Occultation Observations Research Articles

Haze Formation on Triton

Abstract The largest moon of Neptune, Triton, possesses a cold and hazy atmosphere. Since the discovery of the near-surface haze layer during the Voyager fly in 1989, the haze formation mechanism has not been investigated in detail. Here we provide the first haze microphysical model on Triton. Our model solves the evolution of both size and porosity distributions of haze particles in a self-consistent manner. We simulated the formation of sphere and aggregate hazes with and without condensation of the C2H4 ice. The haze particles can grow into fractal aggregates with mass-equivalent sphere sizes of ∼0.1–1 μm and fractal dimensions of D f = 1.8–2.2. The ice-free hazes cannot simultaneously explain both UV and visible observations of Voyager 2, while including the condensation of C2H4 ices provides two better solutions. For ice aggregates, the required total haze mass flux is ∼2 × 10−15 g cm−2 s−1. For the icy sphere scenario, the column-integrated C2H4 production rate is ∼8 × 10−15 g cm−2 s−1, and the ice-free mass flux is ∼6 × 10−17 g cm−2 s−1. The UV occultation observations at short wavelengths, <0.15 μm, may slightly favor the icy aggregates. Observations of the haze optical depth and the degree of forward scattering in UV and visible should be able to distinguish whether Triton’s hazes are icy spheres or ice aggregates in future Triton missions.

Quick-look estimates of ionospheric properties from radio occultation data

Radio occultations are commonly used to determine vertical profiles of ionospheric electron density from measurements of frequency. “Frequency residuals” are important intermediate data products in radio occultation experiments. Frequency residuals are defined as the difference between observed and predicted values of the received frequency, where the predicted frequency includes all effects except refraction at the target object. However, relationships between properties of a vertical profile of ionospheric electron density and properties of time series of frequency residuals are not widely known by the broad community of scientists who may work with frequency residuals. Here we illustrate and explain how properties of a vertical profile of ionospheric electron density affect time series of frequency residuals. We also develop four quantitative relationships between electron density values and frequency residuals. These provide predictions for the topside plasma scale height, topside electron densities, peak altitude, and peak electron density that can be generated directly from a set of frequency residuals without the need to implement the full Abel transform analysis that is usually required to determine electron densities from frequency residuals. They can also be generated prior to the implementation of a high-quality baseline correction, to which the Abel transform analysis is sensitive. The development of these predictions assumed that the ionosphere has an exponential topside and a Chapman-like peak. These predictions are successfully tested on two-way radio occultation observations from the MAVEN Radio Occultation Science Experiment (ROSE) at Mars.

Identifying the Coronal Source Regions of Solar Wind Streams from Total Solar Eclipse Observations and in situ Measurements Extending over a Solar Cycle

Abstract This letter capitalizes on a unique set of total solar eclipse observations acquired between 2006 and 2020 in white light, Fe xi 789.2 nm (T fexi = 1.2 ± 0.1 MK), and Fe xiv 530.3 nm (T fexiv = 1.8 ± 0.1 MK) emission complemented by in situ Fe charge state and proton speed measurements from Advanced Composition Explorer/SWEPAM-SWICS to identify the source regions of different solar wind streams. The eclipse observations reveal the ubiquity of open structures invariably associated with Fe xi emission from Fe10+ and hence a constant electron temperature, T c = T fexi, in the expanding corona. The in situ Fe charge states are found to cluster around Fe10+, independently of the 300–700 km s−1 stream speeds, referred to as the continual solar wind. Thus, Fe10+ yields the fiducial link between the continual solar wind and its T fexi sources at the Sun. While the spatial distribution of Fe xiv emission from Fe13+ associated with streamers changes throughout the solar cycle, the sporadic appearance of charge states >Fe11+ in situ exhibits no cycle dependence regardless of speed. These latter streams are conjectured to be released from hot coronal plasmas at temperatures ≥T fexiv within the bulge of streamers and from active regions, driven by the dynamic behavior of prominences magnetically linked to them. The discovery of continual streams of slow, intermediate, and fast solar wind characterized by the same T fexi in the expanding corona places new constraints on the physical processes shaping the solar wind.

