Midnight Pressure Bulge Research Articles

Airglow observations of nighttime medium‐scale traveling ionospheric disturbances from Yonaguni: Statistical characteristics and low‐latitude limit

AbstractThe characteristics of nighttime medium‐scale traveling ionospheric disturbance (MSTID) features observed over Yonaguni (24.5°N, 123.0°E; 19.3°N dip latitude), Japan are studied using all‐sky imaging of OI 630.0 nm airglow emission. The uniqueness of these observations is that the area observed by the imager covers the transition region between low to middle latitudes in the ionosphere. Typical low‐latitude limit of midlatitude‐type nighttime MSTIDs possessing phase front alignments along the northwest to the southeast occurs in this region. These MSTID features are rarely sighted at dip latitudes below 15°. We selected 2 year period for analysis in which 1 year corresponded to the solar minimum conditions and another year to the solar maximum conditions. The MSTIDs were observed to extend to farther lower latitudes during the solar minimum conditions than during the solar maximum periods. Their observed range of wavelengths, phase velocities, phase front alignment, and propagation directions are similar to those observed at typical midlatitude sites. However, on many occasions the phase fronts of the observed MSTIDs did not extend over the whole field of view of the imager indicating that some process inhibits their extension to further lower latitudes. Detailed investigation suggests that the poleward propagating enhancement of airglow intensity, probably associated with the midnight pressure bulge, causes the MSTID features to disappear when they reach lower latitudes later in the night. When the MSTIDs reach lower latitudes well before midnight, they are found to be inhibited by the equatorial ionization anomaly crest region.

Observations and modeling of post-midnight uplifts near the magnetic equator

Abstract. We report here on post-midnight uplifts near the magnetic equator. We present observational evidence from digital ionosondes in Brazil, a digisonde in Peru, and other measurements at the Jicamarca Radio Observatory that show that these uplifts occur fairly regularly in the post-midnight period, raising the ionosphere by tens of kilometers in the most mild events and by over a hundred kilometers in the most severe events. We show that in general the uplifts are not the result of a zonal electric field reversal, and demonstrate instead that the uplifts occur as the ionospheric response to a decreasing westward electric field in conjunction with sufficient recombination and plasma flux. The decreasing westward electric field may be caused by a change in the wind system related to the midnight pressure bulge, which is associated with the midnight temperature maximum. In order to agree with observations from Jicamarca and Palmas, Brazil, it is shown that there must exist sufficient horizontal plasma flux associated with the pressure bulge. In addition, we show that the uplifts may be correlated with a secondary maximum in the spread-F occurrence rate in the post-midnight period. The uplifts are strongly seasonally dependent, presumably according to the seasonal dependence of the midnight pressure bulge, which leads to the necessary small westward field in the post-midnight period during certain seasons. We also discuss the enhancement of the uplifts associated with increased geomagnetic activity, which may be related to disturbance dynamo winds. Finally, we show that it is possible using simple numerical techniques to estimate the horizontal plasma flux and the vertical drift velocity from electron density measurements in the post-midnight period.

Brightness variations of the northern 630nm intertropical arc and the midnight pressure bulge over Eritrea

Abstract. The nightglow brightness at 630nm from the thermospheric O(1D) layer was monitored nightly at Asmara, Eritrea (15.4° N, 39.9° E, 7° N dip) with an all-sky imager. Averages of north-south strips of the images enabled contour plots of brightness on a latitude vs. local time grid. The contours show the movement of the intertropical arc southward before midnight, staying just north of Asmara after midnight, and gradually brightening to a maximum at 02:00h local civil time, 02:00 LT, after which it disappears before dawn. It is argued that all features of the plots can be explained by known mechanisms capable of driving ions along magnetic field lines, including the fountain effect, summer to winter transequatorial winds, and the midnight pressure bulge. The 02:00 LT brightness maximum is the most striking and the most persistent feature in the data. The persistence of the location of the 02:00 LT brightening is attributed to a pressure bulge centered on the geographic equator at midnight and extending to higher latitudes with increasing local time in both the winter and the summer hemispheres. The bulge is shown to be stronger near solstice than near equinox, confirming earlier work.

