Shrivastava, Pankaj K

Shrivastava, Prashanti

IACS Institutional Repository

Indian J. P hys. 7 6 B  (6 ), 6 9 9 -7 0 3  (2 0 0 2 )UP B' an international journalA  c o m p a ra tiv e  stu d y  o f F o rb u sh  d ecrease events o f F e b ru a ry  a n dA u g u st, 1 9 9 9P rashan ti S h riv astav a  and Pankaj- K  Shrivastava*  Govenimcnl New Science College, Rcwa-486 001, Madhya I^adcsh, India U-mail pankaj-in_2001 (^ ^^ .rcdifimailxoniReceived 16 M y  2001. accepted 24 September 2001Abstract ‘ Forbush decrease in cosmic ray intensity shows a sudden decrease in count rales of neutrons followed by a slow recovery, typically lasting for several days Two such Forbush decrease events noted first in August 19 to 29, 1999 and another in February 15 to 23, 1909 occurred during a high solar activity period in 1999 (i is noteworthy that both the events were accompanied with several intense solar (lares and geomagnetic stroms. In this paper, we made a comparative analysis of the characteristics and production mechanisms o f the events, t urthcr, wc have found out a significant correlation between cosmic ray intensity variation and geomagnetic activity index Dst Decrease in Dat values shows a similar pattern ol cosmic ray decreases, indicating a strong relationship between the two Results of analysis also suggest cl strong relationship between geomagnetic aclivitv and cosmic ray intensity variation on short-term basis.keywords 1‘orbush decrease, solar Hares, magnetic stromsPACS Nos. ; 96.50.Fk, 96 60 Rd, 94 30 Tr1. I n t r o d u c t io nT ran sien t a n d  ra p id  d e c re a s e  in c o s m ic  ra y  in te n s i ty  follow ed by  a  s lo w  reco v e ry  is c a lled  as a  F o rb u sh  decrease  event in  c o sm ic  ray  in ten s ity . T h e  v a rio u s  co sm ic  ray  researchers e s ta b lish e d  th a t th e se  F o rb u sh  d ec reases  are  produced b y  p e rtu rb a tio n  in in te rp lan e ta ry  co n d itio n  and  that th ese  p e rtu rb a tio n s  o rig in a te  e ith e r from  so la r fla res or f ro m  m a g n e t i c  f i e ld  s t r u c tu r e s  a s s o c ia te d  w ith  in terp lanetary  so la r w in d  s tream s [1 ,2 ]. T h e  pertu rb a tio n  could b e  p ro d u c e d  fro m  sh o ck  w aves, co ro n a l m ass e jec tion  and flare  g e n e ra te d  h ig h  so la r  w ind  stream s [3 ,4]. In 1975, B arouch  a n d  B u r la g a  [5 ] h a v e  re p o r te d  th a t th e  h ig h  m ag n e tic  f ie ld  re g io n s  in  th e  in te rp la n e ta ry  sp a c e  a re  a sso c ia te d  w ith  F o rb u s h  d e c re a s e . F u r th e r , th a y  h a v e  dem onstra ted  th a t th e se  co sm ic  ray  dec reases a re  n o t re la ted  to the  tu rb u le n t o r  ra n d o m  m o tio n s  in the  fie ld , w h ile  o n ly  the reg io n s  o f  h ig h  fie ld  s tren g th  in  in te rp lan e ta ry  space  a re  found re sp o n s ib le  fo r  c a u s in g  F o rb u sh  d ec reases . T h ese  regions c o n s is t o f  m a g n e tic  b io b es  and  m ag n e tic  c lo u d s  ejected fro m  a c tiv e  so la r  reg io n s. In  the  en v iro n m en t o fCorresponding Authors h o c k s ,  h ig h  s p e e d  s t r e a m s  a re  s im p ly  t a n g e n t i a l  d isco n tin u ities  [5].lucci e t a l  [6 ] in v es tig a ted  th e  id ea  o f  sp ira l co n e  like reg io n  w h ich  ex ten d s  a lo n g  the  in te rp lan e ta ry  m ag n e tic  fie ld . F urther, they  h av e  show n  th a t in a  tw o -s tep  d ec rease , the first s tep  is sym m etric  and  is d u e  to  sh o ck  p assag e , w h ereas th e  seco n d  step  is s tro n g ly  a sy m m etric  a n d  is due  to  th e  e n tire  m a g n e tic  f ie ld  an d  w ith  a  lo o p  lik e  f ie ld  con figu ra tion .T h ey  have a lso  d e te rm in ed  th e  sep a ra te  co n fig u ra tio n  o f  shock  fro n t and  th e  m ag n e tic  p e rtu rb a tio n  d u rin g  th e  p e rio d  o f  F o rbush  d ecrease . In  th is  study , w e h av e  co m p ared  th e  cha rac te ris tic  o f  F o rb u sh  d ec rea se  ev en ts  o f  A u g u s t 19 to  29 , 1999 an d  F eb ru a ry  15 to  2 3 , 1999.2 . D a ta  a n a ly s isW e h av e  id en tif ied  th e se  tw o  ev en ts  o f  F o rb u sh  d ec rea se s  from  th e  h o u rly  p lo ts  o f  tw o  n e u tro n  m o n ito rs  K ie l an d  C lim ax . D a ily  m ean  v a lu e s  o f  co sm ic  rays h av e  b een  u sed  in th is  an a ly s is  fro m  th e se  tw o  s ta tio n s  s itu a te d  a t  tw oO 2002 lACS700 Prashanti Shnvasiava and Pankaj K Shrivastavad iffe re n t lo c a tio n s  (T a b le  I). W e have u sed  the  A p /D st  in d ices  fo r g e o m a g n e tic  a c tiv ity  and  sun  sp o t n u m b ers /so la r fla re s  are  ta k e n  as re liab le  so la r p a ram ete rs  o f  so la r ac tiv ity .Table 1 .1 .ocalion ol neutron monitors.Cieographicall,ati tilde (degree)No Station name Longitude(degree)C’aigar>- Goose Bay riiriiax Moscow Kiel51 OH N 53.27 N 39 37 N 55.47 N 54.3 N245 91 W 60 40 W 106 IH W 37 32 £ 10 1 BCut-off rigidity GV1 090,692.972412,23C osm ic  ray s , g eo m ag n e tic  and  so la r ac tiv ity  data  fo r th e  p e rio d  o f  F e b ru a iy , 1999 h ave  been  tak en  from  p ro m p t rep o r t o f  S o la r g eo p h y sica l d a ta  book  [7J, A ll these  da ta  sets for A ugust, 1999 ev en t have  been  tak en  from  the p rom p t re p o r t o f  S o la r g eo p h y sica l d a ta  b o o k  [8 ].3 . R e su lts  a n d  d is c u ss io nF igu re  1(a) d isp lay s  th e  d a ily  av e rag e  co sm ic  ray  in tensity  from  1 A u g u s t to  9  S ep tem b er, 1999, as reco rd ed  by K iel an d  C lim ax  g ro u n d  b a sed  n eu tro n  m o n ito rs . M id d le  and u p p e r p an e ls  o f  F ig u re  1(a) sh o w  the  d a ily  va lues o f  cosm ic  ray  in tensity  fo r  K iel an d  C lim ax  s ta tion  resp ec tiv e ly , and  lo w er p an e l o f  F ig u re  1 sh o w s the ra tio  o f  K iel and  C lim axIv\8DaysFigure 1(a). Shows the daily average counts of Kiel and Climax neutron monitors for August, 1999. Ratio of Kiel and Climax neutron counts are also plotted in lower panel of figure.d a ily  co u n ts  o f  n e u tro n  m o n ito r  da ta . S im ila rly , F ig u re  1(b) sh o w s th e  s im ila r  p lo ts  fo r  th e  p e rio d  o f  F eb ru a ry  7 to  28 , 1999. W e can  o b se rv e  s ig n ific an t co sm ic  ray  d ec rease  in b o th  th e  p lo ts . O n ly  d iffe ren ces  a re  n o tic e d  in  th e  ra tio  o f  K ie l an d  C lim a x  d a ta .DaysFigure 1(b). Shows the daily average counts of Kiel and C'limax neutron monitors for Februaiy/, 1999 Ratio of the daily average counts at Kiel and Climax is also plotted in lower panel.W e have p lo tted  the  d a ily  va lues co sm ic  ray  in tensity  of K iel and  C lim ax  n eu tro n  a lo n g w ith  th e  ra tio  o f  K iel and C lim ax  co u n t rates. D ecreases  in co sm ic  ray  in tensity  arc w ell d ep ic ted  in m idd le  and  up p er panel o f  the figu re . Ratios o f  K iel an d  C lim ax  co sm ic  ray  in ten s ity  a re  .shown in the u p p er panne! o f  the  figure. D u rin g  th e  p e rio d  o f  February, 1999 even t, the  ra tio  o f  K iel and  C lim ax  n eu tro n s  vary  by m ore  than  30% . O n th e  o th e r h and , v a ria tio n  in  ratio  is n early  co n stan t d u rin g  th e  pe rio d  o f  A u g u st, 1999 event. T h ese  resu lts  ind ica te  th a t th e  cosm ic  ray  p a rtic le s  get into m o d u la te d  in d if fe re n t w ay  in  d if fe re n t in te rp lan e ta ry  co n d itio n s . T h e  ra tio  o f  co sm ic  ray  co u n ts  o f  K iel/C lim ax n eu tro n  ge t in to  m o d u la ted  d ue  to  th e  la rg e r in fluence  of in te rp lan e ta ry  sh o ck  w av es  d u rin g  th e  p e rio d  o f  February, 1999. 