Shah, G N

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29th International Cosmic Ray Conference Pune (2005) 1, 351354Major Forbush Decrease recorded by Lead Free-Neutron MonitorG. N. Shah, S. Mufti, M. A. Darzi and P. M. IshtiaqNuclear Research Laboratory, Bhabha Atomic Research Centre, Srinagar - 190 006, IndiaPresenter: G. N. Shah (drgnshah@rediffmail.com), ind-shah-GN-abs1-sh26-oralA strong Forbush decrease was observed by the ground-based lead free Gulmarg Neutron Monitor (GNM) atHigh Altitude Research Laboratory - Gulmarg in response to the burst of solar eruptions on the sun at the endof October 2003. The GNM also records bunches of neutrons detected in pre-determined small time intervalsof 50 milli-seconds duration each, in groups of 1 to 9. The hourly integrated neutron intensity decreased toabout 16% and the percentage decrease in neutron bunch recording channels varied from about 9% in bunch-1to about 50% in bunch-5. This paper presents the results using the data recorded by this neutron monitor.1. IntroductionHighly energetic solar eruptions were produced on the sun at the end of October and the beginning of Novem-ber 2003. This interval had everything: very large sunspot regions; intense solar flares; particle events; andgeomagnetic disturbance. A wide range of effects were felt around the world due this remarkable solar activity.Two active regions (NOAA regions 0486 & 0488) located close to the centre of the visible solar disk producedthese eruptions causing a series of very intense geophysical, interplanetary and other magnetospheric effects.Ground based worldwide network of conventional neutron monitors have recorded the Forbush decrease fol-lowing the major solar flare that occurred on the sun on 28th of October and had X-ray importance of 17.2and heliographics co-ordinates as S18-E08 [1, 2, 3]. The inspection of the 3 minute resolution neutron countrecorded with three independent channels of the GNM reveal a strong Forbush decrease in overall integratedcount and also in the neutron bunches, coinciding with the decrease registered by several worldwide neutronmonitors and muon telescopes. This solar burst also resulted in decreased geomagnetic index Dst upto -300 nTat High Altitude Research Laboratory - Gulmarg indicating a large geomagnetic storm.2. Gulmarg Neutron MonitorGulmarg is a high rigidity (11.6 GV), mid latitude (34.07 N), high altitude (2743 m.a.s.l) station with ge-ographic longitude of 74.42 E. The neutron monitor at Gulmarg was previously operated as an IGY typeinstrument for more than two decades and provided many significant results during this period [4, 5, 6]. Thepresent system is a modified version of the original IGY type cosmic ray neutron monitor optimized to recordthe 2.45 MeV fusion neutrons supposed to be produced in the atmospheric lightning discharges and simulta-neously function as a low energy cosmic ray neutron detector. Three major modifications were made in theoriginal GNM to suite these requirements [7, 8, 9, 10, 11, 12]. These include the complete removal of theproducer lead and inner paraffin moderator from the system and the reduction in the thickness of the upperparaffin from 28 cms to 8 cms. This lead free neutron monitor consists of 21 enriched BF  counters, each 90cms long and 3.8 cm in diameter spread in the form of a pile over 28 cms thick paraffin slabs covering a totalsurface area of 3x10cm. The counters are covered with 8 cms paraffin wax slabs from above to sufficientlythermalize the 2.45 MeV D-D fusion neutrons and allow there detection with an overall efficiency of 3%. Theaverage efficiency was found experimentally using a Pu-Be neutron Source. The thick paraffin base helpsrender the monitor essentially opaque to low energy background neutrons entering it from below as it sets anenergy threshold of  50 MeV for Neutrons and  180 MeV for protons that can enter the monitor from belowdue to interaction of high energy cosmic rays with the ground. It also serves as a reflector of neutrons that352 G.N. Shah et. al.enter the pile from above. The count rate of about 3.6x10per hour of the neutron monitor is considerablylower than that of the IGY monitor because of the absence of lead and the presence of just half the mass ofparaffin compared to an IGY type neutron monitor. The monitor is divided into three separate channels eachcomprising of seven counters. The counters are segmented into three sections to prevent any chance of dis-turbance in the counters or electronics being recorded as a count due to neutrons. Each channel is connectedvia the electronic chain comprising of specific preamplifiers, amplifiers and discriminators to the registrationnode, which consists of a PC/AT and an RS-232C data acquisition interface. The registration node also collectsthe atmospheric pressure from a BCD multiplexed and interfaced bus of a digital barometer, which houses aMotorola semiconductors MPX4115A pressure sensor [13]. The registration node in the counting room isconnected to a remote analysis node in the data analysis room via an ethernet interface to allow monitoring ofthe individual channel data without disturbing the registration node and its data acquisition system. This givesan opportunity for checking the quality of data, comparing directly with stations having their data displayedon the internet in realtime, and correction for the meteorological effects. From the analysis node we can checkthe three minute interval data of all channels separately and together. The pressure coefficient of the monitorwas determined by using data on 14 major atmospheric pressure troughs