Observation of the Large Forbush Decrease Event on 1–10 June 2025 at the Tien Shan Cosmic Ray Station

Abstract

An intensive disturbance of the heliosphere originating from a series of solar flares at the end of May 2025 revealed itself as an extremely strong Forbush decrease in the flux of galactic cosmic rays, which was observed at the worldwide network of neutron monitoring starting from 1 June 2025. Together with an effect on the intensity of cosmic rays measured at a height of 3340 m a.s.l. by a standard NM64 neutron supermonitor of the Tien Shan High-Mountain Cosmic Ray Station, the impact of this event was detected here as a synchronous depression in the counting rates of thermal neutrons and hard gamma rays, the background intensity of which is also continuously tracked in the environment of the station. This observation may be profitable for development of an alternative technique applicable for studying large disturbances in the heliosphere, besides using the traditional method of neutron monitor. An analysis of the frequency of the events of multiple-neutron generation in the Tien Shan supermonitor demonstrated that the influence of the heliosphere disturbance on the hadronic component of cosmic rays did most distinctively reveal itself in the energy range of (3–20) GeV.

1. Introduction

Galactic cosmic rays registered with ground-based installations carry information on the properties of solar wind, which is the medium they propagate in. Investigation of the flux and anisotropy variations of cosmic rays allows us to acquire valuable data on both the processes at the Sun and disturbances of solar wind in the interplanetary space.

Forbush decreases (FDs), or Forbush effects, are the most diverse and numerous phenomena in the variation of cosmic rays. Presently, a most complete definition is accepted of a Forbush effect as a change of the density and anisotropy of cosmic rays in a large-scale variation of solar wind [1,2]. Today these phenomena are investigated both by the network of neutron monitoring [3,4] and by the space-based devices, such as the satellite missions AMS-02 [5,6], PAMELA [7,8], and DAMPE [9]. While on-ground detectors, neutron monitors and muon telescopes allow for nearly continuous precision monitoring of the integral intensity of cosmic ray flux over exceptionally prolonged time periods on the order of several decades, the satellite detectors are able to provide detailed information on variation of the different cosmic ray components: protons, helium nuclei, electrons, and positrons.

The solar sources of FD are coronal mass ejections (CMEs) and high-velocity fluxes of solar wind from coronal holes. The former result in sporadic FDs, originating from the CMEs from solar flares or disappearance of solar filaments [10,11,12,13,14,15,16,17]. Speedy fluxes of solar wind from the coronal holes revolving together with the Sun and living during several rotations result in recurrent FDs [18,19,20,21,22,23,24]. Often, FDs have a mixed origin, being caused by both a CME and high-velocity solar wind from a coronal hole. Also, FD may be connected with an interacting plasma flow of fast solar wind from a coronal hole and a CME [25,26].

The study of FDs provides knowledge on the processes in the interplanetary space and near-Earth environment, which is important for the prognosis of the arrival of interplanetary disturbances into the near-Earth space and their influence on the atmosphere and magnetosphere of the Earth. Most interesting is the investigation of the major FDs triggered by large interplanetary disturbances coming to the Earth. As a rule, such effects are connected with the CMEs initiated with the solar flares which occur in the central region of the Sun’s disk and are directed towards the Earth. In accordance with the database of Forbush effects and interplanetary disturbances created in IZMIRAN [27], starting from July 1957 and up to December 2024, the worldwide network of neutron monitoring has registered 31 FDs with a magnitude above 10%, and every such event deserves special consideration with application of all available experimental data.

On 1 June 2025, a Forbush decrease commenced; it happened to be the deepest in the last 20 years. The FD was caused by a series of solar flares and CMEs consequently occurring during 30–31 May 2025 on the Earth-facing hemisphere of the Sun, thus resulting in a complicated structure and prolonged evolution of the FD. The duration of the recovery phase after the FD was nearly a week, and the measurements of the global cosmic ray intensity revealed complex variations in its time behavior, indicating the existence of large-scale disturbances in the heliosphere.

