Comments on “In Situ Atmospheric Turbulence Measurement Using the Terrestrial Magnetic Field—A Compass for a Radiosonde”

Abstract

In this paper, Harrison and Hogan (2006, hereafter HH06) describe a novel application of an inexpensive Hall effect sensor as a magnetometer and use this to monitor the attitude motions of a radiosonde balloon during ascent to infer the presence of turbulence. I wish here to point out some modifications of their technique, which may offer better results, as well as to offer some further comments on data interpretation and related applications. First, they mount the sensitive axis of the sensor horizontally, such that it responds to deviations from “North”—their Fig. 2 shows the magnetic field apparently horizontal. In fact, at midlatitudes characteristic of northern Europe and North America, the magnetic field has an appreciably vertical direction (see, e.g., the models at the National Geophysical Data Center available online at http://www.ngdc.noaa.gov/seg/geomag/ jsp/struts/calcPointIGRF) and is inclined at some 55°– 70° to the horizontal. Thus, the sensor would be more effectively mounted with its sensitive axis along the direction of the tether, that is, nominally vertical. This further makes the sensor exclusively sensitive to swinging motions, rather than the horizontal twisting/ untwisting motions whose relevance to atmospheric motion is not obvious. It is worth noting that while the Hall effect sensor they describe may be a compact and cost-effective solution, it needs some amplification to derive a reasonable angular sensitivity, whereas a variety of small and inexpensive magnetometers (magnetostrictive or fluxgate) are becoming available, produced for automotive, game controller, and mobile robotics applications. Many have amplified analog outputs, such as that used in HH06, although others have pulse frequencymodulated or other digital outputs that are convenient for interfacing to microcontrollers without the additional circuitry whose parts and assembly cost may largely offset the sensor cheapness. As a couple of examples, fluxgate sensors have been used to monitor rapid angular motions on aerospace platforms, including parachute-borne atmospheric probes (Dooley and Lorenz 2005) and recreational flying discs (“Frisbees”; Lorenz 2005). However, it should be underscored that the parameter of interest is turbulence and motion, not the magnetic field per se, and since it would be desired to have a standard sensor suite for use worldwide (where the strength of the magnetic field, and its orientation, can vary substantially) it may be more appropriate to use sensors directly sensitive to motion, such as accelerometers or Micro-Electro-Mechanical Systems (MEMS) gyros (also now very compact, low power, and easy to interface). HH06 make an important contribution in comparing the history of the attitude motion to independent measurements of the atmospheric motions. This is an important problem in planetary exploration, where probes descending by parachutes through the atmospheres of Venus, Jupiter, or Titan have experienced motions (e.g., Seiff et al. 1999), which are at least partially excited by ambient turbulence; quantitatively recovering the intensity of the turbulence from these dynamics measurements is a problem that has not yet been satisfactorily addressed. The measurements of HH06 can be considered in this regard. A challenge in this sort of investigation is that turbulence-excited motions are superimposed on various self-excited motions that can occur in perfectly still air, both for ascending balloons and for descending parachutes (although not, of course, on balloons at a steady float altitude). The spectral analysis of motions in HH06 suggests that pendulum motions in particular may be associated with turbulence, for example, at the base of cloud; my own informal observations of parachute motions are consistent with this association. Corresponding author address: Ralph D. Lorenz, The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723. E-mail: ralph.lorenz@jhuapl.edu AUGUST 2007 N O T E S A N D C O R R E S P O N D E N C E 1519