'Ih e  sh o ck  sh ie ld s m o re  co sm ic  ray  p a rtic le s  in high lo n g itu d e  o f  w este rn  zo n e  th a t low  lo n g itu d e  o f  eastern zone  o f  th e  earth .F igu re  2 (a ) show s th e  F o rbush  d ecrease  ev e n t o f  August, 1999. O nse t tim e  o f  th is ev en t is in ea rly  h o u rs  A ugust, 1999 and  it rem ain s low  d u rin g  fo u r to  five  days. R ecovery p e rio d  o f  th is  F o rbush  dec rease  ev en t is fiv e  days. The F ig u re  show s cosm ic  ra y  in ten sity  p lo t fo r sev e ra l neutron m o n ito r s ta tio n s s itu a ted  at, C a lg a ry , G o o seb ay , M oscow , C lim ax , K iel, B e ijing  a n d  H a leak a la  co v e rin g  a  w id e  range o f  c u t-o f f  rig id ity . D a ily  v a lu es o f  D st in d ex  a re  p lo tted  in u p p e r p ane l o f  F igu re  2 (a ) fo r  th e  e v e n t o f  A u g u st 1999. S evera l in tense  so la r fla res  a re  o b se rv ed  d u rin g  th e  eventA comparative study ofForbush decrease events o f February and Auy;ust, 1999 701period. Solar flares of 17th August which occurred in NOAA region of 8668 seems to be more powerful in producing decrease in cosmic ray intensity. Further, solar flare events of 19, 20 and 21st August in same NOAA region are also supported to enhance the decrease in cosmic ray intensity.We have also noted geomagnetic storms on 17 and 18 February, which incidentally coincides with the maximum decrease of cosmic ray intensity.Aug. 99 Sep. 99Figure 2(a). Shows the plots for hourly values of cosmic rays for several neutron monitors for August, 1999 Forbush decrease event. Dst values aic plotted in upper panel.Major solar flare events of August 20 and 21 are responsible for the maximum decrease on August 23. A series of geomagnetic events occurring on August 16, 17, 18 and 22 provided a possible solar terrestrial relationship during the period of Forbush decrease.Similarly Figure 2(b) shows the plot of Forbush decrease event of same neutron monitor stations for the period of February 15 to 23, 1999. The values of Dst index for the period of February 1999 are also plotted in the Figure. Significant variations in Dst values were noticed during the period of the events. Onset time of this event was February 17 and it remained low during two days. Recovery period of this event was four days. We noticed two major solar flares on February 15 and 16 on Noaa region of 8462 which might have affected large decrease in cosmic ray intensity.F igure 2(b). Shows the plols lor cosmic ray intensity variation for .several neutron monitors tor February, 1999 Dst values arc also plotted in upper panelFurther, we have plotted the daily values of Sunspot number {Rz) and Ap index for both the events as depicted in the Figure 3(a) for August 1999 and 3(b) for February, 1999 respectively. Low Rz values and high Ap values were seen during the period of Forbush decrease event of August 1999 and similar by high Ap values were observed duringOAp «Rz IcS lllLUiiitilAlMAug. 99 DaysB 150-<SL10050-^ ^OAp ORz]1 .C,dLm,dUB4jl jl,lljil 1 1 1 1  ll IIFeb. 99 DaysFigure 3. (a) Shows the daily values of sunspot number Hs and geomagnetic disturbance index Ap for the month of August, 1999, which covers the Fd event (August 19 to 29, 1999). (b) Same as in Figure 3(a) but for February, 1999, which covers the fW event of Februaiy 15 to 23, 1999.702 PrasharUi Shrivastava and Pankaj K Shrivastavathe event period of February, 1999. However, high sunspot numbers were also observed during this event period. In this way, we observed a difference in variation of sunspot numbers during these two events. We also observed a significant pre-increase in Rz values.Conditions of interplanetary medium play an important role in energy transmission from solar wind to magnetosphere which significantly modulated cosmic ray particles. Forbush decrease events as we observed during August and February, 1999, arc examples of such events. To draw a possible relationship between cosmic ray decrease and interplanetary features, we have plotted the daily values of solar wind velocity (SWV). solar magnetic field (SMF) and interplanetary magnetic field B (nT) as shown in Figure 4(a) and 4(b) for August and February events respectively.Aug ,99 Ctoy»f^,99 OtyaFigure 4. (a) Shows the daily values of solar wind velocity, solar magnetic field and interplanetary magnetic field for August. 