Observational support of reconnection in solar flares

We present a detailed analysis of the magnetic topology of flaring active region. TheH kernels are found to be located at the intersection of the separatrices with the chromosphere when the shear, deduced from the fibrils or/and transverse magnetic field direction, is taken into account. We show that the kernels are magnetically connected by field lines passing close to the separator. We confirm, for other flares, previous studies which show that photospheric current concentrations are located at the borders of flare ribbons. Moreover we found two photospheric current concentrations of opposite sign, linked in the corona by field lines which follow separatrices. These give evidence that magnetic energy is released by reconnection processes in solar flares.     

Source Region of High and Low Speed Wind during the Spartan 201-05 Flight

The large-scale coronal magnetic fields of the Sun are believed to play an important role in organizing the coronal plasma and channeling the high and low speed solar wind along the open magnetic field lines of the polar coronal holes and the rapidly diverging field lines close to the current sheet regions, as has been observed by the instruments aboard the Ulysses spacecraft from March 1992 to March 1997. We have performed a study of this phenomena within the framework of a semi-empirical model of the coronal expansion and solar wind using Spartan, SOHO, and Ulysses observations during the quiescent phase of the solar cycle. Key to this understanding is the demonstration that the white light coronagraph data can be used to trace out the topology of the coronal magnetic field and then using the Ulysses data to fix the strength of the surface magnetic field of the Sun. As a consequence, it is possible to utilize this semi-empirical model with remote sensing observation of the shape and density of the solar corona and in situ data of magnetic field and mass flux to predict values of the solar wind at all latitudes through out the solar system. We have applied this technique to the observations of Spartan 201-05 on 1–2 November, 1998, SOHO and Ulysses during the rising phase of this solar cycle and speculate on what solar wind velocities Ulysses will observe during its polar passes over the south and the north poles during September of 2000 and 2001. In order to do this the model has been generalized to include multiple streamer belts and co-located current sheets. The model shows some interesting new results. This revised version was published online in August 2006 with corrections to the Cover Date.     

Jets and brightenings generated by energy deposition in the middle and upper solar chromosphere

Numerical simulations of energy depositions in the middle and upper solar chromosphere result in ejection of chromospheric material into the corona and heating of the chromospheric gas. These simulations may be capable of describing some of the features seen by the soft X-ray telescope on board theYohkoh satellite.     

On the application of the boundary element method in coronal magnetic field reconstruction

Solar magnetic field is believed to play a central role in solar activities and flares, filament eruptions as well as CMEs are due to the magnetic field re-organization and the interaction between the plasma and the field. At present the reliable magnetic field measurements are still confined to a few lower levels like in photosphere and chromosphere. Although IR technique may be applied to observe the coronal field but the technique is not well-established yet. Radio techniques may be applied to diagnose the coronal field but assumptions on radiation mechanisms and propagations are needed. Therefore extrapolation from photospheric data upwards is still the primary method to reconstruction coronal field. Potential field has minimum energy content and a force-free field can provide the required excess energy for energy release like flares, etc. Linear models have undesirable properties and it is expected to consider non-constant-alpha force-free field model. As the recent result indicates that the plasma beta is sandwich-ed distributed above the solar surface (Gary, 2001), care must be taken in modeling the coronal field correctly. As the reconstruction of solar coronal magnetic fields is an open boundary problem, it is desired to apply some technique that can incorporate this property. The boundary element method is a well-established numerical techniques that has been applied to many fields including open-space problems. It has also been applied to solar magnetic field problems for potential, linear force-free field and non-constant-alpha force-free field problems. It may also be extended to consider the non-force-free field problem. Here we introduce the procedure of the boundary element method and show its applications in reconstruction of solar magnetic field problems. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interplanetary Origin of Intense,Superintense and Extreme Geomagnetic Storms

