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.
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 12h 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.
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.
Aims. We seek to identify the primary agents causing Forbush decreases (FDs) in high-rigidity cosmic rays observed from the Earth. In particular, we ask if these FDs are caused mainly by coronal mass ...ejections (CMEs) from the Sun that are directed towards the Earth, or by their associated shocks. Methods. We used the 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 can be reasonably well associated with an Earth-directed CME and its associated shock. We employed two models: one that considers the CME as the sole cause of the FD (the CME-only model) and one that considers the shock as the only agent causing the FD (the shock-only model). We used an extensive set of observationally determined parameters for both models. The only free parameter in these models is the level of MHD turbulence in the sheath region, which mediates cosmic ray diffusion (into the CME for the CME-only model, and across the shock sheath for the shock-only model). Results. We find that good fits to the GRAPES-3 multi-rigidity data using the CME-only model require turbulence levels in the CME sheath region that are only slightly higher than those estimated for the quiescent solar wind. On the other hand, reasonable model fits with the shock-only model require turbulence levels in the sheath region that are an order of magnitude higher than those in the quiet solar wind. Conclusions. This observation naturally leads to the conclusion that the Earth-directed CMEs are the primary contributors to FDs observed in high-rigidity cosmic rays.
We examine the propagation of several coronal mass ejections (CMEs) with well-observed flux rope signatures in the field of view of the SECCHI coronagraphs on board the STEREO satellites using the ...graduated cylindrical shell fitting method of Thernisien et al. We find that the manner in which they propagate is approximately self-similar; i.e., the ratio ( Kappa ) 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 ( gamma ) of the flux rope magnetic field and the misalignment angle ( chi ) 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.
The flux of galactic cosmic rays (GCRs) is isotropic in the interstellar space. However, in the heliosphere, the ram pressure of outward-moving solar wind convects the GCRs away from the Sun, thereby ...producing a density gradient in the radial direction. The diffusion of GCRs due to this gradient and scattering with the irregularities in the interplanetary magnetic field (IMF) induce variations in their flux that can be observed near the Earth. A framework for the diffusion-convection mechanism of GCR propagation developed by Parker and collaborators Phys. Rev. 110, 1445 (1958); Planet. Space Sci. 13, 9 (1965); Astrophys. J. 772, 46 (2013); Space Sci. Rev. 78, 401 (1996); Astrophys. J. 234, 746 (1979); Astrophys. J. 361, 162 (1990); Space Sci. Rev. 176, 299 (2013) offers a good description of this phenomenon. One of the outcomes of this framework is an anticorrelation of the variation in solar wind velocity (VSW) and the GCR flux. A second outcome of this gradient in the presence of IMF is the movement of GCRs perpendicular to the ecliptic plane called “Swinson flow.” Therefore, (i) the correlated variations of VSW and GCR flux and (ii) the GCR radial density gradient obtained from Swinson flow can each be used to independently measure the radial diffusion coefficient of GCRs in the inner heliosphere. In an earlier work Phys. Rev. D 91, 121303(R) (2015), the GCR flux was shown to be anticorrelated with VSW at (−1.33±0.07)×10−3%(km s−1)−1. This anticorrelation yields a radial diffusion coefficient κ=0.97×1019 m2 s−1 at 1 AU. In another work Astropart. Phys. 62, 21 (2015), the measurement of Swinson flow was used to obtain a GCR radial density gradient of 0.65 AU−1 at a median rigidity of 77 GV. Here, we report a measurement of radial diffusion coefficient κ=1.04×1019 m2 s−1 at 1 AU from the above-mentioned density gradient, for a mean VSW of 450 km s−1. Thus, these two distinct approaches essentially yielded similar values of the radial diffusion coefficient κ=1019 m2 s−1 at 1 AU, characterizing the diffusion of GCRs at 77 GV. From this value of κ, the mean free path length for parallel diffusion λ∥ was estimated to be 1.2 AU at 77 GV, consistent with earlier reports Rev. Geophys. Space Phys. 20, 335 (1982); Astrophys. J. 420, 294 (1994); Astrophys. J. 604, 861 (2004).
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 large area (560 m2) tracking muon telescope is operating at Ooty in India since 2001. It records 4 × 109 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.
The large area (560 m2) GRAPES-3 tracking muon telescope has been operating uninterruptedly at Ooty, India since 2001. Every day, it records 4 × 109 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 ‘Teff’ 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 Teff. The anti-correlation between the seasonal variation of Teff, and the muon intensity was used to measure the temperature coefficient αT by fast Fourier transform (FFT) technique. The magnitude of αT 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 αT was found to be (−0.17±0.02)% K−1.
Abstract The heliosphere is full of galactic cosmic rays (GCR), high‐energy charged particles coming isotropically from the galaxy. The GCR interact with the solar wind blown by the Sun carrying out ...plasma, magnetic fields and transient structures such as interplanetary coronal mass ejections (ICMEs) and their associated magnetic flux ropes (MFR). The GCR interaction with ICMEs has been extensively studied particularly the GCR flux attenuation (known as Forbush decreases ) as a result of interacting with the ICME sheath and magnetic field. In this work, we investigate the opposite effect: the MFR's ability to generate GCR anisotropies which an observer may detect as an increase in the GCR flux. To achieve this, we simulated a flux of protons with energies in the 10–160 GeV range arriving from all directions to a cylindrical MFR (with and without sheath) with plasma, magnetic field, and spatial dimensions found in average ICMEs observed at 1 au. By following the individual trajectories of the injected particles we found that the MFR deviates the charged particles preferentially in one direction parallel to the MFR–axis. We also found that the peak of this anisotropic GCR flux depends on: the angle between the MFR and ambient magnetic fields; the presence or not of the sheath region; the energy of the incident particles and the observer location inside the MFR.
Key Points We show that magnetic flux ropes associated to interplanetary coronal mass ejections produce anisotropies in the cosmic rays flux This finding helps to understand the anisotropies and enhancements of galactic cosmic rays (GCR) observed when magnetic clouds reaches the Earth Our numerical simulations can be extended to any scenery where cosmic rays interacts with magnetic flux ropes
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