Solar Maximum Conditions Research Articles

Propagation of medium scale traveling ionospheric disturbances at different latitudes and solar cycle conditions

In this work, an extension in latitude range and time span with respect previous studies on Medium Scale Traveling Ionospheric Disturbances (MSTID) propagation, is presented. So far they have been basically studied at mid latitude and for limited periods (less than few years) at solar maximum conditions. This extension has been possible due to the availability of local Global Positioning System (GPS) networks at mid‐north hemisphere (California), mid‐south hemisphere (New Zealand), high and low latitudes (Alaska and Hawaii), for the last 13, 11, 7 and 4 years respectively. Optimal algorithms specially suitable for mass data processing have been used, such as the Single Receiver Medium Scale Traveling Ionospheric activity index (SRMTID) and the phase difference method for MSTID propagation estimation. The results reveal that several of the main MSTID climatological trends at mid latitude are also shared at low and high latitude, also modulated in intensity also by the Solar Cycle. This is the case for local fall/winter day‐time equatorward propagated MSTIDs with typical velocities and wavelengths of 150–250 m/s and 100–300 km respectively. Moreover the comparison of MSTID propagation estimation using different techniques, and their implications in terms of potential origins of MSTIDs, are also discussed.

BEHAVIOR OF SOLAR CYCLES 23 AND 24 REVEALED BY MICROWAVE OBSERVATIONS

Using magnetic and microwave butterfly diagrams, we compare the behavior of solar polar regions to show that (i) the polar magnetic field and the microwave brightness temperature during the solar minimum substantially diminished during the cycle 23/24 minimum compared to the 22/23 minimum. (ii) The polar microwave brightness temperature (b) seems to be a good proxy for the underlying magnetic field strength (B). The analysis indicates a relationship, B = 0.0067Tb - 70, where B is in G and Tb in K. (iii) Both the brightness temperature and the magnetic field strength show north-south asymmetry most of the time except for a short period during the maximum phase. (iv) The rush-to-the-pole phenomenon observed in the prominence eruption activity seems to be complete in the northern hemisphere as of March 2012. (v) The decline of the microwave brightness temperature in the north polar region to the quiet-Sun levels and the sustained prominence eruption activity poleward of 60oN suggest that solar maximum conditions have arrived at the northern hemisphere. The southern hemisphere continues to exhibit conditions corresponding to the rise phase of solar cycle 24.

The 11 year solar cycle signal in transient simulations from the Whole Atmosphere Community Climate Model

The atmospheric response to the 11 year solar cycle (SC) and its combination with the quasi‐biennal oscillation (QBO) are analyzed in four simulations of the Whole Atmosphere Community Climate Model version 3.5 (WACCM3.5), which were performed with observed sea surface temperatures, volcanic eruptions, greenhouse gases, and a nudged QBO. The analysis focuses on the annual mean response of the model to the SC and on the evolution of the solar signal during the Northern Hemispheric winter. WACCM3.5 simulates a significantly warmer stratosphere under solar maximum conditions compared to solar minimum. The vertical structure of the signal in temperature and ozone at low latitudes agrees with observations better than previous versions of the model. The temperature and wind response in the extratropics is more uncertain because of its seasonal dependence and the large dynamical variability of the polar vortex. However, all four simulations reproduce the observed downward propagation of zonal wind anomalies from the upper stratosphere to the lower stratosphere during boreal winter resulting from solar‐induced modulation of the polar night jet and the Brewer‐Dobson circulation. Combined QBO‐SC effects in the extratropics are consistent with observations, but they are not robust across the ensemble members. During boreal winter, solar signals are also found in tropospheric circulation and surface temperature. Overall, these results confirm the plausibility of proposed dynamical mechanisms driving the atmospheric response to the SC. The improvement of the model climatology and variability in the polar stratosphere is the basis for the success in simulating the evolution and magnitude of the solar signal.

