Local Observation Time Research Articles

Geomagnetic storm effect on equatorial ionosphere over Sri Lanka through total electron content observations from continuously operating reference stations network during Mar–Apr 2022

Abstract The technological advancements in the current era have highlighted the increasing significance of satellite-based positioning, navigation, and timing services in a wide range of dynamic and critical applications. This has led to significant efforts towards enhancing the performance of global navigation satellite systems (GNSS) operating under challenging ionospheric conditions. The Sri Lankan ionosphere region is a focal point of equatorial aeronomy scientists, being situated in the southernmost landmass of the Indian longitude sector within the vicinity of the magnetic equator where a combination of electric, wind, and temperature dynamics exerts a substantial influence on the ionosphere but was relatively unexplored in the past. In the present work, we employed a Kriging interpolation technique on the total electron content (TEC) variables from ten GNSS receivers operating under the Continuously Operating Reference Stations (CORS) network in Sri Lanka first ever of its kind to deliver two-dimensional regional ionospheric TEC maps at hourly intervals, both during quiet and disturbed ionospheric conditions in the equinoctial March and April months of 2022. The latitudinal variation patterns are discernable from the hourly TEC maps. Furthermore, a comparative analysis of the performance of GNSS-derived TEC with that of the routinely published Global Ionospheric Maps (GIMs) confirms overestimation characteristics of the latter irrespective of the local time of observation. The generated regional ionospheric maps are fairly responsive to the onset of the storm and the recovery phase thereafter. The extent of nighttime ionospheric irregularity is also probed through the rate of TEC index (ROTI) variations, demonstrating that the irregularities were insignificant during the selected storm event.

Life cycle resolved observation of radiative properties of mesoscale convective systems

AbstractThe evolution of radiative properties (outgoing longwave radiation, OLR, and albedo at the top of the atmosphere) over Mesoscale Convective System (MCS) life cycle is assessed using five years of Scanner for Radiation Budget (ScaRaB) radiometer onboard the Megha-Tropiques satellites merged with geostationary infrared images. The MCS life cycle is documented using a tracking algorithm. A composite approach is then implemented to document the evolution of radiative properties at each life stage at the scale of the tropical belt, in continental and oceanic regions and in specific regions. Independently of the considered region the composites share similarities with a unique maximum in albedo and a unique minimum in OLR, values of which differ depending on the environment as well as the amplitude of both parameters over the life cycle. The unique precessing orbit of the Megha-Tropiques satellite allows a consideration of the albedo as a function of the local time of observation showing that the magnitude of the albedo signal is mainly controlled by the solar zenithal angle.Sensitivity tests allow to the quantification of the impact of an error in radiative properties showing that even small errors lead to substantial increment on the instantaneous cloud radiative effect. All together these elements point toward the subtle balance between life cycle, cloud radiative properties and phasing within the diurnal cycle to build the atmospheric radiative budget in oceanic or continental regions.

Sunspot Positions and Areas from Observations by Thomas Harriot

The reconstruction of sunspot parameters in the early era of telescopic observations extends our knowledge on visual sunspot activity up to 400 years into the past. 200 digital solar images from 18 December 1610 to 28 January 1613 by the English astronomer Thomas Harriot were analyzed to yield sunspot counts, areas and positions. Harriot generally observed in the early morning when the Sun was near the horizon, and he specified the local time of observation, date, place, number of spots, zenith and ecliptic lines, and other information. We assigned 753 sunspots into 293 groups and defined their areas. We developed two approaches to reconstructing the heliographic coordinates of the sunspots. Method I makes use of the time of observation noted by Harriot, and Method II minimizes the day-to-day variability of sunspot latitudes independently of Harriot’s notes. The results of the two methods are discussed.

New generation of U.S. satellite microwave sounder achieves high radiometric stability performance for reliable climate change detection.

