The HUS solar flare and cosmic gamma-ray burst detector aboard the ULYSSES spacecraft

The HUS-Ulysses team has prepared an instrument aboard the ULYSSES spacecraft consisting of 2 CsI detectors and 2 Si surface barrier detectors for measuring X-rays in the range 5–200 keV up to 8 ms resolution. The prime objectives are the study of solar flares and of cosmic gamma-ray bursts. The ULYSSES mission will leave the ecliptic during the next solar cycle. The solar data can be used in conjunction with other experiments to measure the directivity of the emission and for correlative studies. The cosmic gamma-ray burst data will improve source localizations, allowing sensitive searches for counterparts. The energy range and the 4 field of view is well suited to the detection of the soft gamma-ray repeaters.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain     

Solar flares and the cosmic ray intensity

The relationship between the cosmic ray intensity and solar activity during solar cycle 20 is discussed. A model is developed whereby it is possible to simulate the observed cosmic ray intensity from the observed number of solar flares of importance 1. This model leads to a radius for the modulation region of 60–70 AU. It is suggested that high speed solar streams also made a small contribution to the modulation of cosmic rays during solar cycle 20.     

The acceleration and propagation of solar cosmic rays as deduced from the relative abundance of protons to helium nuclei

Except for protons, the chemical composition of solar cosmic rays is very similar to the abundance of the elements at the photosphere of the Sun. If we consider the relative abundance ratio of protons to -particles (P/) at constant rigidity, this ratio is highly variable from one solar cosmic ray event to another. This ratio observed at the Earth, however, decreases monotonically with time from the onset of solar flares and, furthermore, is dependent on the heliocentric distance of the parent flares from the central meridian of the solar disk. P/'s which have been measured before the onset of SC geomagnetic storms change from 1.5 to 50 or more, being a function of the westward position of the source from the east limb of the Sun. These variations with respect to time and heliocentric distance suggest that the propagation of solar cosmic rays is strongly modulated in the interplanetary space. The major part of the -particles seem to propagate as if they are trapped within the magnetic clouds which produce SC geomagnetic and cosmic ray storms at the earth.The chemical composition and rigidity spectra of solar cosmic rays suggest that solar cosmic rays are mainly accelerated by the Fermi mechanism in solar flares. The observed variation of P/'s is produced mainly through the difference between the propagation characteristics of protons and -particles.NAS-NRC Associate with NASA.     

A study of the modulating effect of solar flares on the cosmic ray intensity using time series analysis

The cosmic ray 11-year variation for solar cycle 20 is attributed to the modulating effect of solar flare-induced shocks propagating through the interplanetary medium to the boundary of the heliosphere. The relative influence of these disturbances upon the cosmic ray intensity as a function of their travel time from the Sun is determined by a deconvolution of a linear system with the number of solar flares (importance 1) and the observed cosmic ray intensity as the input and output respectively of this system. The impulse response function so determined indicates that the solar flare - induced disturbances significantly modulate cosmic rays out to a distance of 70–90 AU where the modulating effect of the disturbances abruptly ends. This is interpreted as the boundary of the heliosphere.     

Determination of the source height and anisotropy of solar hard X-rays by measurements with good time resolution

When emitted at the same time, solar hard X-rays reflected by the photosphere arrive at an observer at later times than primary hard X-rays coming directly from the source. This time lag of albedo photons, therefore, should be taken into account in interpreting fine-scale hard X-ray time profiles. If hard X-ray bursts consist of succession of short-lived elementary bursts, under favorable conditions reflected hard X-rays can be resolved from primary hard X-rays with good time resolution. If so, from the time lag and the ratio of the albedo flux to the primary flux, one can determine the source height and anisotropy of solar hard X-rays.     