We present a review on the interplanetary causes of intense geomagnetic storms (Dst≤−100 nT), that occurred during solar cycle 23 (1997–2005). It was reported that the most common interplanetary structures leading to the development of intense storms were: magnetic clouds, sheath fields, sheath fields followed by a magnetic cloud and corotating interaction regions at the leading fronts of high speed streams. However, the relative importance of each of those driving structures has been shown to vary with the solar cycle phase. Superintense storms (Dst≤−250 nT) have been also studied in more detail for solar cycle 23, confirming initial studies done about their main interplanetary causes. The storms are associated with magnetic clouds and sheath fields following interplanetary shocks, although they frequently involve consecutive and complex ICME structures. Concerning extreme storms (Dst≤−400 nT), due to the poor statistics of their occurrence during the space era, only some indications about their main interplanetary causes are known. For the most extreme events, we review the Carrington event and also discuss the distribution of historical and space era extreme events in the context of the sunspot and Gleissberg solar activity cycles, highlighting a discussion about the eventual occurrence of more Carrington-type storms.     

Coupling from the Photosphere to the Chromosphere and the Corona

The atmosphere of the Sun is characterized by a complex interplay of competing physical processes: convection, radiation, conduction, and magnetic fields. The most obvious imprint of the solar convection and its overshooting in the low atmosphere is the granulation pattern. Beside this dominating scale there is a more or less smooth distribution of spatial scales, both towards smaller and larger scales, making the Sun essentially a multi-scale object. Convection and overshooting give the photosphere its face but also act as drivers for the layers above, namely the chromosphere and corona. The magnetic field configuration effectively couples the atmospheric layers on a multitude of spatial scales, for instance in the form of loops that are anchored in the convection zone and continue through the atmosphere up into the chromosphere and corona. The magnetic field is also an important structuring agent for the small, granulation-size scales, although (hydrodynamic) shock waves also play an important role—especially in the internetwork atmosphere where mostly weak fields prevail. Based on recent results from observations and numerical simulations, we attempt to present a comprehensive picture of the atmosphere of the quiet Sun as a highly intermittent and dynamic system.     

SOHO contribution to the understanding of mass supply and flows in the solar corona

We expect a variety of dynamic phenomena in the quiescent non-flaring corona. Plasma flows, such as siphon flows or convective flows of chromospheric material evaporating into the corona, are expected whenever a pressure differences is established either between the footpoints or between the coronal and chromospheric segments of a coronal loop. Such flows can induce phenomena of spatial and temporal brightness variability of the corona. In particular, evaporation induces a net mass input into the corona and consequently coronal density enhancements. Flows are also expected in the regions where energy is released during magnetic reconnection. From the observational point of view the dynamics of the solar atmosphere has been investigated in great detail mostly in the lower transition region with the HRTS, and during flares with theSolar Maximum Mission andYohkoh. The high spectral, temporal and spatial resolution of theSOHO ultraviolet spectrometers should enable us in the near future to fill the gap providing a continuous coverage from the chromosphere to the corona, in the 10⁴–10⁶ K domain, and therefore to best study the dynamics throughout the solar atmosphere.     

Observed Aspects of Reconnection in Solar Eruptions

The observed magnetic field configuration and signatures of reconnection in the large solar magnetic eruptions that make major flares and coronal mass ejections and in the much smaller magnetic eruptions that make X-ray jets are illustrated with cartoons and representative observed eruptions. The main reconnection signatures considered are the imaged bright emission from the heated plasma on reconnected field lines. In any of these eruptions, large or small, the magnetic field that drives the eruption and/or that drives the buildup to the eruption is initially a closed bipolar arcade. From the form and configuration of the magnetic field in and around the driving arcade and from the development of the reconnection signatures in coordination with the eruption, we infer that (1) at the onset of reconnection the reconnection current sheet is small compared to the driving arcade, and (2) the current sheet can grow to the size of the driving arcade only after reconnection starts and the unleashed erupting field dynamically forces the current sheet to grow much larger, building it up faster than the reconnection can tear it down. We conjecture that the fundamental reason the quasi-static pre-eruption field is prohibited from having a large current sheet is that the magnetic pressure is much greater than the plasma pressure in the chromosphere and low corona in eruptive solar magnetic fields.     