Radial Distributions of Coronal Electron Temperatures: Specificities of the DYN Model

This paper is a follow up of the article where Lemaire and Stegen (2016) introduced their DYN method to calculate coronal temperature profiles from given radial distributions of the coronal and solar wind (SW) electron densities. Several such temperature profiles are calculated and presented corresponding to a set of given empirical density models derived from eclipse observations and in-situ measurements of the electron density and bulk velocity at 1 AU. The DYN temperature profiles obtained for the equatorial and polar regions of the corona challenge the results deduced since 1958 from singular hydrodynamical models of the SW. In these models - where the expansion velocity transits through a singular saddle point - the maximum coronal temperature is predicted to be located at the base of the corona, while in all DYN models the altitude of the maximum temperature is found at significantly higher altitudes in the mid-corona. Furthermore, the maximum of the DYN-estimated temperatures is found at much higher altitudes over the polar regions and coronal holes, than over the equator. However, at low altitudes, in the inner corona, the DYN temperatures are always smaller at high latitudes, than at low equatorial latitudes. This appears well in agreement with existing coronal hole observations. These findings have serious implications on the open questions: what is the actual source of the coronal heating, and where is the maximum energy deposited within the solar corona?

Tropical Temperature Variability in the UTLS: New Insights from GPS Radio Occultation Observations

AbstractGlobal positioning system (GPS) radio occultation (RO) observations, first made of Earth’s atmosphere in 1995, have contributed in new ways to the understanding of the thermal structure and variability of the tropical upper troposphere–lower stratosphere (UTLS), an important component of the climate system. The UTLS plays an essential role in the global radiative balance, the exchange of water vapor, ozone, and other chemical constituents between the troposphere and stratosphere, and the transfer of energy from the troposphere to the stratosphere. With their high accuracy, precision, vertical resolution, and global coverage, RO observations are uniquely suited for studying the UTLS and a broad range of equatorial waves, including gravity waves, Kelvin waves, Rossby and mixed Rossby–gravity waves, and thermal tides. Because RO measurements are nearly unaffected by clouds, they also resolve the upper-level thermal structure of deep convection and tropical cyclones as well as volcanic clouds. Their low biases and stability from mission to mission make RO observations powerful tools for studying climate variability and trends, including the annual cycle and intraseasonal-to-interannual atmospheric modes of variability such as the quasi-biennial oscillation (QBO), Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO). These properties also make them useful for evaluating climate models and detection of small trends in the UTLS temperature, key indicators of climate change. This paper reviews the contributions of RO observations to the understanding of the three-dimensional structure of tropical UTLS phenomena and their variability over time scales ranging from hours to decades and longer.

Analysis of Study Results of the Polar Coronal Hole on the Sun According to Observations in the Microwave Wavelength Range

An overview is presented of the main results of the study of the polar coronal hole over the North pole of the Sun based on observations of the solar eclipse on March 29, 2006, using the RATAN-600 radio telescope in a wide cm-range of wavelengths (1.03, 1.38, 2.7, 6.2, 13.0, 30.7 cm) along with observational and theoretical data about the properties of Solar coronal holes published by various authors. The results obtained are discussed: the distribution of brightness temperatures of polar coronal holes over the North pole of the Sun at distances of 1.005–2.0 in units of the Sun optical disk radius from the center of the solar disk; the increase in microwave radiation of polar coronal holes recorded at short wavelengths; the identity of the temperature properties of the polar coronal holes and low-latitude coronal holes on the Sun during the period of minimum solar activity. Comparison of the obtained brightness temperatures of the polar coronal hole with the brightness temperatures of large low-latitude coronal holes at similar wavelengths observed earlier (1973–1976, 1984–1987 yr) were done. This comparison has indicated an identity of the temperature properties of coronal holes regardless of their location on the Sun and the organization of coronal holes during the period of minimum solar activity.