1999. (b) Same as in Figure 4(a) bur for February, 1999.Here, B (w7) shows the total magnetic field values of interplanetary medium, which were monitored by IMP-g space craft in GSE co-ordinates. Solar wind velocity are also monitored by the detector on board of lMP-8 space craft. Solar magnetic field values were different from these two interplanetary features and magnetic field on solar disc. It is seen from the Figure 4(a) that solar magnetic field is lower during event periods.Simultaneously, we have observed high solar wind speed and B (nT) period to event periods. Occurrence of several inicn.se solar flare along with flare generated solar wind streams are expected to causes of Forbush decrease event c»f August, 1999. Similarly, we have plotted daily values of all the.se parameters in Figure 4(b) for the February, 1999 event Higher values of B (nT) coincide with the event period and lower solar magnetic field strengths also are seen for event February, 1999 event. Solar wind velocity data were not available during event period to draw any meaningful conclusions. Hence, we infer the similar interplanetar}' conditions for both the events. However, variation in cosmu ray intensity of event periods, depend on the larger/smaller disturbances in interplanetary condition. High speed solar wind streams particularly generated by solar flare, are found to be more responsible factors in producing transient decreases in cosmic rays. Most dominatent and important features in interplanetary medium are with solar wind plasma streams (high speeds), which produce large decrea.se in cosmic ray intensity. Ihe dccrea.se in cosmic ray intensity is expected due to the shielding of cosmic ray particles by the shock fronts produced by flare generated high speed solar wind stream. This result is consistent with previous results reported by Shrivastava and Shukla [4] in 1994 ITicy observed that the flare generated high speed solai wind streams are more effective in modulating the cosmic ray intensity.It is clear from the figures that the cosmic ray intensity decreases with the decrease of Dst values. Variational profiles of fXst index for these two event periods vary according to variational profiles of Forbush decrease events. A large and narrow decrease in Dsf index during the period of February* 15 to 23 coincides with the main phase of Forbush decrease event of February, 1999. Similarly, we also observed a significant decrease in Dst index which coincides with decreases of cosmic ray intensity for entire period of Forbush decrease event August, 1999. In both the events, we have observed significant decreases in cosmic ray intensity as well as geomagnetic Dst index during the main phase and recovery phase of storm.The relationships between the solar flare and cosmic ray intensity decrease observed during these two events w ere similar to that found in previous investigation [9]. The so larA  c o m p a r a t i v e  s t u d y  o f  F o r b u s h  d e c r e a s e  e v e n t s  o f  F e b r u a r y  a n d  A u g u s t ,  1 9 9 9 703Hare effect has the north south asymmetry. The flare located ,n northern hemisphere can cause longer disturbance in cosmic ray intensity than in southern hemisphere.One sudden storm commencement (SSC) was observed on February 17, 1999 in association with the Forbush decrease event of Frbniary, 1999. As we known SSC is a signature of arrival of shock waves. Therefore, it can be inferred that shock waves also play an important role in producing large cosmic ray decreases.Significant fluctuations were seen in