Commonalities Between Ionosphere and Chromosphere

Three types of processes, occurring in the weakly ionized plasmas of the Earth’s ionosphere as well as in the solar chromosphere, are being compared with each other. The main objective is to elaborate on the differences introduced primarily by the grossly different magnitudes of the densities, both with respect to the neutral and, even more so, to the plasma constituents. This leads to great differences in the momentum coupling from the plasma to the neutral component and becomes clear when considering the direct electric current component transverse to the magnetic field, called “Pedersen current”; in the ionosphere, which has no quasi-static counterpart in the chromosphere. The three classes of processes are related to the dynamical response of the two plasmas to energy influx from below and from above. In the first two cases, the energy is carried by waves. The third class concerns plasma erosion or ablation in the two respective regions in reaction to the injection of high Poynting and/or energetic particle fluxes.     

The Structure of the Chromosphere Properties Pertaining to Element Fractionation

We review the structure and dynamics of the solar chromosphere with emphasis on the quiet Sun and properties that are relevant to element fractionation mechanisms. Attention is given to the chromospheric magnetic field, its connections to the photosphere, and to the dynamical evolution of the chromosphere. While some profound advances have been made in the “unmagnetized” chromosphere, our knowledge of the magnetically controlled chromosphere, more relevant for the discussion of element fractionation, is limited. Given the dynamic nature of the chromosphere and the poorly understood magnetic linkage to the corona, it is unlikely that we will soon know the detailed processes leading to FIP fractionation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Current dissipation within the chromosphere-corona transition

Studies of sporadic outbursts, ranging from flares to nano-flares, invariably endow the solar corona with steady plasma conditions, prior to seeking a current-flow (or the associated magnetic structure) which induces instability. Such an approach does not incorporate a crucial feature of the natural configuration, namely, that the material is of chromospheric origin, and only resides at coronal altitudes for as long as it can acquire adequate energy. There is clearly a feedback loop involved, which allows plasma to moderate the transfer of energy from the field while making use of this heat to permeate coronal altitudes. An examination of the whole procedure is necessary if the location and threshold-conditions for the energy-conversion mechanism are to be identified.A critical step in the feedback procedure mentioned involves the supply line which links the corona to the chromosphere. Because the solar atmosphere has such large vertical dimensions, even a modest change in average temperature and/or density can place heavy demands on this artery: the problem is that a conventional conduction-dominated transition layer cannot readily accommodate a rapid increase in current-density or plasma-flow. (Restructuring of the temperature gradient, to provide the carriers with extra heat, is a very slow process.) A transition layer of this type is unable to endure for long at the base of a sporadically-heated atmosphere in any case, since it becomes the target for plasma falling in the gravitational field during each intermediate cooling phase. As a result, the gap between the chromosphere and corona is more abrupt than is usually considered, endowing the region with thermo-electric characteristics which allow energy to be extracted when modest current-densities arise. Energy-conversion at this region fulfills two rôles: it supplies at least part of the heat required by the overlying corona, and maintains contact between the chromosphere and corona via non-thermal transport processes.     

Oscillations and Waves in Solar Spicules

Since their discovery, spicules have attracted increased attention as energy/mass bridges between the dense and dynamic photosphere and the tenuous hot solar corona. Mechanical energy of photospheric random and coherent motions can be guided by magnetic field lines, spanning from the interior to the upper parts of the solar atmosphere, in the form of waves and oscillations. Since spicules are one of the most pronounced features of the chromosphere, the energy transport they participate in can be traced by the observations of their oscillatory motions. Oscillations in spicules have been observed for a long time. However the recent high-resolution and high-cadence space and ground based facilities with superb spatial, temporal and spectral capacities brought new aspects in the research of spicule dynamics. Here we review the progress made in imaging and spectroscopic observations of waves and oscillations in spicules. The observations are accompanied by a discussion on theoretical modelling and interpretations of these oscillations. Finally, we embark on the recent developments made on the presence and role of Alfvén and kink waves in spicules. We also address the extensive debate made on the Alfvén versus kink waves in the context of the explanation of the observed transverse oscillations of spicule axes.     

