Arctic D Region Ionospheric Remote Sensing with an Array of VLF Receivers and D Region Tomography

The D region of the ionosphere (60-90 km altitude) is a layer of the atmosphere defined by a relatively weakly ionized and collisional plasma. It is primarily driven by solar activity, but it is maintained at night by background energetic radiation from cosmic rays. Due to varying solar conditions (e.g. diurnal, seasonal, or solar cycle), the present state of the ionosphere also varies. In addition to the predictable solar flux, there are many other unpredictable drivers such as atmospheric gravity waves, solar flares, lightning-ionosphere coupling, electron precipitation, etc. Despite the myriad of effects and perturbations, the D region is still relatively poorly understood and sparsely studied. This is in part due to the difficulty of making direct measurements. The neutral atmosphere of the D region is too dense for in-situ satellite measurements but not dense enough to support weather balloons.

To respond to the inherent challenges of studying the D region, many workers use remote sensing of very low frequency electromagnetic waves (VLF, 3-30 kHz). VLF waves are a unique tool for D region studies because they reflect efficiently from both the D region and the earth ground forming the earth ionosphere waveguide (EIWG). Therefore, VLF waves propagate to global distances with low attenuation, modulated by the present state of the D region along the transmitter-to-receiver path.

D region studies in the arctic have historically been limited due to the harsh conditions and a limited number of detectable VLF sources. This is unfortunate since numerous space weather phenomena is observable in the lower ionosphere over Earth's poles. This work introduces some initial results from the new Georgia Tech VLF receiver network over the arctic using the database of VLF observations.