ESR is frequency dependent.
Electrolytic capacitors have a tendency to increase in ESR over time due to drying out or corrosion.
This increase in ESR has particularly been noted as a problem in switching power supplies. The increase in ESR increases both voltage drops within the capacitor and the heat produced in the caps due to resistive heating. AC at 100 kilohertz through the capacitor. The test signal can be sine, square, or pulse waves. The low voltage prevents damage or interference with associated circuits for in circuit tests.