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.     

Associations between the 11-year solar cycle,the QBO and the atmosphere. Part I: the troposphere and stratosphere in the northern hemisphere in winter

Linear correlations between the three solar cycles in the period 1956–1987 and high-latitude stratospheric temperatures and geopotential heights show no associations. However, when the data are stratified according to the east or west phase of the quasi-biennial-oscillation (QBO) in the equatorial stratosphere significant correlations result: when the QBO was in its west phase the polar data were positively correlated with the solar cycle while those in middle and low latitudes were negatively correlated. The converse holds for the east phase of the QBO. Marked relationships existed throughout the troposphere too.No major mid-winter warming occurred in the west phase of the QBO during a minimum in the three solar cycles. In the east phase major warmings tended to take place in the minima of the cycle. Thus the signal of the quasi-biennial-oscillation in the extratropical stratosphere tends to be strengthened in solar minima, and weakened in solar maxima.     

Electron temperature and electron density in the F-region of the ionosphere. I. Observed relationship

Ionospheric data from three incoherent scatter stations over the height range 225–450 km were studied for all daylight hours over a wide range of solar conditions. The relationship between electron temperature T_e, electron density Nand solar flux at 10.7 cm wavelength S_10.7 was expressed as T_e = A−B·(N−5 × 10¹¹) + C·(S_10.7−750), where N is in units of m⁻³ and S_10.7 in kJy.This provided a very satisfactory expression for all data taken at Malvern and St. Santin between 0800 and 1600 LT. For data taken at Arecibo, however, the linearity broke down at low electron densities. The data from all three stations were therefore divided into two sets according to electron density and reexamined.ForN < 5 × 10¹¹ m⁻³ B increased steadily with height and decreased steadily with latitude.For N > 5 × 10¹¹ m⁻³ B did not appear to vary with height, with season or with latitude. C was approximately constant for all sets of data.The different mechanisms involved in the heat balance of the electron population are discussed and a qualitative explanation for the relationship is proposed.     

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.     

QBO and solar activity effects on temperatures in the mesopause region

This statistical correlation study is based on two upper mesospheric temperature data sets for the region around 90 km, on the phase of the quasi-biennial oscillation (QBO) and on the solar 10.7-cm radiation. The temperatures are measurements from passive OH1-spectrometers and lidar sounding experiments carried out between 1980 and 1988 at geographic latitudes between 50°N and 70° N. An anticorrelation exists betwen the 10.7-cm flux and the temperatures. Two different types of significance tests were used to check the confidence level of the correlation. The anti-correlation is highly significant (confidence ⪢ 99.99%) for latitudes around 50°N if the QBO is in its east phase and for the spring/summer temperatures. The anti-correlation is not significant for the QBO west phase and for fall/winter temperatures (confidence < 95%). For higher latitudes, around 79°N, no significant correlation was found.     

Net radiative heating in the middle atmosphere

The meridional distributions of both total solar and net radiative heating rates have been obtained between 30 and 110 km at both the solstice and equinox using Fomichev et al.'s total radiative long wave cooling data in the calculations of the net radiative heating. The contributions to the solar heating of O₃, O₂, CO₂ and H₂O have been investigated. For the ozone heating, the absorption of diffusive solar radiation from the ground and troposphere has been estimated. The 50–90 km layer is close to radiative equilibrium on a globally averaged basis. The importance of radiative cooling as an energy sink in the 90–110 km layer is apparently not less than that of the vertical eddy heat conduction. The ordered meridional circulation has been obtained under the assumption that the temperature variation, due to net radiative heating, is balanced by the adiabatic and temperature variations due to vertical air motion. The circulation model obtained is compared with other empirical models, which are reviewed. For the hemisphere and the 60–80 km layer, the two-cell circulation with the rising motion near the equator and pole from spring to autumn and above 80 km, the one-cell circulation with the sinking motion near the equator and equinox, seem to be most realistic. Also quite realistic for the period near the solstice is the same type of two-cell circulation in the 40–50 km layer and the sinking motion at low latitudes in the 50–60 km layer.     

