arXiv 1012.35232010-12astro-ph.SR

来自327 MHz噪声风暴源大小对日冕turbulence模式的约束

Constraints on coronal turbulence models from source sizes of noise storms at 327 MHz

Prasad Subramanian, Iver Cairns

备注：: Accepted for publication in the Journal of Geophysical Research (Space Physics). Figures 5, 8 and 9 revised. Conclusions unchanged
DOI：: 10.1029/2010JA015864

摘要：

我们试图调和观测的小源尺寸在太阳日冕在327 MHz的散射模型，将折射率效应，内尺度效应和球面发散波前的预测。我们使用的经验处方的湍流振幅$C_{N}^{2}(R)$的基础上VLBI观测斯潘格勒和同事的紧凑的无线电源对太阳风的日心距离$R \approx$ 10- 50 $R_{\odot}$。我们使用Coles & Harmon模型的内尺度$l_{i}(R)$，这是假定产生的回旋阻尼。鉴于普遍的不确定性幂律指数，表征太阳风湍流在不同的日心距离，我们保留这个指数作为一个自由参数。我们发现，列入球面发散的影响抑制了预测源的大小大幅。我们还发现，内尺度效应显着降低预测源的大小。对太阳源的一个重要的普遍发现是，计算大大低估了观测到的源大小。三种可能的，非排他性的，这个一般结果的解释提出。首先，也是最简单的，未来的观测具有更好的角分辨率，将探测到更小的源。与此相一致的是，以前在米制波长下对日冕中小源的观测受到仪器分辨率的限制。第二，内日冕中湍流$C_{N}^{2}(R)$的空间变化水平比经验公式直接向太阳外推所预测的要大得多，经验公式是根据10- 50 $R_{\odot}$之间的观测结果得出的。函数形式或比例常数可以不同。第三，也许内部尺度比模型小，导致散射增加。

英文摘要：

We seek to reconcile observations of small source sizes in the solar corona at 327 MHz with predictions of scattering models that incorporate refractive index effects, inner scale effects and a spherically diverging wavefront. We use an empirical prescription for the turbulence amplitude $C_{N}^{2}(R)$ based on VLBI observations by Spangler and coworkers of compact radio sources against the solar wind for heliocentric distances $R \approx$ 10--50 $R_{\odot}$. We use the Coles & Harmon model for the inner scale $l_{i}(R)$, that is presumed to arise from cyclotron damping. In view of the prevalent uncertainty in the power law index that characterizes solar wind turbulence at various heliocentric distances, we retain this index as a free parameter. We find that the inclusion of spherical divergence effects suppresses the predicted source size substantially. We also find that inner scale effects significantly reduce the predicted source size. An important general finding for solar sources is that the calculations substantially underpredict the observed source size. Three possible, non-exclusive, interpretations of this general result are proposed. First and simplest, future observations with better angular resolution will detect much smaller sources. Consistent with this, previous observations of small sources in the corona at metric wavelengths are limited by the instrument resolution. Second, the spatially-varying level of turbulence $C_{N}^{2}(R)$ is much larger in the inner corona than predicted by straightforward extrapolation Sunwards of the empirical prescription, which was based on observations between 10--50 $R_{\odot}$. Either the functional form or the constant of proportionality could be different. Third, perhaps the inner scale is smaller than the model, leading to increased scattering.

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