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Papers by Arun Babu

Astroparticle Physics, 2016

Abstract The GRAPES-3 large area (560 m 2 ) tracking muon telescope is operating at Ooty in India... more Abstract The GRAPES-3 large area (560 m 2 ) tracking muon telescope is operating at Ooty in India since 2001. It records 4 × 10 9 muons of energy ≥ 1 GeV every day. These high statistics data have enabled extremely sensitive measurements of solar phenomena, including the solar anisotropies, Forbush decreases, coronal mass ejections etc. to be made. However, prior to such studies, the variation in observed muon rate caused by changes in atmospheric pressure needs to be corrected. Traditionally, the pressure coefficient ( β ) for the muon rate was derived from the observed data. But the influence of various solar effects makes the measurement of β somewhat difficult. In the present work, a different approach to circumvent this difficulty was used to measure β , almost independent of the solar activity. This approach exploits a small amplitude (∼1 hPa) periodic (12 h) variation of atmospheric pressure at Ooty that introduces a synchronous variation in the muon rate. By using the fast Fourier transform technique the spectral power distributions at 12 h from the atmospheric pressure, and muon rate were used to measure β . The value of pressure coefficient was found to be β = ( − 0.128 ± 0.005 ) % hPa − 1 .

Astrophysics and Space Science, 2016

The relationship of Forbush decreases (FDs) observed in Moscow neutron monitor with the interplan... more The relationship of Forbush decreases (FDs) observed in Moscow neutron monitor with the interplanetary magnetic field (B) and solar wind speed (Vsw) is investigated in detail for the FDs associated with Interplanetary Coronal Mass Ejections (ICMEs) during 2001-2004. The classical two-step FD events are selected, and characteristics of the first step (mainly associated with shock), as well as of complete decrease (main phase) and recovery phase, are studied here. It is observed that the onset of FD occurs generally after zero to a few hours of shock arrival, indicating in the post-shock region that mainly sheath and ICME act as important drivers of FD. A good correlation is observed between the amplitude of B and associated FD magnitude observed in the neutron count rate of the main phase. The duration of the main phase observed in the neutron count rate also shows good correlation with B. This might indicate that stronger interplanetary disturbances have a large dimension of magnetic field structure which causes longer fall time of FD main phase when they transit across the Earth. It is observed that Vsw and neutron count rate time profiles show considerable similarity with each other during complete FD, especially during the recovery phase of FD. Linear relationship is observed between time duration/e-folding time of FD recovery phase and Vsw. These observations indicate that the FDs are influenced by the inhibited diffusion of cosmic rays due to the enhanced convection associated with B A. Bhaskar

Astronomy & Astrophysics, 2015

Aims. A Forbush decrease (FD) is a transient decrease followed by a gradual recovery in the obser... more Aims. A Forbush decrease (FD) is a transient decrease followed by a gradual recovery in the observed galactic cosmic ray intensity. We seek to understand the relationship between the FDs and near-Earth interplanetary magnetic field (IMF) enhancements associated with solar coronal mass ejections (CMEs). Methods. We used muon data at cutoff rigidities ranging from 14 to 24 GV from the GRAPES-3 tracking muon telescope to identify FD events. We selected those FD events that have a reasonably clean profile, and magnitude >0.25%. We used IMF data from ACE/WIND spacecrafts. We looked for correlations between the FD profile and that of the one-hour averaged IMF. We wanted to find out whether if the diffusion of high-energy protons into the large scale magnetic field is the cause of the lag observed between the FD and the IMF. Results. The enhancement of the IMF associated with FDs occurs mainly in the shock-sheath region, and the turbulence level in the magnetic field is also enhanced in this region. The observed FD profiles look remarkably similar to the IMF enhancement profiles. The FDs typically lag behind the IMF enhancement by a few hours. The lag corresponds to the time taken by high-energy protons to diffuse into the magnetic field enhancement via cross-field diffusion. Conclusions. Our findings show that high-rigidity FDs associated with CMEs are caused primarily by the cumulative diffusion of protons across the magnetic field enhancement in the turbulent sheath region between the shock and the CME.

