The radio data presented in the current work were observed by e-CALLISTO from May 2021 to December 2022 in solar cycle 25. First, we selected a number of radio events from the observations made by the instrument (https://e-callisto.org, last access: 19 February 2024) and selected 32 well-separated type II radio bursts whose morphologies are clear. We then examined their association with the current solar phenomena to give insights into the status of the ascending phase of the solar cycle 25. In order to investigate the implications of space weather in terms of TEC, each selected type II radio burst was associated with a coronal mass ejection (CME) and an onset of a solar flare. The flare records were checked from the solar monitor (https://solarmonitor.org/, last access: 19 February 2024). The CME parameters were taken from the Large Angle and Spectrometric COronagraph (LASCO C2) on board the Solar and Heliospheric Observatory SOHO; catalogue updated to 30 December 2022.

In this study, first we measured the bandwidth (BDW) of each type II radio burst. Because all of the type II bursts do not show a band-splitting feature, the BDW of the fundamental band is linked to the ambient density jump to ensure consistency in computation. 1BDW=fu-flfl, where fu and fl denote the upper and lower frequencies, respectively, of the fundamental emission band. Figure  shows an example of type II radio burst from 03:28:25 to 03:32:30 UT on 17 April 2022 for which fu and fl are indicated. This burst is associated with an X1.1 flare that started at 03:17 UT and stopped at 03:51 UT in NOAA active region 12994.

The values of BDW were used to estimate the density jump across the shock χ via the relation 2χ=(BDW+1)2. By assuming low plasma ratio (β→0) for a perpendicular shock in the corona , the density jump allows one to compute the Alfvén Mach number (MA) using the Rankine–Hugoniot approximation as follows: 3MA=χ(χ+5)2(4-χ). It has been shown that the rate of change in the frequency of metric type II radio bursts is related to the shock speed and the electron density gradient in the solar corona e.g. via the following: 4Vs=-2rα1fdfdt, where r is the shock formation height, α is a fitted empirical index of density variation over the heliocentric distance, f is the starting frequency, and dfdt is the frequency drift rate. The electron density decreases with heliocentric distance from the Sun, according to the power law ne(r)∝r-α. Three different density models by , and describe the variation in the electron density in the corona and interplanetary medium. With these models, it has been observed that within one to three solar radii (R⊙), the electron density is directly proportional to r-6 in the corona and directly proportional to r-2 beyond few tens of solar radii. Because the type II radio observed has all occurred in the range of ∼1-2R⊙, α is chosen to be 6.13 . The Alfvén velocity is directly related to the shock speed as 5VA=VsMA. In the region surrounding a CME, the ambient magnetic field strength (B) of the plasma can be estimated using the following relation : 6B[G]=5.1×10-5×fl[MHz]×VA[kms-1], where fl is the lower frequency of the fundamental frequency band.

Data from ground-based GPS receiver stations around the world were used to analyse the ionospheric TEC for disturbed days identified by type II radio burst observations in this study. These include the African Geodetic Reference Frame (AFREF) database (http://afrefdata.org/, last access: 12 February 2024) and UNAVCO Archive of GNSS Data (https://www.unavco.org/, last access: 12 February 2024). Figure  maps the geographic locations of some GNSS stations used in the current study for reference.

As GPS data are usually provided in a Receiver Independent EXchange (RINEX) format, TEC were derived from RINEX files using the GPS TEC software developed at Boston College, assuming a thin-shell ionosphere at the altitude of 350 km. Details on the software used to derive TEC are provided in and and references therein. To reduce the multipath effects on slant TEC (STEC), the elevation angle was fixed to 30°. The ROT was calculated using Eq. () proposed by and has been utilized by several researchers to explore ionospheric irregularities . 7ROT=STECk+1-STECkΔtk, where STECk+1 and STECk are the STEC values at two successive epochs, and Δtk is the time difference between them equivalent to 30 s for the International GNSS Service (IGS) given in Fig. . Equation () was used to calculate the ROTI, which was defined as the standard deviation of ROT over 5 min. 8ROTI=<ROT2>-<ROT>2, where the <> stands for the time-averaged value. The solar energetic particles were taken from the database at https://cdaweb.gsfc.nasa.gov/ (last access: 19 February 2024).

During the ascending phase of the solar cycle 25, the e-CALLISTO network observed a series of solar radio bursts in the range from 5 to 870 MHz. With the interest of space weather diagnostics, 32 well-separated type II radio bursts observed are presented in this study. Table  lists the spectrometers used in this study, as well as their geographic locations, frequency range of observation and number of radio bursts taken at each station. All spectrometers are observing in a narrow frequency range of few tens of megahertz.

Using the radio parameters such as bandwidth, drift rate and starting frequency, the shock characteristics from each radio event have been estimated. Table  illustrates each type II radio burst selected and the associated CME, GOES soft X-ray flares and shock characteristics. The first column of this table is the numbering index of the events, the next four columns are the date of the radio events in the format dd/mm/yyyy and hh:mm, their starting frequencies, f (MHz), their drift rates (MHz s-1) and their shock formation heights (R⊙) estimated using the relation f(r)=307.87r-3.78-0.14 . Columns 6 to 9 are the GOES soft X-ray flare parameters (start, class, NOAA region and location), followed by two columns that present the CME onset and speed, respectively. Columns 12 to 15 present the shock characteristics (density jumps, Mach numbers, shock and Alfvén velocities, respectively), while the last column of this table presents the estimated ambient magnetic field strength B (Gauss).

