Sec Ster Research Articles

Simultaneous balloon observations at Fort Churchill and Minneapolis during the solar cosmic ray events of July 1961

The solar cosmic ray events of the period July 12–20, 1961, were studied with many simultaneous balloon flights at Fort Churchill, Manitoba, Canada, and Minneapolis and Bemidji, Minnesota. During the low-energy solar cosmic ray event on July 12–13, γ rays were detected that were produced as the result of nuclear reactions of the solar cosmic rays with air nuclei. The balloon results have been correlated with the Injun 1 results, which showed protons in the range 1 to 15 Mev that could not reach the balloons but that produced the observed γ rays. The observed proton γ-ray correlation agrees well with theoretical calculations of cross sections, and thus we are able with confidence to reconstruct the proton flux between infrequent passes over northern latitudes of Injun 1. This γ-ray inferred time history of the event indicated a gradual increase in proton intensity from about 18 hours before the July 13 sudden commencement until shortly thereafter. Then the fluxes dropped sharply, possibly because of an increased geomagnetic cutoff rigidity at Fort Churchill during the initial phase of the magnetic storm, and showed much time structure during the main phase of the storm. In addition to the γ rays, low-energy X rays were observed coincident with the sudden commencement. These X rays are probably due to electron precipitation, as observed in earlier events. The further events on July 18–20 were studied in their entirety by means of balloons floating at high altitude at Fort Churchill and Minneapolis. Since the complete time history of the solar cosmic rays was obtained, it has been possible to study the mechanisms responsible for the increase and the decay phase of these events. The intensity increase through the time of maximum for particles in a fixed energy region has been found to be well described by the point source infinite medium diffusion equations in interplanetary space. The decay phase, exponential in character, requires the assumption of an escape barrier located at about two astronomical units from the sun. The diffusion mean free path was found to be between 0.025 and 0.04 AU. From diffusion theory, assuming uniform emission from the point source, the number of particles emitted into unit solid angle was found to be about 1032 particles/cm2 sec ster for the event on July 18 and about an order of magnitude lower for the smaller event on July 20. In addition, we may estimate time of ejection of the solar particles from the sun in finding the best fit to diffusion theory. In these events, the simultaneous flights at different latitudes also enabled us once more to study the geomagnetic cutoff changes associated with the magnetic storm periods. The decrease in cutoff was found to correlate well with negative excursions of the equatorial magnetic field, and, because the particle spectrums were known, the cutoffs at Minneapolis could be estimated.

Intensity of electrons in the Earth's inner radiation zone

The intensities of electrons as measured with the SUI satellite Injun 1 during its deepest penetrations into the earth's inner radiation zone are reported for the period July 16 to August 10, 1961. The unidirectional intensity of electrons of energy greater than 40 kev was 5 x 10/sup 6/ electrons/cm/sup 2/ sec ster in a direction perpendicular to B at L = 1.22 in the magnetic equatorial plane. The corresponding omnidirectional intensity is estimated to be 1 x 10/sup 7//cm/sup 2/ sec. No temporal variations greater than the experimental uncertainty of 30 per cent were found in this region during the period of observation of this report. (auth)

Protons in the outer zone of the radiation belt

The trapped protons in the outer zone of the Van Allen radiation belt were measured on October 4, 1960, by means of a Scout rocket payload containing a two-crystal scintillation spectrometer. Protons with energies above 1 Mev were detected, the intensity rising above background at an altitude of 1200 km. Representative intensities for 1≤Ep≤0 Mev at 2, 3, 4, and 5 times; 03 km, through a range of 42°N to 30°N geomagnetic latitudes, were 0.27, 0.90, 2.02, and 4.56 times; 05 protons/cm2 sec ster. These intensities are averages for protons having pitch angles between 90° and ∼60°. The proton energy spectrum at 5000 km is J(E) = 2.0 × 106E−5.2 protons/cm2 sec ster Mev for 1.02≤Ep≤2.24 Mev, and J(E) = 0.71 × 106E−3.9 for 2.24≤Ep≤7.3 Mev. The pitch angle distributions for altitudes of 4600 km and 5150 km were also obtained for angles between 90° and 58°. There was no evidence for a proton intensity maximum coincident with the E2 maximum.

Protons of 0.1 to 5 Mev and electrons of 20 kev at 12 Earth radii during sudden commencement on September 30, 1961

During the period around the sudden commencement of the September 30, 1961, magnetic storm, the ion-electron detector aboard Explorer 12 observed fluxes of low-energy protons and electrons while the satellite was at 12 earth radii and free of the magnetosphere. Whereas the proton intensities initiated an increase about 25 minutes before the sudden commencement, a sudden jump in the omnidirectional intensity occurred eight minutes before the sudden commencement. The maximum flux of about 2×105 protons/cm2 sec ster above 140 kev was attained a half hour after the sudden commencement. Thereafter the intensities varied irregularly with time and displayed fluctuations in anisotropy over the look cone of the detector that were dependent upon the direction of the local magnetic field. The energy spectrum was independent of both time and intensity. The energy density of the proton spectrum (1×10—9 erg/cm3) was too small to account for the observed compression of the earth's field on the sunlit side during the magnetic storm. Time coincident with the sudden jump in proton intensities, an electron flux first appeared and reached its peak value of 3×106 electrons/cm2 sec ster in the 10- to 35-kev range about one minute before the observance of the sudden commencement on the earth's surface. It continued for about 10 minutes and then disappeared. The detection efficiency for electrons places an upper limit of 2×105 electrons/cm2 sec ster for the rest of the storm.

