David Altadill

Characteristics of quasi‐2‐day oscillations in the ƒ oF2 at northern middle latitudes

The temporal variations of the characteristics of ƒoF2 quasi-2-day oscillations at middle latitudes in the northern hemisphere are investigated. The hourly data for the period 1976–1986 for four European ionospheric stations, Kiev (50.5°N, 30.5°E), Kaliningrad (54.7°N, 20.62°E), Lannion (48.75°N, 3.45°W) and Slough (51.48°N, 0.57°W), are used for analysis. Periodogram and complex demodulation analysis are used to evaluate the period and amplitude time variations of this oscillation. It is found that quasi-2-day oscillations in ƒoF2 are present almost continuously during the year. The amplitude is larger in the equinoxes and lower in the solstices. The amplitude variations are modulated by the long-term geomagnetic variations during the solar cycle. There is a clear expressed seasonal variation of the oscillation period with minimum in the winter, November–February (42–47 hours), and maximum near summer solstice (48–55 hours). The observations suggest that there are three types of ƒoF2 quasi-2-day oscillations with behavior of (1) westward traveling planetary wave with zonal wave number one, predominantly during summer half year, with greatest occurrence in August, (2) stationary planetary wave with a maximum occurrence in the summer solstice with unknown wave number, and (3) independent oscillations in separate locations on relatively long distances. The vertical plasma drift variations can explain the annual and long-term modulation effects of ƒoF2 quasi-2-day amplitude variations but encounter significant difficulties in explaining the discrepancy between zonal wave numbers in the mesosphere (three) and upper ionosphere (one). Another possibility is that the arising of quasi-2-day oscillation in the upper ionosphere may be induced by the forcing of such oscillation in the mesosphere with further independent development in the F region.

International Reference Ionosphere 2016: From ionospheric climate to real‐time weather predictions

The paper presents the latest version of the International Reference Ionosphere model (IRI-2016) describing the most important changes and improvements that were included with this version and discussing their impact on the IRI predictions of ionospheric parameters. IRI-2016 includes two new model options for the F2 peak height hmF2 and a better representation of topside ion densities at very low and high solar activities. In addition, a number of smaller changes were made concerning the use of solar indices and the speedup of the computer program. We also review the latest developments toward a Real-Time IRI. The goal is to progress from predicting climatology to describing the real-time weather conditions in the ionosphere.

Planetary wave type oscillations in the ionospheric F region

The existence of quasi-periodic oscillations (from 2- to 6.5-day) in the ionospheric F region, which may be connected with the planetary wave activity in the mesosphere/lower thermosphere, and the main results based on studies of the critical frequency foF2 of the F region in both hemispheres are presented. The existence of quasi-periodic oscillations with periods of about 2, 3, 5, and 6.5 days is statistically evident in the foF2 and they have the following characteristics: the probability of existence and the occurrence frequency are maximum in local summer, the oscillation amplitude is maximum near equinoxes, and the dominant zonal structure of the oscillations at middle latitudes is westward travelling with zonal wave number s=1 and stationary waves with zonal wave numbers s=1 and 2. The zonal structure of the foF2 quasi-2-day oscillation with s=1 differs from the expected s=2 or s=3 of the Rossby normal modes (2,0) and (3,0) in the atmosphere. These quasi-periodic oscillations contribute significantly to the day to day variability of foF2 and their contributions have been evaluated in relation to the solar cycle, season and latitude. The altitude progression of the 2-day oscillation in the northern mid-latitude F region is also presented. The possible mechanisms for generating the wave type oscillations, from 2 to 6.5 days, in the F region are discussed.

Remote Geophysical Observatory in Antarctica with HF Data Transmission: A Review

The geophysical observatory in the Antarctic Spanish Station, Juan Carlos I (ASJI), on Livingston Island, has been monitoring the magnetic field in the Antarctic region for more than fifteen years. In 2004, a vertical incidence ionospheric sounder completed the observatory, which brings a significant added value in a region with low density of geophysical data. Although the ASJI is only operative during the austral summer, the geomagnetic station records the data throughout the year. A High Frequency (HF) transmission system was installed in 2004 in order to have the geomagnetic data available during the whole year. As the power supply is very limited when the station is not operative, we had to design a low-power HF transceiver with a very simple antenna, due to environmental aspects. Moreover, the flow of information was unidirectional, so the modulation had to be extremely robust since there is no retransmission in case of error. This led us to study the main parameters of the ionospheric channel and to design new modulations specially adapted to very low signal to noise scenarios with high levels of interference. In this paper, a review of the results of our remote geophysical observatory and associated transmission system in Antarctica during the last decade is presented.

Some seasonal hemispheric similarities in ƒ o F2 quasi‐2‐day oscillations

The annual variations of the amplitude, period, and probability of existence of ƒoF2 quasi-2-day oscillations at middle latitudes in both northern and southern hemispheres are investigated. The ƒoF2 hourly data for the period 1977–1982 for stations Kiev (50.5° N, 30.5° E) and Wakkanai (45.39° N, 141.69° E) for the northern hemisphere and Kerguelen (49.35° S, 70.24° E) and Campbell Island (52.6° S, 169.1° E) for the southern hemisphere are used for analysis. It is found that there is a strong tendency for hemispheric similarities in the seasonal variations of oscillation amplitude, period, and probability of existence. The annual variations of the oscillation amplitude are modulated by the 12-month hemispheric and semiannual geomagnetic waves; the dominant period of oscillation is maximum in summer (50–51 hours) and minimum in winter (47–49 hours), and the probability of existence has a maximum in the summer half year and minimum in the winter half year. The seasonal similarities in the annual variations of the oscillation period and the probability of existence show the possible influence of the planetary 2-day wave in the middle neutral atmosphere on the electron density variations in the F region.

