D. Morris

Energy Dependence of the Ionic Charge State Distribution During the November 1997 Solar Energetic Particle Event

3 Abstract. Ionic charge state distributions for a variety of species, such as C, O, Ne, Mg, Si and Fe were obtained with the Solar Energetic Particle Ionic Charge Analyzer (SEPICA) on ACE for the strongest of a series of energetic particle events after the November 4 and 7, 1997, flares. The capabilities of SEPICA allow a much more detailed analysis of the charge dis- tributions than previous instrumentation. Over the energy range from ≈ 0.2 to 1 MeV/Nuc a trend is observed that shows charge states increasing with energy, in particular for Mg, Si and Fe. In addition, for Fe a mixed charge state distribution with a distinct peak at lower charge states (10 - 14) is ob- served simultaneously with a tail reaching to charge states up to ≈ 20. This may be an indication of a mixture of different energetic particle populations. 1

The Revised COMPTEL Orion Results

COMPTEL observations of the Orion/Monoceros region have shown distinct evidence for excessive 3-7 MeV emission that was attributed to nuclear de-excitation lines from accelerated 12C and 16O nuclei. Unfortunately, we must conclude now that this appears to be a spurious result. This conclusion follows from a better understanding of the instrumental background, from a better exposure of the region, and from an improved analysis method. We show here how the impact of each of these gradually reduces the signal to a less than 3 σ result. The prime underlying cause seems to be 24Na activation in and around the upper COMPTEL detectors. Combining all available data, we now set a 2 σ flux upper limit on the 3-7 MeV emission of Orion of 3 × 10-5 γ cm-2 s-1, to be compared with the previously derived flux of ~10-4 γ cm-2 s-1.

The COMPTEL instrumental line background

The instrumental line background of the Compton telescope COMPTEL onboard the Compton Gamma- Ray Observatory is due to the activation and/or decay of many isotopes. The major components of this background can be attributed to eight individual isotopes, namely 2 D, 22 Na, 24 Na, 28 Al, 40 K, 52 Mn, 57 Ni, and 208 Tl. The identication of instrumental lines with specic isotopes is based on the line energies as well as on the variation of the event rate with time, cosmic-ray intensity, and deposited radiation dose during passages through the South- Atlantic Anomaly. The characteristic variation of the event rate due to a specic isotope depends on its life-time, orbital parameters such as the altitude of the satellite above Earth, and the solar cycle. A detailed understanding of the background contributions from instrumental lines is crucial at MeV energies for measuring the cosmic diuse gamma-ray background and for observing -ray line emission in the interstellar medium or from supernovae and their remnants. Procedures to determine the event rate from each background isotope are described, and their average activity in spacecraft materials over the rst seven years of the mission is estimated.

Neutron measurements in near‐Earth orbit with COMPTEL

The fast neutron flux in near-Earth orbit has been measured with the COMPTEL instrument on the Compton Gamma Ray Observatory (CGRO). For this measurement one of COMPTELs seven liquid scintillator modules was used as an uncollimated neutron detector with threshold of 12.8 MeV. The measurements cover a range of 4.8 to 15.5 GV in vertical cutoff rigidity and 3° to 177° in spacecraft geocenter zenith angle. One of the measurements occurred near the minimum of the deepest Forbush decrease ever observed by ground-level neutron monitors. After correction for solar modulation, the total flux is well fitted by separable functions in rigidity and zenith angle. With the spacecraft pointed near the nadir the flux is consistent with balloon measurements of the atmospheric neutron albedo. The flux varies by about a factor of 4 between the extremes of rigidity and a factor of 2 between the extremes of zenith angle. The effect of the spacecraft mass in shielding the detector from the atmospheric neutron albedo is much more important than its role as a source of additional secondary neutrons. The neutron spectral hardness varies little with rigidity or zenith angle and lies in the range spanned by earlier atmospheric neutron albedo measurements.

Comptel measurements of solar flare neutrons

Abstract The ability to measure solar flare neutrons by the COMPTEL γ-ray telescope on-board the Compton Gamma Ray Observatory has been demonstrated during the observations of the powerful solar flares of June 1991 from Active Region 6659. We present intensity-time profiles and count rate spectra from the impulsive X10 flare on 1991 June 19 and the prolonged X12+ flare on 1991 June 15. The emission of neutrons during the 9 June flare is coincident in time with the impulsive γ-ray emission, while the neutron emission during the 15 June flare takes place from the time of the X-ray maximum and for at least 30 minutes thereafter. The detected neutrons can also be used to image the Sun in heavy particles, the first time a celestial object has been imaged in radiation other than electromagnetic radiation.

