<p>Solar ultraviolet (UV) irradiance exhibits significant variability driven by magnetic activity in the chromosphere and transition region, particularly near the maximum phase of a solar cycle. A three-channel Silicon Carbide (SiC) UV dosimeter, developed at the Laboratory for Electro-Optics Systems (LEOS), Indian Space Research Organisation, monitored solar UV radiation during the peak of Solar Cycle 25 onboard EOS-08. The instrument employed radiation-hardened 4H-SiC photodiodes and continuously measured solar irradiance in the UV-A (325 – 375 nm), UV-B (275 – 325 nm), and UV-C (250 – 260 nm) bands from August 2024 to September 2025. After correction for Sun–Earth distance, the normalized irradiance data reveal enhanced variability in the shorter wavelength channels, consistent with increased solar activity during this period.</p><p>Correlation analysis with established activity proxies shows a moderate but statistically significant relationship between UV-C irradiance and daily sunspot number (<InlineEquation ID="IEq1"> <EquationSource Format="MATHML"><math> <mi>r</mi> <mo>=</mo> <mn>0.47</mn> </math></EquationSource> <EquationSource Format="TEX">$r = 0.47$</EquationSource> </InlineEquation>, <InlineEquation ID="IEq2"> <EquationSource Format="MATHML"><math> <mi>p</mi> <mo>=</mo> <mn>0.025</mn> </math></EquationSource> <EquationSource Format="TEX">$p = 0.025$</EquationSource> </InlineEquation>). Stronger correlations are observed with the Mg II index, yielding Pearson coefficients of <InlineEquation ID="IEq3"> <EquationSource Format="MATHML"><math> <mi>r</mi> <mo>=</mo> <mn>0.45</mn> </math></EquationSource> <EquationSource Format="TEX">$r = 0.45$</EquationSource> </InlineEquation> (<InlineEquation ID="IEq4"> <EquationSource Format="MATHML"><math> <mi>p</mi> <mo>=</mo> <mn>0.0039</mn> </math></EquationSource> <EquationSource Format="TEX">$p = 0.0039$</EquationSource> </InlineEquation>) for UV-B and <InlineEquation ID="IEq5"> <EquationSource Format="MATHML"><math> <mi>r</mi> <mo>=</mo> <mn>0.75</mn> </math></EquationSource> <EquationSource Format="TEX">$r = 0.75$</EquationSource> </InlineEquation> (<InlineEquation ID="IEq6"> <EquationSource Format="MATHML"><math> <mi>p</mi> <mo>=</mo> <mn>0.0002</mn> </math></EquationSource> <EquationSource Format="TEX">$p = 0.0002$</EquationSource> </InlineEquation>) for UV-C irradiance. During the observation period, distinct UV-C enhancements were detected during three major solar flare events. In the X2.2-class flare on 8 December 2024, the UV-C irradiance peak preceded the GOES soft X-ray maximum by approximately 90 s, consistent with flare models in which impulsive chromospheric heating produces ultraviolet emission prior to the development of thermal soft X-ray emission from hot coronal plasma. These results demonstrate the capability of SiC-based UV detectors for long-term monitoring of solar ultraviolet irradiance and for investigating solar magnetic activity and flare energetics in the ultraviolet domain.</p>

Solar Activity During the Peak of Solar Cycle 25 Captured by SiC UV Dosimeter Onboard EOS-08

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发表日期：

2026年4月13日

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Ishan Gunjal,
Sumesh M. A.,
Vishwasa A.,
Keerthi V.,
Radhika T.,
S. P. Karanth

摘要

Solar ultraviolet (UV) irradiance exhibits significant variability driven by magnetic activity in the chromosphere and transition region, particularly near the maximum phase of a solar cycle. A three-channel Silicon Carbide (SiC) UV dosimeter, developed at the Laboratory for Electro-Optics Systems (LEOS), Indian Space Research Organisation, monitored solar UV radiation during the peak of Solar Cycle 25 onboard EOS-08. The instrument employed radiation-hardened 4H-SiC photodiodes and continuously measured solar irradiance in the UV-A (325 – 375 nm), UV-B (275 – 325 nm), and UV-C (250 – 260 nm) bands from August 2024 to September 2025. After correction for Sun–Earth distance, the normalized irradiance data reveal enhanced variability in the shorter wavelength channels, consistent with increased solar activity during this period.

Correlation analysis with established activity proxies shows a moderate but statistically significant relationship between UV-C irradiance and daily sunspot number ( $r = 0.47$ $r = 0.47$ , $p = 0.025$ $p = 0.025$ ). Stronger correlations are observed with the Mg II index, yielding Pearson coefficients of $r = 0.45$ $r = 0.45$ ( $p = 0.0039$ $p = 0.0039$ ) for UV-B and $r = 0.75$ $r = 0.75$ ( $p = 0.0002$ $p = 0.0002$ ) for UV-C irradiance. During the observation period, distinct UV-C enhancements were detected during three major solar flare events. In the X2.2-class flare on 8 December 2024, the UV-C irradiance peak preceded the GOES soft X-ray maximum by approximately 90 s, consistent with flare models in which impulsive chromospheric heating produces ultraviolet emission prior to the development of thermal soft X-ray emission from hot coronal plasma. These results demonstrate the capability of SiC-based UV detectors for long-term monitoring of solar ultraviolet irradiance and for investigating solar magnetic activity and flare energetics in the ultraviolet domain.