Physics News Update, Number 487

Number 487, June 2, 2000, 2000 by Phillip F. Schewe and Ben Stein

THE FIRST GLOBAL IMAGE OF THE EARTH'S PLASMASPHERE, the shell of positively charged ions and negatively charged electrons lying at the top of our atmosphere and extending far out into space, has been recorded by the Imager for Magnetosphere to Aurora Global Exploration (IMAGE) satellite. The ability to view the Earth and its environs through plasma-colored glasses is important for understanding basic geophysics properties of the Earth and for monitoring "space weather," the general name for disturbances in our planet's vicinity caused by fields and particles coming from the sun.

A violent storm on the sun can, a few days later, pose hazards for satellites and even ground-based power grids. IMAGE performs its sentry duty by photographing the glow caused when light or particles coming directly from the sun or nearby particles whipped up to high energies smash into atoms in our upper atmosphere. Launched in March 2000, the IMAGE spacecraft follows a highly eccentric orbit which takes it far enough from the Earth that at times the whole planet, and its fluorescing plasma, can be captured within the photographic frame.

First data from the IMAGE mission were reported this week by James Burch, Southwest Research Institute (210-522-2526) and several colleagues at the American Geophysical Union meeting in Washington, DC. One surprise: a picture of the helium glow around the Earth at extreme ultraviolet (EUV) wavelengths (see figure at Physics News Graphics) exhibited unexplained lobe structures. Another first: separate ultraviolet movies of electron and proton auroras were shot simultaneously by using filters that discriminate among fluorescence at different wavelengths coming from hydrogen, oxygen, and nitrogen atoms in the atmosphere; EUV at 121 nm, for instance, comes from energetic protons resonantly scattering from hydrogen atoms.

To locate more precisely the position and velocity of the plasma clouds being viewed, IMAGE uses an immense cross-shaped radio antenna (mission scientist James Green of Goddard Space Flight Center called it a "radar cop in the sky") measuring 500 meters from tip to tip (longer than the Empire State Building is tall, or equivalent to three Washington Monuments laid end to end), making it the longest manmade structure in space. (See also the IMAGE website).

DELIVERING GENES AND DRUGS WITH ULTRASOUND-ACTIVATED BUBBLES. At this week's meeting of the Acoustical Society of America, Evan Unger of the University of Arizona and ImaRx Therapeutics in Tucson (520-770-1259, eunger@imarx.com) presented several new uses for ultrasound contrast agents, micron-sized bubbles that are injected into the bloodstream for medical ultrasound purposes. Traditionally used to enhance ultrasound images of the heart, because they reflect sound so well, the bubbles can now dissolve blood clots and potentially deliver genes and drugs to targeted parts of the body.

Introducing the microbubbles into a rabbit's blood vessel, and aiming ultrasound at it, Unger and his coworkers dissolved a blood clot, by causing the bubbles to pop in that location and sweep away the clot in small pieces. In addition, Unger and his colleagues have attached drugs and genes to the microbubbles in several ways. Introducing gene-containing microbubbles into an animal and aiming ultrasound at its heart, the researchers observed significant quantities in the heart of CAT, the protein expressed by the gene.

In traditional gene therapy, the gene is delivered via a modified virus, which may cause serious allergic reactions in some cases. But ultrasound-activated microbubbles may provide a safer alternative, and a more effective one, since the application of ultrasound even without the bubbles seems to enhance the introduction of genes and drugs into cells in many cases. Even without the bubbles, Unger showed that ultrasound enabled the tumor-suppressing drug interleukin-12 to be taken up in 10-1000 times greater amounts in mice. Unger speculated that the microbubbles might someday be used in outpatient heart exams, first to detect plaque, then to dissolve the plaque if it is present. While promising, these applications all require further testing and development.