A Forbush decrease (FD) is the decrease in Galactic cosmic ray (GCR) flux, e.g., as observed by a neutron monitor count rate, in association with a coronal mass ejection (CME) and/or its shock. The FD amplitude is known to decrease at higher cutoff rigidity. During Solar Cycle 24, the Mawson neutron monitor in Antarctica, with a low (atmosphere-limited) cutoff rigidity of ~1 GV, observed numerous FDs, while the Princess Sirindhorn Neutron Monitor (PSNM) located near the Earth's equator at Doi Inthanon, Thailand, with the world’s highest geomagnetic cutoff rigidity (17 GV), observed only a fraction of these as FDs.  Instead, we find that the shock arrival is often followed by repeated dips in the PSNM count rate at only certain times of day, while the GCR flux from other directions remains near the pre-shock level.  We refer to decreases of this type as diurnal dips.  In this work, we have surveyed FDs and diurnal dip events observed by PSNM and the dependence of their maximum amplitude on the solar wind speed, ICME speed, and interplanetary magnetic field.  We acknowledge logistical support from Australia's Antarctic Program for operating the Mawson NM and support from the National Astronomical Research Institute of Thailand and grant RTA6280002 from Thailand Science Research and Innovation.

How to cite: Banglieng, C., Ruffolo, D., Sáiz, A., Mitthumsiri, W., Nutaro, T., Duldig, M. L., and Humble, J. E.: Diurnal Dips and Forbush Decreases in Galactic Cosmic Rays Observed at High Cutoff Rigidity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4352, https://doi.org/10.5194/egusphere-egu22-4352, 2022.

Galactic cosmic rays (GCR) show a small local anisotropy detected as a diurnal variability of neutron monitor (NM) count rates. As the asymptotic directions of different NMs are diverse, the capability of the GCR diurnal variation observation is also various. Here we present that the Dome C (DOMC) NM is barely sensitive to the diurnal variation. Its amplitude is very small, 0.03%, in comparison to other polar NMs, for which the diurnal variability amplitudes vary from 0.16 to 0.4%. This fact is associated to the narrow asymptotic-direction cone of DOMC NM looking almost to the South pole with geographic latitude above 75^o. Thus, DOMC NM is the only existing NM accepting cosmic-ray particles from the off-equatorial region, which makes this station a unique detector.

How to cite: Gil, A., Mishev, A., Poluianov, S., and Usoskin, I.: Diurnal anisotropy of polar neutron monitors, Dome C looks poleward, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1221, https://doi.org/10.5194/egusphere-egu22-1221, 2022.

The first solar proton event of solar cycle 25 was detected on 28 October 2021 by several neutron monitors (NMs) in the polar regions, the strongest signal was registered by the DOMC/DOMB monitors located at the Antarctic plateau at Concordia French-Italian research station. It is identified as the GLE (ground-level enhancement) #73 in the International GLE database. Here, we report the observations of the GLE by the global NM network and present the derived angular and spectral features of solar energetic protons with their dynamical evolution throughout the event employing a state-of-the-art model based on analysis of the neutron monitor data. Using the derived spectra we computed the related terrestrial effects, namely the cosmic rate induced ionization at several altitudes on a global map and discuss possible implications.

How to cite: Mishev, A., Larsen, N., and Usoskin, I.: The first GLE (# 73 – 28-Oct-2021) of solar cycle 25: a study of the related terrestrial effects using neutron monitor data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1243, https://doi.org/10.5194/egusphere-egu22-1243, 2022.

We present an overview of the first ground-level enhancement (GLE) event of solar cycle 25, recorded on 28 October 2021 (GLE73), based on the available neutron monitor (NM) network observations and on data from near-Earth spacecraft (GOES, SOHO, SolO). The maximum increase was ~7.3% for DOMC (Dome C NM at Concordia station) and 5.4% for SOPO (South Pole) conventional NMs located on the Antarctic plateau. Bare (lead-free) NMs at the same sites detected a higher response (14.0% for DOMB and 6.6% for SOPB). The Fort Smith (FSMT) NM shows the earliest increase among the high-latitude NMs, indicating a moderate anisotropy in the first phase of the GLE event. The maximum rigidity of accelerated protons did not exceed 2.4 GV. We estimated the solar release time (SRT) of ≥1 GV protons into open magnetic field lines at ~15:40 UT.  In-situ proton observations from near-Earth spacecraft were combined with the detection of a solar flare in soft X-rays (SXRs), a coronal mass ejection (CME), radio bursts and extreme ultraviolet (EUV) observations to identify the solar origin of the GLE. Around the ≥1 GV proton SRT the CME-driven shock was located at a height of ~2.33 Rs. The timing of the EUV wave evolution towards the field lines magnetically connected to Earth seem to be in good agreement with the inferred release time of ≥1 GV protons.

