We study the spectral energy distribution (SED) and the power spectrum of Galactic cirrus emission observed in the 14 deg^2 Science Demonstration Phase field of the Herschel-ATLAS using Herschel and IRAS data from 100 to 500 μm. We compare the Spectral and Photometric Imaging Receiver (SPIRE) 250, 350 and 500 μm maps with IRAS 100-μm emission, binned in 6-arcmin pixels. We assume a modified blackbody SED with dust emissivity parameter β (F∝λ^(−β)) and a single dust temperature T_d, and find that the dust temperature and emissivity index varies over the science demonstration field as 10 < T_d < 25K and 1 < β < 4. The latter values are somewhat higher than the range of β often quoted in the literature (1 < β < 2). We estimate the mean values of these parameters to be T_d= 19.0 ± 2.4 K and β= 1.4 ± 0.4. In regions of bright cirrus emission, we find that the dust has similar temperatures with T_d = 18.0 ± 2.5 K, and similar values of β, ranging from 1.4 ± 0.5 to 1.9 ± 0.5. We show that T_d and β associated with diffuse cirrus emission are anti-correlated and can be described by the relationship: β(T_d) =NT^α_d with [N= 116 ± 38, α=−1.4 ± 0.1]. The strong correlation found in this analysis is not just limited to high-density clumps of cirrus emission as seen in previous studies, but is also seen in diffuse cirrus in low-density regions. To provide an independent measure of T_d and β, we obtain the angular power spectrum of the cirrus emission in the IRAS and SPIRE maps, which is consistent with a power spectrum of the form P(k) =P_0(k/k_0)^γ, where γ= 2.6 ± 0.2 for scales of 50–200 arcmin in the SPIRE maps. The cirrus rms fluctuation amplitude at angular scales of 100 arcmin is consistent with a modified blackbody SED with T_d 20.1 ± 0.9 K and β= 1.3 ± 0.2, in agreement with the values obtained above
