Forbush decrease

The GRAPES-3 is a dense extensive air shower array operating with ∼400 scintillator detectors and it also contains a 560 m2 tracking muon detector (Eμ>1GeV), at Ooty in India. 25% of scintillator detectors are instrumented with two fast photomultiplier tubes (PMTs) for extending the dynamic range to ∼5×103∼5×103 particles m−2. The scintillators, signal processing electronics and data recording systems were fabricated in-house to cut costs and optimize performance. The muon multiplicity distribution of the EAS is used to probe the composition of primary cosmic rays below the ‘knee’, with an overlap with direct measurements. Search for multi-TeV γ-rays from point sources is done with the aid of the muon detector. A good angular resolution of 0.7° at 30 TeV, is measured from the shadow of the Moon on the isotropic flux of cosmic rays. A sensitive limit on the diffuse flux of 100 TeV γ-rays is placed by using muon detector to filter the charged cosmic ray background. A tracking muon detector allows sensitive measurements on coronal mass ejections and solar flares through Forbush decrease events. We have major expansion plans to enhance the sensitivity of the GRAPES-3 experiment in the areas listed above.

Abstract: Major solar flare events have been selected to study the heliographic distribution of solar flares during solar cycle 23. The occurrence of Forbush decreases (FDs), bright solar flares; solar flare index, coronal mass ejections (CMEs), average solar magnetic field and ...

Since data-taking began in January 2004, the Pierre Auger Observatory has been recording the count rates of low energy secondary cosmic ray particles for the self-calibration of the ground detectors of its surface detector array. After correcting for atmospheric effects, modulations of galactic cosmic rays due to solar activity and transient events are observed. Temporal variations related with the activity of the heliosphere can be determined with high accuracy due to the high total count rates. In this study, the available data are presented together with an analysis focused on the observation of Forbush decreases, where a strong correlation with neutron monitor data is found.

Since data-taking began in January 2004, the Pierre Auger Observatory has been recording the count rates of low energy secondary cosmic ray particles for the self-calibration of the ground detectors of its surface detector array. After correcting for atmospheric effects, modulations of galactic cosmic rays due to solar activity and transient events are observed. Temporal variations related with the activity of the heliosphere can be determined with high accuracy due to the high total count rates. In this study, the available data are presented together with an analysis focused on the observation of Forbush decreases, where a strong correlation with neutron monitor data is found.

The Forbush decrease following the large X2 solar flare on mid-February 2011 has been observed by the muon telescopes of the EEE Project, which are located in several Italian sites and at CERN. Data from two different telescopes of the EEE network have been analyzed and compared to those measured by neutron monitor stations. The variation of the muon counting rate during the Forbush decrease was also extracted for different intervals of the azimuthal angle of the incoming muons.

The cosmic ray variations consequent upon the interplanetary disturbances during the solar active period of late October 1989 have been investigated by using the data recorded with the Lead-Free Gulmarg Neutron Monitor (LFGNM). Apart from recording total neutron count, it is also set to record neutron bunches with occurrence rates of 1-9 in short time intervals of 50 ms duration. Whereas the Forbush decrease (FD) profile observed in the total hourly count rate of this monitor compares well with the conventional neutron monitors, the neutron bunch-1 data reveals clear and large amplitude diurnal variation that persisted for about 10 consecutive days with average phase close to ˜1700h direction. The power spectrum analysis of the neutron bunch-1 data by maximum entropy method (MEM) further revealed presence of semi- and tri-diurnal components also. The most intriguing result on anisotropic variation observed with LFGNM in a distinct mode of recording bunch-1 neutrons is unique thus far, though consistent with the strong unidirectional diurnal anisotropy observations made by conventional neutron monitors during the said period. We report the observations and the results in this work.

Muon rate variations during Forbush effects registered by means of muon detectors DECOR, TEMP and URAGAN operated in the experimental complex NEVOD (MEPhI, Moscow) have been studied, and comparative analysis with neutron monitor data has been performed. The ratio of values of Forbush decreases in muon and in neutron fluxes is about one third, and preliminary rigidity dependence of Forbush decrease amplitude using muon data has been also obtained (for 2.4 GV cut-off rigidity). The detection of muon flux in the hodoscopic mode allows to study the dynamics of muon flux anisotropy related with magnetic field perturbations. Results of analysis of data from the new unique muon detector URAGAN indicate the change of muon flux asymmetry direction during the Forbush decrease. This phenomenon is related with the motion of solar plasma cloud and hodoscope acceptance cone relative to each other.

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.

Coronal mass ejections (CMEs) are the solar events which carry large amounts of plasma and magnetic fields into the heliosphere and are responsible for interplanetary shocks, geomagnetic disturbances in earth's magnetosphere and Forbush decreases in cosmic ray intensity. ...

We have used Simple Denoising Algorithm using Wavelet Transform on the daily Forbush decrease data from the year 1967 to 2003. For this data we observe periodicity around 5-6, 11, 13, 15 and 24 years. For all the obtained peaks corresponding confidence levels are higher than 95%. We observe that the periodicity of around 5-6 years is common to solar flare data, major proton event data and solar neutrino flux data. Because of that common periodicity, it is suggested that Forbush decrease with the solar flare data and major solar proton event data together with solar neutrino flux variations, behave similarly and may have a common origin.