Solar Rotation Time Scales Research Articles

Understanding Sun-as-a-Star Variability of Solar Balmer Lines

Precise, high-cadence, long-term records of stellar spectral variability at different temporal scales lead to better understanding of a wide variety of phenomena including stellar atmospheres and dynamos, convective motions, and rotational periods. Here, we investigate the variability of solar Balmer lines (Hα, β, γ, δ) observed by space-borne radiometers (OSIRIS, SCIAMACHY, OMI, and GOME-2), combining these precise, long-term observations with high-resolution data from the ground-based NSO/ISS spectrograph. We relate the detected variability to the appearance of magnetic features on the solar disk. We find that on solar-rotational timescales (about 1 month), the Balmer line activity indices (defined as line-core to line-wing ratios) closely follow variations in the total solar irradiance (which is predominantly photospheric), thus frequently (specifically, during passages of sunspot groups) deviating from behavior of activity indices that track chromospheric activity levels. On longer timescales, the correlation with chromospheric indices increases, with periods of low correlation or even anticorrelation found at intermediate timescales. Comparison of these observations with estimates from semiempirical irradiance reconstructions helps quantify the contributions of different magnetic and quiet features. We conclude that both the lower sensitivity to network and in part the higher sensitivity to filaments and prominences, may result in complex, time-dependent relationships between Balmer and other chromospheric indices observed for the Sun and solar-like stars. The fact that core and wings contribute in a similar manner to the variability, and current knowledge of Balmer-lines formation in stellar atmospheres, supports the notion that Balmer line core-to-wing ratio indices behave more like photospheric rather than chromospheric indices.

Solar-Cycle Variability Results from the Solar Radiation and Climate Experiment (SORCE) Mission

The Solar Radiation and Climate Experiment (SORCE) was a NASA mission that operated from 2003 to 2020 to provide key climate-monitoring measurements of total solar irradiance (TSI) and solar spectral irradiance (SSI). This 17-year mission made TSI and SSI observations during the declining phase of Solar Cycle 23, during all of Solar Cycle 24, and at the very beginning of Solar Cycle 25. The SORCE solar-variability results include comparisons of the solar irradiance observed during Solar Cycles 23 and 24 and the solar-cycle minima levels in 2008 – 2009 and 2019 – 2020. The differences between these two minima are very small and are not significantly above the estimate of instrument stability over the 11-year period. There are differences in the SSI variability for Solar Cycles 23 and 24, notably for wavelengths longer than 250 nm. Consistency comparisons with SORCE variability on solar-rotation timescales and solar-irradiance model predictions suggest that the SORCE Solar Cycle 24 SSI results might be more accurate than the SORCE Solar Cycle 23 results. The SORCE solar-variability results have been useful for many Sun–climate studies and will continue to serve as a reference for comparisons with future missions studying solar variability.

Solar activity and responses observed in Balmer lines

Context. Many stars show Sun-like magnetic activity cycles, which are frequently observed by tracking changes in the chromospherically sensitive CaII H&K doublet. However, relationships between the line profile changes related to the magnetic activity seen in strong spectral transitions in other portions of a stellar spectrum are yet to be understood. Aims. We follow variability patterns in various solar lines in order to relate them to the emergence, passage, and decay of active solar regions. Methods. The line activity indices (core-to-wing ratio) for the upper Balmer lines – Hβ, Hγ, and Hδ – are constructed from the near-daily solar measurements acquired by the Ozone Monitoring Instrument and the TROPOspheric Monitoring Instrument. Results. On solar rotation timescales, the upper Balmer line activity indices closely follow variations in the total solar irradiance, r ∼ −(0.6 − 0.7), and thus frequently deviate from the behavior of the line activity indices that track chromospheric activity levels (e.g., the CH 430 nm band used in this study), specifically during passages of big sunspot groups.

