Coupling Between the Solar Wind, Magnetosphere, Ionosphere, and
Neutral Atmosphere

D.N. Baker

Laboratory for Atmospheric and Space Physics

U. of Colorado, Boulder, CO 80309

Introduction

The particle flux from the Sun and the magnetosphere represents a large source of energy and ionization for the lower thermosphere and ionosphere. The energy flux, which ranges from <10^-3 J m^-2 s^-1 to over 0.1 J m^-2 s^-1, is deposited by particles with energies that range from hundreds of eV to several hundred MeV. The intensity, spectrum, and localization of the precipitation are functions of solar and geomagnetic activity. The mean values also show longer-term dependence on the solar cycle. The particle-induced ionization leads to dissociation of tightly bound N₂ molecules and to the formation of the reactive nitrogen compounds NO and NO₂. These compounds, when transported to lower altitudes, participate in a catalytic cycle leading to destruction of ozone. Continuing research seeks to understand the impact of magnetospheric and solar energetic particles on the chemistry and electrodynamics of the middle atmosphere.

Atmospheric Processes

During the last 25 years intensive efforts have been underway to develop an understanding of variations in middle atmospheric ozone (O₃) due to natural processes and due to the effects of mankind. It is now reasonably well understood that the global balance of O₃ is governed in part by the balance between the production of O₃ and its destruction by reactions within the NO_y, Cl_y, HO_y, and O_y chemical families. One of the most important catalytic cycles in this global balance is that due to the following reactions:

The bulk of the odd nitrogen (i.e., NO_y, oxides of nitrogen) in the stratosphere is formed from the oxidation of N₂O by O(1D) forming NO. It is known, however, that middle atmospheric NO_y is formed by ion chemistry initiated by the precipitation of energetic particles into the upper atmosphere.

The first step in determining the production of NO_y due to the precipitation of energetic particles is the calculation of the deposition of energy from particle penetration. The particle energy is absorbed by the atmosphere through collisions with, and scattering by, atmospheric constituents which result in either dissociation or ionization of the atmospheric constituents. Production of each electron pair within the atmosphere requires an energy deposition of approximately 35 eV. In addition to the energy imparted directly to the atmosphere during collisions, a certain amount of energy is converted to X-rays by the bremsstrahlung process. This is associated with the rapid deceleration of energetic electrons during their penetration into the atmosphere, providing additional ionization down to altitudes of 20 km.

The ion and neutral odd-nitrogen chemistry initiated by energetic particles produces secondary electrons, e*, which in turn ionize and dissociate the major atmospheric species. This ionization is followed by a series of interchange and recombination reactions involving nitrogen and its ions which produce additional atomic nitrogen. The resulting atomic nitrogen may either be in the ground ⁴S level or excited to the ²D level. Nitric oxide is formed by the reaction of atomic nitrogen with O₂. Reactions involving excited nitrogen are faster than those with the ground state N atom. The destruction of odd nitrogen proceeds through the reaction of N atoms with NO to produce molecular nitrogen and atomic oxygen. In the sunlit atmosphere the photodissociation of NO is important; however, in the fall, winter, and spring in the polar region, the photolytic reaction is negligible and the resulting NO lifetime is sufficient for transport to bring the NO into the mesosphere and stratosphere.

Current numerical models take into account the details of the photochemical complexities and are capable of calculating the odd nitrogen production rates as functions of the energy-dependent ionization and deposition rates. Thus, if energetic particle precipitation can be measured, the auroral-induced emissions and the NO can be specified. As the NO produced in the thermosphere and mesosphere is transported to lower altitudes it reacts with ozone to form NO₂ and remains as this form of odd-nitrogen for the remaining time of transport.

There have been several studies of these effects suggesting that precipitating electrons could lead to the formation of oxides of nitrogen and hydrogen. These could affect O₃ near 60 km and the ion formation rates due to these electrons could dominate those of other processes down to approximately 40 km. Baker et al. [1987] found ion formation rates using electron measurements from geostationary orbit that showed that the fluxes of energetic electrons and hence the ion formation rates were strongly modulated by solar activity. Callis et al. [1991], using estimates of the precipitating electron fluxes derived from geostationary data and 2-D simulations, concluded that the precipitating electrons could have a significant effect on the budgets of both NO_y and O₃ within the stratosphere. This coupling mechanism between solar activity, solar wind structures, the energetic particle populations within the magnetosphere, and the chemical state of the middle atmosphere represents an important linkage which must be understood to assess the natural variations in the global chemical state of the middle atmosphere.