Survey of ionic charge states of solar energetic particle events during the first year of ACE

The ionic charge state distributions of solar energetic particle events are determined with ACE SEPICA on an event by event basis, over the time period from launch through the end of 1998. Because of the large geometric factor of SEPICA the observations can be extended to events with very low fluxes. The study is confined to the most abundant species O, Ne, Mg, and Fe. Mean charge states for Fe are observed to vary between ≈11 for CME related events and ≈20 for small events that carry signatures of impulsive events. For these events all elements up to Mg, appear almost fully ionized. The charge states of all species follow the same trend as that of Fe in their variation from event to event. A comparison of observed mean charge states with a model assuming thermal equilibrium shows a general agreement with temperatures ranging from 1.2–10⋅106 K. However, noticeable deviations from charge states at a unique temperature for all species are seen for O at high and for Mg at both high and low charge states, whi...

COMPTEL as a Solar Gamma Ray and Neutron Detector

The imaging Compton telescope COMPTEL on the Gamma Ray Observatory has unusual spectroscopic capabilities for measuring solar γ-ray and neutron emissions. Flares can be observed above the 800 keV γ-ray threshold of the telescope. The telescope energy range extends to 30 MeV with high time resolution burst spectra available from 0.1 to 10 MeV. Strong Compton tail suppression facilitates improved spectral analysis of solar flare γ-ray emissions. In addition, the high signal-to-noise ratio for neutron detection and measurement provides new neutron spectroscopic capabilities. For example, a flare similar to that of 1982 June 3 will yield spectroscopic data on > 1500 individual neutrons, enough to construct an unambiguous spectrum in the energy range of 20 to 150 MeV. Details of the instrument response to solar γ-rays and neutrons are presented.

Reassessment of the 56Co emission from SN 1991T

The detection of 56Co emission from SN 1991T has been previously reported at a level near the COMPTEL sensitivity threshold. The spectral analysis method, fitting the count spectrum to a background model plus a 56Co emission template, is subject to possible systematic effects which had not been thoroughly studied at that time. To better evaluate the significance of that ∼3.3σ detection, the same method has been applied to a grid of points with 5° spacing, out to 35° from the pointing direction, in each of 103 observing periods from phases 1 through 3. A dozen instances were found with a 56Co signal as significant as that for either of the two observations of SN 1991T alone (∼2σ). Nothing was found as significant as the combined observations of SN 1991T. The strongest instrumental background artifact in the vicinity of the two principal 56Co lines, attributed to 27Mg, falls between the 56Co lines. It fills in the valley between those lines, and so will obscure real 56Co emission rather than producing false...

Implications for source populations of energetic ions in co-rotating interaction regions from ionic charge states

The ionic charge states of He have been observed in several co-rotating interaction regions (CIR) in 1999 and 2000 with ACE SEPICA. For all CIRs under study the He+/He2+ ratio increases consistently from the start of the event towards the end, while the absolute flux of the energetic ions usually reaches a maximum close to the beginning of the event. With time, the spacecraft is magnetically connected to the compression region at increasing distance from the sun. Therefore, the increasing He+/He2+ ratio can be interpreted as an increase in the relative importance of interstellar pickup ions over the solar wind as a source for the energetic ions. In addition, the observed He+/He2+ ratio and its increase with the inferred distance from the sun provide an estimate of the injection and acceleration efficiencies for these species, which is found to be higher by about two orders of magnitude for pickup ions compared with the solar wind.

Ionic charge states of solar energetic particles from solar flare events during the current rise of solar activity as observed with ACE SEPICA

Abstract Ionic charge states of solar energetic ions provide key information on the local temperatures and ionization processes in solar flares. In conjunction with the particle mass this measurement is a tool to study acceleration, fractionation, and transport processes. SEPICA has been designed to resolve charge state distributions in detail and to provide a large collecting power, which allows a much more detailed analysis. During November 4 – 7, 1997, a series of strong flares occurred that were magnetically well-connected. Over the energy range ≈ 0.2 – 1 MeV/nucleon a substantial increase of the charge states with energy was observed. Fe shows a charge state distribution with a distinct peak at Q ≈ 10 – 15 and a tail extending to Q ≈ 20. The newly revealed variation of the charge states with energy will potentially provide insight into modification of charge states during or after acceleration due to stripping and/or rigidity dependent acceleration processes. In addition, a sample of solar particle (SEP) events in 1998, the first year of significant solar activity during the rise of the current solar cycle, is studied for their charge state variations. Typical gradual events that resemble closely coronal composition are found with Fe charge state of ≈11 and impulsive events with high Fe abundance and charge states between 17 and 20. In addition, there are also SEP events with charge states between 12 and 15 that show a moderate, but varying enhancement in Fe. It appears that charge states of SEPs show a much richer variation than the previously reported two groups of charge states, which is also correlated with heavy ion enrichment.