arXiv subjects

E. M. B. Thiemann

Publications and source records attributed to E. M. B. Thiemann.

5 recordsLinked to original sources

arXiv2019

The GOES-R EUVS Model for EUV Irradiance Variability

The Geostationary Operational Environmental Satellite R (GOES-R) series of four satellites are the next generation NOAA GOES satellites. Once on orbit and commissioned, they are renamed GOES 16-19, making critical terrestrial and space weather measurements through 2035. GOES 16 and 17 are currently on orbit, having been launched in 2016 and 2018, respectively. The GOES-R satellites include the EUV and X-ray Irradiance Sensors (EXIS) instrument suite, which measures calibrated solar irradiance in 8 lines or bands between 25 and 285 nm with the Extreme Ultraviolet Sensors (EUVS) instrument. EXIS also includes the X-Ray Sensor (XRS) instrument, which measures solar soft X-ray irradiance at the legacy GOES bands. The EUVS measurements are used as inputs to the EUVS Model, a solar spectral irradiance model for space weather operations that predicts irradiance in twenty-two 5 nm wide intervals from 5 nm to 115 nm, and one 10 nm wide interval from 117 to 127 nm at 30 second cadence. Once fully operational, NOAA will distribute the EUVS Model irradiance with 1 minute latency as a primary space weather data product, ushering in a new era of rapid dissemination and measurement continuity of EUV irradiance spectra. This paper describes the EUVS Model algorithms, data sources, calibration methods and associated uncertainties. Typical model (relative) uncertainties are less than $\sim$5\% for variability at time-scales longer than 6 hours, and are $\sim$25\% for solar flare induced variability. The absolute uncertainties, originating from the instruments used to calibrate the EUVS Model, are $\sim$10\%. Examples of model results are presented at both sub-daily and multi-year timescales to demonstrate the model's capabilities and limitations. Example solar flare irradiances are also modeled.

astro-ph.SR↗

arXiv2019

The effects of crustal magnetic fields and solar EUV flux on ionopause formation at Mars

We study the ionopause of Mars using a database of 6,893 ionopause detections obtained over 11 years by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) experiment. The ionopause, in this work, is defined as a steep density gradient that appears in MARSIS remote sounding ionograms as a horizontal line at frequencies below 0.4 MHz. We find that the ionopause is located on average at an altitude of $363 \pm 65$ km. We also find that the ionopause altitude has a weak dependence on solar zenith angle and varies with the solar extreme ultraviolet (EUV) flux on annual and solar cycle time scales. Furthermore, our results show that very few ionopauses are observed when the crustal field strength at 400 km is greater than 40 nT. The strong crustal fields act as mini-magnetospheres that alter the solar wind interaction and prevent the ionopause from forming.

physics.space-ph↗

arXiv2019

Variations in the Ionospheric Peak Altitude at Mars in Response to Dust Storms: 13 Years of Observations from the Mars Express Radar Sounder

Previous observations have shown that, during Martian dust storms, the peak of the ionosphere rises in altitude. Observational studies of this type, however, have been extremely limited. Using 13 years of ionospheric peak altitude data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on Mars Express, we study how the peak altitude responded to dust storms during six different Mars Years (MY). The peak altitude increased $\sim$10-15 km during all six events, which include a local dust storm (MY 33), three regional regional dust storms (MYs 27, 29, and 32), and two global dust storms (MYs 28 and 34). The peak altitude's orbit-to-orbit variability was exceptionally large at the apexes of the MY 29 and MY 32 dust seasons, and dramatically increased during the MY 28 and MY 34 global dust storms. We conclude that dust storms significantly increase upper atmospheric variability, which suggests that they enhance dynamical processes that couple the lower and upper atmospheres, such as upward propagating gravity waves or atmospheric tides.

astro-ph.EP↗

arXiv2018

Mars Thermospheric Variability Revealed by MAVEN EUVM Solar Occultations: Structure at Aphelion and Perihelion, and Response to EUV Forcing

The Mars thermosphere holds clues to the evolution of the Martian climate, and has practical implications for spacecraft visiting Mars, which often use it for aerobraking upon arrival, or for landers, which must pass through it. Nevertheless, it has been sparsely characterized, even when past accelerometer measurements and remote observations are taken into account. The Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter, which includes a number of instruments designed to characterize the thermosphere, has greatly expanded the available thermospheric observations. This paper presents new and unanticipated measurements of density and temperature profiles (120-200 km) derived from solar occultations using the MAVEN Extreme Ultraviolet (EUV) Monitor. These new measurements complement and expand MAVEN's intended thermospheric measurement capacity. In particular, because the local-time is inherently fixed to the terminator, solar occultations are ideally suited for characterizing long-term and latitudinal variability. Occultation measurements are made during approximately half of all orbits, resulting in thousands of new thermospheric profiles. The density retrieval method is presented in detail, including an uncertainty analysis. Altitude-latitude maps of thermospheric density and temperature at perihelion and aphelion are presented, revealing structures that have not been previously observed. Tracers of atmospheric dynamics are also observed, including a high altitude polar warming feature at intermediate latitudes, and an apparent thermostatic response to solar EUV heating during a solar rotation, which shows heating at high altitudes that is accompanied by cooling at lower altitudes.

astro-ph.EP↗

arXiv2017

Vertical Thermospheric Density Profiles from EUV Solar Occultations made by PROBA2 LYRA for Solar Cycle 24

A new dataset of summed neutral N2 and O number density profiles, spanning altitudes between 150 and 400 km, and observed during Northern Winter from 2010 through 2016 is presented. The neutral density profiles are derived from solar occultation measurements made by the 0.1-20 nm Zr channel on the LYRA instrument onboard PROBA2. The climatology derived from the vertical profiles is consistent with that predicted by the NRLMSISE-00 model, and the systematic error and random uncertainty of the measurements is less than 13% and 6%, respectively. The density profiles are used to characterize the response of thermospheric density to solar EUV irradiance variability. Peak correlation coefficients between neutral density and EUV irradiance occur near 300 km at the dusk terminator and 220 km at the dawn terminator. The sensitivity of thermospheric temperature to EUV irradiance decreases with altitude above 300 km at both terminators.

astro-ph.EP↗