Solar protons and magnetic storms in July 1961

The State University of Iowa's satellite Injun 1 (1961/sub O/sub 2/) was launched at 0422 UT on June 29, 1961, and is in an orbit of 67 deg inclination with apogee 998 km and perigee 881 km. Amorig the Injun instruments are silicon p-n junction detectors sensitive to protons with energy between 1 and 15 Mev, and a Geiger counter sensitive to protons of energy above 40 Mev. These units detected solar protons during the period of considerable solar activity extending from July 11 to 28, an epoch containing twelve flares of class 2 or 3 and six major magnetic storms that can be divided into four distinct solar proton events. In this period the proton intensity varied widely, with a maximum unidirectional flux of 33,000 particles/cm/sup 2/ sec ster for 1to 15-Mev protons occurring during the storm of 1115 UT, July 13, and a maximum omnidirectional flux of 900 particles/cm/sup 2/ sec for protons of energy above 40 Mev in the storm of 1121 UT July 18. In general, the energy spectrum varied from storm to storm and within a single storm. The temporal and spatial characteristics of the solar proton flux are correlated with details of geomagnetic stormsmore » and other solar- terrestrial phenomena such as polar-cap absorption. Some 1- to 15-Mev protons leak out of the main solar stream and enter the geomagnetic fleld many hours before the onset of a magnetic storm. These particles are observed only on magnetic shells of L with values between 4.8 and 6.1 earth radii, the range corresponding to the undisturbed geomagnetic cutoffs. A large influx of 1- to 15- Mev protons occurs during the last part of the initial phase of the storm and during the early part of the main phase, indicating their previous containment in the storm-producing plasma. A marked equatorial shift of the cutoff latitude occurs during the main phase of the storrn. A sharp decrease in the tlux in the main phase is followed by a slower exponential decay with a relaxation time of about one day for the 1- to 15-Mev particles. This decay begins after the peak of the main phase and extends through the recovery period of the storm, during which the geomagnetic cutoff gradually returns to its prestorm value. The possibility that the 1- to 15-Mev particles are geomagnetically trapped during the exponential decay is discussed. Considerable fluxes of protons with energy above 40 Mev were observed during only one of the four solar proton events reported. The storm-time cutoffs of these protons occurred at L va1ues lower than the corresponding cutoffs observed with the 1- to 15-Mev protons. These observations and those reported for similar events by other workers are compared in detail. An over-all picture for such events is proposed. (auth)« less

Note on the electron energy spectrum in the inner Van Allen Belt

The energy spectrum of trapped electrons predicted by the neutron-albedo theory was tested experimentally with a magnetic electron spectrometer for the energy range 40≤E≤110 kev. This detector was flown on the State University of Iowa radiation research satellite Injun I at an altitude of about 1000 km in the inner radiation zone over South America. Another detector on the same satellite measured the directional intensity of electrons with E≥40 kev. The data are consistent with a peak directional intensity of electrons in this region of N(E)∼104 exp ( −E/160) particles (cm2 sec ster kev)−1, or with a power-law spectrum with a differential exponent of −(1.0±0.2) for 40≤E≤110 kev. This result is significantly in disagreement with the predictions of the neutron-albedo theory.

An extremely intense electron flux at 1000-kilometer altitude in the auroral zone

The radiation-research satellite Injun I passed through an extremely large electron flux, at 1000 km altitude and high latitudes, around 2238 UT on September 25, 1961. The peak flux was 400 ergs (cm/sup 2/ sec ster)/sup -1/ from electrons with energy above 1 kev and 10/sup 8/ particles (cm/sup 2/ sec ster)/ sup -1/ with energy above 40 kev. The electron flux varied greatly both in intensity and spectral character during the satellite pass. For example, the intensity changed by a factor of about a million in a time of about 30 seconds or a distance of several hundred kilometers. The electron spectrum changed so greatly and so rapidly that, if a differential power-law spectrum was applied, values of the exponent ranged between above 4 and more than 14. At this time, the electron flux was measured with similar detectors on Explorer XII in the heart of the outer zone on the same magnetic shell but at a different longitude. Since the electron intensity and spectrum measured by Explorer XII were normal, the anomalous flux seen by Injun was confined in longitude. The Injun observations indicated that the flux was essentially isotropic over the upper hemisphere. The energy flux wasmore » sufficient to cause a very bright aurora, and the measurements were tabulated for comparison with auroral theories. The measurements proved that an acceleration mechanism in the ionosphere was not always necessary to produce an aurora. (auth)« less

