One of the most commonly observed solar radio sources in the metric and decametric wavelengths are the so-called solar noise storms. These are generally associated with active regions and are believed to be powered by the plasma emission mechanism. Since plasma emission emits primarily at the fundamental and the harmonic of the local plasma frequency, the apparent angular size of the source can become significantly scatter-broadened due to the multi-path propagation caused by refraction from the density inhomogeneities. Past observational and theoretical estimates suggest some minimum observable source size in the solar corona. The details of this limit, however, depend on the modeling approach and details of the coronal turbulence model chosen. Hence pushing the minimum observed source size to smaller values can help constrain the plasma environment of the observed sources.
There have only been a handful of investigations in the past to investigate the structure of noise storm using observations at high spatial resolution (e.g. Lang et al. 1987, Mercier et al. 2015 ,etc.). Recently Mondal et al. (2024) presented evidence for structures of size 9” in noise storms at 250 MHz. This observed size is about 3-4 times smaller than earlier predictions. In this work, we present multiple instances of very small-scale structures (~10”-20”) in the noise storms. These structures are stable over timescales of 15-30 minutes, and have a bandwidth of ~100 MHz, comparable to the observation frequency.
Figure 1: The left panel shows a low resolution image of a noise storm at 314 MHz, produced by integrating 24 MHz and 25 minutes of data. In the left panel, we use the same dataset and produce an image at high spatial resolution. The blue ellipse at the bottom shows the instrumental angular resolution.
Small angular sizes observed over such long timescales suggest that these small sources are not arising because of chance alignment of the coronal conditions. To understand these observations, we present an illustrative model which can lower the predicted source size by more than a factor of 2. We demonstrate that if the noise storm is arising from a high-density coronal loop, embedded in plasma with lower density, then the observed source size can be much smaller, depending on the density contrast between the loop and the background plasma. A higher density difference makes it easier to observe a smaller source. While our model has many limitations, and uses parameters which are probably more suited for interplanetary conditions, it suggests an interesting avenue to explore for explaining the observations of these unexpectedly compact observed sources.
Conclusions
Thus, this work demonstrates that investigating such small sources not only provides us knowledge about coronal scattering, but can also potentially shed light on the emission conditions of the noise storm source themselves.
References
Lang, K.R., Willson, R.F., 1987, ApJL, 319, 514
Mercier, C., Prasad, S., et al. 2015, A&A, 576, 136
Mondal, S., Kansabanik, D., et al., 2024, ApJ, 975, 122
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