## 4 Discussion
### Cause of Large Ionosphere Spatial Gradients  
The multi-GNSS observations provide further opportunity to get the high resolution of the spatial ionosphere description within a small scale region [49]. The storm at 22-23 June 2015 is typical in this solar cycle, and a larger spatial gradient can be acquired by analysis of the observations from different constellations. Two major ionosphere anomalies were contributed to large ionosphere spatial gradients: (1) the TEC enhancements as studied by Reference [16] during the superstorm in 2003 and (2) the ionosphere irregularities, as revealed in Section 3.5. An ionosphere gradient of 518 mm/km was discovered, generated by a plasma bubble [17]. However, the features of large ionosphere spatial gradients caused by irregularities were more complicated. With a short appearance time, the gradient feature may vary a lot and be hard to be modeled. With multi-GNSS observations, a clear irregularity evolution pattern generated by the second sub-phase of the storm was shown in Figure 12. The PPEFs were considered to be the major cause for the irregularity formation. The comprehensive analysis in Figures 11 and 12 also indicated that the ionosphere gradient represented by GNSS observations largely rely on the longitudinal coverage of the irregularity. Previous studies proved that the ionosphere gradient is influenced by the 'width of gradient'; the conclusion is based on GPS observations. In this work, the multi-GNSS observations help to achieve more detailed results by considering the geometry of the GNSS satellites and the geophysical climatology of the ionosphere irregularity. (1) With multi-GNSS observations, the ionosphere TEC variations in this geomagnetic storm have been better investigated. The European sector has the most severe response; the southern hemisphere of the European sector showed dramatic TEC enhancements. Some MGEX sites that were analyzed showed over 300% of TEC enhancements during the second sub-phase of the storm. Other obvious TEC enhancements were discovered in the southern hemisphere of the Asian sector, with \(\sim\)200% TEC increases observed. The TEC inhibitions were found near the equatorial region in the American sector, influenced by the storm forces. (2) The geomagnetic storm brings strong ionosphere irregularities to both hemispheres. The irregularities were mainly concentrated in the high latitudes, as consequences of the particle precipitations. The concentrated features of the irregularities were analyzed by the behavior of ROTI from multi-GNSS observations. Several upper bubbles were produced, especially during the second sub-phase of the storm. The irregularities had longitudinal extension tendency, covering almost 10 degrees in the east-west directions. The intense appearances of the bubbles in the middle latitudes were related with the propagation of the prompt penetration electric field. (3) Two nearby cites were selected to analyze the ionosphere spatial gradient. A maximum gradient of 247.2 mm/km was found by GPS/GLONASS observations during the second sub-phase of the storm. The possible cause for this spatial gradient was one of the bubbles located in 160\({}^{\circ}\) E-170\({}^{\circ}\) E, 40\({}^{\circ}\) S-50\({}^{\circ}\) S. The influence and statistical features of the ionosphere spatial gradient generated by the irregularities on the GNSS performances was worth further investigation. [PERSON], [PERSON] and [PERSON] conceived the study idea and designed the experiments. [PERSON] and [PERSON] developed the simulation platform and performed the experiments. [PERSON], [PERSON] drafted the manuscript. All authors read and approved the manuscript. This work was supported by the National Key Research and Development Plan by Ministry of Science and Technology 2016 YFC1402502, the National Natural Science Foundation of China Innovation Group 61521091 and National Natural Science Foundation of China under Grant 61771030, 61301087. The contribution is also supported by 2011 Collaborative Innovation Center of Geospatial Technology.