recorded at the site of observatory aswell as uninterrupted data of 120 days of neutron count rate and atmospheric pressure. Based on this data aregression analysis of the atmospheric pressure and neutron count rate reveals a barometric pressure coefficientof -0.845% per mbar, in close agreement with reported values [14].A neutron bunch detector is incorporated with the neutron monitor to record rates of neutron pulses of upto 9detected neutrons in a predetermined small time interval of 50 milli-seconds each. Thus the monitor is capableof providing us rates on the detection of neutrons coming in groups of 1 to 9 in predetermined time interval.The detector receives neutron pulses from 21 BF  counters of the monitor, which appear as a train of neutronpulses. This is accomplished by opening a time gate initiated by first neutron count and adding the subsequentcounts that occur during the gate length. The total number of counts in each event determines the bunch level.3. Data analysis and resultsThe low energy GNM recorded the Forbush decrease on october 29th with 0951 Hours UT as the onset timeas a consequence of X17.2/4B class solar flare on October 28th, 2003.The identification of the onset time hasbeen made from the three-minute resolution data available from the monitor. The pressure corrected hourlyintegrated intensity from the GNM for the period October 26 to November 5 has all the features of the Forbushdecrease as recorded by other worldwide conventional neutron monitors. The decrease in the integrated countsrecorded by the GNM is about 16% following the onset of the event. We present in figure. 1. pressurecorrected hourly neutron count rate (%) for the period October 26 to November 5. The 100% countrate is takenas average of 72 hours of counts occurring before the onset time of major decrease on 29th of October 2003.It is clearly seen that with onset time of 0951 Hours UT, the sudden amplitude decrease of 16% is followed byslow recovery. Also presented in figure. 2. are the variations observed in neutron bunch data from bunch 1 tobunch 5. The decrease in neutron bunches coincides with the decrease registered by the individual channels ofthe neutron monitor. It is seen from the figure. 1. and figure. 2. that the the onset time and profile of Forbushdecrease as observed in integrated neutron count and bunch data is similar. However, the rates recorded inbunches 6 to 9 are very low and scanty and are not presented here. The decrease similar to that observed infigure. 1. is also seen in figure. 2. ( plots b,c,d,e,f). In figure. 2. (plot a) we present bunch no. Vs the %decrease. It is observed that percent decrease increases from about 9% in bunch-1 to about 50% in bunch-5.The percentage error increases as one moves from lower bunches to higher bunches.Major Forbush Decrease ...... 353 80 85 90 95 100 105 11026 27 28 29 30 31 01 02 03 04 05Countrate [%]October / November 2003Integrated CountFigure 1. Pressure corrected hourly neutron count rate [%] of the GNM from October 26 to November 5, 2003 overlappingthe occurrence of a X17.2/4B class Solar are with onset at 9:51 Hrs UT on 28 October.4. DiscussionWe presented experimental evidence of recording a major Forbush decrease as a result of the extremely highactivity on the Sun and in the heliosphere that took place in October-November 2003 during the decliningphase of the 23rd solar cycle. During this period complex plasma and magnetic disturbances were formed asa result of the gushing solar wind. The coronal mass ejection initiated by this flare was emitted at a very highspeed and was directed towards the earth. It passed the Earth after less than a day and was followed by a region,depleted of cosmic rays and possibly resulting in a Forbush decrease of about 16% in the hourly integrated andthe bunch counting rates of the ground based GNM. The results indicate effectiveness of observing the solarmodulation effects like Forbush decrease with the help of a lead-free neutron monitor.354 G.N. Shah et. al. 0 20 40 60 80 100 0  1  2  3  4  5  6% Decrease (a) 90 95 100 10526 27 28 29 30 31 01 02 03 04 05Count rate [%](b) 80 90 100 11026 27 28 29 30 31 01 02 03 04 05Count rate [%]Oct/Nov 2003(c) 60 80 100 12026 27 28 29 30 31 01 02 03 04 05Count rate [%](d) 40 80 12026 27 28 29 30 31 01 02 03 04 05Count rate [%](e) 0 40 80 120 16026 27 28 29 30 31 01 02 03 04 05Count rate [%]Oct/Nov 2003(f)Figure 2. (a) % Decrease in neutron bunches as one moves from bunch-1 to bunch-5. Plots (b,c,d,e,f) show variation in theneutron Bunch countrate from October 26 to November 5 overlapping the occurrence of a X17/4B class Solar are withonset at 9:51 Hrs UT on 28 October.References[1] I. S. Veselovsky et al., Cosmic Research, Vol. 42, No. 5, (2004).[2] M.R. Moser et. al., COSPAR 2004, Paris, (2004).[3] I. M. Chertok et al., Space Weather, (2004).[4] M. M. Bemalkhedkar et al., Proc. of 13th ICRC, Denver, 2 , 1293, (1973).[5] M. M. Bemalkhedkar, Ph. D. Thesis., Univ. of Gujrat, (1974).[6] S. P. Agarwal et al., J. Geophys. Res., 79, 2269, (1974).[7] G. N. Shah et al., 18th ICRC, Banglore, 3, 555, (1983).[8] G. N. Shah, Ph. D. Thesis., Univ of Kashmir, (1985).[9] G. N. Shah et al., Nature Vol. 33, 773, (1985).[10] P. H. Stoker et al., Space Sci. Rev. 93, 361, (2000).[11] P. H. Stoker et al., 16th ICRC, Kyoto, 4, 358, (1985).[12] Z. Y. Bang and S. Kawasaki, 18th ICRC, Banglore, (1983).[13] P. M. Ishtiaq et al., INCON-2004, Pune, (2004).[14] V. E. Sdobnov et al., 17th ICRC, Paris, 4, 338, (1981).

Major Forbush Decrease recorded by Lead Free-Neutron Monitor

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