Francis G. Eparvier

Flare Irradiance Spectral Model (FISM): Flare component algorithms and results

[1] The Flare Irradiance Spectral Model (FISM) is an empirical model developed for space weather applications that estimates the solar irradiance at wavelengths from 0.1 to 190 nm at 1 nm resolution with a time cadence of 60 s. This is a high enough temporal resolution to model variations due to solar flares, where few accurate measurements at these wavelengths exist, as well as the solar cycle and solar rotation variations. The FISM modeling of the daily component variations, including variations from the solar cycle and solar rotation, was the topic of the first FISM paper (Chamberlin et al., 2007). The modeling of the FISM flare component that includes the solar irradiance variations from both the impulsive and gradual phases of solar flares is the topic of this paper. The flare component algorithms and results are discussed, and comparisons show that FISM estimates agree within the stated uncertainties with measurements of the solar vacuum ultraviolet (VUV; 0.1--200 nm) irradiance. Results from FISM show that the relative change of the solar irradiance during flares for some wavelengths can exceed those of the solar cycle relative changes, ranging from factors of 60 times the quiet Sun irradiance during the gradual phase for emissions originating in the solar corona to factors of 10 in the transition region emissions during the flare’s impulsive phase. FISM fully quantifies, on all timescales, the changes in the solar VUV irradiance directly affecting satellite drag, radio communications, as well as the accuracy in the Global Positioning System (GPS).

TIMED Solar EUV experiment

Abstract The Solar EUV Experiment (SEE) on the NASA Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) mission will measure the solar vacuum ultraviolet (VUV) spectral irradiance from 1 to 195 nm. To cover this wide spectral range two different types of instruments are used: a grating spectrograph for spectra above 25 nm and a set of silicon photodiodes with thin film filters for the soft x-ray (XUV) below 35 nm. Redundant channels of the spectrograph and XUV photodiodes provide in-flight calibration checks on the time scale of a week, and annual rocket underflight measurements provide absolute calibration checks traceable to radiometric standards. Both types of instruments have been developed and flight proven as part of a NASA solar EUV irradiance rocket experiment. The SEE instrument is currently on the NASA TIMED spacecraft at the Johns Hopkins University (JHU) Applied Physics Laboratory (APL). The TIMED launch is planned for October 2000 as a two year mission.

TIMED solar EUV experiment

The solar EUV experiment (SEE) selected for the NASA Thermosphere, Ionosphere, and Mesosphere Energetics and Dynamics mission will measure the solar vacuum UV (VUV) spectral irradiance from 0.1 to 200 nm. To cover this wide spectral range two different types of instruments are used: grating spectrograph for spectra above 25 nm and a set of silicon soft x-ray (XUV) photodiodes with thin film filters for below 30 nm. Redundant channels of the spectrograph and XUV photodiodes provide in-flight calibration checks on the time scale of a week, and annual rocket underflight measurements provide absolute calibration checks traceable to radiometric standards. Both types of instrument have been developed and flight proven as part of a NASA solar EUV irradiance rocket experiment.

Next generation x-ray sensor (XRS) for the NOAA GOES-R satellite series

The X-Ray Sensor (XRS) has been making observations of the solar soft X-ray irradiance for over thirty years onboard National Oceanic and Atmospheric Administrations (NOAA) Geostationary Operational Environmental Satellites (GOES). The XRS provides critical information about the solar activity for space weather operations, and the standard X-ray classification of the solar flares is based on its measurements. The GOES-R series of XRSs, with the first in the series to launch in 2014, has a completely new instrument design. The XRS spectral bands remain the same as before by providing the solar X-ray irradiance in the 0.05-0.4 nm and 0.1-0.8 nm bands. The changes include using Si photodiodes instead of ionization cells to improve performance, using multiple channels to allow wider dynamic range, providing quadrant photodiodes for real-time flare location measurements, and providing accurate radiometric calibrations using the National Institute of Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF) in Gaithersburg, Maryland.

Model insights into energetic photoelectrons measured at Mars by MAVEN

Photoelectrons are important for heating, ionization, and airglow production in planetary atmospheres. Measured electron fluxes provide insight into the sources and sinks of energy in the Martian upper atmosphere. The Solar Wind Electron Analyzer instrument on board the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft measured photoelectrons including Auger electrons with 500 eV energies. A two-stream electron transport code was used to interpret the observations, including Auger electrons associated with K shell ionization of carbon, oxygen, and nitrogen. It explains the processes that control the photoelectron spectrum, such as the solar irradiance at different wavelengths, external electron fluxes from the Martian magnetosheath or tail, and the structure of the upper atmosphere (e.g., the thermal electron density). Our understanding of the complex processes related to the conversion of solar irradiances to thermal energy in the Martian ionosphere will be advanced by model comparisons with measurements of suprathermal electrons by MAVEN.