How to cite: Papaioannou, A., Kouloumvakos, A., Mishev, A., Vainio, R., Usoskin, I., Herbst, K., Rouillard, A. P., Anastasiadis, A., Giesler, J., Wimmer-Schweingruber, R., and Kuhl, P.: Characteristics of the First Ground Level Enhancement (GLE) of Solar Cycle 25 on 28 October 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5504, https://doi.org/10.5194/egusphere-egu22-5504, 2022.

A neutron monitor is a large ground-based detector responding to the flux of cosmic ray particles in space by measuring atmospheric secondary neutrons. Any ground-based detector is sensitive to cosmic rays from a specific range of directions in space. In particular, a particle arriving from a specific sky direction with a specific rigidity (momentum per unit charge) was necessarily moving from a certain direction in space, called the asymptotic direction. McMurdo and Jang Bogo neutron monitor stations are Antarctic stations with similar geomagnetic latitudes but slightly different longitudes. From December 17, 2015 to January 9, 2017, six of the eighteen neutron counters from McMurdo had been transferred to Jang Bogo (with full transfer to Jang Bogo completed in December 2017). We present an analysis of the correlation of the cosmic ray flux between the McMurdo and Jang Bogo stations, during the time when both were operating, with ten-second time resolution. Although highly correlated, there are significant differences, including a systematic time lag of approximately 16 minutes between the data from the two stations. Although McMurdo observes with similar asymptotic directions to Jang Bogo, the response-weighted average directions still have a substantial difference of 21.9 degrees in geographic longitude, so with Earth’s rotation, time-independent anisotropy effects should induce a lag of 88 minutes.  Because the observed lag of 16 minutes is intermediate between 0 and 88 minutes, the joint observations reveal structure in the interplanetary cosmic-ray density that is consistent with a combination of simultaneous temporal variations and non-simultaneous variations with direction (i.e., anisotropy). The research is supported in part by a TA/RA scholarship (active recruitment) of Chiang Mai University and Thailand Science Research and Innovation via Research Team Promotion Grant RTA6280002.

How to cite: Kittiya, E., Nuntiyakul, W., Seripienlert, A., Evenson, P., Saiz, A., Ruffolo, D., and Oh, S.: Cosmic Ray Flux Correlation between McMurdo and Jang Bogo Neutron Monitor Stations vs. Time Lag, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4215, https://doi.org/10.5194/egusphere-egu22-4215, 2022.

Neutron monitors were designed to measure atmospheric secondary neutrons from cosmic ray showers in order to track the cosmic ray flux vs. time.  Furthermore, at the Princess Sirindhorn Neutron Monitor (PSNM), an 18-counter NM64 detector at 2560-m altitude at Doi Inthanon, Thailand, the leader fraction (inverse multiplicity) inferred from time delay distribution between successive neutron events in the same counter has been used to track spectral variations.  More recent measurements of time delays between neutron events in different counters, as a function of counter separation, have confirmed that 1) the product neutrons from the interaction of a single atmospheric secondary neutron can spread among neighboring counters, with a cross-counter leader fraction that depends on whether the first counter is an end or middle counter, and 2) coincident counts between distant counters can be produced by multiple atmospheric secondary neutrons from the same primary cosmic ray, with a leader fraction that depends on whether the second counter is an end or middle counter.  Here we report on measurements of neutron signals in amplifier outputs at PSNM using a 4-channel oscilloscope, in order to further investigate these phenomena.  For a pair of neighboring counters located at the edge of the counter array, we find roughly equal event rates in either neighbor following a neutron trigger in one of them, implying that the difference in leader fraction relates to the base count rate of the first counter and is therefore lower if that is an end counter. In addition, an FPGA-based readout system was developed for more efficient collection of neutron events on two distant counters (Tubes 2 and 18) that were coincident within a 250-microsecond time window, while also monitoring Tube 10 in between.  The time distributions and neutron multiplicities indicate that a small fraction of the cosmic ray events that triggered both Tubes 2 and 18 also led to neutron events on Tube 10 with an enhanced rate of high multiplicity, indicating a few air shower events that densely “carpeted” the neutron monitor, while the majority of such coincidences apparently involved a sparse distribution of isolated secondary particles near Tube 2 and near Tube 18 and not near the intermediate Tube 10, which is consistent with a cross-counter leader fraction dependence on the count rate of the second counter.  This research was supported by the postdoctoral research sponsorship of Mahidol University, Thailand, by grant RTA6280002 from Thailand Science Research and Innovation, and by grant NSRF from the Program Management Unit for Human Resources & Institutional Development, Research and Innovation, NXPO, Thailand [grant number B05F640051]. 

How to cite: Chaiwongkhot, K., Ruffolo, D., Chaiwongkhot, P., Sáiz, A., and Banglieng, C.: Coincident signals on multiple counters in a neutron monitor: Sparse vs. dense atmospheric secondary particles from cosmic ray showers , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5650, https://doi.org/10.5194/egusphere-egu22-5650, 2022.