Low-latitude ionospheric scintillations and total electron content obtained with the CITRIS instrument on STPSat1 using radio transmissions from DORIS ground beacons

The primary objective of the Scintillation and Tomography Receiver in Space (CITRIS) is to detect ionospheric irregularities from space at low latitude. For this purpose, the satellite receiver uses the UHF and S-Band transmissions of the ground network of Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) beacons. CITRIS, developed at the Naval Research Laboratory, differs from the normal DORIS receiver by being able to capture and store the complex amplitude of the 401.25 and 2036.25 MHz transmissions at 200 Hz sample rate. Ground processing of the CITRIS data yields total electron content (TEC) and both phase and amplitude scintillations. With CITRIS flying on the US Space Test Program (STP) satellite STPSat1, 2 years of data were collected and processed to determine the fluctuations in ionospheric TEC and radio scintillations associated with equatorial irregularities. CITRIS flights over DORIS transmitters yield direct measurements of the horizontal plasma density fluctuations associated with equatorial plasma bubbles. Future flights of CITRIS can provide valuable complements to other satellite instruments such as GPS occultation receivers used to estimate vertical electron density profiles in the ionosphere.     

海南地区电离层不规则体纬向漂移速度的观测和研究

根据中国海南富克(19.3°N,109.1°E)三点GPS观测系统2007年3月至11月的观测数据,利用互相关方法分析了三站闪烁信号的时间延迟,得出了不规则体纬向漂移的基本特征.在中国海南地区,闪烁主要发生在春秋季节,夜间不规则体的纬向漂移速度以东向为主,大小在50～150 m/s之间;平均东向漂移速度随时间呈下降趋势.另外,在闪烁刚发生时,不规则体纬向速度起伏较大,这可能与不规则体的随机起伏以及等离子体泡产生时垂直速度较大有关.中国海南地区不规则体纬向漂移速度的这些基本特征与低纬其他地区的测量结果较为一致.     

Multi-wavelength coordinated observations of ionospheric irregularity structures from an anomaly crest location during unusual solar minimum of the 24th cycle

The present paper reports coordinated ionospheric irregularity measurements at optical as well as GPS wavelengths. Optical measurements were obtained from Tiny Ionospheric Photometer (TIP) sensors installed onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites. GPS radio signals were obtained from a dual frequency GPS receiver operational at Calcutta (22.58°N, 88.38°E geographic; geomagnetic dip: 32.96°; 13.00°N, 161.63°E geomagnetic) under the SCIntillation Network Decision Aid (SCINDA) program. Calcutta is located near the northern crest of Equatorial Ionization Anomaly (EIA) in the Indian longitude sector. The observations were conducted during the unusually low and prolonged solar minima period of 2008–2010. During this period, four cases of post-sunset GPS scintillation were observed from Calcutta. Among those cases, simultaneous fluctuations in GPS Carrier-to-Noise ratios (C/N_o) and measured radiances from TIP over a common ionospheric volume were observed only on February 2, 2008 and September 25, 2008. Fluctuations observed in measured radiances (maximum 0.95 Rayleigh) from TIP due to ionospheric irregularities were found to correspond well with C/N₀ fluctuations on the GPS links observed from Calcutta, such effects being noted even during late evening hours of 21:00–22:00 LT from locations around 40° magnetic dip. These measurements indicate the existence of electron density irregularities of scale sizes varying over several decades from 135.6 nm to 300–400 m well beyond the northern crest of the EIA in the Indian longitude sector during late evening hours even in the unusually low solar activity conditions.     

The northern and southern mid-latitude ionospheric trough using global IGS vTEC maps

The aim of this work is the analysis of the mid-latitude ionospheric trough (MIT) using the Global Ionospheric Maps from IGS (GIMs) during the solar minimum, year 2008. This study was performed for different local times, 22, 00, 02 and 04 LT on the Northern and Southern hemisphere simultaneously. In the two hemispheres the MIT show asymmetric pattern. The high-latitude troughs are clearly distinguished in autumn and winter. Another feature is the longitudinal development towards the west of the geomagnetic pole covering a wider area in the Northern Hemisphere. Five empirical reference models were tested and compared with the MIT minimum position obtained from GIMs at different local times for both hemisphere. The results show a better agreement with the observations for the Northern Hemisphere specially with the Köehnlein & Raitt model. Fluctuations of 9 days and 27 days of the MIT minimum position are found, which could be related with the solar wind oscillations, especially for 00 and 02 LT in both hemisphere, suggesting a link between them.     

