Solar flux and seasonal variations of the mesopause temperatures at 51°N

Observations of the OH (8-3) band airglow emission, using a multichannel tilting filter type photometer, have been carried out at Calgary (51°N, 114°W), Canada, since 1981. In this paper recent measurements of the nocturnal, seasonal and solar flux variations of the mesopause temperature, obtained from the rotational temperature of the OH (8-3) band observations, are presented. The data presented span the ascending phase of the present solar cycle viz. 1987–1988 (low solar activity) and 1990 (high solar activity). Good correlations (r = 0.73) between the OH (8-3) band rotational temperature and the 10.7 cm solar flux were observed. The mean temperature for the period investigated was about 210 K. The seasonally averaged nocturnal variations show only small irregular excursions, possibly associated with solar tides and the passage of gravity waves in the mesopause region. However, the observed rotational temperatures show considerable night-to-night changes.     

Solar-induced variation of proton precipitation near the equator

This paper reports the solar condition dependences of the quasi-trapped component (low energy) of the proton population of energy 0.65–35 MeV which peaks in the equatorial zone centered on the minimum magnetic field equator in the altitude range 170–850 km. The proton populations compared pertain to AZUR observation in 1969–1970, S81-1 mission observation in 1982 and EXOS-C observation during 1984–1986. In the equatorial zone, the dependence of the flux normalization constant, which represents the absolute proton population, upon factors like L (1.1–1.3), B (0.29–0.32 gauss), latitude ( ± 20°), longitude (0–360°) and anisotropy index q (~6–12) of the pitch angle distribution function is not so significant in the given range of these factors as it is upon the solar epoch. It is found that the absolute proton flux in 1982 was, at least, forty times that in 1984–1986 and, almost, three times that in 1969–1970, possibly, due to, varying solar conditions in those epochs.     

Short term variabilities of ionospheric electron content (IEC) and peak electron density (NP) during solar cycles 20 and 21 for a low latitude station

Hourly values of IEC and of f₀F₂ (critical frequency) for a low latitude station, Hawaii (21.2°N, 157.7°W), during the solar maxima (1969 and 1981) and minima (1965 and 1985) years of two consecutive solar cycles, 20 and 21, are used to study the day to day variabilities of the ionospheric parameters IEC and NP. It is found that there is good correspondence in the day to day variations of IEC and NP from one solar cycle to the other for both solar maximum and minimum years in the two solar cycles. Depending on solar phase and season, while the mean daytime IEC and NP variations range from about 20% to 35%, the mean night time values vary from about 25% to 60%. The mean daytime variations in NP for the solar minimum phase are remarkably higher in all the three seasons compared to the solar maximum phase. However, no such increase is observed in the mean daytime IEC variations, indicating the highly variable nature of the daytime ionospheric F region compared to the topside during solar minimum for this low latitude station. The winter night time IEC also seems to be a relatively stable parameter during the solar minimum. The short term day to day variabilities of the day time peak values of IEC and NP (ie IEC_max and NP_max) are not closely associated with the variations in F_10.7 solar flux. Contrary to the common expectation, the variabilities in both the parameters, particularly in NP_max, are somewhat reduced during the solar maximum (when the variability in F_10.7 solar flux is much higher compared to the solar minimum) which is more evident in the stronger 21 solar cycle. A larger number of significant components are seen in the spectra of the percentage variation of both IEC_max and NP_max during both solar phases of the two solar cycles compared to the corresponding F_10.7 solar flux spectra. The number of additional components for both the parameters with periods less than 15 days are more for the low solar activity years than for the solar maximum years.     

Solar cycle variation of ƒoF2

For solar cycle 19 (1954–1964), the 12 monthly mean values of noon-time ƒoF2 at Ahmedabad (23°N, 73°E) show a large hysteresis effect when plotted against sunspot number or against geomagnetic Ap. However, a multiple regression analysis for the dependence of ƒoF2 on solar 2800 MHz flux and geomagnetic Ap, simultaneously, shows a better matching. Thus, long-term predictions need to take into account not just sunspot number but some solar index and geomagnetic index as two key parameters, simultaneously.     

