On the propagation of low‐latitude whistlers

We review here the recent works on low‐latitude whistler propagation based on ground measurements with a reference to space observations and point out the role of low‐latitude ground whistlers in the general study of whistler propagation. Then we identify the main unsolved problems of low‐latitude whistlers: (1) whether or not ground whistlers propagate along field lines, (2) the characteristics of transmission through the ionosphere, and (3) the properties of low‐latitude whistler ducts (formation and decay times) and their structure. These fundamental problems also concern high‐latitude whistlers, but they have less influence, since the rate of duct occurrence is higher and the duct trapping is much easier at high latitudes. On the other hand, considering that the smaller dip angle of the geomagnetic field at low latitudes makes the duct trapping and the ionospheric transmission very hard and that the rate of duct occurrence itself is not so high, we can say that low‐latitude ground whistlers have suffered severe conditions during the course of their propagation. In other words, the fuller study of low‐latitude whistlers will contribute to solving the most poorly understood problem: how the whistler propagates upward from its source, excites ducts, and leaks from ducts, penetrating to the ground. Furthermore, the characteristics of whistler ducts may be studied effectively by using low‐latitude whistlers. Hence these questions should be investigated during this International Magnetospheric Study period.