ALBERT

Description: DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite data have been used to investigate the global relationship between electron density (Ne) and electron temperature (Te) in the topside ionosphere (~680 km) from 2006-2009. Te and Ne were negatively correlated in most of the low and middle latitude regions at ~10:30 solar local time (LT). In these regions, photoelectron heating of the electrons was balanced by cooling through collisions with the ions. The negative correlation became weaker at mid-latitudes, due to the increasing influence of heat conduction. The correlation was negative in most seasons during the daytime except at high latitudes in the northern winter, where a positive correlation occurred. There were wave-like longitudinal structures in Ne and Te around the geomagnetic equator, but they had different patterns in the day and the night. However, no obvious longitudinal variations in the correlation were associated with these structures. A positive correlation occurred near the magnetic equator at ~22:30 LT, which depended on thermal equilibrium between the electrons, ions and neutrals. A negative correlation occurred at mid-latitudes. Around the September Equinox, at night around the magnetic equator, the positive correlation had a wider latitudinal range. At mid-latitudes, a negative correlation occurred in smaller areas than it did around the March Equinox. Around the December Solstice the direct night-time coupling between Ne and Te was weaker than it was around the June Solstice. The negative correlation depends on the collisions between the electrons and the ions and the heating source.

Description: Abstract The effects of the 13‐14 March 1989 great magnetic storm on the East Asian ionosphere have been re‐investigated using ground‐based and satellite measurements as well as theoretical simulation. Within introduction of new data like the Chinese ionosondes and DMSP F8&F9 and low‐equatorial magnetometer data, we are able to track the ionospheric response at both the bottomside and topside ionosphere from middle to low latitude to obtain an overall understanding of storm‐time ionospheric change. Through the comparative study of different longitude bands, we found that the East‐Asian ionosphere was characterized by a strong westward electron density gradient persisting over a day at both the bottomside and topside ionosphere at mid‐low latitudes during the main and recovery phases of the storm. This feature was not studied in the previous literature for this event at this area. We then examine the effect through a numerical simulation work from the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) under Apex and Dipole geomagnetic fields. It is seen that the model well reproduces the zonal gradients during this event under realistic geomagnetic field model. The conditions favor for this structure requires a hemispheric asymmetry response of storm thermosphere as well as background condition and also the storm development which requires further investigation. This study shows that even very close stations would manifest totally different storm behaviors during the superstorm event suggesting a great challenge in the space weather prediction.

Description: Abstract The Global Ultraviolet Imager (GUVI) aboard the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet airglow emissions in the thermosphere. The retrieved altitude profiles of thermospheric neutral density from GUVI daytime limb scan are significant for ionosphere‐thermosphere study. Here, we use the profiles of the main neutral density to derive the total mass density during the period 2002‐2007 under geomagnetic quiet conditions (ap〈=12). We attempt to compare the obtained total mass density with the Challenging Minisatellite Payload (CHAMP) observations, making use of an empirical model (GUVI model hereafter). This GUVI model is aimed to solve the difficulty of the direct comparison of GUVI and CHAMP observations due to their different local times at a given location in a given day. The GUVI model are in good agreement with CHAMP observations with the small standard deviations of their ratios (less than 10%) except at low solar flux levels. The correlation coefficients are greater than 0.9 and the relative standard errors (RSE) are less than 20%. Comparison between the GUVI model and CHAMP observations during solar minimum show a large bias (~30%). The large bias at low solar flux levels might be due to the limitation of F10.7 as an EUV radiation flux proxy and the fitting method. Our results demonstrate the validity and accuracy of our model based on GUVI data against the density data from the CHAMP satellite.

Description: In this paper, we report the evolution (generation, amplification, and dissipation) of optically observed mesoscale field-aligned irregularity structures (FAIs) (~150 km) associated with a medium-scale traveling ionospheric disturbance (MSTID) event. There have not been observations of mesoscale FAIs of airglow before. The mesoscale FAIs were generated in an airglow-depleted front of southwestward propagating MSTIDs that were simultaneously observed by an all-sky imager, a GPS monitor, and a digisonde around Xinglong (40.4°N, 30.5°MLAT), China, on February 17/18 2012. A normalized cross correlation method has been used to obtain the velocities of mesoscale FAIs and MSTIDs. The mesoscale FAIs had an obvious northwestward relative velocity to main-body MSTIDs (about 87.0 m/s on average). The direction of this relative velocity was roughly parallel to the depleted fronts. Furthermore, the evolution of the mesoscale FAIs was mostly controlled by the intensity of the depleted fronts. Occurred in a highly elevated ionosphere that had a TEC depletion associated with large negative airglow perturbations (−25%), the mesoscale FAIs grew rapidly when they experienced southeastward wind, which had a speed of about 100 m/s and were measured by a Fabry-Perot interferometer. A northeastward polarization electric field within a depleted airglow front can play a controlling role in the development of the mesoscale FAIs. The electric field can significantly elevate the ionosphere, and move the mesoscale FAIs northwestward by the E × B drift. The processes for the generation and development of the polarization electric field and the mesoscale FAIs, however, need further study.