Observations from the satellite microwave sounders play a vital role in measuring the long-term temperature trends for climate change monitoring. Changes in diurnal sampling over time and calibration drift have been the main sources of uncertainties in the satellite-measured temperature trends. We examine observations from the first of a series of U.S. new generation of microwave sounder, the Advanced Technology Microwave Sounder (ATMS), which has been flying onboard the National Oceanic and Atmospheric Administration (NOAA)/NASA Suomi National Polar-orbiting Partnership (SNPP) environmental satellite since late 2011. The SNPP satellite has a stable afternoon orbit that has close to the same local observation time as NASA's Aqua satellite that has been carrying the heritage microwave sounder, the Advanced Microwave Sounding Unit-A (AMSU-A), from 2002 until the present. The similar overpass timing naturally removes most of their diurnal differences. In addition, direct comparison of temperature anomalies between the two instruments shows little or no relative calibration drift for most channels. Our results suggest that both SNPP/ATMS and Aqua/AMSU-A instruments have achieved absolute stability in the measured atmospheric temperatures within 0.04 K per decade. This uncertainty is small enough to allow reliable detection of the temperature climate trends and help to resolve debate on relevant issues. We also analyze AMSU-A observations onboard the European MetOp-A satellite that has a stable morning orbit 8 hours apart from the SNPP overpass time. Their comparison reveals large asymmetric trends between day and night in the lower- and mid-tropospheric temperatures over land. This information could help to improve climate data records for temperature trend detection with improved accuracy. The SNPP satellite will be followed by four NOAA operational Joint Polar Satellite System (JPSS) satellites, providing accurate and stable measurement for decades to come. The primary mission of JPSS is for weather forecasting. Now, with the added feature of stable orbits, JPSS observations can also be used to monitor changes in climate with much lower uncertainty than the previous generation of NOAA operational satellites.

Diurnal variation of tropospheric relative humidity in tropical regions

Abstract. Despite the importance of water vapor especially in the tropical region, the diurnal variations of water vapor have not been completely investigated in the past due to the lack of adequate observations. Measurements from Sondeur Atmosphérique du Profil d'Humidité Intertropicale par Radiométrie (SAPHIR) onboard the low inclination Megha-Tropiques satellite with frequent daily revisits provide a valuable dataset for investigating the diurnal and spatial variation of tropospheric relative humidity in the tropical region. In this study, we first transformed SAPHIR observations into layer-averaged relative humidity, then partitioned the data based on local observation time into 24 bins with a grid resolution of one degree. Afterwards, we fitted Fourier series to the binned data. Finally, the mean, amplitude, and diurnal peak time of relative humidity in tropical regions were calculated for each grid point using either the measurements or Fourier series. The results were separately investigated for different SAPHIR channels as well as for relative humidity with respect to both liquid and ice phases. The results showed that the wet and dry regions are, respectively, associated with convective and subsidence regions which is consistent with the previous studies. The mean tropospheric humidity values reported in this study are generally 10 to 15 % higher than those reported using infrared observations which is because of strict cloud screening for infrared measurements. The results showed a large inhomogeneity in diurnal variation of tropospheric relative humidity in tropical region. The diurnal amplitude was larger over land than over ocean and the oceanic amplitude was larger over convective regions than over subsidence regions. The results showed that the diurnal amplitude is less than 10 % in middle and upper troposphere, but it is up to 30 % in lower troposphere over land. Although the peak of RH generally occurs over night or in early morning, there are several regions where the diurnal peak occurs at other times of the day. The early morning peak time is because of a peak in convective activities in early morning. Additionally, a double peak was observed in tropospheric humidity over some regions which is consistent with double peak in precipitation.

Intercalibrating Microwave Satellite Observations for Monitoring Long-Term Variations in Upper- and Midtropospheric Water Vapor*

Abstract This paper analyzes the growing archive of 183-GHz water vapor absorption band measurements from the Advanced Microwave Sounding Unit B (AMSU-B) and Microwave Humidity Sounder (MHS) on board polar-orbiting satellites and document adjustments necessary to use the data for long-term climate monitoring. The water vapor channels located at 183.31 ± 1 GHz and 183.31 ± 3 GHz are sensitive to upper- and midtropospheric relative humidity and less prone to the clear-sky sampling bias than infrared measurements, making them a valuable but underutilized source of information on free-tropospheric water vapor. A method for the limb correction of the satellite viewing angle based upon a simplified model of radiative transfer is introduced to remove the scan angle dependence of the radiances. Biases due to the difference in local observation time between satellites and spurious trends associated with satellite orbital drift are then diagnosed and adjusted for using synthetic radiative simulations based on the Interim European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-Interim). The adjusted, cloud-filtered, and limb-corrected brightness temperatures are then intercalibrated using zonal-mean brightness temperature differences. It is found that these correction procedures significantly improve consistency and quantitative agreement between microwave radiometric satellite observations that can be used to monitor upper- and midtropospheric water vapor. The resulting radiances are converted to estimates of the deep-layer-mean upper- and midtropospheric relative humidity, and can be used to evaluate trends in upper-tropospheric relative humidity from reanalysis datasets and coupled ocean–atmosphere models.