Experimental daytime VLF ionospheric parameters

Measured field strengths from VLF transmitters are used to determine improved daytime values of ionospheric parameters to enable improved VLF propagation predictions. These parameters are the traditional H′ (height in km) and β (sharpness in km⁻¹) as used by Wait and by NOSC in their Earthionosphere waveguide computer program. They are found by comparing the predictions of the NOSC program with the observed VLF field strengths over both long and short paths.Experimental observations from two nearly north-south paths are used to determine the solar zenith angle dependence of both H′ and β for low latitude (or summer mid-latitude) conditions. These results are then used to predict the daytime variations in VLF field strengths with solar zenith angle (and hence time) on other suitable paths and good agreement is found with measurements made on these paths.The absolute value of β for overhead Sun is found to be 0.45 km⁻¹ and is principally determined by the attenuation on the very long, west to east, fully sunlit, 14.4 Mm path from NWC (Australia, 22°S) to San Francisco (37°N), after applying small corrections for the solar zenith angle variations along the path at midday. Further support is obtained from results from the 8.6 Mm path NDT (Japan) to San Francisco, an 8.2 Mm path NPM (Hawaii) to New Zealand, and an east to west 7.5 Mm path from NPM to Townsville, Australia. The conditions studied are solar maximum. The frequencies studied are 15–30 kHz.     

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.     

Geomagnetic storms in the Antarctic F-region. I. Diurnal and seasonal patterns for main phase effects

New analysis procedures are used to show that the main phase mid-latitude storm effects conform to consistent patterns in local time when suitable selection rules are applied, with averaging over several years. Changes in $\text{ƒoF2}$, with respect to estimated quiet-time values, are analysed in terms of a_p(τ), a new geomagnetic index derived to take account of integrated disturbance. Reduction of $\text{ƒoF2}$ is greatest during the early morning hours, in summer, at higher geomagnetic latitudes, near solar minimum and through the more active periods. The various dependencies are quantitatively determined for the first time by creating an average ‘steady state’ disturbance, rather than following specific storm events. This approach will permit tests of competing theories using available modelling programs.     

On the association between the QBO and the extratropical stratosphere

Although diagnostic studies and mechanistic model experiments have found that, on average, the polar vortex in northern winter is stronger and colder in the west than in the east years of the equatorial Quasi-Biennial Oscillation (QBO), we show with an expanded data base that the results are not statistically significant. The reason for the insignificance is that in 36% of the winters (13 out of 36) the vortex was warm and weak in the west, and cold and strong in the east years. Only at low activity in the 11-yr solar cycle did the difference between the west (cold) and the east (warm) years become statistically significant. At high solar activity the west years had a warm and the east years a cold polar vortex in the mean. We show this association with the 11-yr solar cycle also in terms of the geostrophic wind.     

Theoretical calculations on the changes in the ionospheric composition and temperature profiles during storms

The changes in the ionospheric composition and temperature profiles, in the altitude range of 120–1000 km, due to different mechanisms currently considered important during storms, are estimated quantitatively for a low latitude station, Delhi, for moderate solar activity conditions using the computer method of Stubbe. The theoretical results reported here are discussed in the light of the available ion composition and temperature variations observed at low latitudes during storms with satellite data in the topside ionosphere. The results are presented for the three atomic ions O⁺, H⁺ and He⁺ which are important in the F-region and topside ionsophere. It is found that all the three atomic ions increase or decrease in phase with the change in the concentration of n(O) when there is no change in total neutral density. When the change in the exospheric temperature T_∞ with its consequent change in neutral composition and an additional storm time increase in N₂ by a factor of 2 is considered, O⁺ is found to increase in the topside and decrease in the bottomside ionosphere, whereas H⁺ and He⁺ decrease all throughout except for a small increase in He⁺ above 800 km during day. The effect of eastward electric field or southward (equatorward) winds during the day is to increase all three ions in the topside ionosphere and to decrease them in the bottomside ionosphere and vice versa for westward fields or northward (poleward) winds. At night, O⁺ shows the same type of behaviour as for day, while He⁺ shows an increase above 900 km and a decrease below that height for eastward fields or soutward winds and H⁺ shows an oscillating behaviour.Electron and ion temperature (T_e and T_i) during the day shows anticorrelation with the change in the electron concentration N_e (equal to total ion concentration), whereas at night it does not show any significant change except for the case of change in T_∞ and N₂.     

