Rekha Agarwal

DAILY VARIATION IN COSMIC RAY INTENSITY ON DIFFERENT GEOMAGNETIC CONDITIONS

A new concept of data analysis has been attempted for studying the long/short term daily variations in cosmic ray (CR) intensity recorded with neutron monitors/meson telescopes. Fourier Technique has been applied on four different types of groups of days chosen according to their different geomagnetic conditions. The selected groups are 60 quietest days (60 QD), 120 quiet days (120 QD), continuous quiet days (CQD) and All Days (AD) in a year. CQD is a new set of days selected on the basis of Ap and Kp values. These are the days when transient magnetic variations are supposed to be regular and smooth continuously for a span of at least three days. The criteria of selection of CQD is based upon the mathematical manipulation with Ap index. The data of Deep River neutron monitoring station is used for the period 1985–1995, to investigate for a com-parative study of diurnal anisotropy in CR intensity on 60 QD, 120 QD, CQD and AD. It is observed that 60 QD are most suitable for the anisotropic studies on short/long term basis. The time/spatial variations in the amplitude and phase of the diurnal anisotropy become more pronounced for 60 QD for the period under investigation.

Modulation of cosmic rays at different cutoff rigidity

The present work deals with the study of first three harmonics of cosmic ray intensity on geo-magnetically quiet days over the period 1981-1993 for Deep River and Inuvik neutron monitoring stations having mid and low cutoff rigidity. The amplitude of first harmonic remains high for Deep River having mid cutoff rigidity as compared to Inuvik neutron monitor having low cutoff rigidity on quiet days. The diurnal amplitude significantly decreases and phase shifts towards an earlier time during solar activity minimum years at both Deep River and Inuvik. The amplitude of second harmonic significantly enhanced during solar activity minimum as well as maximum at Deep River and remains low during solar activity maximum at Inuvik, whereas the phase shifts towards an earlier time during solar maximum for both the stations. The amplitude of the third harmonic significantly enhanced during solar activity minimum at Deep River and on solar activity minimum at Inuvik, whereas the phase does not show any significant characteristics and fluctuates quite frequently. The amplitude of semi/tri-diurnal anisotropy has a good positive correlation, while the others (i.e. amplitude and phase) have very weak correlation with solar wind velocity on quiet days at Deep River station during 1981-1993. The solar wind velocity significantly remains in the range 350 to 425 km/s i.e. being nearly average on quiet days. The amplitude and direction of the anisotropy on quiet days are weakly dependent on high-speed solar wind streams for two neutron monitoring station of mid and low cutoff rigidity threshold. The amplitude as well as direction of second harmonic has a good anti-correlation with IMF Bz and the product V x Bz on quiet days at Deep River station. However, the direction of second and third harmonic has a good anti-correlation with IMF Bz and the product V x Bz on quiet days at Inuvik station.

Cosmic ray intensity during the passage of coronal mass ejections

Coronal Mass Ejections are vast structures of plasma and magnetic fields that are expelled from the sun into the heliosphere. This material is detected by remote sensing and in-situ spacecraft observations. The present study deals with the influence of four types of CMEs namely Asymmetric ’Full’ Halo CMEs, Partial Halo CMEs, Asymmetric and Complex ’Full’ Halo CMEs and ’Full’ Halo CMEs on cosmic ray neutron monitor intensity. The data of ground based neutron monitor of Moscow and CME events observed with instruments onboard and Wind spacecraft have been used in the present analysis. The method of superposed epoch (Chree) analysis has been used to the arrival times of these CMEs. It is noteworthy that the frequency of occurrence of Asymmetric ’Full’ Halo CMEs is significantly high, whereas frequency of occurrence of Asymmetric and Complex ’Full’ Halo CMEs is low compared to other CMEs. Significant enhancement in cosmic ray intensity is observed after 4 days of the onset of asymmetric full halo and 6 days after the onset of full halo CMEs. The fluctuations in cosmic ray intensity are more prior to the onset of both types of the CMEs. However, during Partial Halo CMEs the cosmic ray intensity peaks, 8- 9 days prior to the onset of CMEs and depressed 3 days prior to the onset of CMEs, whereas in case of asymmetric and complex full CMEs, the intensity depressed 2 days prior to the onset of CMEs and enhanced 2 days after the onset of CMEs. The deviations in cosmic ray intensity are more pronounced in case for asymmetric and complex full halo CMEs compared to other CMEs.