Measurements of the intensity and spectrum of electrons at 1000-kilometer altitude and high latitudes

The intensity and spectrum of electrons at 1000-km altitude were measured by detectors on the SUI satellite Injun I at magnetic latitudes between about 45° and 80°. A preliminary report is given here for the period July and August 1961; further observations are continuing. Trapped electrons with energy above 40 kev were found between magnetic latitudes 45° and about 75° in typical intensities of 105 to 106 particles (cm2 sec ster)–1, with peak intensity measured at 4×106 particles (cm2 sec ster)−1. The intensity above 75° was less than 102 particles (cm2 sec ster)–1. Occasionally the electron intensity reached a broad peak around 50° and a second peak near the auroral zone. The intensity in the second region often varied by an order of magnitude in times of a few seconds or distances of a few tens of kilometers. If we assumed that the differential electron spectrum followed a power law of the usual form dN = constant E−γ dE, then, for both trapped and dumped electrons in the energy range 40 to 120 kev, γ varied significantly between 0 and 2 in the low-latitude region below 60°, and between 0 and 7 at latitudes above 60°. The spectrum was generally softer at higher latitudes. On several occasions in the daytime, electrons with energy above 1 kev were observed in fluxes of the order of 10 ergs (cm2 sec ster)−1 at such angles to the magnetic field vector that they must have penetrated to auroral altitudes of 200±200 km. Since they could produce weak auroras, it is concluded that auroras can occur in the daytime. Since they were detected at 1000-km altitude, it is concluded that an acceleration mechanism in the ionosphere is not always necessary to produce an aurora in the same hemisphere. The measurements are compared with other satellite, rocket, and balloon measurements. It is tentatively concluded that most of the counts of the type 302 shielded Geiger tube in Explorer VII were due to penetrating particles, very likely electrons with energy of several Mev in peak intensity of order 103 particles (cm2 sec)−1. The significance of the latitude distribution of the low-energy electron fluxes is discussed.

Direct observations of dumping of electrons at 1000-kilometer altitude and high latitudes

Quiescent dumping of electrons with energy above 40 kev in intensities of the order of 104 particles (cm2 sec ster)−1 was detected by the Injun I satellite at altitudes about 1000 km and high latitudes around July 1, 1961. In 16 passes between longitudes of 230°E and 340°E, the angular distribution of electrons about the magnetic vector B showed there was dumping in every one of 8 passes between 1200 and 1500 local time, and in only one of 8 passes between 1900 and 2300. When there was no dumping, the directional intensity of electrons at ∼20° from the normal to B was less than 1 per cent of that normal to B. When there was dumping, the corresponding value was of the order of 100 per cent, and the intensity at about 50° from the normal to B was of the order of 10 per cent of that normal to B. Appreciable intensities of electrons were backscattered. Dumping was seen over very wide ranges of latitude between L∼2 and L∼10 and was uncorrelated with Kp. It is pointed out that the dumped electrons may be either new, freshly injected or accelerated particles, or they may be old, formerly trapped particles, which were perturbed in their orbits. The obervations are further evidence that acceleration mechanisms must occur frequently at altitudes above 1000 km.

Hydrogen nuclei of the primary cosmic radiation

Abstract A small stack of nuclear emulsions has been used to determine the flux of cosmic ray protons over Texas. The experimental procedure was similar to that used previously, but due to the more favourable conditions of the exposure the dependence of the value at the top of the atmosphere on the, rather uncertain, corrections for the overlying matter, was greatly reduced, the correction being 0·902 o.f. the previous 0·694. The value found for the flux at the top of the atmosphere on July 12, 1960 was 540 ± 50 protons/ma ster sec.

The alpha-particle component of the primary cosmic radiation over northern england

Abstract Alpha-particles of the primary cosmic radiation have been studied in a stack of nuclear emulsions exposed over northern England on July 29, 1959. A flux of 167 ± 12 α-particles/m2 ster sec was found. The energy spectrum was examined between the cut-off energy of about 250 MeV per nucleon and an energy of 1·5 BeV per nucleon. It was not found to be significantly different from that observed during solar maximum. The energy spectrum was also examined as a function of zenith angle. An apparently significant linear relationship was established between neutron monitor counts recorded at sea level and primary α-particle flux values. A value of 24·0 ± 2·4cm has been determined for the mean free path of α-particles in nuclear emulsions, which is somewhat higher than previously reported values.

The determination of the flux of cosmic ray protons with nuclear emulsions

Abstract The flux of protons in the primary cosmic radiation has been determined in a stack of nuclear emulsions exposed during 1954 over northern Italy. This determination, which depended on the observation of the nuclear interactions produced by the protons, gave a flux at the top of the atmosphere of 500±70 protons/m2 ster sec, a value in good agreement with that expected from counter measurements. A simple model of the variation of the proton flux with depth in the atmosphere has been derived which appears to be in reasonable agreement with the experimental data.