Anomalous diurnal phase shifts of Omega VLF waves (10–14 kHz) on the east–west low latitude and transequatorial paths

The phase of the Omega HAIKU (Hawaii, U.S.A.) and REUNION (La Reunion) signals were measured at Inubo, Japan and onboard ship at Fremantle. Australia. Strong east–west non-reciprocities of the diurnal phase shift are obtained both on the low latitude and transequatorial paths, and it is found that the non-reciprocity on one path is in an opposite sense to the other. The diurnal phase shift, ϕ_DN for the west-to-east (WE) propagation is 7.8–8.7 µs Mm^–1 at 13.6 kHz on the transequatorial and mid-latitude paths, indicating no significant latitude dependence of the phase velocity in WE propagation. On the other hand, ϕ_DN for the east-to-west (EW) propagation greatly depends on the geomagnetic latitude; at 13.6 kHz ϕ_DN = 11.3µs Mm^–1 on the low latitude path and ϕ_DN = 50 µs Mm^–1 on the transequatorial path, which are 40% greater and 35% less than ϕ_DN in WE propagation, respectively. The east-west non-reciprocities of ϕ_DN on the low latitude and transequatorial paths are interpreted in terms of a single mode propagation in the conventional anisotropic waveguide model with β_D = 0.3 km^–1, β_N = 0.5 km^–1 and h_N–h_D = 12.5 km. In particular, the anomalously small ϕ_DN on the EW transequatorial path is explained as due to the high phase velocity of the night-time first-order mode in the equatorial region within ±12° geomagnetic latitude.     

Antarctic polar cap total electron content observations from Casey Station

In early 1990 a modified JMR-1 satellite receiver system was installed at Casey Station, Antarctica (g.g. 66.28°S, 110.54° E, -80.4°A, magnetic midnight 1816UT, L = 37.8), in order to monitor the differential phase between the 150 and 400 MHz signals from polar orbiting NNSS satellites. Total electron content (TEC) was calculated using the differential phase and Casey ionosonde foF2 data, and is presented here for near sunspot maximum in August 1990 and exactly one year later. The data are used to investigate long-lived ionization enhancements at invariant latitudes polewards of − 80° A, and the ‘polar hole’, a region from −70 to − 80° A on the nightside of the polar cap where reduced electron densitiy exists because of the long transport time of plasma from the dayside across the polar cap. A comparison is made between the Casey TEC data and the Utah State University Time Dependent Ionospheric Model (TDIM) which uses as variables the solar index (F 10.7), season (summer, winter or equinox), global magnetic index (K_p), IMF B_y direction, and universal time (UT) [sojkaet al. (1991) Adv. Space Res.11(10), 39].     

Longitudinal difference in the Sq profiles in the South American region

The average daily variation profiles of the H component at Huancayo on the west coast and Eusébio, Fortaleza, on the east coast in the dip equator region and La Quiaca near the west coast and Vassouras near the east coast in the southern low latitude region of the South American continent indicate that the profiles are broader for the east coast. Thus, distortions in the overhead current systems while passing from one coast to the other are indicated.     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.     

Comparison of total electron content calculated using the IRI with observations in China

Values of total electron content (TEC) calculated using the International Reference Ionosphere (IRI-86 and IRI-90) are compared with the observations at Xinxiang based on the Faraday rotation measurement. It is found that the IRI gives acceptable values with respect to the observations during low solar activity. Generally the IRI-90 is better than the IRI-86 and the URSI coefficients are better than the CCIR coefficients in the calculation of TEC. Making use of the foF2 and M(3000)F₂ calculated using the Asia Oceania Region F₂-layer mapping (AOR) instead of using the CCIR or the URSI coefficients, the IRI gives more accurate TEC values. In October-April during high solar activity, however, the IRI obviously underestimates TEC in the daytime, which could be due to an improper topside electron density profile.     