At the Tien Shan High-Mountain Cosmic Ray Station (Northern Tien Shan, 43.04°N 76.94°E, 3340 m above sea level), the Forbush decrease of 1 June 2025 was clearly detected as a drop in the intensity of the cosmic rays with a cutoff rigidity of 5.9 GV, as measured by a NM64 type neutron supermonitor which resides at the station (see Figure 1). The supermonitor is a part of the world-wide network for monitoring the global intensity of cosmic rays [28]. Together with this standard instrument, the experimental complex of the Tien Shan station includes several other detector facilities, which were also influenced by the Forbush decrease of 1 June 2025. These are the detectors of thermal neutrons and the gamma radiation detectors, sensitive, in particular, to the hard gamma rays with an energy above 1 MeV. Both detector systems were initially intended for long-term monitoring of the neutron and gamma radiation flux in the local environment [29,30,31], as well as for detection of the transient bursts of low-energy neutrons and gamma rays accompanying the passage of extensive air showers [32,33], or close lightning discharges [34,35].

The subject of the present publication is a description of the manifestations of the extremal Forbush decrease of 1 June 2025 detected, firstly, in the counting rate of the Tien Shan neutron supermonitor, and then among the synchronous time series of the data on the thermal neutron and gamma radiation background in the outer environment of the Tien Shan Cosmic Ray Station.

2. The Solar and Geomagnetic Situation

In Figure 2, several data time series are reproduced reflecting the state of the heliosphere and geomagnetic field during the end of May to the beginning of June 2025. The measurements of the solar wind speed and the module of the interplanetary magnetic field (IMF) strength from NASA’s Advanced Composition Explorer (ACE) spacecraft presented in the upper two panels of the figure were taken from the website [36], while the information on the $Kp$ and $ap$ characteristics of the geomagnetic field was obtained from [37].

The prolonged period of Forbush decreases originated from a series of solar flares and coronal mass ejections at the end of May 2025, starting on 29 May with the arrival at Earth of the high-speed solar wind particles from the coronal hole SWPC CH 52 situated on the Earth-facing hemisphere of the Sun. As seen in the upper plots of Figure 2, the velocity of solar wind in the vicinity of the Earth at that time rose to ∼700 km·s⁻¹, and a temporary disturbance in the IMF strength was of about ∼(12–15) nT. According to the time series of the geomagnetic field parameters from [37], around 0 h UT at 29 May, this event resulted in a G3-level geomagnetic storm with the maximum $Kp$ index above 6, and caused a small FD, which has revealed itself as a ∼1.5% recession in the counting rate of the Tien Shan neutron supermonitor, noticeable in Figure 1 between the dates of 29 May and 1 June. During that time, two minima of the counting rate in the neutron supermonitor took place around 29 May, 22:30 UT and 31 May, 19:10 UT.

On 30 May 2025 there were two CMEs at the Sun, at 06:38 and 12:53 UT. The second CME was connected with a M3.4-class solar flare that occurred in the active region AR4100 with a complicated magnetic configuration of the $\beta$-$\gamma$-$\delta$ type [38]. On 31 May, three M-class solar flares took place in the same active region, and the most powerful of them, that which reached its maximum at 00:05 UT in the point with the coordinates N08E12, initiated a Full-Halo CME, first seen by the GOES-19’s CCOR-1 space coronograph [39], and then by the LASCO C2/C3 coronographs at the SOHO space observatory [40]. As seen in Figure 2, in the vicinity of the Earth the effect of this CME began to be felt on 1 June 2025 as a sudden jump in the velocity of solar wind, which went up to ∼1100 km·s⁻¹, and a rise of the IMP from ∼7 nT to above 24 nT. At 05:22 UT, the CME reached the Earth, resulting in a strong geomagnetic storm with the maximum $Kp\approx 7.5$ reflected in the 3–6 h bin of the $Kp$ histogram for the date of 1 June. Most probably, this fast CME did override the former, slower CMEs of 30 May.

Consecutive disturbances in the heliosphere caused by the solar flares on 30–31 May 2025 determined the complicated structure of the prolonged Forbush decrease, which started on 1 June and continued up to 10 June. According to the measurements of the counting rate R in the Tien Shan neutron supermonitor in Figure 1, the relative magnitude of the drop in the intensity of the nuclear active component of cosmic rays $\Delta R/\langle R\rangle$ at that time was of approximately 0.14–0.15, which makes the considered event the most prominent FD in the last 20 years. Here and below, $\langle R\rangle$ is the average counting rate in neutron supermonitor as registered during the quiet period of 25–28 May before the beginning of any disturbance, and $\Delta R$ is the difference between the average level $\langle R\rangle$ and the minimum of the counting rate at the peak of the Forbush decrease (see Figure 1).

