Solar Type J Radio Bursts and the Associated Coronal Loop

The solar type J radio burst is a variant of type III bursts, which are a probe for understanding solar energetic electrons and local electron density. This study investigates a type J burst event on 2017 September 9. We have combined the data from the extreme-ultraviolet (EUV) imaging and the EUV Imaging Spectrometer (EIS) to analyze the event. Within 4 minutes several type J bursts with similar morphology occur. Two of them, with clear fundamental and second harmonic bands, are studied in detail. We find a delay of 2 ± 0.5 s between their different harmonic bands. During type J bursts, only one coronal loop brightens significantly at its northern footpoint, in correlation with the continuous injection of erupting jets into the loop. The EUV intensity of the brightening footpoint is correlated with the radio flux at 245 and 410 MHz, with correlation coefficients of 0.2 and 0.4, respectively. These observations suggest that the type J bursts should originate from this coronal loop. By analyzing the electron number density distribution along the coronal loop diagnosed from the EIS data and the time evolution of the plasma frequency calculated from the type J burst, we determine that the velocities of the energetic electrons exciting the two type Js are 0.10 ± 0.02c and 0.12 ± 0.02c. Our results confirm previous studies on type J bursts.


Introduction
The Sun occasionally produces eruptions such as coronal jets, solar flares, and coronal mass ejections (CMEs).The release of energy associated with these eruptions accelerates and produces energetic electrons that emit radio emissions (Ginzburg & Zhelezniakov 1958;Dulk 1985;Pick & Vilmer 2008).When the accelerated electrons propagate along open or quasi-open magnetic field lines, they produce type III radio bursts (Payne-Scott 1949;Wild 1950;Chen et al. 2013Chen et al. , 2019;;Reid 2020), which drift rapidly from higher to lower frequencies within a few seconds.Since their emission frequency and frequency drift rate correspond to the local electron density and the energetic electron velocity, type IIIs can be used to diagnose them.
Solar type III bursts can have different morphologies.In the solar radio dynamic spectrum, they show the J-, U-, or N-like shapes, which are thought to be radio emissions produced by energetic electrons traveling in closed coronal loops (Maxwell & Swarup 1958;Klein & Aurass 1993;Karlicky et al. 1996;Kong et al. 2016;Mancuso et al. 2023).The emissions form a U-shaped structure as energetic electrons propagate in a corona loop from one footpoint to another.However, in a type J burst, the right-hand part of the "U" disappears at the reverse frequency as it drifts from lower to higher frequencies.On the other hand, in a type N burst, a third part of the U-shaped structure appears due to further reflection of energetic electrons above another footpoint (Caroubalos et al. 1987).
Observational studies have shown that some type U bursts can be repeated within a few minutes (Aschwanden et al. 1992;Aurass & Klein 1997).Recently, Morosan et al. (2017) used the radio and EUV imaging data to analyze the relationship between EUV jets and type J bursts and found that the energetic electrons move along the closed magnetic field lines from the site of the EUV jets and reach the top of a coronal loop with a height of 360 Mm.The reconnection between the emerging magnetic flux and the existing magnetic field lines causes the EUV jets.The energetic electrons that excite the type J burst should also originate from the magnetic field reconnection process.These observations suggest that the energetic electrons can be continuously injected into coronal loops.
Type J and U bursts are commonly used to diagnose the velocities of the energetic electrons.Some researchers took advantage of positional information provided by radioheliograph observations, while others had no relevant radioheliograph data at all.Using radioheliograph data, Aschwanden et al. (1992) measured the velocity of electrons to be 0.14-0.21cat 1.1-1.7 GHz, and Reid & Kontar (2017) found the velocity to be 0.13-0.24cat 30-80 MHz.In the absence of radio heliograph observations, the electron number density models have been used to make the diagnoses (Fokker 1970;Yao et al. 1997;Dorovskyy et al. 2010;Fernandes et al. 2012).For example, Yao et al. (1997) gave the velocity (0.26-0.38 c) of energetic electrons exciting a U-shaped burst at 1.0-2.8GHz, and Fernandes et al. (2012) found the velocity to be 0.16-0.53cfor a U-shaped burst in the frequency range 950-2500 MHz.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Some type J, U, and N bursts have harmonic bands (Labrum & Stewart 1970;Aurass & Klein 1997;Dorovskyy et al. 2015;Morosan et al. 2017).Stewart (1974) pointed out that the frequency ratio of the second harmonic band to the fundamental is less than 2.More recently, Dorovskyy et al. (2015) used observational data from the Giant Ukrainian Radio Telescope to study type U bursts with harmonic structures in the frequency range 10-70 MHz and found that the time delay of the fundamental relative to the second harmonic is 7 s.Kong et al. (2016) also found a rather large time delay (∼3-5 s) between the fundamental and harmonic components for a type N burst.The differences in group velocity of different harmonics well explain the delay.
Recently, radio imaging observations have been used by several researchers to study type J bursts and their associated coronal loops (e.g., Mancuso et al. 2023;Zhang et al. 2023).However, none of these works have identified visible coronal loops in the EUV bands or used EUV data to determine the electron density of the associated coronal loops.The density is a crucial parameter in the study of type J bursts.In this paper, we present our study of type J bursts, where we use a combination of EUV spectroscopy and imaging data to establish the correlation between a visible coronal loop and type J bursts.The velocities of the energetic electrons causing the type J bursts and the electron density of the coronal loop are determined.The following section describes the observational data.In Section 3, the type J bursts with harmonics are presented.Section 4 shows the associated coronal loop and the diagnosis of coronal parameters.Finally, conclusions and discussion are given.

