An observational study of the D-region winter anomaly in lower latitudes during sudden stratopsheric warmings

An observational study of the D-region winter anomaly of HF radio wave absorption in lower latitudes has been made during the period of a sudden stratospheric warming of the 1967/1968 winter. By means of large-scale isopleth analysis of the absorption index, ƒ_min, and of meridional winds near 70 km height along 60°N, it is found that there exists a winter anomaly in lower latitudes which is comparable in order to that in middle latitudes, resulting from a nitric oxide (NO) increase due to southward transport from higher latitudes by well-developed planetary wave winds. From the daily changes of absorption in the equatorial region, it is found that the enhanced absorption reveals an oscillation with a period of about 2 weeks and has its maximum in the region south of 20°N. The period is similar to that of planetary wave amplitudes in the winter stratosphere and mesosphere, suggesting that an effect of planetary waves could contribute to the equatorial anomaly of the absorption in the D-region.     

Recent advances in the study of the D-region winter anomaly

D-region work as concerns the winter anomaly of electron density is screened for the period 1974–78. The following topics are dealt with: electron density distribution; ion production processes; neutral atmosphere effects by trace constituents, temperature and transports; ion composition and chemistry; coupling between atmospheric/ionospheric layers. A summary is given which might be read first, and which leads to some aspects of future work.     

A mid-latitude ground-based lidar study of stratospheric warmings and planetary wave propagation

A survey of stratospheric and mesospheric temperature was performed on a regular basis during winter conditions from the lidar station of the Observatory of Haute Provence (44°N, 6°E). The method uses the range resolved Rayleigh scattering of a laser beam and its latest performances are given. The data presented in this paper cover two winter periods and include the major stratospheric warmings of 1980 and 1981; the results obtained by lidar are compared with global stratospheric data obtained by the radiosonde network and from the SSU satellite experiment. It is shown how the extension of the measurements into the mesosphere (and simultaneously in the stratosphere) and the height resolution achievable with the lidar technique permits one to estimate the time-delay of the downward propagation of a warming from the mesosphere to the low stratosphere in about 20 days. The influence of planetary wave propagation is also shown to produce a periodic variation in the stratopause height even in the absence of the stratospheric warming reported by the STRATALERT messages.     

The role of stratospheric minor warmings in producing the total ozone deficiencies over Europe in 1992 and 1993

During 1992 and 1993, record low total ozone values were observed over the middle and high northern latitudes. The ozone data from the long-operating station at Belsk, Poland, have been used to examine their departures from climatological behaviour in 1992 and 1993. It seems that not only do the exceptionally low ozone amounts recorded over the northern mid-latitudes need an explanation but also their occurrence for two years in a row. One of the possible mechanisms which may be responsible for this event is suggested to be connected with the occurrence of stratospheric minor warmings. They occur without a breakdown of the polar vortex but only with the displacement of very cold air towards lower latitudes (as in January 1992 and February 1993). It is known that air masses in the polar vortex have been chemically disturbed and, when they arrive over the sunlit middle latitudes, chemical destruction of ozone is likely to occur. During the periods under study, the strongest negative total ozone deviations correspond to strong negative temperature deviations at 30 hPa and to large potential vorticity values; this points to the presence over Europe of air masses of polar vortex origin. It has been shown that the characteristics of mid-winter stratospheric warmings and the interannual variability of winter-spring total ozone averages at Belsk are associated with each other.     

Variations of transport conditions and winter anomaly in the D-ionospheric region

The system of differential equations, describing the behaviour of the concentrations of minor neutral constituents and charged particles within the height range 30–150 km have been numerically solved and the effect of variations in the turbulent diffusion coefficient upon the ion structure of D-region evaluated. Two cases have been analysed. In the first complete mixing of the atmosphere is assumed to 90 km. In the second the effect of rapid transport through the 95–105 km region is considered. In the latter case the concentration of nitric oxide is increased compared with the first in the height interval 70–100 km, i.e. at those altitudes where the winter anomaly phenomenon is localized. The maximum excess is about 19 times. The distribution of concentrations of charged particles in the second case is also in agreement with values observed experimentally during the winter anomaly period.     