Mesopause temperatures and integrated band brightnesses calculated from airglow OH emissions recorded at Maynooth (53.2°N, 6.4°W) during 1993

Spectra of the hydroxyl emissions in the wavelength range 1.0–1.6 μm, which originate at mesopause altitudes, have been obtained, using a Fourier transform spectrometer at Maynooth (53.2°N, 6.4°W), on all suitable nights during the period January–December 1993. Rotational temperatures and integrated band brightnesses have been calculated from the spectra of the OH(3, 1) and (4, 2) vibration-rotation bands. The mean annual temperatures calculated over all measurements were T(3, 1)=200±19 K and T(4, 2)=206±19 K, where the uncertainty represents the standard deviation on the measurements. Harmonic analysis of the nightly averaged temperature values revealed an amplitude of 27 ± 1 K and a phase of 95 ± 2 days in the annual variation of the (3, l) band at our latitude. The semiannual component was found to have an amplitude of 7 ± 1 K and a phase of −51 ± 9 days for this band. Results for the (4, 2) band were identical in both amplitude and phase for the annual component, while the semiannual component gave an amplitude of 8 ± 1 K and a phase of − 43 ± 7 days. These results are compared with data recorded by the SME satellite, and with the predictions of the MSISE-90 model for a station at 53° latitude. Temperatures predicted by the MSISE-90 model for Maynooth are consistently below the values obtained in this study by 15–20 K. Excellent agreement is observed between the absolute value of temperature, in the case of the SME satellite, and in the amplitude and phase of the annual variation predicted by MSISE-90. The phase of the semiannual component observed in our data deviates somewhat from the −99 ± 1 days predicted by MSISE-90.The annual mean brightness of the OH (3, 1) band was found to be 75 ± 18 kR, while that of the (4, 2) band was 106±26 kR. Diurnal variations generally showed a steady decrease from dusk to dawn, apart from a brief period in June and July. Monthly average values of band brightness have been calculated for each band and are compared with the predictions of a recent photochemical model (Le Texier et al., 1987). The model shows some elements of agreement with our observations, particularly a pair of maxima near the equinoxes, but it does not predict the broad winter maximum observed in both bands at this latitude.     

Temperatures and tides in the high latitude mesopause region as observed in the OH night airglow emissions

The OH (6-2) band night airglow emissions have been observed from two sites at 60 and 70°N, respectively, in Norway during the December–April periods 1985–1986 and 1986–1987. Variations in rotational temperatures at ~90 km on time scales from tens of minutes to days show similar patterns as at 80°N. The semi-diurnal tide is dominant with average peak to peak amplitudes of ~5 K over the observing periods. There is a negligible difference in average tidal amplitude at 60 and 70°N. The phase of the tide is changing slowly through the December–February period. The mid-winter to early spring average temperatures are ~10 K higher than predicted by the CIRA 1972 90 km model atmosphere for the respective sites.     

Wave characteristics in the troposphere and stratosphere over the Indian Tropics during the DYANA period

One of the important scientific objectives of the international DYANA campaign was to obtain the characteristics of planetary scale waves in the low-latitude middle atmosphere. India participated in this campaign by way of launching several rockets and high-altitude balloons from a number of locations to determine the vertical structure of different wave modes present during January–March 1990. Rocket launchings were conducted from two stations, namely Thumba (8.5°N, 77.0°E) with M-100 rockets and Balasore (21.5°N, 87.0°E) with RH-200 rockets, while balloons were launched from three stations, i.e. Trivandrum near Thumba, Minicoy (8.2°N, 73.0° E) and Port Blair (11.7°N, 92.7°E). In addition, there were balloon flights from Hyderabad (17.3°N, 78.3°E) and Bhubaneshwar (20.2°N, 85.5°E). The results of the synoptic scale wave activity as obtained from various data sources are given here.Three prominent peaks with wave periods near 6–8 days (short periods), 10–12 days (medium periods) and 30–45 days (long periods) are found to occur at all the stations. The medium- and long-period waves appear to be forced Rossby modes penetrating from midlatitudes while short period waves all have characteristics matching those of mixed Rossby-gravity waves. A very interesting result is the presence of long-period oscillations in the upper stratosphere and mesosphere, with very large amplitude, contrary to earlier observations.     

The middle atmospheric response to short and long term solar UV variations: analysis of observations and 2D model results