Narrowband and Wideband Channel Sounding of an Antarctica to Spain Ionospheric Radio Link

La Salle and Ebro Observatory have been involved in remote sensing projects in Antarctica for the last 11 years (approximately one solar cycle). The Ebro Observatory has been monitoring and analyzing the geomagnetic and the ionospheric activity in the Antarctic Spanish station Juan Carlos I (ASJI) (62.7°S, 299.6°E) for more than eighteen and ten years, respectively. La Salle has two main goals in the project. The first one is the data transmission and reception from Antarctica to Spain to obtain a historical series of measurements of channel sounding of this 12,760-km ionospheric HF (high frequency) radio link. The second one is the establishment of a stable data low power communication system between the ASJI and Cambrils, Spain (41.0°N, 1.0°E), to transmit the data from the remote sensors located on the island. In this paper, both narrowband and wideband soundings have been carried out to figure out the channel availability performed using a frequency range from 2 to 30 MHz with 0.5 MHz step during the 24 h of the day, encompassing wider channel measurements than previously done, in terms of hours and frequency. This paper presents the results obtained for the austral summer in 2014, using a monopole antenna at the transmitter and an inverted V on the receiver side. These results led us to the final physical layer design for the long Remote Sens. 2015, 7 11713 haul link, dividing the day into two parts: daytime, with low data throughput design, and nighttime, reaching high data throughput.

Validation of GPS Ionospheric Radio Occultation results onboard CHAMP by Vertical Sounding Observations in Europe

Ionospheric radio occultation (IRO) measurements have a big potential for monitoring the ionospheric behavior on global scale for now- and forecasting the ionospheric impact on radio systems. In this article we validate the retrieved vertical electron density profiles (EDPs) derived from IRO measurements onboard CHAMP by using vertical sounding measurements at five European vertical sounding stations — Athens, Dourbes, Juliusruh, Rome and Tortosa. Since first IRO measurement onboard CHAMP in April 2001, more than 70000 electron density profiles have been retrieved by a model assisted technique so far. The comparison of IRO retrieved EDPs with ionosonde profiles obtained from the above mentioned stations will be discussed.

Remote sensing and skywave digital communication from antarctica.

This paper presents an overview of the research activities undertaken by La Salle and the Ebro Observatory in the field of remote sensing. On 2003 we started a research project with two main objectives: implement a long-haul oblique ionospheric sounder and transmit the data from remote sensors located at the Spanish Antarctic station Juan Carlos I to Spain. The paper focuses on a study of feasibility of two possible physical layer candidates for the skywave link between both points. A DS-SS based solution and an OFDM based solution are considered to achieve a reliable low-power low-rate communication system between Antarctica and Spain.

Physical Layer Definition for a Long-Haul HF Antarctica to Spain Radio Link

La Salle and the Observatori de l’Ebre (OE) have been involved in a remote sensing project in Antarctica for the last 11 years. The OE has been monitoring the geomagnetic activity for more than twenty years and also the ionospheric activity of the last ten years in the Spanish Antarctic Station Juan Carlos I (ASJI) (62.7 ° S, 299.6 ° E). La Salle is finishing the design and testing of a low-power communication system between the ASJI and Cambrils (41.0 ° N, 1.0 ° E) with a double goal: (i) the transmission of data from the sensors located at the ASJI and (ii) the performance of an oblique ionospheric sounding of a 12,760 km HF link. Previously, La Salle has already performed sounding and modulation tests to describe the channel performance in terms of availability, Signal-to-Noise Ratio (SNR), Doppler spread and delay spread. This paper closes the design of the physical layer, by means of the channel error study and the synchronization performance, and concludes with a new physical layer proposal for the Oblique Ionosphere Sounder. Narrowband and wideband frames have been defined to be used when the oblique sounder performs as an ionospheric sensor. Finally, two transmission modes have been defined for the modem performance: the High Robustness Mode (HRM) for low SNR hours and the High Throughput Mode (HTM) for the high SNR hours.

An analysis of the scale height at the F2-layer peak over three middle-latitude stations in the European sector

This paper presents the results of an analysis of the variations of the scale height at the F2-layer peak (Hm) under different seasonal and solar-activity conditions. The database includes hourly Hm values derived from ionograms recorded at three middle-latitude stations in the European sector: El Arenosillo (37.1°N; 353.3°E), Ebro (40.8°N, 0.5°E) and Pruhonice (50.0°N; 15.0°E). The results show that, in general: (1) Hm exhibits diurnal variation with higher values during daytime than during night-time and secondary peaks around sunrise and sunset; (2) during winter time the scale height is lower than in summer time; (3) the scale heights increase with increasing solar activity; (4) Hm decreases when the latitude increases; (5) Hm shows a low correlation with the F2-region peak parameters NmF2 and hmF2 and a high correlation with the thickness parameter B0 and the equivalent slab thickness EST; (6) the day-to-day variability is greater at low solar activity than at high solar activity—it reaches maximum values around sunrise or sunset and it is lower around midnight than around noon at low solar activity. The results of this study are similar to those reported by other authors and can be useful for estimating the topside ionosphere from bottomside measurements and modelling.