Correcting diurnal cycle aliasing in satellite microwave humidity sounder measurements

Microwave humidity measurements from polar orbiting satellites are affected by diurnal sampling biases which are caused by changes in the local observation time of the satellites. The long‐term data records available from these satellites thus have spurious trends, which must be corrected. Diurnal cycles of the microwave measurements have been constructed by combining data over the period 2001–2010 from five different satellite platforms (NOAA‐15, ‐16, ‐17, ‐18, and MetOpA). This climatological diurnal cycle has been used to deduce and correct the diurnal sampling bias in Advanced Microwave Sounding Unit‐B and microwave humidity sounder measurements. Diurnal amplitudes for channels which are sensitive to surface temperature variations show a sharp land‐sea contrast with the amplitudes exceeding 10 K for land regions but less than 1 K for oceanic regions. The humidity channels sensitive to the upper and middle troposphere exhibit a seasonal variation with large diurnal amplitudes over convective land regions (often above 3 K) in comparison to oceanic regions. The diurnal peak times of these channels over land occur in the early mornings. The diurnal sampling bias correction has a greater impact over land regions when compared to oceanic regions due to the large diurnal amplitudes over land. The diurnal cycle of humidity generated as a part of this study could be used to evaluate diurnal cycles in climate models.

Construction of Stratospheric Temperature Data Records from Stratospheric Sounding Units

AbstractIn recognizing the importance of Stratospheric Sounding Unit (SSU) onboard historical NOAA polar-orbiting satellites in assessment of long-term stratospheric temperature changes and limitations in previous available SSU datasets, this study constructs a fully documented, publicly accessible, and well-merged SSU time series for climate change investigations. Focusing on methodologies, this study describes the details of data processing and bias corrections in the SSU observations for generating consistent stratospheric temperature data records, including 1) removal of the instrument gas leak effect in its CO2 cell; 2) correction of the atmospheric CO2 increase effect; 3) adjustment for different observation viewing angles; 4) removal of diurnal sampling biases due to satellite orbital drift; and 5) statistical merging of SSU observations from different satellites. After reprocessing, the stratospheric temperature records are composed of nadirlike, gridded brightness temperatures that correspond to identical weighting functions and a fixed local observation time. The 27-yr reprocessed SSU data record comprises global monthly and pentad layer temperatures, with grid resolution of 2.5° latitude by 2.5° longitude, of the midstratosphere (TMS), upper stratosphere (TUS), and top stratosphere (TTS), which correspond to the three SSU channel observations. For 1979–2006, the global mean trends for TMS, TUS, and TTS, are respectively −1.236 ± 0.131, −0.926 ± 0.139, and −1.006 ± 0.194 K decade−1. Spatial trend pattern analyses indicated that this cooling occurred globally with larger cooling over the tropical stratosphere.

Regional assessment of microphysical properties of marine boundary layer cloud using the PATMOS‐x dataset

Cloud droplet number concentration and geometrical thickness of marine boundary layer clouds are inferred from 25 years of National Oceanic and Atmospheric Administration's AVHRR Pathfinder Atmospheres‐Extended (PATMOS‐x) Level 2b retrievals of optical thickness and cloud droplet effective radius over the period 1982 through 2009. A novel approach to addressing nonphysical values of cloud droplet number concentration N owing to satellite orbital drift is applied by normalizing estimated droplet number concentrations with respect to local observation time. Cloud geometrical thickness H is also normalized to a common reference time by scaling H against diurnal values from a passive microwave liquid water path climatology. The effectiveness of the methods applied to correct N and H are evaluated. Both quantities are spatially and temporally characterized in several subtropical subsidence regions for likely drizzle‐free observations. Estimated liquid water path from PATMOS‐x is further validated against 20 years of liquid water path values from the Special Sensor Microwave/Imager (SSM/I). Good agreement between SSM/I and PATMOS‐x is found in coastal regions. Cloud droplet number concentrations in excess of 300 cm−3 are found along the western boundaries of the American and African continents, with greatly lower values found further out to sea with no observed long‐term trends in cloud properties.