A Three-Dimensional Simulation of Collisional-Interchange-Instability in the Equatorial-Low-Latitude Ionosphere

We present a three-dimensional simulation of collisional-interchange-instability in the equatorial-low-latitude F region of ionosphere. The instability is responsible for the generation of large plasma density depletion or bubble. General 3D potential and continuity equations are derived to simulate the phenomena. To understand the linear aspect of the instability, the general 3D linear growth rate is derived using 3D potential equation. The linear growth characteristics are compared with earlier field-aligned-integrated growth analyses. The numerical simulation of bubble in the 3D spherical polar geometry is finally presented. Distinct evolution characteristics of bubble in 3D and 2D simulation are further discussed.     

Mid-latitude amplitude scintillation of GPS signals and GPS performance slips

Degradation of transionospheric radio signals and operation failures during ionospheric disturbances constitute a crucial factor of space weather influence on radio engineering satellite systems performance. We found that during the main phase of strong magnetic storms in 2000–2003 when the auroral oval expands into mid-latitudes, its southern boundary develops a region with intense small-scale electron density irregularities. Such irregularities may cause strong amplitude scintillations of GPS signals at both GPS operating frequencies. The another consequence of it was significant random GPS signal phase fluctuations, breaking-down of signal tracking, and sharp increasing of GPS positioning errors as a result.     

Effects of a “day-time” substorm on the ionosphere and radio propagation

Propagation mechanisms of lateral (non-great-circle) signals on a high-latitude HF radio path during magnetospheric substorms that occurred in the day-time have been considered. The path is equipped with oblique ionospheric sounding (OIS) from Murmansk to St. Petersburg. The OIS method gives the possibility to determine propagation modes, MOF (maximum observed frequency) values, signal delays, etc. Data of the CUTLASS radar, the IMAGE magnetometer system, the Finnish riometer chain, and the Tromso ionosonde were also used for the analysis. The main results are the following: (1) the lateral signal propagation takes place, as a rule, if the path midpoint is located near the irregularity region that moves sharply from high to low latitudes. The lateral signal propagation appearing during day-time is a new effect. (2) Formation of dense field-aligned irregularities during a substorm leads to decreasing F2MOF values on radio paths. These results can be useful for problems of radiolocation, HF communications and navigation.     

Early morning enhancement in ionospheric electron density during intense magnetic storms

This study reports on ionospheric disturbances that occurred in the early morning hours in the South American–Atlantic sector during a few intense/super storm events. The events were observed at latitudes close to the southern crest of the equatorial ionization anomaly (EIA) as an unusual intensification of the F region electron density peak at local times when the EIA is not usually developed. All the events were observed at pre dawn-morning hours, under conditions of northward interplanetary geomagnetic field. Large scale traveling ionospheric disturbances that are launched during highly disturbed conditions and/or equatorward surges in the thermospheric meridional winds seem to be the most probable causes of the observed disturbances.     

On the uncertainties in the measurement of absolute (true) TEC over Indian equatorial and low latitude sectors

The Indian sector encompasses the equatorial and low latitude regions where the ionosphere is highly dynamic and is characterized by the equatorial ionization anomaly (EIA) resulting in large latitudinal electron density gradients causing errors and uncertainties in the estimation of range delays in satellite based navigation systems. The diurnal and seasonal variations of standard deviations in the TEC data measured during the low sunspot period 2004–2005 at 10 different Indian stations located from equator to the anomaly crest region and beyond are examined and presented. The day-to-day variability in TEC is found to be lowest at the equatorial station and increases with latitude up to the crest region of EIA and decreases beyond.     

GPS广域增强系统电离层延迟网格修正算法的研究

建立了一种广域增强系统地面监测网对电离层延迟的观测数据生成网格修正值算法,并在天津,长春,西安三地通过用双频接收机同时采集的电离层延迟实测数据,对广域增强系统的电离层网格修正法的修正精度进行验证,并与一般用户目前采用的Ｋｌｏｂｕｃｈａｒ模型修正法进行了比较,结果表明,电离层延迟网格修正法的修正精度普遍大于８０％,优于Ｋｌｏｂｕｃｈａｒ模型修正法,是一种精度较高的电离层实时修正法。