FORMOSAT-3/COSMIC Research Articles

Profiles of temperature responses to the 22 July 2009 total solar eclipse from FORMOSAT‐3/COSMIC constellation

Due to the difficulty in making reliable and timely four‐dimensional observations of atmospheric temperature in the vicinity of the path of the total solar eclipse, direct measurements of temperature changes from the troposphere to the stratosphere during a total solar eclipse still haven't been reported before. In this work we use profiles of temperature measurements from a constellation of the FORMOSAT‐3/COSMIC (FS3/C) LEO satellites to estimate temperature changes during the 22 July 2009 total solar eclipse. The FS3/C data reveals a very important temperature response structure in the vertical. The cooling in the troposphere can be attributed to the reduction in solar heating, which is thermally driven. The warming in the stratosphere, especially for those that occurred between 13 and 23 km altitudes could be dynamically driven, which can be induced by the thermal contraction of the troposphere.

Observations of Equatorial Kelvin Wave Modes in FORMOSAT-3/COSMIC GPS RO Temperature Profiles

In this study, we analyze FORMOSAT-3/COSMIC (F3/C) GPS radio occultation (RO) derived temperature components for the period September 2006 to February 2008. Results show the presence of slow Kelvin waves (wave period ＞ 10 days) with higher zonal wavenumbers (either one or two) in the upper troposphere and lower stratosphere (UTLS). The vertical wavelengths of these waves are found to be in the range of 5-12 km. The predominant Kelvin waves observed in the temperature fluctuations are in the altitude range between 15 and 28 km and centered on the tropical tropopause. The downward phase progression of these waves suggests that the derived waves are propagating upward, with the source region located at lower altitudes possibly due to tropical convective heating. The zonal winds retrieved using radiosonde observations over Singapore (1°N, 104°E) during this period show a periodicity of~24-26 months in the stratosphere, and quasi-biennial oscillation (QBO) characteristics with eastward zonal winds from March 2006 to May 2007 and westward winds from June 2007 to July 2008 respectively. Our results further show that the Kelvin wave characteristics are enhanced during the westward phase of QBO and diminish during the eastward phase, in line with the previous reported results. Furthermore, an examination of outgoing longwave radiation (OLR) data shows that deep convection activity is developed episodically over the Indonesian archipelago during the observation period, thereby indicating that the Kelvin wave events observed in temperature fluctuations are either driven by convective activity (convectively coupled waves) or by a broad spectrum of convective variability (free waves) over the Indonesian region.

Electron density profiles in the equatorial ionosphere observed by the FORMOSAT-3/COSMIC and a digisonde at Jicamarca

We examine for the first time the ionospheric electron density profiles concurrently observed by the GPS occultation experiment (GOX) onboard the FORMOSAT-3/COSMIC (F3/C) and the ground-based digisonde portable sounder DPS-4 at Jicamarca (12°S, 283°W, 1°N geomagnetic) in 2007. Our results show that the F3/C generally underestimates the F2-peak electron density NmF2 and the F2-peak height hmF2. On the other hand, when the equatorial ionization anomaly (EIA) pronouncedly appears during daytime, the total electron content (TEC) derived from the radio occultation of the GPS signal recorded by the F3/C GOX is significantly enhanced. This results in the NmF2 at Jicamarca being overestimated by the Abel inversion on the enhanced TEC during the afternoon period.

First tomographic observations of the Midlatitude Summer Nighttime Anomaly over Japan

Recently, a chain of digital beacon receivers has been established over Japan, mainly for the tomographic imaging of the ionosphere. These receivers are installed at Shionomisaki (33.45°N, 135.8°E), Shigaraki (34.85°N, 136.1°E), and Fukui (36.06°N,136°E), which continuously track the Low Earth Orbiting Satellites (LEOS), and the simultaneous line‐of‐sight Total Electron Content (TEC) data are used for tomographic reconstruction. In the images obtained during July 2008, it is seen that the nighttime electron densities exceed the daytime values on almost all days over latitudes >33–34°N. On several days, these northern latitudes show enhanced electron densities compared to the low‐latitude region during nighttime. These are the prominent features of the “Midlatitude Summer Nighttime Anomaly (MSNA)” that is recently observed in the northern hemisphere and is considered similar to the nighttime Weddell Sea Anomaly (WSA). This is the first study of the MSNA using tomographic technique and found its significant day‐to‐day variability. The ionosonde data from Wakkanai (45.4°N, 141.7° E), ground‐based GPS TEC observations using the GEONET, CHAMP in situ electron density measurements, and Formosat3/COSMIC (F3/C) occultation measurements are also used to confirm the presence of MSNA over this region and to examine its variability. It is seen that, in general, during the local summer period, electron density over the northern latitudes is highest at ∼2000–2100 LT and the latitudinal enhancement in electron density also begins to appear around the same time, which continues to exist even at later hours.