Nighttime thermospheric meridional winds at Cachoeira Paulista (23°S, 45°W): Evidence for effects of the equatorial midnight pressure bulge

We have studied the local time and seasonal variations of thermospheric nighttime meridional winds at the low‐latitude station, Cachoeira Paulista, Brazil, corresponding to quiet geomagnetic conditions of high solar activity (Fl0.7 > 150 units). The meridional winds are derived from F layer peak height (hmax) data using a modified form of the “servo” model wherein plasma transport due to electric field is taken into account. It is found that during the solstices the meridional winds exhibit the transequatorial neutral airflow from the summer to the winter hemisphere, i.e., the nighttime winds are primarily equatorward (poleward) during the December (June) solstice. It is shown that this neutral wind flow pattern is effectively modulated by winds related to equatorial midnight temperature maximum (MTM) and the associated midnight pressure bulge, especially in the December solstice (local summer). The seasonal visibility of the effect of MTM on low/equatorial latitude meridional winds seems to be different in the east Brazil (45°W) and Indian (75°E) sectors.

Local time and altitude variation of equatorial thermosphere midnight density maximum (MDM): San Marco drag balance measurements

We present the first study of the local time and altitude variation of the MDM, the density component of the equatorial midnight pressure bulge, an important feature in the nighttime motions of the ionosphere‐thermosphere system in the equatorial region. The neutral density data of the San Marco 3 (SM3) and San Marco 5 (SM5) satellites were averaged to obtain 24 hr density variations from April to December in 1971 and 1988. From these variations, the MDM amplitude as a function of altitude and local time was obtained at altitudes from 220 to 400 km at the geographic equator. We show the first evidence of downward phase propagation of the MDM together with a vertical structure not observed before, which may suggest an ionospheric interaction or perhaps a viscous dissipation effect. In 1971, the MDM propagates rather gradually from about 350 to 220 km in about two hours. In 1988, there appear two regions in which the MDM time shows little change with height with a discontinuous jump in between the two regions. The net effect is still a downward displacement of the MDM with local time. The results show significant seasonal and solar activity effects. During low solar activity periods (1971), the MDM occurs earlier in equinox than in solstice for all altitudes, consistent with the seasonal variation of the midnight temperature maximum (MTM). However, with higher solar activity (1988) and at altitudes below 340 km, the MDM occurs earlier in solstice than in equinox, raising new questions on solar activity effects on the relative phases of the tidal oscillations of the neutral density and the neutral temperature.

Modeling the Arecibo nighttime F2 layer: 2. Ionospheric gradients

The servo model is extended and used to fit horizontal gradients in the F2 layer height and density and to estimate the zonal Pedersen current and its zonal and meridional gradient. Horizontal gradients were measured from the Arecibo Observatory during the following five nights: August 16‐17 and 17‐18, 1982; and October 4‐5, 5‐6, and 9‐10, 1983. The model gradients are driven by nonzero current gradients, which are applied as needed to fit the measured gradients in the F2 peak. The gradient in the peak height is accurately reproduced; the peak density gradient is calculated self‐consistently in the model. The divergence of the Pedersen current can be deduced when the current flows zonally and is found to differ from zero. This is a consequence of zonal divergence of the model zonal current. Expressions are derived for the divergence of the Hall current and for the curl of the current in the presence of ionospheric gradients. The vertical vorticity of the F region current is determined from the radar and optical measurements and the mass spectrometer/incoherent scatter (MSIS) neutral densities. Both neutral and plasma motions generate current vorticity equally as expected from the F region dynamo. The measured velocity gradients produce more current gradients and vorticity than the measured conductance gradients. The measured height gradient in the perpendicular‐north plasma drift (∂zυ⊥N) is the dominant term in the vorticity and drives the two current shears that cause vorticity. Geometrical factors increase (∂zυ⊥N) by about 13% in the zonal gradient of the meridional current over the meridional gradient of the zonal current. This causes anticorrelation between the former current shear and the vorticity. The two measured shears generally follow each other, have opposing vorticities and large uncertainties. The nighttime current may be irrotational or have constant vorticity. Large current gradients occur in conjunction with observed descents of the F2 peak height. The gradients are interpreted as due to the midnight pressure bulge at low latitudes. Short‐period gravity waves of meteorological origin are ruled out as they were not observed and are limited in their ability to reach ionospheric heights. The harmonic analysis used to obtain horizontal wind gradients is largely unaffected by spatially uniform wind accelerations. Therefore the deduced spatial variations in the measured winds are unlikely to be due to temporal variations.

