The generation of vertical thermospheric winds and gravity waves at auroral latitudes—I. Observations of vertical winds

Observations of vertical and horizontal thermospheric winds, using the OI (3P-1D) 630 nm emission line, by ground-based Fabry-Perot interferometers in Northern Scandinavia and in Svalbard (Spitzbergen) have identified sources of strong vertical winds in the high latitude thermosphere. Observations from Svalbard (78.2N 15.6E) indicate a systematic diurnal pattern of strong downward winds in the period 06.00 U.T. to about 18.00 U.T., with strong upward winds between 20.00 U.T. and 05.00 U.T. Typical velocities of 30 m s^?1 downward and 50 m s^?1 upward occur, and there is day to day variability in the magnitude (30–80 m s^?1) and phase (+/- 3 h) in the basically diurnal variation. Strong and persistent downward winds may also occur for periods of several hours in the afternoon and evening parts of the auroral oval, associated with the eastward auroral electrojet (northward electric fields and westward ion drifts and winds), during periods of strong geomagnetic disturbances. Average downward values of 30–50 m s^?1 have been observed for periods of 4–6 h at times of large and long-lasting positive bay disturbances in this region. It would appear that the strong vertical winds of the polar cap and disturbed dusk auroral oval are not in the main associated with propagating wave-like features of the wind field. A further identified source is strongly time-dependent and generates very rapid upward vertical motions for periods of 15–30 min as a result of intense local heating in the magnetic midnight region of the auroral oval during the expansion phase of geomagnetic disturbances, and accompanying intense magnetic and auroral disturbances. In the last events, the height-integrated vertical wind (associated with a mean altitude of about 240 km) may exceed 100–150 m s^?1. These disturbances also invariably cause major time-dependent changes of the horizontal wind field with, for example, horizontal wind changes exceeding 500 m s^?1 within 30 min. The changes of vertical winds and the horizontal wind field are highly correlated, and respond directly to the local geomagnetic energy input. In contrast to the behaviour observed in the polar cap or in the disturbed afternoon auroral oval, the ‘expansion phase’ source, which corresponds to the classical ‘auroral substorm’, generates strong time-dependent wind features which may propagate globally. This source thus directly generates one class of thermospheric gravity waves. In this first paper we will consider the experimental evidence for vertical winds. In a second paper we will use a three-dimensional time-dependent model to identify the respective roles of geomagnetic energy and momentum in the creation of both classes of vertical wind sources, and consider their propagation and effects on global thermospheric dynamics.     

The generation of vertical thermospheric winds and gravity waves at auroral latitudes—II. Theory and numerical modelling of vertical winds

Recent observations of strong vertical thermospheric winds and the associated horizontal wind structures, using the 01(3P-1D)nm emission line, by ground-based Fabry-Perot interferometers in Northern Scandinavia have been described in an accompanying paper (Paper I). The high latitude thermosphere at a height of 200–300 km displays strong vertical winds (30–50m ms^?1)of a persistent nature in the vicinity of the auroral oval even during relatively quiet geomagnetic conditions. During an auroral substorm, the vertical (upward) wind in the active region, including that invaded by a Westward Travelling Surge, may briefly(10–30 min)exceed 150 m s^?1. Very large and rapid changes of horizontal wind structure (up to 500 m^?1 in 30 min) usually accompany such large impulsive vertical winds. Magnetospheric energy and momentum sources generate large vertical winds of both a quasi-steady nature and of a strongly time-dependent nature. The thermospheric effects of these sources can be evaluated using the UCL three-dimensional, time-dependent thermospheric model. The auroral oval is, under average geomagnetic conditions, a stationary source of significant vertical winds (10–40 m s^?1). In large convective events (directly driven by a strong momentum coupling from the solar wind) the magnitude may increase considerably. Auroral substorms and Westward Travelling Surges appear to be associated with total energy disposition rates of several tens to more than 100 erg cm^?2s^?1, over regions of a few hours local time, and typically 2–5° of geomagnetic latitude (approximately centred on magnetic midnight). Such deposition rates are needed to drive observed time-dependent vertical (upward) winds of the order of 100–200m s^?1.The response of the vertical winds to significant energy inputs is very rapid, and initially the vertical lifting of the atmosphere absorbs a large fraction (30% or more) of the total substorm input. Regions of strong upward winds tend to be accompanied in space (and time) by regions of rather lower downward winds, and the equatorward propagation of thermospheric waves launched by auroral substorms is extremely complex.     

