Maximum Mission Satellite Research Articles

Fe XVIII EMISSION LINES IN SOLAR X-RAY SPECTRA

Abstract : We have calculated intensity ratios for emission lines of Fe XVIII in the 19-94A wavelength range at electron temperatures characteristic of the solar corona, Te = 2-10 x 10 to the 6th power K. Our model ion includes data for transitions among the 2s22p5, 2s2p6, 2s22p43l, and 2s2p53l (l = s, p, and d) states. Test calculations that omit the 2s2p53l levels show that cascades from these are important. We compare our results with observed ratios determined from four solar X-ray instruments, a rocket-borne spectrograph, and spectrometers on the P78-1, OV1-17 and Solar Maximum Mission (SMM) satellites. In addition, we have generated synthetic spectra which we compare directly with flare observations from SMM. Agreement between theory and observation is generally quite good, with differences that are mostly less than 30%, providing limited support for the accuracy of the atomic physics data used in our calculations. However, large discrepancies are found for ratios involving the 2s22p5 2P3/2-2s2p6 2S line at 93.84A, which currently remain unexplained. Our analysis indicates that the Fe XVIII feature at 15.83A is the 2s22p5 2P3/2-2s22p4(3P)3s4 P3/2 transition, rather than 2s22p5 2P3/2-2s22p4(3P)3s 2P3/2, as suggested by some authors.

X-ray emission-line ratios in MG XI as electron temperature diagnostics for solar flares and active regions

Electron impact excitation rates for transitions in helium-like Mg XI, calculated with the R-matrix code, are used to derive the electron temperature-sensitive emission-line ratios G = [I(1s2 1S-1s2s 3S) + I(1s2 1S-1s2p 3P1,2)]/I(1s2 1S-1s2p 1P) = [I(9.314 angstrom) + I(9.232 angstrom)]/I(9.169 angstrom), R1 = I(1s2 1S-1s3p 1P)/I(1s2 1S-1s2p 1P) = I(7.850 angstrom)/I(9.169 angstrom), and R2 = I(1s2 1S-1s4p 1P)/I(1s2 1S-1s2p 1P) = I(7.472 angstrom)/I(9.169 angstrom). These are found to be significantly different from the earlier diagnostic calculations of Keenan, Tayal, & Kingston, and Keenan, Kingston, & McKenzie, and will lead to electron temperature estimates up to a factor of approximately 1.6 larger. Values of T(e) deduced from R1 and R2 ratios measured from solar flare and active region spectra obtained with the Flat Crystal Spectrometer (FCS) on board the Solar Maximum Mission (SMM) satellite are consistent, and furthermore are in good agreement with temperatures estimated from the observed G ratios for these solar features. This provides support both for the validity of the theoretical R1 and R2 diagnostics, and for the FCS calibration curve in the wavelength region covering the Mg XI transitions, 7.472-9.314 angstrom.

Sunspot proper motions in active region NOAA 2372 and its flare activity during SMY period of 1980 April 4–13

Solar Active Region NOAA 2372 was observed extensively by the Solar Maximum Mission (SMM) satellite and several ground-based observatories during 1980 April 4–13 in the Solar Maximum Year. After its birth around April 4, it underwent a rapid growth and produced a reported 84 flares in the course of its disc passage. In this paper, we have studied photospheric and chromospheric observations of this active region together with Marshall Space Flight Center magnetograms and X-ray data from HXIS aboard the SMM satellite. In particular, we discuss the relationship of the flare-productivity with sunspot proper motions and emergence of new regions of magnetic flux in the active region from its birth to its disappearance at the W-limb.

Frequencies of low-degree solar acoustic oscillations and the phase of the solar cycle

A study of the solar total irradiance data of the Active Cavity Radiometer Irradiance Monitor (ACRIM) on the Solar Maximum Mission (SMM) satellite shows a small but formally significant shift in the frequencies of solar acoustic (p-mode) oscillations between the epochs of maximum and minimum solar activity. Specifically, the mean frequency of the strongest p-mode resonances of low spherical-harmonic degree (l = 0–2) is approximately 1.3 parts in 104 higher in 1980, near the time of sunspot maximum, than in ∼1985, near sunspot minimum. The observed frequency shift may be an 11-yr effect but the precise mechanism is not clear.