We have investigated the middle atmospheric response to the 27-day and 11-yr solar UV flux variations at low to middle latitudes using a two-dimensional photochemical model. The model reproduced most features of the observed 27-day sensitivity and phase lag of the profile ozone response in the upper stratosphere and lower mesosphere, with a maximum sensitivity of +0.51% per 1% change in 205 nm flux. The model also reproduced the observed transition to a negative phase lag above 2 mb, reflecting the increasing importance with height of the solar modulated HO_x chemistry on the ozone response above 45 km. The rnodel revealed the general anti-correlation of ozone and solar UV at 65–75 km, and simulated strong UV responses of water vapor and HO_x species in the mesosphere. Consistent with previous 1D model studies, the observed upper mesospheric positive ozone response averaged over ±40° was simulated only when the model water vapor concentrations above 75 km were significantly reduced relative to current observations. Including the observed temperature-UV response in the model to account for temperature-chemistry feedback improved the model agreement with observations in the middle mesosphere, but did not improve the overall agreement above 75 km or in the stratosphere for all time periods considered. Consistent with the short photochemical time scales in the upper stratosphere, the model computed ozone-UV sensitivity was similar for the 27-day and 11-yr variations in this region. However, unlike the 27-day variation, the model simulation of the 11-yr solar cycle revealed a positive ozone-UV response throughout the mesosphere due to the large depletion of water vapor and reduced HO_x-UV sensitivity. A small negative ozone response at 65–75 km was obtained in the 11-yr simulation when temperature-chemistry feedback was included,In agreement with observations, the model computed a low to middle latitude total ozone phase lag of +3 days and a sensitivity of +0.077% per 1% change in 205 nm flux for the 27-day solar variation, and a total ozone sensitivity of +0.27% for the 11-yr solar cycle. This factor of 3 sensitivity difference is indicative of the photochemical time constant for ozone in the lower stratosphere which is comparable to the 27-day solar rotation period but is much shorter than the 11-yr solar cycle.     

Diurnal variations in the flux of ionisation above the F2 peak in the northern and southern hemispheres

The flux of ionisation at 850 km height is calculated using the MSIS atmospheric model, a simplified form for the continuity equation at the peak of the F2-layer, and observed values of NmF2. Results are given for stations at latitudes of 32°N, 21°N, 21°S and 37°S during 1971 and for Tahiti (18°S) in 1980. Changes in the neutral atmosphere and in the hmF2 model have minor effects at low latitudes, where the fluxes are larger, but can appreciably alter the results at mid latitudes. Increased recombination due to N₂ vibrational excitation produces a large afternoon decrease in NmF2 in summer, near solar maximum, and an increased downward flux. At all stations the day-time flux has a much larger downward component in winter than in summer. Because of the eastward magnetic declination, zonal winds produce opposite effects on the diurnal variations of hmF2, NmF2 and flux in the northern and southern hemispheres. Downward fluxes are largest in the morning in the southern hemisphere and in the late afternoon and evening in the north. At ± 21° latitude, neutral winds have a major effect on the distribution of ionisation from the equatorial fountain. Thus, at the solstices the day-time flow is about 4 times larger in winter than in summer. Averaged over both hemispheres, the total flow at 21° latitude is approximately the same for solstice and equinox conditions. At mid latitudes there is a downwards flux of about 1–2 × 10¹² m² s⁻¹ into the night ionosphere.     

Types of ionospheric scintillations in southern mid-latitudes during the last sunspot maximum

A 5-yr study (1987–1992) has been undertaken at a southern mid-latitude station, Brisbane (35.6°S invariant latitude) on scintillation occurrences in radio-satellite transmission (at a frequency of 150 MHz) from polar orbit Transit satellites, within a sub-ionospheric invariant latitude range 20–55°S. Over 7000 recorded passes were used to define the spatial and temporal occurrence pattern of different types of scintillation events. Two predominant scintillation types were found: so-called type P (associated with a scintillation patch close to the magnetic zenith) and type S (characteristic of the equatorward edge of auroral scintillation oval). Type S was by far the most frequent during sunspot maximum (1988–1992), with sharp occurrence peaks in the summer-autumn period. Its seasonal occurrence showed a high degree of correlation (correlation coefficient r = 0.8) with the seasonally averaged 10.7 cm solar radio flux. This type occurred mainly at night-time except in austral summer where 40% of scintillations were detected in daytime, coinciding with the well-known summer peak of sporadic-E occurrence. Type P was more predominant during a year (1987) of ascending sunspot activity but decreased to a much lower level during the sunspot maximum.     

Observation of low energy particle precipitation at low altitude in the equatorial zone

Precipitation of protons (~ 1 MeV) in the equatorial zone was investigated by the Phoenix-1 experiment on board the S81-1 mission from May–November, 1982. The protons show a precipitation peak along the line of minimum magnetic field strength with a full width at half maximum (FWHM) of 13°. The index of equatorial pitch angle distribution is q ~ 19. The peak proton flux shows a fifth-power altitude dependence, and the proton flux shows approximately a factor of 3 times increase in 1982 compared to that in 1969 due, possibly, to the stronger (~ 1.2 times) solar maximum conditions of 10.7cm radiation in 1982.     

