On the Origin of Far‐Aspect Angle Irregularity Regions Seen by HF Radars at 100‐km Altitude

A special summer midday population of echoes has been reported by high‐frequency (HF) radars at ranges less than 300 km. These echoes come from a narrow altitude region near 100 km. Their spectral width indicates that they decay at a rate controlled by ambipolar diffusion. We label the echo regions as “far‐aspect angle irregularity regions” by contrast with “high‐aspect angle irregularity regions” (HAIR) documented in the past. Both types are from structures that decay after an initial growth phase. We show that each type decays at a rate controlled by the altitude where it is produced. The large aspect angles and decaying process come from a monotonic aspect angle increase due to a weak altitude dependence of the “eigenfrequency.” HAIR echoes are produced by the decay of otherwise standard E ‐region irregularities. However, HAIR echoes are usually not far‐aspect angle irregularity regions because the latter are the result of a Gradient Drift Instability in the bottomside of the E ‐region where the vertical density gradients can sometimes be so strong that a less than 10 mV/m northward electric field is able to destabilize 10‐m structures in the afternoon sector. Owing to smaller dip angles, the Gradient Drift Instability requirement is smaller at midlatitudes although it still needs a combined electron drift and eastward neutral wind contribution of the order of 100 m/s. With northward electric fields of 2 to 3 mV/m, the minimum midlatitude eastward wind requirement is of the order of 50 m/s.