Energetic Particle Sources

The energetic particle flux into the atmosphere has characteristic spectral features and temporal and spatial scales depending on latitude and local time. The most important sources include the solar energetic particles within the polar cap, the diffuse and discrete auroral precipitation within the auroral zone, the energetic magnetospheric electrons extending from mid-latitudes throughout the auroral zone, and the ring current particles precipitating at mid-latitudes. Table 1 summarizes the known particle input characteristics; in the following sections we discuss each type of particle input separately.

Table 1. Particle Input Characteristics

Solar Energetic Particle Influx. Solar energetic particle (SEP) events extending in energy up to hundreds of MeV can produce dramatic enhancements in the ionization and NO_y production rates in the polar regions. Fig. 1 shows instantaneous production rates of NO calculated for several solar particle events. These particles enter via the open magnetic field line regions. Also shown are the average production rates due to Galactic Cosmic Rays (GCRs) at solar maximum, and estimated production rates due to precipitating electrons [Brasseur and Solomon, 1984]. Note that solar protons can have a large effect between the altitudes of ~20 and 100 km.

Of the solar proton events shown in Fig. 1, that of 4 August 1972 was one of the largest on record, the kind that occurs roughly once a decade. However, SEP events of the size of the other two typically occur several times per year during solar maximum. Reid and Solomon [1991] have calculated the ionization rates and resulting effects on NO_y and O₃ for the series of large solar proton events in late 1989 that were comparable in intensity to those of August 1972. They predicted NO increases as large as a factor of 20 near 60 km altitude, with corresponding peak O₃ depletion as large as 20% near 40 km in late October 1989. Furthermore, calculation of the latitudinal range over which the deposition occurs requires information of the geomagnetic cutoff. Because the cutoff can be lowered by several degrees at the time of these events due to associated geomagnetic storm activity, the cutoff needs to be monitored at the same time. The scale on the right of Fig. 1 indicates the minimum energy proton that can penetrate to a given altitude, which shows that calculation of the ionization rate due to solar protons from 20 to 80 km requires information on the proton energy spectrum from about 3 to 500 MeV.

Figure 1: NO production from Solar Proton Events (SPE), Relativistic Electron Precipitation (REP), and Galactic Cosmic Rays (GCR).

Relativistic Electron Precipitation. Baker et al. [1986, 1987] reported observations of the fluxes of energetic (1 - 15 MeV) electrons at geostationary orbit (6.6 R_E) for the period 1979 to 1986, from solar maximum conditions to solar minimum. Their results indicated that large enhancements in the electron population were sporadic in the 1979 - 1981 solar maximum period. However, during the declining phase of the solar cycle, from late 1981 until mid-1986, frequent, large increases in this population were observed. The average intensity and frequency of these events increased dramatically so that the 6-month average of the flux of these electrons was larger by a factor of 8 or 9 than a similar average observed during 1979 - 1980.

The population of relativistic electrons is known to be closely related to the occurrence of high speed (V_SW ³ 600 km/s) solar wind streams and often shows a periodicity of 27 days. Since high speed solar wind can be associated both with coronal mass ejections and with the cyclical development of solar coronal holes, their occurrence is temporally associated with the 11-year solar cycle. The maximum frequency of energetic electrons develops during the declining sunspot phase, but even near solar maximum conditions, very strong relativistic electron events appear in association with coronal mass ejections and the related magnetic clouds interacting with the Earth’s magnetosphere. A dramatic example is the event of March 24, 1991, when a strong interplanetary shock struck the Earth’s magnetosphere and generated several orders of magnitude increases in the fluxes of multi-MeV electrons and protons in the inner magnetosphere. These results provide support for and strengthen the suggestion that relativistic electron precipitation may significantly influence the photochemical state of the middle atmosphere.

Auroral Particle Influx. The particle flux precipitating from the magnetospheric plasma sheet represents a large source of energy and ionization for the lower thermosphere and ionosphere. The energy flux, which ranges from less than 10^-3 J m^-2 s^-1 to over 0.1 J m^-2 s^-1, is mainly deposited by particles with energies that vary from hundreds of eV to several hundred keV. The intensity, energy, and localization of the precipitation are functions of geomagnetic activity: higher, localized fluxes and higher mean energies are found during substorms, and the mean values also show longer-term dependence on the solar cycle.

