Search

One of the goals of the EUVE observation of AB Doradus was to search for rotational modulation of the EUV emissions. In support of this goal we carried out optical photometry in Chile, Australia and South Africa and radio observations in Australia. In addition, an ASCA observation of AB Dor was scheduled to occur during the campaign. Several spectacular X-ray and optical flares with accompanying brightening in the EUV were seen, but no rotational modulation of the EUV emission was evident, except for a dip in the He II 304 A line coincident with the optical minimum. An X-ray flare was seen with no accompanying EUV flare.

We have extended the proxy relationship between irradiance and microwaves by using the daily solar fluxes from Toyokawa Observatory at 1000, 2000, 3750 and 9400 MHz in addition to the Ottawa 2800 MHz flux for the years 1980–1989. It turns out that the flux at 1000 MHz is better correlated with irradiance than the flux at higher frequencies-an unexpected result. We have also found that the spectrum of the flux shows shape changes that are related to the number and type of active regions. Because of this the five-frequency spectral measurements of microwave flux allow one to separate the sunspot and coronal features, providing an improved proxy of solar variability.

We report preliminary results of the partial eclipse seen from the BIMA millimeter-wavelength interferometer in northern California. The use of an interferometer has many advantages over previous single-dish eclipse observations at 3 mm, and in particular it allows a quite different type of measurement of the height of the “3-mm limb” which is direct and precise: we find it to be 11.9”±0.4” above the optical limb.

We describe a set of millimeter interferometric observations of solar flares carried out in conjunction with GRO experiments during the 1991 June Campaign of the Max’91 Program. We show evidence that millimeter emission probes the most energetic (MeV) electrons in solar flares; we also find that in the same flare there can be both impulsive nonthermal and gradual thermal millimeter emission. Millimeter emission usually occurs at the steep rise phase of the hard X-ray emitting electrons (25–100 KeV). There appears to exist some delay between BIMA mm-emission onset and GRO-BATSE 25–100 KeV X-ray emission. Both results have implications for the particle acceleration process.

The non-linear coalescence instability of current carrying solar loops can explain many of the characteristics of the solar flares such as their impulsive nature, heating and high energy particle acceleration, amplitude oscillations of electromagnetic and emission as well as the characteristics of 2-D microwave images obtained during a flare. The plasma compressibility leads to the explosive phase of loop coalescence and its overshoot results in amplitude oscillations in temperatures by adiabatic compression and decompression. We note that the presence of strong electric fields and super-Alfvenic flows during the course of the instability play an important role in the production of non-thermal particles. A qualitative explanation on the physical processes taking place during the non-linear stages of the instability is given.

Radio observations offer an important means for providing estimates of magnetic fields in post-flare loops.

The Hat Creek Observatory’s two-element interferometer and the NRAO 3-element interferometer have been used at 1.3, 3.7- and 11.1-cm wavelengths respectively, to study the fine structure of the radio emissive regions on the Sun. Observations of the quiet Sun at 1.3 cm show sudden increases followed by a gradual decrease in the fringe amplitude lasting for typically about 5–8 min. Assuming these events are identical in nature, a plot of peak amplitude against the projected baseline at the time of the event suggests emission from a region of angular size of about 10″. The corresponding brightness temperature is 50000 K. It is possible that these events may be related to the appearance and disappearance of groups of spicules or mottles.

Observations from SKYLAB have shown that coronal transients, which involve mass ejections occur quite frequently, possibly up to three times a day at solar maximum (Hildner et al., 1976). An estimated mass of ∼1015 − 1016 g (Stewart et al., 1974) and a total mechanical energy in excess of 2×1031ergs (Webb et al., 1978) is expelled from the Sun during each event. The transients therefore play a major role in the dynamics of the outer corona and of the interplanetary medium. Joint radio and white light observations provide the best opportunity to derive the physical parameters, such as the electron density and magnetic field in different parts of the transients, and consequently to estimate the forces driving the ejecta.

