Influence of Magnetic Clouds on Variations of Cosmic Rays in November 2004

We investigate the effects of two magnetic clouds on hourly cosmic-ray intensity profiles in the Forbush decrease events in November 2004 observed by 47 ground-based neutron-monitor stations. By using a wavelet decomposition, the start time of the main phase in a Forbush decrease event can be defined, and then clearer definitions of initial phase, main phase, and recovery phase are proposed. Our analyses suggest that the main phase of this Fd event precedes the arrival time of the first magnetic cloud by about three hours, and the Fds observed at the majority (39/47) of the stations were found to originate from the sheath region as indicated by large fluctuations in magnetic field vectors at 19:00 UT on 7 November 2004, regardless of the station location. In addition, about 45% of the onset times of the recovery phase in the Forbush decreases took place at 04:00 UT on 10 November, independent of the station position. The results presented here support the hypothesis that the sheath region between the shock and the magnetic cloud, especially the enhanced turbulent magnetic field, results in the scattering of cosmic-ray particles, and causes the following Forbush decreases. Analysis of variation profiles from different neutron monitors reveals the global simultaneity of this Forbush decrease event. Moreover, we infer that the interplanetary disturbance was asymmetric when it reached the Earth, inclined to the southern hemisphere. These results provide several observational constraints for more detailed simulations of the Forbush decrease events with time-dependent cosmic-ray modulation models.     

Significance of the turbulent sheath following the interplanetary shocks in producing forbush decreases

The physical processes responsible for transient cosmic-ray decreases have been investigated for two types of interplanetary shock events associated with helium enhancement (He-shocks) and those not associated with helium enhancement (non-He-shocks). The Calgary cosmic-ray neutron monitor data and the interplanetary field data have been subjected to a superposed-epoch Chree analysis. The difference in the profiles of the cosmic-ray intensity have been compared with the interplanetary field data and its variance. It is suggested that the turbulence sheath following the shock front is very effective and of major importance for producing cosmic-ray decreases. A simple model has been proposed to explain the observations which show that a Forbush decrease modulating region consists of a shock front associated with a plasma sheath in which the magnetic field is turbulent and the sheath, in turn, is followed by an ejected plasma cloud having ordered structure and high magnetic field strength.     

Study and effect of magnetic clouds on the transient modulation of cosmic-ray intensity

Data of cosmic-ray intensity from the Calgary Super Neutron Monitor and interplanetary plasma and field data are divided into three groups corresponding to the magnetic clouds preceded by shocks, followed by interaction region and clouds without any such association, observed during the period 1967–1982. A superposed epoch analysis of these data, in addition to the field variance data, have been performed. The results suggest the hypothesis that the Forbush decreases are caused by the scattering of particles in the region of enhanced turbulence, observed during the passage of shocked plasma (i.e., sheath) between the shock front and the magnetic cloud.     

Interplanetary Transients and Cosmic-Ray Anisotropy

Cosmic-ray intensity data recorded with the ground-based neutron monitor at Deep River have been investigated taking into account the associated interplanetary magnetic field and solar-wind plasma data during 1981 – 1994. A large number of days having abnormally high or low amplitudes for five or more successive days as compared to the annual average amplitude of diurnal anisotropy have been taken as high- or low-amplitude anisotropic wave-train events. The amplitude of the diurnal anisotropy of these events is found to increase on days with a magnetic cloud as compared to the days prior to the event, and it is found to decrease during the later period of the event as the cloud passes the Earth. The high-speed solar-wind streams do not play any significant role in causing these types of events. However, corotating solar-wind streams produce significant deviations in cosmic-ray intensity during high- and low-amplitude events. The interplanetary disturbances (magnetic clouds) are also effective in producing cosmic-ray decreases. Hα solar flares have a good positive correlation with both the amplitude and direction of the anisotropy for high-amplitude events, while the principal magnetic storms have a good positive correlation with both amplitude and direction of the anisotropy for low-amplitude events. The source responsible for these unusual anisotropic wave trains in cosmic rays has been proposed.     

