Wave Injection Experiments At Arecibo, Puerto Rico

Abstract

The equations of motion are solved for ions moving in a model electric field which corresponds to the nightside equatorial region of the magnetosphere. Features of the substorm onset such as its association with the Harang discontinuity (HD), the presence of enhanced 0' in the plasma sheet, the creation of a substorm current wedge, and the local weakening of the magnetic field are treated. The divergence of inertia currents carried by the oxygen ions at thc HD rcquircs thc formation of a substorm current wedge. A spontaneous onset occurs where the current wedge is driven to the point when sufficient east-west current is diverted to the ionosphere. A triggered onset occurs when IMFB, turns from south to north causing the cross-tail electric field to pass through zero. Under those conditions an analytic treatment in terms of elliptic functions shows that the ions may go chaotic thereby producing a current interruption.We will discuss our wave injection experiments at Arecibo, Puerto Rico, investigating the spectral characteristics of HF enhanced Langmuir waves among other phenomena. The obliquely propagating HF waves were transmitted from a tilted heater in experiments during August 5-11, 1992, while the vertically propagating HF waves were employed in experiments during September 17-22, 1992. In the "oblique heating " experiments, the main beam of the HF heater waves was tilted by 12' to the north, while the first sidelobe pointed 13' to the south. The radiated power of the first sidelobe was estimated to be only slightly different (-3 dB) from that of the main beam. Thus, the illuminated ionospheric region was significantly much greater than with the vertically transmitted waves from the heater. The cascading spectra of the PDI (parametric decay instabilities) produced Langmuir waves were observed in the vertical heating experiments as expected. The radar measurements of the HF enhanced plasma lines in the oblique heating experiments also showed the excitation of PDI. In addition, a broad, highly structured spectrum separated from the PDI produced spectrum by 15 kHz or so with higher power was generated. It indicates that other process(es) excited by the obliquely propagating heater waves can compete with the PDI. Our comparative studies of Langmuir waves produced by the vertically and obliquely transmitted HF heater are aimed at investigating the source mechanisms responsible for the generation of HF enhanced Langmuir waves. Parametric instabilities and nonlinear scattering processes will be examined as the potential mechanisms. We will show that weak lower hybrid waves with a broad range of frequencies induced presumably by lightning can lead to the generation of frequency-upshifted Langmuir waves as observed in our Arecibo experiments.We review some new features observed in recent critical ionization velocity (CIV) space experiments: CRIT1, CRITZ, CRRES, IBSS, ATLAS-1, and APEX. In the two relcases of CRIT1, the one with higher ionization featured a doublc pulse electric field structure while the other feature...