Impact of the 2018 Mars Global Dust Storm on the Ionospheric Peak: A Study Using a Photochemical Model

AbstractWe report the impact of the 2018 Mars global dust storm on the ionosphere, in particular, its peak altitude during the onset and growth phases of the event. A one‐dimensional photochemical model is applied to estimate the ionospheric electron densities below 200 km using the measurements of in situ neutral densities and ionizing solar radiation by the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Our model results show that while the peak density of the ionosphere on the dawn side during the dust storm is hardly impacted, the altitude of the peak layer may be lifted by ∼10 km. The simulation results support the MAVEN radio occultation observations that there is a time delay in the response of the ionospheric peak altitude in the southern hemisphere (SH), compared with the northern hemisphere (NH) during the dust storm development. Although the 2018 global dust storm developed in the northern Martian mid‐latitudes on 1 June and developed into a global dust event on 17 June, our study suggests that the impact on the ionospheric peak altitude near the equatorial and southern mid‐latitudes became important only after 22 June. This suggests that there was a time‐lag of about 3 weeks in the response of the Martian upper atmosphere of the SH to the dust storm that originated in the NH.

Using machine learning method on calculation of boundary layer height

Traditional observation methods of the boundary layer are susceptible to external factors. In particular, detection data are relatively scarce in special areas such as plateaus and oceans. However, occultation observation data have the unique advantage of global coverage, and the boundary layer information of the occultation profile is rich. Based on the machine learning algorithm, this paper combines GPS occultation technology to improve the algorithm and constructs the boundary layer height simulation model based on the actual situation and analyzes its functional structure one by one. Moreover, in order to verify the validity of the simulation model, a simulation analysis is performed in combination with actual data. In addition, the performance of the model is verified from multiple aspects, and the results of the research are calculated by mathematical statistics, and the corresponding statistical diagram is drawn. Through experimental analysis, it can be known that the model constructed in this paper can effectively simulate the boundary height and has certain practical effects.

Sensing Horizontally Oriented Frozen Particles With Polarimetric Radio Occultations Aboard PAZ: Validation Using GMI Coincident Observations and Cloudsat a Priori Information

The sensitivity of PAZ’s Polarimetric Radio Occultation (PRO) observations to horizontally oriented frozen particles is assessed using coincident measurements from the Global Precipitation Measurement (GPM) core satellite’s radiometer GPM Microwave Imager (GMI) and ancillary information from Cloudsat. The difference between the horizontal and vertical polarizations of the GMI observed radiances at 166 GHz, indicative of horizontally oriented particles, is compared with the PAZ differential phase shift observations. A clear positive trend is observed, exhibiting a good correlation between the two observations. The radiometer absolute observations and polarization differences (PD) are then used to build a lookup table of ice water content (IWC) vertical profiles, using coincident observations between GPM and Cloudsat. The vertical profiles of IWC, along with the information of the radiometric PDs and some simple assumptions, are used to simulate the expected PAZ differential phase shift observations. The validation with the PAZ and GPM coincident measurements shows a high correlation between the simulated and observed differential phase shift, therefore proving the sensitivity of polarimetric radio occultations to horizontally oriented frozen particles. These results demonstrate that PRO can not only sense the precipitation near the surface (the original goal of the mission) but also the frozen particles near and well above the freezing level, providing additional detail of the cloud vertical structure.

Ultraviolet-Based Science in the Solar System: Advances and Next Steps

We review the importance of recent UV observations of solar system targets and discuss the need for further measurements, instrumentation and laboratory work in the coming decade. In the past decade, numerous important advances have been made in solar system science using ultraviolet (UV) spectroscopic techniques. Formerly used nearly exclusively for studies of giant planet atmospheres, planetary exospheres and cometary emissions, UV imaging spectroscopy has recently been more widely applied. The geyser-like plume at Saturn's moon Enceladus was discovered in part as a result of UV stellar occultation observations, and this technique was used to characterize the plume and jets during the entire Cassini mission. Evidence for a similar style of activity has been found at Jupiter's moon Europa using Hubble Space Telescope (HST) UV emission and absorption imaging. At other moons and small bodies throughout the solar system, UV spectroscopy has been utilized to search for activity, probe surface composition, and delineate space weathering effects; UV photometric studies have been used to uncover regolith structure. Insights from UV imaging spectroscopy of solar system surfaces have been gained largely in the last 1-2 decades, including studies of surface composition, space weathering effects (e.g. radiolytic products) and volatiles on asteroids (e.g. [2][39][48][76][84]), the Moon (e.g. [30][46][49]), comet nuclei (e.g. [85]) and icy satellites (e.g. [38][41-44][45][47][65]). The UV is sensitive to some species, minor contaminants and grain sizes often not detected in other spectral regimes. In the coming decade, HST observations will likely come to an end. New infrastructure to bolster future UV studies is critically needed. These needs include both developmental work to help improve future UV observations and laboratory work to help interpret spacecraft data. UV instrumentation will be a critical tool on missions to a variety of targets in the coming decade, especially for the rapidly expanding application of UV reflectance investigations of atmosphereless bodies.