Interplanetary magnetic clouds and cosmic ray modulation

We studied the cosmic ray intensity variation due to interplanetary magnetic clouds during an unusual class of low amplitude anisotropic wave train events. The low amplitude anisotropic wave train events in cosmic ray intensity have been identified using the data of ground based Deep River neutron monitor and studied during the period 1981–1994. Even though the occurrence of low amplitude anisotropic wave trains does not depend on the onset of interplanetary magnetic clouds, but the possibility of occurrence of these events cannot be overlooked during the periods of the interplanetary magnetic cloud events. It is observed that the solar wind velocity remains higher (> 300) than normal and the interplanetary magnetic field B remains lower than normal on the onset of the interplanetary magnetic cloud during the passage of low amplitude wave trains. It is also noted that the proton density remains significantly low during high solar wind velocity, which is expected. The north south component of interplanetary magnetic field Bz turns southward to one day before the arrival of cloud and remains in the southward direction after the arrival of a cloud. During these events the cosmic ray intensity is found to increase with increase of solar wind velocity. The superposed epoch analysis of cosmic ray intensity for these events during the onset of interplanetary magnetic clouds reveals that the decrease in cosmic ray intensity starts not at the onset of the cloud but after a few days. The cosmic ray intensity increases on arrival of the magnetic cloud and decreases gradually after the passage of the magnetic cloud.     

Effect of interplanetary shocks and magnetic clouds on onset of cosmic-ray decreases

A detailed analysis has been carried out to study the onset times of cosmic-ray decreases occurring during 1978–1982 with respect to the arrival times of interplanetary shocks and magnetic clouds. The observations demonstrate that shocks, magnetic clouds and a combination of both could effectively trigger a cosmic-ray decrease when they are associated with turbulent sheaths of maximum thickness 15.0 hr (0.15 AU). Further, the shocks associated with enhanced solar wind velocity produce a fast decrease and the magnetic clouds accompanied by extended and enhanced magnetic field produce a slow decrease. The decrease, non-correlated with the arrival times of shocks and magnetic clouds, represents a corotating cosmic-ray decrease produced by corotating streams.     

Study of the Cosmic-Ray Modulation During the Passage of ICMEs and CIRs

We compare the cosmic-ray response to interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) during their passage in near-Earth space. We study the relative importance of various structures/features identified during the passage of the ICMEs and CIRs observed during Cycle 23 (1995?–?2009). The identified ICME structures are the shock front, the sheath, and the CME ejecta. We isolate the shock arrival time, the passage of the sheath region, the arrival of ejecta, and the end time of their passage. Similarly, we isolate the CIR arrival, the associated forward shock, the stream interface, and the reverse shock during the passage of a CIR. For the cosmic-ray intensity, we utilize the data from high counting rate neutron monitors. In addition to neutron monitor data, we utilize near-simultaneous and same time-resolution data of interplanetary plasma and field, namely the solar-wind velocity, the interplanetary magnetic field (IMF) vector, and its variance. Further, we also utilize some derived interplanetary parameters. We apply the method of the superposed-epoch analysis. As the plasma and field properties are different during the passage of different structures, both in ICMEs and CIRs, we systematically vary the epoch time in our superposed-epoch analysis one by one. In this way, we study the role and effects of each of the identified individual structures/features during the passage of the ICMEs and CIRs. Relating the properties of various structures and the corresponding variations in plasma and field parameters with changes of the cosmic-ray intensity, we identify the relative importance of the plasma/field parameters in influencing the amplitude and time profiles of the cosmic-ray intensity variations during the passage of the ICMEs and CIRs.     

Predicting the Arrival Time of Shock Passages at Earth

The purpose of this parametric study is to predict the arrival time at Earth of shocks due to disturbances observed on the Sun. A 3D magnetohydrodynamic (MHD) simulation code is used to simulate the evolution of these disturbances as they propagate out to 1 AU. The model in Han, Wu and Dryer (1988) uses solar data for input at 0.08 AU (18 solar radii). The initial shock speed (ISS) is assumed to be constant from the corona to 0.08 AU. We investigate how variations of this ISS affect the arrival times of the shock at Earth. This basic parametric study, however, does not consider inhomogeneous background solar wind structures such as corotating interaction regions and their precursor stream–stream interactions, nor interplanetary manifestations of complex coronal mass ejecta such as magnetic clouds. In the latter case, only their associated shocks are considered. Because the ambient (pre-existing background) solar wind speed is known to affect the shock arrival time at 1 AU, we also simulated events with various background solar wind speeds (BSWS) to investigate this effect. The results show that the shock arrival time at Earth depends on the BSWS, the speed of solar disturbances, their size, and their source location at the Sun. However, it is found that for a sufficiently large momentum input, the shock arrival time at Earth is not significantly affected by the pre-existing solar wind speed.     