3. Methods

The main instrument used for the long-term precision measurement of the intensity of cosmic rays at the Tien Shan Cosmic Ray Station is a standard NM64 neutron supermonitor of the type especially invented for this purpose in the mid-1960s [41,42]. The Tien Shan supermonitor consists of three $(2\times 3)$ m² units, each with six $(0.15\times 2)$ m² gas discharge ionization counters inside. The counters have a special fill including the BF₃ gas enriched with the ¹⁰B isotope, which makes them sensitive to thermal neutrons due to the nuclear reaction n(¹⁰B,⁷Li)$\alpha$. From outside, the counters are surrounded with interchanging layers of the heavy lead target and light hydrogen-containing neutron moderator material. High-energy particles of the nuclear active (hadronic) cosmic ray component, when penetrating into the monitor, cause nuclear reactions upon their interaction with the lead nuclei in the target, while the evaporation neutrons born in these reactions diffuse inside the moderator and rapidly loose their energy down to thermal values in collisions with light nuclei, becoming available for detection by the counters. Thus, the summed counting rate of the pulse signals at the output of all neutron counters in the supermonitor is proportional to the intensity of cosmic ray flux (more precisely, to the intensity of the nuclear active component of cosmic rays). Both the comparatively large geometrical dimensions of the neutron supermonitor and considerable multiplicity of evaporation neutrons produced upon the interaction of a high-energy hadronic particle allow the reduction in the random statistical fluctuations of the counting rate, and ensure a measurement accuracy sufficient for precision investigation of tiny variations in the cosmic ray intensity with a typical relative magnitude below 1%. In the case of the Tien Shan neutron supermonitor, an advantageous circumstance is also its high-altitude mountain disposition, which ensures a much larger, up to 2–3 times higher, flux of cosmic ray hadrons reaching the supermonitor, in comparison with the sea level installations, and thus increases even more the statistical reliability of the measurements of cosmic ray intensity.

Since the neutron supermonitor registers mostly the secondary hadrons originating from interaction of high-energy primary cosmic ray particles in the upper layers of the atmosphere, its counting rate depends on the amount of substance met by the hadronic products of these reactions on their way to the supermonitor. A single parameter characterizing the thickness of that substance is the atmospheric pressure [43,44], so in the study of cosmic ray variation with the technique of neutron supermonitor it is customary to make a correction of the immediately determined counting rate values $R_I$ to the pressure p, which is also regularly registered together with the intensity of neutron signals:

$$R=R_I·exp(0.0072·(p-p_0)).$$

(1)

Here, $p_0=675$ mbar is the average atmospheric pressure at the altitude of the Tien Shan Cosmic Ray Station. The effect of Formula (1) is illustrated by the left plot in Figure 3, which was built over the data collected at the Tien Shan neutron supermonitor during the latter minimum epoch of solar activity, when excessive disturbances of cosmic ray flux were rare.

As it is a standard in the world-wide network for monitoring the intensity variation of cosmic rays, the total number of output pulses from all ionization counters of the Tien Shan neutron supermonitor is accumulated during one-minute-long consecutive time intervals, and the resulting values of the counting rate R thus defined are sent into a common database [4] for further use as an intensity measure of cosmic rays. In Figure 1, the original one-minute measurements of the counting rate are indicated with a thin gray line, while the bold curve corresponds to the same data additionally averaged with a 1 h long step.

Another type of neutron-sensitive device applied at the Tien Shan Cosmic Ray Station is the detector of thermal neutrons. Each detector consists of a box containing 12 $(0.03\times 1)$ m² ionization counters filled with ³He gas, such that the registration of thermal neutrons goes through the reaction n(³He,³H)p. In contrast to the supermonitor, the neutron counters in these detectors are not surrounded with either a lead target or any moderator. Because of that, the detectors of the second type are sensitive to the local background flux of neutrons, both born in the interaction of cosmic ray particles with the matter of the environment and originating from the processes of natural radioactivity. Thus, the role of a target necessary for generation of evaporation neutrons by cosmic ray particles in this case is as heavy substances in the detector surrounding primarily the rock ground beneath.

The detectors of the thermal neutron background are installed at nearly the same height above sea level at 3 different points in the territory of the Tien Shan Cosmic Ray Station (the neutron detectors #1, #2, and #3), such that the maximum distance between the detector points in the horizontal plane reaches ∼180 m.

