Analysis of Low-frequency solar Radio Bursts from the solar Corona and their space weathe rimplications

Abstract

Space Weather is the term used to describe changes in the space environment around Earth, often occurring within a day or less, driven by solar activity. The most powerful effects arise when massive bursts of solar material known as Coronal Mass Ejections (CMEs) and their shock waves interact with Earth’s magnetic field. The extent of impact depends on the speed, size, and magnetic strength of these CMEs. Solar flares also modify the amount of solar radiation reaching Earth’s atmosphere, affecting its lower layers. High-energy particles from the Sun, known as solar energetic particles (SEPs), are also a major concern for astronauts traveling to the Moon and beyond. Solar radio observations are essential for space weather research, as solar radio bursts (SRBs) originate from regions where solar flares erupt, SEPs are accelerated, and CMEs are launched. SRBs arise from different altitudes in the solar atmosphere and span wavelengths from millimeters to decameters. Coronal properties such as electron density, magnetic field strength, and turbulence affect the generation of SRBs and vary with both the solar cycle phase and overall solar activity. This study investigated low-frequency SRBs and their space weather implications during the progression of Solar Cycle 25 (SC 25). Initially, type II SRBs were analyzed alongside their impact on the ionosphere, particularly enhancementsintotalelectroncontent(TEC)measuredthroughtherateofTECindex(ROTI).Observations were primarily conducted using the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (CALLISTO). A dataset of 32 type II bursts was used to estimate shock and Alfv´en speeds, ranging from 504 to 1282 km/s and 368 to 826 km/s, respectively, at heliocentric distances of 1 – 2 solar radii (R⊙). The ambient magnetic fieldstrength,rangingfrom7.8to0.7Goverthisradialspan,wasmodeledasB(r) = 6.07r−3.96 G. The analysis showed that 19 of the 32 type II bursts were directly associated with radio blackouts and polar cap absorption events. For the first time, type II bursts were demonstrated to be reliable indicators of subsequent ionospheric irregularities in TEC. ROTI-based assessments revealed that diurnal TEC variability was influenced by the strength of associated solar flares and SEPs, with observed longitudinal variations linked to GPS station locations. In the second part of the study, 35 geomagnetic storms (Dst ≤−50 nT) were analyzed to characterize magnetic activity during SC 25. Correlation between SRBs and geomagnetic disturbances confirmed intense magnetic activity during the cycle’s ascending phase. The time delay between v SRBs and CME-driven magnetospheric impacts ranged from 48 to 120 hours, with an average of 79 hours, highlighting the potential of solar radio emissions as forecasting tools. The analysis confirms that space weather responses to geomagnetic storms exhibit event-specific variability, yet ionospheric storm activity remains a persistent feature, independent of geomagnetic storm magnitude. The third part examined the relationship between SRBs and large SEP events, focusing on their terrestrial impacts. This analysis covered three solar cycles (1997 – 2024) and 122 large SEP events were analyzed. Statistical results showed that SC 25 behaves similarly to SC 23 and exhibits higher activity than SC 24 regarding large SEP occurrences. Velocity dispersion analysis (VDA) revealed that 35 of the 122 SEP events were released either before or concurrently with the peaks of associated GOES X-ray solar flares, without any time lag. The observed correlation betweenSRBs andSEPevents provides insightsintothebehaviorof particlepopulationsdriven by solar flares and CMEs. WAVES/STEREO dynamic spectra indicated that 76% of SRBs extended into interplanetary space, demonstrating the dynamics of associated shocks and electron beams propagating along open or quasi-open magnetic field lines. As a whole, this study underscored the value of SRBs observation for a better understanding of the Sun – Earth interaction dynamics. The ascending phase of SC 25 has been comprehensively monitored and characterized, paving the way for future advancements in space weather modeling and forecasting.