Observational Data
The observational imaging data used in this study are from the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamics Observatory (SDO) and the EUV Imaging Spectrometer (EIS; Culhane et al. 2007) on board Hinode.The AIA observes the Sun in 10 different passbands in the soft X-ray, EUV, and UV regions to study the processes occurring in different atmospheric layers, i.e., the chromosphere, transition region, and corona with a pixel size of 0 6 and a 12 s cadence.
The EIS is a high-spectral-resolution spectrometer observing in the wavelength ranges of 170-210 and 250-290 Å aimed at studying dynamic phenomena in the corona with high spatial resolution and sensitivity.To align the AIA and EIS, solar features taken by the two instruments at similar formation temperatures are compared.
For the solar radio dynamic spectrum, we use the data recorded by the Assembly of Metric-band Aperture TElescope and Real-time Analysis System (AMATERAS; Iwai et al. 2012) with a frequency range of 100-500 MHz, a temporal resolution of 10 ms, and a sensitivity of 0.6 SFU.

Solar Type J Radio Bursts
On 2017 September 9, type J bursts were recorded by several solar radio spectrometers, including AMATERAS, ORFESS,5 Chashan Solar Observatory6 (Du et al. 2017;Feng et al. 2018;Feng 2019), and Learmonth of the Radio Solar Telescope Network (RSTN). 7The AMATERAS data were found to have higher sensitivity and temporal resolution and were therefore used for this paper.
The associated soft X-ray flare class was C1.7 recorded by the GOES15.The flare began at 06:51 UT, peaked at 06:56 UT, and ended at 06:58 UT, as shown in Figure 1(a).In 2017 September, the associated NOAA active region (AR) 12673 was more explosive, with an X8.2 flare and several M-class flares within a few days.
Figures 1(b) and (c) display the normalized intensities in six AIA passbands for AR 12673, in addition to the radio emissions from the Sun at 245, 410, and 610 MHz observed by RSTN/Learmonth.Notably, the intensity curves at 94 and 131 Å are comparable to those observed in the X-ray, which indicates a high level of activity in AR 12673 during this period.Consequently, it led to an increase in both X-ray and EUV intensities.Figure 1(d) shows that the type III bursts were observed from about 06:52 UT to 06:56 UT, which occurred during the peak of the radio emission, indicated by the two vertical dashed lines.Furthermore, few high-intensity radio bursts were detected outside this time frame.These type IIIs are thought to be a result of the AR 12673 eruption, as they occurred just before the maximum X-ray and EUV intensities were reached.
Figure 2 shows these solar radio bursts, consisting of about 20 type III bursts in 4 minutes.The type III bursts can be divided into two distinct groups.The first group, which occurred before 06:54 UT, shows inverted "J" shapes with similar start, reverse, and end frequencies.The second group, which occurred after 06:54 UT, also has a similar morphology but without any discernible J-shaped features.
In Figure 2(a), we have identified two episodes of type J bursts, E1 and E2, marked by white boxes.These two episodes have two distinct features in common: (1) J-shaped features, and (2) fundamental and harmonic bands.We have provided enlarged images in Figures 2(b) and (c) to visualize both episodes better.Solid and dotted contours indicate the fundamental and harmonic bands of the type J bursts.We have also plotted the harmonic bands in Figures 2(d) and (e), divided by 2 in frequencies, together with the fundamental bands.The vertical dashed lines in both figures indicate the times at the reverse frequency of the type J bursts.
For E1, the descending branch begins at a frequency of approximately 320 MHz and ends at 185 MHz.The segment lasts for ∼4 s and experiences a frequency drift rate of −33.8 MHz s −1 .The results of the second segment (E2) show a frequency range of approximately 180-330 MHz.This segment lasts for ∼5 s and experiences a frequency drift rate of −30 MHz s −1 .All parameters, including the ascending frequency branch after the reverse frequency, have been detailed for both the fundamental (F) and harmonic (H) bands in Table 1.
Figures 2(d) and (e) show that the starting frequencies of the fundamental bands are later than those of the harmonics.To determine this delay, reverse frequency times were used.It has been discovered that the time delay for both E1 and E2 is 2 ± 0.5 s.The reasons behind this delay will be discussed in the final section.