The decay phase of solar flare events

A study of the properties of the cosmic radiation of energy - 10 MeV generated by solar flares is reported. Data from four Pioneer spacecraft in interplanetary orbits, and separated by 180° in heliocentric longitude are employed. Attention is restricted to the properties evident at times in excess of 1 day after the occurrence of the parent flare. The anisotropic character of the radiation; the gradients in heliocentric longitude; the decay time constants; and the energy spectra of the radiation are all studied in detail.It is found that the equilibrium anisotropy assumes a direction - 45° E of the satellite-Sun line at very late times. It is suggested that the anisotropy at such times is parallel to E × B. This observation confirms that convection is the determining process in the escape of the solar cosmic rays from the solar system. It indicates that a positive radial gradient of solar cosmic radiation density has builtup at orbit of Earth some 4 days after a flare. This results in an effective convective velocity of approximately 1/2 the solar wind velocity. Direct measurements indicate the presence of strong gradients in heliocentric longitude even at very late times ( 4 days). These gradients are essentially invariant with respect to time, e-folding angles of n - 30° have been observed at - 10 MeV. The presence of these gradients has a major effect on the temporal variation of the cosmic ray flux during the decay phase of the flare effect. Thus, the observed decay time constant is either increased or decreased relative to the convective value depending on the position of the observer relative to the centroid of the cosmic ray population injected by the flare. The effect of the gradient becomes more pronounced at lower energies, and may even exceed the convective removal rate. The observed decay time constant, the characteristics of the anisotropy, and the gradient in longitude are shown to be inter-related as demanded by theory. It is shown that the exponent of the cosmic ray spectrum is dependent on the location of the observer relative to the centroid of the cosmic ray population injected by the parent flare. At a given point in the frame of reference of the cosmic ray population, the spectral exponent is invariant with time.Now at CSIRO, G.P.O. Box 124, Port Melbourne, Victoria 3207, Australia.On leave from Physical Research Laboratory, Ahmedabad, India.     

UH cosmic rays and solar system material: The elements just beyond iron

The origin of the elements from Cu to As in the UH (ultra-heavy) cosmic rays is investigated and related to current concepts of the nucleosynthesis of solar system material. The charge spectrum of the UH cosmic rays in the interval 29Z60 is studied via a fully developed propagation calculation for source abundances given by solar system material, ther-process, the massive-star core helium-burnings-process, and explosive carbon burning. None of these sources considered individually can explain the cosmic ray observations. However a combination of material produced in ther-process, the core helium-burnings-process and in explosive carbon burning provides a good representation of the experimental data. The cosmic-rayr-process is found to differ from solar systemr-process events by an underproduction of the low-massr-process peaks relative to theA195 peak. The large cosmicray abundance forZ=40–44 may be due to anr-process fission component, but this explanation is by no means certain. Improved cosmic-ray data, especially for Zn–Sr, can provide limits to the various source contributions. The model described here gives a consistent picture for the origin of both the cosmic rays and the solar system elements just beyond iron, and adds additional evidence for the importance of massive stars as a site of nucleosynthesis and the birthplace of the cosmic rays.Enrico Fermi Institute.     

Studies of the correlation between the global radiation,ultraviolet radiation,cosmic radiation,sunspot number and meteorological parameters

The correlation between the ratio of the global irradiation to the extraterrestrial solar radiation (H/H ₀), and the ratio of the ultraviolet solar irradiation to the extraterrestrial solar radiation (H _u /H ₀) on a horizontal surface at Bahrain (=26°), and some terrestrial and solar parameters (the monthly average relative humidity, temperature, relative sunshine duration, cosmic radiation intensity, and sunspot number) have been studied. Moreover, the role of the solar effects and the terrestrial effects on the global and the solar ultraviolet radiation has been studied. A detailed investigation has been carried between the level of the cosmic radiation received at Bahrain and the sunspot number. It was concluded that as the solar activity increases, cosmic radiation and sunspot number play a predominant effect on the correlation of (H/H ₀) and (H _u /H ₀). Furthermore, the correlation between cosmic radiation and sunspot number also increases.     

Cosmic-ray Periodicity at 170 Days

Power-spectral analysis of cosmic-ray indices (CRI) data for the years 1989–1991 shows a 170-day periodicity of cosmic rays. The periodicity is related to a strong magnetic field. Power-spectral analysis of the long-term periodicity (11 years) of the CRI data for the years 1953–1997 shows that the period 1989–1991 is a unique one in the sense of the discussed pronounced periodicity. The 170-day periodicity of cosmic rays was interpreted in the base of six solar rotations (1 SR = 28.3-day periodicity of 10.7 cm solar radio flux) and may be connected to the instability of the solar core.     

