Space Weather dependence of the air drag as observed by CHAMP Hermann Lühr 1) and Huixin Liu 2) 1) GeoForschungsZentrum Potsdam, Germany 2) Dept. Earth and Planetary Science, Hokkaido Univ., Sapporo, Japan 2 nd European Space Weather Week, ESWW 2005 ESTEC, 14 – 18 Nov. 2005

 For satellites in low-Earth orbit air drag is generally the most important disturbance force determining lifetime, fuel consumption and predictability of ephemeris.  Current atmospheric models are not capable of predicting the dynamics of the thermosphere adequately. This is, in particular, true during magnetically disturbed periods.  Recent satellite missions, such as CHAMP, carrying sensitive accelerometers provide detailed observations and offer the possibility to study the relevant forcing mechanisms. Motivation

CHAMP Payload Instruments

Prölss (2001) Physik des erdnahen Weltraums Global Density and Wind Distribution at 300 km Altitude

m=520 km, C d =2.2, V 2 =V s 2 +V c 2 Deriving the Thermospheric Density from the Accelerometer

Average Thermospheric Density at 400 km (10^-12 kg m^-3) Liu et al., 2005

Mid-latitude density enhancement

Low latitude density variation, 2003

Propagation of the Density Disturbance Thermospheric density vs. time (unit:10s) during three orbits on Oct. 29, 2003.

Thermospheric Density During magn. Storm Liu and Lühr, 2005

Response of Thermospheric Density: Change over quiet day ρ experienced large disturbance, ρ↑at high latitudes first, then propagated to lower latitudes. It recovered quickly within 12 h after the storm main phase, It reached pre-storm condition within 26 h after Dst minimum. Thermospheric density variation. Time starting at 00 UT on Nov. 20, 2003.

Density features at high latitude At auroral latitudes distinct features of enhanced density are present. On the dayside, cusp region: density peak shows little dependence on magnetic activity. On the nightside, pre-midnight: density enhancement depends on substorm activity. Percentage difference in polar regions Liu et al., 2005

Air Drag Spikes in Cusp Region Lühr et al., 2004

Density and Currents

Heating and Up-welling

Deriving the Thermospheric Winds from the Accelerometer Underlaying concept: The acceleration vector is parallel to the relative velocity of the air Zonal wind: at polar regions special considerations are required.

Zonal Winds at Equator Liu et al. (2005)

A super rotation of the upper atmosphere was deduced from satellite orbit inclination changes (King-Hele, 1964). There has been a long debate about the driver for this net motion of the air. A very promising candidate mechanism is the low-latitude F-region dynamo proposed by Rishbeth (1971). Super Rotation of the Atmosphere

Equatorial eastward windjet at night [m/s]

Conclusions  The CHAMP satellite has provided for the first time insight into the full spatial and temporal variability of thermospheric density and winds.  Space weather related modifications of the thermospheric density is not well predicted by models like MSIS. This is in particular true for effects driven by currents and plasma interaction.  Upper atmospheric winds are even less understood. They play a key role in the interaction between charged and neutral particles. For a low drag spacecraft like GOCE they are an important factor in the disturbance balance.  For signification progress in predictability of air drag conditions it is required that thermodynamic and electrodynamic effects are considered at the same time. A mission like Swarm has the ability to provide the required information on a near-realtime basis.

Winds across the pole, Summer 2003 Vx: sunward Vy: dawnward

Thermospheric Density During magn. Storm Liu and Lühr, 2005

Responses at Different Latitudes