Prediction of the Sun’s Coronal Magnetic Field and Forward-modeled Polarization Characteristics for the 2019 July 2 Total Solar Eclipse

Abstract On 2019 July 2 a total solar eclipse—visible across parts of the Southern Pacific Ocean, Chile, and Argentina—enabled observations of the Sun’s corona. The structure and emission characteristics of the corona are determined by underlying magnetic fields, which also govern coronal heating and solar eruptive events. However, coronal magnetic field measurements remain an outstanding challenge. Coronal magnetic field models serve an important purpose in this context. Earlier work has demonstrated that the large-scale coronal structure is governed by surface flux evolution and memory buildup, which allows for its prediction on solar rotational timescales. Utilizing this idea and based upon a 51 day forward run of a predictive solar surface flux transport model and a potential field source surface model, we predict the coronal structure of the 2019 July 2 solar eclipse. We also forward model the polarization characteristics of the coronal emission. Our prediction of two large-scale streamer structures and their locations on the east and west limbs of the Sun match eclipse observations reasonably well. We demonstrate that the Sun’s polar fields strongly influence the modeled corona, concluding that accurate polar field observations are critical. This study is relevant for coronal magnetometry initiatives envisaged with the Daniel K. Inouye Solar Telescope, Coronal Multichannel Polarimeter and upcoming space-based instruments such as Solar Orbiter, Solar Ultraviolet Imaging Telescope and the Variable Emission Line Coronagraph on board the Indian Space Research Organisation’s Aditya-L1 space mission.

Hale cycle and long-term trend in variation of galactic cosmic rays related to solar rotation

Context. Galactic cosmic ray (GCR) intensities around solar minimum times are modulated by magnetic drifts that depend on the overall solar polarity. GCR intensities reach a higher but more narrow peak during negative minima than during positive minima. However, despite these higher intensities, the variation of GCRs over timescales of solar rotation is smaller during negative minima than during positive minima. Aims. We study the variation of GCR intensity over the 27-day synodic solar rotation and over the 14-day half-rotation, in particular the long-term trend and cyclic pattern of this variation, and propose a unifying explanation for the observations. Methods. We used two high-latitude neutron monitors, Oulu and Apatity, which are most sensitive to the low-energy part of the GCR spectrum and thereby more strongly affected by the changes in the conditions of the local heliosphere. We calculated the yearly mean amplitudes of the GCR intensity variation during the full solar rotation ( A 27 ) and half-rotation ( A 14 ) in 1964–2016. Results. We verify that the A 27 and A 14 amplitudes exhibit a clear 22-yr Hale cycle during solar minima at both stations, with larger amplitudes in positive minima. We find that the mean amplitude of the Hale cycle is about 30–45% of the mean amplitude for A 14 , while is only about 15–30% for A 27 . We also find that all amplitudes depict a declining long-term trend, which we suggest is due to the weakening of solar polar magnetic fields during the last four solar cycles and the ensuing latitudinal widening of the heliospheric current sheet (HCS) region. An exceptionally wide HCS region during the last solar minimum, when A 14 reached its all-time minimum, is demonstrated by Ulysses probe observations. Conclusions. Our results emphasize the effect of polarity-dependent drift and the properties of the HCS in modulating the variation of GCR intensity during solar rotation in solar minimum times. The second rotation harmonic yields a larger Hale amplitude than the first because it is more probable for the Earth to be outside the HCS only once during the rotation than twice or more, which more strongly reduces A 14 during negative polarity times than A 27 . With the HCS region widening from minimum to minimum, the decrease in A 14 is relatively faster than in A 27 .

