PROCEEDINGS OF THE 31 st ICRC, L ´ OD ´ Z 2009 1 Calculation of the barometric coefficients for the particle detectors belonging to the world-wide networks at the start of the 24 th solar activity cycle Ashot Chilingarian, Tigran Karapetyan and Bagrat Mailyan Yerevan Physics Institute, Alikhanyan Brothers 2, Yerevan 36, Armenia Abstract. After major modernization of the data acquisition electronics of the Aragats Space Envi- ronmental Center (ASEC) particle detectors the cal- culations of the barometric coefficients of all ASEC particle monitors were performed in the beginning of 24 th solar activity cycle. The time periods of minimal disturbances of the interplanetary magnetic field (IMF) were selected to avoid biases due to transient solar events. The barometric coefficients of different particle detectors measuring various secondary cosmic ray fluxes located at altitudes of 1000 m, 2000 and 3200 m. a.s.l. are calculated and compared. The barometric coefficients for the several Neutron Monitors of recently established Eurasian data base (NMDB) and SEVAN particle detector networks also were calculated. The latitude and alti- tude dependencies of the barometric coefficients were investigated, as well as the dependence of coefficients on the most probable energy of the primary protons generated definite species of the secondary fluxes. Keywords: Atmospheric pressure effects, Sec- ondary cosmic rays, Instrumentation and techniques I. I NTRODUCTION Particle detectors of the Aragats Space Environmen- tal Center [1], [2] are located at slopes of mountain Aragats and in CRD headquarters in Yerevan, Armenia; geographic coordinates: 40 o 30’N, 44 o 10’E, altitudes - 3200m, 2000m and 1000m a.s.l.. Various ASEC de- tectors, measuring fluxes of diverse secondary cosmic rays, are sensitive to different energetic populations of primary cosmic rays. Two neutron monitors (18NM- 64) operating at Nor-Amberd and at Aragats research stations detect secondary neutrons. The Nor-Amberd muon multidirectional monitor (NAMMM) detects low energy charged particles and muons with energies above 350 MeV. The Aragats Multidirectional Muon Monitor (AMMM) registers high energy muon flux (threshold energy - 5 GeV). The Aragats Solar Neutron Telescope (ASNT) is measuring neutrons and charged particles. ASNT is part of a world-wide network coordinated by the Solar-Terrestrial Laboratory of the Nagoya Univer- sity. Another monitoring system, based on the scin- tillation detectors of the Extensive Air Shower (EAS) surface arrays, MAKET-ANI and GAMMA (3200 m a.s.l.), detects low energy charged particles. New world- wide particle detector networked, named SEVAN, is in operation now in Armenia, Bulgaria and Croatia [3], [4]. SEVAN detectors are measuring low energy charged particles, neutral particles (gammas and neutrons) and high energy muons. NAMMM and ASNT measuring channels are equipped with Amplitude-to-Digital (ADC) convertors and microcontroller based advanced electron- ics. Data Acquisition (DAQ) electronics and flexible software triggers allow to not only register the count rates of the detector channels, but also histograms of energy releases; correlations of the charged and neutral fluxes; and many other physical phenomena. Details of detector operation can be found in [4], [5]. Cosmic Ray flux incident on the terrestrial atmosphere and measured elementary particles on the Earth’s surface comprise very different entities although genetically connected with each other. Primary particles interactions with atmospheric nuclei and different meteorological effects can hide genuine variations of the primary flux and prevent from understanding of dynamics of ongoing physical processes in solar-terrestrial chain. For recovering the primary particles fluxes incident on the Earths atmosphere it is necessary to know the rela- tionship between observed count rates of the detectors and the primary particles fluxes, as well as the influence of the meteorological effects on the flux of secondary particles reaching Earth surface. Dorman’s theory of meteorological effects [6] gives detailed classification of the effects; mentioned the barometric one has a major influence on particle fluxes (at least for highest energies 10-100 GeV). Therefore, it is of greatest importance to accurately measure the barometric coefficients to ”unfold” the solar modulation effects. Besides this main goal there are several independent research problems connected with barometric coefficient dynamics: • rigidity dependence; • solar cycle phase dependence; • height dependence; • detected particle type dependence. All these dependences can be investigated at ASEC and by SEVAN network due to different altitudes, various detected particle fluxes and planned long-term operation. The main drivers of these dependences are changing