臭氧浓度变化对馆藏银器文物腐蚀变色的影响

博物馆环境中的臭氧作为氧化剂,对文物起到氧化腐蚀的作用,会使馆藏银器文物腐蚀变色。本研究主要采用石英晶体微天平(QCM)反应性监测技术,监测电沉积金属银石英晶振片在不同臭氧浓度、不同相对湿度中的质量变化,从而推测银在臭氧中的腐蚀机理。研究结果表明,臭氧浓度对银的腐蚀具有较大的影响。随着臭氧浓度的升高,银表面质量不断增大,腐蚀程度加剧。银对相对湿度具有较小的敏感性,在不同的相对湿度下,银表面质量变化相差不大。据此推断银在臭氧中的腐蚀首先表现为银与臭氧分解产生的原子氧反应生成氧化银(Ag2O),随着Ag2O膜的不断变厚,氧化膜顶层生成过氧化银(AgO),AgO是由Ag2O与原子氧反应生成。     

Possible linkage between atmospheric total ozone and solar magnetic sector boundary passage

Superposed epoch analysis of the daily values of atmospheric total ozone at 70 stations for the period 1972–1975, with solar magnetic sector boundary passage (M.S.B.) past the earth as the key day, suggests large variations in high latitudes and larger variations in winter than in summer. Similar analysis with the day on which the boundary crossed the central meridian of the sun as key day does not reveal discernible ozone variations on the day of sector crossing. It is inferred that the link between solar activity and total ozone variation may be corpuscular radiation. A speculative hypothesis is proposed that the Mev solar Protons in the solar wind may be causing the variation in atmospheric ozone in association with the solar magnetic sector boundary passage.     

Diurnal,semi-diurnal and terdiurnal Hough components of surface pressure

Diurnal and semi-diurnal Hough components of surface pressure are evaluated by classical tidal theory for previously presented profiles of water vapour and ozone heating. Values are compared with the observational results of Haurwitz and Cowley (1973) and show significant discrepancies which are considered to indicate the need for additional heating to be identified. The present calculations are in satisfactory agreement with those for semi-diurnal modes evaluated by Walterscheid et al. (1980).Profiles of terdiurnal heating due respectively to ozone and water vapour absorption of solar radiation are calculated for the (3,3,3) to (3,3,6) Hough modes and corresponding surface pressure oscillations are evaluated by classical tidal theory. Comparisons are made with earlier evaluations and with observationally derived results. The calculated solstitial (3, 3, 4) mode, which is the mode of the largest surface pressure amplitude, shows good agreement in phase with observation but underpredicts observed amplitudes by about 36% in contrast to earlier evaluations which were based on a now unacceptable basic temperature profile.     

Comparison of incoherent scatter observations of electron density,and electron and ion temperature at Millstone Hill with the International Reference Ionosphere

Millstone Hill incoherent scatter (IS) observations of electron density (N_e, electron temperature (T_e) and ion temperature (T_i) are compared with the International Reference Ionosphere (IRI-86) for both noon and midnight, for summer, equinox and winter, at both solar maximum (1979–1980) and solar minimum (1985–1986). The largest difference inN_e is found in the topside, where values of N_e given by IRI-86 are generally larger than those obtained from IS measurements, by a factor which increases with increasing height, and which has a mean value near two at 600 km. Apart from the bottom of the profile, which is tied to the CIRA neutral temperature, the IRI-86 T_e model has no solar cycle variation. However, the IS measurements during the summer reveal larger T_e at solar maximum than at solar minimum. At other seasons higher T_e at solar maximum occurs only during the daytime at the greater heights. Nighttime T_e is shown by the IS radar to be generally larger in winter than in summer, an effect not included in the IRI. This is apparently due to photoelectron heating during winter from the sunlit ionosphere conjugate to Millstone Hill. The day-night difference in T_i given by IRI-86 above 600km is not as large in the IS measurements.