A theoretical assessment of the feasibility of potential Lunar Reconnaissance Orbiter radio occultation observations of the lunar ionosphere

The existence of a “dense” lunar ionosphere has been controversial for decades. Positive ions produced from the lunar surface and exosphere are inferred to have densities that are ≲ 10 6 – 10 7 m −3 near the surface and smaller at higher altitudes, yet electron densities derived from radio occultation measurements occasionally exceed these values by orders of magnitude. For example, about 4% of the single-spacecraft radio occultation measurements from Kaguya/SELENE were consistent with peak electron densities of ~ 3 × 10 8 m −3 . Space plasmas should be neutral on macroscopic scales, so this represents a substantial discrepancy. Aditional observations of electron densities in the lunar ionosphere are critical to resolving this longstanding paradox. Here we theoretically assess whether radio occultation observations using two-way coherent S-band radio signals from the Lunar Reconnaissance Orbiter (LRO) spacecraft could provide useful measurements of electron densities in the lunar ionosphere. We predict the uncertainty in a single LRO radio occultation measurement of electron density to be ~ 3 × 10 8 m −3 , comparable to occasional observations by Kaguya/SELENE of a dense lunar ionosphere. Thus an individual profile from LRO is unlikely to reliably detect the lunar ionosphere; however, averages of multiple ( ~ 10) LRO profiles acquired under similar geophysical and viewing conditions should be able to make reliable detections. An observing rate of six ingress occultations per day ( ~ 2000 per year) could be achieved with minimal impact on current LRO operations. This rate compares favorably with the 378 observations reported from the single-spacecraft experiment on Kaguya/SELENE between November 2007 and June 2009. The large number of observations possible for LRO would be sufficient to permit wide-ranging investigations of spatial and temporal variations in the poorly understood lunar ionosphere. These findings strengthen efforts to conduct such observations with LRO.

Methodology for the Observations of Stellar Occultations by Small Bodies of the Solar System

The observation of stellar occultations constitutes one of the most important techniques for determining the dimensions and establishing the physical parameters of small Solar System bodies. The most substantial calculations are obtained from multiple observations of the same event, which turns the observation of stellar occultations into highly collaborative work and groups teams of observers through international networks. The above situation also requires the participation of both professional and amateur observers in these collaborative networks. With the aim of promoting the participation of professional and amateur groups in the collaborative observation of stellar occultations, we present the methodology developed by the Astronomical Observatory of the Technological University of Pereira (OAUTP) for the observations of occultations due small Solar System bodies. We expose the three fundamental phases of the process: the plan to make observations, the capture of the events, and the treatment of the data. We apply our methodology using a fixed station and a mobile station to observe stellar occultations due to MBAs (354) Eleonora (61) Danae (15112) Arlenewolfe (3915) Fukushima (61788) 2000 QP181 (425) Cornelia (257) Silesia (386) Siegena and (41) Daphne, and due to TNOs 1998BU48 and (529823) 2010 PP81. The positive detections for the objects (257) Silesia (386) Siegena and (41) Daphne allow us to derive lower limits in the diameter of the MBAs of 63.1 km, 166.2 km and 158.7 km and offsets in the astrometric position (Δαc cosδc, Δδc) of 622.30 ± 0.83, 15.23 ± 9.88 mas, 586.06 ± 1.68, 43.03 ± 13.88 mas and –413.44 ± 9.42, 234.05 ± 19.12 mas, respectively.