The discrete auroras are highly structured and localized, and their occurrence frequency maximizes in the premidnight sector at auroral latitudes. Individual auroral arc widths vary from a few km to 100 km in latitude, but during substorms the discrete auroras can cover a region of about 1000 km in latitude and several thousand km in longitude. The discrete precipitation consists mainly of electrons in the energy range from a few hundred eV to a few tens of keV, often accelerated by field-aligned potential structures which produce a characteristic monoenergetic peak in their spectrum. Electrons in the 0.5-30 keV energy range make a significant contribution to the energy input to the lower ionosphere, and are important for maintaining ionization levels in the ~100 km altitude region during the night when photoionization stops and particle impact ionization is dominant. Their energy and ionization inputs are thus important for the study of external influences on the mesosphere.

The diffuse auroras are less structured and persist in the auroral latitudes almost continuously. This more uniform precipitation consists mostly of electrons and ions with energies from a few hundred eV to many tens of keV. Especially, during the substorm recovery phase, energetic (tens of keV) electron precipitation is often found in the morning sector of the auroral oval affecting the chemistry at about 80 km altitude.

Ring Current Ion Precipitation. At mid-latitudes the precipitation from the ring current just inside the plasmapause consists mainly of ions in the 10-200 keV energy range. The ions are anisotropic, with a peak near 90° pitch angle, and have been observed down to an altitude of about 120 km. The energy flux increases with magnetic activity from 10^-7 to 10^-5 J m^-2 s^-1. For disturbed conditions, the precipitating ions and neutrals are the primary source of nighttime ionization at these altitudes and latitudes. Ring current ions interact with the outer edge of the plasmasphere, where they excite ion cyclotron waves. These waves lead to scattering of the ring current ions, which most likely produces the observed precipitation. Outside the plasmapause there is another region of ion precipitation, where the observed precipitating flux is more isotropic. The filled loss cone suggests that a mechanism for strong pitch angle diffusion exists in the outer magnetosphere with energy deposition near 10^-3 J m^-2 s^-1.

Precipitation of energetic ions and neutrals ionizes and heats the thermosphere. Good agreement is found between the ionization rate based on the energetic ion and neutral precipitation with measurements of the electron density profile. The ionization profiles show a nearly constant rate of ionization from 120 to 200 km, and the ionization rates increase with magnetic activity from 10^-1 to 10² cm^-3 s^-1.

Present Evidence of Energetic Particle Coupling to Atmospheric Chemistry

The extensive thermospheric NO data set acquired by the Solar Mesosphere Explorer (SME) satellite has quite clearly shown that NO is produced above 100 km by impact of auroral particles with energies less than 10 keV. On the other hand, the observational evidence of the significance of NO_y production in the middle atmosphere and its subsequent downward transport has been limited: Regular mesospheric NO measurements have become available only after the 1991 launch of the Halogen Occultation Experiment (HALOE) onboard the UARS satellite. These data have provided evidence that large amounts of NO are being produced in the mesosphere. Further evidence suggests transport to the stratosphere.

Callis et al. [1996] looked at specific events and compared the observed NO changes with concurrent electron measurements by the SAMPEX satellite. They found significant NO increases (up to a factor of ten from 60 to 120 km) during long-lasting electron events. A good correlation was found between the E > 30 keV electron measurement and the NO response measured by HALOE. Backed by 2D model calculations of ozone decreases due to enhanced NO_y, Callis et al. [1996] made the provocative suggestion that NO_y enhancements such as discussed above were responsible for the ozone depletions in the lower stratosphere. Thus, the current observational database suggests that the influence of energetic particle precipitation on atmospheric chemistry extends to as low as 25 km. Dedicated measurements of both NO_y and O₃ throughout the polar winter are clearly required to prove this contention.

In summary, as shown in Fig. 2, there are a wide variety of coupling processes between the sun, solar wind, and magnetosphere on the one hand and the various layers of the atmosphere on the other. This coupling affects both the chemical and the electrodynamic properties of the middle and upper atmosphere. Thus, it is quite possible that solar wind and magnetospheric coupling to the lower layers of the atmosphere can have substantial effects on NO_y and O₃ concentrations.

Figure 2: Comparison of energy inputs and subsequent ionization rates in the atmosphere due to energetic particle sources, solar EUV and x-ray, and galactic cosmic rays.

Table 1: Particle Input Characteristics

Figure 2: Comparison of energy inputs and subsequent ionization rates in the atmosphere due to energetic particle sources, solar EUV and x-ray, and galactic cosmic rays.