Estimation of probability of occurrence of F1 layer or L condition using tables and electron density profile models

An algorithm is proposed for evaluation of the probability of occurrence of an F1 layer or L condition, based on tables. Observations independent of the tables database are used for comparison between the estimated probability of occurrence, the formulation used at present in IRI, and the occurrence actually observed. The importance of the inclusion of L condition in the electron density profile model is shown.     

Plasmaspheric electron content derived from GPS TEC and FORMOSAT-3/COSMIC measurements: Solar minimum condition

The plasmaspheric electron content (PEC) was estimated by comparison of GPS TEC observations and FORMOSAT-3/COSMIC radio occultation measurements at the extended solar minimum of cycle 23/24. Results are retrieved for different seasons (equinoxes and solstices) of the year 2009. COSMIC-derived electron density profiles were integrated up to the height of 700 km in order to retrieve estimates of ionospheric electron content (IEC). Global maps of monthly median values of COSMIC IEC were constructed by use of spherical harmonics expansion. The comparison between two independent measurements was performed by analysis of the global distribution of electron content estimates, as well as by selection specific points corresponded to mid-latitudes of Northern America, Europe, Asia and the Southern Hemisphere. The analysis found that both kinds of observations show rather similar diurnal behavior during all seasons, certainly with GPS TEC estimates larger than corresponded COSMIC IEC values. It was shown that during daytime both GPS TEC and COSMIC IEC values were generally lower at winter than in summer solstice practically over all specific points. The estimates of PEC (h > 700 km) were obtained as a difference between GPS TEC and COSMIC IEC values. Results of comparative study revealed that for mid-latitudinal points PEC estimates varied weakly with the time of a day and reached the value of several TECU for the condition of solar minimum. Percentage contribution of PEC to GPS TEC indicated the clear dependence from the time with maximal values (more than 50–60%) during night-time and lesser values (25–45%) during day-time.     

A study of occurrence characteristics of plasma bubbles over Hong Kong area

Plasma bubble is one of the important weather events of the ionosphere. In past research, many studies on its occurrence characteristics have been done based on various observations, such as ionosondes, topside sounders, radio scintillations and in situ measurements by satellites. In recent years, GPS has become an important tool for the studies in this aspect. In this research, a study is made on occurrence characteristics of plasma bubbles with the GPS observations collected by the Hong Kong local GPS network for about 12 years.     

Comparison of model and experimental ionospheric parameters at high latitudes

Modern use and study of the auroral region needs to attract a wider class of models for describing conditions of radio wave propagation in the ionosphere. In this paper the possibilities of the International Reference Ionosphere model, well-proven and widespread in the mid-latitudes, are investigated in the high latitude zone. Model and measured values of the critical frequency foF2 for two mid-latitude stations (Juliusruh and Goosebay) and four high-latitude ones (Loparskaya, Sodankyla, Sondrestrom, Thule) are compared. Deviations of medians, variations from day to day and solar activity trends seemed to be similar for both areas. This similarity is irrespective of the RZ12 index. Special attention is paid to the TEC parameter and its determination using 6 versions of models, a new version of the model IRI2010 (IRI-Plas) among them. It is shown that the IRI-Plas model significantly improves the definition of TEC in contrast to the versions of IRI2007 and the new model NeQuick. The use of the median of the experimental equivalent slab thickness, together with the current values of the TEC, increases by a factor of two the agreement between calculated and measured foF2 values as compared with the variations from day to day. This allows foF2 to be defined in near-real time.     