Types of interplanetary shocks and forbush decreases

Two types of interplanetary shocks have been identified and classified into two groups, those associated with a helium-enhancement and those not associated with any helium-enhancement. The cosmic-ray intensity decreases at Calgary neutron monitor are studied with respect to the arrival time of the two groups of shocks. The observations show that large Forbush decreases are caused by shocks associated with the helium-enhancement; and those not associated with He shocks show comparatively a small decrease in cosmic-ray intensity.     

Structure of the July 1982 event in relation to the magnetosphere's response

A large Forbush-type decrease with an amplitude of 16–22% was observed by the world-wide network of cosmic-ray detectors during the period 13–14 July, 1982. Combined neutron-monitor measurements with interplanetary plasma and magnetic field data, auroral data, and Earth's magnetospheric data are used for the study of this event. It is suggested that this interesting event is probably a consequence of the dynamic interactions of the solar wind with the Earth's magnetosphere as it is obvious from the large magnetic storm which was recorded in the auroral electrojet indices.     

Interplanetary Coronal Mass Ejections,Associated Features,and Transient Modulation of Galactic Cosmic Rays

Interplanetary structures such as shocks, sheaths, interplanetary counterparts of coronal mass ejections (ICMEs), magnetic clouds, and corotating interaction regions (CIRs) are of special interest for the study of the transient modulation of galactic cosmic rays (GCRs). These structures modulate the GCR intensity with varying amplitudes and recovery-time profiles. It is known that ICMEs are mainly responsible for Forbush decreases in the GCR intensity. However, not all of the ICMEs produce such decreases in GCR intensity. We utilize GCR intensity data recorded by neutron monitors and solar-wind plasma/field data during the passage of ICMEs with different features and structures, and we perform a superposed-epoch analysis of the data. We also adopt the best-fit approach with suitable functions to interpret the observed similarities and differences in various parameters. Using the GCR-effectiveness as a measure of the cosmic-ray response to the passage of ICMEs, about half of the ICMEs identified during 1996?–?2009 are found to produce moderate to very large intensity depressions in GCR intensity. The ICMEs associated with halo CMEs, magnetic-cloud (MC) structures, bidirectional superthermal electron (BDE) signatures, and those driving shocks are 1.5 to 4 times more GCR effective than the ICMEs not associated with these structures/features. Further, the characteristic recovery time of GCR intensity due to shock/BDE/MC/halo-CME-associated ICMEs is larger than those due to ICMEs not associated with these structures/features.     

Are interplanetary magnetic clouds manifestations of coronal transients at 1 AU?

Using proxy data for the occurrence of those mass ejections from the solar corona which are directed earthward, we investigate the association between the post-1970 interplanetary magnetic clouds of Klein and Burlaga (1982) and coronal mass ejections. The evidence linking magnetic clouds following shocks with coronal mass ejections is striking; six of nine clouds observed at Earth were preceded an appropriate time earlier by meter-wave type II radio bursts indicative of coronal shock waves and coronal mass ejections occurring near central meridian. During the selected control periods when no clouds were detected near Earth, the only type II bursts reported were associated with solar activity near the limbs. Where the proxy solar data to be sought are not so clearly suggested, that is, for clouds preceding interaction regions and clouds within cold magnetic enhancements, the evidence linking the clouds and coronal mass ejections is not as clear; proxy data usually suggest many candidate mass-ejection events for each cloud. Overall, the data are consistent with and support the hypothesis suggested by Klein and Burlaga that magnetic clouds observed with spacecraft at 1 AU are manifestations of solar coronal mass ejection transients.     

The Bastille day Shock and Merged Interaction Region at 63 au: Voyager 2 Observations

A transient flow system containing several streams and shocks associated with the Bastille Day 2000 solar event was observed by the WIND and ACE spacecraft at 1 AU. Voyager 2 (V2) at 63 AU observed this flow system after it moved through the interplanetary medium and into the distant heliosphere, where the interstellar pickup protons strongly influence the MHD structures and flow dynamics. We discuss the Voyager 2 magnetic and plasma observations of this event. Increases in the magnetic field strength B, density N, temperature T and speed V were observed at the front of a stream at V2, consistent with presence of a shock related to the Bastille Day shock at 1 AU. However, the jumps occurred in a 16.9-hour data gap, so that the shock was not observed directly, and the properties of the candidate shock cannot be determined precisely. The candidate shock was followed by a merged interaction region (MIR) that moved past V2 for at least 10 days. The first part of this MIR contains a structure that might be a magnetic cloud. Just ahead of the shock there was an abrupt increase in density associated with a decrease in temperature such that the solar wind thermal pressure was constant across it. Just behind the shock there was an abrupt decrease in density associated with a net increase in magnetic field strength. This appears to be a pressure balanced structure in which the interstellar pickup protons make a significant contribution.     

