Steep Energy Spectrum Research Articles

SAS-2 gamma ray observations related to a galactic halo

An examination of the intensity, energy spectrum, and spatial distribution of the diffuse γ radiation observed by SAS 2 away from the galactic plane in the energy range above 35 MeV has revealed no evidence supporting a cosmic ray halo surrounding the galaxy in the general shape of a sphere. The diffuse γ radiation does consist of two components. One component is related to the galactic disk on the basis of its correlation with the 21-cm measurements, the continuum radio emission, and galactic coordinates. Further its energy spectrum is similar to that in the plane, and its intensity distribution joins smoothly to the intense radiation from the plane. The other component appears isotropic, at least on a coarse scale, and has a steep energy spectrum. The degree of isotropy which has been established for the “isotropic” radiation and the steep energy spectrum, which distinguishes it from the galactic disk radiation, place strong constraints on galactic halo models for the origin of this component. Theoretical models involving a galactic halo have generally postulated a halo with dimensions of the order of the Galaxy and hence a radius, at least in the plane, of about 15 kpcs. Since the Sun is about 10 kpc from the galactic center, if such a halo exists and is responsible for the γ rays (through, for example, black body Compton radiation), a very marked anisotropy would be seen, with the γ ray intensity from the general direction of the galactic center being much larger than that from the same latitudes in the anticenter direction. In fact, no such anisotropy is seen; specifically the ratio of the average intensity in the (300° < ℓ < 60°, 20° < |b| < 40°) region to that in the (100° < ℓ < 250°, 20° < |b| < 40°) region was found to be 1.10±0.19 compared to a calculated value for a model with a uniform cosmic ray sphere with a 15 kpc radius of 2.85. The ratio between the average γ-ray intensity from regions with |b| < 60° to that from 20° < |b| < 40° is found to be 0.87±0.09. If the region is assumed to be spherical, but with a larger radius and a uniform cosmic ray density, the upper limit (2σ) set for the anisotropy demands that the radius be at least 45 kpc. An extragalactic origin for the isotropic component currently appears to be a more plausible explanation.

Diffuse gamma radiation

Results are reported for an investigation of the intensity, energy spectrum, and spatial distribution of the diffuse gamma radiation detected by SAS 2 away from the galactic plane in the energy range above 35 MeV. The gamma-ray data are compared with relevant data obtained at other wavelengths, including 21-cm emission, radio continuum radiation, and the limited UV and radio information on local molecular hydrogen. It is found that there are two quite distinct components to the diffuse radiation, one of which shows a good correlation with the galactic matter distribution and continuum radiation, while the other has a much steeper energy spectrum and appears to be isotropic at least on a coarse scale. The galactic component is interpreted in terms of its implications for both local and more distant regions of the Galaxy. The apparently isotropic radiation is discussed partly with regard to the constraints placed on possible models by the steep energy spectrum, the observed intensity, and an upper limit on the anisotropy.

Celestial Diffuse Gamma Radiation above 30 MeV Observed by SAS-2

The Small Astronomy Satellite (SAS)-2, launched on November 15, 1972, carried into orbit a 32-deck magnetic-core digitized spark chamber gamma ray telescope to study celestial gamma radiation in the energy range above 30 MeV. In the study of several regions with b sub 2 15 deg, a finite, diffuse flux of gamma rays with a steep energy spectrum in the energy region from 35 to 200 MeV is observed. Representing the energy spectrum by a power law of the form dJ/dE = AE to - alpha power over this energy range, alpha is found along with the integral flux above 100 MeV. Combining this result with existing low energy gamma ray data yields an energy spectrum which is not a simple power law in energy, as in the X-ray region, but which demonstrates first an increase and then a decrease in slope, consistent within uncertainties with that predicted by cosmological theories, including the continuous production of high energy gamma rays primarily from neutral pi mesons throughout the history of the universe.

On observations of auroral-zone X-rays with energies up to and greater than 200 keV

Balloon measurements of auroral-zone X-rays revealed several precipitation events of electrons with energies up to some hundred keV. These events can be divided in two groups according to their energy spectrum. Events with relatively steep energy spectrum occur during the daytime. Their spectrum can be approximated in an exponential form with characteristic energies well below 50 keV. Nighttime events have a flat energy spectrum, which deviates from an exponential form. In these cases the calculated characteristic energies of exponential fittings over limited energy intervals were generally above 50 keV and a considerable variability of the spectrum was observed. Maximum values of the characteristic energy are often related to maximum changes of intensity, not to maximum intensities. These nighttime events are not related in a unique way to a certain phase of the polar substorm.

Re-interpretation of OSO-III scintillation counter measurements of hard solar X-ray spectra

Laboratory tests show that pulse pile-up requires a significant correction to the response of scintillation counters to solar X-ray bursts with steep energy spectra. The interpretation of the OSO-III and other satellite data in view of these corrections indicates that steep solar X-ray spectra with varying emission measure produce an effect similar to that produced by relatively hard spectra with a constant emission measure.