The present‐day decadal solar cycle modulation of Earth's radiative forcing via charged H2SO4/H2O aerosol nucleation

The decadal solar cycle modulation of Earth's radiative forcing via ionization of the atmosphere by galactic cosmic rays, aerosol formation from the gas phase, and the response of clouds to aerosol is quantified for the first time with a climate model that represents and couples the relevant processes. Simulations are conducted for solar maximum and minimum conditions, with present‐day anthropogenic aerosol and aerosol precursor gas emissions, and contemporary large‐scale meteorology. The solar cycle signal appears in atmospheric ionization, aerosol formation from the gas phase, aerosol concentrations, aerosol optical depth, and in cloud properties, and is most pronounced at mid‐ and high latitudes. The resulting solar cycle modulation of Earth's radiative forcing exhibits a distinct hemispheric asymmetry, with peak values of −0.14 W m−2 in the southern and −0.06 W m−2in the northern mid‐latitudes. Globally and annually averaged, the solar cycle modulation of Earth's radiative forcing, arising from the increase in atmospheric ionization by galactic cosmic rays from solar maximum to minimum, via charged nucleation of aerosol, the direct aerosol effect, and the cloud albedo effect, amounts to −0.05 W m−2. A limited relevance of this variation for the Earth's atmosphere and climate can be inferred, given that Earth's radiative forcing changes by −0.24 W m−2 from solar maximum to minimum because of a decrease in total solar irradiance.

Analysis of the Characteristics of Low-Latitude GPS Amplitude Scintillation Measured During Solar Maximum Conditions and Implications for Receiver Performance

Ionospheric scintillations are fluctuations in the phase and/or amplitude of trans-ionospheric radio signals caused by electron density irregularities in the ionosphere. A better understanding of the scintillation pattern is important to make a better assessment of GPS receiver performance, for instance. Additionally, scintillation can be used as a tool for remote sensing of ionospheric irregularities. Therefore, the study of ionospheric scintillation has both scientific as well as technological implications. In the past few years, there has been a significant advance in the methods for analysis of scintillation and in our understanding of the impact of scintillation on GPS receiver performance. In this work, we revisit some of the existing methods of analysis of scintillation, propose an alternative approach, and apply these techniques in a comprehensive study of the characteristics of amplitude scintillation. This comprehensive study made use of 32 days of high-rate (50 Hz) measurements made by a GPS-based scintillation monitor located in Sao Jose dos Campos, Brazil (23.2°S, 45.9°W, −17.5° dip latitude) near the Equatorial Anomaly during high solar flux conditions. The variability of the decorrelation time (τ0) of scintillation patterns is presented as a function of scintillation severity index (S4). We found that the values of τ0 tend to decrease with the increase of S4, confirming the results of previous studies. In addition, we found that, at least for the measurements made during this campaign, averaged values of τ0 (for fixed S4 index values) did not vary much as a function of local time. Our results also indicate a significant impact of τ0 in the GPS carrier loop performance for S4 ≥ 0.7. An alternative way to compute the probability of cycle slip that takes into account the fading duration time is also presented. The results of this approach show a 38% probability of cycle slips during strong scintillation scenarios (S4 close to 1 and τ0 near 0.2 s). Finally, we present results of an analysis of the individual amplitude fades observed in our set of measurements. This analysis suggests that users operating GPS receivers with C/N0 thresholds around 30 dB-Hz and above can be affected significantly by low-latitude scintillation.

The combined effects of ENSO and the 11 year solar cycle on the Northern Hemisphere polar stratosphere

