Generally, the solar and interplanetary radio radiation is considered to be generated by plasma emission. In such processes, suprathermal electrons excite high frequency electrostatic waves (e. g. Langmuir waves and upper hybrid waves), which convert into electromagnetic (radio) waves by nonlinear interaction with low frequency plasma waves and/or scattering off ion density fluctuations (Melrose 1985). This mechanism is responsible for the fundamental radiation. Furthermore, the coalescence of two high frequency electrostatic plasma waves leads to the harmonic emission (Melrose 1985). Thus, the radio waves are emitted near the local electron plasma frequency (e, elementary charge; N, electron number density; , electron mass) or its harmonics. Then, high frequency radio waves (e. g. 400 MHz) are generated in the low corona, while the low frequency waves (e. g. 20 kHz) are emitted from sources roughly located at 1 AU (1 AU = ). A radial motion of a radio source would appear as a drift in the corresponding dynamic radio spectrum. The relationship between the drift rate measured at the frequency f and the radial source velocity is given by
Such radio sources are sub-relativistic electron beams and travelling shock waves, for instance, appearing as type III and II bursts in dynamic radio spectra, respectively (cf. e. g., Suzuki & Dulk (1985) and Gurnett (1995) as reviews). The knowledge of a heliospheric density model would provide the distance of the radio source from the Sun. Consequently, the radial velocity of the radio source can be deduced from the drift rate of the associated radio signature in dynamic radio spectra according to (1). These facts illustrate the importance of the knowledge of a heliospheric density model.
The heliosphere is highly structured, i. e., it is filled by high and low speed solar wind streams, the so-called heliospheric current sheet, corotating interaction regions and travelling disturbances (e. g. coronal mass ejections and interplanetary shocks) (cf. e. g. Schwenn (1990) as a review). Because of these reasons, a global radial density model of the heliosphere is an approximation. Nevertheless, such a global model would be useful for the interpretation of solar and interplanetary radio data as presently received by the instruments URAP (Stone et al. 1992) and WAVES (Bougeret et al. 1995) aboard the spacecrafts ULYSSES and WIND, respectively. Therefore, we derive a heliospheric density model, which is an average but in agreement with observations between the low corona and 5 AU.
In Sect. 2 the heliospheric density model is derived by means of the magnetohydrostatic equations supplemented by an isothermal equation of state and the gravitational force of the Sun. The resulting density model is compared with different measurements of the density in the low corona and the interplanetary space in Sect. 3. Finally, the model is applied for estimating the velocities of energetic electrons associated with the solar and interplanetary type III radio bursts on December 27, 1994.
© European Southern Observatory (ESO) 1999
Online publication: July 26, 1999