Fig 1. The CBS dynamic spectra (a) and the CBS and NoRP flux densities (b-c) obtained from 03:52 UT to 03:58 UT. Taken from Yan et al. 2023.
Insightful Information in Microwave Spectra of Solar Flares
Microwave emission of solar flares can be excited by energetic electrons through the gyro synchrotron (GS) radiation. Thus, the microwave spectra contain valuable/unique information not only about energetic electrons accelerated during solar flares, but also about the underlying magnetic field and energy release process.
he microwave spectra of solar flares reach their maximum intensity (peak) at frequencies below or around 10 GHz. At this frequency range, there is a transition in the spectral slope from positive to negative.
The “optically thick regime” refers to a condition where the plasma in the flaring region is dense enough to cause significant absorption of the microwave radiation. In this regime, the spectral slope of the microwave emission is positive, meaning that the intensity increases with increasing frequency.
On the other hand, the “optically thin regime” refers to a condition where the plasma density is lower, resulting in less absorption of the microwave radiation. In this regime, the spectral slope becomes negative, indicating that the intensity decreases with increasing frequency.
Overall, this line highlights that the microwave spectra of solar flares provide valuable information about the plasma density, the underlying magnetic field, and the energy release processes occurring during the flares.
Limitations in Previous Flare Studies and Data Gap in Millimeter Wavelength Observations
Earlier studies were limited to flares with a turnover frequency being less than 9.4 GHz or 17 GHz. This is to ensure two data points are available in the optically thin regime, mainly with the measurements of the Nobeyama Radiopolarimeter (NoRP). Yet, the turnover frequency can go beyond 20 GHz around the peak time of large flares, and microwave spectra with unusual shapes, such as flat or rising continuously above tens of GHz, have been reported, while the present solar microwave spectrometers mainly provide dynamic spectrum below ~ 20 GHz. Above that, data exist only at few discrete frequencies. This means there exists significant data gap of flare observations in the millimeter wavelength.
First Flare Observation by Chashan Broadband Solar Millimeter Spectrometer (CBSmm) at High Frequencies
The newly-built Chashan Broadband Solar millimeter spectrometer (CBSmm, CBS for short) started its routine observation since 2020, working from 35 to 40 GHz (Shang et al. 2022). It is operated by the Institute of Space Sciences of Shandong University. The X2.2 flare on 2022 April 20 was observed by both NoRP and CBS (see Fig. 1). This provides the first flare observation of CBS since its routine operation. The CBS data are first calibrated with the new moon observations, and then cross-calibrated with the simultaneous NoRP data at 35 GHz. The flare is of special interest due to its strong millimeter emission and the high turnover frequency (>20 GHz) of the spectra during the impulsive stage. Such kind of events has not been well studied due to the large data gap beyond ~ 20 GHz.
Three distinct local intensity peaks exist during the impulsive stage of this flare (see Fig. 1 and Fig. 2). The middle peak is the strongest one, with the largest flux density reaching ~9300 SFU at 35-40 GHz. The gyrosynchrotron turnover frequency (nt) is above 35-40 GHz for this major peak, according to the positive spectral indices of the CBS data there. The turnover frequency (nt) is larger than 20 GHz for most of the other two peaks. We found the turnover intensity (It) correlates well with the turnover frequency (nt) according to the power-law relation (with an index ~ 4.8) during the impulsive stage with nt < 35 GHz. During the decay stage, both the CBS spectral index and the fitted optically-thin spectral index present a gradual hardening trend.
Fig 2. Spectra obtained by fitting the combined NoRP and CBS data at selected moments of T1 – T13 (see Fig 1.). The asterisks and squares represent the NoRP and CBS data, respectively. The spectral parameters, including the optically-thick and -thin indices atk and atn, and the turnover frequency nt are given in each panel. The filled circles denote the location of vt. Taken from Yan et al. 2023.
This study demonstrates the spectral turnover frequency and other fitting parameters can be better constrained with the CBS data covering the range of 35 ~ 40 GHz. This is true for the spectra with the turnover frequency < 35 GHz. During the second peak of the impulsive stage with a higher turnover frequency, valuable information can still be inferred according to the unique CBS data though the exact turnover frequency cannot be determined. Such data (covering 35-40 GHz) are available for the first time in the millimeter observations of solar radio bursts. Data with a broader spectral coverage are still demanded to better understand flares with a high turnover frequency. In the near future, we plan to upgrade the CBS to cover 26 – 40 GHz, so to better constrain the nonthermal microwave spectra of solar flares.
Based on a recently published article: Fabao Yan, Zhao Wu, Ziqian Shang, Bing Wang, Lei Zhang, and Yao Chen, The First Flare Observation with a New Solar Microwave Spectrometer Working in 35-40 GHz, The Astrophysical Journal Letters, 942:L11 (2023), DOI: https://doi.org/10.3847/2041-8213/acad02
References:
Shang, Z., Xu, K., Liu, Y., et al. 2022, ApJS, 258, 25
*Full list of authors: Fabao Yan, Zhao Wu, Ziqian Shang, Bing Wang, Lei Zhang, and Yao Chen
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