PHOTOMETRIC TRENDS IN THE VISIBLE SOLAR CONTINUUM AND THEIR SENSITIVITY TO THE CENTER-TO-LIMB PROFILE

Solar irradiance variations over solar rotational time-scales are largely determined by the passage of magnetic structures across the visible solar disk. Variations on solar cycle time scales are thought to be similarly due to changes in surface magnetism with activity. Understanding the contribution of magnetic structures to total solar irradiance and solar spectral irradiance requires assessing their contributions as a function of disk position. Since only relative photometry is possible from the ground, the contrasts of image pixels are measured with respect to a center-to-limb intensity profile. Using nine years of full-disk red and blue continuum images from the Precision Solar Photometric Telescope at the Mauna Loa Solar Observatory (PSPT/MLSO), we examine the sensitivity of continuum contrast measurements to the center-to-limb profile definition. Profiles which differ only by the amount of magnetic activity allowed in the pixels used to determine them yield oppositely signed solar cycle length continuum contrast trends; either agreeing with the result of Preminger et al. (2011) showing negative correlation with solar cycle or disagreeing and showing positive correlation with solar cycle. Changes in the center-to-limb profile shape over the solar cycle are responsible for the contradictory contrast results, and we demonstrate that the lowest contrast structures, internetwork and network, are most sensitive to these. Thus the strengths of the full-disk, internetwork, and network photometric trends depend critically on the magnetic flux density used in the quiet-sun definition. We conclude that the contributions of low contrast magnetic structures to variations in the solar continuum output, particularly to long-term variations, are difficult, if not impossible, to determine without the use of radiometric imaging.

Sensitivity of tropical stratospheric ozone to rotational UV variations estimated from UARS and Aura MLS observations during the declining phases of solar cycles 22 and 23

The correlation between tropical stratospheric ozone and UV radiation on solar rotational time scales is investigated using daily satellite ozone observations and reconstructed solar spectra. We consider two 3-year periods falling within the descending phases of two 11-year solar cycles 22 (1991–1994) and 23 (2004–2007). The UV rotational cycle is highly irregular and even disappears for half a year during cycle 23. For the 1991–1994 period, ozone and 205nm UV flux are found to be correlated between about 10 and 1hPa with a maximum of 0.29 at ~5hPa; ozone sensitivity (percentage change in ozone for 1% change in UV) peaks at ~0.4. Correlation during cycle 23 is weaker with a peak ozone sensitivity of 0.2. The correlation is found to vary widely, not only with altitude, but also from one year to the next with a rotational signal in ozone appearing almost intermittent. Unexpectedly, the correlation is not found to bear any relation with the solar rotational forcing. For instance, solar rotational fluctuations are by far the strongest during 1991–1992 whereas the correlation peaks at the end of 1993, a rotationally quiescent period. When calculated over sliding intervals of 1-year, the sensitivity is found to vary very strongly within both 3-year periods; it is almost negligible over the entire vertical profile during some 1-year intervals or reaches close to 1 around 2–5mb for other intervals. Other sources of variability, presumably of dynamical origin, operate on the rotational spectral range and determine to a large extent the estimated solar rotational signal. Even considering 3 years of observations (corresponding to about 40 solar cycles), the extraction of the rotational solar signal does not appear to be robust during declining phases of 11-year solar cycles. As observational studies cover at best three 11-year solar cycles, it must be challenging to produce a reliable estimation of the 11-year solar cycle signal in stratospheric ozone, especially in the presence of decadal climate variability.

Discrepant responses of the global electron content to the solar cycle and solar rotation variations of EUV irradiance

In this paper, the responses of the ionosphere to the solar cycle and solar rotation variations of extreme ultraviolet (EUV) irradiance are comparatively investigated using daily mean global electron content (GEC) and 0.1–50 nm EUV daily flux. GEC is well correlated with EUV on both the solar cycle and solar rotation timescales; however, the responses of GEC to the solar cycle and solar rotation variations of EUV are significantly different in terms of the following two aspects: (1) There is a significant time lag between the solar rotation variations of GEC and EUV; the lag is dominated by a 1-day lag and generally presents a decrease trend with solar activity decreasing. For the solar cycle variations of GEC and EUV, however, there are no evident time lags. (2) The GEC versus EUV slopes are different for the solar cycle and solar rotation variations of GEC and EUV; the solar cycle GEC versus EUV slope is higher than the solar rotation GEC versus EUV slope, and this difference occurs in different seasons and latitudinal bands. The results present an aspect of the difference between ionospheric climatology and weather.