The Astrophysical Journal, 2014

We examine the propagation of several CMEs with well-observed flux rope signatures in the field o... more We examine the propagation of several CMEs with well-observed flux rope signatures in the field of view of the SECCHI coronagraphs aboard the STEREO satellites using the GCS fitting method of Thernisien, Vourlidas & Howard (2009). We find that the manner in which they propagate is approximately self-similar; i.e., the ratio (κ) of the flux rope minor radius to its major radius remains approximately constant with time. We use this observation of self-similarity to draw conclusions regarding the local pitch angle (γ) of the flux rope magnetic field and the misalignment angle (χ) between the current density J and the magnetic field B. Our results suggest that the magnetic field and current configurations inside flux ropes deviate substantially from a force-free state in typical coronagraph fields of view, validating the idea of CMEs being driven by Lorentz self-forces.

Astroparticle Physics, 2017

The large area (560 m 2) GRAPES-3 tracking muon telescope has been operating uninterruptedly at O... more The large area (560 m 2) GRAPES-3 tracking muon telescope has been operating uninterruptedly at Ooty, India since 2001. Every day, it records 4 × 10 9 muons of ≥ 1 GeV with an angular resolution of ∼ 4 °. The variation of atmospheric temperature affects the rate of decay of muons produced by the galactic cosmic rays (GCRs), which in turn modulates the muon intensity. By analyzing the GRAPES-3 data of six years (2005-2010), a small (amplitude ∼ 0.2%) seasonal variation (1 year (Yr) period) in the intensity of muons could be measured. The effective temperature 'T eff ' of the upper atmosphere also displays a periodic variation with an amplitude of ∼ 1 K which was responsible for the observed seasonal variation in the muon intensity. At GeV energies, the muons detected by the GRAPES-3 are expected to be anticorrelated with T eff. The anti-correlation between the seasonal variation of T eff , and the muon intensity was used to measure the temperature coefficient α T by fast Fourier transform (FFT) technique. The magnitude of α T was found to scale with the assumed attenuation length ' λ' of the hadrons in the range λ = 80-180 g cm −2. However, the magnitude of the correction in the muon intensity was found to be almost independent of the value of λ used. For λ = 120 g cm −2 the value of temperature coefficient α T was found to be (−0. 17 ± 0. 02)% K −1 .

Physical Review Letters, 2016

arXiv: Solar and Stellar Astrophysics, 2014

Owing to our dependance on spaceborne technology, an awareness of disturbances in the near-Earth ... more Owing to our dependance on spaceborne technology, an awareness of disturbances in the near-Earth space environment is proving to be increasingly crucial. Earth-directed Coronal mass ejections (CMEs) emanating from the Sun are the primary drivers of space weather disturbances. Studies of CMEs, their kinematics, and their near-Earth effects are therefore gaining in importance. The effect of CMEs near the Earth is often manifested as transient decreases in galactic cosmic ray intensity, which are called Forbush decreases (FDs). In this thesis we probe the structure of CMEs and their associated shocks using FD observations by the GRAPES-3 muon telescope at Ooty. We have established that the cumulative diffusion of galactic cosmic rays into the CME is the dominant mechanism for causing FDs (Chapter 3). This diffusion takes place through a turbulent sheath region between the CME and the shock. One of our main results concerns the turbulence level in this region. We have quantitatively established that cross-field diffusion aided by magnetic field turbulence accounts for the observed lag between the FD and the magnetic field enhancement of the sheath region (Chapter 4). We have also investigated the nature of the driving forces acting on CMEs in this thesis. Using CME data from the SECCHI coronagraphs aboard STEREO sapcecraft, we have found evidence for the non-force-free nature of the magnetic field configuration inside these CMEs, which is the basis for the (often-invoked) Lorentz self-force driving (Chapter 5). Taken together the work presented in this thesis is a comprehensive attempt to characterise CME propagation from typical coronagraph fields of view to the Earth.

Cosmic ray Forbush decreases (FDs) are usually thought to be due to Earth-directed coronal mass e... more Cosmic ray Forbush decreases (FDs) are usually thought to be due to Earth-directed coronal mass ejections (CMEs) from the Sun and their associated shocks. When CMEs and their shocks reach the Earth, they cause magnetic field compressions. We investigate the relation between these magnetic field compressions and FDs at rigidities between 12 and 42 GV using data from the GRAPES-3 instrument at Ooty. We find that the shapes of the Forbush decrease profiles show a startling similarity to that of the magnetic field compression in the near-Earth IP medium. We seek to understand the implications of this interesting result.