Detection and Correction of Diurnal Sampling Bias in HIRS/2 Brightness Temperatures

Abstract Diurnal sampling biases arise in the High-Resolution Infrared Radiation Sounder (HIRS) satellite observations because some of the NOAA polar-orbiting satellites drift significantly from their original local observation time. Such bias adversely affects interpretation of these data for climate studies. Twenty-six years of HIRS/2 radiance satellite data (1979–2004) were examined by creating monthly mean gridded data that categorize the observations by local observing time through averaging ascending and descending orbits separately. Corresponding HIRS/2 simulated radiance data from the Geophysical Fluid Dynamics Laboratory (GFDL) climate model were constructed using HIRS/2 satellite sampling and were found to accurately represent the diurnal sampling bias. Correction of the HIRS/2 observations from the observed diurnal sampling bias was using the model simulations of HIRS brightness temperatures to adjust the observed brightness temperatures to the model daily mean. The diurnal bias was found to vary with channel, surface type, latitude, satellite, and cloud cover, but showed little dependence on satellite scan angle. Diurnal bias is most pronounced for ascending orbit observations of the afternoon [1400 local solar time (LST)] satellites with 60°N to 60°S domain averaged brightness temperatures variations up to 0.78 K yr−1. Lower tropospheric temperature and water vapor channels contained the largest bias, and biases over land were more than twice as large as those over the ocean. Brightness temperature adjustments of up to 10 K were needed in the most extreme situations.

Radiosonde observations of vertical wave number spectra for gravity waves in the lower atmosphere over Central China

Abstract. Vertical wave number spectra of inertial gravity waves in the troposphere and lower stratosphere over six stations at latitudes from 20° N to 40° N were statistically studied by using the data from Radiosonde observation on a twice daily basis at 08:00 and 20:00 LT. Statistically, the spectral characteristics seem to be independent of the local observation time, and show considerable conformity between the spectral of zonal and meridional kinetic energy densities. Compared with the spectra of the kinetic energy density, the spectra of the potential energy density are steeper. in addition the characteristic wave numbers of the spectra also show considerable consistency among the observations at different stations. As for the spectral slopes, they are systematically smaller (in magnitude) than the canon value of –3, and exhibit slight height, seasonal and latitudinal variability. In addition to these universal characteristics, the spectral structures also exhibit departures and variations, and most of the departures and variations are related to the strong tropospheric jets. Generally, in the case of strong shear due to the tropospheric jet, there usually occur larger characteristic wave numbers and smaller spectral slopes. These departures seem to be persistent and climatological rather than transitory, indicating the significant impacts of the sheared background winds on the spectral structures of gravity waves.

Development of a daily long term record of NOAA-14 AVHRR land surface temperature over Africa

We developed a new 6-year daily, daytime and nighttime, NOAA-14 AVHRR based land surface temperature (LST) dataset over continental Africa for the period 1995 through 2000. The processing chain was developed within the Global Inventory Modeling and Mapping System (GIMMS) at NASA's Goddard Space Flight Center. This paper describes the processing methodology used to convert the Global Area Coverage Level-1b data into LST and collateral data layers, such as sun and view geometries, cloud mask, local time of observation, and latitude and longitude. We used the Ulivieri et al. [Ulivieri, C., M.M. Castronuovo, R. Francioni, and A. Cardillo (1994), A split window algorithm for estimating land surface temperature from satellites, Adv. Space Research, 14(3):59–65.] split window algorithm to determine LST values. This algorithm requires as input values of surface emissivity in AVHRR channels 4 and 5. Thus, we developed continental maps of emissivity using an ensemble approach that combines laboratory emissivity spectra, MODIS-derived maps of herbaceous and woody fractional cover, and the UNESCO FAO soil map. A preliminary evaluation of the resulting LST product over a savanna woodland in South Africa showed a bias of < 0.3 K and an uncertainty of < 1.3 K for daytime retrievals (< 2.5 K for night). More extensive validation is required before statistically significant uncertainties can be determined. The LST production chain described here could be adapted for any wide field of view sensor (e.g., MODIS, VIIRS), and the LST product may be suitable for monitoring spatial and temporal temperature trends, or as input to many process models (e.g., hydrological, ecosystem).