ISOCAM Long Wave Detector Glitches: Data and Ray-Tracing Simulations

The affected pixel number distribution of the ISOCAM Long-Wave (LW) detector in cosmic ray-induced glitchesis calculated. The methods employed are Monte Carlo ray-tracing techniques and the taxi metric, which allows direct calculation of the affected pixels based on the knowledge of entry and exit points of the ray. The simulation results are compared with long-term experimental data. Based on the simulations combined with the CREME96 cosmic ray model for solar quiet period, it is estimated that the detector is on average traversed by 0.3 cosmic ray protons per second. For the experimental data, a corresponding minimum bound of 0.14 cosmic ray protons per second is obtained. Deviations in the simulated and measured pixel number distributions are discussed.     

Dynamics of electron beams in a coronal loop and the hard X-ray burst

A numerical simulation has been made for the dynamics of non-thermal electrons (> 10keV) injected with spatial, temporal and velocity distributions into a model coronal loop. The time variations of the spatial intensity distribution and the spectrum for the expected hard X-rays are computed for many models in order to find the important physical parameters for those characteristics.The most important one is the column density of plasma, CD, along the loop. If CD is smaller than 10²⁰ cm^–2, the expected X-rays behave like the solar impulsive hard X-ray bursts, that is the spatial maximum of X-rays shifts to the top of the loop in the later phase of the burst accompanying a spectral softening. On the other hand, if CD is greater than this value, quasi-steady decay appears in the later phase. In this case the intensity distribution of X-rays above about 20 keV along the loop shows a broad maximum away from the loop top giving an extended spatial distribution of hard X-rays, and spectral hardness is kept constant. These characteristics are similar to the solar gradual hard X-ray bursts (the so-called extended burst which is not a hot thermal gradual burst).     

An X-ray observation in the atmosphere during the August 7, 1972 solar flare

An observation carried out with a balloon-borne detector of an additional flux of secondary X-rays (E 30 keV) at large depths in the atmosphere is described. This excess is attributed to the emission of very hard X-rays during the solar flare of August 7, 1972. The propagation in the atmosphere of the secondary photons resulting from their electromagnetic interactions in the air is computed by utilizing the Monte Carlo method. The computations agree with the observed flux when a very hard solar X-ray spectrum is assumed.     

Production of metagalactic X-rays by relativistic dust grains

The relativistic dust grains which may be responsible for ultra-high energy cosmic rays, as suggested by the present author, interact with the cosmic black-body radiation. This results in the energy loss of the relativistic dust grains, so that their energy spectrum is cut-off at the Lorentz factor as large as 2×10³ (0.1/a), wherea is the grain radius. The black-body radiation is scattered and absorbed by the dust grains. The photons scattered and reemitted contribute to metagalactic X-rays. The X-ray intensity estimated is comparable to the observed one in the soft X-ray region.     

A new technique for estimating D-region effective recombination coefficients under different solar flare conditions

Under solar flare conditions, the intensity of the solar X-rays below 10 Å increases by several orders of magnitude, while the increase in intensity of H-L is small. Photo-ionization rates in the various wavelength bands in theXUV spectrum have been presented graphically as a function of altitude under quiet, M3, X4, and outstanding flare conditions to show the relative importance of solar X-rays below 10 Å in the height range between 50 and 90 km. Presuming the total time constant for recombination of the ions with electrons remains constant at each altitude under different flare conditions, one can obtain the effective recombination coefficient _eff under these conditions with a knowledge of the quiet time recombination coefficient, the production rate profiles and profiles. The importance of the ratio of negative ions to electrons below 70 km in lowering the effective electron loss rates has been highlighted.     