Investigations of the winter anomaly in ionospheric absorption in January 1981

Ground-based and rocket-borne investigations were carried out in January 1981 in the Volgograd region to study space-time peculiarities of the winter anomaly in ionospheric radio wave absorption (WA). Electron-density altitude profiles N_e(h) were measured with rockets, by the coherent frequency method and by using electrostatic probes; temperature profiles T(h) were measured by a resistance thermometer: wind velocity and direction were measured by radio-observations of a chaff cloud and of the payload parachute drift. At the same time, ionospheric radio wave absorption was measured in Volgograd at two frequencies, 2.2 and 2.7 MHz, by the A1 method. The condition of the lower ionosphere could be determined from absorption data and from f min parameter data obtained from vertical sounding ionograms. “Salvo” launchings of the rockets were performed on 14 January, when absorption was anomalously large, and on 21 and 28 January, which were days of normal winter absorption.Data analysis has shown that N_e values on the day with excessive absorption exceeded the same values on a normal day at altitudes from 72 to 95 km; on 21 January N_c values exceeded those of 29 February 1980 (without WA) at all altitudes below ~ 90 km. The absorption at Volgograd on 28 January was somewhat higher than on 21 January and than at stations at higher latitude, which may be due to a stable local increase of N_e values in the altitude range 80–90 km. The temperature in the region of the N_e-enhanced values (up to the limit altitude of measurements, about 80 km) was below the standard temperature (COSPAR, 72), both on 14 January and on the normal days. Measurements carried out at night have shown that winter N_c values considerably exceeded those during the autumn. The zonal and meridional wind profiles (up to about 80 km) at Volgograd exhibit a stable eastward flux, both in the stratsophere and in the mesosphere. The value of the wind velocity meridional component on 21 January is close to zero at all altitudes. On 14 and 28 January the wind profiles show an irregular structure with large velocity gradients at all altitudes above about 50–60 km.The absorption data and f min data from a number of stations, viz. from Juliusruh to Yakutsk (in longitude) and from Arkhangel'sk to Rostov-on-Don (in latitude), show that anomalously excessive absorption occurred over a vast distance exceeding 100° of longitude at ~ 55° latitude and that, based on the dates of absorption maxima (f min), one may conclude that the source of the disturbance was moving from west to east. Data on the motion of the air as shown by rocket and radiometeoric observations, indicate the same wind direction in the stratosphere as in the mesosphere. These data and the constant pressure charts point to the conclusion that the enhanced radio absorption values at mid-latitudes may be explained by a transport of dry air rich in nitric oxide from the auroral zone towards lower latitudes. The transport is provided by a stable circumpolar vortex existing in winter time. This mechanism may explain both the normal and anomalous winter absorption, as well as the post-storm effect.     

An attempt to identify the obscured paths of water cluster ions build-up in the D-region

Existing ion-chemical scheme has been applied to reproduce the positive ion composition measured on 30 June 1973 by a rocket-borne mass spectrometer flown from Wallops Island. It is found that reactions analogous to the conversion of NO⁺ to NO⁺·H₂O are able to reproduce the observed densities of NO⁺·(H₂O)₂ and NO⁺·(H₂O)₃. The theoretical values of O₂⁺ and O₂⁺·H₂O are found to be about an order of magnitude higher and lower respectively than those measured. The direct hydration rates of H⁺·(H₂O)_n ions given by Good et al (1970) yield much less hydrates for n > 2. To properly reproduce the observed positive ions we need the following:

• 1.(a) a reaction which will convert O₂⁺ to O₂⁺·H₂O in the form
  O with U1 = 10⁻³¹[M]²(300/T)^4.4s⁻¹ and U2 = 10⁻⁹[H₂O]s⁻¹; and
• 2.(b) formation of hydrated protons via intermediate ions in the form
  with S=10⁻⁹[H₂0]s⁻¹, F = 8 × 10⁻³²[M]²(300/T)^4.4s⁻¹ and B increasing from 10⁻³ to 10²S⁻¹ as n increases from 2 to 6.

     

Hemispheric synoptic analysis of 95 km winds during the winter of 1983/1984 and comparison with stratospheric parameters

For the winter 1983/1984 the close coupling between stratospheric and mesospheric disturbances is discussed, using derived dynamical quantities of the stratosphere and prevailing winds of the upper mesosphere/lower thermosphere.     

A study of ground diffraction pattern parameters associated with D-region partial reflections

D-region drift observations have been made at Adelaide in recent years using the three-receiver technique. The amplitude fading records are analysed by a method known as ‘full correlation analysis.’ The resultant drift values have been reported elsewhere. However, in the process of correlation analysis several other parameters related to the geometry and variability of the ground diffraction pattern are defined and calculated.These parameters often fluctuate markedly from one record to the next but on the average they show consistent changes with height, time of day and with season. These variations, and some aspects of the physical interpretation of the parameters themselves are discussed.     