[1] The combined effects of El Niño–Southern Oscillation (ENSO) and the 11 year solar cycle on the Northern Hemisphere polar stratosphere have been analyzed in the Whole Atmosphere Community Climate Model version 3 in the absence of the quasi-biennial oscillation. The polar response to ENSO agrees with previous studies during solar minimum; composites of warm minus cold ENSO events show a warmer polar stratosphere and a weaker polar vortex, propagating downward as the winter evolves. During solar maximum conditions, little downward propagation of the ENSO signal is simulated, leading to colder temperatures and stronger winds in the polar lower stratosphere. The analysis of the Eliassen-Palm flux and wave index of refraction shows that this is mainly due to a reduction of upward propagating extratropical planetary wave number 1 component caused by changes in the background winds in the subtropics related to a warmer tropical upper stratosphere during solar maximum. The effect of the 11 year solar cycle variability on the polar stratosphere is not significant during cold ENSO events until February. During warm ENSO events, a statistically significant colder polar lower stratosphere and stronger polar vortex are simulated throughout the winter, and no downward propagation of this signal occurs. This is mainly due to the combined effects of solar maximum and warm ENSO conditions on the wave mean flow interaction. These results show a nonlinear behavior of the extratropical stratosphere response to the combination of the two forcings and highlight the need to stratify with respect to ENSO and solar conditions and analyze the seasonal march throughout the winter.

Dynamical amplification of the stratospheric solar response simulated with the Chemistry-Climate Model LMDz-Reprobus

The impact of the 11-year solar cycle on the stratosphere and, in particular, on the polar regions is investigated using simulations from the Chemistry Climate Model (CCM) LMDz-Reprobus. The annual solar signal clearly shows a stratospheric response largely driven by radiative and photochemical processes, especially in the upper stratosphere. A month-by-months analysis suggests that dynamical feedbacks play an important role in driving the stratospheric response on short timescales. CCM outputs on a 10 days frequency indicate how, in the northern hemisphere, changes in solar heating in the winter polar stratosphere may influence the upward propagation of planetary waves and thus their deposition of momentum, ultimately modifying the strength of the mean stratospheric overtuning circulation at middle and high latitudes. The model results emphasize that the main temperature and wind responses in the northern hemisphere can be explained by a different timing in the occurrence of Sudden Stratospheric Warmings (SSWs) that are caused by small changes in planetary wave propagation depending on solar conditions. The differences between simulations forced by different solar conditions indicate successive positive and negative responses during the course of the winter. The solar minimum simulation generally indicates a slightly stronger polar vortex early in the winter while the solar maximum simulation experiences more early SSWs with a stronger wave-mean flow interaction and reduced zonal wind at mid-latitudes in the upper stratosphere. The opposite response is observed during mid-winter, in February, with more SSWs simulated for solar minimum conditions while solar maximum conditions are associated with a damped planetary wave activity and a reinforced vortex after the initial stratospheric warming period. In late winter, the response is again reversed, as noticed in the temperature differences, with major SSW mostly observed in the solar maximum simulation and less intense final warmings simulated for solar minimum conditions. Due to the non-zonal nature of SSWs, the stratospheric response presents high regional variability during the northern hemisphere winter. As a result, successive positive and negative responses are observed during the course of the winter.

Effects of the 11 year solar cycle on middle atmospheric stationary wave patterns in temperature, ozone, and water vapor

[1] The influence of the 11 year cycle in solar irradiation on middle atmospheric stationary wave patterns in temperature, ozone, and water vapor, as indicated by the deviations from zonal mean T*, O3*, and H2O*, is investigated on the basis of time-slice simulations with the general circulation and chemistry model HAMMONIA for solar maximum and minimum conditions. For northern winter, the long-term means of the three parameters are characterized by a pronounced wave-one pattern in the middle atmosphere, but for each of the parameters with a different shift in phase with increasing height. We find a significant increase in amplitude and a horizontal shift in phase of these wave-one patterns when changing from solar minimum to maximum, i.e., regional changes of about ±2–3 K in T*, ±4%–5% in O3*, and ±2%–3% in H2O*. We demonstrate that the solar variability induces these changes by modulating the effect of zonally asymmetric radiative heating due to the stationary wave-one patterns in ozone and other absorbers and to subsequent modulations in planetary wave propagation and wave-driven transport. A comparison with ensemble means for solar maximum and minimum derived from European Centre of Medium-Range Weather Forecasts (ECMWF) Reanalysis data (ERA-40) shows reasonable agreement but also some differences in the significance and location of the solar signals, which is discussed in relation to the different setups of the two data sets. Overall, the results indicate a remarkable effect of the solar cycle on local changes in temperature, wave dynamics, and transport at northern midlatitudes and polar latitudes.