Instability of the structure of strong oblique MHD cosmic-ray shocks

A criterion of the instability of a flow of a thermal plasma and cosmic rays in front of an oblique MHD shock wave with respect to short-wavelength magnetosonic disturbances is derived. The dependence of a cosmic-ray diffusion tensor on a plasma density and a large-scale magnetic field is taken into account. The most unstable disturbances propagate at an angle to the magnetic field if diffusion is strongly anisotropic. In some cases the most strong instability connects with the off-diagonal terms of the diffusion tensor.     

Shock Orientations,Magnetic Turbulence and Forbush Decreases

We have studied the effects of quasi-parallel and quasi-perpendicular shocks on the transient modulation of cosmic-ray intensity. Interplanetary magnetic field strength, its variance and solar wind velocity during their passage have also been considered for the analysis in this work. It has been demonstrated that magnetically turbulent quasi-parallel shocks are much more effective in producing Forbush decreases in cosmic-ray intensity than the non-turbulent quasi-perpendicular shocks. From these results it is inferred that turbulence in the shock environment is an important factor in causing Forbush decreases by scattering particles due to magnetic field fluctuations. Results presented in this study provide more specific information about structures responsible for Forbush decreases, physical processes mainly responsible for this phenomenon and the possibility of predicting the likely occurrence of Forbush decreases from observations in space.     

First evidence of IMF control of Jovian magnetospheric boundary locations: Cassini and Galileo magnetic field measurements compared

The Cassini spacecraft, en route to Saturn, passed close to Jupiter while the Galileo spacecraft was completing its 28th and 29th orbits of Jupiter, thus offering a unique opportunity for direct study of the solar wind-Jovian interaction. Here evidence is given of response of the Jovian magnetopause and bow shock positions to changes of the north-south component of the solar wind magnetic field, a phenomenon long known to occur in equivalent circumstances at Earth. The period analyzed starts with the passage over Cassini of an interplanetary shock far upstream of Jupiter. The shock's arrival at Galileo on the dusk-flank of the magnetosphere caused Galileo to exit into the solar wind. Using inter-spacecraft timing based on the time delay established from the shock arrival at each spacecraft, we point out that Galileo's position with respect to the Jovian bow shock appears to correlate with changes in the disturbed north-south reversing field seen behind the shock. We specifically rule out the alternative of changes in the shape of the bow shock with rotations of the interplanetary magnetic field as the cause.     

The Bastille day Magnetic Clouds and Upstream Shocks: Near-Earth Interplanetary Observations

The energetic charged particle, interplanetary magnetic field, and plasma characteristics of the `Bastille Day' shock and ejecta/magnetic cloud events at 1 AU occurring over the days 14–16 July 2000 are described. Profiles of MeV (WIND/LEMT) energetic ions help to organize the overall sequence of events from the solar source to 1 AU. Stressed are analyses of an outstanding magnetic cloud (MC2) starting late on 15 July and its upstream shock about 4 hours earlier in WIND magnetic field and plasma data. Also analyzed is a less certain, but likely, magnetic cloud (MC1) occurring early on 15 July; this was separated from MC2 by its upstream shock and many heliospheric current sheet (HCS) crossings. Other HCS crossings occurred throughout the 3-day period. Overall this dramatic series of interplanetary events caused a large multi-phase magnetic storm with min Dst lower than −300 nT. The very fast solar wind speed (≥ 1100 km s⁻¹) in and around the front of MC2 (for near average densities) was responsible for a very high solar wind ram pressure driving in the front of the magnetosphere to geocentric distances estimated to be as low as ≈ 5 R _E, much lower than the geosynchronous orbit radius. This was consistent with magnetic field observations from two GOES satellites which indicated they were in the magnetosheath for extended times. A static force-free field model is used to fit the two magnetic cloud profiles providing estimates of the clouds' physical and geometrical properties. MC2 was much larger than MC1, but their axes were nearly antiparallel, and their magnetic fields had the same left-handed helicity. MC2's axis and its upstream shock normal were very close to being perpendicular to each other, as might be expected if the cloud were driving the shock at the time of observation. The estimated axial magnetic flux carried by MC2 was 52×10²⁰ Mx, which is about 5 times the typical magnetic flux estimated for other magnetic clouds in the WIND data over its first 4 years and is 17 times the flux of MC1. This large flux is due to both the strong axially-directed field of MC2 (46.8 nT on the axis) and the large radius (R ₀=0.189 AU) of the flux tube. MC2's average speed is consistent with the expected transit time from a halo-CME to which it is apparently related.     

