Bei-Chen Zhang

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).

Hemispheric asymmetry of the structure of dayside auroral oval

A comprehensive analysis of long-term and multispectral auroral observations made in the Arctic and Antarctica demonstrates that the dayside auroral ovals in two hemispheres are both presented in a two-peak structure, namely, the prenoon 09:00 magnetic local time (MLT) and postnoon 15:00 MLT peaks. The two-peak structures of dayside ovals, however, are asymmetric in the two hemispheres; i.e., the postnoon average auroral intensity is more than the prenoon one in the Northern Hemisphere but less in the Southern Hemisphere. The hemispheric asymmetry cannot be accounted for by the effect of the interplanetary magnetic field By component and the seasonal difference of ionospheric conductivities in the two hemispheres, which were used to interpret satellite-observed real-time auroral intensity asymmetries in the two hemispheres in previous studies. We suggest that the hemispheric asymmetry is the combined effect of the prenoon-postnoon variations of the magnetosheath density and local ionospheric conductivity.

Earth's ion upflow associated with polar cap patches: Global and in situ observations

We report simultaneous global monitoring of a patch of ionization and in situ observation of ion upflow at the center of the polar cap region during a geomagnetic storm. Our observations indicate strong fluxes of upwelling O+ ions originating from frictional heating produced by rapid antisunward flow of the plasma patch. The statistical results from the crossings of the central polar cap region by Defense Meteorological Satellite Program F16–F18 from 2010 to 2013 confirm that the field-aligned flow can turn upward when rapid antisunward flows appear, with consequent significant frictional heating of the ions, which overcomes the gravity effect. We suggest that such rapidly moving patches can provide an important source of upwelling ions in a region where downward flows are usually expected. These observations give new insight into the processes of ionosphere-magnetosphere coupling.

Aurora image segmentation by combining patch and texture thresholding

The proportion of aurora to the field-of-view in temporal series of all-sky images is an important index to investigate the evolvement of aurora. To obtain such an index, a crucial phase is to segment the aurora from the background of sky. A new aurora segmentation approach, including a feature extraction method and the segmentation algorithm, is presented in this paper. The proposed feature extraction method, called adaptive local binary patterns (ALBP), selects the frequently occurred patterns to construct the main pattern set, which avoids using the same pattern set to describe different texture structures in traditional local binary patterns. According to the different morphologies and different semantics of aurora, the segmentation algorithm is designed into two parts, texture part segmentation based on ALBP features and patch part segmentation based on modified Otsu method. As it is simple and efficient, our implementation is suitable for large-scale datasets. The experiments exhibited the segmentation effect of the proposed method is satisfactory from human visual aspect and segmentation accuracy.

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.

The simulation of the coronal mass ejection‐shock system in the inner corona

In the concept that coronal mass ejections (CME) are usually originated in large closed magnetic field regions which are found in the coronal streamer belt near the solar surface, we have used a thermal driving force so strong that portions of the closed magnetic fields were carried away by the strong disturbance. A CME-shock system is obtained in the inner corona. The “legs” of loop-like CMEs are again obtained at the interface between the coronal open and closed magnetic fields. However, there is no counterpart in outer space. The shock is a combined one with an intermediate shock near the equator at its early stage. Ultimately, it becomes a pure fast shock. A plasmoid with higher density and bubble-like magnetic fields is formed behind the MHD shock wave. It propagates at high speed. The results show that the high-speed plasmoid does not propel the MHD shock in front of it; rather, the plasmoid forms behind the MHD shock.

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.