The GRAPES-3 muon telescope in Ooty, India had claimed detection of a 2 hour (h) high-energy (∼20... more The GRAPES-3 muon telescope in Ooty, India had claimed detection of a 2 hour (h) high-energy (∼20 GeV) burst of galactic cosmic-rays (GCRs) through a > 50σ surge in GeV muons, was caused by reconnection of the interplanetary magnetic field (IMF) in the magnetosphere that led to transient weakening of Earth's magnetic shield. This burst had occurred during a G4-class geomagnetic storm (storm) with a delay of 1 2 h relative to the coronal mass ejection (CME) of 22 June 2015 [P. K. Mohanty et al., Phys. Rev. Lett. 117, 171101 (2016)]. However, recently a group interpreted the occurrence of the same burst in a subset of 31 neutron monitors (NMs) to have been the result of an anisotropy in interplanetary space [P. Evenson et al., Proc. Sci., ICRC2017 (2017) 133] in contrast to the claim in P. K. Mohanty et al., [ Phys. Rev. Lett. 117, 171101 (2016)]. A new analysis of the GRAPES-3 data with a fine 10.6°angular segmentation shows the speculation of interplanetary anisotropy to be incorrect, and offers a possible explanation of the NM observations. The observed 28 minutes (min) delay of the burst relative to the CME can be explained by the movement of the reconnection front from the bow shock to the surface of Earth at an average speed of 35 km=s, much lower than the CME speed of 700 km=s. This measurement may provide a more accurate estimate of the start of the storm.

The large area (560 m 2) GRAPES-3 tracking muon telescope has been operating uninterruptedly at O... more The large area (560 m 2) GRAPES-3 tracking muon telescope has been operating uninterruptedly at Ooty, India since 2001. Every day, it records 4 × 10 9 muons of ≥ 1 GeV with an angular resolution of ∼ 4 °. The variation of atmospheric temperature affects the rate of decay of muons produced by the galactic cosmic rays (GCRs), which in turn modulates the muon intensity. By analyzing the GRAPES-3 data of six years (2005–2010), a small (amplitude ∼ 0.2%) seasonal variation (1 year (Yr) period) in the intensity of muons could be measured. The effective temperature 'T eff ' of the upper atmosphere also displays a periodic variation with an amplitude of ∼ 1 K which was responsible for the observed seasonal variation in the muon intensity. At GeV energies, the muons detected by the GRAPES-3 are expected to be anti-correlated with T eff. The anti-correlation between the seasonal variation of T eff , and the muon intensity was used to measure the temperature coefficient α T by fast Fourier transform (FFT) technique. The magnitude of α T was found to scale with the assumed attenuation length ' λ' of the hadrons in the range λ = 80–180 g cm −2. However, the magnitude of the correction in the muon intensity was found to be almost independent of the value of λ used. For λ = 120 g cm −2 the value of temperature coefficient α T was found to be (−0. 17 ± 0. 02)% K −1 .

The relationship of Forbush decreases (FDs) observed in Moscow neutron monitor with the interplan... more The relationship of Forbush decreases (FDs) observed in Moscow neutron monitor with the interplanetary magnetic field (B) and solar wind speed (Vsw) is investigated in detail for the FDs associated with Interplanetary Coronal Mass Ejections (ICMEs) during 2001–2004. The classical two-step FD events are selected, and characteristics of the first step (mainly associated with shock), as well as of complete decrease (main phase) and recovery phase, are studied here. It is observed that the onset of FD occurs generally after zero to a few hours of shock arrival, indicating in the post-shock region that mainly sheath and ICME act as important drivers of FD. A good correlation is observed between the amplitude of B and associated FD magnitude observed in the neutron count rate of the main phase. The duration of the main phase observed in the neutron count rate also shows good correlation with B. This might indicate that stronger interplanetary disturbances have a large dimension of magnetic field structure which causes longer fall time of FD main phase when they transit across the Earth. It is observed that Vsw and neutron count rate time profiles show considerable similarity with each other during complete FD, especially during the recovery phase of FD. Linear relationship is observed between time duration/e-folding time of FD recovery phase and Vsw. These observations indicate that the FDs are influenced by the inhibited diffusion of cosmic rays due to the enhanced convection associated with the interplanetary disturbances. We infer that the inhibited cross-field diffusion of the cosmic rays due to enhanced B is mainly responsible for the main phase of FD whereas the expansion of ICME contributes in the early recovery phase and the gradual variation of Vsw beyond ICME boundaries contributes to the long duration of FD recovery through reduced convection–diffusion.