Description: This paper presents an epoch analysis of global ionosphere responses to recurrent geomagnetic activity during 79 corotating interaction region (CIR) events from 2004-2009. The data used were GPS-TEC data from the Madrigal Database at the MIT Haystack Observatory and the electron density (Ne) data obtained from CHAMP observations. The results show that global ionosphere responses to CIR events have some common features. In high and middle latitudes, the total electron content (TEC) showed a significant positive response (increased electron densities) in the first epoch day. A negative TEC response occurred at high latitudes of the American Sector following the positive response. The CHAMP Ne showed a daytime positive response in all latitudes and a nighttime negative response in the subauroral region. These negative TEC and Ne responses were found to be related to thermospheric composition (O/N 2 ) changes during the storms. At all latitudes, the maximum of the TEC positive effect always occurred at 2-6 hours after the CIR starting during local daytime and 10-18 hours later for the CIR onset during local nighttime. Case studies indicate that the TEC and Ne positive response had a strong dependence on the southward component (Bz) of the interplanetary magnetic field and solar wind speed. This suggests that penetration electric fields that were associated with changes in solar winds might play a significant role in the positive ionospheric response to storms. During the recovery time of the CIR-produced geomagnetic activity, the TEC positive disturbance at low latitudes sometimes could last for 2-4 days, whereas that at middle to high latitudes the disturbance lasted only for 1 day in most cases. A comparison of the ionospheric responses between the American, European and Asian sectors shows that the ionosphere response in the North American sector was stronger than that in the other two regions. The response of foF 2 to the CIR events in middle to high latitude showed a negative response for 2-3 days after the first epoch day. This is different from the response of TEC which was mostly positive during the same period of time.

Description: [1]  The global configuration of the geomagnetic field shows that the maximum east-west difference in geomagnetic declination of northern middle latitude lies in the US region (~32°), which produces the significant ionospheric east-west coast difference in terms of total electron content first revealed by Zhang et al. (2011). For verification, it is valuable to investigate this feature over the Far East area, which also shows significant geomagnetic declination east-west gradient but smaller (~15°) than that of the US. The current study provides evidence of the longitudinal change supporting the thermospheric zonal wind mechanism by examining the climatology of peak electron density (NmF2), electron density (Ne) of different altitudes in the Far East regions with a longitude separation of up to 40–60° based on ground ionosonde and space-based measurements. Although the east-west difference ( R ew ) over the Far East area displays a clear diurnal variation similar to the US feature, that is negative R ew (West Ne 〉 East Ne) in the noon and positive at evening-night, the observational results reveal more differences including: (1) The noontime negative R ew is most pronounced in April–June while in the US during February–March. Thus, for the late spring and summer period negative R ew over the Far East region is more significant than that of the US. (2) The positive R ew at night is much less evident than in the US, especially without winter enhancement. (3) The magnitude of negative R ew tends to enhance toward solar maximum while in the US showing anticorrelation with the solar activity. The altitude distribution of pronounced negative difference (300–400 km) moves upward as the solar flux increases and hence produces the different solar activity dependence at different altitude. The result in the paper is not simply a comparison corresponding to the US results but raises some new features that are worth further studying and improve our current understanding of ionospheric longitude difference at midlatitude.

Description: Ionospheric F 2 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 F 2 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 o N, 71.5 o W, 72 o dip angle) show a quiet different picture. Compared with the quiet-time ionosphere, TEC and the F 2 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 F 2 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 F 2 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 F 2 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 non-storm time profiles, indicating less daytime plasma flow between the ionosphere and the plasmasphere.

Description: 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 F2 layer maximum electron density (NmF2), peak height (hmF2), and Chapman scale height (Hm). During the deep solar minimum of 2008–2009, NmF2, hmF2, and Hm 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 hmF2 and Hm appear at southern middle latitudes in the region centered at 70°E in the daytime and hmF2 at 70°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.

Description: 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.

Description: The F3 layer is a common feature within ±10° of the magnetic equatorial ionosphere in the daytime. According to Balan et al. (1998) the F3 layer occurs mainly during the morning-noon period due to the combined effect of the upward E × B drift and the neutral wind that provides upward plasma drifts at and above the F2 layer. The F3 layer occurrence rate is higher in summer and decreases with increasing solar activity. In this study, the characteristic of the sunset F3 layer is first investigated using a solar cycle of ionosonde data (1995–2010) from the magnetic equatorial station at Jicamarca, and compared with the features derived from the four subtropical stations at Sao Luis, Fortaleza, Kwajalein, and Vanimo. Evidence shows that the local time distribution of the occurrence of the F3 layer can extend to the postsunset time (1800–2100 local time). The sunset F3 layer has a strong seasonal dependence occurring mainly during the summertime. Unlike the daytime F3 layer, the occurrence of the sunset F3 layer clearly increases and the virtual height of the bottom side of the F3 layer statistically increases from 620 to 1000 km with increasing solar activity. In addition, the occurrence of the sunset F3 layer at the other stations is much less than that at Jicamarca. These features of the dependence on the season, solar activity, and latitude are clearly related to the geomagnetic control of the evening prereversal enhancement of the equatorial zonal electric field and geomagnetic configuration.