 January 1998

The interplanetary magnetic field (IMF) was northward for an extended period on 19 January 1998. This caused the open polar cap of the ionosphere to become very small and the auroral emission to move poleward. The auroral emission at 630 nm was observed by the meridian scanning photometer located at Eureka near the north magnetic pole. Effects of changes in sign of the dawn–dusk component of the IMF were also observed. A magnetohydrodynamic simulation model of the magnetosphere and ionosphere was used to study these events. The model was driven using data from the Wind and IMP-8 spacecraft. The simulation results show a very small open polar cap indicating that the magnetosphere is nearly closed. Moreover, in response to the shift from dawnward to duskward IMF, a narrow strip of closed field breaks off from the dawn boundary and convects across the polar cap and into the dusk boundary.

Observations of mesospheric neutral wind 12-hour wave in the Northern Polar Cap

Abstract Combined three-station neutral wind observations from 70° to 80°N are used to study the 12-h oscillation from 87 to 130 km altitude. A strong 12-h wave with a 37 km vertical wavelength was observed at Troms o (69.6°N). The observed phases and vertical wavelength are consistent with the predictions of the Global Scale Wave Model-98 (GSWM-98) for the westward zonal wavenumber two semi-diurnal migrating tide (SDW2). However, the observed amplitudes are much greater than the model prediction at Troms o . At Resolute (74.9°N), the observed 12-h oscillation in neutral winds appears to be have large contribution from the SDW2, based on the zonal phase shift from Troms o to Resolute. At Eureka (81.1°N), the 12-h oscillation (not the strongest wave) does not have the predicted phase shift from Troms o based on the zonal wavenumber of the SDW2. The amplitudes of the 12-h oscillation at Resolute and Eureka are much smaller than those predicted by the GSWM-98 for the SDW2. We believe that the contribution from non-migrating semi-diurnal tide (perhaps, the zonal wavenumber one SDW1) is the likely cause of the inconsistency between the observed and predicted SDW2 in phases and amplitudes at high latitudes.

Quasi-periodic poleward motions of Sun-aligned auroral arcs in the high-latitude morning sector: A case study

This is the first paper which reports the characteristics of quasi-periodic poleward motions of Sun-aligned auroral arcs in the high-latitude morning sector. The moving arcs are observed from ground-based stations at magnetic latitudes (MLAT) of 78° and 84° during magnetically quiet intervals (interplanetary magnetic field Bz ∼ 0 or > 0). The arcs move poleward repeatedly with a period of several minutes and a velocity of ∼400–500 m/s and disappear at around 85° MLAT. For the event observed at 78° MLAT, the arcs are repeatedly detached from a stable aurora which is located at the equatorward of the arcs. The moving arcs correspond to accelerated electrons observed by the Exos D satellite. The stable aurora corresponds to continuous precipitation of high-energy electrons which probably originate from the inner part of the plasma sheet. The ion drift data from the DMSP-F11 satellite show that the poleward moving arcs are located around the boundary of the large-scale sunward flowing region at lower latitudes and the antisunward flowing region at higher latitudes. From these results, we conclude that the arcs are connected to the boundary region between the plasma sheet and the low-latitude boundary layer in the morningside tail flank. Several mechanisms which can produce the observed motions of the arcs are discussed.

Quasi‐periodic poleward motions of morningside Sun‐aligned arcs: A multievent study

We study eight events of the quasi-periodic poleward motions of Sun-aligned auroral arcs in the high-latitude morning sector using all-sky image data and meridian scanning photometer data obtained at Resolute Bay (RSB; 84° magnetic latitude), Canada, during four winters from December 1992 to February 1996. More than 10% of morningside ares observed at RSB move poleward repeatedly. The periodic arc motions occur during intervals of interplanetary magnetic field B >0 or ∼0, B x >0 or 0 or ∼0. Most of the events occur around the end of substorm-like magnetic activity. suggesting that the source of moving arcs is related to the plasma sheet particles. Comparison with a global MHD simulation suggests that the observed poleward motion of arcs corresponds to inward (+Y GSM direction) motion of source plasmas in the morningside tail flank where the plasmas from the low-latitude boundary layer and from the plasma sheet are mixed.

A statistical study of the motions of auroral arcs in the high-latitude morning sector

Motions of auroral arcs in the high-latitude morning sector (0300-0900 magnetic local time) have been studied statistically for magnetically quiet periods, using the data from all-sky images for two winters from two ground-based stations at magnetic latitudes (MLAT) of 84° and 78°. Most of the observed arcs are in a sum-aligned direction. Most of the arcs observed at 84° MLAT move duskward with a typical velocity of ∼500 m/s. A similar duskward motion of the arcs with a slightly higher velocity is dominant at 78° MLAT, though some arcs move antisunward at this latitude. It is suggested that the observed duskward auroral motion is related to poleward shrinkage of the morningside oval on closed field lines during magnetically quiet conditions.

Ground‐based optical observations from the north magnetic pole during the January 1997 magnetic cloud event

Dynamic and intense polar auroras were observed over Eureka, Canada (88.9°N magnetic) through January 10 and 11 following the solar coronal mass ejection (CME) of January 6, 1997. A dusk sector arc appeared by 0200 UT on January 10 following the arrival of the magnetic shock at 0110 UT; aurora filled the Eureka field of view between 0225 and 0300 UT. Spectral easurements with allsky cameras, meridian scanning photometers and a CCD spectrograph showed this aurora to be excited by electrons of E0 ≈ 3 keV. After an interval of F-layer patches, faint F-layer arcs (E0 ≈ 300 eV) reappeared by 1900 UT and continued until 0700 UT on January 11, intensifying between 0200 and 0700 UT. The initial auroral display was remarkable in its spectral characteristics, while the prolonged F-layer aurora which followed was similar to the aurora observed during the October 1995 magnetic cloud event.

A continuous view of the dawn-dusk polar cap

Continuous monitoring of airglow and auroral emissions over Eureka (89° magnetic latitude) through each winter has shown that the dusk and dawn boundaries of the polar cap can be routinely seen when the IMF B 2 is northward. When this condition prevails the auroral oval shrinks poleward, and the polar boundaries are clearly and continuously seen in photometric dusk-dawn meridian scans of OI 630 nm emission brightness. This fact is illustrated with two 24-hr days of records on December 19, 1996 and January 19, 1998. These examples show, and others similarly, that polar arcs emerge from the dusk or dawn flanks of the auroral oval (depending on the sign of B y ) and thus occur on closed field lines. In a few cases polar arcs emerge from the midnight or noon sector of the oval and extend along the sun-earth line to cross the Eureka meridian, bifurcating the polar cap. This new technique for monitoring the polar cap dusk-dawn extent and all auroral activity within promises to be valuable for diagnostic studies of both polar auroral activity and magnetospheric topology during solar wind changes and major sun-earth events.

The all-sky camera revitalized

An all-sky camera, a ground imager used since the 1950s in the aeronomy and space physics studies, was refurbished with a modern control, digitization, and archiving system. Monochromatic and broadband digital images of airglow and aurora are continuously integrated and recorded by the low-cost unmanned system, which is located in northern Canada. Radiometric corrections applied to the data include noise subtraction, normalization to a flat-field response, and absolute calibration. The images are geometrically corrected with star positions and projected onto a geographic or geomagnetic coordinate system. An illustration of the application of corrected all-sky camera images to the study of auroral spirals is given.