The impact of electron precipitation on Earth's thermospheric NO production and the drag of LEO satellites

Abstract

We investigate the response of space weather events on Earth's upper atmosphere over the polar regions by studying their effect on the drag of the CHAMP and GRACE satellites. Increasing solar activity that results in heating and the expansion of the upper atmosphere threatens low Earth orbit (LEO) satellites. Auroral events are closely related to the stellar energy deposition of solar EUV radiation and precipitating energetic electrons, which influence photochemical processes such as the production of nitric oxide (NO) in the upper atmosphere. To study the production of NO molecules and their influence on the thermospheric structure and satellite drag, we first model Earth's background thermosphere with the 1D upper atmosphere model Kompot by considering the incident X-ray, EUV, and IR radiation during selected space weather events. To investigate the effect of electron precipitation in the production of NO molecules in the polar thermosphere, we apply a Monte Carlo model accounting for the stochastic nature of collisional scattering of auroral electrons in collisions with the surrounding N$_2$-O$_2$ atmosphere, including the production of suprathermal N atoms. The observed effect of the atmospheric drag on CHAMP and GRACE during the two studied events indicates that a sporadic enhancement of NO molecule production in the polar thermosphere and its IR-cooling capability, which counteracts thermospheric expansion and can lead to an ``overcooling'' with decreased density after the space weather event, can have a protective effect on LEO satellites. Their production efficiency, however, is highly dependent on the energy flux of the precipitating electrons. Our results have direct implications for empirical satellite orbit prediction models, as our simulations highlight the need to consider precipitation-induced NO production to improve the predictive power of these models.

Figures (9)

• Figure 1: Evolution of the altitude of the satellite for the CHAMP (orange) and GRACE (blue) missions over their entire mission duration. Thick lines correspond to the mean altitude per revolution. For both satellites, the shaded areas illustrate their orbits' eccentricity, i.e., the difference between apogee and perigee, which decreases over time. The sudden changes in CHAMP's orbit are due to orbital maneuvers.
• Figure 2: Impact of a CME in November 2004 on the CHAMP (left) and GRACE-A satellites (right) at altitudes of 370 km and 490 km, respectively. The top and bottom panels show the neutral density [kg/m$^3$] evolution and the storm-induced orbit decay [m]. Additionally illustrated are the calculated background density (orange line), the start time of the disturbance (red line), i.e., the time of the arrival of a shock or the leading edge of the CME at the Earth, and the observed start and end times of the responsible near-Earth interplanetary CME plasma/magnetic field (blue lines), as specified by the R&C catalog. CHAMP shows a larger decline in altitude than GRACE-A. This is expected, since the absolute increase in the thermospheric density, $\Delta\rho$, between the onset of such an event and the maximum thermospheric density during the event will typically be larger at the lower compared to the higher orbit (i.e., roughly $\Delta \rho_{\rm CHAMP}\sim10^{-11}\rm\,kg/m^3$ and $\Delta \rho_{\rm GRACE-A}\sim10^{-12}\rm\,kg/m^3$ for the orbits of CHAMP and GRACE-A, respectively).
• Figure 3: There are two distinct measurement configurations for TIMED/SABER, indicated by blue and green. To avoid solar infrared radiation, SABER does not view towards the Sun. The dashed blue line shows the measurement range when the Sun is on the right side of the sketch. Due to the precession of the orbital plane, after some time, the Sun will appear on the left side. When that happens, SABER will observe the green area for the next 60 days. Polar gaps are hence existing in the data, which must be kept in mind when interpreting differences in the NO maps.
• Figure 4: Altitude-integrated Nitric oxide (NO) flux observed by the SABER instrument on board the TIMED satellite during the CME event in November 2004. The top and bottom rows illustrate the measurements taken on the northern and southern hemispheres, respectively. These maps are generated by considering all SABER measurements taken during the day, and by then using an interpolation scheme Mlynczak2003.
• Figure 5: Impact of a CME in May 2005 on the CHAMP satellite (left) and GRACE-A satellite (right) 360 km and 480 km. The top and bottom panels show the neutral density [kg/m$^3$] evolution and the storm-induced orbit decay [m], respectively. Additionally illustrated are the calculated background density (orange line), the start time of the disturbance (red line), i.e., the time of the arrival of a shock or the leading edge of the CME at the Earth, and the observed start time of the responsible near-Earth interplanetary CME plasma/magnetic field (blue lines), as specified by the R&C catalog. As for the first event (Fig. \ref{['Fig2']}), CHAMP again shows a larger decline in altitude than GRACE-A, since the absolute increase in the thermospheric density is again larger at the lower compared to the higher orbit, as expected (both again in the order of $\Delta \rho_{\mathrm{CHAMP}}\sim10^{-11}\mathrm{\,kg/m}^3$ and $\Delta \rho_{\mathrm{GRACE-A}}\sim10^{-12}\mathrm{\,kg/m}^3$).