C. G. Rapley

Impulsive phase of flares in soft X-ray emission

Observations using the Bent Crystal Spectrometer instrument on the Solar Maximum Mission show that turbulence and blue-shifted motions are characteristic of the soft X-ray plasma during the impulsive phase of flares, and are coincident with the hard X-ray bursts observed by the Hard X-ray Burst Spectrometer. A method for analysing the Ca xix and Fe xxv spectra characteristic of the impulsive phase is presented. Non-thermal widths and blue-shifted components in the spectral lines of Ca xix and Fe xxv indicate the presence of turbulent velocities exceeding 100 km s-1 and upward motions of 300–400 km s-1.The April 10, May 9, and June 29, 1980 flares are studied. Detailed study of the geometry of the region, inferred from the Flat Crystal Spectrometer measurements and the image of the flare detected by the Hard X-ray Imaging Spectrometer, shows that the April 10 flare has two separated footpoints bright in hard X-rays. Plasma heated to temperatures greater than 107 K rises from the footpoints. During the three minutes in which the evaporation process occurs an energy of 3.7 × 1030 ergs is deposited in the loop. At the end of the evaporation process, the total energy observed in the loop reaches its maximum value of 3 × 1030 ergs. This is consistent with the above figures, allowing for loss by radiation and conduction. Thus the energy input due to the blue-shifted plasma flowing into the flaring loop through the footpoints can account for the thermal and turbulent energy accumulated in this region during the impulsive phase.

The response of lake levels and areas to climatic change

The levels and areas of lakes, particularly closed lakes (those without outlet) are known to be sensitive indicators of changes in climate, and so in principle they can play a role in monitoring current and future climatic changes. In this paper we derive solutions to the water balance equation giving the response of the level and area of closed lakes to steps, spikes and sinusoidal variations in aridity, and show that such a lake acts approximately as a simple low pass filter having a characteristic equilibrium response timeτe. We also review the similar response of open lakes to changes in runoff. It is shown howτe depends on lake geomorphology and climate, and for a number of lakes we calculateτe values, ranging from 1.5 to 350 yr for closed lakes, and from 2.9 days to 2 yr for open lakes. We propose methods involving Fourier analysis for inverting closed lake level or area records to obtain plots of a simple aridity index C. It is shown that for all ≈ 200 of the worlds large (⩾ 100 km2) closed lakes, satellite remote sensing of lake levels and areas is currently sensitive enough to monitor variations in C (e.g. in average basin precipitation) of order 1% to 10% on the time-scale of years to decades.

The soft X-ray polychromator for the Solar Maximum Mission

The 1.4–22.4 Å range of the soft X-ray spectrum includes a multitude of emission lines which are important for the diagnosis of plasmas in the 1.5–50 million degree temperature range. In particular, the hydrogen and helium-like ions of all abundant solar elements with Z > 7 have their primary transitions in this region and these are especially useful for solar flare and active region studies. The soft X-ray polychromator (XRP) is a high resolution experiment working in this spectral region. The XRP consists of two instruments with a common control, data handling and power system. The bent crystal spectrometer is designed for high time resolution studies in lines of Fe i-Fe xxvi and Ca xix. The flat crystal scanning spectrometer provides for 7 channel polychromatic mapping of flares and active regions in the resonance lines of O viii, Ne ix, Mg xi, Si xiii, S xv, Ca xix, and Fe xxv with 14″ spatial resolution. In its spectral scanning mode it covers essentially the entire 1.4–22.5 Å region.This paper summarizes the scientific objectives of the XRP experiment and describes the characteristics and capabilities of the two instruments. Sufficient technical information for experiment feasibility studies is included and the resources and procedures planned for the use of the XRP within the context of the Solar Maximum Mission is briefly discussed.

Observations of a post-flare radio burst in X-rays

More than six hours after the two-ribbon flare of 21 May 1980, the hard X-ray spectrometer aboard the SMM imaged an extensive arch above the flare region which proved to be the lowest part of a stationary post-flare noise storm recorded at the same time at Culgoora. The X-ray arch extended over 3 or more arc minutes to a projected distance of 95 000 km, and its real altitude was most probably between 110 000 and 180 000 km. The mean electron density in the cloud was close to 109 cm−3 and its temperature stayed for many hours at a fairly constant value of about 6.5 × 106 K. The bent crystal spectrometer aboard the SMM confirms that the arch emission was basically thermal. Variations in brightness and energy spectrum at one of the supposed footpoints of the arch seem to correlate in time with radio brightness suggesting that suprathermal particles from the radio noise regions dumped in variable quantities into the low corona and transition layer; these particles may have contributed to the population of the arch, after being trapped and thermalized. The arch extended along the H∥ = 0 line thus apparently hindering any upward movement of the upper loops reconnected in the flare process. There is evidence from Culgoora that this obstacle may have been present above the flare since 15–30 min after its onset.

