I don't want to take a stand on this but I would like to present what DARPA presents officially to the public as Explanation of HAARP:
http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=1634"(...)
(U) An April 2001 Defense Threat Reduction Agency (DTRA) Study indicated that a single low-yield (10-20 kt), high-altitude (125 - 300 km)
nuclear detonation (HAND) would enhance the magnetospheric high energy particle density by three orders of magnitude or more, and persist for
over a year. This event would disable all low earth orbit (LEO) satellites not specifically hardened to withstand radiation generated by that
explosion. The resultant loss of communications, imaging, and weather satellites would have catastrophic global security and economic impacts.
Replacing the lost inventory after the fact would cost in the trillions of dollars. The cost of hardening new satellites is also prohibitive. The
probability of such an asymmetric threat is not negligible, given the recent nuclear weapon activities in Asia, and the consequences of such a threat
are disastrous. HAARP will provide a ground-based testbed for early feasibility examination and tradeoffs of concepts for accelerating the
reduction of HAND-enhanced charged particle populations in the radiation belts. Two concepts will be considered: (1) space-based propagation
of very low frequency (VLF) electromagnetic waves into the ionosphere in the magnetic field line where the trapped particles are oscillating
between magnetic mirror points, causing the particles to be “swept away” ten times faster than the natural decay; and (2) high voltage multikilometer
Electrodynamic/Static (ED/ES) space tethers to remove highly charged trapped radiation particles as they traverse the magnetic field
lines. In both concepts, HAARP would be used to establish the optimum electromagnetic wave/tether parameters for an operational system that
could be based on the ground, in space, or both.
(...)"
Page 255
Department of Defense Fiscal Year (FY) 2005 Budget Estimates
February 2004
RESEARCH, DEVELOPMENT, TEST AND EVALUATION, DEFENSE-WIDE
Volume 1 - Defense Advanced Research Projects Agency