Response of Mars's Topside Ionosphere to Changing Solar Activity and Comparisons to Venus

AbstractStudying how planetary ionospheres respond to changes in solar irradiance can inform our understanding of how planetary atmospheres have reacted to our changing Sun over the course of solar system history. The response of Mars's main ionospheric peak to changing solar irradiance has been well‐studied. Less well understood are the altitudes above the main peak. We use radio occultation observations by Mars Global Surveyor and ground‐based solar radio flux measurements to quantify the response of Mars's electron densities to changing solar irradiance at these altitudes. This analysis shows that the electron densities respond modestly at and just above the main peak, with the response increasing above or below the peak. We use a one‐dimensional photochemical equilibrium model to reproduce the observed electron densities and investigate the influence of minor ions, changing electron temperatures, and changing neutral densities and composition on the resultant electron density. We find that the neutral atmosphere is primarily responsible for the observed behavior. We also make comparisons to Venus, which shows a more dramatic increase in its ionospheric response as a function of altitude. We show that while the ionospheric responses of Mars and Venus as a function of altitude are markedly different, recasting the analysis in terms of number of scale heights above the main peak brings the two planets into closer agreement, especially at the main peak. The agreement is worse at higher altitudes and this is attributed to Venus's larger proportion of atomic oxygen.

Two Years of Observations of the Io Plasma Torus by Juno Radio Occultations: Results From Perijoves 1 to 15

AbstractThe Io plasma torus is the primary source for plasma in Jupiter’s magnetosphere. Observations of fundamental properties of the torus place valuable constraints on how the Io plasma torus behaves. Yet measurements of the density structure of the torus that precisely determine its location, total electron content, and scale height are rare. Here, we report measurements of four profiles of the total electron content through the torus by the Juno spacecraft. These profiles were acquired by radio occultation observations during Juno Perijoves 10, 11, 14, and 15. They complement previously reported profiles from Perijoves 1, 3, 6, and 8, for which an initial ad hoc correction has been verified by improved calibration at the ground station. Fits to each profile determined the location, peak total electron content, and scale height of two torus regions: the cold torus and the torus beyond 5.5 RJ. Measurements show the torus locations depend on longitude, but not time. The peak total electron content values vary with time but do not exhibit any clear dependence on longitude. The scale heights may depend on longitude and/or time. There is no apparent dependence of these properties on the Io phase angle. Future analysis of more observations by the Juno spacecraft will clarify the respective roles of longitude, time, and the Io phase angle in shaping the density structure of the Io plasma torus. Observations at previously sampled longitudes will be particularly valuable, as demonstrated here by Perijoves 8 and 15, which sampled the same longitude 1 year apart.

Generalized canonical transform method for radio occultation sounding with improved retrieval in the presence of horizontal gradients

Abstract. By now, a series of advanced wave optical approaches to the processing of radio occultation (RO) observations are widely used. In particular, the canonical transform (CT) method and its further developments need to be mentioned. The latter include the full spectrum inversion (FSI) method, the geometric optical phase matching (PM) method, and the general approach based on the Fourier integral operators (FIOs), also referred to as the CT type 2 (CT2) method. The general idea of these methods is the application of a canonical transform that changes the coordinates in the phase space from time and Doppler frequency to impact parameter and bending angle. For the spherically symmetric atmosphere, the impact parameter, being invariant for each ray, is a unique coordinate of the ray manifold. Therefore, the derivative of the phase of the wave field in the transformed space is directly linked to the bending angle as a single-valued function of the impact parameter. However, in the presence of horizontal gradients, this approach may not work. Here we introduce a further generalization of the CT methods in order to reduce the errors due to horizontal gradients. We describe, in particular, the modified CT2 method, denoted CT2A, which complements the former with one more affine transform: a new coordinate that is a linear combination of the impact parameter and bending angle. The linear combination coefficient is a tunable parameter. We derive the explicit formulas for the CT2A and develop the updated numerical algorithm. For testing the method, we performed statistical analyses based on RO retrievals from data acquired by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) and collocated analysis profiles of the European Centre for Medium-Range Weather Forecasts (ECMWF). We demonstrate that it is possible to find a reasonably optimal value of the new tunable CT2A parameter that minimizes the root mean square difference between the RO retrieved and the ECMWF refractivity in the lower troposphere and allows the practical realization of the improved capability to cope with horizontal gradients and serve as the basis of a new quality control procedure.