Thermosphere and F-region plasma dynamics in the equatorial region

The dynamics of the equatorial thermosphere and the F-region ionospheric plasma are reviewed highlighting several features observed with in-situ satellite and ground-based experiments. Attention is given to the midnight temperature maximum (MTM) and related phenomena and to recent results on zonal neutral and plasma flows at F-region heights. The midnight temperature maximum and its midnight pressure bulge above 250 km altitude lead to neutral wind variations which significantly affect the F-region equilibrium height and its airglow emissions. During magnetically active periods, enhanced meridional winds from the poles lead to strong meridional intensity gradients (MIGs) in the atomic oxygen emission at 6300Å; MIGs have been used to estimate the magnitude of meridional wind gradients during active periods, and these estimates are consistent with measurements using incoherent scatter radar and optical Fabry-Perot interferometry. The pressure gradients which drive the thermospheric wind have been estimated using averaged density and temperature data, and the results have been used to check the consistency of the current data base in terms of the momentum equation. New analyses of the AE-E data are presented as further evidence of the effect of the MTM on the latitude-local time distribution of the meridional wind reversal. The tidal decomposition studies of the neutral temperature and of both ion and neutral flows are reviewed. The zonal plasma flow is found to be closely coupled to the zonal neutral wind as a consequence of the F-region dynamo, and more recently, the F-region dynamo has been found to play an important role in an anomaly in the latitudinal distribution of the equatorial zonal plasma flow.

Interaction of zonal winds with the equatorial midnight pressure bulge in the Earth's thermosphere: Empirical check of momentum balance

A minimum is observed at midnight in the time variation of averaged zonal winds measured in‐situ near the equator at an average altitude of about 350 km, and we inquire whether observations of the equatorial midnight pressure bulge are consistent with this feature. The observed zonal wind maintains an eastward direction all night. It increases steadily in the early evening reaching a maximum of about 160 m/s at 2100 hrs local time (LT), dropping to a minimum of about 70 m/s around 2400 LT, then up to about 120 m/s around 0300 LT, decreasing thereafter and turning westward in the morning around 0500 LT. The nighttime eastward ion drift at Jicamarca, averaged over periods of similar solar activity (1970‐71), shows the same signature, but generally with amplitude between 70 and 90% of the zonal wind amplitude. The 1978‐81 Jicamarca ion drifts are similar except from 2400 to 0300 LT when they are about 30% larger than the in‐situ zonal wind averages. The ion‐drift is used together with the measured zonal winds to estimate the ion‐drag effect in the momentum equation. The pressure term of this equation is estimated using in‐situ measurements of the neutral temperature and density. The pressure term is found to be large before 2100 LT (∼0.05 m/s²), consistent with the steady increase in zonal wind before 2100LT, drops rapidly to near zero (roughly 0.005 m/s) before 2200 LT, and then increases slowly beginning at 2300 to about 0.015 m/s² just before 0300 LT when it is observed to reverse. The small pressure term observed from 2100 to 2400 LT is associated with the midnight pressure bulge and accounts for the decay of the wind from 160 m/s at 2100LT to 70 m/s at midnight. The larger value at 0300LT is consistent with the second maximum in the wind. It is found that the existing data sets are consistent in terms of zonal momentum balance. The striking similarity between the zonal wind and the eastward ion‐drift is responsible for the small ion‐drag term before midnight which is required to maintain the large zonal winds observed.

Equatorial airglow depletions induced by thermospheric winds

Interferometric observations of the 630.0 nm nightglow brightness at the equatorial station of Arequipa, Peru [16.2°S, 71.4°W geographic, 3.2°S dip latitude] have revealed widespread areas of airglow depletion, with reductions in intensity as large as factors of 3 or 4. These depletions correlated closely with large increases of the equatorward (northward) wind and the 630.0 nm kinetic temperature. On occasion, the usually small meridional wind reached a velocity of 100 m/s near 22h LT lasting for 1 to 2 hours. The temperature increases of 100 K or more existed only in the poleward (southward) direction. Comparisons with modelling calculations suggest that this effect results from an upward movement of the ionosphere along the inclined magnetic field lines, driven by the equatorward neutral wind. The airglow column integrated emission rate is consequently decreased by the slower rate of formation and subsequent dissociative recombination of molecular oxygen ions within the higher F‐layer. We conclude that the transient period of equatorward wind is a result of the passage of the midnight pressure bulge.

