Annu. Rev. Astron. Astrophys. 1992. 30: 653-703
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3.3.4 BALLOON OBSERVATIONS AT MILLIMETER AND SUBMILLIMETER WAVELENGTHS A number of very sensitive bolometer measurements at millimeter and submillimeter wavelengths has been carried out by Page et al (1990) and by Meyer et al (1991a, b). This group has reported the results from two sessions of observations in October 1988 (Page et al 1990) and October 1989 (Meyer et al 1991a, b). The telescope beamwidth was 3°. 8 FWHM. The instrument scanned the sky at 45° zenith angle (first session) or 44° zenith angle (second session) at a rotation rate of 2 rpm (first session) or 1.5 rpm (second session). The radiometer operated in four passbands centered at wavelengths 1.8, 1.1, 0.63 and 0.44 mm with bandwidths of 0.4, 0.2, 0.07 and 0.03 mm, respectively. They switched between the sky and an internal reference load at a frequency of 4.5 Hz. Each detector output was sampled synchronously with the switch.

In their first session the observations lasted for only 8.7 minutes before a balloon failure aborted the experiment, but they were nevertheless able to place interesting constraints on the anisotropy (see Table 4). The observations after subtraction of the dipole term are shown in Figure 3b. In the second session the observations lasted for ten hours. For the lowest frequency channel, after correction for offsets and drifts, the best calibration was provided by the dipole anisotropy of the microwave background radiation itself. Meyer et al assumed a value for the dipole amplitude of 3.3 mK [Lubin et al 1985; D. A. Cottingham (private communication); Smoot et al 1991b]. They allowed for one additive and one multiplicative parameter for each one-hour segment of data, and two parameters for the direction of the dipole, so that 6 hours of data was fitted with 14 parameters. A map of about half the northern hemisphere was then made, and they then excised the data with Galactic latitude |b| < 15° and analyzed the residuals for anisotropy. The chi2 fit to the dipole term is 1.7 per degree of freedom, which shows that there is definitely excess variance in these observations.

Meyer et al assumed a Gaussian form for the spectrum W(k), and followed the method of Boughn et al (1991) described above. A statistic, S'obs, was again formed. Meyer et al assumed a certain correlation angle, phic, and used Monte Carlo tests to determine both the sensitivity of their observations using the null statistic described above and the 95% confidence upper limits which they could place on the microwave background radiation variations. In their simulations they ran series of 300 Monte Carlo tests for a number of values of phic covering the range 3-22°. The values of DeltaT/T derived from the null assumption and from their measured {DeltaTi} are shown in Figure 4b. One can see immediately from this figure that the expected 95% confidence upper limit (indicated by the line) is significantly below the 95% confidence upper limit derived from their observations. The power of the tests for each upper limit derived is greater than 90%. The fact that the 95% upper limits based on the data are about a factor two higher than the expected 95% limits based on the null assumption is clear evidence for excess variance.

Figure 4b
Figure 4b. Limits on the anisotropy set by Meyer et al (1991a) at 168 GHz. The circles represent 95% confidence upper limits determined by the same method as in Figure 4a and assuming a Gaussian form for W(k), and the line represents the sensitivity of the observations, determined from a Monte Carlo simulation in which the flucutaions due to the microwave background radiation are set to zero. It is clear that fluctuations have been detected over the whole range of angular scales from 3-22°. The detected anisotropy might all be attributable to Galactic thermal emission from dust, but intrinsic microwave background radiation fluctuations at a level of DeltaT/T ~ 10-5 cannot be ruled out.

Meyer et al considered the possible sources of this anisotropy, apart from the microwave background radiation itself. The most likely candidates are Galactic dust emission, quasi-stationary atmospheric emission or systematic errors due to instrumental effects. They pointed out that the expected level of Galactic dust emission, DeltaT/T ~ 10-5, was not far from their measured levels. Also, a ``stationary'' atmospheric cloud could have produced the excess, and they did see clouds at high altitudes during part of their observing time. Finally, their instrument could be susceptible to a number of different effects, ranging from diffracted earthshine to magnetic effects on their switches, but these were estimated to be significantly smaller than the observed effect.

Meyer et al concluded that there was definitely some anisotropy at the level DeltaT/T = 1-3 x 10-5 on scales greater than 4°, but that reasonable expectations of the level of anisotropy in Galactic dust emission and in atmospheric emission were close to this level and so could not be ruled out. They therefore used their observations only to place upper limits on any intrinsic anisotropy in the microwave background radiation.

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