Analysis of Sampling Errors for Climate Monitoring Satellites

Abstract Sampling retrievals of high-accuracy first-moment statistics constitute a central concern for climate research. Considered here is the important case of brightness temperature retrievals from a selection of possible orbits. Three-hourly global satellite brightness temperature data are used to predict the sampling error of monthly to annual mean brightness temperature retrieved by one or more satellites in low earth orbits. A true polar orbit is found to offer substantial advantages over a sun-synchronous orbit in the retrieval of annual mean brightness temperature, since the rotation of the local time of observation through two full diurnal cycles greatly reduces the error due to imperfect sampling of diurnal variations. Thus, a single polar orbiting satellite can produce annual mean, zonal mean brightness temperatures with typical sampling errors of less than 0.1 K, while even three sun-synchronous orbiters have high-latitude errors of up to 0.4 K. The error in retrievals of the annual mean diurnal cycle of brightness temperature is also discussed. In this case, high accuracy (&amp;lt;0.1 K) requires three cross-track scanning satellites in precessing orbits, or else a very large number (∼10) of nadir-viewing satellites in precessing orbits. The large sampling errors of sun-synchronous satellites are highly correlated from year to year, so that if equator-crossing times are held fixed, sampling errors in year-to-year differences of annual means are similar for sun-synchronous and precessing orbits.

Altitudinal Distribution Of 20 Persistent Meteor Trains: Estimates Derived From Metro Campaign Archives

Observations of persistent meteor trains are limited because of the extreme rarity of the phenomenon. The altitudinal distribution of persistent trains has previously been investigated via limited instances of simultaneous observation from multiple sites, however, a statistical study of persistent trains has yet to be realized. The meteor train observation (METRO) campaign was established in Japan in 1998 to obtain images of persistent trains. From 1998 to 2002, the METRO campaign, involving Japanese amateur collaborators, captured more than 400 image sequences of persistent trains, resulting in 53 simultaneous multi-site observations. Several Japanese observers were involved in imaging persistent trains prior to the METRO campaign, producing 6 simultaneous observations over the period 1988–1997. In this paper, simultaneous multi-site observations of high spatial and temporal resolution are used to determine, via triangulation, the altitudinal distribution of persistent trains for 20 events. The altitudinal range of 2 Orionid trains was slightly higher than that of 18 Leonid trains. The Leonid train data reveal no clear dependence of upper and lower altitude on the brightness of the associated fireball. The upper altitude of the 18 observed Leonid trains were almost invariant with respect to local time (LT) of observation, however, a possible dependence of lower altitude on LT of observation was also found, indicating a near-constant penetrating path-length in the upper atmosphere for each train. The average upper altitude of persistent Leonid trains was 99.8 km, while the average central altitude was 93.0 km. These trends are probably determined by atmospheric conditions such as the abundance of O and O3.

On Magnetospheric Transmissivity of Cosmic Rays

Results of computations of cosmic-ray trajectories in the model magnetic field obtained for a high-latitude neutron-monitor station (Oulu) and a middle-latitude one (Lomnický Stit) are summarized. The method is slightly different from previous computations by controlling the smoothness of the trajectory. At the lowest rigidity edge of allowed trajectories for Oulu (above the atmospheric cut-off), the external field and the addition ofDst shows variations of the asymptotic directions with the level of geomagnetic activity and with local time of observations. The transmissivity function introduced here, accounting for variability of the magnetic field with changingKp and with local time, may be used as a reference for description of the average magnetospheric transparency at middle latitudes and it is probably better approximation than IGRF only. There are indications of appearance of windows of allowed trajectories at very low rigidities in the middle latitudes at 500 km (altitude of, e.g., CORONAS satellites or ISS) during the geomagnetic disturbance which is not the case for trajectories computed from the ground.