The Extreme Ultraviolet Sensor (EUVS) for GOES-R

Recognizing that the solar extreme ultraviolet (EUV) irradiance is an important driver of space weather, the National Oceanic and Atmospheric Administration (NOAA) has added an Extreme Ultraviolet Sensor (EUVS) to its Geostationary Operational Environmental Satellite (GOES) program, starting with the recently launched GOES-N, now designated GOES-13. For the GOES-R series (slated for launch starting in 2015) , the EUVS measurement concept has been redesigned. Instead of measuring broad bands spanning the EUV, the GOES-R EUVS will measure specific solar emissions representative of coronal, transition region, and chromospheric variability. From these measurements, the geo-effective EUV wavelength range from 5 to 127 nm can be reconstructed using models based on spectrally resolved measurements gathered over the full range of solar variability. An overview of the GOES-R EUVS design is presented. A description of the in-flight degradation tracking utilizing similar measurement and modeling techniques used to generate the EUV irradiance is also provided.

Proton cyclotron waves occurrence rate upstream from Mars observed by MAVEN: associated variability of the Martian upper atmosphere

Measurements provided by the Magnetometer and the Extreme Ultraviolet Monitor (EUVM) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft together with atomic H exospheric densities derived from numerical simulations are studied for the time interval from October 2014 up to March 2016. We determine the proton cyclotron waves (PCWs) occurrence rate observed upstream from Mars at different times. We also study the relationship with temporal variabilities of the high altitude Martian hydrogen exosphere and the solar EUV flux reaching the Martian environment. We find that the abundance of PCWs is higher when Mars is close to perihelion, and decreases to lower and approximately constant values after the Martian Northern Spring Equinox. We also conclude that these variabilities cannot be associated with biases in MAVENs spatial coverage or changes in the background magnetic field orientation. Higher H exospheric densities on the Martian day side are also found when Mars is closer to perihelion, as a result of changes in the thermospheric response to variability in the ultraviolet flux reaching Mars at different orbital distances. A consistent behavior is also observed in the analyzed daily irradiances measured by the MAVEN EUVM. The latter trends point towards an increase in the planetary proton densities upstream from the Martian bow shock near perihelion. These results then suggest a method to indirectly monitor the variability of the H exosphere up to very high altitudes during large time intervals (compared to direct measurements of neutral particles), based on the observed abundance of PCWs.

Photoelectrons and solar ionizing radiation at Mars: Predictions versus MAVEN observations

Understanding the evolution of the Martian atmosphere requires knowledge of processes transforming solar irradiance into thermal energy well enough to model them accurately. Here we compare Martian photoelectron energy spectra measured at periapsis by Mars Atmosphere and Volatile Evolution MissioN (MAVEN) with calculations made using three photoelectron production codes and three solar irradiance models as well as modeled and measured CO2 densities. We restricted our comparisons to regions where the contribution from solar wind electrons and ions were negligible. The two intervals examined on 19 October 2014 have different observed incident solar irradiance spectra. In spite of the differences in photoionization cross sections and irradiance spectra used, we find the agreement between models to be within the combined uncertainties associated with the observations from the MAVEN neutral density, electron flux, and solar irradiance instruments.

Electron energetics in the Martian dayside ionosphere: Model comparisons with MAVEN data

This paper presents a study of the energetics of the dayside ionosphere of Mars using models and data from several instruments onboard the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft. In particular, calculated photoelectron fluxes are compared with suprathermal electron fluxes measured by the Solar Wind Electron Analyzer (SWEA), and calculated electron temperatures are compared with temperatures measured by the Langmuir Probe and Waves (LPW) experiment. The major heat source for the thermal electrons is Coulomb heating from the suprathermal electron population, and cooling due to collisional rotational and vibrational CO2 dominates the energy loss. The models used in this study were largely able to reproduce the observed high topside ionosphere electron temperatures (e.g., 3000 K at 300 km altitude) without using a topside heat flux when magnetic field topologies consistent with the measured magnetic field were adopted. Magnetic topology affects both suprathermal electron transport and thermal electron heat conduction. The effects of using two different solar irradiance models were also investigated. In particular, photoelectron fluxes and electron temperatures found using the HESSR (Heliospheric Environment Solar Spectrum Radiation) irradiance were higher than those with the FISM-M (Flare Irradiance Spectrum Model - Mars). The electron temperature is shown to affect the O2+ dissociative recombination rate coefficient, which in turn affects photochemical escape of oxygen from Mars.

SDO-EVE EUV spectrograph optical design and performance

The NASA Solar Dynamics Observatory (SDO), scheduled for launch in 2009, incorporates a suite of instruments including the EUV Variability Experiment (EVE). The EVE instrument package contains grating spectrographs that will measure the solar extreme ultraviolet (EUV) irradiance from 0.1 to 105 nm. The Multiple EUV Grating Spectrograph (MEGS) channels use concave reflection gratings to image solar spectra onto CCDs. MEGS will provide 0.1nm spectral resolution between 5-105nm every 10 seconds with an absolute accuracy of better than 25% over the SDO 5- year mission. MEGS-A utilizes a unique grazing-incidence, off-Rowland circle (RC) design to minimize angle of incidence at the detector while providing ≥ 0.1nm resolution between 5-37 nm. MEGS-B utilizes a double-pass, cross-dispersed double-Rowland circle design while providing ≥ 0.1nm resolution between 35-105 nm. We present the as-built performance of the MEGS optical design, including spectral resolution, wavelength shift, focus and alignment.