Longitudinal behaviors of the IRI-B parameters of the equatorial electron density profiles retrieved from FORMOSAT-3/COSMIC radio occultation measurements

The electron density profiles in the bottomside F₂-layer ionosphere are described by the thickness parameter B0 and the shape parameter B1 in the International Reference Ionosphere (IRI) model. We collected the ionospheric electron density (N_e) profiles from the FORMOSAT-3/COSMIC (F3/C) radio occultation measurements from DoY (day number of year) 194, 2006 to DoY 293, 2008 to investigate the daytime behaviors of IRI-B parameters (B0 and B1) in the equatorial regions. Our fittings confirm that the IRI bottomside profile function can well describe the averaged profiles in the bottomside ionosphere. Analysis of the equatorial electron density profile datasets provides unprecedented detail of the behaviors of B0 and B1 parameters in equatorial regions at low solar activity. The longitudinal averaged B1 has values comparable with IRI-2007 while it shows little seasonal variation. In contrast, the observed B0 presents semiannual variation with maxima in solstice months and minima in equinox months, which is not reproduced by IRI-2007. Moreover, there are complicated longitudinal variations of B0 with patterns varying with seasons. Peaks are distinct in the wave-like longitudinal structure of B0 in equinox months. An outstanding feature is that a stable peak appears around 100°E in four seasons. The significant longitudinal variation of B0 provides challenges for further improving the presentations of the bottomside ionosphere in IRI.     

Feasibility of developing an ionospheric E-region electron density storm model using TIMED/SABER measurements

We present a new technique for improving ionospheric models of nighttime E-region electron densities under geomagnetic storm conditions using TIMED/SABER measurements of broadband 4.3 μm limb radiance. The response of E-region electron densities to geomagnetic activity is characterized by SABER-derived NO⁺(v) 4.3 μm Volume Emission Rates (VER). A storm-time E-region electron density correction factor is defined as the ratio of storm-enhanced NO⁺(v) VER to a quiet-time climatological average NO⁺(v) VER, which will be fit to a geomagnetic activity index in a future work. The purpose of this paper is to demonstrate the feasibility of our technique in two ways. One, we compare storm-to-quiet ratios of SABER-derived NO⁺(v) VER with storm-to-quiet ratios of electron densities measured by Incoherent Scatter Radar. Two, we demonstrate that NO⁺(v) VER can be parameterized by widely available geomagnetic activity indices. The storm-time correction derived from NO⁺(v) VER is applicable at high-latitudes.     

Near real-time assimilation in IRI of auroral peak E-region density and equatorward boundary

The paper describes the method and initial results of assimilating the auroral peak E-region density (NmE) and the auroral equatorward boundary (EB) into the International Reference Ionosphere (IRI). The NmE and EB are obtained using a FUV based auroral model or FUV measurements in near real-time. Initial results show that the auroral NmE is often significantly larger than the NmE due to the solar EUV. This indicates the importance of including the contribution of precipitating electrons in IRI. The global equatorial boundary helps to improve the specification of the sub-auroral ionosphere trough in IRI. An IDL software package has been developed to interactively display the IRI parameters with assimilated NmE and EB. It can serve as an operational tool for space weather monitoring.     

Time and spatial variations of the foF2(night)/foF2(day) values

The ratio of daytime and nighttime values of the foF2 critical frequency is analyzed on the basis of the data of 35 ionospheric stations of the eastern hemisphere and 7 stations of the western hemisphere. The analysis confirms the results of Danilov [Danilov, A.D. Variations in the ratio of nighttime and daytime critical frequencies of the F2 layer. Geomagn. Aeronomy, 47 (6), 551–561 (in Russian) 2007; Danilov, A.D. Time and spatial variations in the ratio of nighttime and daytime critical frequencies of the F2 layer, J. Atmos. Sol-Terr. Phys. 70, 1201–1212, 2008.] that after about 1980, a systematic behavior of this ratio with time is observed: an increase with time (a positive trend) or a decrease with time (a negative trend). The sign of this trend is shown to be governed by the signs of the magnetic declination D and magnetic inclination I at the given ionospheric station. This fact makes it possible to assume that the above indicated trend is caused by long-term variations in the zonal component V_ny of the horizontal wind in the thermosphere, the latter component contributing into the vertical drift velocity W.     

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.