Correlations between variations in solar EUV and soft X‐ray irradiance and photoelectron energy spectra observed on Mars and Earth

AbstractSolar extreme ultraviolet (EUV; 10–120 nm) and soft X‐ray (XUV; 0–10 nm) radiation are major heat sources for the Mars thermosphere as well as the primary source of ionization that creates the ionosphere. In investigations of Mars thermospheric chemistry and dynamics, solar irradiance models are used to account for variations in this radiation. Because of limited proxies, irradiance models do a poor job of tracking the significant variations in irradiance intensity in the EUV and XUV ranges over solar rotation time scales when the Mars‐Sun‐Earth angle is large. Recent results from Earth observations show that variations in photoelectron energy spectra are useful monitors of EUV and XUV irradiance variability. Here we investigate photoelectron energy spectra observed by the Mars Global Surveyor (MGS) Electron Reflectometer (ER) and the FAST satellite during the interval in 2005 when Earth, Mars, and the Sun were aligned. The Earth photoelectron data in selected bands correlate well with calculations based on 1 nm resolution observations above 27 nm supplemented by broadband observations and a solar model in the 0–27 nm range. At Mars, we find that instrumental and orbital limitations to the identifications of photoelectron energy spectra in MGS/ER data preclude their use as a monitor of solar EUV and XUV variability. However, observations with higher temporal and energy resolution obtained at lower altitudes on Mars might allow the separation of the solar wind and ionospheric components of electron energy spectra so that they could be used as reliable monitors of variations in solar EUV and XUV irradiance than the time shifted, Earth‐based, F10.7 index currently used.

Solar EUV and XUV energy input to thermosphere on solar rotation time scales derived from photoelectron observations

Solar radiation below ∼100 nm produces photoelectrons, a substantial portion of the F region ionization, most of the E region ionization, and drives chemical reactions in the thermosphere. Unquantified uncertainties in thermospheric models exist because of uncertainties in solar irradiance models used to fill spectral and temporal gaps in solar irradiance observations. We investigate uncertainties in solar energy input to the thermosphere on solar rotation time scales using photoelectron observations from the FAST satellite. We compare observed and modeled photoelectron energy spectra using two photoelectron production codes driven by five different solar irradiance models. We observe about 1.7% of the ionizing solar irradiance power in the escaping photoelectron flux. Most of the code/model pairs used reproduce the average escaping photoelectron flux over a 109‐day interval in late 2006. The code/model pairs we used do not completely reproduce the observed spectral and solar rotation variations in photoelectron power density. For the interval examined, 30% of the variability in photoelectron power density with equivalent wavelengths between 18 and 45 nm was not captured in the code/model pairs. For equivalent wavelengths below ∼16 nm, most of the variability was missed. This result implies that thermospheric model runs based on the solar irradiance models we tested systematically underestimate the energy input from ionizing radiation on solar rotation time scales.

The discrepancy in solar EUV‐proxy correlations on solar cycle and solar rotation timescales and its manifestation in the ionosphere

This study investigates the correlations between SOHO/Solar EUV Monitor 26–34 nm EUV and the F10.7 and Mg II proxies on solar cycle (long‐term) and solar rotation (short‐term) timescales. The long‐term components of EUV and proxies are well correlated, and the general relation between them can be captured by the 81 day averaged EUV and proxies. Short‐term EUV‐proxy correlation is poorer and variable during the solar cycle. The slopes of short‐term EUV against proxies vary from solar rotation to solar rotation, and they are generally lower than those of long‐term EUV against proxies. EUV and proxies show discrepant evolutions during the episode of major active regions, which should be a primary reason for the poorer short‐term EUV‐proxy correlation and the variable short‐term EUV‐proxy slope. Mg II is a better proxy than F10.7 for 26–34 nm EUV. Its superiority mainly comes from better indications for short‐term EUV. Global electron content (GEC) significantly responds to the long‐term and short‐term variations of EUV. Accordingly, the correlations between short‐term GEC and proxies are poorer, and they are obviously lower than those between short‐term EUV and proxies owing to ionospheric day‐to‐day variability. Short‐term GEC‐proxy slopes are also lower than the long‐term slopes. F10.7 and Mg II are improved by combining the daily and 81 day averaged components of them with weighted factors that are designed to decrease the difference between long‐term and short‐term EUV‐proxy slopes. The improved proxies can effectively upgrade the indications of proxies for EUV though they cannot solve the variability of short‐term EUV‐proxy slope. This method is also used to improve proxies for better indicating GEC, but the improved proxies for GEC differ from those for 26–34 nm EUV.

