One of the most commonly observed solar radio sources in metric and decametric wavelengths are the so -called solar noise storms. These are generally associated with active regions and are believed to be driven by the plasma emission mechanism. Since plasma emission emits mainly in the fundamental and harmonic of the frequency of local plasma, the apparent angular size of the source can be significantly-notation of dispersion due to multiple propagation caused by the refraction of density homogeneities. Past observational and theoretical estimates suggest a minimum source size observable in the solar crown. However, the details of this limit depend on the modeling approach and the details of the chosen coronal turbulence model. Therefore, pushing the minimum source size observed at smaller values can help restrict the plasma environment of the observed sources.
There have only been a handful of investigations in the past to investigate the structure of the noise storm using observations at high space resolution (e.g. Lang et al. 1987, Mercier et al. 2015 ,etc.). Recently Mondal et al. (2024) Evidence presented of structures of size 9 “In noise storms at 250 MHz. This size observed is approximately 3-4 times smaller than the previous predictions. In this work, we present multiple instances of structures of very small scale (~ 10” -20 “) in the noise storms. These structures are stable during the times of 15-30 minutes and we have a general route of ~ 100 MHz, comparable with the frequency of observation.
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 data set and produce an image at high spatial resolution. The blue ellipse at the bottom shows the instrumental angular resolution.
The small angular sizes observed in such long scales suggest that these small sources are not arising due to the casual alignment of coronal conditions. To understand these observations, we present an illustrative model that can reduce the size of the predicted source in more than one factor of 2. We demonstrate that if the noise storm arises from a coronal loop of high density, embedded in plasma with less density, then the size of the source observed can be much smaller, depending on the density contrast between the loop and the background plasma. A difference of greater density facilitates the observation of a smaller source. While our model has many limitations and uses parameters that are probably more suitable for interplanetary conditions, it suggests an interesting way to explore to explain the observations of these unexpectedly compact sources.
Conclusions
Therefore, this work shows that investigating such small sources not only provides us with knowledge about coronal dispersion, but can also shed light on the emission conditions of the noise storm source.
References
LANG, KR, WILLSON, RF, 1987, APJL, 319, 514
#Observation #modeling #small #spatial #structures #solar #radio #noise #storms #UGMRT #Mondal #European #astronomer #radio #community
