Modeling of SEP induced auroral emission at Mars: Contribution of precipitating protons and effects of crustal fields

<p>Solar Energetic Particle (SEP) and the Imaging UltraViolet Spectrograph (IUVS) instruments on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft have discovered diffuse aurora that spans across nightside Mars, which resulted from the interaction of Solar Energetic Particles (SEPs) with Martian atmosphere [Schneider et al., 2015]. Previous models showed that 100 keV monoenergetic electron precipitation should have been at the origin of the low altitude (~60 km) peak of the limb emission, however, no models were able to reproduce the observed emission profiles by using the observed electron energy population [e.g. Haider et al., 2019]. Previous auroral emission models did not take into account the contribution of MeV proton precipitation, although MeV proton can penetrate down to ~60 km altitude as well [e.g., Jolitz et al., 2017]. This study aims to model SEP induced diffuse auroral emission by both electrons and protons.</p><p>We have developed a Monte-Carlo collision and transport model of SEP electrons and protons with magnetic fields on Mars. We calculated limb intensity profile of CO<sub>2</sub><sup>+</sup> ultraviolet doublet (UVD) due to precipitation of electrons and protons with energy ranging 100eV-100keV and 100eV-5MeV, respectively, during December 2014 SEP event and September 2017 SEP event by using electron and ion fluxes observed by MAVEN/SEP, SWEA and SWIA.</p><p>The calculated peak limb intensity of CO<sub>2</sub><sup>+</sup> UVD due to precipitation of protons is 3-5 times larger than that due to precipitation of electrons during both December 2014 and September 2017 SEP events, which suggests that protons can make brighter CO<sub>2</sub><sup>+</sup> UVD emission than electrons. Peak altitude of limb intensity profiles of CO<sub>2</sub><sup>+</sup> UVD due to precipitation of electrons and protons are both 10 - 20 km higher than the observation, a discrepancy could be explained by the uncertainty in the electron and proton fluxes that precipitate into the nightside Mars.</p><p>We have tested an effect of crustal field on the emission of CO<sub>2</sub><sup>+</sup> UVD. CO<sub>2</sub><sup>+</sup> UVD emission due to the precipitating electrons are depleted by a factor of 10 in the region of open crustal field and disappeared in the region of closed and parallel crustal field, whereas emission due to the precipitating protons does not change significantly. Further observations of diffuse aurora in the crustal field region should be needed to constrain the origin of diffuse aurora on Mars.</p>

CO+ first-negative band emission: A tracer for CO in the Martian upper atmosphere

Context. Recently, the Imaging Ultraviolet Spectrograph (IUVS) on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite observed CO+ first-negative band limb emission in the Martian upper atmosphere. Aims. We aim to explore the photochemical processes in the Martian upper atmosphere, which drive this band emission. Methods. A photochemical model was developed to study the excitation processes of CO+ first-negative band emission (B2Σ+ → X2Σ+) in the upper atmosphere of Mars. The number density profiles of CO2 and CO from two different models, namely, Mars Climate Database (MCD) and Mars Global Ionosphere-Thermosphere (MGIT), were used to determine the limb intensity of this band emission. Results. By increasing the CO density by a factor of 4 and 8 in MCD and MGIT models, respectively, the modelled CO+ first-negative band limb intensity profile is found to be consistent with the IUVS/MAVEN observation. In this case, the intensity of this band emission is significantly determined by the ionisation of CO by solar photons and photoelectrons, and the role of dissociative ionisation of CO2 is negligible. Conclusions. Since CO is the major source of the CO+(B2Σ+), we suggest that the observed CO+ first-negative band emission intensity can be used to retrieve the CO density in the Martian upper atmosphere for the altitudes above 150 km.

Intensity of emission lines of the quiescent solar corona: comparison between calculated and observed values

The paper reports the results of calculations of the center-to-limb intensity of optically thin line emission in EUV and FUV wavelength ranges. The calculations employ a multicomponent model for the quiescent solar corona. The model includes a collection of loops of various sizes, spicules, and free (inter-loop) matter. Theoretical intensity values are found from probabilities of encountering parts of loops in the line of sight with respect to the probability of absence of other coronal components. The model uses 12 loops with sizes from 3200 to 210000 km with different values of rarefaction index and pressure at the loop base and apex. The temperature at loop apices is 1 400 000 K. The calculations utilize the CHIANTI database. The comparison between theoretical and observed emission intensity values for coronal and transition region lines obtained by the SUMER, CDS, and EIS telescopes shows quite satisfactory agreement between them, particularly for the solar disk center. For the data acquired above the limb, the enhanced discrepancies after the analysis refer to errors in EIS measurements.

Intensity of emission lines of the quiescent solar corona: comparison between calculated and observed values

The paper reports the results of calculations of the center-to-limb intensity of optically thin line emission in EUV and FUV wavelength ranges. The calculations employ a multicomponent model for the quiescent solar corona. The model includes a collection of loops of various sizes, spicules, and free (inter-loop) matter. Theoretical intensity values are found from probabilities of encountering parts of loops in the line of sight with respect to the probability of absence of other coronal components. The model uses 12 loops with sizes from 3200 to 210000 km with different values of rarefaction index and pressure at the loop base and apex. The temperature at loop apices is 1 400 000 K. The calculations utilize the CHIANTI database. The comparison between theoretical and observed emission intensity values for coronal and transition region lines obtained by the SUMER, CDS, and EIS telescopes shows quite satisfactory agreement between them, particularly for the solar disk center. For the data acquired above the limb, the enhanced discrepancies after the analysis refer to errors in EIS measurements.

Stellar radii

Observing a stellar radius basically means observing a center-to-limb intensity variation. The significance and properties of center-to-limb variations, common approximations, the correlation with optical-depth radii in extended-photophere stars, and direct measurements of angular (interferometry, lunar occultation) and absolute diameters (binary eclipses) are discussed. Spectrophotometric and doppler techniques of diameter determination are also briefly outlined.

Observation of Global 160-min Infrared (Differential) Intensity Variation of the Sun

The method developed and the instrument designed for detecting variations of the solar limb darkening at the atmospheric transparency window of the solar opacity minimum region of λ 1.65 µ are described. This differential technique proved to be successful in rejecting undesirable low frequency noises due to the atmosphere and to the instrument. Analysis of observations made in 1977, 1978, and 1981 indicates the persistance of global fluctuations of the IR differential, center-to-limb intensity at the wellknown 160 min period with an average amplitude of about ± 2 x 10-4 in units of the ‘average Sun’ intensity near 1.65 µm.