Intercomparison of SCIAMACHY and SIM vis-IR irradiance over several solar rotational timescales

The two satellite spectrometers SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) aboard ENVISAT (Environmental Satellite), and SIM (Spectral Irradiance Monitor) aboard SORCE (Solar Radiation and Climate Experiment) observe since 2002 and 2003, respectively, daily solar spectral irradiance (SSI) not only in UV but extending to the visible and near- infrared (vis-NIR) regions. In this work, we intercompare (1) spectra and (2) timeseries of SSI measurements from SCIAMACHY and SIM. In (1) same-day (April 21, 2004) SSI measurements from these two instruments are compared to reference spectra from ground (new Kurucz), high-altitude (Hall and Anderson, Neckel and Labs, and Wehrli composite), and space (SOLSPEC/ATLAS 3, and SUSIM/UARS). In (2) timeseries of measurements (July 3 to August 21, 2004) covering several solar rotations in 2004 are compared to VIRGO sunphotometers (SPM) aboard SOHO. In general, SCIAMACHY and SIM are in agreement to within 4% over the common spectral domain and with respect to the other reference data. Apart from SSI and its variability, we integrate SSI over selected wavelength intervals and compare qualitatively to total solar irradiance (TSI) variability from PMOD/WRC and TIM/SORCE. Timeseries of integrated SSI in the vis (400–700 nm), NIR (700–1600 nm), and UV-vis-NIR (240–1600 nm) bands are compared. The overall rise and fall of integrated SCIAMACHY and SIM irradiances over several solar rotations are in good agreement and agree in most cases qualitatively with TSI variations in the visible and near IR. The application of White Light Source (WLS) corrections brings SCIAMACHY irradiances in closer agreement with SIM. Since WLS is also degrading with time, the WLS lamp ratios cannot be used for SSI degradation corrections after 2004.

Spectral irradiance variations: comparison between observations and the SATIRE model on solar rotation time scales

Aims: We test the reliability of the observed and calculated spectral irradiance variations between 200 and 1600 nm over a time span of three solar rotations in 2004. Methods: We compare our model calculations to spectral irradiance observations taken with SORCE/SIM, SoHO/VIRGO and UARS/SUSIM. The calculations assume LTE and are based on the SATIRE (Spectral And Total Irradiance REconstruction) model. We analyse the variability as a function of wavelength and present time series in a number of selected wavelength regions covering the UV to the NIR. We also show the facular and spot contributions to the total calculated variability. Results: In most wavelength regions, the variability agrees well between all sets of observations and the model calculations. The model does particularly well between 400 and 1300 nm, but fails below 220 nm as well as for some of the strong NUV lines. Our calculations clearly show the shift from faculae-dominated variability in the NUV to spot-dominated variability above approximately 400 nm. We also discuss some of the remaining problems, such as the low sensitivity of SUSIM and SORCE for wavelengths between approximately 310 and 350 nm, where currently the model calculations still provide the best estimates of solar variability.