Are the equatorial electrojet and counterelectrojet centered invariably on the dip equator?

The H, D and Z variations at Huancayo (dip angle 2°N) in the South American dip equator region were compared during a sequence of days (11–16 January 1964) having extraordinarily large afternoon counterelectrojets. It was noticed that both the electrojet and counterelectrojet showed large latitudinal excursions (exceeding 2° dip angle or 1° latitude) on different days as also at different hours of the same day.     

Vertical plasma drifts in the post-sunset F-region at the magnetic equator

HF doppler observations of vertical plasma drifts in the post-sunset equatorial F-region at Trivandrum (dip 0.9°S), conducted over a range of solar and geomagnetic conditions, are presented. The observations show that under magnetically quiet conditions, the characteristic post-sunset enhancement in the vertical plasma drift is quite sensitive to solar activity; the peak velocity drops by about a factor of 3 as the solar flux index (S_10.7) changes from about 125 to 70. It is found that the drift velocity enhancement has strong magnetic activity dependence only during high solar activity; the drift velocity drops by more than a factor of 2 from quiet to moderate activity, but builds back to the quiet day level for high magnetic activity. The occurrence of equatorial spread-F (ESF) is seen to be closely linked to the post-sunset enhancement in the vertical drift velocity, both showing essentially the same dependence on solar and magnetic activities. A comparison with Jicamarca observations shows that while the gross characteristics of the drift velocity pattern are about the same for the two stations, there are significant differences in the detailed variations, particularly for magnetically disturbed conditions.     

Results inferred from electron density measurements at Saskatoon,Canada (L = 4.4) by a partial reflection technique—I. Variations of nitric oxide in the D-region during quiet periods

Measurements of electron densities at Saskatoon (52°N, 106° W, L = 4.4) from 1976 to 1979 reveal seasonal variations which cannot be explained solely by solar zenith angle variations. These profiles have been used to infer the variations of nitric oxide density with season and solar activity for quiet-time conditions. It is found that while nitric oxide varies with season, it remains unchanged with the change in solar activity. The summer and spring profiles are much lower than the measured values of Baker et al. (1977) for heights below 85 km, while the winter estimated values show differences from the measured values in the height range 77.5–85 km. Above 85 km the values for all the seasons are close to the measured values. A dip in the nitric oxide distribution is obtained in all the cases around 80–82 km and the values of nitric oxide at the minimum are less than those measured by Baker et al. (1977) or Meira (1971).     

Equatorial night-time spread-F characteristics observed from stations in the Western Pacific

A comparative study of the ionospheric spread-F observed at an equatorial station. Port Moresby (9°24.5′S, 147°9.9′E; magn. dip lat. 18°S). Papua New Guinea, with corresponding data from the East Asian stations during a period 27 May–19 June 1983, as well as the same periods for 1981 and 1982, is presented. The Port Moresby spread-F condition is particularly outstanding and persistent at night and the occurrence of spread-F between 23 LT and 05 LT near June solstice is very high, reaching 100 % of all observed nights. The latitudinal variation of nocturnal spread-F, its duration and connection with sunspot number are analyzed. The relationship between long-term variation of night-time spread-F occurrence and solar activity during June and December solstices from 1954 up to 1984 at a typical lower magnetic latitudinal station, Wuhan (30°32.7′N, 114°21.5′E; magn. dip lat. 26°N). is also investigated.     

Rocket measurements of nitric oxide in the equatorial region

Measurements of nitric oxide (NO) concentrations were carried out from Thumba, India, using rocket-borne radiometers. The technique of measurement is based on the detection of day-glow emissions of the NO gamma (1,0) band. The results obtained are presented in this paper. The peak NO concentration shows a very good correlation with integrated value of the solar X-ray flux in the 0.1–0.8 nm band, thereby indicating the influence of the X-ray flux on the NO concentration. The observed variability of NO is thought to be mainly due to solar activity and partly due to different X-ray flux values on the days of the flights. Theoretical model calculations for rocket flight conditions were found to be in fairly good agreement with the observed profiles. The differences below 90 km altitude in the NO profiles are thought to be due to eddy turbulence. This model is also used to study changes in the NO concentration with solar activity and latitude.     

