## 1 Introduction
The ionosphere gradients under the storm case can exceed 400 mm/km, measured from a 5 km distance limited baseline, as previous research revealed [16]. Recently, a method was explored to measure a very short baseline ionosphere gradient and a gradient of 518 mm/km was discovered, caused by an ionosphere plasma bubble [17]. However, the details of the bubble structure were not well studied, based on the given satellite measurements. Moreover, since the plasma bubbles or the irregularities can easily generate ionosphere scintillation, cautions should be taken for the ionosphere gradient calculation, which has been omitted in previous literature. The global coverage and the high temporal and spatial resolution GNSS ionosphere measurements provide abundant and high-quality Earth-ionosphere observables. By calculating the GNSS TEC estimation from GNSS observables, the global and regional ionosphere TEC morphology can be further investigated [18]. With the study of statistics of TEC enhancement and depletion, the ROTI was defined and used to describe the intensity of the ionosphere irregularities [19]. The resolution of the TEC estimation, to portray regional ionosphere variation, relies greatly on the spatial distribution of IPP. For the multiple constellation case, the amounts of ionosphere pierce points (IPP) have increased tremendously [20;21]. It is worth studying the features of the TEC derived from GNSS observables under geomagnetic storms, to show the benefits of the multiple constellation measurements. The geometry of the GNSS constellation also contributes to a better coverage of IPEPs, with the mixed orbits of the BeiDou Navigation Satellite System (BDS), to the TEC derived from the BDS enriched spatial diversity of IPP observations, and to set new opportunities to comprehensively discover the variations of the ionosphere under geomagnetic storms. The Multi-GNSS Experiment(MGEX) set-up by the International GNSS Service (IGS) [22] aims to track, collate, and analyze all available GNSS measurements, including BDS, Galileo, as well as any space-based augmentation system (SBAS) of interest [23; 24]. The MGEX provides a data source for pioneering studies on the assessments of multi-GNSS performance as well as related applications. The geomagnetic storm caused by coronal mass ejection (CME) on 22 June 2015 is a typical case for this work. The geomagnetic storm has a minimum of \(-204\) nT Dst index at sudden storm commencement, mainly lasting for two days [25; 26]. It happened at the summer solstice, when the occurrence of the ionosphere irregularities is considered quite low at the middle and low latitudes, thus, setting a good background for the investigations of any unusual phenomena. In this work, the ionosphere responses on 22 June 2015 geomagnetic storm are analyzed using multi-constellation GNSS observations from the MGEX data. The ionosphere irregularities morphology during the event is further investigated with abundant observations from GPS, GLONASS, and BDS measurements. The temporal and geophysical behaviors of TEC from 22 June to 23 June 2015 are analyzed. Section 2 describes the data representation and the proposed methodology. Section 3 presents the experiments conducted to verify the responses with aspects of ionosphere TEC variation and irregular ROTI behaviors. The results are discussed in Section 4, which is followed by Section 5 with concluding remarks.