Proton Flare Project, 1966

The paper summarizes observations of solar and space phenomena related to the McMath region Number 8461 which passed over the solar disk during the 1966 Proton Flare Project period, from August 21 to September 4, and produced two important solar particle events on August 28 and September 2. The most important results are reviewed and interpretation of some of them is suggested. Items of particular interest: Occurrence of proton-active regions when two or more ‘rows’ of activity approach each other (Section 3). Possible stimulation of activity by magnetic fields of decaying regions that had been active before (4.2a, 5.1a). Significantly increased correlation of flares with X-ray bursts during the proton-active transit of the region (5.3b). Striking difference in the flare response in radio frequency range before and after August 26 (5.2b). Hardening of the X-rays (5.3a), increase in radio flux (5.2a), change in sunspot configuration (5.1c), and increased capability of the region for particle acceleration (5.1b, 5.2b), starting about three days prior to the proton flare. Clear evidence that some flares that occurred on or after August 26, but prior to the proton flare of August 28, already were sources of ≈ 1 MeV protons (5.2b, 8). Anomalous deficiency in metric component of radio bursts produced in the region (5.2c, 9.4d, 11.4b). Strong radio storm on meter waves immediately preceding the proton flare on August 28 (5.2a, 9.1b), coincident with preflare rising dark filament (9.1a) and slight preflare rise in flux of ≈ 1 MeV protons (10.2). Two phases of expansion (fast and slow) of the bright flare ribbons (9.2c). Coincidence of hard X-ray burst with the formation and fast separation of the bright flare ribbons. It is suggested that this is the time of particle acceleration in the flare (9.5b). Short-lived burst of UV radiation (9.6). Visible flare wave in the flare of August 28 (9.3b), and complexity of motions in this flare (9.4b). Suggested electron release by means of a blast wave (10.1a). Electron-proton splitting in the delayed shock-wave-associated maximum of the particle flux on August 29 (10.2c). First brightening of both proton flares in a similar position between the regions 8461 and 8459 (11.2c). Existence of a unique, low elevation coronal condensation three days after proton flare occurrences (7.2). Very strong flux of protons in energy range of the order of 100 MeV producing the largest PCA since July 1961, and unusually steep energy spectrum above 100 MeV in the flare of September 2 (12.2a, b, 12.4). Unusually long rise to the maximum flux, inconsistent with Burlaga's theory of anisotropic diffusion (12.2b). Interpretation of the undisturbed flux decay from September 2 to September 8 (12.2c). A corotating modulation phenomenon on September 8 (12.2d). Detection of medium nuclei, with He/M ratio 50 ± 11 (12.3a). Evidence against a purely velocity-dependent mode of particle propagation (12.3b). Electrons as the possible cause of the first PCA phase (12.4). Plasma disturbance due to permanent proton flux from the region (13.1). Electron injection into inner radiation belt during the geomagnetic storm associated with the September 2 flare (13.3). Section 14 brings a time scheme of the most important phenomena associated with the complex of activity and the active region in question, and some unsolved problems of particular interest are pointed out in Section 15.

The general pattern of auroral particle precipitation

Gross statistical data on optical- and radio-auroral phenomena are examined to establish the latitude-versus-time pattern for the causal particle precipitation. Although these phenomena may occur at any time of day at auroral-zone latitudes, they are shown to occur most frequently at about 08 and 22 hours geomagnetic time. These two dominant diurnal maxima exhibit radically different characteristics, indicating that two distinct particle populations are involved on the average. Further, the two types of precipitation are recognizable at other times of day in the statistical data, although at a reduced intensity, leading to a two-zoned precipitation pattern in which there is a clear separation between the zones during most of the day. The most prominent features of the precipitation in the zone with the night-time maximum are shown to be the impulsive character of the precipitation and its steep energy spectrum, whereas the zone with the morning maximum is best described as having a continuous influx with a much flatter particle spectrum. On the basis of detailed studies of these characteristics, of observations on other related ionospheric parameters, and of direct particle measurements made with rockets and satellites it seems plausible to associate the two particle populations with different regions of the magnetosphere. This relationship to the magnetospheric structure and to the processes therein is discussed for both the ‘splash’ and ‘drizzle’ types of precipitation.

Pion production in high-energy cosmic-ray collisions

The Landau-Milekhin hydrodynamic model of very-high energy nucleon interactions is extended to calculate the pion energy and angle distributions in the center-of-mass system (c.m.s.) and laboratory frames. Consideration of the physical limitations on the average energy of the pions produced in the decomposition of the nucleonic fluids leads to truncation of the pion distribution and a rather slow energy dependence of the average c.m.s. pion energy on the incident proton energy. When a power law dependence is chosen for incident primary cosmic-ray protons, the pion production spectrum found in the laboratory frame has a steeper energy spectrum than those found using simpler models of nucleon interactions.