Mechanisms for the discrete aurora — A review

The V-shock is identified as the primary mechanism for the acceleration of electrons responsible for the discrete aurora. A brief review of the evidence supporting the V-shock model is given, including the dynamics of auroral striations, anomalous motion of barium plasma at high altitudes and in-situ observations of large electric fields. The V-shock is a nonlinear, n = 0 ion cyclotron mode soliton, Doppler shifted to zero frequency. The V-shock is also shown to be a generalization of the one-dimensional double layer model, which is an ion acoustic soliton Doppler shifted to zero frequency. The essential difference between the double layer theory and the theory for the oblique, current-driven, laminar electrostatic shock is that the plasma dielectric constant in directions perpendicular to the magnetic field is c ²/V _a ^/2 , where V _a is the Alfvén velocity; but the plasma dielectric constant parallel to the magnetic field is unity. Otherwise, in the limit that the shock thickness perpendicular to the magnetic field is much larger than an ion gyroradius, the equations describing the double layer and the oblique shock are the same. The V-shock, while accounting for the acceleration of auroral electrons, requires an energy source and mechanism for generating large potential differences perpendicular to the magnetic field. An energy source is the earthward streaming protons coming from the distant magnetospheric tail. It is shown how these protons can be energized by the cross-tail electric field, which is the tailward extension of the polar cap dawn-to-dusk electric field. The local, large cross-field potential differences associated with the V-shock are seen to be the result of a non-linear, E × B drift turbulent cascade which transfers energy from small- to large-scale sizes. Energy at the smallest scale sizes comes from the kinetic energy in the ion cyclotron motion of the earthward streaming protons, which are unstable against the zero-frequency flute-mode instability. The review points out the gaps in our understanding of the mechanism of the diffuse aurora and the mechanism of the auroral substorm.     

Magnetic storms and associated phenomena

This review will not merely be a précis of the literature in this field though a partial survey is attempted. A critical stand will be taken and a point of view put forward. Experiments to test this point of view and others will be suggested. Several new ideas are introduced.Two broad conditions of the magnetosphere are discussed, the quiet and the disturbed. During the quiet condition, the polar cap F region either glows red or is filled with a family of red auroral arcs parallel roughly to L-contours. Auroras near the auroral zone have an increasing amount of green (5577) coloration. The ionospheric F region exists even in winter over the polar caps despite the absence of solar ionizing radiation or obvious corpuscular bombardment. The red polar glow and the maintenance of the quiet polar winter F region are suggested to be accounted for by the cooling of plasma in the geomagnetic tail. These phenomena consume less than 0.01 of the energy and flux of the solar wind impinging on the magnetosphere. The relevance of dynamo theory to this quiet condition is discussed.During the disturbed condition, many phenomena such as polar magnetic substorms, auroral substorms, the sudden appearance of islands of energetic particles in the magnetosphere, and the rapid acceleration of auroral particles appear to call for the operation of an instability deep in the magnetosphere.The energetics of various facets of geomagnetic disturbance are discussed, and joule dissipation of ionospheric current is found to be a major sink of energy during storms. This causes significant heating of the ionosphere particularly at the site of auroral electrojets. Corpuscular bombardment may consume as much energy, but its heating effect is likely to be less.The stable auroral red arc (SAR-arc) observed equatorwards of normal active aurora during magnetic storms is a major sink of energy of a magnetospheric ring current. It is contended that the ring current generally consists of particles of energy of less than a few keV. It is suggested that the ring current is caused by the irreversible pumping and energisation of plasma from the outer to the inner magnetosphere. This pumping is achieved by the random electrostatic fields associated with the noisy component of geomagnetic disturbance. The SAR-arc must be a major feature of ring current theory.The consumption of energy in polar magnetic and auroral substorms, during a complete storm, is tentatively concluded to be far greater than that of the ring current. The ring current is considered to be a byproduct of magnetic disturbance on higher L-shells.The main phase of a storm should be considered, in storm analysis, as a separate entity from the initial phase, for physically they bear a tenuous and unpredictable relationship to one another. A new system of analysis is proposed in which the onset of geomagnetic noise rather than sudden commencement is taken as the origin of time, both for magnetic and ionospheric storms. This will enable analysis of storms with both gradual and sudden commencements to be made on a common basis.No reliable evidence is found to support the contention that magnetic storms are caused dominantly by neutral H-atoms ejected from the sun. In fact much evidence can be amassed to deny this hypothesis.     