Seismoionospheric GPS total electron content anomalies observed before the 12 May 2008 Mw7.9 Wenchuan earthquake

The global ionospheric map (GIM) is used to observe variations in the total electron content (TEC) of the global positioning system (GPS) associated with 35 M ≥ 6.0 earthquakes that occurred in China during the 10‐year period of 1 May 1998 to 30 April 2008. The statistical result indicates that the GPS TEC above the epicenter often pronouncedly decreases on day 3–5 before 17 M ≥ 6.3 earthquakes. The GPS TEC of the GIM and electron density profiles probed by six microsatellites of FORMOSAT3/COSMIC (F3/C) are further employed to simultaneously observe seismoionospheric anomalies during an Mw7.9 earthquake near Wenchuan, China, on 12 May 2008. It is found that GPS TEC above the forthcoming epicenter anomalously decreases in the afternoon period of day 6–4 and in the late evening period of day 3 before the earthquake, but enhances in the afternoon of day 3 before the earthquake. The spatial distributions of the anomalous and extreme reductions and enhancements indicate that the earthquake preparation area is about 1650 km and 2850 km from the epicenter in the latitudinal and longitudinal directions, respectively. The F3/C results further show that the ionospheric F2 peak electron density, NmF2, and height, hmF2, significantly decreases approximately 40% and descends about 50–80 km, respectively, when the GPS TEC anomalously reduces.

Seasonal variations of the ionospheric electron densities retrieved from Constellation Observing System for Meteorology, Ionosphere, and Climate mission radio occultation measurements

We collected the ionospheric electron density (Ne) profiles from the FORMOSAT‐3/COSMIC (F3/C) radio occultation measurements to investigate the seasonal behaviors of daytime Ne in the altitude range of 200–560 km. Harmonic analysis of the Ne at different altitudes provides unprecedented detail of the seasonal behaviors of Ne at low solar activity (LSA). Global maps of seasonal harmonic components indicate that there are strong annual and semiannual variations in daytime Ne, which have distinct latitudinal and altitudinal dependency. The semiannual component predominates over the annual variation in the equatorial regions and at high latitudes in the East Asian and South Atlantic sectors at low altitudes, and at higher altitudes the semiannual component predominates in the equatorial region, but recedes in other regions. The semiannual variation peaks in equinoctial months in most regions, while it has maxima in solstice months, first in the South Pacific region (around 30°S, 120°W) at 250 km altitude and expanding over the South Pacific and South Atlantic oceans at higher altitudes. Moreover, there is a region around 45°S, 30°W with a dominant semiannual component, moving toward east‐north with increasing altitude in the range of 200–270 km. These two interesting features are novel but are not reported yet. The relative amplitude of the annual component of Ne has hemispheric asymmetry, which is prominent at high altitudes in the Southern Hemisphere. The winter/seasonal anomaly widely exists in the Northern Hemisphere and southern low latitudes and in Indian Ocean region at low altitudes but gradually disappears at higher altitudes. Further, in equatorial regions, a new finding is the obvious wave‐like pattern in the longitudinal structure of the amplitudes of seasonal harmonic components in equatorial regions, which supports possible couplings of sources with lower atmospheric origins in the longitudinal variations of Ne.

Quality Control, Error Analysis, and Impact Assessment of FORMOSAT-3/COSMIC in Numerical Weather Prediction

Following several years of experimentation with the GPS radio occultation technique, the 6-satellite FORMOSAT-3/COSMIC (F3C) mission was launched mid-2006 and has been collecting data since then. In this paper we present early findings of research performed at Meteo-France regarding the use of these data for assimilation in numerical weather prediction. Benefiting from the dense global coverage allowed by F3C refraction-induced observations, we first assess the quality of these data at four levels: bending angle, refractivity, refractivity lapse rate, and temperature. We compare them with calculations from Meteo-France numerical weather forecasts. Leaning from these various levels of data we devise quality control procedures that rely on the refractivity lapse rate. Applying a recent methodology developed in data assimilation we calculate observation bending angle error variances for our assimilation system. Using these new quality control procedures and observation error estimates we run an assimilation and forecast experiment with Meteo-France's operational global 4DVAR data assimilation system used as a reference. Our results indicate a very clear positive impact of the assimilation of F3C bending angle data in the Southern hemisphere for the prediction of geo potential heights and winds. We also observe an improvement in wind forecast skill in the Northern hemisphere, albeit such an improvement is smaller than in the Southern hemisphere.