The GRAPES-3 tracking muon telescope in Ooty, India measures muon intensity at high cutoff rigidi... more The GRAPES-3 tracking muon telescope in Ooty, India measures muon intensity at high cutoff rigidities (15–24 GV) along nine independent directions covering 2.3 sr. The arrival of a coronal mass ejection on 22 June 2015 18:40 UT had triggered a severe G4-class geomagnetic storm (storm). Starting 19:00 UT, the GRAPES-3 muon telescope recorded a 2 h high-energy (∼20 GeV) burst of galactic cosmic rays (GCRs) that was strongly correlated with a 40 nT surge in the interplanetary magnetic field (IMF). Simulations have shown that a large (17×) compression of the IMF to 680 nT, followed by reconnection with the geomagnetic field (GMF) leading to lower cutoff rigidities could generate this burst. Here, 680 nT represents a short-term change in GMF around Earth, averaged over 7 times its volume. The GCRs, due to lowering of cutoff rigidities, were deflected from Earth’s day side by ∼210° in longitude, offering a natural explanation of its night-time detection by the GRAPES-3. The simultaneous occurrence of the burst in all nine directions suggests its origin close to Earth. It also indicates a transient weakening of Earth’s magnetic shield, and may hold clues for a better understanding of future superstorms that could cripple modern technological infrastructure on Earth, and endanger the lives of the astronauts in space.

Earth-directed Coronal mass ejections (CMEs) emanating from the Sun and the shock associated with... more Earth-directed Coronal mass ejections (CMEs) emanating from the Sun and the shock associated with it are the primary drivers of space weather disturbances. Forbush decrease precursors are advance warning of these upcoming magnetic field disturbances. GRAPES-3 tracking muon telescope which is a part of GRAPES-3 experiment located in Ooty, India, provides high statistics measurement of the muon flux with good temporal resolution. In this study we are using data from GRAPES-3 muon telescope and making use of its multidirectional observations to study the Forbush decrease precursors in greater detail. We have identified few Forbush decrease precursor signatures in muon flux well before the arrival of the actual shock. We can use these Forbush decrease precursors to study the characteristics of magnetic field compression associated with the upcoming CME shock-sheath system.

Astroparticle Physics, 2016

Astrophysics and Space Science, 2016

Astronomy & Astrophysics, 2015

The Astrophysical Journal, 2014

Astroparticle Physics, 2017

Physical Review Letters, 2016

arXiv: Solar and Stellar Astrophysics, 2014

The large area (560 m 2) GRAPES-3 tracking muon telescope has been operating uninterruptedly at O... more The large area (560 m 2) GRAPES-3 tracking muon telescope has been operating uninterruptedly at Ooty, India since 2001. Every day, it records 4 × 10 9 muons of ≥ 1 GeV with an angular resolution of ∼ 4 °. The variation of atmospheric temperature affects the rate of decay of muons produced by the galactic cosmic rays (GCRs), which in turn modulates the muon intensity. By analyzing the GRAPES-3 data of six years (2005–2010), a small (amplitude ∼ 0.2%) seasonal variation (1 year (Yr) period) in the intensity of muons could be measured. The effective temperature 'T eff ' of the upper atmosphere also displays a periodic variation with an amplitude of ∼ 1 K which was responsible for the observed seasonal variation in the muon intensity. At GeV energies, the muons detected by the GRAPES-3 are expected to be anti-correlated with T eff. The anti-correlation between the seasonal variation of T eff , and the muon intensity was used to measure the temperature coefficient α T by fast Fourier transform (FFT) technique. The magnitude of α T was found to scale with the assumed attenuation length ' λ' of the hadrons in the range λ = 80–180 g cm −2. However, the magnitude of the correction in the muon intensity was found to be almost independent of the value of λ used. For λ = 120 g cm −2 the value of temperature coefficient α T was found to be (−0. 17 ± 0. 02)% K −1 .

