S. Biswas

Ionization states of the anomalous cosmic rays

Ionization states of individual N, O, and Ne ions in the low energy (10-30 MeV/n) anomalous cosmic rays (ACR) are determined in the Anuradha experiment conducted onboard Space Shuttle Spacelab-3 mission during 1985 April-May. The geomagnetic field is utilized as a momentum filter in this experiment to obtain upper limits on the ionization states of individual ACR ions. A combination of an extremely sensitive passive plastic detector and an active electromechanical system allow us to obtain accurate information on the arrival locations and directions of individual ACR ions and hence the threshold rigidity needed by each of these ions to reach the point of their detection in the Spacelab-3 orbit.

Ionization states of low-energy cosmic rays - Results from Spacelab 3 cosmic-ray experiment

The Indian cosmic ray experiment Anuradha, conducted onboard Spacelab 3 during April 29-May 6, 1985 was designed to obtain information on the ionization states of low-energy cosmic rays, using the geomagnetic field as a rigidity filter to place an upper limit on the ionization state of individual cosmic ray particles. This paper presents data confirming the presence of three distinct groups of energetic particles in the near-earth space: (1) low-energy (15-25 MeV/nucleon) anomalous cosmic rays that are either singly ionized or consistent with their being in singly ionized state, (2) fully ionized galactic cosmic ray ions, and (3) partially ionized iron and sub-iron group ions (which account for about 20 percent of all the iron and sub-iron group ions detected at the Spacelab 3 orbit within the magnetosphere in the energy interval 25-125 MeV/nucleon). It is argued that these partially ionized heavy ions are indeed a part of the low-energy galactic cosmic rays present in the interplanetary space.

Anuradha and low-energy cosmic rays

After critically reviewing observational results obtained by astronomical spacecraft in the interplanetary medium for several aspects of galactic cosmic rays (GCRs) and anomalous cosmic rays (ACRs), attention is given to spacecraft data gathered in the magnetosphere and a detailed description is given of the Anuradha cosmic-ray experiment carried by Spacelab-3. The Anuradha results discussed concern the orbit average flux and ionization state of ACRs, the origins of partially ionized galactic cosmic-ray sub-Fe and Fe ions, and the significance of enhanced abundance ratios of sub-Fe and Fe ions in GCRs inside the magnetosphere.

The ionization state of oxygen ions in anomalous cosmic rays: results from the Anuradha experiment in Spacelab-3

We report the first results on the determination of the ionization states of oxygen ions in the anomalous cosmic rays (ACR) from the measurements of their flux in the cosmic-ray experiment in Spacelab-3 (SL-3) mission of NASA flown at 350 km altitude during 29 April–6 May, 1985. The detectors used were specially prepared CR-39 plastics of very high sensitivity for recording tracks of ions withZ>2. The measured orbit averaged flux of ACR oxygen is (2.9±1.3)×10−4 particles m−2sr−1s−1 (MeV N−1) at an energy of 23 MeV N−1. We made an independent estimate of the expected ACR oxygen flux at SL-3 orbit from interplanetary data and compared this with the measured flux to infer the ionization states of ACR oxygen ions. The flux and energy spectra of ACR oxygen at 1 AU outside the magnetosphere is obtained from the data of Voyager-2, during the same epoch as the SL-3 flight, and using the measured radial intensity gradient of 15%/AU for ACR oxygen between 1–17 AU. We calculate the geomagnetic transmission factors for ACR oxygen ions of charge states O+1, O+2, etc., from the known cut-off rigidities in the world grid and using the SL-3 trajectories for 116 orbits in the 6-day mission to obtain the expected flux at SL-3 for different charge states. When these flux values are compared with our measured flux, the averge ionization state of ACR oxygen ions in the energy interval of 20–26 MeV N−1 is obtained as O+1.

Observation of low-energy (30-100 MeV/nucleon) partially ionized heavy ions in Galactic cosmic rays

Experimental evidence is presented that shows the existence of low-energy (30-100 MeV/nucleon) partially ionized heavy ions in Galactic cosmic rays. The measurements were made by a CR-39 plastic nuclear detector module aboard the Spacelab-3 mission during the cosmic-ray experiment Anuradha (April 29 - May 6, 1985), in which the ionization states of low-energy cosmic-ray ions of elements O to Fe were determined by utilizing the earths magnetic field as a momentum analyzer. It was found that, while most of the ions of the Fe group in this energy range were fully ionized, Ti, Cr, and Fe were observed in partially ionized states equal to or lower than 6, 8, and 20, respectively. The possible sources of these particles are discussed. 24 refs.