Geometry of Coronal Mass Ejections in the Context of Recent Solar Cycle

Geometry of Coronal Mass Ejections in the Context of Recent Solar Cycle Coronal mass ejections (CMEs) disrupt the flow of the solar wind and produce disturbances that strike the Earth with sometimes catastrophic results. These ejections are often associated with solar flares and prominence eruptions, but they can also occur in the absence of either of these processes. The frequency of CMEs varies with the sunspot cycle. At the solar minimum we observed about one CME a week. Near the solar maximum we could observe an average of 2 to 3 CMEs per day. We have studied different CME characteristics based on the observation with Large Angle and Spectrometric Coronagraph (LASCO) on board of the Solar and Heliospheric Observatory (SOHO) space craft during the period 1996-2006. It is noteworthy that the rate of occurrence of class B CMEs (with the measurement position angle (MPA) in the range 200°-360°) is greater than that for class A CMEs (the MPA in the range 50°-200°). The CME occurrence spectrum for both classes follows the trend of the phase of solar cycle, and the maximum number of both type CMEs seems to occur during the maximum solar activity. It is also observed that the maximum numbers of class A, class B CMEs are in a speed range of 0-500 km/s. We have observed that the maximum number of class A, class B CMEs occurred in the apparent angular width range 0°-90°. It is also found that the maximum numbers of class A and class B CMEs occur when the position angle ranges in 5°-100° and 250°-300° respectively. Koronālo Masu Izvirdumu (KMI) Geometrija Pēdējo Saules Ciklu Kontekstā Koronālie masas izvirdumi (KMI) sagrauj Saules vēja plūsmu un noved pie perturbācijam, kas var radīt uz Zemes spēcīgas katastrofas. KMI frekvence mainās atkarībā no saules plankumu cikla. Pie minimālās saules aktivitātes mēs novērojām vienu KMI dienā, bet tuvu maksimālai Saules aktivitātei - 2-3 KMI dienā. Mēs pētījām dažādus raksturlielumus, kas iegūti novērojumos uz Saules ar platleņķa korongrafa spektrometru (LASCO) no heliosfēriskās observatorijas (SOHO) kosmosa kuga borta laika periodā no 1996. gada līdz 2006. gadam. Iegūts rezultāts, ka KMI klases B rašanās frekvence (pie mērījuma leņķa diapazonā 200° līdz 300°), lielāka nekā KMI klasei A (50°-200°). KMI rašanās spektrs abām klasēm atbilst Saules ciklu tendencēm; maksimālais abu tipu KMI skaits ir pie Saules maksimālās aktivitātes.

Cosmic Ray Nucleonic Intensity in Low-Amplitude Days During the Passage of High-Speed Solar Wind Streams

Cosmic Ray Nucleonic Intensity in Low-Amplitude Days During the Passage of High-Speed Solar Wind Streams One of the most striking features of solar wind is its organization into high- and low- speed streams. It is now well established that the passage over the Earth of high-speed solar wind streams leads to geomagnetic disturbances. The high-speed plasma streams are thus a key element in the complex chain of events that link geomagnetic activity to the solar activity and are therefore of great interest to the solar terrestrial physics. Two types of high-speed solar wind streams - coronal-hole-associated (or corotating) and flare-generated - were studied based on magnetic field and solar wind plasma parameters. In the work, the dependence was obtained for cosmic ray (CR) depressions due to high-speed solar wind streams during low-amplitude days. The CR nucleonic intensity data were subjected to the superposed epoch analysis with respect to the start time of high-speed solar wind streams. It was found that streams of both types produce significant deviations in the CR intensity during low-amplitude anisotropic wave train events. At the onset of such streams the CR intensity reaches its minimum during low-amplitude events and then increases statistically. Viena no Saules vēja pārsteidzošākajām īpatnībām ir tā, ka veidojas divu tipu plūsmas - liela un maza ātruma plūsmas. Noteikts, ka nonākot uz Zemes liela ātruma Saules vēja plūsmām, rodas Geomagnētiskās vētras. Ātri darbīgās plazmu plūsmas ir galvenie elementi, kas izsauc geomagnētisko darbību saistītu ar Saules darbību, un tāpēc ir liela interese par Zemes fiziku un Sauli. Šie ātrdarbīgās plūsmas divi tipi tiek klasificēti divās kategorijās, kuru pamatā ir magnētisko lauku un Saules vēja parametri. Mēs pētījām kosmiskā starojuma depresiju saistībā ar ātrdarbīgām Saules vēja plūsmām mazu amplitūdu dienās. Kosmisko staru intensitātes dati analizēti no Saules vēja ātri darbīgo plūsmu parādīšanās sākuma. Saules vēja divu tipu plūsmas rada būtisku novirzi kosmiskā starojuma intensitātē mazu amplitūdu laikā. Sākumā kosmiskā starojuma intensitāte sasniedz minimumu mazu amplitūdu dienās, bet pēc tam palielinās statistiski.