The relationship of Forbush decreases (FDs) observed in Moscow neutron monitor with the interplan... more The relationship of Forbush decreases (FDs) observed in Moscow neutron monitor with the interplanetary magnetic field (B) and solar wind speed (Vsw) is investigated in detail for the FDs associated with Interplanetary Coronal Mass Ejections (ICMEs) during 2001–2004. The classical two-step FD events are selected, and characteristics of the first step (mainly associated with shock), as well as of complete decrease (main phase) and recovery phase, are studied here. It is observed that the onset of FD occurs generally after zero to a few hours of shock arrival, indicating in the post-shock region that mainly sheath and ICME act as important drivers of FD. A good correlation is observed between the amplitude of B and associated FD magnitude observed in the neutron count rate of the main phase. The duration of the main phase observed in the neutron count rate also shows good correlation with B. This might indicate that stronger interplanetary disturbances have a large dimension of magnetic field structure which causes longer fall time of FD main phase when they transit across the Earth. It is observed that Vsw and neutron count rate time profiles show considerable similarity with each other during complete FD, especially during the recovery phase of FD. Linear relationship is observed between time duration/e-folding time of FD recovery phase and Vsw. These observations indicate that the FDs are influenced by the inhibited diffusion of cosmic rays due to the enhanced convection associated with the interplanetary disturbances. We infer that the inhibited cross-field diffusion of the cosmic rays due to enhanced B is mainly responsible for the main phase of FD whereas the expansion of ICME contributes in the early recovery phase and the gradual variation of Vsw beyond ICME boundaries contributes to the long duration of FD recovery through reduced convection–diffusion.

The effect of CMEs near the Earth is often manifested as transient decreases in galactic cosmic ray intensity, which are called Forbush decreases (FDs). In this thesis we probe the structure of CMEs and their associated shocks using FD observations by the GRAPES-3 muon telescope at Ooty. We have established that the cumulative diffusion of galactic cosmic rays into the CME is the dominant mechanism for causing FDs (Chapter 3).

This diffusion takes place through a turbulent sheath region between the CME and the shock. One of our main results concerns the turbulence level in this region. We have quantitatively established that cross-field diffusion aided by magnetic field turbulence accounts for the observed lag between the FD and the magnetic field enhancement of the sheath region (Chapter 4).

We have also investigated the nature of the driving forces acting on CMEs in this thesis. Using CME data from the SECCHI coronagraphs aboard STEREO sapcecraft, we have found evidence for the non-force-free nature of the magnetic field configuration inside these CMEs, which is the basis for
the (often-invoked) Lorentz self-force driving (Chapter 5).

Taken together the work presented in this thesis is a comprehensive attempt to characterise CME propagation from typical coronagraph fields of view to the Earth.

The large area (560 m 2) GRAPES-3 tracking muon telescope in Ooty, India recorded a 2 hour (h) mu... more The large area (560 m 2) GRAPES-3 tracking muon telescope in Ooty, India recorded a 2 hour (h) muon burst on 22 June 2015 starting at 19:00 UT in the midst of a continuing Forbush decrease. The burst occurred following the arrival of a coronal mass ejection containing a 40 nT southward interplanetary magnetic field (IMF) that had triggered a G4 class geomagnetic storm. During the 2 hour, the muon telescope recorded an excess of ∼ 10 6 muons with a significance of 54σ. Simultaneous occurrence of the burst was observed in 9 directions covering 2.3 sr field of view suggesting its origin close to Earth. Trajectory computation of cosmic ray primary protons followed by atmospheric simulation by superimposing the 2 hour IMF variation on geomagnetic field (GMF) could reproduce the observed profiles with a high degree of correlation (0.89 ± 0.05). The simulations could explain the burst profiles due to lowering of cutoff rigidity (0.5-0.7 GV) for 2 h which allowed an excess near threshold galactic cosmic rays (GCRs) to enter into the atmosphere. It showed that the near cutoff rigidity GCRs were deflected 210 • in longitude offering a natural explanation of its midnight detection by the GRAPES-3 experiment. This exciting observation has attracted considerable global attention following its publication in Physical Review Letters. Further studies of this event since then have shed more light on this phenomenon which will be presented during the conference.