A study of the strong linear relationship between the equatorial ionization anomaly and the prereversal E × B drift velocity at solar minimum

It is known that there exists a linear relationship between the maximum velocity of the prereversal enhancement (PRE) of the E × B drift and the strength of the equatorial ionization anomaly (EIA) crests at night. This can be a particularly useful relationship in the event that only one of the quantities is observed. But it is important to understand the drivers of the linear relationship in order to determine its range of validity. In this study, we use the SAMI2 model of the ionosphere together with measurements of vertical E × B drift velocity at Jicamarca to show that daytime drifts significantly affect the slope and linearity of the relationship. To validate the model, nighttime O I 135.6 nm radiances measured with the Tiny Ionospheric Photometer (TIP) aboard the Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT‐3/COSMIC) are used in coincidence with the E × B drift measurements at Jicamarca. From the observations, we derive a linear relationship between the crest‐to‐trough ratio of the EIA NmF2 and the PRE under solar minimum conditions. The model simulations demonstrate that the influence of daytime drifts on the nighttime ionosphere varies with longitude and solar cycle conditions; and therefore the linear relationship also varies with these parameters. In particular, we show that the late afternoon drifts at solar minimum have the effect of steepening the slope of the linear relationship. On the other hand, the extended effect of daytime drifts under solar maximum conditions, including contributions from midmorning hours, tends to weaken the relationship.

Chemistry climate model simulations of the effect of the 27 day solar rotational cycle on ozone

[1] The results from two simulations with the coupled chemistry climate model (CCM) ECHAM5/MESSy (EMAC-FUB) are analyzed for the effect of solar variability at the 27 day rotational time scale on ozone. One simulation is forced with constant spectral irradiances at the top of the atmosphere and the other one with daily varying irradiances using data of 1 year for solar maximum conditions. Consistent changes are applied to the photolysis scheme of the model. The model results show the main features of observed correlations between ozone and solar irradiance variability with a maximum positive correlation in the upper stratosphere and an anticorrelation in the mesosphere. The relative sensitivity of upper stratospheric ozone to changes in the solar ultraviolet flux is estimated to be 0.3 to 0.4% per 1% change in 205 nm flux. During periods of strong 27 day variability, a similar upper stratospheric ozone sensitivity is derived. However, when the daily solar irradiance variability is weak and dominated by the 13.5 day period, the ozone sensitivity is reduced in the subtropics. The modeled temperature response is consistent with the ozone signal. When averaged over one rotational cycle, the ozone and temperature response to a neglect of the 27 day cycle is weak and statistically insignificant in the stratosphere but of nonnegligible magnitude and statistically significant in the equatorial mesosphere. Our results suggest that ignoring daily solar flux variations on the 27 day time scale in transient CCM simulations does not lead to a significant degradation of the time mean ozone response in the stratosphere, while in the tropical mesosphere, significant errors of up to 3% may occur. This result does not exclude potential additional effects of 27 day solar cycle variability on stratospheric dynamics in winter which were, however, not the subject of this study.

On the robustness of the solar cycle signal in the Pacific region

[1] The potential role of the stratosphere for the 11-year solar cycle signal in the Pacific region is investigated by idealized simulations using a coupled atmosphere-ocean general circulation model. The model includes a detailed representation of the stratosphere and accounts for changes in stratospheric heating rates from prescribed time dependent variations of ozone and spectrally high resolved solar irradiance. Three transient simulations are performed spanning 21 solar cycles each. The simulations use slightly different ozone perturbations representing uncertainties of solar induced ozone variations. The model reproduces the main features of the 20th century observed solar response. A persistent mean sea level pressure response to solar forcing is found for the eastern North Pacific extending over North America. Moreover, there is evidence for a La Nina-like response assigned to solar maximum conditions with below normal SSTs in the equatorial eastern Pacific, reduced equatorial precipitation, enhanced off-equatorial precipitation and an El Nino-like response a couple of years later, thus confirming the response to solar forcing at the surface seen in earlier studies. The amplitude of the solar signal in the Pacific region depends to a great extent on the choice of the centennial period averaged.