New flare model using recent measurements of the solar ultraviolet irradiance

The solar photon output from the Sun, which was once thought to be constant, varies considerably over time scales from seconds during solar flares to years due to the solar cycle. This is especially true in the wavelengths shorter than 190 nm. These variations cause significant deviations in the Earth and space environment on similar time scales, which then affects many things including satellite drag, radio communications, atmospheric densities and composition of particular atoms, molecules, and ions of Earth and other planets, as well as the accuracy in the Global Positioning System (GPS). The Flare Irradiance Spectral Model (FISM) is an empirical model that estimates the solar irradiance at wavelengths from 0.1 to 190 nm at 1 nm resolution with a time cadence of 60 s. This is a high enough temporal resolution to model variations due to solar flares, for which few accurate measurements at these wavelengths exist. This model also captures variations on the longer time scales of solar rotation (days) and solar cycle (years). Daily average proxies used are the 0–4 nm irradiance, the Mg II c/w, F10.7, as well as the 1 nm bins centered at 30.5 nm, 121.5 (Lyman Alpha), and 36.5 nm. The GOES 0.1–0.8 nm irradiance is used as the flare proxy. The FISM algorithms are given, and results and comparisons are shown that demonstrate the FISM estimations agree within the stated uncertainties to the various measurements of the solar Vacuum Ultraviolet (VUV) irradiance.

Solar radiation‐induced changes in ionospheric height and the Schumann resonance waveguide on different timescales

This study draws together the available observations in the Schumann resonance frequency range to examine the general issue of sensitivity of ionospheric height variations to changes in ionizing radiation from the Sun on different timescales. Ionospheric height can be formally defined, and two characteristic heights are recognized in the Schumann resonance frequency range. In general, order of magnitude changes in radiation are needed to cause relative changes in ionospheric height as large as 10%, as is the case on both the diurnal and 11‐year timescales. Changes in EUV radiation are probably insufficiently strong to account for either the 27‐day or the 11‐year variation in ionospheric height. More ionization‐effective X radiation is needed, but much smaller height changes are expected on the solar rotation timescale because the variations in X radiation on this timescale are only tens of percent and not orders of magnitude. The annual variation in radiation from the Sun is only 7%, with an expected height variation less than 100 m, and this remains to be verified by observations. The general insensitivity of the Schumann resonance cavity to changes in ionizing radiation lends stability to the medium that is valuable toward quantifying absolute changes in the global lightning activity on various timescales within the cavity.

Solar cycle evolution of the contrast of small photospheric magnetic elements

Solar irradiance variations produced on the solar rotation time-scale are known to be driven by the passage of active regions while, during the last years, the origin of variations on the solar cycle time-scale has been under debate. Nowadays, there is an agreement that the magnetic network has an important contribution to these long-term variations, although it has not been fully quantified. This important role motivated us to study its physical properties along the solar cycle, such as contrast and population. We combine magnetograms and intensity images from the MDI instrument on board the SOHO spacecraft to analyze the radiative properties of small magnetic elements. We determine the contrast of faculae and network elements as a function of position over the disk, magnetic flux and time, finding that these elements exhibit a very different center-to-limb variation of the contrast. This implies that their contribution to irradiance variability is distinct. By extending this analysis through the rising phase of solar cycle 23, we conclude that the functional dependence of the contrast of small elements results to be time independent, implying that the physical properties of the underlying flux tubes may not vary with time. We decompose magnetograms into two structures identifying both faculae and network features and we examine their populations along the solar cycle.

How much of the solar irradiance variations is caused by the magnetic field at the solar surface?

The contribution to total solar irradiance variations by the magnetic field at the solar surface is estimated. Detailed models of the irradiance changes on the basis of magnetograms show that magnetic features at the solar surface account for over 90% of the irradiance variations on a solar rotation time scale and at least 70% on a solar cycle time scale. If the correction to the VIRGO record proposed by Fröhlich & Finsterle (2001) is accepted, then magnetic features at the solar surface are responsible for over 90% of the solar cycle irradiance variations as well.