Seasonal variation in the occurrence time of the equatorial midnight temperature bulge

We have reexamined incoherent scatter radar data on electron and ion temperature from the Jicamarca Radar Observatory (12°S, 77°W; 1°N diplat) for the period 1967‐68 and early 1969. We find clear evidence of systematic nighttime temperature increases presumably associated with the “midnight pressure bulge” and the collapse of the middle latitude F region. A significant new finding is that the temperature enhancements tend on the average to occur about two hours before midnight in the local summer and about one hour after midnight in the local winter. The bulges appear in various shapes and the enhancement magnitudes vary considerably from barely detectable to many tens of degrees Kelvin, but show a definite tendency to be smaller in winter than in summer, when increases of up to 200 K are sometimes observed. Changeover from “summer” to “winter” behavior occurs within two months or less around equinox. Our results are consistent with the hypothesis of a nighttime equatorial temperature bulge moving with the antisolar meridian, but, at Jicamarca, systematically centered ∼ 30° west of this meridian ( ∼ 2 hrs before midnight) near the December solstice and ∼ 15° east of it near the June solstice. Our data have relevance to the questions of seasonal changes in the phase and amplitude of the tidal modes responsible for the midnight pressure and temperature bulge.

Equatorial night-time F-region events: a survey of 6300 Å airglow intensity maps at Arecibo

6300 Å nightglow intensities have been mapped along an arc following the emission layer. The resulting time vs zenith angle airglow maps show significant structure which is classified into three main types: meridional intensity gradients (MIG), F-region southward propagating waves, and short wavelength (about 100 km) events. Interpretation of winter MIG data suggests that the equatorial midnight pressure bulge is in the northern hemisphere in winter and in the southern hemisphere in summer.

A comparison of the ionospheric equatorial anomaly in the East Asian and the American regions at Sunspot minimum

Using ƒoF2 and M(3000)F2 data, taken at stations spread over narrow longitude ranges and giving a good coverage of dip location near the magnetic equator, a comparison has been undertaken of the diurnal variations of the ionospheric equatorial anomaly at different seasons in East Asia and America. The very significant differences observed have been explained in terms of (1) the relative inferred locations of the thermal pressure bulges with respect to the magnetic equator, (2) magnetic declination effects (only significant at high latitudes in South America) and (3) the greater magnetic field in Asia. For both regions maximum symmetry of the equatorial anomaly crests has been observed in the particular month for which the midday overhead sun occurred near the magnetic equator. The equatorial anomaly was found to persist later at the solstices in America but at equinox it persisted until 0300–0400 LT in both regions. Some re-generation of the anomaly seems to take place after sunset but in addition it appears that poleward winds from the so-called ‘midnight pressure bulge’ converge with the winds from the poles thereby delaying the decay of the anomaly crests.

6300‐Å airglow meridional intensity gradients

Airglow meridional intensity gradients (MIG) that moved from east to west with the antisolar meridian have been observed during times of considerable magnetic activity. The magnitude of the gradient is around 0.1 R/km or more. We believe the MIG phenomenon develops as a result of a velocity gradient that is produced by the meeting of opposing meridional neutral winds. The northern wind system arises from the auroral heating at high latitude that produces meridional winds penetrating to low latitudes in the summer hemisphere. The southern wind system is produced by the equatorial midnight pressure bulge that is formed by the thermal tides of the lower atmosphere combined with the in situ forcing by EUV radiation absorption in the daytime. In a magnetic storm the merging of the two wind systems produces a sharp front over a latitudinal width of several hundred kilometers. North of the front the ionosphere is elevated, and the airglow is weak. South of the front the ionosphere is depressed, and the airglow is enhanced with a surface brightness of the order of 100 R. The conceptual model that evolved from these observations accounts for the overall behavior of the MIG as well as the normal midnight collapse. A simple calculation shows that the MIG can be used to estimate the meridional velocity gradient as well as the magnitude of the winds in the midnight pressure bulge.