The Hard X-Ray Sun in Stereo Tailoring Liposome Structure

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814 Nature Vol. 283 28 February 1980 stable reverse current to neutralise the The hard X-ray Sun in stereo descending beam. Reverse current stability n?:: 9 3 from John C. Brown demands 5 x 10 cm - which is consistent with the density enhancements VIRTUALLY on the eve of launch (14 typical of active regions. It will be of great February, 1980) of the first ever solar hard interest to see whether a similar source X-ray imaging telescope, aboard NASAs distribution with height occurs in other Solar Maximum Mission (SMM), a group bursts or if these are more consistent with of US researchers has scooped some of its one of the other source models. expected results without the use of imaging In the coming months SMM will instruments. Kane and Anderson of disgorge vastly more hard X-ray source Berkeley and Evans, Klebesadel and Laros spatial data than are contained in Kane et of Los Alamos (Astrophys. J. Lett. 233, al. 's results. Specifically it will yield LI 51; 1979) have obtained quantitative detailed information on horizontal source information on the spatial structure of a structure and so test whether electrons solar hard X-ray source by simultaneous really do bombard the chromosphere. On observations from two separate spacecraft. the other hand the SMM telescope is Their results provide invaluable insight limited to photon energies below 30 keV into the process of electron acceleration and, furthermore, has no spatial resolution which is central to the solar flare problem. along the line of sight. Thus the 'stereo' At the time of observation (5 October, data of Kane et al. will remain unique until 1978 around 0632 UT) an International we are again fortunate enough to have two Sun Earth Explorer (ISEE-3) was located spacecraft and a flare in the right places at near the Earth while Pioneer Venus Orbiter the righttime. D (PYO) was 12.5 degrees eastward as seen from the Sun (see Figure). Fortuitously, a flare occurred in an active region A some 15 degrees behind the solar limb which PVO EARTH consequently occulted different parts of ISEE-3 Schematic showing directions of the two Tailoring liposome the flaring solar atmosphere from the two spacecraft as seen from the Sun and the spacecraft. As a result (see Figure) ISEE-3 different heights in the X-ray source (hatched structure observed only X rays emanating from area) seen from each spacecraft. from Gregory Gregoriadis altitude (C-D) in the source greater than 25,000 km above the photosphere (A) while Two possible cases of the latter situation THE idea that drugs entrapped in lipid PYO observed all emissions (B-D) from are the descending electron beam (thick envelopes (liposomes) may provide a better above 700 km altitude. At burst peak the target) model (with acceleration near Din means of delivery to specific sites within the PYO flux above 50 keV photon energy the Figure) and the trap model where fast body has aroused wide interest. Much of exceeded the ISEE flux, extrapolated to the electrons are confined in a magnetic arch the interest in the liposomal carriers has no same energy range, by a factor of 600 and after acceleration. Although these latter doubt stemmed from the almost unlimited had a spectrum harder by about + 2 in the models pose problems for acceleration number of potential versions in terms power-law exponent. We can thus mechanisms, they provide a plausible of composition, size and other conclude that the hard X-ray source is interpretation of impulsive chromospheric characteristics. Indeed, correct choice of concentrated at altitudes below 25,000 km, emissions in flares in terms of electron these parameters has, in some instances, particularly at higher photon energies, with bombardment. The greatest single source led to liposomal preparations best suited a factor of 600above 50 keV. This result is a of ambiguity in modelling hard X-ray for optimal drug action (for reviews of great advance on previous behind-the-limb bursts to date has been the lack of spatial various aspects of liposomes see results from single spacecraft which resolution. Liposomes and their uses in Biology and established the existence of a high altitude There seems to be no ready Medicine (D. Papahadjopoulos ed.) Ann. source component but gave no information interpretation of Kane et al. 's results in N. Y. Acad. Sci. 308, 1978; Liposomes in on the differential height structure. terms of the trap model since this predicts Biological Systems (eds G. Gregoriadis & The importance of hard X-ray bursts lies comparable X-ray fluxes from the trapped A.C. Allison) John Wiley and Sons Ltd, not in the bremsstrahlung X rays (coronal) electrons and those precipitating 1980). Recently, attempts have been made themselves but in the energy of the down the limbs to the chromosphere to rationalise liposome development by electrons emitting them. These electrons (unless by chance ISEE just glimpsed part tailoring their structure to the particular represent a significant, and perhaps a large, of the arch summit at C). A thermal model biological milieu in which they are intended fraction of the total energy released by just might fit the facts if quasi-thermal to act. magnetic field dissipation in a solar flare. emission emanated from CD at energies up The ability to control the extent to which How severe are the demands that this to around 50 ke V while the high energy liposomes in the blood retain their places on the electron acceleration process electron tail escaped downward. The most entrapped drugs is obviously important to depends strongly on the hard X-ray source convincing interpretation at present, those interested in applying them model assumed. In particular the however, is the thick target model which systemically. Although in many cases drugs acceleration efficiency involved is modest predicts a difference of + 2 in spectral must be transported intact to where they if the bulk of the source electrons are in a index between coronal and chromospheric are needed, in others their gradual release relaxed quasi-thermal state. On the other components, and much greater total flux at in the circulation may be preferable, hand, if the source electrons are non low altitudes (Brown & McClymont Solar especially when membrane barriers will thermal they lose most of their energy by Phys. 41, 135; 1975). The PVO/ISEE flux· prevent liposomes from reaching target collisions to the cooler thermal plasma ratio would then require an acceleration sites. rather than to bremsstrahlung and a far site D located at a plasma column depth of In the blood, drugs are released from greater total electron energy is required. 2.5 x 10 18 protons cm·2 above the 25,000 liposomes much more rapidly than km level C. For quiet Sun conditions the expected from normal solute diffusion J.C. Brown is a Senior Lecturer in the 8 2 Department of Astronomy, University of plasma density n here is around 10 cm· , Gregory Gregoriadis is at the MRC's Clinical Glasgow. but such a low density could not sustain a Research Centre, Harrow, Middlesex.