Xinan Yue

A study of the Weddell Sea Anomaly observed by FORMOSAT‐3/COSMIC

[1] More than two years of COSMIC electron density profiles at low solar activities are collected to study the evolution of the Weddell Sea Anomaly (WSA), which appears as an evening enhancement in electron density during local summer. Observations show that the change in NmF2 (the F2 peak electron density) is associated with the change in hmF2 (the F2 peak height), while the latter is correlated closely with the components of the geomagnetic field. We find that (1) in the afternoon, hmF2 is more liable to rise drastically in regions with a larger jsin(2I)j value, which would occur early at certain declinations, eastward in the southern hemisphere and westward in the northern hemisphere; (2) subsequently, a larger increment of hmF2 is coincidentally followed by a stronger enhancement of NmF2 and the enhancement ends just around the local sunset; and (3) in midlatitudes, the evolution pattern of hmF2 in the evening of equinoxes and winter is similar to that in summer, albeit without a lasting NmF2 enhancement as that in summer. These features suggest that the NmF2 enhancement and the hmF2 increase could arise from the thermospheric wind effect, and solar photoionization plays a crucial role in the enhancement as well. The general midlatitude F2 layer enhancement in local summer evening is consistent with the WSA on the above features, indicating that the WSA is a manifestation, with a particular geometry of the magnetic field, of the evening enhancement induced by the winds.

Longitudinal variations of electron temperature and total ion density in the sunset equatorial topside ionosphere

Based on the DMSP F13 Satellite observations from 1995 to 2005, the longitudinal distributions of the electron temperature (T-e) and total ion density (N-i) in the sunset equatorial topside ionosphere are examined. The results suggest that the longitudinal variations of both T-e and N-i exhibit obvious seasonal dependence as follows: (1) wavenumber-four longitudinal structure in equinox, (2) three peaks structure in June solstice, and (3) two peaks structure in December solstice. Moreover, the longitudinal variations of T-e and N-i show significant anti-correlation, and we speculate that the longitudinal variation of T-e may result from that of N-i which can control T-e through the electron cooling rate. The wavenumber-four longitudinal structures of both T-e and N-i in equinox may relate to the eastward propagating zonal wavenumber-3 diurnal tide (DE3), which has effect on the amplitude of the daytime zonal electric field. The longitudinal variation of T-e and N-i in the two solstices may be caused both by longitudinal variation of geomagnetic declination and DE3.

Features of the middle- and low-latitude ionosphere during solar minimum as revealed from COSMIC radio occultation measurements

In this study, the ionospheric electron density profiles retrieved from radio occultation measurements of the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) mission are analyzed to determine the F-2 layer maximum electron density (NmF2), peak height (h(m)F(2)), and Chapman scale height (H-m). During the deep solar minimum of 2008-2009, NmF2, h(m)F(2), and H-m show complicated seasonal variations, which are generally consistent with those in previous solar minima. Besides the equinoctial asymmetry, nonseasonal and semiannual anomalies are present in daytime NmF2; the Weddell Sea anomaly appears in nighttime NmF2 in all seasons except the June solstice. Unusually higher values of h(m)F(2) and H-m appear at southern middle latitudes in the region centered at 70 degrees E in the daytime and h(m)F(2) at 70 degrees W in the nighttime. Wave-like longitudinal patterns are evidently present at low latitudes in all three parameters, showing diurnal and seasonal nature. The values of the parameters under study are smaller in 2008-2009 than the rest of the COSMIC period examined in this study. The seasonal and latitudinal pattern of daytime NmF2 on the solar sensitivity not only confirms our earlier investigation but also explains the observed small NmF2 in 2008-2009 in response to the reduced solar extreme ultraviolet radiance.