Studies of anomalous cosmic ray oxygen ions in space and their ionization states in ANURADHA experiment in spacelab-3

Abstract During April 29 to May 6, 1985, the Spacelab-3 mission of NASA was in orbit carrying among others, the Indian cosmic ray experiment ANURADHA. The major objectives of the experiment were the measurements of the fluxes and ionization states of anomalous cosmic rays by means of a time resolution device which in turn provided information of the arrival location and direction of the particles. This report presents the results of the measurements of ionization states of anomalous cosmic ray heavy ions by trajectory computation method. In the energy interval 15–20 MeV/N, we obtained ionization states of oxygen as +1. Also the orbit-average flux of oxygen ion indicates the charge state of +1. Both the methods show consistent results. Trajectory computations for few other ions of nitrogen and neon was also carried out which shows their ionization states as +1 or ≦ +2.

Observations of enhanced sub-iron (Sc-Cr) to iron abundance ratios in the low energy galactic cosmic rays in Spacelab-3 and their implications

The Anuradha cosmic ray experiment in Spacelab-3, flown in the orbit at 350 km with an inclination of 57° for about six days, was used to measure the low energy galactic cosmic ray (GCR) heavy ions using a specially designed CR-39 detector module incorporating the arrival time information of the particles. The abundances of sub-iron (Sc-Cr) and iron particles in the low energy interval of 30–300 MeV/N were determined from the measurements made in four different depths of the CR-39 detector module of 150 layers. From these studies we obtained sub-iron (Sc-Cr) to iron abundance ratios of 0.8 to 1.2 in 30–300 MeV/N energy range. It is found that these ratios are enhanced by a factor of two as compared to interplanetary ratios of about 0.5. It is shown that the enhancement of the ratio inside the earth’s magnetosphere is probably due to the degree of ionization of low energy Sc to Cr and Fe ions in the galactic cosmic rays and to the rigidity filtering effects of the geomagnetic field. Further studies are needed to understand fully the phenomena and their implications.

Observation of enhanced sub-iron (ScCr) to iron ration in low energy cosmic rays of 50–100 MeV/N in spacelab-3

Abstract Relative abundances of sub-iron (Sc-Cr) to iron nuclei in low energy (50–100 MeV/N) galactic cosmic rays have been determined from an analysis of about 100 events of heavy ions (Z = 10−28) recorded in a detector assembly flown in the Anuradha cosmic ray experiment in the Spacelab-3 on a six day mission in April–May 1985. The measured abundance ratio of (Sc-Cr)/Fe nuclei in 50–100 MeV/N energy range is 1.1 ± 0.3, and the present result of enhanced ratio of sub-iron to iron nuclei is in agreement with other experimental results in 200–800 MeV/N range. The over-abundance of iron secondaries at these low energies cannot be explained in the conventional models for propagation of cosmic rays. Available experimental data indicate a very different time history for the low energy iron-group, as compared to those of lighter nuclei in galactic cosmic rays.

Interstellar propagation and electron capture processes of galactic cosmic ray heavy ions in space

Abstract The new information on galactic cosmic rays (GCR) derived from the Spacelab-3 cosmic ray experiment “Anuradha” shows that at 25–125 MeV/N GCR sub-iron and iron (Z = 21–28) particles consists of a mixture of partially ionized and fully ionized ions. Computation of electron capture and loss cross sections in hydrogen in 1–50 MeV/N energy range are made for Fe, Cr, Ti and Ni. From these it is concluded that: (1) these GCR particles must have captured orbital electrons at energies of about 1–5 MeV/N and (2) these particles are then reaccelerated to 300–500 MeV/N most probably in interstellar medium by collisions with SNR shock fronts. Some reacceleration may take place also in heliospheric boundary region. It is suggested that these observations of partially ionized GCR ions of about 100 MeV/N in Spacelab-3 provide a direct evidence of reacceleration of GCR.

Observation and implications of sub-iron and iron abundance ratios in low energy galactic cosmic rays

Abstract The Spacelab-3 cosmic ray experiment Anuradha was used to measure the sub-iron (Scue5f8Cr) to iron abundance ratios in the low energy galactic cosmic rays. Measurements made in four different depth of the detector yielded the (Sc-Cr) Fe ratios of 0.8 to 1.2 in 30 to 300 MeV/N. These are in agreement with results from Skylab and Soyuz-6 experiments and establishes that this abundance ratio is about 1.0 inside the magnetosphere. It is seen that this abundance ratio is about a factor of two higher than values of about 0.5 measured in space crafts in interplanetary space. It is shown that the enhancement of the ratio is probably due to geomagnetic transmission effect and the degree of ionization of the low energy Sc to Cr and Fe ions in galactic cosmic rays. Further studies are needed to fully understand the phenomena and their implications.