The abnormal LF disturbances for the period 1974--1992 for an auroral radio trace Aldra--Apatity in the frequency range 10--16 kHz are analyzed. The disturbances are associated with relativistic and ultrarelativistic electron precipitations into the middle atmosphere. The statistics of events indicate that one may expect the 22--year periodicity in the activity of relativistic electron precipitations. The morphology of the most intensive disturbances (powerful disturbances) for the period is presented, and according to their maximum durations the estimation of the events space range is given (some thousand kilometers in the auroral zone). Solving of the corresponding inverse LF problem for the events has indicated, just as in our previous publications, a high ionization of the atmosphere at 20--40 km altitudes. Such ionization is generated by the ultrarelativistic electrons (with energy more the some tens of MeV) in the presented events.