The measurement procedure of the intensity of the flux of environmental thermal neutrons is quite analogous to the case of neutron supermonitor; the total number of output pulses from all ionization counters of each neutron detector is determined in subsequent fixed time intervals, then the current counting rate of the neutron background is calculated. The accumulation time of neutron pulses in every single measurement at these detectors is set to 10 s. Just as for the neutron supermonitor, before any further analysis the initially measured counting rate values are corrected to the current atmospheric pressure, in accordance with the above-mentioned Formula (1). As follows from the right plot in Figure 3, the formula also remains fairly relevant in the case of the background of thermal neutrons, such that its application to correction of these data leads to a reasonable result. Obviously, this is a consequence of the fact that the origin source of a significant portion of neutrons in the environment is connected with the same secondary hadrons from interaction of cosmic rays in the upper atmosphere layers, as of the neutrons registered in neutron supermonitor.

More precisely the system of neutron detectors of the Tien Shan Cosmic Ray Station, as well as the results of a simulation of the detectors’ operation and the registration technique of the neutron counting rate, are described in [32,45].

The evaporation neutrons produced under the influence of nuclear active cosmic ray particles, when propagating through the environment in the process of their diffusion, are captured by the nuclei of surrounding matter, such that the gamma ray quanta with a MeV order energy are emitted as a result of those captures. Thus, one more instrument applicable for monitoring the variation of secondary neutron flux may be a gamma detector able to register the gamma rays with an energy threshold of several MeV.

Presently, there exist gamma ray detectors at the Tien Shan Cosmic Ray Station suitable for continuously monitoring the intensity of MeV gamma radiation. The detectors are installed in three spatially separated points, one of which is hosted at the territory of the station, at an altitude of 3340 m a.s.l., and the other two are placed at the slopes of a neighboring mount, at the heights of 3750 m and 3900 m a.s.l. correspondingly. Each detector is based on an inorganic NaI scintillation crystal of a cylindrical form with the dimensions of $(110\times 110)$ mm², viewed through by a photomultiplier tube with a 100 mm-sized input window. The analogue output pulses of the photomultiplier, after proper shaping, come to a set of amplitude discriminators, which allow the selection, in particular, of the signals corresponding to the gamma ray quanta with an energy above (1.3–1.5) MeV. Quite analogously to the case of thermal neutron detectors, the counting rate of the discriminator’s output signals is continuously determined with a 10 s time periodicity.

A more detailed description of the gamma radiation detectors used at the Tien Shan Cosmic Ray Station may be found in [34].

4. The Measurement Data and Discussion

4.1. The Neutron and Gamma Radiation Background in the Environment

The monitoring records of the background intensity of thermal neutrons and hard gamma radiation in the environment of the Tien Shan Cosmic Ray Station registered during the time period both before and after the 1 June 2025 Forbush decrease are shown in Figure 4, together with the variation of the intensity of cosmic rays simultaneously measured by the Tien Shan neutron supermonitor. For convenience of comparison of the data from different detectors, the counting rates here are plotted as a difference between the momentary counting rate R fixed in every point of corresponding distribution, and its mean value $\langle R\rangle$ calculated over the whole time series. All distributions are scaled in the units of the number of pulse signals obtained from each detector per one second (p.p.s.). The original 10 s time resolution data of the thermal neutron and gamma radiation measurements are represented in the plots of Figure 4 as thin lines, while the smooth colored curves overlapping those quickly fluctuating distributions indicate the behavior of the counting rate after its averaging over the longer time periods with a 1 h duration. One-hour time series of the counting rate data in the neutron and gamma detectors are also placed separately in Figure 5 and Figure 6 in absolute form for illustration of the influence of statistical fluctuations on their shape.

As follows from Figure 4, the Forbush effect in the flux of cosmic rays detected on 1 June 2025 (top panel) can also be clearly traced over the monitoring records of the thermal neutron background. The imprint of the effect in thermal neutrons generally repeats the shape of the FD in the neutron supermonitor; at first, a sharp drop is observed at the beginning of the depression in the counting rate of all three neutron detectors, then a slow restoration up to the initial undisturbed level follows, which continued until approximately 10–11 June 2025. According to the Pearson correlation coefficients in

Table 1. The Pearson correlation coefficients r between the time series on the intensity of cosmic rays, as measured at the Tien Shan neutron supermonitor, and the data of the simultaneous monitoring of the neutron and radiation background in the environment of the Tien Shan station. By calculation, the averaged data series with a 1 h time resolution were used.