Detectability of Rocky-Vapour atmospheres on super-Earths with Ariel

Ariel will mark the dawn of a new era as the first large-scale survey characterising exoplanetary atmospheres with science objectives to address fundamental questions about planetary composition, evolution and formation. In this study, we explore the detectability of atmospheres vaporised from magma oceans on dry, rocky Super-Earths orbiting very close to their host stars. The detection of such atmospheres would provide a definitive piece of evidence for rocky planets but are challenging measurements with currently available instruments due to their small spectral signatures. However, some of the hottest planets are believed to have atmospheres composed of vaporised rock, such as Na and SiO, with spectral signatures bright enough to be detected through eclipse observations with planned space-based telescopes. In this study, we find that rocky super-Earths with a irradiation temperature of 3000 K and a distance from Earth of up to 20 pc, as well as planets hotter than 3500 K and closer than 50 pc, have SiO features which are potentially detectable in eclipse spectra observed with Ariel.

The observation of stars occultation by asteroid (259) Alethea and comet 21P/Jacobini- Zinner

The observation of stars occultation by asteroid (259) Alethea and comet 21P/Jacobini- Zinner

Intensive GNSS radio occultation observations by FORMOSAT-7/COSMIC-2 in the dawn, noon, dusk, and midnight ionosphere

Intensive GNSS radio occultation observations by FORMOSAT-7/COSMIC-2 in the dawn, noon, dusk, and midnight ionosphere

Langevin's influence on relativity theories

A century ago, Paul Langevin [C. R. 173, 831 (1921)], through his influence, convinced the scientific community that Einstein's theories of relativity were correct and could explain the Sagnac effect. A simple note in Comptes Rendus was all it took to silence many prominent skeptical scientists. The relativity skeptics had pointed to Sagnac's experiment [C. R. 157, 1410 (1913)] with the interference of counter rotating light beams as proof that the speed of light was not the same in both directions, contrary to the key postulate in Einstein's theory. Langevin showed that the result was also explained by relativity. The rest is history, and relativity has remained a center piece of theoretical physics ever since. Langevin had been captivated by solar eclipse observations of a shifted star pattern near the sun as reported by Eddington [Report on the Relativity Theory of Gravitation (Fleetway Press, Ltd., London, 1920)]. This was taken as proof positive for Einstein's General Theory of Relativity. The case of a light beam split into two beams, which propagate in opposite directions around a circuit, has an analog in a simple thought experiment—a speed test for runners. Two runners can be timed on a running track with the runners going around the track in opposite directions. Two stop watches will display the time for each runner's return to the starting position. The arithmetic difference in time shown on each stop watch will provide the differences in speed between the two runners. If the two speeds are the same, the time difference will be zero. It would not make any sense for one of the stop watches to measure a negative time, that is, time moving into the past. In fact, the idea is absurd! However, Langevin did just that, assigned the time for light to travel in one direction as positive while the time for the light to traverse in the opposite direction as negative, moving into the past! By so doing, Langevin reproduced Sagnac's expression and declared that relativity explains Sagnac's experiment. Langevin was wrong!

Global 3D Features of Error Variances of GPS Radio Occultation and Radiosonde Observations

Global Positioning System (GPS) radio occultation (RO) and radiosonde (RS) observations are two major types of observations assimilated in numerical weather prediction (NWP) systems. Observation error variances are required input that determines the weightings given to observations in data assimilation. This study estimates the error variances of global GPS RO refractivity and bending angle and RS temperature and humidity observations at 521 selected RS stations using the three-cornered hat method with additional ERA-Interim reanalysis and Global Forecast System forecast data available from 1 January 2016 to 31 August 2019. The global distributions, of both RO and RS observation error variances, are analyzed in terms of vertical and latitudinal variations. Error variances of RO refractivity and bending angle and RS specific humidity in the lower troposphere, such as at 850 hPa (3.5 km impact height for the bending angle), all increase with decreasing latitude. The error variances of RO refractivity and bending angle and RS specific humidity can reach about 30 N-unit2, 3 × 10−6 rad2, and 2 (g kg−1)2, respectively. There is also a good symmetry of the error variances of both RO refractivity and bending angle with respect to the equator between the Northern and Southern Hemispheres at all vertical levels. In this study, we provide the mean error variances of refractivity and bending angle in every 5°-latitude band between the equator and 60°N, as well as every interval of 10 hPa pressure or 0.2 km impact height. The RS temperature error variance distribution differs from those of refractivity, bending angle, and humidity, which, at low latitudes, are smaller (less than 1 K2) than those in the midlatitudes (more than 3 K2). In the midlatitudes, the RS temperature error variances in North America are larger than those in East Asia and Europe, which may arise from different radiosonde types among the above three regions.