The GRAPES-3 experiment is an array of ∼400 plastic scintillator detectors with a compact configu... more The GRAPES-3 experiment is an array of ∼400 plastic scintillator detectors with a compact configuration at Ooty in India. An accurate measurement of the gain of each detector is an important requirement for accurate determination of the energy of primary cosmic rays. The old method used earlier relied on manual placement of muon paddles for determining the signal produced by single muons in the detector. However, the number of detectors being rather large, it takes around 40 days to completete one round of calibration. Therefore each detector gets calibrated only 6-8 times in a year. However, the gain of detectors may vary significantly over this period. Also these detectors display gain variations due to changes in temperature. Since the estimated particle densities in an EAS depend on detector gains, their infrequent measurement may lead to incorrect estimate of shower size. In new method the gains are calculated on an hourly basis from the EAS data, which avoids the problems listed above.

The GRAPES-3 experiment, a high density extensive air shower (EAS) array located at Ooty, India i... more The GRAPES-3 experiment, a high density extensive air shower (EAS) array located at Ooty, India is designed for high precision measurements of cosmic ray energy spectrum and nuclear composition in energy range of 10 13 –10 16 eV. It consists of tightly packed 1 m 2 area 400 plastic scintillation detectors covering an effective area of 25,000 m 2. The trigger rate and particle density measured by EAS array shows strong correlation with the atmospheric parameters such as the pressure, and temperature. By employing linear regression, and Fast Fourier Transform techniques, the pressure and temperature coefficients for the dependence of trigger rate and particle density, respectively were obtained. Thereafter, by applying corrections for these effects the contribution of atmospheric effects was largely eliminated in the EAS data. These corrections are expected to result in a better measurement of EAS parameters which will be presented during the conference.

A large area (560 m 2) tracking muon detector operating in the GRAPES-3 experiment at Ooty in Ind... more A large area (560 m 2) tracking muon detector operating in the GRAPES-3 experiment at Ooty in India has been recording cosmic ray muons at a rate of 1.7× 10 8 h −1 since 2000. The high statistics data have enabled sensitive measurements of several solar phenomena to be made including the solar and sidereal anisotropy and Forbush decreases associated with coronal mass ejections. Prior to studying of any of these phenomena, an important task is to correct the variation in measured muon rates due to atmospheric pressure. Unfortunately, the pressure coefficient usually deduced from the observed data is not very reliable due to the presence of various solar phenomena listed above. Here, we present an alternative method which avoids complications arising from solar effects. Since the pressure at Ooty displays a 12 h periodicity, using which we could separate its contribution from other effects in the muon data though a power spectrum analysis. The method yielded a clear dependence of muon rate on pressure providing an accurate estimate of the pressure coefficient almost independent of the solar modulation effects.

The large area (560 m 2) GRAPES-3 tracking muon telescope has been operating at Ooty, India unint... more The large area (560 m 2) GRAPES-3 tracking muon telescope has been operating at Ooty, India uninterruptedly from 2001. It records 4 × 10 9 muons of energy > 1 GeV with an angular resolution of ∼4 • every day. The high statistical accuracy of this instrument helps in measuring the variations of galactic cosmic rays (GCRs) with a statistical precision of < 0.01% every hour. Here a study of solar modulation of GCR using GRAPES-3 is presented. We analysed uninterrupted GRAPES-3 data for six years (2005-2010) after applying the detector efficiency, pressure, and temperature corrections on this data to remove the effects of instrumental and atmospheric variations. Muon intensity, after applying these corrections shows an anti-correlation with the interplanetary magnetic field (IMF) from ACE/WIND spacecrafts. This anti-correlation of muon intensity with the IMF indicates that the solar modulations of GCRs may be caused by variations in the IMF. The dependence of GCR intensity variation and IMF is estimated. Further calculations were performed to study these variations by assuming the diffusion of GCRs through the turbulent magnetic field. The results show that the solar modulations of GCRs are largely due to the diffusion of GCRs though turbulent IMF in the heliosphere.