Instrument	r, 25–28 May 2025	r, 31 May–2 June 2025
thermal neutron detectors #1, #2, and #3	0.29 0.14 0.23	0.99 0.98 0.99
gamma detectors at 3340, 3750, and 3900 m a.s.l.	−0.08 0.18 −0.40	0.69 0.64 0.72

, which were calculated between the hourly smoothed time series of the neutron supermonitor and the data of other neuron detectors, during the FD period between 31 May and 2 June 2025, the behavior of the environmental neutron background practically repeated that of the cosmic ray flux, such that the correlation r between the time series of the neutron supermonitor and thermal neutron detectors remained around 0.99, in contrast with an insignificant correlation of $r$ ≈ (0.14–0.30) at the preceding undisturbed epoch. In Figure 5, an evident drop of neutron background is seen at the beginning moment of the FD, the amplitude of which is nearly on order of magnitude above the typical level of statistical fluctuations in hourly intensity measurements.

The time series of gamma radiation in Figure 4 obtained in the period of Forbush decrease also reveal distinct similarity with the neutron supermonitor, and their correlation with the cosmic ray data reaches the level of $r$ ≈ (0.60–0.70). In contrast, the data on gamma radiation registered during a same-length period immediately preceding the event, at 25–28 May 2025, either do not noticeably correlate with the variation of cosmic ray counts ($r$ ≈ (0.10–0.30)), or even have a noticeable anticorrelation. The latter may be a consequence of a complex combination of various local factors which influence the gamma radiation background without any connection with cosmic rays: thunderstorm activity [34], precipitation [46,47,48], exhalation of radioactive gases from under the surface of the ground modulated by seismic processes [31]. One-hour time series in Figure 6 also demonstrate a significant diurnal variation, which seemingly reflects the temperature dependence of the radiation background. In spite of the susceptibility of gamma detectors to a variety of side interferences, a pulsed imprint of the 1 June 2025 Forbush effect can be undoubtedly traced there, and its amplitude also exceeds by an order of magnitude the level of statistical fluctuations in hourly measurements of the MeV gamma ray intensity.

The quantitative metrics of the effect of Forbush decrease of 1 June 2025 on the intensity of neutrons and gamma rays in the environment of the Tien Shan Cosmic Ray Station are listed in

Table 2. Parameters of the Forbush decrease on 1 June 2025, as observed with different detector facilities of the Tien Shan Cosmic Ray Station.

Instrument	Background Counting Rate, $\langle \mathit{R}\rangle$, p.p.s.	Standard Deviation, $\mathit{\sigma }$, p.p.s.	Magnitude of the Forbush Decrease, $\mathbf{\Delta }\mathit{R}$, p.p.s.	Relative Magnitude, $\mathbf{\Delta }\mathit{R}/\langle \mathit{R}\rangle$	Magnitude in Standard Deviations, $\mathbf{\Delta }\mathit{R}/\mathit{\sigma }$
the NM64 neutron supermonitor	1222	6.0	175	0.14	29
thermal neutron detectors #1, #2, and #3	19.4 20.2 14.8	0.21 0.24 0.17	2.9 2.9 2.1	0.15 0.14 0.14	14 12 12
gamma detectors at 3340, 3750, and 3900 m a.s.l.	30.7 45.2 45.8	0.57 0.48 1.2	2.2 3.6 5.3	0.07 0.08 0.11	3.8 7.6 4.4

together with the mean counting rate $\langle R\rangle$ and its standard deviation $\sigma$ typical for each detector facility in the quiet period immediately preceding the Forbush effect. The difference $\Delta R$ between the average level of the counting rate $\langle R\rangle$ and its minimum at the beginning of the FD is considered as a magnitude of the effect in the corresponding detector.

The definition of the parameters $\Delta R$, $\langle R\rangle$, and $\sigma$ is explicitly illustrated in Figure 1.

In Table 2, the magnitudes of FD are presented both in absolute and relative form, as relations $\Delta R/\langle R\rangle$ and $\Delta R/\sigma$. To reduce the influence of random fluctuations on the magnitude estimate, the calculations were made using the time series smoothed with the 1 h time resolution.