Associated Coronal Loop and Diagnosis of Coronal Parameters
The images captured by the AIA during the E1 and E2 are presented in Figures 3(a) and (c).The active region can be found on the western limb of the Sun, highlighted by the white boxes.Figures 3(b) and (d) display an expanded view of the active region, revealing a coronal loop brightening at one footpoint, indicated by the white arrows.This brightening may be linked to reconnections of small-scale magnetic fields (Golub et al. 1975;Zhang et al. 2001).It is worth noting that the two type J bursts potentially correspond with the coronal loop, as only one such loop brightens at its footpoint in the two instances displayed in Figure 3.The footpoint's brightening should be the source of the energetic electrons creating E1 and E2.
An animation was created using different AIA images taken at various passbands and time intervals.The animation shows an increase in brightness at the northern footpoint of the coronal loop, from which EUV jets are continuously injected into the loop.The series of type III bursts, seen in Figure 2(a), may be attributed to this brightening footpoint, which is consistent with previous studies on type III bursts (e.g., Aschwanden et al. 1992;Aurass & Klein 1997;McCauley et al. 2017;Morosan et al. 2017).
To further confirm the link between the coronal loop and type J bursts, a correlation analysis was carried out.Specifically, we analyzed the intensities of two EUV brightening regions-F3 (located at the footpoint of the coronal loop) and F2 (outside of the footpoint)-during the type J bursts.The two chosen regions are located within AR 12673, as presented in Figure 3   and determined correlation coefficients, which are displayed in Figures 4(c) and (d).
According to the analysis, there is a quite strong correlation between F3 and the radio flux at 245 and 410 MHz.The correlation coefficients are 0.2 and 0.4, respectively.This correlation implies that the footpoint of the coronal loop is the origin of the energetic electrons that cause radio bursts.As a result, the type III radio bursts should be located in the coronal loop.On the other hand, the correlation between F2 and the radio flux indicates weakness with correlation coefficients of 0 and 0.2, respectively.Hence, there is a significant correlation between the coronal loop and type III bursts, and it is likely that the footpoint of the coronal loop is the source of the energetic electrons that trigger the type III bursts.Note.Columns (3), (5), and (7) show the frequency range, duration, and frequency drift rate of the fundamental band, while columns (4), (6), and (8) show the same for the second harmonic band.
In Figures 4(b) and (d) the EUV intensity at F3 in the 94 Å passband is most strongly correlated with the 410 MHz radio flux.The time-slice image generated in this passband is shown in Figure 5.In the figure, each peak represents a type III burst.We can see that there is a correlation between the EUV jets and type III bursts, but it is not a close one-to-one relationship.About three-quarters of the type III bursts have corresponding EUV jets, while some jets have no associated type III bursts.It is notable that the radio flux of the type III burst is strongest when the height of the EUV jet is at its maximum, such as during the fourth type III burst.
The coronal loop observed by the AIA and EIS is shown in Figures 6(a)-(c).The height of the loop apex is about 0.14 R e above the photosphere.In the EIS images, the northern and upper parts of the coronal loop show higher emission.This allows a diagnosis of the electron number density.
In Figure 6(d), we can see the electron density in the white squares that are positioned along the coronal loop.Each square is numbered from 1 to 12, starting from the northern footpoint, as shown in Figure 6(b).The density gradually decreases from the higher to the lower values.Figure 6(e) shows the distance of each square from the footpoint marked by a white cross in Figure 6(b).By combining Figures 6(d) and (e), we can obtain the electron density that changes with the distance of the white squares, as illustrated in Figure 6(f).
In Figures 7(a) and (c), we compared the frequency of type J bursts over time with the plasma frequency over distance, which was calculated from the electron density of the coronal loop.The two panels unambiguously demonstrate the similarity between the frequency of type J bursts and the plasma frequency of the coronal loop.The close proximity of the inverse frequency of type J bursts (about 200 MHz) to the plasma frequency at the top of the coronal loop, where the electron density is around 5.0 × 10 8 cm −3 , indicates that type J bursts are located in the coronal loop.
To create time versus frequency graphs for the two type J bursts, we manually selected the central part of the bursts.We marked the selected area with dashed curves in Figures 7(a) and (c).After that, we used the electron density as a function of distance for the coronal loop to calculate the distance-time relationships for the type J burst sources.These relationships show the distance traveled by the energetic electrons that cause the type J bursts along the coronal loop over time.We have displayed the relationships in Figures 7(b) and (d).To determine the initial velocity of the energetic electrons, we fitted a second-order polynomial to both relationships.
The initial velocities of the energetic electrons that emitted the two type Js were calculated to be 0.10 ± 0.02c and 0.12 ± 0.02c.It should be noted that these electrons experience deceleration within the coronal loop.It is important to note that the rapid decrease in the velocity of the energetic electrons shown in Figures 7(b) and (d) can be attributed to the small fluctuations in density at the top of the coronal loop, resulting in a relatively small change in the calculated distance over time.