Transmission of galactic cosmic rays through Mars atmosphere

For human operations on the surface of Mars, methods of estimating radiation exposures from galactic cosmic rays (GCRs) are needed. To facilitate making estimates of human radiation exposures for crew operations in the Martian atmosphere, lookup tables have been generated that provide doses for critical body organs and effective doses for exposures from galactic cosmic rays anywhere on the surface of Mars. The organ doses and effective doses are tabulated for carbon dioxide atmospheric shielding areal densities ranging from 0 to 300 g cm−2 followed by aluminum spacecraft or habitat shield areal densities ranging from 0 to 100 g cm−2. The Badhwar‐O'Neill GCR model for interplanetary magnetic field potentials, ranging from the most probable solar minimum (currently 417 MV) to solar maximum conditions (1800 MV) in the solar cycle, is used as input into the calculations. This model is the standard one used for space operations at the Space Radiation Analysis Group at NASA Johnson Space Center. Use of the tables is illustrated for an environment consisting of the current galactic cosmic radiation spectrum impinging on an aluminum habitat on the surface of Mars.

Three-dimensional, multifluid, high spatial resolution MHD model studies of the solar wind interaction with Mars

[1] Our newly developed 3‐D, multifluid MHD model is used to study the interaction of the solar wind with Mars. This model is based on the BATS‐R‐US code, using a spherical grid and a radial resolution equal to 10 km in the ionospheric regions. We solve separate continuity, momentum, and energy equations for each ion fluid and run our model for both solar minimum and maximum conditions. We obtain asymmetric densities, velocities, and magnetic pileup in the plane containing both the direction of the solar wind and the convective electric field. These asymmetries are the result of the decoupling of the individual ions; therefore, our model is able to account for the respective dynamics of the ions and to show new physical processes that could not be observed by the single‐fluid model. Our results are consistent with the measured bow shock and magnetic pileup locations and with the Viking‐observed ion densities. We also compute the escape fluxes for both solar minimum and solar maximum conditions and compare them to the single‐fluid results and the observed values from Mars Express.

Parameterized Regional Ionospheric Model and a comparison of its results with experimental data and IRI representations

We describe a Parameterized Regional Ionospheric Model (PARIM) to calculate the spatial and temporal variations of the ionospheric electron density/plasma frequency over the Brazilian sector. The ionospheric plasma frequency values as calculated from an enhanced Sheffield University Plasmasphere–Ionosphere Model (SUPIM) were used to construct the model. PARIM is a time-independent 3D regional model (altitude, longitude/local time, latitude) used to reproduce SUPIM plasma frequencies for geomagnetic quiet condition, for any day of the year and for low to moderately high solar activity. The procedure to obtain the modeled representation uses finite Fourier series so that all plasma frequency dependencies can be represented by Fourier coefficients. PARIM presents very good results, except for the F region peak height (hmF2) near the geomagnetic equator during times of occurrence of the F 3 layer. The plasma frequency calculated by IRI from E region to bottomside of the F region present latitudinal discontinuities during morning and evening times for both solar minimum and solar maximum conditions. Both the results of PARIM and the IRI for the E region peak density show excellent agreement with the observational values obtained during the conjugate point equatorial experiment (COPEX) campaign. The IRI representations significantly underestimate the foF2 and hmF2 compared to the observational results over the COPEX sites, mainly during the evening–nighttime period.