Long term measurements of radiation belts by SAMPEX and their variations

The Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), a low‐altitude and polar‐orbiting satellite, has provided a long‐term global picture of the radiation belts since its launch on July 3, 1992. While the inner belt (L < 2) protons appear to have only a solar cycle variation, the outer radiation belt (L > 2) electrons vary on solar cycle, semiannual, and solar rotation time scales, and with geomagnetic storms. Recently developed models of predicting MeV electron at geostationary orbit [Li et al., 2001] and the Dst index [Temerin and Li, 2001] based on solar wind measurements are used to examine the cause of the prominent semiannual variations of outer belt electrons and the Dst index. The equinoctial effect (the angle between the Earth's dipole and the flow direction of the solar wind) contributes most to the semiannual variation of the Dst and MeV electrons deep in the inner magnetosphere (L < 5). The semiannual variation of MeV electrons at geostationary orbit is attributed mostly to the semiannual variation of solar wind velocity.

NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989–1994: 2. Results, validation, and comparisons

Accurately measuring long‐term solar UV variability is an experimental challenge because instrument response degradations are typically large enough to obscure solar change. For satellite instruments, one solution is a series of regular comparisons with a well‐calibrated reference. The NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument made solar spectral irradiance measurements between 170 and 400 nm from December 1988 to October 1994, covering the maximum and most of the decline of solar cycle 22. The NOAA 11 irradiance data were corrected for long‐term instrument sensitivity changes using comparisons with coincident flights of the Shuttle SBUV (SSBUV) instrument. The NOAA 11 data show a decrease of 7.0(±1.8)% in smoothed 200–208 nm irradiance from Cycle 22 maximum in mid‐1989 to October 1994, near solar minimum. The long‐term decrease in solar irradiance at 250 nm was ∼3.5(±1.8)%. Longward of 300 nm, no solar variations were observed to within the 1% accuracy of the data. The NOAA 11 measurements overlap observations from the Upper Atmosphere Research Satellite (UARS) Solar Stellar Irradiance Comparison Experiment (SOLSTICE) and Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instruments from October 1991 to October 1994, providing the first opportunity to compare three coincident long‐term solar UV irradiance data sets. We find reasonable agreement between the NOAA 11, SOLSTICE, and SUSIM results at all wavelengths in the 170–400 nm region. Power spectral analysis gives consistent results for all three instruments on solar rotational timescales, and reveals the evolution of solar rotation periodicity and strength during a solar cycle. We find significant differences between instruments in both period and spectral location when the spectral irradiance data are analyzed on intermediate (50–250 days) timescales. The NOAA 11 spectral irradiance data provide a valuable complement to the UARS solar data, and capture the entire maximum of solar cycle 22.

Coherency between solar UV radiation and equatorial total ozone

Influences of solar UV variability on the total ozone column densities as measured by the total ozone mapping spectrometer (TOMS) on the Nimbus 7 satellite are investigated and distinguished from the noticeable signals of dynamical origin. We confine our analysis to the equatorial region where the purest observable periodic signals that relate to the solar UV variability at the solar cycle and solar rotational timescales appear. The Mg II index is used as a proxy for solar variability in the UV region of the solar spectrum. Applying a multiple taper method of power spectral estimation to the entire TOMS observation period (14.4 years), evidence for a solar cycle variation is obtained at the 95% confidence level. The estimated solar cycle variation is 1.5±0.3% from solar minimum to maximum, corresponding to a 7.6±0.7% change in the Mg II index with no significant phase lag. The most significant line contributions to the power spectrum of the daily total ozone values in the proximity of solar rotation frequencies have periods of 28.4±0.2 and 14.6±0.1 days during maximum solar activity in solar cycle 22. F tests show both lines to be significant at or above the 99% level. Identical analysis of the Mg II index for the same period shows the strongest signals appearing at 28.4±0.5 and 13.5±0.1 days. The latter signal is more difficult to confirm because of possible interference with other lines at close frequencies (14.8 and 12.3 days). The sensitivity of total ozone to the major solar rotational mode is comparable to the solar cycle effect, and the results are consistent with solar signals of pure photochemical origin. Strong coherency appears in a couple of narrow frequency bands around 26 and 13 days showing that total ozone responds to solar UV radiation in both of the major solar rotation modes.