Conclusions and Discussion
We have conducted a study that combines observations from EUV spectroscopy and imaging to demonstrate that two type J bursts originate from a specific coronal loop on the solar limb.Our findings show that the initial velocities of the energetic electrons resulting in the two type J bursts are 0.10 ± 0.02c and 0.12 ± 0.02c.The results are consistent with previous research, which further validates both our study and previous studies.Despite the lack of solar radio imaging data, our innovative approach has enabled us to obtain satisfactory results.
Type J bursts have both fundamental and harmonic bands.The time delay between the two bands is 2 ± 0.5 s.Several studies, including Dorovskyy et al. (2015), Kong et al. (2016), andFeng et al. (2018), respectively, have investigated the causes of delays between various harmonics of type J bursts, type N bursts, and solar radio spikes, respectively.It is generally agreed that differences in the group velocity of electromagnetic waves are responsible for these delays.
Previous studies have reported the electron density at the top of the coronal loop.According to Gupta et al. (2015), the electron density was measured to be 3.2 × 10 8 cm −3 in two loops, which is slightly lower than the value of about 5.0 × 10 8 cm −3 reported in this paper.However, Xie et al. (2017) reported a density range of 4.0 × 10 8 -2.8 × 10 9 cm −3 , while Huang (2018) found the density to fall between 1 × 10 9 cm −3 and 4 × 10 9 cm −3 .Both studies found higher densities at the top of loops in comparison to the value in this paper.Therefore, the density in this paper is credible but not typical.Not all coronal loops exhibit an electron density at the top that aligns with the type J bursts.It is plausible that the other unexamined coronal loop may not be appropriately correlated.
In addition, we measured the height of the loop apex to be about 0.14 R e above the photosphere.In the study by Zhang et al. (2023), the coronal loop tops were found to be at an average height of 1.37 R e , with an average scale height of 0.36 R e .The average plasma density at the loop apex changed from 0.45 × 10 7 cm −3 to 1.8 × 10 7 cm −3 .The difference in the frequencies of the type J bursts studied in the two papers leads to this marked discrepancy in the density and height of the coronal loop apex.
The relationship between type III bursts and coronal jets is a fascinating topic that has been studied extensively in the past (e.g., Aurass et al. 1994;Innes et al. 2011;Chen et al. 2013;Zhang et al. 2021).A study by Innes et al. (2011) found that the EUV jets lag behind the radio signals and the footpoint brightening by about 30 s because the jets take some time to develop.Our research has also found that type III bursts are associated with EUV brightening/jets and that the radio flux of the type III burst is strongest when the height of the EUV jet is at its maximum.However, there is not a close one-to-one relationship between them.It should be noted that EUV jets and radio bursts are two different phenomena representing energy release.It is acceptable to observe that they are not closely related, since one represents the ejection of material during the solar eruption, whereas the other must go through the process of electron acceleration and radio emission.Alternatively, it is possible that the EUV jets themselves can drive the generation of shock waves, thereby accelerating the energetic electrons, which in turn can excite radio bursts.This is similar to the type II bursts associated with blowout jets reported by Hou et al. (2023).Further research is needed to study the relationship between EUV jets and type III bursts in conjunction with radio imaging observations to better understand them.
Recently, studies have shown that type J bursts can occur in locations other than the active region (Reid & Kontar 2017;Morosan et al. 2022;Dabrowski et al. 2023).Without the solar radio imaging observations presented in this paper, the possible origin of type J bursts from an unseen coronal loop cannot be completely ruled out.
In the future, it will be possible to employ radio imaging observations from instruments such as LOFAR (van Haarlem et al. 2013), DSRT (Yan et al. 2023), and MUSER (Yan et al. 2020) for precise identification of type J burst sources.These observations will help to recognize the associated coronal loops.The conjunction of EUV spectroscopy and imaging observations will allow the diagnosis of the physical parameters of these coronal loops, including the density and temperature.This research will enhance our comprehension of the brightness temperature, size, and polarization of type J bursts.Ultimately, it will contribute to a better understanding of the plasma emission that generates type J bursts and the EUV emission processes occurring in the associated coronal loops. (b).
Figure 4(b) illustrates the normalized intensities.We correlated the intensities with the 245 and 410 MHz flux