Nonpotential waves in the auroral electrojet

An ionospheric plasma instability in the auroral electrojet region believed to be responsible for electromagnetic waves with angular frequency smaller thanv _i is discussed. The threshold current velocityU _t and oscillation growth rate are found for the realistic physical situation when a nonuniform plasma density and an ionization function are considered. It is found that the gradient of plasma density n may be neglected in the expressions forU _t and and that the spectral density of fluctuations in the density of the plasma agrees well with the experimental data.     

Television imaging of fast auroral waves

Fast auroral waves have been recorded with an Image Intensifier-TV system and a new characteristic of this phenomenon has been revealed, namely, an abrupt transition from wave motion equatorwards to an E to W motion associated with an auroral band. For equatorward motion speeds ranged from 15 to 150 km/sec which is significantly lower than values previously reported. For the E to W motion speeds ranged from 20 to 90 km/sec.     

Note on the parallel propagation effects of unstable Farley-Buneman waves at high latitudes

We have studied the effect that substantially enhanced electron temperatures produce on the higher-frequency shorter-wavelength modes of the Farley-Buneman instability at high latitudes. In all cases the value of the parallel wave vector is reduced and the results are closer to those obtained from fluid theory than previous calculations had revealed. The large electron temperatures are themselves the product of heating by unstable plasma waves so that the corrections described in the present work become increasingly important as the linear growth rate increases.     

Interpretation in terms of gravity waves of structures observed at the mesopause level by photogrammetry and lidar

Experiment data of the vertical distribution of sodium density measured by lidar are compared with the horizontal structures observed by photogrammetry on the sodium emissive layer. The gravity wave induced variations seen in the vertical distribution are used to define a 2-dimensional model of the layer, which is shown to explain the observed horizontal patterns. It is strongly suggested that the structures currently seen on OH can be also related to gravity waves and that their sporadic occurrence in specific angular directions is explained by the wave characteristic and by wave or wind induced chemistry.     

Excitation of whistler waves by reflected auroral electrons

Excitation of electron cyclotron waves and whistlers by reflected auroral electrons which possess a loss-cone distribution is investigated. Based on a given magnetic field and density model, the instability problem is studied over a broad region along the auroral field lines. This region covers altitudes ranging from one quarter of an Earth radius to five Earth radii. It is found that the growth rate is significant only in the region of low altitude, say below the source region of the auroral kilometric radiation. In the high altitude region the instability is insignificant either because of low refractive indices or because of small loss cone angles.     

Propagation of auroral infrasonic waves studied by ray-tracings

On the basis of a ray tracing method the propagation and the attenuation of an auroral infrasonic wave are studied. Relations between the direct and reflected waves recorded at the Syowa Station, Antarctica, are clarified with regard to; (1) the delay time, (2) the intensity ratio, and (3) trace velocities. The time required for a wave to travel from the source to the ground is calculated as a function of a source altitude. The retardation time of the wave arrival behind the zenith crossing of the source current is deduced. A method is proposed for estimating the altitude of a source current from the retardation and a trace velocity of the wave. It is concluded that the existence of a supersonic equatorward motion of an electrojet which continues for a certain distance is necessary for the observation of auroral infrasonic waves. This distance must exceed at least 60 km equatorwards from the zenith to enable the direct wave to be observed and with total length of 930 km to enable the reflected wave to be observed. From these conditions it is also concluded that the infrasonic wave is not seen in mid latitudes and the reflected wave is a rare phenomenon.     

Optical detection of electrostatic ion cyclotron waves in the auroral plasma

Spectral analysis of recent photometric observations has revealed the existence of narrow frequency band pulsations in the brightness of some auroras. Peaks in the power spectra, between 25 and 32 Hz have been observed. We suggest that these observations are associated with electrostatic ion cyclotron waves, which are excited when the relative drift between ions and electrons due to field aligned (Birkeland) currents, exceeds a certain critical value.     

Ion-induced nucleation of atmospheric water vapor at the mesopause

The possibility of aerosol formation at the mesopause by ion-induced water vapor nucleation is investigated. A simple kinetic model considering new information on ion-growth and -recombination processes as well as on mesospheric water vapor and temperatures is put forward. It predicts ion-nucleation to be possible only at high latitudes during local summer in a narrow height region, extending from about 88 to 91 km. Derived nucleation rates increase steeply with decreasing temperatures and electron number densities. If for the latter typical values are considered nucleation rates may become sufficiently high to account for the observed mesospheric aerosol layer. Various observed characteristic temporal and spatial variations of the aerosol layer including its response to geomagnetic activity may be explained by the model.     

