The Large Yield Radiometer (LYRA) instrument onboard the PRoject for OnBoard Autonomy (PROBA2) observes the solar irradiance in four channels in the UV–EUV. One of these channels is centered around the hydrogen line at 121.6 nm. The solar Lyman- $\alpha $ emission line is an optically thick line mostly formed in the chromosphere. Although it is one of the strongest lines of the solar spectrum, only a limited number of instruments provided observations of solar flares in Lyman- $\alpha $ , and those observations differ significantly in shape, durations, and amplitude. We focus on an event that happened on 8 September 2011 (SOL2011-09-08T15:46). This event, an M6.7 flare, was associated with a filament eruption that happened during the decaying phase of the flare. Most of the irradiance fluctuations observed in the Lyman- $\alpha $ time series are synchronized with nonthermal emission fluctuations, as is predicted by flare models. However, there is a late-phase peak in Lyman- $\alpha $ observations that rather correlates with the timing of the filament eruption. We demonstrate that the eruption of the filament is at the origin of this peak.

The space environment is considered hazardous to spacecraft, resulting in materials degradation. Understanding the degradation of space-based instruments is crucial in order to achieve the scientific objectives, which are derived from these instruments. This paper discusses the on-orbit performance degradation of recent spacebased solar instruments. We will focus on the instruments of three space-based missions such as the Project for On-Board Autonomy 2 (PROBA2) spacecraft, the Solar Monitoring Observatory (SOLAR) payload onboard the Columbus science Laboratory of the International Space Station (ISS) and the PICARD spacecraft. Finally, this paper intends to understand the degradation processes of these space-based solar instruments.

The spectra of the PROBA2/LYRA data, similarly to every other solar time series, show predominant periodicities that can be of solar or instrumental origin. In this article, we compare the main periodicities characterizing the LYRA spectrum to those found in the sunspot number, in the 10.7 cm flux, in an X-ray flare index, and in the sunspot area evolution. We focused on the 2010 to 2014 time range, for which the LYRA data are available, although we also briefly address the evolution of the main periodicities in the longer range. The mid-term periodicities at \({\sim}\,28\), \({\sim}\,44\), \({\sim}\,54\), \({\sim}\,59\), \({\sim}\,100\), \({\sim}\,110\), and \({\sim}\,150\) days appear as highly significant in several analyzed datasets. The consistency of distinct periodicities between datasets provides characteristics for the global Sun. This consistency also strengthens the reliability of LYRA data.

We present a long-term assessment of the radiometric calibration and degradation of the Large Yield Radiometer (LYRA), which has been on orbit since 2009. LYRA is an ultraviolet (UV) solar radiometer and is the first space experiment using aboard a pioneering diamond detector technology. We show that LYRA has degraded after the commissioning phase but is still exploitable scientifically after almost 5 years on orbit thanks to its redundancy design and calibration strategy correcting for instrument degradation. We focus on the inflight detector’s calibration and show that diamond photodetectors have not degraded while silicon reference photodiodes that are even less exposed to the Sun show an increase of their dark current and a decrease of their photoresponse.

There are very few reports of flare signatures in the solar irradiance at H i Lyman α at 121.5 nm, i.e. the strongest line of the solar spectrum. The LYRA radiometer onboard PROBA2 has observed several flares for which unambiguous signatures have been found in its Lyman-α channel. Here we present a brief overview of these observations followed by a detailed study of one of them: the M2 flare that occurred on 8 February 2010. For this flare, the flux in the LYRA Lyman-α channel increased by 0.6 %, which represents about twice the energy radiated in the GOES soft X-ray channel and is comparable with the energy radiated in the He ii line at 30.4 nm. The Lyman-α emission represents only a minor part of the total radiated energy of this flare, for which a white-light continuum was detected. Additionally, we found that the Lyman-α flare profile follows the gradual phase but peaks before other wavelengths. This M2 flare was very localized and had a very brief impulsive phase, but more statistics are needed to determine if these factors influence the presence of a Lyman-α flare signal strong enough to appear in the solar irradiance.

We analyze the variability of the spectral solar irradiance during the period from 7 January, 2010 until 20 January, 2010 as measured by the Herzberg channel (190-222 nm) of the Large Yield RAdiometer (LYRA) onboard PROBA2. In this period of time observations by the LYRA nominal unit experienced degradation and the signal produced by the Herzberg channel frequently jumped from one level to another. Both these factors significantly complicates the analysis. We present the algorithm which allowed us to extract the solar variability from the LYRA data and compare the results with SORCE/SOLSTICE measurements and with modeling based on the Code for the Solar Irradiance (COSI).