Unusual coronal activity following the flare of 6 November 1980

For almost 30 hr after the major (gamma-ray) two-ribbon flare on 6 November 1980, 03:30 UT, the Hard X-Ray Imaging Spectrometer (HXIS) aboard the SMM satellite imaged in > 3.5 keV X-rays a gigantic arch extending above the active region over the limb. Like a similar configuration on 22 May 1980, this arch formed the lowest part of a stationary post-flare radio noise storm recorded at metric wavelengths at Nançay and Culgoora. 6.5 hr after the flare a coronal region below the arch started quasi-periodic pulsations in X-ray brightness, observed by several SMM instruments. These brightness variations had no response in the chromosphere (Hα), very little in the transition layer (O v), but they clearly correlated with similar variations in brightness at 169 MHz. There were 13 pulses of this kind, with apparent periodicity of about 20 min, until another flare occurred in the active region at ∼ 15:00 UT. All the brightenings appeared within a localized area of about 30000 km2 in the northern part of the active region, but they definitely did not occur all at the same place.The top of the X-ray arch, at an altitude of ∼ 155 000 km, was continuously and smoothly decaying, taking no part in the striking variations below it. Therefore, the area variable in brightness does not seem to be the footpoint of the arch, as we supposed for similar variations on 22 May. More likely, it is a separate region connected directly with the source of the radio storm; particles accelerated in the storm may be dumped into the low corona and cause the X-ray enhancements. The X-ray arch was enhanced by two orders of magnitude in 3.5–5.5 keV X-ray counts and the temperature increased from ≲ 7.3 × 106 to 9 × 106 K when the new two-ribbon flare occurred at 15:00 UT. Thus, it is possible that energy is brought into the arch via the upper parts of the reconnecting flare loops - a process that can continue for hours.

OBSERVATIONS OF THE SURFACE-PROPERTIES OF THE ICE SHEETS BY SATELLITE RADAR ALTIMETRY

By comparing modelled and averag ed satellite altimeter return, it is demonstra ted that time profiles of altimeter return can be used to provide important information on the surface proper ties of the ice sheets. Alt imeter ice-sheet radar echoes from low altit udes and/or rela tively low latitudes are, in gene ral, domin ated by surface scat tering and, in Greenl and, the area of surface-d ominated return broadly coi ncides with the zone of summer melting . Seasonal variations in the echo wave-form shapes are negligible in all regions studied, with the possib le exception of an area near the margin of the Greenland dry-snow zone. In general, the model expla ins well the observed variations in mean wave- form shape, but small discrepances between the model wave forms and the recorded wave forms indicate that sub-sur face layers may be influ encing the shape of the return. The poss ibility of deriving quantitative estima tes of surface proper ties is explored by fitting model returns to averaged altimeter wave forms from the Wilkes Land plateau in Anta rctica. Surface roug hness can be meas ured unambig uously from the wave-form data, but estim ations of other parameters, such as gra in-size, snow density, and snow temperature are found to be ambiguous because different surface para meters have a similar infl uence on the shape of the retur n. Despite this, the derived estimates compare well with ground-b ased observations and suggest that the satellite alti meter may have an important role to play in providing info rmation on the surface properties of the ice sheets.

X-ray line widths and coronal heating

We present preliminary results of spectroscopy and imaging of a solar active region and flare plasma in soft X-ray emission lines. Observed X-ray line widths in a nonflaring active region are broader than the Doppler width corresponding to the local electron temperature. An analysis of 41 soft X-ray flares within a single active region reveals a preference for flares to occur at locations that already show enhanced X-ray emission and to favor magnetic complexity over high gradient. However, flares do not appear to be directly responsible for the heating and X-ray production of the active regions.

X-ray spectra of solar flares obtained with a high-resolution bent crystal spectrometer

Preliminary results obtained for three solar flares with the Bent Crystal Spectrometer on the SMM are presented. Resonance and satellite lines of Ca XIX and XVIII and FeXXV and XXIV are observed together with the Fe XXVI Ly..cap alpha.. line. Plasma properties are deduced from line ratios and evidence is presented for changes of line widths coincident with the occurrence of a hard X-ray impulsive burst. Fe K..cap alpha.. spectra from a disk center and a limb flare agree with the predictions of a fluorescence excitation model. However, a transient Fe K..cap alpha.. burst observed in a third flare may be explained by the collisional ionization of cool iron by energetic electrons.

A comparison of the performance of the ice and ocean tracking modes of the ERS-1 radar altimeter over non-ocean surfaces

The European Space Agencys ERS-1 radar altimeter is the first to include separate operating modes to optimise performance over both ocean and non-ocean surfaces. As part of the ERS-1 commissioning activities, we have carried out a study of the tracking performance of this instrument over non-ocean surfaces. Statistics for land ice, sea ice, arid lands, and inland water are presented. Performance in both operating modes is shown to be better than that of previous missions.

Swath altimetry of oceans and terrain

Abstract Satellite radar altimeters have demonstrated a wide range of scientific capabilities over oceans and ice, and have considerable potential over land and inland water /1/. However, the universal adoption of the single-beam, pulse-limited mode of operation limits the spatial and temporal sampling achievable, and makes the generation of surface elevation maps critically dependent on the accuracy of the satellite orbit reconstruction. Also, over topographic surfaces, the data can be difficult, sometimes impossible, to interpret. With the advent of the Columbus polar platform, previous limitations on the size and complexity of space instruments will not apply. We consider what types of swath altimeter might take advantage of this possibility and conclude that both multi-feed, large antenna instruments and an interferometric design could provide valuable advances. Ultimately, a scanning beam, phased array instrument could provide full global coverage with high spatial resolution.