Magnetic Fields, Rotational periods, and Wind Speed

When measuring the rotational period of Jupiter, astronomers have noted a disparity arises between results obtained by radio observations and those taken using visible and infrared radiation. The difference, explains lead author Allers, arises because the radio emission is created by electrons interacting with Jupiter’s magnetic field, deep within its interior, whilst the infrared emission originates from the top of the gas giant’s atmosphere.

In-between, these points the atmosphere of Jupiter is rotating faster than its interior as a result of its atmospheric winds. Thus the difference in measurements is also a result of said winds. That means that calculating the speed of these winds should be as simple as subtracting the value of the interior velocity — given by radio observations — from the velocity measurement given by infrared observations.

Brown dwarf, left, and Jupiter, right. An artist’s conception of brown dwarf illustrates magnetic field and atmosphere’s top, which were observed at different wavelengths to determine wind speeds (Bill Saxton, NRAO/AUI/NSF)

“If you measure the rotation period of Jupiter using its radio emission, you’ll find a period of 9h~55m~30s, but if you measure the period from the rotation of its cloud features in the optical or near-IR, you’ll find a slightly different period of 9h~50m~30s,” Biller says. “The approximate 5-minute difference corresponds to a velocity difference of roughly 100 m/s. In other words, what you measure here is the equatorial wind of Jupiter itself.”

The team expect the same mechanisms at play on Jupiter are also a factor on 2M1047, hence why they chose to make complementary observations with the VLT and the Spitzer space telescope — radio and infrared telescopes respectively — to assess the brown dwarf’s wind speeds.

“We did the same thing for a brown dwarf — measured the rotation period from the magnetic field (radio) and also from the clouds (infrared). The difference between those two measured rotation periods then gives the wind speed,” Biller says.

To this end, the team observed 2M1047 with Spitzer in 2017 and again the following year, finding that its infrared brightness displays a regular variation — likely as a result of a long-standing feature preserved in its upper atmosphere. The team then took radio observations of the interior in 2018.

“We measured a wind speed of approximately 1425 miles per hour— quite fast, and several times faster than Jupiter’s wind — which has been measured at around 230 mph,” says Biller.

“This is the first wind speed measured for any cool, extrasolar object.”

The higher wind-speed measured by the astronomers conforms with theoretical predictions for atmospheric conditions on brown dwarfs.

In addition to measuring wind speeds on more brown dwarfs, the team also hopes to turn their attention — and their new technique — to the study of exoplanets. The method may require a few adjustments when applied to planets outside the solar system, however. This is mainly because exoplanets have weaker magnetic fields than brown dwarfs, meaning that the measurements will have to be taken in lower-frequencies.

In addition to this, Biller suggests the team may have also uncovered an indirect way to study exoplanets: “Because brown dwarfs are often good analogues for exoplanets we can study their atmospheric properties as a proxy for atmospheres that are harder to access, such as directly imaged gas giant planets that are hidden in the glare of their parent star.