The heating of the solar corona

In this paper a discussion is given of the present state of the theory of the heating of the solar corona by shock waves. Arguments are presented why the main contribution to the mechanical energy flux is of acoustic origin, while estimates for the amount of acoustic energy generated in the convection zone as well as the deviations from isotropy are given. During propagation through the atmosphere acoustic waves develop into shock waves after a distance of a few scale heights in the chromosphere. The heating of the outer layers by dissipation of shock waves is found to be sufficient to account for the observed radiative and corpuscular energy losses.Much emphasis is laid on the competitive role played by the four fundamental processes of energy transfer: mechanical heating, radiation, heat conduction and convection of energy in establishing the equilibrium structure of the corona. The atmosphere may be divided in several regions according to the predominance of one of the energy processes mentioned above.The physical properties of the chromosphere and the solar wind are discussed only where they are intimately connected with the problem of the heating of the corona.The most important aspects of the influence of a magnetic field on the structure and the heating of the corona in magnetically active regions are briefly mentioned. Special attention is paid to the strong channelling of heat flow along the field lines and its consequences for the structure and dynamics of the chromosphere-corona transition layer.     

Solar flares and particle acceleration

Energy release in solar flares occurs during the impulsive phase, which is a period of a few to about ten minutes, during which energy is injected into the flare region in bursts with durations of various time scales, from a few tens of seconds down to 0.1 s or even shorter. Non-thermal heating is observed during a short period, not longer than a few minutes, in the very first part of the impulsive phase; in average flares, with ambient particle densities not larger than a few times 10¹⁰ cm^–3 it is due to thick-target electron beam injection, causing chromospheric ablation followed by convection. In flares with larger densities the heating is due to thermal fronts (Section 1). The average energy released in chromospheric regions is a few times 10³⁰ erg, and an average number of 10³⁸ electrons with E 15 keV is accelerated. In subsecond pulses these values are about 10³⁵ electrons and about 10²⁷ erg per subsecond pulse. The total energy released in flares is larger than these values (Section 2). Energization occurs gradually, in a series of fast non-explosive flux-thread interactions, on the average at levels about 10⁴ km above the solar photosphere, a region permeated by a large number ( 10) of fluxthreads, each carrying electric currents of 10¹⁰–10¹¹ A. The energy is fed into the flare by differential motions of magnetic fields driven by photospheric-chromospheric movements (Section 3). In contrast to these are the high-energy flares, characterized by the emission of gamma-radiation and/or very high-frequency (millimeter) radiobursts. Observations of such flares, of the flare neutron emission, as well as the observation of ³He-rich interplanetary plasma clouds from flares all point to a common source, identified with shortlived ( 0.1 s) superhot ( 10⁸ K) flare knots, situated in chromospheric levels (Section 4). Pre-flare phenomena and the existence of homologous flares prove that flare energization can occur repeatedly in the same part of an active region: the consequent conclusions are that only seldom the full energy of an active region is exhausted in one flare, or that the flare energy is generated anew between homologous flares; this latter case looks more probable (Section 5). Flare energization requires the formation of direct electric fields, in value comparable with, or somewhat smaller than the Dreicer field (Section 6). Such fields originate by current-thread reconnection in a regime in which the current sheet is thin enough to let resistive instability originate (Section 7). Particle acceleration occurs by fast reconnection in magnetic fields 100 G and electric fields exceeding about 0.3 times the Dreicer field at fairly low particle densities ( 10¹⁰ cm^–3); for larger densities plasma heating is expected to occur (Section 8). Transport of accelerated particles towards interplanetary space demands a field-line configuration open to space. Such a configuration originates mainly after the gradual gamma-ray/proton flares, and particularly after two-ribbon flares; these flares belong to the dynamic flares in Sturrock and vestka's flare classification. Acceleration to GeV energies occurs subsequently in shock waves, probably by first-order Fermi acceleration (Section 9).     

