Magnetospheric control of the energy input into the thermosphere

Received 8 November 2001; revised 16 January 2002; accepted 17 January 2002; published 4 July 2002. [1] A thermospheric photochemical model has been used in conjunction with satellite observations of nitric oxide to determine the flux of precipitating electrons into the thermosphere in the northern auroral region. Satellite measurements have shown that electron precipitation occurs predominantly between 1800 and 2400 hours magnetic local time. This time dependence has been incorporated into the thermospheric model calculation of nitric oxide density. Polar displays of the results of the model calculations show that the electrons deposit their energy between geomagnetic latitudes 60–70N. The longitudinal distribution is asymmetric with more energy being deposited at western geomagnetic longitudes and less at eastern longitudes. A calculation for equinox conditions shows that energy input as a function of longitude is 50% greater at 90W than at 90E. Therefore, the tilted, offset magnetic dipole field of the Earth seems to be controlling the precipitation of electrons in the northern auroral region. INDEXTERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 0358 Atmospheric Composition and Structure: Thermosphere—energy deposition; 2704 Magnetospheric Physics: Auroral phenomena (2407); 2716 Magnetospheric Physics: Energetic particles, precipitating; 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions [2] The Earth’s magnetosphere is strongly asymmetric in local time with a compressed dayside structure and an elongated nightside tail region. This leads to a pronounced day-night asymmetry in auroral properties and large-scale current systems. The upper atmosphere of the Earth, of course, also shows very obvious local time asymmetries, but the corotation of the atmosphere with the Earth itself means that a decoupling of the magnetosphere from the atmosphere must ultimately occur to at least some degree. This decoupling has important consequences that we wish to explore in this paper. [3] Satellite observations of nitric oxide in the thermosphere show that the flux of precipitating auroral electrons has an asymmetric distribution in geomagnetic longitude [Barth et al., 2001]. These results have been interpreted as being the result of the asymmetric distribution of the magnetic field strength of the Earth’s offset, tilted dipole field. To better understand the relationship between the magnetosphere and the thermosphere, we have used a thermospheric photochemical model to calculate the expected nitric oxide distribution. We start with well-known satellite measurements of the magnetospheric distribution of precipitating electrons. With the photochemical model, we calculate the thermospheric distribution of nitric oxide which results from the interaction of the precipitating electrons with the constituents of the upper atmosphere. We pay particular attention to the use of the appropriate coordinate systems for the magnetosphere and for the thermosphere. In the magnetosphere, the appropriate coordinates are geomagnetic latitude and magnetic local time (MLT). The Earth with its magnetic field is rotating beneath this magnetospheric coordinate system. The results of the electron precipitation are imprinted on the rotating thermosphere and are best described in geomagnetic latitude and geomagnetic longitude coordinates. The goal here is to clarify how the magnetosphere controls the energy input into the thermosphere.