In the GRAPES-3 experiment, consisting of an array of ∼400 scintillator detectors, the arrival di... more In the GRAPES-3 experiment, consisting of an array of ∼400 scintillator detectors, the arrival direction of the shower is determined from the relative arrival times of particles at different detectors. The fixed arrival time of the signal from the detector to the measuring device, referred as time offset, is crucial for an accurate measurement of shower direction. In the older method the time offset of various detectors was measured with respect to a common detector. But this method proved to be ineffective since it took a long time (∼40 days) to complete one round of measurements. However, the time offsets vary with temperature due to change in the propagation delay in signal cables. Hence, a technique was devised to determine the time offsets on an hourly basis by using the shower data. In this method, the time offset between two neighbouring detectors was determined from the distribution of their relative arrival times. A random walk method was used to effectively determine the time offset with respect to a common detector. The accuracy of the new method was validated by using both the simulations, and EAS data which will be presented at the conference.

The large area (560 m 2) GRAPES-3 tracking muon telescope located at Ooty, India has been operati... more The large area (560 m 2) GRAPES-3 tracking muon telescope located at Ooty, India has been operating uninterruptedly since 2001. Everyday it records 4 × 10 9 muons of energy > 1 GeV with an angular resolution of ∼ 4 •. Atmospheric temperature variation affects the rate of decay of these GeV muons produced by the galactic cosmic rays (GCRs), which in turn modulates the intensity of detected muons. Since the daily temperature induced variation combines with the diurnal modulation of the GCRs by the magnetized solar wind, it becomes rather difficult to segregate the respective contributions of these two phenomena. A small seasonal variation in the intensity of cosmic ray muon (∼ 0.4%) with periodicity ∼ 1 year (Yr) was measured by analyzing the GRAPES-3 data of six years (2005-2010). The effective temperature 'T eff ' of the upper atmosphere above Ooty also displayed a similar periodic variation with an amplitude of ∼ 1K, which was responsible for the observed seasonal variation in the muon intensity. At GeV energies, the muons detected by the GRAPES-3 show an anti-correlation with T eff calculated by using a hadronic attenuation length λ. Using the fast Fourier transform (FFT) technique and making use of the anti-correlation between the seasonal variation of T eff with the muon intensity, we calculated the temperature coefficient α T. The magnitude of α T was found to scale with the assumed attenuation length λ , which we varied within a range of 80-180 gcm −2. However, the actual magnitude of the correction was found to be independent of the value of λ .

The GRAPES-3 experiment in Ooty, India has been operating a large area (560 m 2) tracking muon te... more The GRAPES-3 experiment in Ooty, India has been operating a large area (560 m 2) tracking muon telescope since 2000. It consists of 16 identical modules and each one is designed to measure the flux of muons in 13 × 13 directions covering 2.3 sr field of view. The high statistics data has enabled to probe transient space weather events on time scale of minutes. Due to independent nature of operation of the modules, despite intermittent failure of individual modules, a continuity in rate could still be achieved. By correcting for transient instrumental problems and gradual efficiency variations, an uninterrupted muon record is being assembled which may prove to be a valuable database for probing both transient and long-term solar phenomena. Details of the efficiency correction technique will be presented during the conference.

The GRAPES-3 experiment is a unique facility to study cosmic ray energy spectrum and composition ... more The GRAPES-3 experiment is a unique facility to study cosmic ray energy spectrum and composition with high precision. It consists an array of 400 plastic scinitillation detectors and a muon telescope of 3712 proportional counters to study extensive air shower (EAS) phenomenon around knee (≳ 10 15 eV). Study of energy spectrum and composition of primary cosmic rays (PCR) can improve the understanding about the nature of the sources accelerating PCRs to energies ≳ 10 15 eV which may be studied by using high statistics data produced by GRAPES-3. Measurement of particle densities (≳ 5000 m −2) is achieved by viewing each scintillator with two photomultiplier tubes (PMTs). The results obtained till date will be presented at the conference.