Gerhard Rank

Extended gamma-ray emission of the solar flares in june 1991

During the solar flares on 9, 11, and 15 June 1991 the COMPTEL instrument measured extended {radiation in the 2.223 MeV neutron-capture line, in prompt nuclear deexcitation lines and in pion-decay ra- diation for several hours after the flares. The long-term time proles can be described by a double exponential decay with decay constants on the order of 10 min for the fast and several 100 min for the slow components. We studied the 11 June 1991 flare in more detail and found that during the extended phase the accelerated proton and ion spectrum is harder, the e/p ratio is lower, and the emission prole is smoother, compared to those of the impulsive phase. Pion-decay radiation was not detected before the onset of the extended emission phase. When comparing the three flares to one another, we found a striking similarity in the time proles of the nuclear line and the neutron capture line emission. However, the pion-decay radiation varied in intensity signicantly from flare to flare. The impulsive-phase emissions of the flares show no such similarity. Our measurements indicate that the processes taking place during the extended phase dier from those during the impulsive phase, or in other {ray line flares. Based on these results long-term trapping of energetic particles from the impulsive phase seems unlikely, as opposed to continuous particle acceleration.

Neutron and gamma‐ray measurements of the solar flare of 1991 June 9

The COMPTEL Imaging Compton Telescope on‐board the Compton Gamma Ray Observatory measured significant neutron and γ‐ray fluxes from the solar flare of 9 June 1991. The γ‐ray flux had an integrated intensity (≳1 MeV) of ∼30 cm−2, extending in time from 0136 UT to 0143 UT, while the time of energetic neutron emission extended approximately 10 minutes longer, indicating either extended proton acceleration to high energies or trapping and precipitation of energetic protons. The production of neutrons without accompanying γ‐rays in the proper proportion indicates a significant hardening of the precipitating proton spectrum through either the trapping or extended acceleration process.

Extended γ‐ray emission in solar flares

During the solar flare events on 11 and 15 June 1991, COMPTEL measured extended emission in the neutron capture line for about 5 hours after the impulsive phase. The time profiles can be described by a double exponential decay with decay constants on the order of 10 min for the fast and 200 min for the slow component. Within the statistical uncertainty both flares show the same long‐term behaviour. The spectrum during the extended phase is significantly harder than during the impulsive phase and pions are not produced in significant numbers before the beginning of the extended emission. Our results with the measurements of others allow us to rule out long‐term trapping of particles in non‐turbulent loops to explain the extended emission of these two flares and our data favour models based on continued acceleration.

Observations of the 1991 June 11 solar flare with COMPTEL

The COMPTEL instrument onboard the Compton Gamma Ray Observatory is well suited for the observation of solar flares. It is sensitive to γ‐rays in the energy range from 0.75 to 30 MeV. In addition COMPTEL has the novel capability to detect individual neutrons from solar flares. During the period of unexpectedly high solar activity in June 1991 several flares from active region 6659 were observed by COMPTEL. For one of these—that of June 11th—we present the latest results from the analysis of COMPTEL data. This includes the time history of the γ‐ray emission extending for at least two hours after the impulsive phase.

COMPTEL’s solar flare catalog

COMPTEL, the imaging gamma‐ray telescope, capable of detecting gamma rays in the range of 0.1–30 MeV, is one of four instruments aboard NASA’s Compton Gamma‐Ray Observatory. The Comptel burst detectors (single Defector Mode) have a field of view of ∼2.5 π sr. These detectors of COMPTEL permit measurements of energy spectra and time histories of solar flare gamma‐ray emission. A search through the Single Detector Mode’s data is being conducted. We summarize the preliminary results of this search.

COMPTEL gamma ray and neutron measurements of solar flares

COMPTEL on the Compton Gamma Ray Observatory has measured the flux of x‐rays and neutrons from several solar flares. These data have also been used to image the Sun in both forms of radiation. Unusually intense flares occurred during June 1991 yielding data sets that offer some new insight into of how energetic protons and electrons are accelerated and behave in the solar environment. We summarize here some of the essential features in the solar flare data as obtained by COMPTEL during June 1991.

COMPTEL gamma-ray observations of the C4 solar flare on 20 January 2000

The “Pre-SMM” (Vestrand and Miller 1998) picture of gamma-ray line (GRL) flares was that they are relatively rare events. This picture was quickly put in question with the launch of the Solar Maximum Mission (SMM). Over 100 GRL flares were seen with sizes ranging from very large GOES class events (X12) down to moderately small events (M2). It was argued by some (Bai 1986) that this was still consistent with the idea that GRL events are rare. Others, however, argued the opposite (Vestrand 1988; Cliver, Crosby and Dennis 1994), stating that the lower end of this distribution was just a function of SMM’s sensitivity. They stated that the launch of the Compton Gamma-ray Observatory (CGRO) would in fact continue this distribution to show even smaller GRL flares. In response to a BACODINE cosmic gamma-ray burst alert, COMPtonTELescope on the CGRO recorded gamma rays above 1 MeV from the C4 flare at 0221 UT 20 January 2000. This event, though at the limits of COMPTEL’s sensitivity, clearly shows a nuclear line e...

Improved Analysis of COMPTEL Solar Neutron Data, with Application to the 15 June 1991 Flare

Direct solar flare neutrons are a valuable diagnostic of high- energy ion acceleration in these events, and COMPTEL improves over all previous cosmic neutron detectors in its capacity for neutron energy measurement. Previous studies of COMPTEL neutron data have worked with an incomplete model of the instrumental response, applying energy-by-energy detection efficiencies. Here we employ statistical regularisation techniques with the full (Monte Carlo simulation derived) response matrix to produce improved estimates of neutron numbers and energy distribution. These techniques are applied to data from the well-observed 15 June 1991 flare. Our improved treatment of the instrumental response results in a reduction of 73% in total neutron numbers, compared with previously deduced values. Implications for the picture of primary ion acceleration in this flare are briefly discussed.

Gamma ray measurements of the 1991 November 15 solar flare

The 1991 November 15 X1.5 flare was a well observed solar event. Comprehensive data from ground-based observatories and spacecraft provide the basis for a contextual interpretation of gamma-ray spectra from the Compton Gamma Ray Observatory (CGRO). In particular, spectral, spatial, and temporal data at several energies are necessary to understand the particle dynamics and the acceleration mechanism(s) within this flare. X-ray images, radio, Ca XIX data and magnetograms provide morphological information on the acceleration region [4,5], while gamma-ray spectral data provide information on the parent ion spectrum. Furthermore, time profiles in hard X-rays and gamma-rays provide valuable information on temporal characteristics of the energetic particles. We report the results of our analysis of the evolution of this flare as a function of energy (∼25 keV–2.5 MeV) and time. These results, together with other high energy data (e.g. from experiments on Yohkoh, Ulysses, and PVO) may assist in identifying and und...

COMPTEL observations of gamma‐ray flares in October 1991

The COMPTEL experiment on GRO images 0.75–30 MeV celestial gamma‐radiation that falls within its 1 steradian field of view. During observation 12 (primary target Cen A) in October 1991 the sun had been in the fov and several solar flares associated with the active region 6891 had been observed. Time profile and energy spectra had been produced, using COMPTEL’s primary mode of operation (the telescope mode). Additionally the number of counts received in the D2‐single burst detector (the secondary mode of operation) are given. We summarize the preliminary results on all of these flares.