geomagnetic D s t  index during both the periods of Forbush decrease. The values of D s t  index show significant transient decrease in similar pattern as that of Forbush decrease. The geomagnetic distrubance index D s t  recorded at low latitude which comes from the outward blowing zonal current system called the ring current. The D s t  index gives the average depression of iiorizontal component in unit of n L  which is proportional to the total kinetic energy of symmetrically distributed particles injected and trapped in the Van Allen belt. It is assumed that the massive compression of the magnetosphere and enormous intensification of the large scale magnetosphere current system reflected in D s t  lead to a significant geomagnetic effect on cosmic ray measurement near the earth.It is known that coronal mass ejections (CMEs) events are generally associated with solar flares. CMEs have considerable importance towards our understanding of the heliospheric disturbances because of the open out and expand amout of mass and energy injected into interplanetary medium. In a number of recent studies CMlis are investigated as geomagnetic and cosmic ray disturbances [10]. We have noticed a number of CMEs events during both August and February events of Forbush decreases. Wc observed two CME events on 17 and 22 August. Similarly, wc observed a series of CME events on 12, 13,14, 20 and 21 February. Now, we can say that the CMEs are also one of the factors in producing short-term decreases on short-term basis.4. ConclusionsFrom the comparative analysis of these two Forbush decrease events of August and Frbruary, 1999, it can be concludedthat these events were found very much influenced by the magnetic field of the earth. Both the events were found to be associated with geomagnetic storms. Their main phases also coincide with maximum decrease in D s t  values. This leads to confirm the ring current effect on cosmic ray intens|y modulation. We can distinguish these events only on th^  basis of Sudden Storm Commencements (SSCs). Occui|ence of SSC event on 17th February 1999, clearly exhibit the effects of interplanetary shock waves on cosmic ray in|ensily which shows a step like decrease in shorter time slaie. However, effects of intense solar flare on cosmic ray delcrease are significantly seen in both these events. It is fouhd that the CMEs are also one of the factors in interplanetary medium to produce geomagnetic disturbances and d^ ression in cosmic ray intensity. However, occurrence of large number of CMEs during February, 1999 produce larger decrease in comparison to Forbush decrease event of August, 1999, vifhere we observed less number of CMEs.AcknowledgmentThe authros are thankful to World Data Centre (A) for providing the cosmic rays and solar data.References| l ]  E N Parker In le rp la n e la i^  D yn a m ica l P ro cesses  (New York : Inlcrsciencc, John V iley and Sons) (196.1)[2| A Kadokura and A Nishida J  Geophys Res 91 13 (1986)|3| H V Cane and I G Richardson J  G eophys Res 100 1755 (199.5)|4] P K Shrivastava and R P Shukla S o lar P hysics 154 177(1994)|5| 1- llarouch and I. F Buriagad. Geophys Res 80 449 (1975)(6J N lucci, M  Parisi, C Signorini and G Velio Rest Nuov Cim 7C(1984)(7| Solar Geophysical Data (Prompt Report Compiled by N f i lX ’, Boulder, Colorado) Vol. 653 Part I p i 14 (1999)|S) Solar Geophysical D ata  (Prompt Report Compiled by N G IX ', Boulder, Calorado) Vol. 663 Part I p 121 (1999)[91 Xue Shunshong and Yc Zhonghei Proc 2 l- s t  Inti Cosm ic Ray C onf (Adeliad, Australia) Vol. 6 p 241 (1990)|101 P K Shrivastava Proc 27-th In ti Cosmic Ray C onf (Germany Hamburg) Vol. S l l  9 p 3481 (2001)

A Comparative Study of Forbush Decrease Events of February and August, 1999

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A Comparative Study of Forbush Decrease Events of February and August, 1999

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