Temporally resolved Type III solar radio bursts in the frequency range 3-13 Mhz 

(Solar Orbiter Nugget #41 by Antonio Vecchio^{1, 2}, Milan Maksimovic², Nicolina Chrysaphi^3,4, Eduard P. Kontar⁴, Vratislav Krupar^{5, 6})

Introduction

Radio waves occupy the longest wavelength region of the electromagnetic spectrum. Different astronomical objects, characterized by a varying magnetic field, produce radio waves. The first quasars were identified in the late 50s as radio sources with no corresponding signal in the visible light. Astronomers produced the first image of a black hole in 2019 by visualizing the radio emission from hot gas swirling around it under the effect of strong gravity fields close to the event horizon.

The Sun is one of the strongest radio sources in the sky producing several kinds of radio emissions differing in characteristics, intensity and mechanism of production. Type III radio bursts are the most common coherent radio emission produced by the Sun. They are characterized by a rapid drift in time towards lower frequencies (f) and represent an indirect signature of energetic electrons produced at the Sun during a flare and propagating through the plasma of the corona and the interplanetary medium with subluminal speed (typically 0.3-0.1c). Type III radio bursts are observed over a wide range of frequencies ranging from about ∼500 MHz down to tens of kHz close to 1 au, corresponding to a wide range of heliocentric distances.

Density fluctuations along the path of the solar radio waves can strongly affect the propagation and the properties of the detected type III bursts. Scattering of radio waves on random density irregularities has long been recognized as an important process for the interpretation of radio source sizes (e.g. [1]), positions (e.g. [2,3]), directivity (e.g. [4-6]), and intensity-time profiles (e.g. [7]). Due to the scattering, the intensity-time profiles of a radio burst are characterized by a very fast rising phase followed by a long-lasting exponential decrease [8] (see Figure 1). Since decay times are directly related to the radio-wave scattering, their analysis provides useful information about the strength and anisotropy of the scattering process [9,10] and the level of density fluctuations [7,11].

When the measured decay time τ is represented as a function of frequency, it follows a power law f^(−β), where β=-0.970 +/- 0.003 [9]. However, a data gap, marking the separation between measurements from space and on ground, is present between 3 and 13 MHz due to the lack of temporally resolved measurements.

Decay time measurements in this frequency range can only be made from space as the Earth’s ionosphere partially reflects and absorbs the signals below ∼10 MHz. Accurate decay time measurements in this frequency range are therefore needed to confirm the expected trend and characterize, through observational data, the scattering in the radial distance belt between 2 and 5 R☉ that currently remains unexplored. The few observations present in the literature [12-14], performed with sampling time larger than 3.5 s, found quite different β values due to the various low temporal resolutions of the data sets, not allowing for the measurements to be resolved. A sufficiently high temporal resolution (lower than 0.5s) is indeed needed to properly sample signals with expected decay times of the order of 1-10 s.

The SO/RPW/HFR Observations

The High Frequency Receiver (HFR) [15,16] of the RPW instrument on board Solar Orbiter was configured to acquire five frequencies [3.225, 5.225, 6.875,10.125, 12.225] MHz for ten times followed by a sweep on 50 frequencies between 0.425 and 16.325 MHz. This novel configuration, including performing an average on the lowest possible number of spectra [16] and measuring at only one sensor V1–V2, allows to reach, for each of the five chosen frequencies, a time resolution of ∼0.07s and an average resolution of∼0.18 s (Figure 1), significantly better than previous measurements.

Figure 1: Intensity-time profiles of the radio flux density measured by SO/RPW/HFR at five frequency channels during a type III burst on 2023 November 26. The exponential decrease is clearly visible in the data and is highlighted by a red line showing the results of the decay time fitting.

Results and Discussion

The dataset obtained over about 13 months of observation, is unique in the framework of space observations since the achieved sample time is up to 2 orders of magnitude higher than any other spacecraft measurements. By analyzing HFR power spectral densities from calibrated level 2 (L2) data, more than 350 type III bursts have been identified. Decay times were obtained through an exponential fit on the data.

The large number of detected events allowed to statistically characterize the decay time in the range 3-13 MHz, with the following conclusions:

1. For each of the considered frequency the decay time does not depend on the radial distance of the spacecraft but only on the frequency (Figure 2).

Figure 2: Decay time as a function of the radial distance for the five considered frequencies. No radial distance dependence is observed.

2. the time resolution of the data set is the decisive factor for the proper measurement of decay times and the accurate determination of the τ spectral index in the considered frequency range. The time resolution of the measurements used in previous works was insufficient to accurately characterize the decay time at frequencies higher than 6 MHz since the sampling time is comparable to the expected decay time (Figure 3).

Figure 3: Decay times for the five considered frequencies obtained as the median value from the sample of measurements. Red: full time resolution original data set; black: data set with time resolution reduced to 3.5 s; green: PSP data set with 3.5 s resolution. Error bars represent the standard error. The black, red, and lime lines correspond to the power-law fit on the respective five data points. The blue dashed line shows the power-law function with β= -0.970 from Equation 1 of [9]. The comparison of the decay time trends at different time resolutions shows that in the frequency range 3-13 MHz the discrepancy with Equation 1 increases when the time resolution of the data set decreases, and a flatter trend is obtained.

3. HFR measurements allowed to fully characterize the τ behavior as a function of frequency in the range 3-13 MHz and to fill the long-standing gap in the observations of type III burst decay times. Our observations show that the τ-f power law trend does not change in the radial distance range 2-5 R⊙, and a spectral index β=-1 is representative in the full 1-100 R⊙ range (Figure 4).

Figure 4: Median decay time values from measured type III bursts in the frequency range of 3–13 MHz (shaded region), superimposed on the data shown in Figure 10 of [9] and the newly added data from [17]. Error bars represent the standard error obtained from observations. The best-fit function is also printed.

Acknowledgements

The RPW instrument has been designed and funded by CNES, CNRS, the Paris Observatory, the Swedish National Space Agency, ESA-PRODEX and all the participating institutes.

N.C. acknowledges funding support from the Initiative Physique des Infinis (IPI), a research training program of the Idex SUPER at Sorbonne Université.

EPK was supported via the STFC/UKRI grants ST/T000422/1 and ST/Y001834/1.

Afilliations

[1] Radboud Radio Lab, Department of Astrophysics, Radboud University, Nijmegen, The Netherlands

[2] LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris,5 place Jules Janssen,92195 Meudon, France

[3] Sorbonne Université, Ecole Polytechnique, Institut Polytechnique de Paris, CNRS, Laboratoire de Physique des Plasmas(LPP), 4 Place Jussieu, 75005 Paris, France

[4] School of Physics & Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK

[5] Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County, Baltimore, MD 21250, USA

[6] Heliospheric Physics Laboratory, Heliophysics Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771,USA

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