Short-period pulsations observed simultaneously by X-ray and radio waves

From simultaneous high-time-resolution observations of solar X-rays from Hinotori and the millimeter waves at Itapetinga Radio Observatory in Brazil during a solar flare on November 4, 1981 at 1827 UT, short period ( 300 ms) pulsations have been detected in five time intervals of 2 s each. Both a cross-correlation analysis between X-rays and microwaves and a Fourier analysis were made to verify the significance of the quasi-periodic pulsations. The cross-correlation is significant but the pulsations could not be periodic oscillation.on leave of absence from Physical Res. Lab., Ahmedabad, India     

Anisotropic solar cosmic ray propagation in an inhomogeneous medium,II

The author's model for anisotropic solar cosmic ray propagation gives 2 coupled, partial differential equations for the intensity and anisotropy of solar cosmic rays propagating with finite speed V in an inhomogeneous medium. The model is used to study the effect of the solar shell on solar cosmic ray propagation. It predicts an exponential decay, regardless of the observer's position. It predicts that when the observer is near the center of the shell, t _D/t ₀ 20 to 30, (t _D= decay time, t ₀ = onset time) and A _m(anisotropy) 15%, if t _m/t ₀ 3 to 5 (t _m= time of maximum), consistent with observations of relativistic particles on Feb. 23, 1956. When the observer is between the shell and the sun, the model predicts that oscillations might be observed near maximum intensity. When the observer moves away from the sun and the shell, the propagation is diffusive, but there is an increasingly large persistent anisotropy which serves as a measure of the width of the shell.     

Mid-term periodicities and heliospheric modulation of coronal index and solar flare index during solar cycles 22–23

We applied fast Fourier transform techniques and Morlet wavelet transform on the time series data of coronal index, solar flare index, and galactic cosmic ray, for the period 1986–2008, in order to investigate the long- and mid-term periodicities including the Rieger (\({\sim }130\) to \({\sim }190\) days), quasi-period (\({\sim }200\) to \({\sim }374\) days), and quasi-biennial periodicities (\({\sim }1.20\) to \({\sim }3.27\) years) during the combined solar cycles 22–23. We emphasize the fact that a lesser number of periodicities are found in the range of low frequencies, while the higher frequencies show a greater number of periodicities. The rotation rates at the base of convection zone have periods for coronal index of \({\sim }1.43\) years and for solar flare index of \({\sim }1.41\) year, and galactic cosmic ray, \({\sim }1.35\) year, during combined solar cycles 22–23. In relation to these two solar parameters (coronal index and solar flare index), for the solar cycles 22–23, we found that galactic cosmic ray modulation at mid cut-off rigidity (\(\hbox {Rc} = 2.43\hbox {GV}\)) is anti-correlated with time-lag of few months.     

Cosmic-ray flow lines and energy changes

In this paper we discuss the particle flow or streaming and energy changes of cosmic rays in the interplanetary region via flow lines in momentum-position space. We consider the steady-state case where particles are released monoenergetically from the Sun or from infinity and study the cosmic-ray traffic pattern in momentum and position arising from monoenergetic sources. The analysis makes extensive use of the result (wherep is the particle momentum,V the solar wind velocity andG the cosmic-ray density gradient) for the mean time rate of change of momentum of cosmic rays reckoned for a fixed volume in a reference frame fixed in the solar system, developed by us in several recent papers.Deceased.     

Energy changes of cosmic rays in the interplanetary region

A clarification and discussion of the energy changes experienced by cosmic rays in the interplanetary region is presented. It is shown that the mean time rate of change of momentum of cosmic rays reckoned for a fixed volume in a reference frame fixed in the solar system is 〈p〉 =p V·G/3 (p=momentum,V is the solar wind velocity andG=cosmic-ray density gradient). This result is obtained in three ways:

1. by a rearrangement and reinterpretation of the cosmic-ray continuity equation;
2. by using a scattering analysis based on that of Gleeson and Axford (1967);
3. by using a special scattering model in which cosmic-rays are trapped in ‘magnetic boxes’ moving with the solar wind.

The third method also gives the rate of change of momentum of particles within a moving ‘magnetic box’ as 〈p〉_ad = ?p ?·V/3, which is the adiabatic deceleration rate of Parker (1965). We conclude that ‘turnaround’ energy change effects previously considered separately are already included in the equation of transport for cosmic rays.     

The expected pulsations of the heliosphere relevant to cosmic ray variations

The possible natural large-scale pulsations of the solar wind cavity are examined. The period of the pulsationsT2R/a (R is the size of the cavity,a is the sound velocity in the interstellar space near the solar system) may vary from a year to tens of years. The relevant new type of cosmic ray variations is predicted.