Density fluctuations populating the heliosphere interfere with propagating radio photons, altering their trajectories through frequency-dependent effects like scattering. Crucially, these density fluctuations are anisotropic, leading to anisotropic scattering and directional radio-wave propagation. This means that observers at different positions may obtain different estimations of radio properties. Such effects are particularly evident in solar radio bursts which are emitted through the plasma emission mechanism. It has been shown that detectors at various locations will measure vastly different radio burst properties, including the flux (which can vary by orders-of-magnitude), source sizes, and source positions (Kontar et al. 2019, Kuznetsov et al. 2020, Musset et al. 2021). However, until recently, it was unknown whether the measured decay and rise times of solar radio bursts also vary with the observer’s location. Decay times are of particular interest since they are dominated by scattering effects, and have thus been used to approximate the level of small-scale density fluctuations in the heliosphere (e.g. Krupar et al. 2020). Chrysaphi et al. (2024) address this open question using stereoscopic observations of Type III solar radio bursts from four non-collinear, angularly-separated spacecraft: Solar Orbiter (SolO), Parker Solar Probe (PSP), STEREO-A (STA), and WIND.

Methodology & Results

Each Type III burst is simultaneously observed (at comparable frequencies) by at least 3 spacecraft at different vantage points. The radio light curves are fit with a single function that can describe the entirety of the signal (Figure 1), allowing for improved estimations of the decay time and enabling a simultaneous estimation of the rise time and peak time/amplitude. The reliability of the proposed function is also illustrated, notably in comparison to the normally-favoured single-exponential fit to the decay phase. By successfully fitting the entirety of observed radio light curves with the proposed function, Chrysaphi et al. (2024) demonstrate that the rise phase of radio bursts is non-exponential, growing at a non-constant rate. Instead, the rise phase’s growth rate is a function of time, initially growing faster than an exponential.

The angular separation between the spacecraft and the radio source is calculated considering the 3D positions of the spacecraft (and not simply their longitudinal positions), further allowing for an estimation of the Euclidean separation between the spacecraft and the source (instead of merely comparing their heliocentric distances). Therefore, any dependence of measured rise and decay times is examined in terms of both the angular and Euclidean separations between the source and spacecraft. Chrysaphi et al. (2024) find that the rise and decay times do not vary with the position of the observer (whether the variation is angular or Euclidean; Figure 1). Therefore, rise and decay times are identified as the only radio burst properties whose measurements remain independent of the observer’s vantage point.

Figure 1. Left: Light curve of a Type III solar radio burst recorded by a spacecraft (black datapoints) being fit in its entirety with the proposed function (blue curve). Right: Measurements of the decay and rise times as a function of the angular separation (top) and the Euclidean distance (bottom) between the spacecraft and the radio source. Figure adapted from Chrysaphi et al. (2024).

The frequency dependence of the rise and decay times of radio bursts is also examined (Figure 2). More than 300 light curves are analysed, covering frequencies (f) from 60 – 1725 kHz. Both the rise and decay times are found to have an approximately 1/f relation, as has been previously shown for decay times (Kontar et al. 2019). Furthermore, Chrysaphi et al. (2024) find that the ratio of rise-to-decay times is independent of the frequency (Figure 2). This result is compared against rise-to-decay time ratios calculated using already-available data, overall covering four decades of frequencies ranging from 0.06 – 130 MHz, confirming the obtained frequency independence. Given that decay times are significantly affected by the (frequency-dependent) scattering effects, this finding implies that rise times are affected by scattering effects in a proportionate manner. This provides evidence that scattering not only impacts the decay phase, but is also an important contributor to the rise phase of solar radio bursts.

Figure 2. Measurements of the rise time (left), decay time (middle), and the rise-to-decay time ratio (right) as a function of the observed frequency. Figure adapted from Chrysaphi et al. (2024).

Conclusions

1. Rise and decay time measurements of solar radio bursts depend on neither the angular nor the Euclidean separation of the detector from the radio source.
2. The rise-to-decay time ratio is independent of frequency, indicating that the rise phase of solar radio bursts is affected by scattering in a proportional manner to their decay phase. Therefore, scattering effects are identified as a significant contributor to the rise phase, adding to our understanding of the plasma emission process.
3. The rise phase of solar radio bursts grows at a non-constant, non-exponential rate.
4. Functions that successfully fit the entirety of the radio burst light curves should be used for reliable estimations of the rise and decay times.

Based on a recent paper by Nicolina Chrysaphi, Milan Maksimovic, Edurard P. Kontar, Antonio Vecchio, Xingyao Chen, and Aikaterini Pesini. First determination of the angular dependence of rise and decay times of solar radio bursts using multi-spacecraft observations. Astronomy & Astrophysics, 687, L12 (2024). DOI: https://doi.org/10.1051/0004-6361/202348175

References

Kontar, E. P., Chen, X., Chrysaphi, N., et al. 2019, ApJ, 884, 122

Krupar, V., Szabo, A., Maksimovic, M., et al. 2020, ApJS, 246, 57

Kuznetsov, A. A., Chrysaphi, N., Kontar, E. P., et al. 2020, ApJ, 898, 94

Musset, S., Maksimovic, M., Kontar, E. P., et al. 2021, A&A, 656, A34