According to Table 2, the Forbush decrease of 1 June 2025 is fairly distinctly seen among the data of all three thermal neutron detectors. The relative magnitude $\Delta R/\langle R\rangle$ of the drop in the counting rate in all these detectors is nearly one and the same, and also coincides with the corresponding magnitude of the effect registered in the neutron supermonitor. This observation confirms the applicability of the monitoring data on the flux of thermal neutrons in the environment at mountain heights as an alternative method to detect large variations in the intensity of cosmic rays, in addition to precision measurements at standard neutron supermonitor.

The statistical reliability of the FD observed by the flux of thermal neutrons, as expressed in the units of standard deviation, $\Delta R/\sigma$, is approximately twice as bad as that for the measurements with the neutron supermonitor. This is a consequence of an essentially lesser total counting rate, which is characteristic for the thermal neutron detectors, in comparison with the supermonitor, and correspondingly higher relative deposit of random fluctuations in their counts.

The effect of fluctuation is even more significant in the case of gamma detectors, where it acts together with essential influence on the intensity of the registered radiation background of the above-mentioned side factors. The complicated composition of different effects results in a relatively enhanced standard deviation of the background quiet time counts in radiation detectors, and in a correspondingly reduced value of the significance relation $\Delta R/\sigma$, even in comparison with the detectors of thermal neutrons. Nevertheless, the FD remains fairly noticeable in the intensity time series of hard radiation, with an amplitude $\Delta R$ steadily exceeding (3–7)$\sigma$.

Thus, continuously monitoring the intensity of gamma radiation with an energy threshold of a MeV order opens one more way for the detection of strong FD events in cosmic rays and this reveals its influence on the level of hard radiation in the environment. In connection with this the results could be mentioned of an observation of the same 1 June 2025 Forbush event with the detectors of the SEVAN network hosted at Mount Aragats, Armenia, nearly at the same altitude above sea level as the Tien Shan Cosmic Ray Station. The data presented in [49] demonstrate the time variations in monitoring records of neutrons, gamma rays, and muons similar to the ones considered above. This observation confirms the applicability of diverse types of particle detectors to revealing large disturbances of solar and heliospheric origin in the flux of cosmic rays, and to the investigation of the effects such disturbances may cause on the environment.

4.2. Intensity of the Events of Multiple-Neutron Generation in the Neutron Supermonitor

The mean multiplicity of evaporation neutrons arising in the nuclear reaction initiated by a hadronic cosmic ray particle in lead target depends on the energy of interacting hadron $E_h$, so a rough estimation of that energy is possible based on the number M of pulse signals acquired from all counters of a neutron monitor installation during a short time period after the interaction. For the case of a NM64-type neutron supermonitor, the functional dependence $E_h\left(M\right)$ was studied both theoretically [50,51,52], experimentally [53,54], and using the modern simulation methods based on the Geant4 toolkit [55]; all these attempts have given results fairly agreeable with each other. As a result, a nearly quadratic function connecting the average energy of incoming hadron with the mean number of registered neutron pulses M,

$$E_h\left(M\right)\approx 0.5·M^{1.8}\mathrm{GeV},$$

(2)

was determined for the NM64 neutron supermonitor set-up. As shown in the works cited, the amount of neutron pulses is largely influenced by the fluctuations of the process of neutron production inside the monitor, so the values of average hadron energy predicted by Formula (2) should be understood in the sense of a rough order of magnitude estimate.

The registration technique of neutron signals accepted at the Tien Shan neutron supermonitor provides a possibility to count separately the events, when strictly 1, 2, 3, 4–5, 6–7, or 8–10 pulses from the neutron counters of a single 6-counter supermonitor unit were detected in a 800 μs long time gate. The latter duration was selected as a comparable one with the mean life time of evaporation neutrons in the NM64 neutron supermonitor (∼660 μs) [42].

In Figure 7, the time series for the end of May to the beginning of June 2025 are shown of the intensity of the events of multiple-neutron generation in the Tien Shan neutron supermonitor with various values of the multiplicity M. The effect of the Forbush decrease of 1 June 2025 is rather obvious in all these distributions, reflecting the influence of the heliosphere disturbance connected with the solar flares of 30–31 May 2025 separately on the cosmic ray components with different energy.