Validation of Refractivity Profiles Retrieved from FORMOSAT-3/COSMIC Radio Occultation Soundings: Preliminary Results of Statistical Comparisons Utilizing Balloon-Borne Observations

The GPS radio occultation (RO) soundings by the FORMOSAT-3 COSMIC (Taiwan's Formosa Satellite Misssion #3/Constellation Observing System for Meteorology, Ionosphere and Climate) satellites launched in mid-April 2006 are compared with high-resolution balloon-borne (radiosonde and ozonesonde) observations. This paper presents preliminary results of validation of the COSMIC RO measurements in terms of refractivity through the troposphere and lower stratosphere. With the use of COSMIC RO soundings within 2 hours and 300 km of sonde profiles, statistical comparisons between the collocated refractivity profiles are performed for some tropical regions (Malaysia and Western Pacific islands) where moisture-rich air is expected in the lower troposphere and for both northern and southern polar areas with a very dry troposphere. The results of the comparisons show good agreement between COSMIC RO and sonde refractivity profiles throughout the troposphere (1-1.5% difference at most) with a positive bias generally becoming larger at progressively higher altitudes in the lower stratosphere (1-2% difference around 25 km), and a very small standard deviation (about 0.5% or less) for a few kilometers below the tropopause level. A large standard deviation of fractional differences in the lowermost troposphere, which reaches up to as much as 3.5-5% at 3 km, is seen in the tropics while a much smaller standard deviation (1-2% at most) is evident throughout the polar troposphere.

Longitudinal structure of the equatorial ionosphere: Time evolution of the four‐peaked EIA structure

Longitudinal structure of the equatorial ionosphere during the 24 h local time period is observed by the FORMOSAT‐3/COSMIC (F3/C) satellite constellation. By binning the F3/C radio occultation observations during September and October 2006, global ionospheric total electron content (TEC) maps at a constant local time map (local time TEC map, referred as LT map) can be obtained to monitor the development and subsidence of the four‐peaked longitudinal structure of the equatorial ionosphere. From LT maps, the four‐peaked structure starts to develop at 0800–1000 LT and becomes most prominent at 1200–1600 LT. The longitudinal structure starts to subside after 2200–2400 LT and becomes indiscernible after 0400–0600 LT. In addition to TEC, ionospheric peak altitude also shows a four‐peaked longitudinal structure with variation very similar to TEC during daytime. The four‐peaked structure of the ionospheric peak altitude is indiscernible at night. With global local time maps of ionospheric TEC and peak altitude, we compare temporal variations of the longitudinal structure with variations of E × B drift from the empirical model. Our results indicate that the observations are consistent with the hypothesis that the four‐peaked longitudinal structure is caused by the equatorial plasma fountain modulated by the E3 nonmigrating tide. Additionally, the four maximum regions show a tendency of moving eastward with propagation velocity of several 10 s m/s.

Motions of the equatorial ionization anomaly crests imaged by FORMOSAT‐3/COSMIC

The equatorial ionization anomaly (EIA) structures and evolutions are imaged using radio occultation observation of the newly launched FORMOSAT‐3/COSMIC (F3/C) satellite constellation. Three‐dimensional ionospheric images provide unprecedented detail of the EIA structure globally. This paper presents images of the EIA structures during July–August 2006 and discusses the development and subsidence of the EIA. Clear seasonal asymmetries in both ionospheric electron density and layer height are observed. Two‐dimensional (cross section) maps at a meridian provide dynamic variations and motions of the northern and southern EIA crests. Results suggest that in addition to the asymmetric neutral composition effect, interactions between the summer‐to‐winter (transequatorial) neutral winds and strength of the equatorial plasma fountain effect play important roles in producing asymmetric development of the EIA crests as imaged by the F3/C.

Plausible effect of atmospheric tides on the equatorial ionosphere observed by the FORMOSAT‐3/COSMIC: Three‐dimensional electron density structures

The plausible effect of atmospheric tides on the longitudinal structure of the equatorial ionosphere is observed by the FORMOSAT‐3/COSMIC (F3/C) constellation during September Equinox, 2006, near solar minimum. The longitudinal structure was first reported in IMAGE satellite airglow observations at the far‐ultraviolet (FUV) 135.6‐nm wavelength during March Equinox, 2002, near solar maximum. The global three‐dimensional ionospheric electron density observed by F3/C shows a prominent four‐peaked wave‐like longitudinal enhancement in the equatorial ionization anomaly (EIA). The vertical electron density structures observed by F3/C reveal that the feature exists mainly above 250 km altitude indicating that the feature is an F‐region phenomenon. The four longitudinal F‐region enhancements of the EIA peaks may result from a stronger equatorial plasma fountain at each longitude region produced by a stronger F‐region eastward electric field transmitted along the magnetic field lines from E‐region where longitudinal variations in atmospheric tides affect the ionospheric dynamo process.