Solar Magnetism: The State of Our Knowledge and Ignorance

We review some longstanding scientific mysteries related to solar magnetism, with final attention to the mystery of the “turbulent diffusion” essential for the theoretical α ω-dynamo that is believed to be the source of the magnetic fields of the Sun. Fundamental difficulties with the concept of turbulent diffusion of magnetic fields suggest that the solar dynamo problem needs to be reformulated. An alternative dynamo model is proposed, but it remains to be shown that the model can provide the quantitative aspects of the cyclic magnetic fields of the Sun.     

Magnetic energy storage and conversion in the solar atmosphere

A review of the theoretical problems associated with preflare magnetic energy storage and conversion is presented. The review consists of three parts; preflare magnetic energy storage, magnetic energy conversion mechanisms, and preflare triggers. In Section 2, the relationship between magnetic energy storage and the electrodynamic coupling of the solar atmosphere is developed. By accounting for the electrodynamic coupling of the solar atmosphere, we are able to examine the fundamental problems associated with the concept of in situ versus remote magnetic-energy storage. Furthermore, this approach permits us to distinguish between the roles of local and global parameters in the storage process.Section 3 is focused on the conversion mechanisms that can explain, in principle, the rapid energy release of a flare. In addition, we discuss how electrodynamic coupling eventually dictates which mechanism(s) is responsible for releasing the stored magnetic energy, and how the global coupling dictates the final evolution of the relevant mechanism. Section 4 examines preflare triggers and Section 5, we examine the most promising directions for future research into the problem of magnetic-energy storage and conversion of the Sun.     