High-energy continuum emission from solar flares

The properties of solar flare continuum emission at energies >300 keV are discussed. Emphasis is placed on observations made during the 21st Solar Maximum by γ-ray detectors aboard the Solar Maximum Mission and Hinotori satellites. The statistical properties of high-energy flares are presented, including their size-frequency distribution, spectral-index distribution, position distribution, and associated soft X-ray size. The temporal structure of the high-energy continuum is reviewed as well as attempts to model the structure by two-step acceleration and particle trapping. Evidence for the directivity of flare radiation is presented and statistical and stereoscopic analysis techniques are compared and contrasted. The first observations of flare γ-rays at energies > 10 MeV are examined. We show that the very high-energy emission must be a mixture of pion-decay radiation and primary electron bremsstrahlung. Finally, we present high-energy observations from the extended phase of the giant 3 June, 1982 flare which seem to require a new acceleration component.

Time Variations of the Calcium Elemental Abundance in Flares from NOAA Active Regions 2562 and 2779

AbstractAnalysis of flare spectra obtained with the Bent Crystal Spectrometer aboard the Solar Maximum Mission (SMM) satellite enables the determination of the fluxes in the resonance (w) line of Ca XIX and the near-by continuum. The line-to-continuum intensity ratio is a sensitive measure of the calcium elemental abundance (Aca) relative to hydrogen in the emitting plasma. In previous investigations we found the calcium abundance varies from flare to flare. In the present investigation we analyse the variations ofAcafor flares which occurred in two active regions well observed bySMM. We conclude that it is not possible to correlate the abundance variations with the time of the flare occurrence as suggested in an earlier paper for flares produced from a single active region. Further, we find no convincing correlation of abundance variation with any other flare characteristics. This negative result will stimulate future work on the physical interpretation of the calcium abundance variation.

On the energy balance of solar active regions

The cause of sunspots has long been an important, unsettled problem in solar physics. Biermann1 suggested that the strong magnetic field of a sunspot inhibited convection, allowing the sunspot to cool. Parker2, on the other hand, proposed that a sunspot was cooled by the generation of waves that carried away the thermal energy. The solar ‘constant’ has been measured with a daily uncertainty of about 10 parts per million (p.p.m.) by the Active Cavity Radiometer Irradiance Monitor (ACRIM) on board the Solar Maximum Mission (SMM) satellite launched in February 1980. The ACRIM has shown that the solar constant changes with solar activity, showing dips of the order of 0.1–0.3% associated with sunspots. This discovery has provoked renewed attempts to understand the sunspot phenomenon. Two divergent views are emerging: that the missing energy is stored in the convection zone for long periods of time3 or that the missing energy is re-radiated fairly quickly at different angles by faculae4, non-spot magnetic fields that often accompany sunspots. The consequences are that in the first case an 11-yr modulation is expected in the solar output whereas in the second case it is not. I point out here that facular emission may equal the missing energy from sunspots, over a period of some weeks or months, suggesting that the missing energy is stored by being converted from kinetic into magnetic energy after which it decays back into thermal energy in the faculae.

Imaging of impulsive solar flare phenomena

We review some recent advances in our understanding of impulsive solar flare phenomena obtained through new hard X-ray and radio imaging instruments (the Solar Maximum Mission and Hinotori satellites, the VLA and VLBI).

Comparison of observed Ca xix and Ca xviii relative line intensities with current theory

A comparison is made between Ca xix and Ca xviii line ratios observed in solar flares with the Bent Crystal Spectrometer (BCS) on the Solar Maximum Mission (SMM) satellite and currently available atomic data. Close agreement is found with the excitation rates recently published by Pradhan et al. (1981). The observations show little dependence of line ratios on electron temperature, supporting a further conclusion that cascade contributions to the 23P and 23S levels are not significant.