Signal degradation of the AVHRR shortwave channels of NOAA 11 and NOAA 14 by daily monitoring of desert targets

The only indication of sensor degradation affecting the shortwave channels of the AVHRR is the changing response of targets which are assumed to be invariant in time. Using daily data of the whole lifetime of NOAA 11 three different long term trends in the signal response could be identified and quantitatively separated: real sensor degradation, atmospheric influence caused by the changing local time of observation, and surface anisotropy by increasing solar zenith distance. Together they cause the decrease of the measured signal and should be called effective signal degradation. If atmospheric corrections are applied to get surface reflectances it is important to avoid duplicate corrections that would lead to long term trends caused by changing atmospheric attenuation. The degradation coefficients were calculated by postulating a constant NDVI of the desert area. Corrections for surface ansisotropy to the time series of the reflectances were applied. Thus a precise quantitative inter-calibration of the NOAA 14 to the NOAA 11 time series could be performed. The resulting degradation coefficients can be updated and applied in real time.

A model for meteoric magnesium in the ionosphere

We present a model of neutral and ionized magnesium in the equatorial ionosphere at altitudes of 50–350 km. The model was developed in an attempt to understand recent spectroscopic measurements made from the space shuttle. The model incorporates ablation of cosmic dust particles, ion and neutral chemistry, diffusion, and vertical drift due to the equatorial electric field. We find that in the absence of electric field drift, there is little or no variation in Mg and Mg+ density profiles above 120 km. With the electric field included, the high‐altitude behavior of the Mg+ ion changes dramatically. At dawn, almost all the ion density is confined to the 90‐ to 125‐km altitude range, while at dusk there is significant ion density as high as 350 km. A radiance model is developed to allow for direct comparison with measurements. The model incorporates shielding of incident solar UV by ozone and variable local time of observation and tangential height of instrument line of sight. Comparisons between model and measurement are encouraging.

Inversion and space-time-averaging algorithms for ScaRaB (Scanner for the Earth Radiation Budget). Comparison with ERBE

Abstract. Establishment of a uniform long-term record of "top-of-the atmosphere" (TOA) Earth radiation budget (ERB) components, on a scale appropriate to the study of cloud radiation interactions, requires that the data obtained from different observation missions satisfy two basic conditions: (1) the broadband shortwave (SW:0.2–4 µm) and longwave (LW: 4–50 µm) radiances must be demonstrably made on the same absolute scale; and (2) the methods used first to convert the instantaneous (filtered) radiances into (unfiltered) SW and LW radiant fluxes, and then to perform the space-time integrations to yield regional monthly means, must be consistent. Here we consider mainly the second point, with regard to the ScaRaB/Meteor mission in orbit since 25 January 1994 and observing the Earth since 24 February 1994. The objective of this mission is to determine the TOA ERB components and so to provide a continuation of the NASA ERBE scanner mission (November 1984–February 1990). We show how results compatible with ERBE can be obtained by taking into account the instrumental characteristics and the satellite orbit parameters: spectral response of the broadband channels, Earth local time of observation. Considering the spectral response of the ScaRaB broadband channels, we show that no spectral correction is required in the longwave domain, whereas a correction of +4.5% must be applied in the shortwave domain for clear and partly cloudy ocean, in order to compensate for underestimation at the shortest wavelengths. Despite possible differences between ERBE and ScaRaB procedures in values assumed for certain parameters of the scene/cloud identifications, application of these procedures to the same set of ERBE data (spectrally corrected, i.e. "unfiltered" radiances) shows that scene identification agreement is close to 90% and that, where there is disagreement, resulting differences in LW fluxes are negligible, those in SW fluxes small. We show that regional and global mean quantities are in excellent agreement, considering that differences between (ERBS+NOAA-9) and (NOAA-9 only) results may be taken as illustrating time-sampling effects. We find that biases may occur from the undersampling, specifically for the night-time clear-sky estimation over land and desert. Preliminary results using ScaaB data of March 1994 show that clear-sky regional estimates may be less numerous than in ERBE scanner products, due to either the larger pixel size or the auxiliary parameters used in the scene identification, and that expected uncertainties in the global monthly mean values depend mainly on the instrument radiometric calibration.