On the formation of electrostatic shock waves associated with auroral electron precipitation

Models of acceleration of auroral electrons by electrostatic shock waves are considered based on the model electron beam, calculated by Evans (1974), to account for the observed precipitating electron fluxes. Electron populations in our models include a primary accelerated beam, originating from the plasma sheet, the secondary electrons and the energy-degraded and backscattered primary electrons produced by precipitating electrons of that beam. We find a feasible electrostatic shock model with appropriate ion populations from considerations on the conditions for the existence of shock solutions.     

Disentangling the Galaxy at low Galactic latitudes

We have used the field stars from the open cluster survey BOCCE (Bologna Open Clusters Chemical Evolution), to study three low-latitude fields imaged with the Canada–France–Hawaii telescope (CFHT), with the aim of better understanding the Galactic structure in those directions. Because of the deep and accurate photometry in these fields, they provide a powerful discriminant among Galactic structure models. In the present paper we discuss if a canonical star count model, expressed in terms of thin and thick disc radial scales, thick disc normalization and reddening distribution, can explain the observed colour–magnitude diagrams (CMDs). Disc and thick disc are described with double exponentials, the spheroid is represented with a De Vaucouleurs density law. In order to assess the fitting quality of a particular set of parameters, the colour distribution and luminosity function of synthetic photometry is compared to that of target stars selected from the blue sequence of the observed CMDs. Through a Kolmogorov–Smirnov test, we find that the classical decomposition halo-thin/thick disc is sufficient to reproduce the observations – no additional population is strictly necessary. In terms of solutions common to all three fields, we have found a thick disc scalelength that is equal to (or slightly longer than) the thin disc scale.     

Filamentary structure of the ISM at high latitudes

Different mechanisms for filament formation are studied with a view to account for the results of Verschuur (1991a, b). It is shown that pinch phenomena can produce a filamentary structure of H I without any pressure contribution from the warm component of the interstellar medium (ISM). The mechanisms of filament formation involving the Parker instability and magnetoconvection are also discussed. It is shown that instabilities corresponding to all these mechanisms may result in a picture similar to that observed by Verschuur.     

Explorer 34 magnetic field measurements near the tail current sheet and auroral activity

Explorer 34 (Imp 4) 2.56 s magnetic data during 131 traversals of the tail current sheet are presented along with simultaneous 2.5 min auroral electrojet indices AE and AL. The normal magnetic field,B , satellite crossing times and positions are tabulated for these 131 crossings.B is defined in the center of the sheet: it is the vector magnetic field at the time of field minimum during the crossing (B _x component changes sign). It is remarkable that the only normal components too large in magnitude to be classified as fine structure occur near the time of onset of an AE event. Cases are discussed where the normal component, defined near the plasma sheet edges, has the opposite sign compared to the normal component defined at the sheet center. For quiet times, the current sheet may be only about 1000 km thick within a 3R _e (Earth-radii) plasma sheet, and may carry some 10–15% of the total tail current.     

Observation of mesospheric ozone at low latitudes

Stellar ultraviolet light near 2500 Å is attenuated in the Earth's upper atmosphere due to strong absorption in the Hartley continuum of ozone. The intensity of stars in the Hartley continuum region has been monitored by the University of Wisconsin stellar photometers aboard the OAO-2 satellite during occultation of the star by the Earth's atmosphere. These data have been used to determine the ozone number density profile at the occultation tangent point. The results of approximately 12 stellar occultations, obtained in low latitudes, are presented, giving the nighttime vertical number density profile of ozone in the 60- to 100-km region. The nighttime ozone number density has a bulge in its vertical profile with a peak of 1 to 2×10⁸ cm^?3 at approximately 83 km and a minimum near 75 km. The shape of the bulge in the ozone number density profile shows considerable variability with no apparent seasonal or solar cycle change. The ozone profiles obtained during a geomagnetic storm showed little variation at low latitudes.     

Studies of blue objects at high galactic latitudes

Continuing the survey for faint blue objects at high galactic latitudes, a field of 25 square degrees centered at the Coma Cluster of Galaxies has been examined on two-color plates taken with the 90-67 cm Schmidt telescope of Asiago. 487 objects have been identified. A selected list of those with the largest negative color index is given in Table I. Identification charts, notes and comments follow.     