Recent observations of the fast time variability in the hard X-ray emission from solar flares are interpreted. The fast spikes are assumed to be superimposed on the thermal X-ray emission. The rise and fall of a spike are caused by disruptions in the plasma. The rise time represents the impulsive heating time and the decay or fall time represents a quick cooling of the plasma due to the accelerating growth rate of the m=1 tearing mode. The estimated characteristic time durations of the spike are found to be in good agreement with the observed ones.

We summarize the current state and future problems of research on stars that exhibit continuum radio emission. This summary is based upon comments presented by the authors during the meeting together with material suggested by the papers and discussions.

We review the results of simultaneous two-frequency imaging observations of solar microwave bursts with the Very Large Array. Simultaneous 2 and 6 cm observations have been made of bursts which are optically thin at both frequencies, or optically thick at the lower frequency. In the latter case the source structure may differ at the two frequencies, but the two sources usually seem to be related. However, this is not always true of simultaneous 6 and 20 cm observations. The results have implications for the analysis of non-imaging radio data of solar and stellar flares.

In this paper, we first discuss a set of 6 cm observations made with the NRAO Very Large Array (VLA) (spatial resolution ∼ 2″) that pertain to changes in the coronal magentic field configurations that took place before the onset of an impulsive burst observed on 14 May 1980. We also discuss a second set of 6 cm VLA observations (spatial resolution 18″; arc) where several interacting loops were involved in triggering the onset of an impulsive burst observed on June 24, 1980, 19:57:00 UT. Both sets of observations are examples of magnetic reconnection process being involved in accelerating microwave emitting electrons.

1: Preflare Activity.- 1.1 Introduction.- 1.2 Magnetohydrodynamic Instability..- 1.3 Preflare Magnetic and Velocity Fields.- 1.4 Coronal Manifestations of Preflare Activity.- 1.5 References.- 2: Particle Acceleration.- 2.1 Introduction.- 2.2 Phenomena Associated with Mildly-Relativistic Electrons.- 2.3 Phenomena Associated with Ions and Relativistic Electrons in Solar Flares.- 2.4 Theoretical Studies Of Particle Acceleration.- 2.5 Achievements - Outstanding Questions.- 2.6 Acknowledgements.- 2.7 References.- 3: Impulsive Phase Transport.- 3.1 Introduction.- 3.2 Impulsive Phase Observations and their Interpretation.- 3.3 Theoretical Studies of Transport Processes.- 3.4 Summary.- 3.5 References.- 4: Chromospheric Explosions.- 4.1 Introduction.- 4.2 Crystal Spectrometer Experiments.- 4.3 Debate of Issue 1.- 4.4 Debate of Issue 2.- 4.5 Debate of Issue 3.- 4.6 References.- 5: Flare Energetics.- 5.1 Introduction.- 5.2 Energetics of the Impulsive Phase.- 5.3 The Energetics of the Gradual Phase.- 5.4 Relationships among the Phases.- 5.5 Characterization of Total Flare Energy.- 5.6 Conclusions.- 5A. Flares Chosen for the Energetics Study.- 5B. A Review of Impulsive Phase Phenomena.- 5.7 Acknowledgements.- 5.8. References.- 6: Coronal Mass Ejections and Coronal Structures.- 6.1 Intrdocution.- 6.2 Observations.- 6.3 The Initiation of Coronal Mass Ejections.- 6.4 Modelling of Coronal Mass Ejections and Post-Flare Arches.- 6.5 Interplanetary Effects of Coronal Mass Ejections.- 6.6 The Slowly Varying Corona Near Solar Activity Maximum.- 6.6 Summary.- 6.7 Acknowledgements.- 6.8 References.- 7: Intercomparison of Numerical Models of Flaring Coronal Loops.- 7.1 Introduction.- 7.2 The Original Benchmark Problem.- 7.3 Initial Solution Comparisons.- 7.4 Final Benchmark Problem and Solution Comparison.- 7.5 Conclusions.- 7.6 References.- SMM Participants.

