## 2 The HORIZON-T Installation for Detection of High-Energy EAS and the Multi-modal Shower Events  
The electron component of secondary cosmic rays at the Tien Shan mountain station is detected by a set of synchronously operating large-sized plastic scintillators and Cherenkov detectors which both constitute the HORIZON-T installation for registration of the high-energy EAS [9]. The general layout of this installation is shown in Figure 1. As it follows from the figure, the system of charged particles detectors currently comprises 12 detector points distributed over an area of ~3 km\({}^{2}\). The lower energy threshold of EAS is ~10 PeV, while the upper limit of its effective operation is defined by the steeply diminishing intensity of the energy spectrum of cosmic rays, and for an installation of such geometrical sizes it is close to ~(3-5) \(\times\) 10\({}^{3}\) PeV. As an active medium for the detectors of the installation (1 \(\times\) 1) m\({}^{2}\) sized polystyrene scintillator plates are used. The thickness of the plates is 10 cm in the points No (1-5), and 5 cm in the points No (6-12) (refer to Figure 1). Besides the scintillator, there is an additional Cherenkov detector in the point No 1. Figure 1: Disposition of the HORIZON-T detector points over the mountain pass of the Tien Shan station: an aerial view and a graph scheme. One of the unusual results of the HORIZON-T installation due to an enhanced temporal resolution of its detectors is bi- and multi-modal EAS, which is shown in Figure 2. As it is seen there, instead of a single scintillation corresponding to the passage of the main particles in front of EAS there are three closely allocated pulses of comparable amplitude in the high-resolution time series of signal amplitude of the scintillation detectors. Absolute amplitudes of the pulses correspond to the passage of bunches of 316, 327, and 363 relativistic particles which are successively hitting the detector with the mutual time delays of a few hundreds of nanoseconds. Such a time raw of the scintillation signal can be naturally interpreted as a passage of three particle disks during an extremely narrow time gate of the order of a microsecond. Nevertheless, because of a low general intensity of the EAS with primary energy of \(\sim\)100 PeV, the probability for a number of such independent EAS to hit one and the same time window of microsecond duration is quite negligible, as it can be easily estimated by applying the modern simulation tools of EAS development, such as the CORSIKA package [10]. According to simulations, which were made using this package for the HORIZON-T experiment, all the detected events should consist of a single scintillation flash which corresponds to the passage of a unique particle disk of the shower. Consequently, successive separate showers in the cases like the event shown in Figure 2 must be somehow casually connected with each other. The relative frequency to meet a multi-modal EAS among the flux of all detected showers can be estimated from the general statistics of all observed events. Thus, in the period from 21 March to 12 May 2018, during 1137 operation hours of the HORIZON-T installation it had registered 15,725, showers with the energy above 20 PeV, 13.8 events per hour. Of them, more than 500 showers were found to belong to the multi-modal type. All the multi-modal showers had primary energy above 100 PeV which in fact can be an indication of the threshold nature of the discussed effect. Another peculiarity of multi-modal showers is the exceedingly prolonged lateral distribution of the density of charged particles in those events. An illustration of this effect is shown in Figure 3, where the distributions of the particle density measured in 217 multi-modal EAS, shown with points, are compared with the distribution functions resulting from a CORSIKA simulation based on the standard EAS development models. It  
Figure 2: A sample of EAS event with multi-modal (triple) scintillation response to the passage of successive shower fronts.