Interplanetary origin of geomagnetic storms

Around solar maximum, the dominant interplanetary phenomena causing intense magnetic storms (Dst<−100 nT) are the interplanetary manifestations of fast coronal mass ejections (CMEs). Two interplanetary structures are important for the development of storms, involving intense southward IMFs: the sheath region just behind the forward shock, and the CME ejecta itself. Whereas the initial phase of a storm is caused by the increase in plasma ram pressure associated with the increase in density and speed at and behind the shock (accompanied by a sudden impulse [SI] at Earth), the storm main phase is due to southward IMFs. If the fields are southward in both of the sheath and solar ejecta, two-step main phase storms can result and the storm intensity can be higher. The storm recovery phase begins when the IMF turns less southward, with delays of ≈1–2 hours, and has typically a decay time of 10 hours. For CMEs involving clouds the intensity of the core magnetic field and the amplitude of the speed of the cloud seems to be related, with a tendency that clouds which move at higher speeds also posses higher core magnetic field strengths, thus both contributing to the development of intense storms since those two parameters are important factors in genering the solar wind-magnetosphere coupling via the reconnection process. During solar minimum, high speed streams from coronal holes dominate the interplanetary medium activity. The high-density, low-speed streams associated with the heliospheric current sheet (HCS) plasma impinging upon the Earth's magnetosphere cause positive Dst values (storm initial phases if followed by main phases). In the absence of shocks, SIs are infrequent during this phase of the solar cycle. High-field regions called Corotating Interaction Regions (CIRs) are mainly created by the fast stream (emanating from a coronal hole) interaction with the HCS plasma sheet. However, because the B_z component is typically highly fluctuating within the CIRs, the main phases of the resultant magnetic storms typically have highly irregular profiles and are weaker. Storm recovery phases during this phase of the solar cycle are also quite different in that they can last from many days to weeks. The southward magnetic field (B_s) component of Alfvén waves in the high speed stream proper cause intermittent reconnection, intermittent substorm activity, and sporadic injections of plasma sheet energy into the outer portion of the ring current, prolonging its final decay to quiet day values. This continuous auroral activity is called High Intensity Long Duration Continuous AE Activity (HILDCAAs). Possible interplanetary mechanisms for the creation of very intense magnetic storms are discussed. We examine the effects of a combination of a long-duration southward sheath magnetic field, followed by a magnetic cloud B_s event. We also consider the effects of interplanetary shock events on the sheath plasma. Examination of profiles of very intense storms from 1957 to the present indicate that double, and sometimes triple, IMF B_s events are important causes of such events. We also discuss evidence that magnetic clouds with very intense core magnetic fields tend to have large velocities, thus implying large amplitude interplanetary electric fields that can drive very intense storms. Finally, we argue that a combination of complex interplanetary structures, involving in rare occasions the interplanetary manifestations of subsequent CMEs, can lead to extremely intense storms. This revised version was published online in June 2006 with corrections to the Cover Date.     

Solar modulation of galactic cosmic radiation

In this review an attempt is made to present an integrated view of the solar modulation process that cause time variation of cosmic ray particles. After briefly surveying the relevant large and small scale properties of the interplanetary magnetic fields and plasma, the motion of cosmic ray particles in the disordered interplanetary magnetic fields is discussed. The experimentally observed long term variations of different species of cosmic ray particles are summarised and compared with the theoretical predictions from the diffusion-convection model. The effect of the energy losses due to decelaration in the expanding solar wind are clearly brought out. The radial density gradient, the modulation parameter and their long term variation are discussed to understand the dynamics of the modulating region. The cosmic ray anisotropy measurements at different energies are summarised. At high energies (E 1 GeV), the average diurnal anisotropy is shown to be energy independent and along the 18.00 h direction consistent with their undergoing partial corotation with the sun. The average semi-diurnal anisotropy seems to vary with energy as E ⁺¹ and incident from a direction perpendicular to the interplanetary field line, consistent with the semi-diurnal component being produced by latitudinal gradients. Both the diurnal and semi-diurnal components are shown to be practically time invariant. On a day to day basis, however, the anisotropy characteristics such as the exponent of variation, the amplitude and the phase show very high variability which are interpreted in terms of convection and variable field aligned diffusion due to the redistribution of the galactic cosmic ray density following transient changes in the interplanetary medium. The anisotropy observation at low energies (E 100 MeV) are, however, not explained by the theory.The rigidity dependence and the anisotropies during short term variations such as Forbush decreases are discussed in terms of the proposed field models for the interplanetary field structure and are compared with the observed rigidity dependence of long term variations. The data pertaining to the 27 day corotating Forbush decreases and their association with enhanced diurnal variation are also presented. The relationship between the energetic storm particle events which are caused by the acceleration of particles in the shock fronts and the Forbush decreases which are caused by the exclusion of galactic particles by the enhanced field structure in the same fronts are clearly brought out. Thus the recurrent increases at low energies and recurrent decreases at high energies may both be caused by the field structure in the shock front. In conclusion, the properties of the very short period fluctuations (18–25 cph) are summarised.