The winter anomaly as seen in the propagation of low frequency 40 kHz radio waves

An analysis of propagation data for LF 40 kHz radio waves shows that the field strength of the sky wave is enhanced during day-time on winter anomaly days (WAD), in striking contrast to the severe attenuation of HF radio waves. This peculiar enhancement of the field strength may be ascribed to an increase in the reflection coefficient. The analysis also demonstrates that the reflection height is lower on WAD, which seems to be associated with enhancements of ionization in the D-region. Moreover, it was found that WAD are characterized by an earlier occurrence in the morning and a delayed occurrence in the evening of pronounced interference maxima and minima, respectively.     

Model studies of the D-region negative-ion composition during day-time and night-time

A model for the negative-ion composition of the D-region is constructed, based on gas-phase reactions and photodissociation and photodetachment processes. The height distributions of negative-ion species, electrons and total positive ions for day-time and night-time are examined using time-dependent solutions of their continuity equations which incorporate simultaneous solutions for the minor neutral constituents involved. A distinction is drawn between the two isomers of NO₃⁻ and particular attention is paid to the influence of possible chemical and photodestruction of the more stable form of this ion.     

Some characteristics of the quiet polar D-region and mesosphere obtained with the partial reflection method

It is shown that from a detailed analysis of the amplitude behaviour of electromagnetic waves partially reflected in the D-region valuable information about the reflection process and the dynamics of the mesosphere can be obtained. The results indicate that the process which is responsible for the partial reflections is different for different heights in the D-region and also changes with the season. From the temporal fluctuations of the amplitudes of the reflected waves, estimates of various turbulence parameters of the mesosphere have been deduced. Their mean values are in satisfactory agreement with results obtained by other methods.     

Eiscat observations of the ionospheric D-region during auroral radio absorption events

Electron densities in the D-region have been observed with EISCAT during energetic electron precipitation events. Sample results are presented which demonstrate the value of the technique in studying variations of electron density with fine temporal and spatial resolution. Different types of absorption event can be characterized in terms of the changes in the incoming electron spectrum inferred from profiles of electron density. We contrast the D-region behaviour of night- and day-time events in terms of precipitating spectrum and absorption profile. A softening of the electron spectrum during the course of a morning event is clearly seen.     

D-region signatures of substorm growth phase and onset observed by EISCAT

High resolution electron density measurements by EISCAT during the pre-onset phase and onset of an auroral absorption substorm are used to investigate the characteristics of electron precipitation during these substorm phases. The development of the pre-onset phase is the result of a uniform increase of electron fluxes with energies of a few tens of keV, with no particularly hard component. The absorption spike observed at substorm onset contains fine structure when investigated at 10 s resolution, indicating a rapid hardening of the precipitating spectrum at the onset.     

Incoherent scatter spectral measurements of the summertime high-latitude D-region with the EISCAT UHF radar

Measurements of incoherent scatter spectra from the auroral D-region were obtained during the summer of 1985 using a sophisticated pulse-to-pulse correlation technique with the EISCAT UHF radar. The spectral width variations with altitude are interpreted in terms of ion-neutral collision frequency, neutral temperature, mean positive ion mass and negative ion number density. Close agreement with predictions of currently available atmospheric models is obtained, except for a narrow layer around 86 km altitude. This layer showed evidence of increased positive ion mass for most of the experiment, and for short intervals indicated a mean ion mass close to 200 a.m.u. It is suggested that the layer is composed of proton hydrates in the vicinity of a structured noctilucent cloud, and that the index of hydration is occasionally large.     

Long-term variation of stratospheric temperature at the North Pole

The (30 mb) stratospheric temperatures at the North Pole during the winter months (November–February) showed large QBO (Quasibiennial Oscillations) but of an intermittent nature, different for different months and not matching in phase with the QBO of the tropical zonal wind. When the QBO was minimized by moving averages over two successive (yearly winter) values and then further over three successive values, the resultant series showed clear solar cycle variations with lags or leads of ~ (0–2) yr for temperature maxima with respect to sunspot maxima. However, in solar cycle 21 from 1978 onwards, temperatures seem to be more depressed, indicating enhanced stratospheric cooling in recent years, probably due to an increase of greenhouse gases. No relationship with El Nino events is indicated.     

Large scale coherence of the mesospheric and upper stratospheric temperature fluctuations

A large set of temperature profiles has been obtained in the upper stratosphere and the mesosphere over Europe during the MAP/WINE compaign by the use of different techniques: datasondes and falling spheres launched by metrockets, ground-based OH spectrometers and a Rayleigh lidar. These data have been used to study the large scale variability of the middle atmosphere during the winter 1983–1984. The temperature variations with periods longer than 25 days are clearly related to the succession of minor upper stratospheric warmings observed during this winter. The variations in the period range 10–20 days are at least partially due to westward propagating Rossby waves, of which one mode, with a 12.5 days period, is tentatively identified as the second symmetric mode of the wave number 2.