That Unpredictable Sun
The ebb and flow of the sun's magnetic activity, which spans periods of about 11 years, has not been playing out as expected. The current solar cycle, No. 24 since scientists have been keeping count, was predicted to be a whopper, but instead has proved to be a dud, with sunspot and solar flare activity at a nadir. While that might seem good news for telecommunications and satellite operators—since strong solar magnetic activity can wreak havoc with transmissions—in the long run everyone would prefer reliably accurate predictions.
That's the mission of NASA's Solar Dynamics Observatory, which was launched into geosynchronous orbit in February 2010 carrying the Stanford-designed and operated Helioseismic and Magnetic Imager (along with two other instruments). By capturing Doppler radar images every 45 seconds for the past two years, the HMI has enabled researchers to map the patterns and determine the velocity of plasma waves on the sun's surface. These currents are connected to the star's internal dynamics and the generation and distribution of its magnetic field.
In September, Stanford solar physicists led by Junwei Zhao, a senior research scientist at the Hansen Experimental Physics Laboratory, reported details for one such current, known as meridional flow, in Astrophysical Journal Letters. The HMI observations provide evidence that the flow—which moves from the equator toward the poles on the sun's surface—returns to the equator more rapidly and at a shallower depth than previously thought. More surprising, they also revealed a second, deeper meridional current.
"Considered together, this means that our previously held beliefs about the solar cycle are not totally accurate, and that we may need to make accommodations," Zhao says.
He speculates that one possible explanation for the current solar cycle differing so dramatically from predictions is that current simulations have incorrect or incomplete pa-rameters for mechanisms such as meridional flow. Plugging data obtained by the HMI and other SDO instruments into these simulations should help improve their accuracy and the validity of their predictions.
Typhoid Mary, Quite Contrary
The bacterial genus salmonella is responsible for 100 million cases of food poisoning and 16 to 20 million cases of typhoid fever worldwide each year—resulting in hundreds of thousands of deaths. But in a small fraction of cases, such as that of the infamous Typhoid Mary, who worked as a cook in New York in the early 1900s, a person infected with salmonella shows no symptoms. These carriers can harbor the bacteria in their bodies for years and unknowingly infect others.
"Between 1 and 6 percent of people infected with S. typhi, the salmonella strain that causes typhoid fever, become chronic, asymptomatic carriers," says Denise Monack, an associate professor of immunology and microbiology. "That's a huge threat to public health."
Monack's team previously showed that in these carriers salmonella hides in an unlikely place: inside immune cells called macrophages, whose job, among other things, is to gobble up pathogens and cellular debris. Recently, using a mouse model infected with a salmonella strain related to S. typhi, the group looked at how the microbe is able to make its home in the belly of the beast. They found that the bacterium S. typhimurium preferentially associates with macrophages that have shifted from "seek and destroy" mode (M1) to nonaggressive functions such as wound healing (M2). What's more, the presence of salmonella appears to facilitate the macrophage's transition from M1 to M2.
Further investigation revealed that S. typhimurium's ability to replicate within macrophages is tied to a class of genetic switches called PPARs. Monack's group showed that a strain of mice lacking PPAR-delta, a regulator of fatty acid metabolism, developed acute but not chronic salmonella infections. Monack suspects that the bacterium is activating PPAR-delta for its own ends. Understanding how, she says, "could lead to some great anti-salmonella therapeutics with relatively fewer side effects."
