Energy Input into the Auroral Region Lower Thermosphere

C A Barth, D N Baker, K D Mankoff, and S M Bailey*
Laboratory for Atmospheric and Space Physics , University of Colorado
*Geophysical Institute, University of Alaska

Introduction

This is an online version of a poster presentation at the Fall 2001 AGU Meeting. The poster number was SA41B-0745, and has the title and authors shown above.

The six main sections of the poster are each presented in a section below. Use the following links to view these sections:
  1. Abstract
  2. Introduction
  3. Discussion
  4. Summary
  5. References
  6. Related Web Sites

Abstract

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 60o-70o N. 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 90o W than at 90o E. Therefore, the tilted, offset magnetic dipole field of the Earth seems to be controlling the precipitation of electrons in the northern auroral region.

Introduction

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.

Discussion



Figure 1. Probability of observing electron precipitation with energy flux greater than 5 erg cm-2 s-1 as a function of geomagnetic latitude and MLT [Newell, et al., 2001].


Figure 2. A map of average auroral ultraviolet intensity as a function of geomagnetic latitude and MLT [Liou, et al., 1997].


Electron precipitation associated with auroral arcs occurs predominately in the 1800-2400 MLT sector. A comprehensive survey [Newell et al., 1996] described a number of satellite measurements of the flux of precipitating electrons as a function of magnetic local time. Figure 1 which is taken from a review [Newell et al., 2001] shows quite clearly that the precipitating electron flux is concentrated in the 1800-2400 MLT sector. Satellite measurements of the ultraviolet aurora similarly indicate that the most intense ultraviolet emissions from the aurora occur in this same MLT sector [Liou et al., 1997]. The intensity of the average auroral ultraviolet emissions is greatest at 67o N geomagnetic latitude and 2230 hours MLT (Figure 2).


Figure 3. Model calculation of the nitric oxide density at 65o N, 216o W geographic coordinates at the time of equinox.


The MLT dependence of the satellite electron flux measurements has been used as a guide to determine the energy input into a thermospheric photochemical model [Bailey et al., 2001]. The input into the model has the auroral electron flux turned on during the 1800-2400 MLT period and off for the remaining time of the day. During the daylight hours, 0600-1800 of the first day, the nitric oxide density is nearly constant at this latitude. At 1800 hours, the auroral electron precipitation is turned on. The nitric oxide density increases rapidly during the time that the auroral flux is switched on reaching a value four times greater at midnight than existed at sunset. At midnight (0000 hours) the electron precipitation is turned off. For the remaining hours of the night there is no auroral precipitation and the nitric oxide density remains nearly constant. From sunrise throughout the daylight hours, the nitric oxide density decreases because of photodissociation.

Figure 4. Polar image of the flux of precipitating electrons at 106 km as a function of corrected geomagnetic latitude and longitude (CGM). The 0o longitude line crosses the east coast of the United States. The lines of longitude are spaced 24o apart.


A polar display of the resulting flux is presented in Figure 4. This is an orthographic projection in geomagnetic coordinates with the magnetic north pole at the center. The outlines of the continents are plotted in geomagnetic coordinates and, hence, are distorted compared to how they look in an orthographic projection in geographic coordinates. The maximum in the electron flux generally follows the 60o-70o N geomagnetic latitude band, but there are important deviations from this circular pattern. The electron flux varies as function of geomagnetic longitude: the maximum lies between 108 W and 36o W and the minimum between 36o E and 108o E. This longitude dependence may be seen more clearly in Figure 5a where the flux at geomagnetic latitude 65o N is plotted as a function of longitude. The maximum electron flux is 50% greater than the minimum. The average flux as a function of latitude is plotted in Figure 6 which shows the maximum to occur at 65o N with the half-intensity levels at 55o N and 75o N geomagnetic latitude.


Figure 5. (a) A plot of the electron flux at geomagnetic latitude 65o N as a function of longitude. (b) The magnetic field strength at 65o N and 65o S geomagnetic latitude as a function of geomagnetic longitude.


Figure 6. A plot of the average electron flux as a function of geomagnetic latitude.


The magnetic field strength at 65o N and S geomagnetic latitude has been plotted in Figure 5b as a function of geomagnetic longitude. Between geomagnetic longitudes 30o W to 120o E, the magnetic field strength is lower in the southern auroral region than in the northern region. Mirroring electrons penetrate deeper into the atmosphere in the south and preferentially deplete the supply of electrons available for precipitation in the north. Since the mirroring electrons drift eastward, the effect of the weak magnetic field in the south Atlantic region is seen in the north eastward of 90o E geomagnetic longitude where the SNOE observations show the nitric oxide density to be the lowest.

Summary

Based on the results presented in this paper, a paradigm may be constructed that describes the magnetospheric control of the energy input into the thermosphere. In the magnetosphere, auroral electron precipitation occurs most strongly between 1800 and 2400 MLT. Magnetospheric activity causes variations in the flux of precipitating electrons. As the Earth rotates beneath the magnetosphere, the magnetic field lines of the offset, tilted dipole field determine where the electrons impact the thermosphere. The magnetic field strength controls the distribution of the precipitating electrons in geomagnetic latitude and longitude. The result is that there is a larger energy input into the northern auroral thermosphere in the western geomagnetic hemisphere than in the eastern. This means that there is greater heating, ionization, and auroral luminosity in the west than in the east.

References

Bailey, S.M., C.A. Barth, and S.C. Solomon, A model of nitric oxide in the lower thermosphere, J. Geophys. Res., submitted, 2001.

Barth, C.A., D.N. Baker, K.D. Mankoff, and S.M. Bailey, The northern auroral region as observed in nitric oxide, Geophys. Res. Lett., 28, 1463, 2001.

Liou, K., P.T. Newell, and C.-I. Meng, Synoptic auroral distribution: A survey using Polar ultraviolet imagery, J. Geophys. Res., 102, 27,197, 1997.

Newell, P.T., K.M. Lyons, and C.-I. Meng, A large survey of electron acceleration events, J. Geophys. Res., 101, 2599, 1996.

Newell, P.T., R.A. Greenwald, and J.M. Ruohonieme, The role of the ionosphere in aurora and space weather, Rev. Geophys., 39, 137-149, 2001.

Related Web Sites

The SNOE Data Website

The SNOE Data Viewer Webpage

The SNOE Spacecraft Website

Auroral Arc Probability Maps

The POLAR UVI Website