In the astrophysics community, “the solar flare problem” is generally considered to be how to accumulate sufficient magnetic energy in one active region and to subsequently release it on a sufficiently short time scale. Satisfactory solution of the solar flare problem will require at least two achievements by the solar physics community: first, convincing theoretical demonstration that one or more mechanisms of energy storage and release can occur; second, convincing observational demonstration that one (or more) of these theoretical processes actually does occur in the solar atmosphere. The contents of this Chapter essentially relate to the second problem, being largely concerned with how the energy released from magnetic form is transported through the solar atmosphere before escaping in the form of the radiant and mechanical energy signatures which we must interpret.

Over the past decade two large arrays — the Westerbork Synthesis Radio Telescope (WSRT) and the Very Large Array (VLA) built primarily for sidereal radio astronomy have been used for solar radio astronomical studies with spatial resolution of a few seconds of arc. In this review, we discuss some results obtained at Maryland using these instruments.

We present simultaneous VLA observations of two late type dwarf stars UV Ceti and YZ CMi at 6 and 20 cm, Multiwavelength observations put sufficient constraints on existing interpretations of quiescent radio emission from these stars. We find that the microwave emission is due to gyro-synchrotron radiation of nonthermal electrons having a power law energy distribution. This emission originates from a source whose size is 2–3 times larger than the star. From the lifetime of several hours of the nonthermal particles against radiation and collisional losses we estimate a magnetic field of ~ 140 G and a density of ≲ 2 × 10 cm-3 in the microwave source.

In recent years large aperture synthesis instruments such as the Westerbork Synthesis Radio Telescope (WSRT) and the Very Large Array (VLA) with resolution of a few arc seconds at cm wavelengths have been used for solar observations. These observations have provided valuable information on the structures of radio emissive regions in active regions, and flares on scales of a few arc seconds. As a result we have much improved understanding of the origin of radio emission in these regions, and consequently of the processes of electron acceleration in the flaring regions and of the build-up of energy in the preflare active regions.

Some properties of solar active regions at 9 and 3.5 mm wavelengths are discussed. The regions have excess brightness temperatures of up to 1000 and 700K at 9 and 3.5 mm wavelengths respectively. The background radiation at 3.5 mm is often seen to be ‘absorbed’ in regions closely coincident with Hα dark filaments on the disk. Interpretation of this ‘absorption’ as due to the large optical thickness of the overlying filamentary material leads to an estimate of electron density in the filaments. The active regions at millimeter wavelengths show almost one-to-one correspondence with the Ca-plage regions as well as with the regions of longitudinal magnetic fields on Mt. Wilson magnetograms. A comparison of the mm-λ maps with the magnetograms ‘smoothed’ with the beams of mm observations shows this correspondence in a striking manner. This relationship suggests the possibility of measuring chromospheric magnetic fields from the measurement of polarization at millimeter wavelengths.

The flare associated continuum bursts of Type IV in the 500–1000 Mc/s range in general consist of a smooth continuum with different variabilities superimposed upon it (Kundu and Spencer, 1963). These variabilities can take various forms. They are often short-duration fast-drift bursts which appear to be different in nature from the high frequency components of meter-wave bursts of Type III. The intense fast-drift elements which are the most dominant radiation in some dm-λ continuum events are sometimes so blended together as to make the continuum radiation appear to be composed of a succession of fast-drift bursts of fine structure. Even in such events, the smooth background continuum appears clearly whenever the superimposing fast-drift elements are weak or absent (Figure la). Occasionally the fast-drift elements occur in the form of “sprays” (Figure lb). Sometimes drifting patches occur over short periods in course of the continuum event. The individual patches, usually of duration less than 1/2 minute, often drift at the rate of several Mc/s per sec from low to high frequencies over bandwidths of the order of 200–400 Mc/s in the 500–1000 Mc/s range (Figure 1c). In addition, intermediate-drift bursts are occasionally superimposed upon the continuum (Young et al. 1961); these are short duration (< 1 sec at one frequency) bursts drifting from high to low frequencies at rates of about 30 Mc/s per second. Often the superimposing variabilities are so mixed-drifting patches, fine-structure bursts, or a combination of both-as to make the continuum event appear rather complex. However, in the later stages of such events, the continuum becomes very smooth with practically no variabilities superimposed upon it. Because of the superimposing variabilities, the dm-λ Type IV appears more variable than the corresponding cm-λ and meter-λ Type IV.