Matthew Todd DeLand

A census of magnetograms acquired by the space-based Helioseismic and Magnetic Imager (HMI) during solar activity minimum between 2019 January 1 and 2020 February 29 reveals a population of small, typically ∼1 Mm across, predominantly unipolar magnetic sources, embedded in extensive same-polarity halos. The sources carry ∼10 ¹⁸ Mx fluxes and have median lifetimes of 400–670 s. Some vary on ∼1 minute timescales, rapidly doubling their magnetic flux densities (MFDs), then returning to the preoutburst level. Emission from a slowly changing mix of network and small-scale sources modulates the total solar irradiance (TSI) at 0.005%–0.01% levels on rotational timescales. We find evidence of a nonlinear TSI–magnetic field relationship at very low activity levels. During the 2019 cycle minimum, as in the prior activity minimum in 2009, the total MFD in ground-based magnetograms measured by the Global Oscillation Network Group (GONG) persistently remained in excess of ∼1.5 G above that of the total flux in the Gaussian core. Using Wilcox Solar Observatory (WSO) synoptic photospheric magnetic field maps, we identify an overall decline of ∼2 G in total MFD from solar minimum in 1976 to the 2009 and 2019 minima, which we attribute to the decline of the Centennial Gleissberg Cycle of 1910 to 2009. We speculate that the MFD during the upcoming solar activity minimum in 2029 may be higher than in 2019 as the next Gleissberg cycle manifests.

The Ozone Monitoring Instrument (OMI) on the Aura satellite makes near‐daily measurements of solar spectral irradiance (SSI) at wavelengths between 265 and 500 nm for instrument calibration purposes. The low degradation and exceptional stability of OMI's calibration during its lifetime facilitates the creation of a new 18‐year OMI Version 7 data set. This data set presented here, which extends our previous data set by more than six years, is constructed using an improved OMI degradation model, and enables discussion of SSI variability during the extended 2009 solar activity minimum and subsequent Solar Cycles 24 and 25. OMI observations of solar irradiance variability in the mid‐ultraviolet (265–300 nm), near‐ultraviolet (300–400 nm), and visible (400–500 nm) spectral regions, consistent with concurrent measurements from other satellite instruments, show that, contrary to initial predictions, Solar Cycle 25 is significantly more active than Solar Cycle 24. Solar activity indices created from OMI SSI observations agree with both satellite and ground‐based indices, and are similarly higher in Solar Cycle 25 than 24. We demonstrate good agreement between the OMI irradiance observations and the new observation‐based NOAA‐NASA‐LASP (NNLSSI1) model of solar irradiance variability, including during Solar Cycle 25. The agreement is even better when the model is revised using an improved facular index that is consistent with OMI activity indices. We use the revised NNLSSI1 rev 1 model reconstruction of solar irradiance in the past century to make a simple statistical projection of SSI variability during the remaining portion of Solar Cycle 25 and beyond.

¿p#1 The Ozone Monitoring Instrument (OMI) on the Aura satellite makes near-daily measurements of solar spectral irradiance (SSI) at wavelengths between 265-500 nm for instrument calibration purposes. The low degradation and exceptional stability of OMI’s calibration during its lifetime facilitates the creation of a SSI data product that covers more than 18 years and spans almost three of the Sun’s 11-year activity cycles. OMI observations of solar irradiance variability in the mid-ultraviolet (265-300 nm), near-ultraviolet (300-400 nm), and visible (400-500 nm) spectral regions mostly agree with concurrent measurements from other satellite instruments. Solar activity indices created from OMI SSI observations also show good agreement with both satellite and ground-based indices. We further compare the OMI irradiance observations with semi-empirical model results from the NOAA-NASA-LASP version 1 (NNLSSI1) model of solar irradiance variability that estimates SSI for the NOAA Solar Irradiance Climate Data Records (CDR). The agreement between the observations and model confirms that solar cycle 25 is more active than originally predicted, and allows us to project SSI behavior during the remaining portion of Cycle 25 and beyond.

The frequency and spatial distribution of polar stratospheric clouds (PSCs) has a strong influence on the magnitude of springtime ozone depletion in polar regions. We use Ozone Mapping and Profiler Suite (OMPS) Limb Profiler (LP) satellite measurements to identify PSCs and characterize their distribution. Concurrent water vapor and nitric acid profiles from Aura Microwave Limb Sounder measurements are used to determine the approximate temperature threshold for PSC formation. Ozone Mapping and Profiler Suite LP daytime measurements have limited coverage of the polar vortex region during winter months, with increasing coverage as spring equinox approaches. Suomi National Polar‐orbiting Partnership (NPP) OMPS LP observations since 2012 show a relatively consistent seasonal pattern of PSC formation and evolution in the Southern Hemisphere (SH), but significantly more variability during each season and between seasons in the Northern Hemisphere (NH). Comparisons of OMPS LP PSC detections with concurrent Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) observations show good agreement in spatial distribution and seasonal evolution when the CALIOP latitude coverage is screened to the range viewed by OMPS LP daytime measurements. Limb Profiler measurements from current and future Joint Polar Satellite System (JPSS) satellites will enable us to follow long‐term changes in PSC behavior, such as possible effects of the increased stratospheric water vapor from the Hunga Tonga‐Hunga Ha'apai eruption.