Precursor Effects in Different Cases of Forbush Decreases

Over the last few years, the pre-decreases or pre-increases of the cosmic-ray intensity observed before a Forbush decrease, called the precursor effect and registered by the worldwide neutron monitor network, have been investigated for different cases of intense events. The Forbush decreases presented in this particular study were chosen from a list of events that occurred in the time period 1967?–?2006 and were characterized by an enhanced first harmonic of cosmic-ray anisotropy prior to the interplanetary disturbance arrival. The asymptotic longitudinal cosmic-ray distribution diagrams for the events under consideration were studied using the “Ring of Stations” method, and data on solar flares, solar-wind speed, geomagnetic indices, and interplanetary magnetic field were analyzed in detail. The results revealed that the use of this method allowed the selection of a large number of events with well-defined precursors, which could be separated into at least three categories, according to duration and longitudinal zone. Finally, this analysis showed that the first harmonic of cosmic-ray anisotropy could serve as an adequate tool in the search for precursors and could also be evidence for them.     

Profile of an Average Magnetic Cloud at 1 au for the Quiet Solar Phase: Wind Observations

Using WIND magnetic field (MFI) and plasma (SWE) data, an `average' profile of an interplanetary magnetic cloud was developed in terms of five physical (scalar) quantities based on appropriately selected individual clouds. The period of study was from early 1995 to late in 1998, primarily during the quiet part of a solar cycle. The physical quantities are: magnetic field magnitude, proton density, solar wind bulk speed, proton thermal speed, and proton plasma beta. Selection of the clouds was based on two considerations: (1) their `quality', determined objectively from the application of a static magnetic field model of cloud field structure, had to be good, and (2) distant spacecraft approaches from the cloud axes were not accepted. Nineteen clouds resulted out of 35 original cases. A superposed epoch analysis was performed on the 5 parameters generating summary profiles of a generic magnetic cloud at 1 AU. The density within the generic magnetic cloud reached a distinct minimum near the center and peaked in the trailing part (closest to Sun) after a slow rise. The individual clouds fall into two classes, those that have such an enhanced density feature (about of them) and those that have an overall nearly flat density profile. For the first 85% of the generic magnetic cloud the bulk speed decreased almost uniformly by 45 km s^–1 indicating marked expansion over 1 AU. The field intensity peaked very near the cloud's center but was noticeably asymmetric. Proton thermal speed was quite symmetric with local maxima at the front, center, and rear. Proton plasma beta was low throughout the cloud (0.12 on average), but had a broad minimum at its center. The relative degree of fluctuation level for the parameters ranged from the most quiet for both speed and field magnitude, to the most `noisy' for proton plasma beta, with fluctuations in density and thermal speed at intermediate levels, all being below 0.2, based on a sample-scale of frac1100 of the cloud duration. These profiles may be useful in constraining future structural and thermodynamic models of clouds with regard to their solar birth conditions and interplanetary evolution.     

Geoeffectiveness of the interplanetary manifestations of coronal mass ejections and solar-wind stream–stream interactions

Coronal mass ejections and high-speed streams from the Sun, and related structures formed and evolved in interplanetary space, i.e. interplanetary manifestations of CMEs (ICMEs) and stream interaction regions (SIRs)/corotating interaction regions (CIRs), are mainly responsible for geomagnetic disturbances in the Earth’s magnetic environment. However, the presence or absence of associated/finer structures of ICMEs (e.g., shock/sheath, magnetic cloud) and SIRs/CIRs (forward and reverse shocks, stream interface) might influence their geoeffectiveness as these features within large-scale structures of ICMEs and SIRs display different and varying plasma and field characteristics. In this work, we analyze the solar-wind plasma and field parameters (plasma velocity, density and pressure, magnetic field, its north-south component and electric field) together with geomagnetic activity parameters (kp and Dst), applying the method of superposed epoch analysis. By systematically changing the time of passage of different features as epochs, e.g. discontinuities/shocks, CMEs/magnetic clouds in ICMEs and discontinuities/forward shocks in SIRs/CIRs, we study the relative geoeffectiveness of not only the large-scale structures (ICMEs/SIRs/CIRs), but of their finer features also. We critically analyze the differences in geoeffectiveness due to different structures and features, with distinct plasma/field characteristics, and we utilize these results to understand the mechanism during their interaction with geospace.