Variabilities of the equatorial electrojet in Brazil and Perú

This report presents seasonal and longitudinal variabilities of the equatorial electrojet in the east (Brazil, São Luís: 2.3° S; 315.8° E; 0.5° S dip latitude) and west (Jicamarca, Perú: 11.95° S; 283.13° E; 0.6° N dip latitude) coasts of the continent of South America. Ground‐based magnetic field perturbation measurements ΔH for solar maximum (2001/2002) and solar minimum (2006/2007) conditions from the two equatorial stations (São Luís and Jicamarca) have been used for the study. The ΔH signal which is a measure of the strength of the equatorial electrojet is spectrally analyzed using wavelet analysis. The results of our analysis show that (1) the equatorial electrojet has maxima around equinoxes in Jicamarca, Perú but it has a prominent maximum during Southern Hemisphere summer (centered about December/January) in São Luís, Brazil. The observed seasonal behavior of the equatorial electrojet in São Luís is highly likely due to the large magnetic declination angle (about 20° west) there. (2) The equatorial electrojet is stronger in the west coast (Jicamarca) compared to the east coast (São Luís), irrespective of solar activity condition. (3) The magnitude of the equatorial electrojet is more variable with season and solar cycle over São Luís than over Jicamarca.

The 27-day solar rotational cycle in the Freie Universität Berlin Climate Middle Atmosphere Model with interactive chemistry (FUB CMAM CHEM)

We simulate twenty, perpetual Januaries each under solar minimum and maximum conditions using the Freie Universität Berlin Climate Middle Atmosphere Model with interactive chemistry (FUB CMAM CHEM), including the 27-day solar rotational cycle. Cross-correlation functions ( r) of tropical ozone and ultraviolet (UV) suggest peak values of +0.5 to +0.6 (with % changes in ozone of 0.4–0.6%) in the middle atmosphere. Temperature in the middle atmosphere varies with UV by up to 0.5 K, which appears to be mainly dynamical in origin. We calculate a signal associated with the rotational cycle in the troposphere, although this requires further investigations.

Solar activity variations of thermospheric temperatures on Mars and a problem of CO in the lower atmosphere

Long-term MGS drag density observations at 390 km reveal variations of the density with season L S (by a factor of 2) and solar activity index F 10.7 (by a factor of 3 for F 10.7 = 40–100). According to Forbes et al. (Forbes, J.M., Lemoine, F.G., Bruinsma, S.L., Smith, M.D., Zhang, X. [2008]. Geophys. Res. Lett. 35, L01201, doi:10.1029/2007GL031904), the variation with F 10.7 reflects variations of the exospheric temperature from 192 to 284 K. However, the derived temperature range corresponds to variation of the density at 390 km by a factor of 8, far above the observed factor of 3. The recent thermospheric GCMs agree with the derived temperatures but do not prove their adequacy to the MGS densities at 390 km. A model used by Forbes et al. neglects effects of eddy diffusion, chemistry and escape on species densities above 138 km. We have made a 1D-model of neutral and ion composition at 80–400 km that treats selfconsistently chemistry and transport of species with F 10.7, T ∞, and [CO 2] 80 km as input parameters. Applying this model to the MGS densities at 390 km, we find variation of T ∞ from 240 to 280 K for F 10.7 = 40 and 100, respectively. The results are compared with other observations and models. Temperatures from some observations and the latest models disagree with the MGS densities at low and mean solar activity. Linear fits to the exospheric temperatures are T ∞ = 122 + 2.17 F 10.7 for the observations, T ∞ = 131 + 1.46 F 10.7 for the latest models, and T ∞ = 233 + 0.54 F 10.7 for the MGS densities at 390 km. Maybe the observed MGS densities are overestimated near solar minimum when they are low and difficult to measure. Seasonal variations of Mars’ thermosphere corrected for the varying heliocentric distance are mostly due to the density variations in the lower and middle atmosphere and weakly affect thermospheric temperature. Nonthermal escape processes for H, D, H 2, HD, and He are calculated for the solar minimum and maximum conditions. Another problem considered here refers to Mars global photochemistry in the lower and middle atmosphere. The models gave too low abundances of CO, smaller by an order of magnitude than those observed. Our current work shows that modifications in the boundary conditions proposed by Zahnle et al. (Zahnle, K., Haberle, R.M., Catling, D.C., Kasting, J.F. [2008]. J. Geophys. Res. 113, E11004, doi:10.1029/2008JE003160) are reasonable but do not help to solve the problem.

