Rui‐Yuan Liu

Direct Observations of the Evolution of Polar Cap Ionization Patches

Patchy Polar Cap Patches of enhanced density plasma in the polar ionosphere (or polar cap patches) disturb radio communications and satellite positioning at high latitudes during magnetospheric storms. Using data from Global Positioning System satellites and a high-frequency radar network, Q.-H. Zhang et al. (p. 1597) analyzed a magnetospheric storm driven by a strong coronal mass ejection from the Sun and followed the evolution and motion of a patch of ionization throughout the polar cap. The localized dayside flow response to the solar disturbance allowed a patch to be stored and grow in the dayside polar cap, and the gaps between patches were controlled by the onset of magnetic reconnection in the magnetospheres tail. Observations of ionospheric perturbations after a solar burst hit Earth show how a patch of ionization formed and evolved. Patches of ionization are common in the polar ionosphere, where their motion and associated density gradients give variable disturbances to high-frequency (HF) radio communications, over-the-horizon radar location errors, and disruption and errors to satellite navigation and communication. Their formation and evolution are poorly understood, particularly under disturbed space weather conditions. We report direct observations of the full evolution of patches during a geomagnetic storm, including formation, polar cap entry, transpolar evolution, polar cap exit, and sunward return flow. Our observations show that modulation of nightside reconnection in the substorm cycle of the magnetosphere helps form the gaps between patches where steady convection would give a “tongue” of ionization (TOI).

Polar cap patch transportation beyond the classic scenario

We report the continuous monitoring of a polar cap patch, encompassing its creation and a subsequent evolution that differs from the classic behviour. The patch was formed from the storm enhanced density (SED) plume, by segmentation associated with a subauroral polarization stream (SAPS) generated by a substorm. Its initial anti-sunward motion was halted due to a rapidly changing of interplanetary magnetic field (IMF) conditions from strong southward to strong eastward with weaker northward components and the patch subsequently very slowly evolved behind the duskside of a lobe reverse convection cell in afternoon sectors, associated with high-latitude lobe reconnection, much of it fading rapidly due to an enhancement of the ionization recombination rate. This differs from the classic scenario where polar cap patches are transported across the polar cap along the streamlines of twin-cell convection pattern from day to night. This observation provides us new important insights into patch formation and control by the IMF, which has to be taken into account in F-region transport models and space weather forecasts.

Absorption events associated with solar flares

During the upward period of solar cycle 23, the Imaging Riometer at Zhongshan, Antarctica (geomag. lat. 74.5 S) was used to study the solar proton events and the Xray solar flares which are associated with the absorption events. In our study, the relationship between the absorption intensity and X-ray flux is found in a power form which is consistent with the theoretical result. The imaging riometer absorption data at Ny-Ålesund, Svalbard reconfirm the above relationship. We also argue that only M-class flares can generate a significant daytime absorption.

F-lacuna at cusp latitude and its associated TEC variation

As a frequent phenomenon occurring during summer days in high-latitude ionosphere, the F-lacuna manifests itself as disappearance of F region ionogram traces. Based on the 7.5 min interval Digisonde ionograms recorded at Zhongshan station (69.4°S, 76.4°E geographic coordinates; 74.5°S, 96.0°E corrected geomagnetic coordinates), we present temporal characteristics of the F-lacuna, as well as its correlation with geomagnetic activity, interplanetary magnetic field, and colocated TEC. Magnetic Local Time (MLT) distribution of the F-lacuna occurrence exhibits a dawn-dusk asymmetry. All types of F-lacuna favor the dawn sector, mainly occurring at 08:00–11:00MLT for F1 and total lacuna, 6:00–8:00MLT for F2-lacuna. The magnetic activity is found to have a strong positive correlation with the F2 and total lacuna. F2-lacuna occurrence is favored by southward component of interplanetary magnetic field (IMF), and total lacuna by high values of either eastward or westward component. It is worth to mention that the F-lacuna associates with the simultaneous total electron content (TEC) condition which has a positive correlation with F1-lacuna occurrence, while a strong negative correlation with the F2 and total lacuna. The associated TEC variation may provide a significant evidence for interpreting the F-lacuna phenomenon.

Ionospheric TEC disturbances based on GPS observations

To indicate the ionospheric TEC variability, it is introduced in this paper an ionospheric disturbance index DI, which is defined as the relative deviation of the vertical total electron content. Based on the GPS observations at Qingdao and 5 stations along 116 degree longitude, specifications of the TEC variations are discussed, particularly for the upper and lower 5% values of DI. The criteria for identifying TEC storm event are proposed by combining the intensity and the duration of the DI variations. Moreover, the characters of TEC storm events in China region are presented.

