HF-driven currents in the polar ionosphere

Abstract

Ionospheric heaters located in the polar-regions (EISCAT, HAARP, HIPAS) have generated waves in the ULF/ELF/VLF range by modulating the auroral electrojet at D/E region altitudes in a concept termed by the EISCAT group as the Polar Electrojet (PEJ) antenna. The PEJ antenna requires an ionospheric heater located under regions of naturally driven D/E region currents, such as in the auroral or equatorial electrojet. In a simplified fashion the PEJ antenna operates as following: In the presence of an electrojet current periodic electron heating in the Hall dominated region of the lower ionosphere reduces the conductivity of the heated spot building oscillatory polarization charges at the boundaries. This acts as an essentially Horizontal Electric Dipole (HED) at the modulation frequency. The HED excites the Earth-Ionosphere Waveguide (EIW) from above while the current closes by launching upwards helicon (whistler) waves carrying field-aligned current. A major problem with the PEJ is its reliance in the presence and strength of the electrojet current. This restricts not only the location of the heater but makes their generation highly unpredictable. We will present theoretical/computational results indicating that modulated HF heating can generate ionospheric currents in the ULF/ELF range even in the absence of electrojet currents. The new concept relies in using HF heating of the F-region to modulate the electron temperature and has been given the name Ionospheric Current Drive (ICD). The ICD is a two-step process: First, the pressure gradient associated with anomalous or collisional F-region electron heating drives a local diamagnetic current that acts as an antenna to inject mainly Magneto-Sonic (MS) waves in the ionospheric plasma. Second, the electric field associated with the MS wave drives Hall currents when it reaches the E region of the ionosphere. The Hall currents act as a secondary antenna that inject waves in the Earth-Ionosphere Waveguide (EIW) below and Shear Alfven (SA) waves upwards towards the conjugate regions. We will examine the scaling and limitations of the concept with simulation and suggest proof-of-principle experiments using the HAARP ionospheric heater. We will discuss the implications of freeing the ionospheric ULF/ELF generation from the requirement of electrojet availability and strength.