Solar cycle signal in a general circulation and chemistry model with internally generated quasi‐biennial oscillation

Simulations with the HAMMONIA general circulation and chemistry model are analyzed to improve the understanding of the atmospheric response to solar cycle variations and the role of the quasi‐biennial oscillation of equatorial winds (QBO) for this response. The focus is on the Northern Hemisphere winter stratosphere. Owing to the internally produced QBO, albeit with a too short period of 24 months, the model is particularly suited for such an exercise. The simulation setup with only solar and QBO forcing allows an unambiguous attribution of the simulated signals. Two separate simulations have been performed for perpetual solar maximum and minimum conditions. The simulations confirm the plausibility of dynamical mechanisms, suggested earlier, that propagate the solar signal from the stratopause region downward to the troposphere. One feature involved in this propagation is a response maximum of temperature and ozone in the lower equatorial stratosphere. In our model, this maximum appears as a pure solar signal independent of the QBO and of other forcings. As observed, the simulated response of the stratospheric polar vortex to solar cycle forcing depends on the QBO phase. However, in the model this is statistically significant only in late winter. The simulation for early and mid winter suffers probably from a too strong internal variability of the polar vortex in early winter.

Modulations of the 27 day solar rotation signal in stratospheric ozone from Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) (2003–2008)

Here we investigate the modulating nature of the 27 day solar rotation forcing on stratospheric ozone using a new ozone profile data set from Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY, 2003–2008). Continuous wavelet transform (CWT), fast Fourier transform (FFT), and cross correlations (CC) have been applied to SCIAMACHY ozone in the tropics (<20° latitude) between 20 and 60 km altitude. The maximum correlation between Mg II index and ozone is weaker during the maximum of solar cycle 23 (r = 0.38) than in the previous two solar cycles that have been investigated in earlier studies using different data sets. The magnitude of the ozone signal is highly time dependent and may vanish for several solar rotations even close to solar maximum conditions. The FFT analysis reveals, besides the 27 day signal, several frequencies close to 27 days. The ozone sensitivity (ozone change in percent per percent change in 205 nm solar flux) is on average about 0.2%/% above 30 km altitude and smaller by about a factor of 2 compared to earlier studies. For selected 3 month periods the sensitivity may rise beyond 0.6%/% in better agreement with earlier studies. The analysis of the 27 day solar forcing was also carried out with stratospheric temperatures from European Centre for Medium‐Range Weather Forecasts operational analysis. Although direct radiation effects on temperature are weak in the upper stratosphere, temperature signals with statistically significant periods in the 25–35 day range similar to ozone could be found with the applied methods.

Drag and energy accommodation coefficients during sunspot maximum

Conditions appropriate to gas-surface interactions on satellite surfaces in orbit have not been successfully duplicated in the laboratory. However, measurements by pressure gauges and mass spectrometers in orbit have revealed enough of the basic physical chemistry that realistic theoretical models of the gas-surface interaction can now be used to calculate physical drag coefficients. The dependence of these drag coefficients on conditions in space can be inferred by comparing the physical drag coefficient of a satellite with a drag coefficient fitted to its observed orbital decay. This study takes advantage of recent data on spheres and attitude stabilized satellites to compare physical drag coefficients with the histories of the orbital decay of several satellites during the recent sunspot maximum. The orbital decay was obtained by fitting, in a least squares sense, the semi-major axis decay inferred from the historical two-line elements acquired by the US Space Surveillance Network. All the principal orbital perturbations were included, namely geopotential harmonics up to the 16th degree and order, third body attraction of the Moon and the Sun, direct solar radiation pressure (with eclipses), and aerodynamic drag, using the Jacchia-Bowman 2006 (JB2006) model to describe the atmospheric density. After adjusting for density model bias, a comparison of the fitted drag coefficient with the physical drag coefficient has yielded values for the energy accommodation coefficient as well as for the physical drag coefficient as a function of altitude during solar maximum conditions. The results are consistent with the altitude and solar cycle variation of atomic oxygen, which is known to be adsorbed on satellite surfaces, affecting both the energy accommodation and angular distribution of the reemitted molecules.