Diurnal variation of winter F-region ionosphere for solar minimum at both Zhongshan Station, Antarctica and Svalbard Station, Arctic†

Diurnal variation features of winter-time F2-peak electron density (NmF2) representative for solar minimum at both Zhongshan station, Antarctica, and Svalbard station, are compared and analyzed. Both stations are located around cusp latitude, and are almost on the same geomagnetic meridian plane in both hemispheres. For quiet time period, typical NmF2 diurnal variation features at Svalbard station show double peaks with a decrease of NmF2 around magnetic local noon (~UT+3 hour), NmF2 diurnal variation at Zhongshan station shows one major peak around magnetic local noon (~UT+1.75 hour), followed by a sharp decrease of NmF2 and a subpeak around 1500 UT. Simulation results of the high-latitude ionospheres in both hemispheres agree well with observations at both stations. It is found that the major difference of NmF2 variation between both stations can be explained by the unique location of each station relative to the sunlit demarcation line during the day. For quiet time period, photoionization from lower latitude contributes to the major peak of NmF2 in the diurnal variation at Zhongshan station, while the interaction between horizontal convection and auroral precipitation is the main cause for NmF2 variation at Svalbard station. For active time period, both stations show the increase of NmF2 due to transportation of higher plasma density from lower latitudes on the dayside with the expansion of the polar cap, and the additional ionization from soft precipitating electrons.

Comparisons of the variation of the ionospheric TEC with NmF2 over China

Comparative studies have been made between the variations of the ionospheric total electron content (TEC) and the variation of the maximum electron density of F2 layer (NmF2) over China, based on observations from 34 GPS/TEC sites and 10 ionosonde stations in 2004. The variations of TEC are mainly similar to that of NmF2, such as a clear single-peak structure in the diurnal variation with the peak time in the afternoon, a common “semi-annual anomaly” in the season variation, similar latitude and longitude variations, and so on. On the other hand, there are some differences. The peak time in the diurnal variation of NmF2 is usually about 1 or 2 hours later than that of TEC. This is possible attributed to the westward electric field in F2 layer in the afternoon, which causes the E×B driven downward plasma movement from the plasmasphere and enhance the electron density in F2 layer. The other difference is that there is no obvious “winter anomaly” in TEC variation, but still exist for NmF2 in China area.

Ionospheric TEC short-term forecasting in CHINA

A short-term forecasting method has been developed for the ionospheric TEC in China region. This method consists of two parts: the single station forecasting and the regional ionospheric reconstruction. Based on the measurement data from 27 stations in China and its surrounding area, the short-term forecasting of the ionospheric TEC has been realized, giving the estimate of the prediction errors. The possible ways are discussed for improving the accuracy of the prediction.

Intensity correction in all-sky auroral image projection transform

BY using all-sky(fish-eye)lens and highly sensitive TV camera,the effective monitoring ra-dius and time resolution of ground aurora observation have now been raised to 600 km and1/30 s(for instance,SIT-TV camera recording in NTSC format).By incorporating high-sen-sitivity imaging device(integrated from image intensifier and CCD(charge coupled device)

Statistical characteristics of ionospheric backscatter observed by SuperDARN Zhongshan radar in Antarctica: Statistical characteristics of ionospheric backscatter observed by SuperDARN Zhongshan radar in Antarctica

Zhongshan HF radar, as one component of SuperDARN, has been establis hed and in operation since April, 2010. Using data from the first two years of its operation, this paper investigates the radar’s performance, the diurnal and seasonal variations of ionospheric echoes, and the ir dependence on geomagnetic activity. Statis tical studies show that the occurrence of echoes in different beams varies at different frequencies, which aris es from the direction of the beam and the area over which the beam can achieve the orthogonality condition between the wave vecto r and the Earth’s magnetic field. The diurnal variation is obvious with double peak structures both in the occurrence rate and average power at 04–08 UT and 16–17 UT. The line- of -sight velocities are mainly positive on the day side and negative on the night side for Beam 0, which is the opposite of the trend for Beam 15. The spectral widths on the day side are of ten higher than those on the night side owing to the high energy particle precipitation in the cusp region. The seasonal variations are more obvious for those beams with larger numbers. The occurrence, the average power, the line- of -sight velocity, and the spectral widths are generally larger in the winter months than in the summer months. The influence of geomagnetic activity on radar echoes is significant. The peak echo occurrence appears on the day side during geomagnetically quiet times, and shifts to ward the night side and exhibits an obvious decrease with increasing K p. With increasing geomagnetic activity, the line- of -sight velocities increase, whereas the spectral widths decrease. The frequency dependence is investigated and it is found that in the operating frequency bands in 2010, 9–10 MHz is the most appropriate band for the SuperDARN Zhongshan radar.