Analytical theory of tachocline transport at mid latitudes

We provide a theory of magnetic diffusion, momentum transport, and mixing in the solar tachocline by considering magnetohydrodynamics (MHD) turbulence on a β plane subject to a large scale shear (provided by the latitudinal differential rotation). In the strong magnetic field regime, we find that the turbulent viscosity and diffusivity are reduced by magnetic fields only, similarly to the two-dimensional MHD case (without Rossby waves). In the weak magnetic field regime, we find a crossover scale (L_R) from a Alfvén dominated regime (on small scales) to a Rossby dominated regime (on large scales). For parameter values typical of the tachocline, L_R is larger than the solar radius so that Rossby waves are unlikely to play an important role in the transport of magnetic field and angular momentum. This is mainly due to the enhancement of magnetic back-reaction by shearing which efficiently generates small scales, thus strong currents. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stereoscopic imaging of the hydroxyl emissive layer at low latitudes

The hydroxyl nightglow layer is an excellent tracer of the dynamical processes occurring within the mesosphere. A new stereo-imaging method is applied that not only measures the altitude of the airglow layer but also provides a three-dimensional map of the OH-layer centroid heights. A campaign was conducted in July 2006 in Peru to obtain NIR images of the OH nightglow layer which were simultaneously taken for two sites separated by 645 km: Cerro Cosmos (12°09′08.2″S, 75°33′49.3″W, altitude 4630 m) and Cerro Verde Tellolo (16°33′17.6″S, 71°39′59.4″W, altitude 2330 m). Data represented by pairs of images obtained during the nights of July 26-27 and 28-29 are analyzed to yield satellite-type views of the wave field. These are obtained by application of an inversion algorithm. In calculating the normalized cross-correlation parameter for the intensity, three-dimensional maps of the OH nightglow layer surface are retrieved. The mean altitude of the emission profile barycenter is found to be at 87.1 km on July 26 and 89.5 km on July 28. In these two cases the horizontal wavelengths determined are 21.1 and 24.6 km with periods of 18 and 34 min, respectively. A panoramic view of the OH nightglow emission obtained on July 29 at 8 h51-9 h26 UT is presented, in which the overall direction of the waves is found to be N-NW to S-SE, azimuth 150°-330° (counted from South). The wave kinetic energy density at the OH nightglow layer altitude is 3.9×10⁻⁴ W/kg, which is comparable to the values derived from partial reflection radiowave data.     

Solar transients disturbing the terrestrial magnetic environment at higher latitudes

Geomagnetic field variations during five major Solar Energetic Particle (SEP) events of solar cycle 23 have been investigated in the present study. The SEP events of 1 October 2001, 4 November 2001, 22 November 2001, 21 April 2002 and 14 May 2005 have been selected to study the geomagnetic field variations at two high-latitude stations, Thule (77.5^° N, 69.2^° W) and Resolute Bay (74.4^° E, 94.5^° W) of the northern polar cap. We have used the GOES proton flux in seven different energy channels (0.8–4 MeV, 4–9 MeV, 9–15 MeV, 15–40 MeV, 40–80 MeV, 80–165 MeV, 165–500 MeV). All the proton events were associated with geoeffective or Earth directed CMEs that caused intense geomagnetic storms in response to geospace. We have taken high-latitude indices, AE and PC, under consideration and found fairly good correlation of these with the ground magnetic field records during the five proton events. The departures of the H component during the events were calculated from the quietest day of the month for each event and have been represented as ΔH _THL and ΔH _RES for Thule and Resolute Bay, respectively. The correspondence of spectral index, inferred from event integrated spectra, with ground magnetic signatures ΔH _THL and ΔH _RES along with Dst and PC indices have been brought out. From the correlation analysis we found a very strong correlation to exist between the geomagnetic field variation (ΔHs) and high-latitude indices AE and PC. To find the association of geomagnetic storm intensity with proton flux characteristics we derived the correspondence between the spectral indices and geomagnetic field variations (ΔHs) along with the Dst and AE index. We found a strong correlation (0.88) to exist between the spectral indices and ΔHs and also between spectral indices and AE and PC.     

Propagational features of higher harmonic tweeks at low latitudes

Higher harmonic tweeks observed for the first time at the low latitude station Varanasi (geomag. lat. 14 55 N) are reported. The analysis of data shows that higher harmonic tweeks are usually not associated with whistlers and occur when the ionization in the lower ionosphere would not increases with height. The Earth-ionosphere waveguide dispersion features play an important role in the propagation of broad band lightning generated signals and their occasional observation as higher harmonic tweeks. It is shown that the conductivity of ground and sea mixed path, forming the lower surface of the waveguide, provide an estimate of the travelled distances of higher harmonic tweeks in the waveguide. The attenuation factors are computed which shows that as the harmonic number increases their probability of observation decreases. The attenuation increases as the frequencies approach the cut-off frequencies and also as the layer height falls. The tweek activity is found to increase during periods of magnetic disturbances.