According to

Table 3. The Forbush decrease of 1 June 2025 as seen through the intensity of neutron generation events in the Tien Shan neutron supermonitor. The energy estimates in second column were acquired from (2). Parameters $\langle R\rangle$, $\Delta R$, and $\sigma$ correspond to the definitions shown in Figure 1.

Neutron Multiplicity, M	Effective Hadron Energy, ${\mathit{E}}_{\mathit{h}}$, GeV	Background Counting Rate, $\langle \mathit{R}\rangle$, p.p.s.	Standard Deviation, $\mathit{\sigma }$, p.p.s.	Magnitude of the Forbush Decrease, $\mathbf{\Delta }\mathit{R}$ p.p.s.	Relative Magnitude, $\mathbf{\Delta }\mathit{R}/\langle \mathit{R}\rangle$	Magnitude in Standard Deviations, $\mathbf{\Delta }\mathit{R}/\mathit{\sigma }$
1	0.5–1	452	2.8	43	0.10	15
2	2	175	0.96	28	0.16	29
3	3	62.1	0.46	11	0.19	25
4, 5	6–10	33.8	0.29	6.7	0.20	23
6, 7	13–17	7.1	0.076	1.3	0.18	17
8, 9, 10	20–30	2.7	0.035	0.39	0.15	11

, the drop in intensity of the events of multiple neutron production $\Delta R$ is about an order of magnitude above the level of standard deviation $\sigma$ of its variation typical for the preceding quiet time, so detection of the effect of Forbush decrease among these events is statistically reliable. Most influenced by the FD is the intensity of the events with multiplicities M equal to 2, 3, and 4, for which the relative magnitude of the effect $\Delta R/\langle R\rangle$ occurred of about ∼0.20, up to (30–40)% above its average value determined over the counting rate in the neutron supermonitor as a whole (0.14, see Table 2). From (2), it follows that the events with such multiplicity correspond to arrival into the neutron supermonitor of (3–10) GeV secondary hadrons born upon the interaction in the atmosphere of the cosmic rays with even higher energies.

The possibility of selecting the cosmic ray component with higher energy threshold through detection of the events of multiple-neutron production allowed us to observe peculiar features in the time behavior of cosmic rays of the 1 June 2025 Forbush effect. As such, a quasi-diurnal variation in counting rates during the maximum of the FD between 1 and 3 June could be mentioned, which most distinctively reveals itself in Figure 7 among the events with multiplicities 2–5. According to the publication [56], where attention to this feature was drawn, such variation could reflect the existence and evolution of large-scale complex structures in the magnetic field of plasma bundles which caused the Forbush event.

5. Conclusions

The event of the extreme Forbush decrease which started on 1 June 2025 was simultaneously observed at different detector facilities of the Tien Shan Cosmic Ray Station: by the NM64 neutron supermonitor, with the detectors of thermal neutrons, and with the detectors of the background hard gamma radiation in the environment. All detectors exhibited mutual agreement in the behavior of their counting rates at the time of the FD. Though the most proper instrument for precision monitoring of the cosmic ray flux remains the neutron supermonitor, the FD has also distinctly revealed itself in the time series of the intensity of environmental neutrons, where the relative magnitude of the effect, as indicated in Table 2, was quite comparable with that of the supermonitor. Also, an imprint of the change in cosmic ray flux was found among intensity records of the hard gamma radiation background with typical energy of about 1 MeV. This observation demonstrates the possibility of using the thermal neutrons and gamma radiation detectors for spotting the inhomogeneities in cosmic ray flux caused by large disturbances in the heliosphere and magnetosphere, and, inversely, of studying the effect from such disturbances on the environmental radiation background, thus extending the capabilities of the experimental complex of the Tien Shan Cosmic Ray Station.

The results presented in Table 3 on the intensity time series of the events of neuron production with different multiplicities in the Tien Shan supermonitor demonstrated that the Forbush decrease of 1 June 2025 has mostly influenced the atmospheric flux of secondary cosmic ray hadrons in the energy range of about (3–10) GeV. Systematic tracing of the intensity of interaction events of such hadrons in the neutron supermonitor, which generally correspond to the bursts of 3–5 pulse signals from its ionization counters closely grouped within a 800 μs time window at the time axis, may be considered as a mean to reveal most effectively small-amplitude Forbush decrease events with a neutron supermonitor residing at mountain heights.