Panagiotis Vergados

The 2013 Chelyabinsk meteor ionospheric impact studied using GPS measurements

On 15 February 2013, the Chelyabinsk meteor event (the largest in size since 1908) provided a unique opportunity to observe ionospheric perturbations associated with the ablation and ionospheric impact of the meteor using GPS measurements. The hypersonic bolide generated powerful shock waves while acoustic perturbations in the atmosphere led to the upward propagation of acoustic and gravity waves into the ionosphere. In our research, we applied two different techniques to detect ionospheric disturbances in dual-frequency global positioning system (GPS) measurements during the meteor impact event. The data were collected from near-field GPS networks in Russia, GPS Earth Observation Network (GEONET) in Japan, and Plate Boundary Observatory (PBO) stations in the coterminous U.S. Using a novel wavelet coherence detection technique, we were able to identify three different wave trains in the measurements collected from the nearest GPS station to the meteor impact site, with frequencies of approximately 4.0–7.8 mHz, 1.0 −2.5 mHz, and 2.7–11 mHz at 03:30 UTC. We estimated the speed and direction of arrival of the total electron content (TEC) disturbances by cross-correlating TEC time series for every pair of stations in several areas of the GEONET and PBO networks. The results may be characterized as three different types of traveling ionospheric disturbances (TIDs). First, the higher-frequency (4.0–7.8 mHz) disturbances were observed around the station ARTU in Arti, Russia (56.43°N, 58.56°E), with an estimated mean propagation speed of about 862 ± 65 m/s (with 95% confidence interval). Another type of TID disturbance related to the wave trains was identified in the lower frequency band (1.0–2.5 mHz), propagating with a mean speed of 362 ± 23 m/s. The lower frequency ionospheric perturbations were observed at distances of 300–1500 km away from Chelyabinsk. The third type of TID wave train was identified using the PBO stations in the relative short-period range of 1.5–6 min (2.7–11 mHz) with a mean propagation speed of 733 ± 36 m/s. The observed short-period ionospheric perturbations in the U.S. region is, to the best of our knowledge, the first observational evidence of the coincident the long-range meteor-generated infrasound signals propagating in the ionosphere.

Assessing the performance of GPS radio occultation measurements in retrieving tropospheric humidity in cloudiness: A comparison study with radiosondes, ERA-Interim, and AIRS data sets

We assess the impact that the Global Positioning System radio occultations (GPSRO) measurements have on complementing different data sets in characterizing the lower-to-middle tropospheric humidity in cloudy conditions over both land and oceans using data from 1 August 2006 to 31 October 2006. We use observations from rawinsondes, Global Positioning System radio occultations (GPSRO), Atmospheric Infrared Sounder (AIRS), and the European Center for Medium-Range Weather Forecasts Reanalysis Interim (ERA-Interim). During the selected time period, Constellation Observing System for Meteorology, Ionosphere, and Climate data were not assimilated in ERA-Interim. From each data set, we estimate a zonally averaged tropospheric specific humidity profile at tropical, middle, and high latitudes. Over land, we use rawinsondes as the ground truth and quantify the specific humidity differences and root-mean-square-errors (RMSEs) of the GPSRO, AIRS, and ERA-Interim profiles. GPSRO are beneficial in retrieving lower tropospheric humidity than upper tropospheric profiles, due to their loss of sensitivity at high altitudes. Blending GPSRO with ERA-Interim produces profiles with smaller humidity biases outside the tropics, but GPSRO data do not improve the humidity RMSE when compared to rawinsondes. Combining GPSRO with AIRS leads to smaller humidity bias at the tropics and high latitudes, while reducing humiditys RMSEs. Over oceans, no rawinsonde information is available, and we use ERA-Interim as a reference. Combining GPSRO with AIRS leads to smaller humidity RMSEs than AIRS. We conclude that cross-comparisons and synergies among multi-instrument observations are promising in advancing our knowledge of the tropospheric humidity in cloudy conditions. GPSRO data can contribute to improving humidity retrievals over cloud-covered regions, especially over land and within the boundary layer.

Consistency of seven different GNSS global ionospheric mapping techniques during one solar cycle

In the context of the International GNSS Service (IGS), several IGS Ionosphere Associated Analysis Centers have developed different techniques to provide global ionospheric maps (GIMs) of vertical total electron content (VTEC) since 1998. In this paper we present a comparison of the performances of all the GIMs created in the frame of IGS. Indeed we compare the classical ones (for the ionospheric analysis centers CODE, ESA/ESOC, JPL and UPC) with the new ones (NRCAN, CAS, WHU). To assess the quality of them in fair and completely independent ways, two assessment methods are used: a direct comparison to altimeter data (VTEC-altimeter) and to the difference of slant total electron content (STEC) observed in independent ground reference stations (dSTEC-GPS). The main conclusion of this study, performed during one solar cycle, is the consistency of the results between so many different GIM techniques and implementations.

Using GPS radio occultations to infer the water vapor feedback

The air refractive index at L-band frequencies depends on the airs water vapor content and density. Exploiting this relationship, we derive for the first time a theoretical model to infer the specific humidity response to surface temperature variations, dq/dTs, given knowledge of how the air refractive index and temperature vary with surface temperature. We validate this model using 1.2–1.6 GHz Global Positioning System Radio Occultation (GPS RO) observations from 2007 to 2010 at 250 hPa, where the water vapor feedback on surface warming is strongest. The dq/dTs estimation from GPS RO observations shows excellent agreement with previously published results and the responses estimated using Atmospheric Infrared Sounder (AIRS) and NASAs Modern–Era Retrospective Analysis for Research and Applications (MERRA) data sets. Because of their high sensitivity to fractional changes in water vapor, current and future GPS RO observations show great promise in monitoring climate feedbacks and their trends.

Electron number density profiles derived from radio occultation on the CASSIOPE spacecraft

This paper presents electron number density profiles derived from high-resolution Global Positioning System (GPS) radio occultation (RO) observations performed using the Enhanced Polar Outflow Probe payload on the high inclination CAScade, Smallsat and IOnospheric Polar Explorer (CASSIOPE) spacecraft. We have developed and applied a novel inverse Abel transform algorithm on high rate RO total electron content measurements performed along GPS to CASSIOPE radio links to recover electron density profiles. The high-resolution density profiles inferred from the CASSIOPE RO are (1) in very good agreement with density profiles estimated from ionosonde data, measured over stations nearby to the latitude and longitude of the RO tangent points; (2) in good agreement with density profiles inferred from GPS RO measured by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC); and (3) in general agreement with density profiles estimated using the International Reference Ionosphere climatological model. Using both CASSIOPE and COSMIC RO observations, we identify, for the first time, that there exist differences in the characteristics of the electron number density profiles retrieved over landmasses and oceans. The density profiles over oceans exhibit widespread values and scale heights compared to density profiles over landmasses. We provide an explanation for the ocean-landmass discrepancy in terms of the unique wave coupling mechanisms operating over oceans and landmasses.