Combining the near-daily Ozone Monitoring Instrument (OMI) and Tropospheric Monitoring Instrument (TROPOMI) measurements of solar spectra, we construct line indices (line-core to line-flanks ratios) for various transitions (mainly Fe i) in the 300–2390 nm spectral domain. The indices are supplemented by the wavelength-binned fluxes from OMI and Total and Spectral Solar Irradiance Sensor (TSIS-1). To study the short-term (solar-rotational) patterns, we normalize the indices and fluxes to the minimum-activity epoch, then de-trend them with 81 day running means. Comparisons of the de-trended TSIS-1 and OMI fluxes with the NASA-NOAA-LASP SSI (NNLSSI1) model show excellent agreement, to (0.5–2.2) × 10⁻⁴ in the normalized and de-trended data. The data are subjected to a multiregression analysis against quantities representing the facular brightening and the sunspot darkening. The de-trended line indices and average fluxes show different sensitivities to these two solar magnetic-activity manifestations, with the fluxes being far more susceptible to the sunspot component. The de-trended line indices experience a rapid drop of activity levels towards longer wavelengths, albeit with a large rebound in the short-wave infrared (SWIR) domain that is caused by the ubiquitous, temperature-sensitive CO transitions. The wavelength-dependent activity also falls, however much slower, in the de-trended average fluxes. Qualitatively similar behavior is captured by semiempirical models.

We evaluate a decadal ozone profile record derived from the Suomi National Polar‐orbiting Partnership (SNPP) Ozone Mapping and Profiler Suite (OMPS) Limb Profiler (LP) satellite instrument. In 2023, the OMPS LP data were re‐processed with the new version 2.6 retrieval algorithm that combines measurements from the ultraviolet (UV) and visible (VIS) parts of the spectra. It employs the second order Tikhonov regularization to retrieve a single vertical ozone profile between 12.5 km (or cloud tops) and 57.5 km with a vertical resolution of about 1.9–2.5 km between 20 and 55 km. The algorithm uses radiances measured at six UV ozone‐sensitive wavelengths (295, 302, 306, 312, 317, and 322 nm) paired with 353 nm, and one VIS wavelength at 606 nm combined with 510 and 675 nm to form a triplet. Each wavelength pair or triplet is used over a limited range of tangent altitudes where the sensitivity to ozone change is strongest. A new aerosol correction scheme was implemented based on a gamma‐function particle size distribution. In addition, numerous calibration changes that affected ozone retrievals were applied to measured LP radiances, including updates in altitude registration, radiometric calibration, stray light, and spectral registration. The key version 2.6 enhancement is the improved stability of the LP ozone record confirmed by the reduction in relative drifts between LP ozone and correlative measurements, linked previously to a drift in the version 2.5 LP altitude registration.

Atmospheric aerosols represent an important component of the Earth’s climate system because they can contribute both positive and negative forcing to the energy budget. We are developing the Aerosol Radiometer for Global Observations of the Stratosphere (ARGOS) instrument to provide improved measurements of stratospheric aerosols in a compact package. ARGOS makes limb scattering measurements from space in eight directions simultaneously, using two near-IR wavelengths for each viewing direction. The combination of forward and backward scattering views along the orbit track gives additional information to constrain the aerosol phase function and size distribution. Cross-track views provide expanded spatial coverage. ARGOS will have a demonstration flight through a hosted payload provider in the fall of 2024. The instrument has completed pre-launch environmental testing and radiometric characterization tests. The hosted payload approach offers advantages in size, weight, and power margins for instrument design compared to other approaches, with significant benefits in terms of reducing infrastructure requirements for the instrument team.

We evaluate a decadal ozone profile record derived from the Suomi National Polar-orbiting Partnership (SNPP) Ozone Mapping and Profiler Suite (OMPS) Limb Profiler (LP) satellite instrument. In 2023, the OMPS LP data were re-processed with the new version 2.6 retrieval algorithm that combines measurements from the ultraviolet (UV) and visible (VIS) parts of the spectra and employs the second order Tikhonov regularization to retrieve a single vertical ozone profile between 12.5 km (or cloud tops) and 57.5 km with the vertical resolution of about 1.9 - 2.5 km between 20-55 km. The algorithm uses radiances measured at six UV ozone-sensitive wavelengths (295, 302, 306, 312, 317 and 322 nm) paired with 353 nm, and one VIS wavelength at 606 nm combined with 510 nm and 675 nm to form a triplet. Each wavelength pair or triplet is used over a limited range of tangent altitudes where the sensitivity to ozone changes are strongest. A new implemented aerosol correction scheme is based on a gamma-function particle size distribution. Numerous calibration changes that affected ozone retrievals were also applied to measured LP radiances, including updates in altitude registration, radiometric calibration, stray light, and spectral registration. The key version 2.6 improvement is the reduction in relative drifts between LP ozone and correlative measurements, linked previously to a drift in the version 2.5 LP altitude registration. We compare LP ozone profiles with those from Aura Microwave Limb Sensor (MLS) to quantify ozone changes in version 2.6.

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.