Effect of the electric field on the equatorial ionospheric plasma distribution

It is known that on a counter electrojet day the noontime electron density at the equator shows enhanced values with no bite-out. The consequences of the absence of the normal equatorial electrojet on the electron density distribution at the equatorial station Kodaikanal (dip latitude 1.4°N, long. 77.5°E) and at an anomaly crest location Ahmedabad (dip latitude 18°N, long. 73°E) are discussed for a strong electrojet (SEJ) day and a counter electrojet (CEJ) day. The electron density distribution with height for a pair of SEJ and CEJ days at the two equatorial stations Kodaikanal and Huancayo (dip latitude 1°N, long. 75°W) are studied. The F-region peak height, hm and the semi-thickness parameter ym on the SEJ day followed a similar variation pattern. On the CEJ days ym exhibited a substantially low and mostly flattened daytime variation compared to the peaked values on the SEJ day. An attempt is made to interpret these differences in terms of the changes in the vertical drift pattern resulting from the E × B drift of plasma at the equator and the varying recombination rate β, which is also a height dependent and a local time dependent parameter.     

Plasma convection and auroral precipitation processes associated with the main ionospheric trough at high latitudes

Intervals of F-region electron density depletions associated with the main (mid-latitude) ionospheric trough have been studied using latitude scanning experiments with the EISCAT UHF radar. From 450 h of measurements over a one year period at solar minimum (April 1986–April 1987) the local time of appearance of the trough at a given latitude is observed to vary by up to about 8 h. No seasonal dependence of location is apparent, but troughs are absent in the data from summertime experiments. A weak dependence of trough location on K_p is found, and an empirical model predicting the latitude of the trough is proposed. The model is shown to be more appropriate than other available quantitative models for the latitudes covered by EISCAT. Detailed studies of four individual days show no relationship between local magnetic activity and time of observation of the trough. On all four of these days, however, the edge of the auroral oval, evidenced by enhanced electron densities in the E-region, is found to be approximately co-located with, or up to 1° poleward of, the F-region density minimum. Simultaneous ion drift velocity measurements show that the main trough is a region of strong (> several hundred metres per second) westward flow, with its boundary located approximately 1°–2° equatorward of the density minimum. Within the accuracy of the observations this relationship between the convection boundary, the trough minimum and the precipitation boundary is independent of local time and latitude. The relevance of these results is discussed in relation to theoretical models of the F-reregion at high latitudes.     

Annual variations in the electron content and height of the F layer in the northern and southern hemispheres,related to neutral composition

Total electron content (TEC) data is presented for similar sites at ±35° latitude, and conjugate sites at ±20°, for several years near solar maximum. Comparison with the MSIS atmospheric model shows that the large seasonal anomaly at 35°N (an increase of 80% in TEC from October to April) is fully explained by changes in neutral composition. The small seasonal anomaly at 35°S also agrees with the MSIS model. Composition changes fail to account for the generally higher TEC in the northern hemisphere; this suggests the presence of an overall south-to-north atmospheric wind. Eastern declinations also contribute to enhanced TEC in the northern hemisphere, in the Pacific zone. The MSIS model predicts a semiannual variation of about ±25% in TEC at all sites, while observed changes are only about ±8%; thus we require some enhanced loss process near the equinoxes, particularly in September and October.Peak height calculations assuming a constant pressure level give a large semiannual variation in the F2 region: this is replaced by an annual variation when h_m F2 is calculated from diffusion theory. Heights calculated from the MSIS model are similar to observed values at ±35° latitude on summer days. A decrease of about 20km in observed heights on winter days is attributed to a poleward neutral wind; this wind also reduces the observed TEC. At night the height changes correspond to an equatorial wind, which is largest in summer and equinox. Observed day time TEC is greater at 20°N than at 20°S at all times of year, suggesting a northward transequatorial wind which is strongest near January and gives increased TEC and decreased peak height at 20°N.     

The influence of ionospheric absorption on mid-latitude whistler mode signal occurrence from VLF transmitters

Whistler mode signals from VLF transmitters received at Faraday, Antarctica (65° S, 64° W) during 1986–1991 show an annual variation in the number of hours over which signals are observed, with a maximum in June and a minimum in December. The variation was larger at solar minimum than at maximum and can be understood in terms of changes in absorption of VLF signals in the D-region, where the high geographic latitude of Faraday plays an important role in producing low attenuation levels during the austral winter. In contrast, very little such variation was observed at Dunedin, New Zealand (46° S, 171° E) in 1991. Nighttime whistler mode signals have start and end time trends that are consistent with the influence of F-region absorption. Increases in whistler mode occurrence appear to be associated with periods of high geomagnetic activity at solar maximum but not during solar minimum. A possible mechanism involving decreased F-region absorption is discussed.