There is a strong correlation (CC =0.98) between the drift rates and starting frequencies of the type II radio bursts (Fig. ), which are the key parameters to estimate the shock speeds from the dynamic spectra. Higher starting frequency have higher drift rates . Such a correlation agrees well with the previous studies, giving slopes of ϵ=1.89 and ϵ=1.33, respectively e.g..

From Table , it is clearly observed that 4 out of 32 radio events are not associated with any solar flare because they originate from the far side on the solar surface, but the shocks generating these bursts were excited by associated CMEs. It is also noticed that 19 out of 28 are connected with intense GOES X-ray flares (M and X classes), which is compatible with their speeds, as well as estimated shock speeds. We derived the shock and Alfvén speeds of these type II radio bursts of the order of 504–1282 and 368–826 km-1, respectively, at heliocentric distance ∼1–2R⊙. Comparatively, values are consistent with the measurements reported by about 590–810 and 250–550 km-1, respectively, at ∼1.2–1.8R⊙. The Alfvén speeds from the current work are also in agreement with the range of the Alfvén speeds of 140–460 km-1 over 1.2–1.5 R⊙ and 259–982 km-1 over 3–15 R⊙ given in and in , respectively. Figure presents the correlation between the speeds from the LASCO field of view (FOV) and the speeds derived from the dynamic spectra.

The Table  observations and Fig.  show that there are estimated shock speeds that are faster than CME speeds from LASCO FOV, and vice versa. The difference in CME speed between dynamic spectra and LASCO is attributed to the CME's central position angle as observed by LASCO, implying that the shock may be weakened and dissipated before entering LASCO FOV . On the other hand, the shock decelerates in the case of a decline in its intensity or when it breaks. The type II burst only serves as a time marker for when the shock occurs. It should be noted that type II radio emission can come from anywhere on the shock front, namely the nose or the flanks, depending on which location is best for electron acceleration . Solar radio type II bursts associated with slow CMEs are thought to be generated from non-thermal electrons accelerated by a moving magnetic reconnection when slow CMEs interact with the background magnetized coronal plasma . Furthermore, a recent study confirmed that observing a type II radio burst is evidence of shock acceleration in the solar corona . The Alfvén Mach numbers in the range ∼1.2–1.8 at ∼1–2R⊙ are consistent with the measurements of about 1.1–1.9 at ∼1.3–2.5R⊙ reported by and that of of the order of 1.4 to 1.7 at ∼1.2–1.8R⊙. The magnetic field strength is an important parameter that influences the dynamical eruption of CMEs in the solar atmosphere . High-starting type II radio bursts are associated with coronal shocks that are closer to the solar surface. As a result, high magnetic field values are expected. Figure  demonstrates the variation in the magnetic field strength estimated in this study (Eq. ) relative to the quiet Sun magnetic field model B(r)=ar2 with a=2.2 and empirical model for the magnetic field above active region B(r)=0.5r-1-1.5. The magnetic field has been calculated in the range 0.5<B<8G at ∼1–2R⊙, which shows excellent consistency with earlier research and is fitted with a single power law distribution of the type B(r)=6.07r-3.96G, as represented by the dotted black curve in Fig. .

However, Rankine–Hugoniot jump relation has been used by a number of researchers to derive shock parameters. For example, with this technique, found 1.2≤MA≤1.7 and 0.3≤B≤4G. The same technique was applied by , who reported a magnetic field strength in the range 1–8 G at heliocentric distance of ∼1.6R⊙. A field strength of 6–5 G at ∼1.5–1.77R⊙ is reported by . and have given a detailed review on solar coronal magnetic fields measured using different techniques and at different wavelengths of the electromagnetic spectrum. A recent work has reported that two necessary conditions for type II radio emissions, (i) relatively intense shock waves (the Mach number should exceed a certain value Mcr) and (ii) perpendicular shock waves, are required . Our values of Mach numbers 1.2≤MA≤1.8 agree well with these conditions. In Table , the statistical findings from this study and earlier research that examined more than two radio events are summarized and compared.

The ascending phase of solar cycle 25 is characterized by more intense solar activity than expected e.g.. and show that solar cycle 25 is more active than the previous cycle and is more consistent with actual observations as predicted. Furthermore, estimated that the maximum peak of cycle 25 would be 30 % stronger than that of cycle 24. These indicate that the activity would be high, and we use this advantage to track the intensity of early space weather events in the current cycle. To account for ionospheric irregularities caused by concurrent GOES X-ray flares, type II solar radio bursts were utilized as selection criteria for disturbed days due to their association with solar phenomena such as radio blackouts. The ROTI were examined on 25 type II radio bursts, which are linked to both solar flares and CMEs, by selecting GNSS stations in either equatorial, mid-latitude or high-latitude regions. Furthermore, the ROTI classifies the irregularities of the ionospheric TEC as no TEC irregularity (ROTI <0.25 TECU min-1), weak (0.25≤ ROTI <0.5 TECU min-1), moderate (0.5≤ ROTI <1.0 TECU min-1) and strong (ROTI ≥1.0 TECU min-1) . It is worth noting that four major solar energetic particles (>10 MeV; SEPs) occurred on days when type II radio bursts are observed, and these dates are used as illustrative examples in this study.