Global 3-D ionospheric electron density reanalysis based on multisource data assimilation

[1] We report preliminary results of a global 3-D ionospheric electron density reanalysis demonstration study during 2002–2011 based on multisource data assimilation. The monthly global ionospheric electron density reanalysis has been done by assimilating the quiet days ionospheric data into a data assimilation model constructed using the International Reference Ionosphere (IRI) 2007 model and a Kalman filter technique. These data include global navigation satellite system (GNSS) observations of ionospheric total electron content (TEC) from ground-based stations, ionospheric radio occultations by CHAMP, GRACE, COSMIC, SAC-C, Metop-A, and the TerraSAR-X satellites, and Jason-1 and 2 altimeter TEC measurements. The output of the reanalysis are 3-D gridded ionospheric electron densities with temporal and spatial resolutions of 1 h in universal time, 5 in latitude, 10 in longitude, and 30 km in altitude. The climatological features of the reanalysis results, such as solar activity dependence, seasonal variations, and the global morphology of the ionosphere, agree well with those in the empirical models and observations. The global electron content derived from the international GNSS service global ionospheric maps, the observed electron density profiles from the Poker Flat Incoherent Scatter Radar during 2007–2010, and foF2 observed by the global ionosonde network during 2002–2011 are used to validate the reanalysis method. All comparisons show that the reanalysis have smaller deviations and biases than the IRI-2007 predictions. Especially after April 2006 when the six COSMIC satellites were launched, the reanalysis shows significant improvement over the IRI predictions. The obvious overestimation of the low-latitude ionospheric F region densities by the IRI model during the 23/24 solar minimum is corrected well by the reanalysis. The potential application and improvements of the reanalysis are also discussed.

Profiles of ionospheric storm‐enhanced density during the 17 March 2015 great storm

Ionospheric F2 region peak densities (NmF2) are expected to have a positive correlation with total electron content (TEC), and electron densities usually show an anticorrelation with electron temperatures near the ionospheric F2 peak. However, during the 17 March 2015 great storm, the observed TEC, NmF2, and electron temperatures of the storm-enhanced density (SED) over Millstone Hill (42.6°N, 71.5°W, 72° dip angle) show a quiet different picture. Compared with the quiet time ionosphere, TEC, the F2 region electron density peak height (hmF2), and electron temperatures above ~220 km increased, but NmF2 decreased significantly within the SED. This SED occurred where there was a negative ionospheric storm effect near the F2 peak and below it, but a positive storm effect in the topside ionosphere. Thus, this SED event was a SED in TEC but not in NmF2. The very low ionospheric densities below the F2 peak resulted in a much reduced downward heat conduction for the electrons, trapping the heat in the topside in the presence of heat source above. This, in turn, increased the topside scale height so that even though electron densities at the F2 peak were depleted, TEC increased in the SED. The depletion in NmF2 was probably caused by an increase in the density of the molecular neutrals, resulting in enhanced recombination. In addition, the storm time topside ionospheric electron density profiles were much closer to diffusive equilibrium than the nonstorm time profiles, indicating less daytime plasma flow between the ionosphere and the plasmasphere.

Quantitative evaluation of the low Earth orbit satellite based slant total electron content determination

[1] With the increased number of low Earth orbit (LEO) satellites equipped with GPS receivers, LEO based GPS observations play a more important role in space weather research because of better global coverage and higher vertical resolution. GPS slant total electron content (TEC) is one of the most important space weather products. In this paper, the LEO based slant TEC derivation method and the main error sources, including the multipath calibration, the leveling of phase to the pseudorange TEC, and the differential code bias (DCB) estimation, are described systematically. It is found that the DCB estimation method based on the spherical symmetry ionosphere assumption can obtain reasonable results by analyzing data from multiple LEO missions. The accuracy of the slant TEC might be enhanced if the temperature dependency of DCB estimation is considered. The calculated slant TEC is validated through comparison with empirical models and analyzing the TEC difference of COSMIC colocated clustered observations during the initial stage. Quantitatively, the accuracy of the LEO slant TEC can be estimated at 1–3 tecu, depending on the mission. Possible use of the LEO GPS data in ionosphere and plasmasphere is discussed.