Statistical studies of impulsive events at high latitudes

A statistical study has been made of the high‐latitude impulsive events that were observed during the 1985–1986 South Pole Balloon Campaign. The events were selected by searching for unipolar pulses ≥ 10 nT above background in the vertical component of the magnetic field on the ground and/or pedestal or “W” shaped horizontal electric field perturbations ≥ 10 mV/m in amplitude and accompanied by perturbations in the vertical electric field at balloon altitude. A main event list comprising 112 events was compiled from the 468 hours of data available. Three aspects of the events were examined: the solar wind conditions prior to the event, local time of observation, and intrinsic properties of the events. The local time distribution was obtained from the 112 entry main event list and was found to be nearly uniform across the dayside, with no midday gap. The event rate found using our low‐amplitude selection criteria was 0.7 event/hr, comparable to expectations based on in situ studies of the magnetopause. A total of 42 events were found for which data were available from Interplanetary Monitoring Platform (IMP) 8. Of these events, 12 occurred when the ZGSM component (BZ) of the interplanetary magnetic field (IMF) was northward and 30 occurred when BZ was southward or fluctuating. Only three of the BZ northward cases and only five of the BZ southward cases were preceded by pressure pulses greater than 0.4 nPa in amplitude. Ten of the events were studied in detail by means of a model‐fitting method discussed elsewhere. This method infers values of several parameters, including the total current flowing in a coaxial or monopole system and a two‐dimensional dipole system. The intrinsic properties of the events showed that only ∼ 10% of the total current contributed to momentum transfer to the high‐latitude ionosphere, that the direction of the motion depended more on local time of observation than IMF By, and that events were usually several hundred kilometers in size. The observed Bz control found in the 42 event list and the prevalence of coaxial current dominated events are inconsistent with the predictions of the pressure pulse model.

Inversion and space-time-averaging algorithms for ScaRaB (Scanner for the Earth Radiation Budget). Comparison with ERBE

Establishment of a uniform long-term record of "top-of-the atmosphere" (TOA) Earth radiation budget (ERB) components, on a scale appropriate to the study of cloud radiation interactions, requires that the data obtained from different observation missions satisfy two basic conditions: (1) the broadband shortwave (SW:0.2–4 µm) and longwave (LW: 4–50 µm) radiances must be demonstrably made on the same absolute scale; and (2) the methods used first to convert the instantaneous (filtered) radiances into (unfiltered) SW and LW radiant fluxes, and then to perform the space-time integrations to yield regional monthly means, must be consistent. Here we consider mainly the second point, with regard to the ScaRaB/Meteor mission in orbit since 25 January 1994 and observing the Earth since 24 February 1994. The objective of this mission is to determine the TOA ERB components and so to provide a continuation of the NASA ERBE scanner mission (November 1984–February 1990). We show how results compatible with ERBE can be obtained by taking into account the instrumental characteristics and the satellite orbit parameters: spectral response of the broadband channels, Earth local time of observation. Considering the spectral response of the ScaRaB broadband channels, we show that no spectral correction is required in the longwave domain, whereas a correction of +4.5% must be applied in the shortwave domain for clear and partly cloudy ocean, in order to compensate for underestimation at the shortest wavelengths. Despite possible differences between ERBE and ScaRaB procedures in values assumed for certain parameters of the scene/cloud identifications, application of these procedures to the same set of ERBE data (spectrally corrected, i.e. "unfiltered" radiances) shows that scene identification agreement is close to 90% and that, where there is disagreement, resulting differences in LW fluxes are negligible, those in SW fluxes small. We show that regional and global mean quantities are in excellent agreement, considering that differences between (ERBS+NOAA-9) and (NOAA-9 only) results may be taken as illustrating time-sampling effects. We find that biases may occur from the undersampling, specifically for the night-time clear-sky estimation over land and desert. Preliminary results using ScaaB data of March 1994 show that clear-sky regional estimates may be less numerous than in ERBE scanner products, due to either the larger pixel size or the auxiliary parameters used in the scene identification, and that expected uncertainties in the global monthly mean values depend mainly on the instrument radiometric calibration.