Figure 1 .
Figure 1.Light curves at different wavelengths.(a) X-ray flux.(b) Normalized intensity in six AIA passbands around AR 12673, with "Eclipse" denoting the AIA's eclipse.(c) 245, 410, and 610 MHz solar radio flux.(d) Solar radio dynamic spectrum recorded by the AMATERAS.The two vertical dotted lines indicate the duration of the studied type J bursts.

Figure 2 .
Figure 2. (a) Solar radio bursts recorded by the AMATERAS.(b) and (c) Enlargement of type J bursts indicated by squares in panel (a).Solid and dotted contours outline the fundamental and harmonic bands.(d) and (e) Time delay between the fundamental and second harmonic bands.The contours shown in panels (b) and (c) are reproduced as solid and dashed lines, respectively, with the dashed contours halved in frequency.Dashed vertical lines mark the times of reverse frequencies for the fundamental and harmonic bands.

Figure 3 .
Figure 3. AIA observations during the type J bursts.Panels (b) and (d) show enlarged views of the image in the white boxes in panels (a) and (c).White arrows indicate the brightening coronal loop.F1, F2, and F3 mark the areas selected to determine the EUV intensities shown in Figures 1(b) and 4(b).Two black dotted lines indicate the slice selected to generate Figure 5.An animation illustrating the evolution of the coronal jet in six passbands is accessible.It covers 5 minutes between 06:51 and 06:56 UT on 2017 September 9. (An animation of this figure is available.)

Figure 4 .
Figure 4. Correlation between solar radio flux and EUV intensity.(a) Solar radio flux at 245 and 410 MHz.(b) Normalized intensity at the F2 and F3 in the six passbands of the AIA.F2 and F3 are displayed in Figure 3(b).(c) Correlation coefficient between the 245 MHz solar radio flux and the intensity of F2 and F3.(d) Same as (c) except for the 410 MHz solar radio flux.

Figure 5 .
Figure 5. Correlation between EUV jets and type III bursts.The background is the image of the time slice in bandpass 94 Å, and the position of the slice is shown in Figure 3(d).The yellow curve represents the solar radio flux at 410 MHz.The eight type III bursts are labeled 1-8, with correlated EUV jets in yellow and uncorrelated jets in blue.

Figure 6 .
Figure 6.(a)-(c) Coronal loop observed by the AIA and EIS.The white squares labeled 1-12 indicate the regions selected for electron density diagnosis, and the white cross represents the site of the brightening footpoint.(d) Electron density in the 12 white squares.(e) Distance of the 12 white squares from the brightening footpoint.(f) Electron density in the 12 white squares vs. distance.

Figure 7 .
Figure 7.Comparison of type J burst frequency and plasma frequency calculated from electron density in the coronal loop.The contours match those shown in Figures 2(b) and (c).The dashed curves show the selected central part of the type J bursts.The asterisks denote the plasma frequency that was calculated based on the electron density of the coronal loop, as shown in Figure 5(f).(b) and (d) Distance of the type J burst sources along the coronal loop as a function of time.

Table 1
Characteristics of Type J Bursts (E1 and E2) for Both Descending and