Latitudinal dependence of the ionospheric response to solar eclipses

[1] In this study, we statistically analyze the latitudinal dependence of F2-layer peak electron densities (NmF2) and total electron content (TEC) responses to solar eclipses by using the ionosonde observations during 15 eclipse events from 1973 to 2006 and the GPS TEC observations during six solar eclipse events from 1999 to 2006. We carried out a model study on the latitudinal dependence of eclipse effects on the ionosphere by running a theoretical ionospheric model with the total eclipse occurring at 13 latitudes from 0 Nt o 60N at intervals of 5. Both the observations and simulations show that the NmF2 and TEC responses have the same latitudinal dependence: the eclipse effects on NmF2 and TEC are smaller at low latitudes than at middle latitudes; at the middle latitudes (>40), the eclipse effect decreases with increasing latitude. The simulations show that the smaller NmF2 responses at low latitudes are mainly because of much higher heights of hmF2 at low latitudes and electron density response decreasing rapidly with increasing height. For the eclipse effects at the middle latitudes (>40), the simulations show that the smaller NmF2 or TEC response at higher latitude is mainly ascribed to the larger downward diffusion flux induced by the larger dip angle at this region, which can partly make up for the plasma loss and alleviate the depression of electron density in the F region. The simulated results show that there is an overall decrease in electron temperature throughout the entire height range at the middle latitude, but for the low latitudes the eclipse effect on electron temperature is much smaller at high heights, which is mainly because of the much smaller reduction of photoelectron production rate at its conjugate low heights where only a partial eclipse with small eclipse magnitude occurs.

Data assimilation retrieval of electron density profiles from radio occultation measurements

[1] In this paper, the Kalman filter is used to retrieve the electron density profile along the tangent points by assimilating the slant total electron content data observed during a radio occultation (RO) event into an empirical background model. The RO data observed by COSMIC satellites on day of year 266 in 2009 are selected to do both the simulation work and the real data retrieval test. The results show that the data assimilation technique can improve the electron density retrieval in comparison with the Abel inversion. It is less influenced by the ionospheric inhomogeneity than the Abel method. Some pseudo‐large‐scale features made by the Abel retrieval, such as the plasma cave underneath the equatorial ionization anomaly region and the three peaks along the latitude direction in the E layer, disappear in the data assimilation retrieval results. Independent validation by ground‐based ionosonde observations confirms the improvement of data assimilation retrieval below the F2 peak. In addition, some potential research on RO data assimilation is also discussed.

Space Weather Observations by GNSS Radio Occultation: From FORMOSAT-3/COSMIC to FORMOSAT-7/COSMIC-2

The joint Taiwan-United States FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) mission, hereafter called COSMIC, is the first satellite constellation dedicated to remotely sense Earths atmosphere and ionosphere using a technique called Global Positioning System (GPS) radio occultation (RO). The occultations yield abundant information about neutral atmospheric temperature and moisture as well as space weather estimates of slant total electron content, electron density profiles, and an amplitude scintillation index, S4. With the success of COSMIC, the United States and Taiwan are moving forward with a follow-on RO mission named FORMOSAT-7/COSMIC-2 (COSMIC-2), which will ultimately place 12 satellites in orbit with two launches in 2016 and 2019. COSMIC-2 satellites will carry an advanced Global Navigation Satellite System (GNSS) RO receiver that will track both GPS and Russian Global Navigation Satellite System signals, with capability for eventually tracking other GNSS signals from the Chinese BeiDou and European Galileo system, as well as secondary space weather payloads to measure low-latitude plasma drifts and scintillation at multiple frequencies. COSMIC-2 will provide 4–6 times (10–15X in the low latitudes) the number of atmospheric and ionospheric observations that were tracked with COSMIC and will also improve the quality of the observations. In this article we focus on COSMIC/COSMIC-2 measurements of key ionospheric parameters.

Applying artificial neural network to derive long-term foF2 trends in the Asia/Pacific sector from ionosonde observations

[ 1] An artificial neural network ( ANN) method is first used for deriving long-term trends of the F2-layer critical frequency (foF2) at 19 ionospheric stations in the Asia/Pacific sector. It is found that the ANN method can eliminate the geomagnetic activity effect on foF2 more effectively than usual regression methods. Of the selected 19 stations, there are significant long-term trends corresponding to a confidence level >= 90% at 14 stations and 12 of these stations present negative trends. An average trend of - 0.05% per year in the selected area can be obtained if the 12 stations with significant negative long-term trends be considered. No pronounced diurnal and latitudinal effects in trends and no uniform pattern of seasonal variation in most stations are detected. The long-term trends for low latitude and equatorial stations differ from other stations suggest that some special dynamical processes may take effects in the equatorial anomaly region. Many factors which can influence ionosphere, such as the greenhouse effect, solar and geomagnetic activity, and neutral background gas, might contribute to the trend.